NIST Technical Note 1896

High-Speed Monitoring of Multiple

Grid-Connected Photovoltaic

Array Configurations

Matthew T. Boyd

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1896

NIST Technical Note 1896

High-Speed Monitoring of Multiple 

Grid-Connected Photovoltaic

Array Configurations

Matthew T. Boyd

Energy and Environment Division

Engineering Laboratory

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1896

October 2015

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie May, Under Secretary of Commerce for Standards and Technology and Director 
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1896

Natl. Inst. Stand. Technol. Tech. Note 1896, 60 pages (October 2015)

CODEN: NTNOEF

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1896
ii

Preface

This effort was conducted by the Energy and Environment Division in the Engineering Laboratory at the National 

Institute of Standards and Technology (NIST). This document describes the data acquisition systems (DASs) for 

three photovoltaic (PV) arrays on the NIST campus in Gaithersburg, Maryland, USA, and includes the rationale 

behind choices made for the sensors, data loggers, control software, and supplementary devices that comprise the 

DASs. The intended audiences are modelers who wish to use the data gathered from these specific systems and 

researchers who build and utilize similar DASs. Modelers and analysts who are interested in using the data in 

collaboration on new research before a public data portal is made available can contact the author.

Author Information

Matthew T. Boyd

Mechanical Engineer

National Institute of Standards and Technology

Engineering Laboratory

100 Bureau Drive, Mailstop 8632

Gaithersburg, MD 20899-8632

Tel.: 301-975-6444

Email: matthew.boyd@nist.gov
iii

Abstract

Three grid-connected monocrystalline silicon photovoltaic arrays have been instrumented with research-grade 

sensors on the Maryland campus of the National Institute of Standards and Technology (NIST). These arrays range

from 73 kW to 271 kW and are of different tilts, orientations, and configurations. Irradiance, temperature, wind, and 

electrical measurements at the arrays are recorded, and images are taken of the arrays to monitor any shading or 

anomalies. Nearly all measurements are sampled and saved every 1 second, with monitoring having started 

August 1, 2014. This report describes the instrumentation approach to capture the performance of these photovoltaic 

systems for use in computer model validation or performance monitoring. 

Keywords

data acquisition, solar energy, photovoltaic array, inverter, meteorology
iv

Table of Contents

Preface .......................................................................................................................................................................... ii

Abstract........................................................................................................................................................................ iii

Table of Contents..........................................................................................................................................................iv

List of Figures...............................................................................................................................................................vi

List of Tables............................................................................................................................................................. viii

Glossary........................................................................................................................................................................ix

1. Introduction ...............................................................................................................................................................1

2. Overview ...................................................................................................................................................................4

2.1. Summary.............................................................................................................................................................4

2.2. Canopy Array .....................................................................................................................................................5

2.3. Ground Array......................................................................................................................................................8

2.4. Roof Array..........................................................................................................................................................9

3. Measurements..........................................................................................................................................................11

3.1. Summary...........................................................................................................................................................11

3.2. Irradiance ..........................................................................................................................................................12

3.3. Ambient Temperature .......................................................................................................................................15

3.4. Module Temperature ........................................................................................................................................17

3.5. Wind .................................................................................................................................................................20

3.6. DC Electrical ....................................................................................................................................................22

3.7. AC Electrical ....................................................................................................................................................25

4. Data Acquisition and Control ..................................................................................................................................27

4.1. Data loggers......................................................................................................................................................27

4.2. Reference Modules...........................................................................................................................................36

4.3. Cameras............................................................................................................................................................38

5. Data Archiving and Monitoring...............................................................................................................................39

6. Uncertainty ..............................................................................................................................................................41

7. Maintenance ............................................................................................................................................................41

Acknowledgements .....................................................................................................................................................41

References ...................................................................................................................................................................41

Appendix A. Oblique Images of the Arrays ............................................................................................................ A-1

Appendix B. Manufacturer Data Sheets ...................................................................................................................B-1

Appendix C. Copper Wire Gauges and Lengths.......................................................................................................C-1

Appendix D. Diagrams of the Arrays...................................................................................................................... D-1
v

Appendix E. The Saved Data Values From Each Array...........................................................................................E-1

Appendix F. Anonymized Diagram of the Data Acquisition Systems’ Communications Network ......................... F-1
vi

List of Figures

Figure 1-1 A map of the instrumented PV arrays and weather station on the NIST campus in Gaithersburg, 

Maryland. The horizontal roof array shown will be instrumented as part of a future project. The vertical 

edge of this image corresponds to 1 km (0.6 miles or 3300 ft)..................................................................1

Figure 1-2 Canopy Array..............................................................................................................................................2

Figure 1-3 Ground Array..............................................................................................................................................2

Figure 1-4 Roof Array ..................................................................................................................................................3

Figure 2-1 Diagram of the Canopy Array ice guards....................................................................................................6

Figure 2-2 Canopy Array electrical layout and measurement locations. The numbered squares are the combiner 

boxes and the hatched regions are the source circuits. Note that “POA Pyra.” stands for a pyranometer 

in the plane of the array, “GHI” stands for a pyranometer measuring the global horizontal irradiance, 

and “RTD” stands for resistance temperature detector, which are mounted on the backsides of the 

respective modules. The integrator also has an RTD mounted on the back of a cell on the module north 

of combiner box 3, the same module with a NIST RTD............................................................................7

Figure 2-3 The Canopy Array inverter, identical to the Ground Array inverter...........................................................7

Figure 2-4 Ground Array electrical layout and measurement locations. The numbered squares are the combiner 

boxes and the hatched regions are the source circuits. The gray regions on the front, back, and east sides 

of the array are bioretention areas. Note that “POA Pyra.” stands for a pyranometer in the plane of the 

array, “GHI” stands for a pyranometer measuring the global horizontal irradiance, and “RTD” stands for 

resistance temperature detector, which are mounted on the backsides of the respective modules. The 

integrator also has an RTD mounted on the back of a cell on the module east of combiner box 3, the 

same module with a NIST RTD.................................................................................................................8

Figure 2-5 Wind deflectors on the rear of the Roof Array module racks .....................................................................9

Figure 2-6 Roof Array electrical layout and measurement locations. The numbered squares are the combiner boxes 

and the hatched regions are the source circuits. Note that “POA Pyra.” stands for a pyranometer in the 

plane of the array, “GHI” stands for a pyranometer measuring the global horizontal irradiance, and 

“RTD” stands for resistance temperature detector, which are mounted on the backsides of the respective 

modules....................................................................................................................................................10

Figure 2-7 The Roof Array inverter............................................................................................................................10

Figure 3-1 The Canopy Array weather instruments at the north end of the center canopy, showing the irradiance 

sensors and mounts as well as the ambient temperature sensor, wind sensor, and wind sensor surge 

protector...................................................................................................................................................13

Figure 3-2 The Ground Array irradiance sensors at the center of the array, showing the sensors and mounts ..........13

Figure 3-3 The Roof Array irradiance sensors at the southeast corner of the array, showing the sensors and mounts

.................................................................................................................................................................14

Figure 3-4 The irradiance sensors at the center of the older horizontal PV array, showing the sensors and mounts .14

Figure 3-5 The temperature probe and radiation shield used to measure the outdoor ambient air temperature at each 

array .........................................................................................................................................................16

Figure 3-6 The temperature probe and enclosure used to measure the ambient air temperature in the inverters at the 

Canopy and Ground Arrays .....................................................................................................................17

Figure 3-7 A visual comparison of the three types of module temperature sensors used at the arrays and for the 

reference modules: the RTD used at the arrays (left), the RTD used for the reference modules (center), 

and the thermocouple used for the reference modules (right)..................................................................18

Figure 3-8 An RTD mounted on the back of a module using thermally conductive epoxy and an adhesive film 

overlay .....................................................................................................................................................19

Figure 3-9 The integrator’s module temperature sensor mounted on the back of a module at the Ground Array......20

Figure 3-10 The wind sensor at the Ground Array .....................................................................................................21

Figure 3-11 The wind sensor and outdoor ambient temperature sensor at the Roof Array ........................................21
vii

Figure 3-12 The custom voltage divider circuit for measuring the array DC voltage ................................................22

Figure 3-13 The inverter DC combiner compartment at the Ground Array with the installed DC measurement 

components, including the current shunts, and left-to-right on the upper DIN rail: the ambient 

temperature sensor, shunt RTU, analog transmitter, voltage divider, and fuse holder...........................24

Figure 3-14 The outdoor DC measurement enclosure at the Roof Array, which includes the current shunts, and leftto-right on the upper DIN rail: the fuse holder, voltage divider, analog transmitter, and shunt RTU....24

Figure 3-15 The AC power meter at the Ground Array (formerly branded by Square D), also showing the separate 

dedicated enclosure for the voltage tap fuses.........................................................................................26

Figure 4-1 The data acquisition enclosure at the Canopy Array showing the data logger, measurement and 

communication peripherals, battery backup, and power converters and distribution blocks................27

Figure 4-2 The integrator’s RTU and a terminal block in their outdoor enclosure at the Ground Array ...................29

Figure 4-3 Breakdown of the DAS energy draws and UPS backups..........................................................................30

Figure 4-4 The indoor communication hubs at the Ground Array for the NIST (top) and integrator (bottom) 

equipment...............................................................................................................................................31

Figure 4-5 Diagram of local network at each array ....................................................................................................32

Figure 4-6 Half-bridge for measuring RTDs..............................................................................................................35

Figure 4-7 The reference modules installed at the weather station.............................................................................36

Figure 4-8 The I-V curve tracer with maximum power tracker and load ...................................................................37

Figure 4-9 The PTZ camera that records images of the Canopy Array ......................................................................38

Figure 4-10 The enclosure for the Canopy and Roof Array camera midspans and network switch...........................39

Figure 5-1 The main data dashboard for the Ground Array .......................................................................................40

Figure A-1 Oblique Aerial View of the Canopy Array Site .....................................................................................A-1

Figure A-2 Oblique Aerial View of the Ground Array Site......................................................................................A-2

Figure A-3 Oblique Aerial View of the Roof Array site ..........................................................................................A-3

Figure B-1 Manufacturer Datasheet for the PV Module at All Three Arrays...........................................................B-1

Figure B-2 Manufacturer Datasheet for the Inverter at the Canopy and Ground Arrays..........................................B-2

Figure B-3 Manufacturer Datasheet for the Inverter at the Roof Array....................................................................B-3

Figure D-1 Diagram of the Canopy Array Strings and Pertinent Array Dimensions ...............................................D-1

Figure D-2 Diagram of the Ground Array Strings and Pertinent Array and Fence Dimensions ..............................D-2

Figure D-3 Diagram of the Roof Array Strings and Pertinent Array Dimensions....................................................D-3

Figure F-1 Anonymized Diagram of the Data Acquisition Systems’ Communications Network ............................ F-1
viii

List of Tables

Table 2-1 Summary of the Campus PV Arrays ............................................................................................................5

Table 3-1 Summary of the Measurements and Respective Instruments Installed at the Campus PV Arrays.............11

Table 3-2 Inverter DC Measurements ........................................................................................................................23

Table 3-3 Inverter and Power Meter AC Measurements............................................................................................26

Table C-1 Copper Wire Gauges and Lengths...........................................................................................................C-1

Table E-1 The Saved Data Values From Each Array ...............................................................................................E-1
ix

Glossary

AC alternating current

ADC analog-to-digital converter

AWG American wire gauge

BORCAL Broadband Outdoor Radiometer Calibration

bps bits per second

comm. communication

CT current transformer

CW clockwise

DAS data acquisition system

DC direct current

DHI diffuse horizontal irradiance

DNI direct normal irradiance

DST daylight saving time

EMI electromagnetic interference

GHI global horizontal irradiance

IC integrated circuit

IR infrared

I-V current-voltage

LIU light interface unit

LST local standard time

NIST National Institute of Standards and Technology

NREL National Renewable Energy Laboratory

NZERTF Net-Zero Energy Residential Test Facility

POA plane-of-array

PoE Power over Ethernet

ppm parts-per-million

PTZ pan-tilt-zoom

PV photovoltaic

pyra. pyranometer

RTC Regional Test Center

RTD resistance temperature detector

RTU remote terminal unit

temp. temperature

UPS uninterrupted power supply

UTC Coordinated Universal Time

UV ultraviolet

VAC volts, alternating current

VDC volts, direct current

VDV Vista Data Vision

α temperature coefficient of resistance

Ω ohms
1

1. Introduction

Three grid-connected solar photovoltaic (PV) arrays were constructed on the National Institute of Standards and 

Technology (NIST) campus in Gaithersburg Maryland, with construction finishing around July of 2012. A map of 

the sites is shown in Figure 1-1. Oblique aerial views of the three array sites are provided in Appendix A.

Figure 1-1 A map of the instrumented PV arrays and weather station on the NIST campus in Gaithersburg, 

Maryland. The horizontal roof array shown will be instrumented as part of a future project. The 

vertical edge of this image corresponds to 1 km (0.6 miles or 3300 ft)

The three arrays use the same monocrystalline silicon module, but all are mounted in different configurations: the 

Canopy Array (Figure 1-2) is mounted on east/west facing canopies over a parking lot, the Ground Array (Figure 

1-3) is mounted on tilted ground supports in an open field, and the Roof Array (Figure 1-4) is mounted on tilted, 

weighted racks on a flat building roof.
2

Figure 1-2 Canopy Array

Figure 1-3 Ground Array
3

Figure 1-4 Roof Array

Research-grade sensors have been installed at each array and at the onsite weather station to measure various 

electrical, temperature, and meteorological values. These data acquisition systems (DASs) were created to address 

the needs for long-term, high accuracy, sub-minute data sets from large scale, calibrated, well-maintained and 

documented systems in a variable environment, as expressed in [1], [2], [3], and [4]. The only other known arrays 

instrumented to a comparable level with public to semi-public data sets in the northeast of the United States are the

65 kW to 907 kW arrays at the Northeast Solar Energy Research Center (NSERC) [5] and the 32 MW Long Island 

Solar Farm [6], both installed at Brookhaven National Laboratory in Upton, New York, on Long Island, and the 

6 kW and 60 kW Regional Test Center (RTC) arrays installed at an IBM facility in Williston, Vermont [7].

The weather station, at the location shown in Figure 1-1, measures various other meteorological quantities,

providing supplementary data to the PV data sets. These measurements include:

 direct normal irradiance (DNI)

 diffuse horizontal irradiance (DHI)

 global horizontal irradiance (GHI)

 infrared (IR) irradiance (net and total)

 ultraviolet (UV) irradiance (A, B, and total)

 spectral irradiance curves

 snow depth

 wind speed and direction

 humidity

 precipitation

 barometric pressure

 hail count

 ambient temperature

 whole sky images

The weather station DAS will be described in a forthcoming technical report.
4

The PV integrator who constructed the arrays has also installed a separate DAS at each PV array, monitoring a much 

smaller scope of parameters. Hereafter, if a differentiation is not made between the systems, then the NIST system is 

the one being referred to. At each array, both systems share communications with the inverter and AC power meter, 

with the NIST system also recording all other measurements made by the integrator’s system.

The NIST systems save measurement values, including those from the integrator’s systems, every 1 second along 

with the status of select components. Cameras near each array capture still images every 5 minutes, and a currentvoltage (I-V) tracer with a maximum power tracker and load saves curve traces from reference modules installed at 

the weather station in the orientations of each array every 1 minute, along with one-minute averaged maximum 

power measurements from one-second samples.

2. Overview

2.1. Summary

The arrays are located in Gaithersburg, Maryland in the United States, which is in the Eastern Time Zone (-5 

hours from Coordinated Universal Time (UTC)). A summary of the arrays is given in Table 2-1. Location 

information is for the center of the array, elevation is for the base of the array support (e.g., ground, roof) above sea 

level, and height is the distance from the base of the array support to the bottom edge of the bottom module.

Manufacturer data sheets for the module and inverters are provided in Appendix B and the average lengths and 

gauges of the wires are provided in Appendix C.
5

Table 2-1 Summary of the Campus PV Arrays

Canopy Ground Roof

Array Rated DC Power [kW] 243 271 73

Latitude [°N] 39.1385 39.1319 39.1354

Longitude [°E] -77.2155 -77.2141 -77.2156

Elevation*[m] 137 138 149

Height** [m] 5.11 0.67 0.08

Tilt [°] 5 20 10

Azimuth [° CW from N.] 90, 270 180 180

Number of Modules 1032 1152 312

Module Manufacturer Sharp1

Module Model NU-U235F2

Module Technology monocrystalline silicon – front contact

Module Rated Power [W] 235

Modules Per String 12

Number of Source Circuits 86 96 26

Number of Combiner Boxes 7 7 4

Number of Inverters 1 1 1

Inverter Manufacturer PV Powered (now Advanced Energy) Satcon

Inverter Model PVP260kW PVS-75 480

Inverter Rated Power [kW] 260 75

* Elevation of the array support (e.g., ground, roof) above sea level

** Height of bottom edge of the bottom module above the array support (e.g., ground, roof)

2.2. Canopy Array

The Canopy Array in the parking lot northeast of NIST Building 227 is composed of five canopies running northsouth. The middle canopies have modules tilted nominally 5° east and west and the two end canopies have modules 

tilted 5° toward the center of the array. Periodic local shading on the array is caused by the five-story Building 227 

southwest of the array, a line of trees east of the array, and several trees south and southwest of the array. Some local 

shading is also caused by ice guards along the outer longitudinal edges of the canopies that were designed to stop 

large sheets of ice from falling off the array. A diagram of the ice guards is shown in Figure 2-1.

 

1

Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such 

identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor 

does it imply that the materials or equipment identified are necessarily the best available for the purpose.
6

Figure 2-1 Diagram of the Canopy Array ice guards

The module strings in the array run predominantly north to south longitudinally in the five canopies. Individual 

strings contain modules that are either all east facing or all west facing. These strings combine in parallel at seven 

combiner boxes, at module height, to form seven output circuits that run to the inverter. Two of these output circuits 

contain strings in the same canopies and orientations, three contain strings in the same canopies but in both 

orientations, and two contain strings from four different canopies and in both orientations. This layout was designed 

to keep modules that are in the same strings at the same tilts to avoid limiting their power output due to current 

mismatch. This layout does, however, result in relatively longer source circuit wire lengths to the respective 

combiner box DC disconnects, as the wires have to run down the 5.35 m (17.5 ft) high canopies and through 0.9 m 

(3 ft) deep trenches and back up the other canopies. A diagram of the Canopy Array wiring layouts and the locations 

of the combiner boxes, inverter, and all instruments and sensors is shown in Figure 2-2. An image of the inverter is 

shown in Figure 2-3. A diagram of the string layouts and pertinent dimensions of the array is provided in Appendix 

D.
7

Figure 2-2 Canopy Array electrical layout and measurement locations. The numbered squares are the 

combiner boxes and the hatched regions are the source circuits. Note that “POA Pyra.” stands for 

a pyranometer in the plane of the array, “GHI” stands for a pyranometer measuring the global 

horizontal irradiance, and “RTD” stands for resistance temperature detector, which are mounted 

on the backsides of the respective modules. The integrator also has an RTD mounted on the back 

of a cell on the module north of combiner box 3, the same module with a NIST RTD.

Figure 2-3 The Canopy Array inverter, identical to the Ground Array inverter
8

2.3. Ground Array

The Ground Array in the field east of Building 216 is composed of five tilted rows, or sheds, running east-west. All 

sheds are tilted at nominally 20° due south. The four front southernmost sheds are five modules wide while the back 

north shed is four modules wide. Periodic local shading on the array is caused by inter-row shading, the three-story 

Building 216 directly west of the array, vegetation in the bioretention area south of the array before it is periodically 

trimmed, and a 1.2 m (48 in.) high cable rail fence surrounding the array. This fence, shown in Figure 1-3, has a 6.4 

cm (2.5 in.) diameter horizontal top rail and 4.8 cm (1.9 in.) diameter vertical posts spaced 2.7 m (9 ft) apart.

The module strings in the array run east to west longitudinally in the five sheds. These strings combine in parallel at 

seven combiner boxes approximately 1.5 m (5 ft) above the ground to form seven output circuits that run to the 

inverter. Four of these output circuits contain strings in the same shed and the remaining three contain strings from 

two different sheds. This layout does result in relatively longer source circuit wire lengths to the respective combiner 

box DC disconnects, like the Canopy Array, as the wires have to run down the 2.06 m to 2.40 m (6.8 ft to 7.9 ft) 

high ground supports and through 0.9 m (3 ft) deep trenches and back up the other sheds. A diagram of the Ground 

Array wiring layouts and the locations of the combiner boxes, inverter, and all of the instrument and sensor locations 

is shown in Figure 2-4. The inverter is identical to the one at the Canopy Arrays, as shown in Figure 2-3. A diagram 

of the string layouts and pertinent dimensions of the array as well as dimensions of the fence are provided in

Appendix D.

Figure 2-4 Ground Array electrical layout and measurement locations. The numbered squares are the 

combiner boxes and the hatched regions are the source circuits. The gray regions on the front, 

back, and east sides of the array are bioretention areas. Note that “POA Pyra.” stands for a 

pyranometer in the plane of the array, “GHI” stands for a pyranometer measuring the global 

horizontal irradiance, and “RTD” stands for resistance temperature detector, which are mounted 

on the backsides of the respective modules. The integrator also has an RTD mounted on the 

back of a cell on the module east of combiner box 3, the same module with a NIST RTD.
9

2.4. Roof Array

The Roof Array is located on a one-story roof of Building 101 just south of the 14 story tower section of the 

building. The array is composed of three sections arranged around various vents and other roof elements. The 

modules are tilted nominally 10° due south; however, the north section of modules has a slight 0.5° clockwise 

azimuthal offset. All of the module racks have an integrated sheet metal wind deflector at the back of the modules to 

inhibit wind forces, as shown in Figure 2-5; however, these deflectors also inhibit convective cooling of the modules

from the wind.

Figure 2-5 Wind deflectors on the rear of the Roof Array module racks

Periodic local shading on the array is caused by inter-row shading, 1.4 m to 4 m (4.6 ft to 13 ft) high building 

elements to the east and west of the array, the combiner boxes, and roof vents. The 14 story (13 stories above array) 

building tower directly north of the array blocks a fraction of the diffuse irradiance but reflects and augments the 

irradiance incident on the array during sunny conditions [8].

The module strings in the array run predominantly east to west. The two farthest east modules in the most southern 

two rows are not connected to the array and will serve as replacements for any future broken modules in the three 

arrays. The module strings combine in parallel at four combiner boxes 0.5 m (1.6 ft) above the roof to form four 

output circuits that run to the inverter. Two of these output circuits contain strings in the same sections and the 

remaining two contain strings from different sections. The sections in this array layout are relatively close together,

so there is significantly less additional wire needed to span the sections than that at the Canopy and Ground Arrays. 

The inverter, however, is farther from the array than at the other two arrays, located approximately 60 m (200 ft) 

north of the array in the tower section of the building at the same height of the array. A diagram of the Roof Array 

wiring layouts and the locations of the combiner boxes and all of the instrument and sensor locations is shown in 

Figure 2-6. An image of the inverter is shown in Figure 2-7. A diagram of the string layouts and pertinent 

dimensions of the array is provided in Appendix D.
10

Figure 2-6 Roof Array electrical layout and measurement locations. The numbered squares are the combiner boxes and the 

hatched regions are the source circuits. Note that “POA Pyra.” stands for a pyranometer in the plane of the array,

“GHI” stands for a pyranometer measuring the global horizontal irradiance, and “RTD” stands for resistance 

temperature detector, which are mounted on the backsides of the respective modules.

Figure 2-7 The Roof Array inverter
11

3. Measurements

3.1. Summary

The measurements at each array and their corresponding sensor or instrument are summarized in Table 3-1. A 

complete listing of the saved data values from each array is provided in Appendix E.

Table 3-1 Summary of the Measurements and Respective Instruments Installed at the Campus PV Arrays

Measurement Instrument Make/Model

AC Voltage, Current, Freq., Power, Energy AC Power Meter with Current 

Transformers (CTs)

Schneider Electric 

Powerlogic ION8600 with

Unknown Make CTs

Ambient Temperature (inside inverter) RTD Air Probe in Custom 

Ventilated Enclosure Omega RTD-806

Ambient Temperature (outdoors) (1)

RTD Probe in a Multi-Plate 

Passively Ventilated Radiation 

Shield

R. M. Young 41342LC

in an

R. M. Young 41003

Ambient Temperature (outdoors) (2)

RTD Probe in a Multi-Plate 

Passively Ventilated Radiation 

Shield

Unknown Make Pt1000

in an

Unknown Make Shield

Current Signal Shunt Resistor Campbell Scientific 

CURS100

DC Voltage Voltage Divider in a Custom 

Circuit and Enclosure Caddock 1776-C6815

DC Current (1) Shunt Resistor Empro W-200-100

DC Current (2) Shunt Resistor Empro HA/LAB-120-100

Empro HA/LAB-90-100

Digital Image (1) PTZ Camera Axis Q6032-E

Digital Image (2) Campbell Scientific CC5MPX

Global Horizontal Irradiance (GHI) Thermopile Pyranometer Eppley PSP

Module Backsheet Surface Temperature (1) RTD Omega RTD-3-F3102-72-T

Module Backsheet Surface Temperature (2) RTD Omega SA1-RTD-4W-80

Module Backsheet Surface Temperature (3) Thermocouple Omega CO1-T-72

Module Backsheet Surface Temperature (4) RTD Unknown Make Pt1000

Plane-of-Array (POA) Irradiance (1) Thermopile Pyranometer Kipp & Zonen CMP 11

Plane-of-Array (POA) Irradiance (2) Flat-Plate Silicon Irradiance 

Sensor IMT Solar Si-420TC

Plane-of-Array (POA) Irradiance (3)

Domed Diffused Silicon-Cell 

Pyranometer Apogee SP-110

Reference Module I-V Curve and Maximum 

Power Point

I-V Curve Tracer with

Maximum Power Tracker and 

Load

Daystar Multi-Tracer 5

RTD Current Shunt Resistor Campbell Scientific 

4WPB100
12

Shunt Voltage Remote Terminal Unit Gantner Q.bloxx A104

Voltage Signal Data Logger Campbell Scientific Inc. 

CR1000-ST

Voltage Signal and RTD Remote Terminal Unit Advantech ADAM-4015 

(custom)

Voltage Divider Output Voltage Analog Transmitter Acromag TT337

Wind Speed and Direction Ultrasonic Wind Sensor Vaisala WMT52

3.2. Irradiance

3.2.1. Sensors

The global horizontal irradiance (GHI) and the plane-of-array (POA) irradiance are measured at each array. The 

GHIs are measured using Eppley Precision Spectral Pyranometers (PSPs) and the POA irradiances are measured 

using both Kipp & Zonen CMP 11 pyranometers and IMT Solar Si-420TC flat-plate silicon irradiance sensors. Both 

pyranometers are all-black thermopile, secondary-standard pyranometers, while the silicon sensor is a shunted 

monocrystalline silicon cell under a thin glass cover with a cell temperature-compensated output signal.

Pyranometers were installed to most accurately measure the irradiance incident on the array in the two standard 

orientations (horizontal and POA). Flat-plate silicon irradiance sensors were also installed to capture the irradiance 

under fast changing conditions at nearly instantaneous speeds, unlike the pyranometers which have multiple-second 

response times. The flat-plate irradiance sensors also better approximate the array’s effective irradiance, or that 

absorbed by the PV cells (compared to the pyranometers which measure the incident irradiance,) because they have 

similar reflective properties due to the flat glass covers and similar spectral responses since both utilize

monocrystalline silicon cells.

The PV integrator has also installed Apogee SP-110 domed diffused silicon-cell pyranometers at each array. These 

instruments are measuring the POA irradiance, with the single pyranometer at the Canopy Array measuring the 

irradiance in the east-tilted plane. These sensors function very similarly to the flat-plate irradiance sensors but have a 

dome-shaped diffuser over the cell that minimizes reflective losses, which better approximates the incident, rather 

than the absorbed, irradiance.

3.2.2. Calibrations

All of the thermopile pyranometers are calibrated at the National Renewable Energy Laboratory (NREL) according 

to their Broadband Outdoor Radiometer Calibration (BORCAL) procedure [9]. This calibration provides net-IR 

corrected responsivities (μV/W/m2) at every 2° in the measured solar zenith angle range. The POA pyranometers are 

recalibrated every year while the GHI measuring pyranometers are calibrated approximately every (18 to 24)

months. The silicon sensors were calibrated in a solar simulator at the factory and verified to be within 

specifications. The flat-plate irradiance sensors have a stated yearly drift of 0.5 %, and the domed diffused silicon 

pyranometers have a yearly drift of less than 2 %.

3.2.3. Locations and Orientations

The locations of the irradiance sensors including those from the PV integrator are shown in the previous array 

diagrams. The sensors are positioned in the actual module plane as near to the modules as possible without shading 

them. As shown in the diagrams, none of the integrator’s pyranometers are co-located with other irradiance sensors 

at the same tilt.

At the Canopy Array (Figure 2-2) there are two sets of POA irradiance sensors in both the east and west tilted 

module planes. These sensors, along with the pyranometer measuring the GHI, are co-located at the north end of the 
13

center canopy, as shown in Figure 3-1. This location is near the center of the array to measure the most 

representative array irradiance but is on the north end of the array opposite the sun to avoid any shading on the 

modules from the instruments. The integrator’s pyranometer is located at the south end of the center canopy in close 

proximity to their outdoor data acquisition enclosure and positioned in the east-tilted module plane.

Figure 3-1 The Canopy Array weather instruments at the north end of the center canopy, showing the irradiance sensors and

mounts as well as the ambient temperature sensor, wind sensor, and wind sensor surge protector

At the Ground Array (Figure 2-4) there is a single set of irradiance sensors located in the middle of the array to 

measure the most representative array irradiance, as shown in Figure 3-2. The integrator’s pyranometer is located at 

the west end of the center shed in close proximity to their outdoor data acquisition enclosure and positioned in the 

plane of the array.

Figure 3-2 The Ground Array irradiance sensors at the center of the array, showing the sensors and mounts

At the Roof Array (Figure 2-6) there is a set of GHI and POA irradiance measuring sensors in the southeast corner 

of the array, as shown in Figure 3-3, located in an area that is least shaded by surrounding building and array 

elements. Pyranometers measuring the GHI are also installed at the other three corners of the array to capture the 

spatial variability of the irradiance caused by the 14 story (13 stories above array) building tower directly north of 
14

the array. The building has a large number of windows and a light-colored brick façade with reflective aluminum 

mullions (see Figure 1-4). The integrator’s pyranometer is located at the southwest corner of the array in relatively 

close proximity to their outdoor data acquisition enclosure and positioned in the plane of the array.

Figure 3-3 The Roof Array irradiance sensors at the southeast corner of the array, showing the sensors and mounts

There are three more locations not pictured in the Roof Array diagram that are nearer to the building where the GHI 

is also measured with pyranometers and flat-plate irradiance sensors. These additional sensors are situated around 

and at the center of an older horizontal PV array, which is the subject of a study on aging that will be discussed in 

later publications. The sensors at the center of this array are shown in Figure 3-4 and include two side-by-side flatplate irradiance sensors to quantify the level of soiling: the east one is regularly cleaned and the west one is not. The

irradiance measurements from these additional sensors are included in this dataset.

Figure 3-4 The irradiance sensors at the center of the older horizontal PV array, showing the sensors and mounts

3.2.4. Mounting and Alignment

The GHI and POA sensors are mounted using Campbell Scientific Inc. (CSI) CM245 Adjustable-Angle Mounting 

Stands, as shown in previous figures, or mounted directly to an aluminum plate for the single location at the center

of the older horizontal roof array (Figure 3-4). The Eppley PSPs are mounted using nylon leveling and fastening 
15

screws to avoid creating ground loops and the associated signal noise they can induce because the body of a PSP is

electrically connected to the ground pin on their cable connector. The GHI pyranometers are leveled using the 

integrated bubble levels, ensuring that the bubble is in the middle of the center circle, with the bubble anywhere in 

the circle corresponding to a leveling accuracy of approximately ±0.1° for the CMP 11’s and ±0.2° for the PSPs. 

The GHI and POA flat-plate irradiance sensors are aligned to horizontal and the array nominal tilt, respectively, 

using a Mitutoyo Pro 360 inclinometer to a reading within ±0.1°.

The alignments of the POA pyranometers are more challenging because there is no accessible flat surface on a 

CMP 11 that is parallel to its sensor on which to rest an inclinometer. To align these instruments, a laboratory-grade 

granite surface plate is first leveled to within a reading of ±0.1° using the inclinometer. Then, the pyranometers are

leveled on the surface plate using the pyranometers’ leveling feet and integrated bubble levels. The mounts for the 

pyranometers are then aligned to a reading ±0.1° from the nominal array tilts using the inclinometer, and lastly the 

pyranometers are fastened to them.

3.2.5. Wiring

The irradiance sensors are wired directly to the data logger multiplexers using UV and moisture protected cables run 

through metal dedicated data conduit, separating them from power conductors that may generate high EMI. The 

cables enter the conduit through cable glands at pull boxes or conduit access ports. The sensors at the Roof Array, 

however, initially pass through shorter sections of nonmetallic conduit. The pyranometers are wired using shielded 

twisted-pair cable grounded at one end on the data logger side to reduce and allow the electromagnetic interference 

(EMI) to be later rejected when differentially measured, which is especially important for their ~10 μV resolution 

analog voltage output. The flat-plate irradiance sensors are similarly wired using shielded twisted three-wire cable. 

These silicon sensors communicate over an unpowered 4 mA to 20 mA current loop, which has a low sensitivity to 

electrical noise, and runs on one pair, with the third wire supplying power to the temperature compensation and loop 

transmitter circuits.

The integrator’s pyranometers are wired directly to their nearby outdoor data acquisition enclosures using the cable 

pigtail connected to each sensor, entering through cable glands. These sensors communicate using an analog voltage 

signal.

3.3. Ambient Temperature

3.3.1. Outdoor

The outdoor ambient air temperature is measured at each array using one R. M. Young 41342LC 1000 Ω platinum 

RTD probe and transducer in an R. M. Young 41003 multi-plate passively-ventilated radiation shield, shown 

disassembled in Figure 3-5. The radiation shields are used to shield the temperature probes from radiative heat 

transfer from the sun and surroundings, which would affect the temperature of the probe, while still allowing 

ambient airflow around the probe. Aspirated, or fan ventilated, radiation shields were not used due to backup power 

considerations. The PV integrator has also installed a similar style two-wire, 1000 Ω RTD probe (without a 

transducer) and radiation shield at each array of otherwise unknown specifications. Both sets of sensors were 

calibrated at the factory and verified to be within specifications, and they have an unspecified yearly drift.
16

Figure 3-5 The temperature probe and radiation shield used to measure the outdoor ambient air temperature at each array

The sensors are located next to the arrays in locations that do not shade the modules, are not in stagnant air, and are 

not near potentially heat-releasing sources like roof vents, with the specific locations of both sets of sensors shown 

in the previous array diagrams. Only at the Ground Array are the NIST and integrator sensors co-located. Both sets 

of sensors are mounted to poles and vertically aligned.

The ambient temperature sensors are wired directly to the data logger multiplexers using UV and moisture protected

cables, and at the Canopy and Ground Arrays these cables run through metal dedicated data conduit, separating them 

from power conductors that may generate EMI. The cables enter the conduit through cable glands at pull boxes. The 

sensor at the Roof Array is located immediately adjacent to the data acquisition enclosure, and its cable enters a 

short nonmetallic conduit through a conduit access port. The sensors at all arrays are wired using shielded cable 

grounded at one end on the data logger side to reduce EMI. These sensors communicate via a transducer over a 

powered 4 mA to 20 mA current loop, which has a low sensitivity to electrical noise and also supplies power to the 

sensor. The integrator’s ambient temperature sensing two-wire RTDs do not have transducers and are wired directly

to their nearby outdoor data acquisition enclosures, with the cables entering through cable glands.

3.3.2. Inverter

The ambient air temperatures inside the inverters at the Canopy Array and the Ground Array are each measured 

using one Omega RTD-806 exposed, three-wire, class “A”, 100 Ω platinum RTD air probe with an α = 0.00385. The 

exposed sensing element of these RTDs results in a fast response time. The RTDs are mounted inside a custom 

perforated enclosure, as shown in Figure 3-6, with a fourth terminal split off from the three-wire RTD at the 

enclosure to allow for more accurate four-wire measurements. The perforated enclosure mounts on a DIN rail and 

physically protects the sensor and limits any radiative heat transfer from the surroundings, which would affect the 

temperature of the probe, while still allowing ambient airflow around the probe. The sensors were calibrated at the 

factory and verified to be within specifications, and they have an unspecified yearly drift. The ambient temperature 

inside of the inverter at the Roof Array is measured by the inverter itself and is one of the measurements polled by 

the data acquisition system, as discussed in a later section.
17

Figure 3-6 The temperature probe and enclosure used to measure the ambient air temperature in the inverters at the Canopy and 

Ground Arrays

The sensors are located in the top center of the DC combiner compartment, with generally unseparated airflow to the 

other inverter compartments including the AC and power module assembly compartments. The temperature sensors 

are wired directly to the data logger multiplexers using UV and moisture protected cables run through metal 

dedicated data conduit, separating them from power conductors that may generate EMI. The cables are also shielded 

and grounded at one end on the data logger side to reduce EMI. The RTD cable has four wires and has the inner 

wire insulation (and outer jacket) made of PTFE (Teflon). PTFE has a much lower permittivity and subsequent 

capacitance than PVC and nearly all other wire insulation materials, resulting in faster settling times after excitation.

3.4. Module Temperature

3.4.1. Sensors

The module backsheet surface temperatures are measured at each array using Omega RTD-3-F3102-72-T exposed, 

thin-film, four-wire, class “A”, 100 Ω platinum RTDs with an α = 0.00385. The sensing elements are flat, with only 

a thin ceramic base material covering the resistive element, thereby reducing the thermal gradient in the sensor, 

bringing it closer to the module surface temperature. The RTDs have relatively small sensing areas of 3.7 mm x 

4.7 mm (0.147 in. x 0.185 in.) that reduce the insulating effect on the module, which affects its temperature local to 

the RTD, while still providing enough area for adhesion and thermal contact. The relatively small size of the RTD 

also results in faster response times due to the smaller thermal mass. A four-wire RTD was chosen instead of a two 

or three-wire RTD because the long cable leads, the large changes in temperature and subsequent resistance of the 

sensing element, and using a static, unbalanced resistance bridge would have resulted in significant measurement 

errors.

The backsheet temperatures of the reference modules at the weather station are measured using a different RTD, the 

Omega SA1-RTD-4W-80, which is very similar to the aforementioned RTD, but has a large 19 mm x 26 mm (0.75

in. x 1.0 in.) adhesive backing to the sensing element. This different RTD was chosen because the other model is 

relatively delicate and rather difficult to install due to its small size. Thermocouples were also installed on the 

reference modules, Omega model CO1-T-72. These thermocouples are ungrounded, flat, low mass, type T, ANSI 

“Special Limits of Error” grade foil sensors with fast response times and high accuracy. These sensors were installed 

to connect to the built-in thermocouple temperature channels on the I-V tracer with maximum power tracker and 

load and provide a temperature measurement more easily correlated with the module measurements. A visual 

comparison of the three temperature sensors is shown in Figure 3-7.
18

Figure 3-7 A visual comparison of the three types of module temperature sensors used at the arrays and for the reference 

modules: the RTD used at the arrays (left), the RTD used for the reference modules (center), and the thermocouple

used for the reference modules (right).

RTDs were chosen instead of thermocouples or other temperature sensors for the primary module temperature 

measurements because they are the most stable and accurate temperature sensing technology [10]. RTDs also do not 

require a temperature reference as do thermocouples, which for the higher accuracy and many channel units can be 

rather large, and in the case of ice point references require significant amounts of power. RTDs do, however, need

excitation, which requires a current source, increases the sampling time, and causes self-heating of the resistance 

sensor that affects the measurement. These issues were addressed and minimized, as later described in the Data 

Acquisition and Control section.

The PV integrator has also installed two-wire, 1000 Ω adhesive backed RTDs of otherwise unknown specifications 

at each array that appear similar to the NIST adhesive backed RTDs.

3.4.2. Calibrations

All of the module temperature sensors were calibrated in the laboratory according to ASTM E220 [11]. According 

to the standard, dry calibrations were performed, with the test and reference temperature sensors packed in brass test 

tubes with thermally conductive aluminum oxide powder and immersed in a circulating temperature bath. With the 

exception of one defective RTD, all RTDs and thermocouples were within specifications, and the proportional and 

offset calibration coefficients and associated uncertainty were calculated for each sensor. 

3.4.3. Locations

The locations of the temperature sensors including those from the PV integrator are shown in the previous array 

diagrams. The sensors are mounted on the backsheets of the modules, with each sensor centered under a cell. Four 

sensors are installed on one module in the center of each array and on each reference module at the weather station, 

positioned according to IEC 60891 [12] (see previous array diagrams). The rest of the sensors in the arrays and the 

additional thermocouples on the reference modules at the weather station are installed behind a center cell on 

separate modules.

The sensor locations are distributed in the array with the intent of best capturing temperature gradients in the array 

and best calculating a single representative array temperature. At the Canopy Array, nine temperature sensors plus 

one from the integrator are installed, spanning the full length and half-width of the center canopy. It is assumed that 

the other canopies will have similar temperature distributions. The integrator’s temperature sensor is co-located on
19

one of the same modules as a NIST temperature sensor, near the south end of the center canopy by the integrator’s 

outdoor data acquisition enclosure.

At the Ground Array, nine temperature sensors plus one from the integrator are installed, spanning the full length 

and width of the northwest quarter of the array. It is assumed that the four quarters of the array will have similar 

temperature distributions. The integrator’s temperature sensor is co-located on one of the same modules as a NIST 

temperature sensor, at the west end of the center shed by the integrator’s outdoor data acquisition enclosure.

At the Roof Array, seven temperature sensors plus one from the integrator are installed, spanning the full length and 

width of the array. The integrator’s temperature sensor is not co-located with a NIST temperature sensor at this 

array, and is located near the south end of the array by the integrator’s outdoor data acquisition enclosure.

3.4.4. Mounting and Alignment

The temperature sensors are mounted to the module backsheet using Omegabond OB-101 thermally conductive 

epoxy and an Innotec 248DR, 57.2 mm (2.25 in.) diameter, 83 μm (3.3 mil) thick round polyester adhesive film 

overlay, as shown in Figure 3-8. This method was derived as the combination of two of the best module back 

surface temperature measurement methods as determined by Smith, et al. [13], specifically the methods designated 

as positions C and H. This combination allows accurate measurement of the surface temperature, minimally affects 

the surface temperature, results in a more precise bias from the true cell temperature, and provides reliably secure 

attachment to the surface. The best practices recommended by Smith, et al. were also followed when mounting the 

sensors, including minimizing the thickness of the epoxy under the sensor, minimizing the amount spread beyond 

the sides of the sensor, and using additional adhesive films and ties on the cable for strain relief to prevent the sensor 

from detaching. For the adhesive backed sensors, the thermally conductive epoxy is applied to the adhesive surface, 

first removing the adhesive covers. The temperature sensors were aligned under the center of the respective cells by 

distance measurements and visual means.

Figure 3-8 An RTD mounted on the back of a module using thermally conductive epoxy and an adhesive film overlay

The integrator’s sensors were mounted by the integrator, adhering the adhesive surface directly to the module 

backsheet and overlaying with insulated foil tape, as shown in Figure 3-9. No thermally conductive epoxy was 

employed.
20

Figure 3-9 The integrator’s module temperature sensor mounted on the back of a module at the Ground Array

3.4.5. Wiring

The module temperature sensors are wired directly to the data logger multiplexers using UV and moisture protected 

cable run through metal dedicated data conduit, separating them from power conductors that may generate EMI. The 

cables enter the conduit through cable glands at pull boxes or conduit access ports. The sensors at the Roof Array, 

however, initially pass through shorter sections of nonmetallic conduit. The sensors are wired using shielded cable

grounded at one end on the data logger side to reduce EMI. The RTD cable has four wires and has the inner wire 

insulation (and outer jacket) made of PTFE (Teflon). PTFE has a much lower permittivity and subsequent 

capacitance than PVC and nearly all other wire insulation materials, resulting in faster settling times after excitation.

The RTDs are connected to the extension cables using waterproof IP67 connectors. The TCs are connected to the 

extension cables using standard flat pin connectors housed in custom waterproof cable connector enclosures.

The integrator’s temperature sensors are wired directly to their nearby outdoor data acquisition enclosures using no 

extension cables, entering through cable glands.

3.5. Wind

3.5.1. Sensor

The horizontal wind speed and direction is measured at each array using one Vaisala WMT52 heated ultrasonic 

wind sensor. An ultrasonic sensor was chosen instead of a traditional vane and cup or windmill anemometer because 

it is has a smaller physical profile, is easily heated and kept free of snow and ice, has no moving parts and therefore 

a higher reliability, and has a higher sensitivity because of the faster response time and virtually zero starting 

threshold (even very low wind speeds will be detected). The wind sensor is configured to sample readings at 4 Hz 

and average every 1 s, and to perform maximum and minimum calculations instead of gust and lull. The wind sensor 

controls its level of heating as a function of the ambient and its internal temperatures. The wind sensors 

communicate over one wire pair using an RS485 serial ASCII protocol at 19200 bits per second (bps). Note that the 

PV integrator did not install any wind sensors. The sensors were calibrated at the factory and verified to be within 

specifications, and they have an unspecified yearly drift.

3.5.2. Locations and Orientation

The sensors are located next to the arrays in horizontal orientations in locations 56 cm to 150 cm (21.9 in. to 59.0 

in.) above and 83 cm to 160 cm (32.5 in. to 62.8 in.) to the north of the modules. The wind sensors at the Canopy 

(see Figure 3-1) and Ground Arrays (Figure 3-10) are north of the entire respective arrays while the sensor at the 

Roof Array (Figure 3-11) is near the middle of the array, north of the middle section of modules. These locations 
21

were chosen to measure the wind that is near to but out of the immediate boundary layer while minimally shading 

the modules, which does occur but is limited to tens of minutes at sunrise and sunset in the summertime. The sensor 

at the Roof Array is not north of the entire array like at the other arrays because it would shade the older horizontal 

array north of this array and it would likely not be as representative of the wind over the array due to its closer 

proximity to the 14 story (13 stories above array) building tower directly north of the array.

Figure 3-10 The wind sensor at the Ground Array

Figure 3-11 The wind sensor and outdoor ambient temperature sensor at the Roof Array

3.5.3. Mounting and Alignment

The wind sensors are mounted on top of vertical poles, aligned to true north using visual references and checked 

using a compass (adjusted for the local magnetic declination). The wind sensors at the Canopy and Ground Arrays 

are higher than any nearby sensor and structure so a Vaisala WSP150 surge protector, designed for this type of 

application, was installed near each wind sensor in-line with the power and data cable to suppress power surges 

caused by nearby lightning. Metal spikes designed for these specific sensors are installed on top of the sensor at each 

array to deter birds from perching on them and affecting the readings.
22

3.5.4. Wiring

The wind sensors are wired to the surge protectors at the Canopy and Ground Arrays. These surge protectors and the 

wind sensor at the Roof Array are wired directly to the data logger serial communication modules. All cables are 

UV and moisture protected, and at the Canopy and Ground Arrays run through metal dedicated data conduit, 

separating them from power conductors that may generate high EMI. The cables enter the conduit through cable 

glands at pull boxes. The sensor at the Roof Array is located immediately adjacent to the data acquisition enclosure 

and enters a short nonmetallic conduit through a conduit access port. The sensors are wired using shielded cable 

grounded at one end on the data logger side to reduce EMI. These sensors communicate over one wire pair, with 

power for the sensor and the heater on separate wire pairs. To reduce power consumption, termination resistors are 

not wired in parallel at the ends of the RS485 communication wires to reduce signal reflections because they are not 

needed, as the cable lengths are less than 600 m (2000 ft) and the data rates are relatively low (19200 bps). The

surge protectors for the wind sensors are grounded to local earth ground using a short length of 8 American wire 

gauge (AWG) (8.37 mm2) wire. The wind sensor at the Roof Array is near higher building structures but is also 

locally grounded like the other two wind sensors to prevent and mitigate surges caused by nearby lightning.

3.6. DC Electrical

3.6.1. Sensors

The direct current (DC) voltage of the arrays is measured at each array using one Caddock 1776-C6815 voltage 

divider integrated circuit (IC) on a custom circuit board housed in a DIN rail mounted enclosure, as shown with the 

cover off in Figure 3-12. Voltage dividers were chosen instead of other types of DC voltage transducers or sensors

because they are much more accurate and stable, with little long-term drift or sensitivity to their environment, and 

therefore need no periodic adjustments to their gain and offset. Voltage dividers do however need to be connected to 

very high input impedance (~10 MΩ) measurement devices to minimize current flow to the devices, which can 

affect the measurements.

Figure 3-12 The custom voltage divider circuit for measuring the array DC voltage

The output circuit currents of the arrays are measured using seven Empro W-200-100 (0.5 mΩ) current shunts at

each of the Canopy and Ground Arrays and three Empro HA/LAB-120-100 (0.833 mΩ) and one HA/LAB-90-100 

(1.111 mΩ) current shunt at the Roof Array. (These shunts are custom and have an HA style body and LAB style 

base). These shunts were specified to best match the current range of the output circuits and the voltage range of the 

isolating remote terminal units (RTUs) to which they are wired, and to be the most appropriate for the manner of

their installation. Current shunts were chosen instead of current transformers (CTs) or Hall effect sensors because 
23

they are much more accurate and stable, with little long-term drift or sensitivity to their environment. Current 

shunts, however, are often more difficult to install and may need more precautions than the other sensor options.

Isolators need to be used with both the voltage dividers and current shunts to electrically separate them from the 

measurement circuits to prevent inadvertent secondary conductive paths from the array circuits to ground, which 

would suppress the inverters’ ground fault detection. The voltage divider at each array is isolated with an Acromag 

TT337, which is a high-voltage isolator and analog output transmitter. This transmitter was chosen because of its 

high voltage isolation, high input impedance (15 MΩ), low uncertainties, and small form factor. The transmitter is 

configured to sample readings at 13 Hz and employ various filters, including for rejecting 60 Hz noise. The 

transmitter outputs a 0 V to 5 V signal proportional to the input voltage signal.

The current shunts at each array are isolated with a Gantner Q.bloxx A104 RTU, which is a high-voltage isolator 

and 8-channel analog-to-digital converter (ADC). This RTU was chosen because of its high voltage isolation, small 

form factor, and very low uncertainties at the millivolt levels output from the shunts, due partly to it being 

specialized for measuring only millivolt signals, typically from thermocouples. The RTU samples readings at 

100 Hz and averages every 10 ms. None of the configurable RTU filters are employed as tests showed that the RTUs 

were more accurate and had a faster response time without them. The RTU communicates over one wire pair using 

the RS485 serial Modbus RTU protocol at 19200 bps. A combination ADC and isolator RTU was chosen to 

eliminate any EMI that would be picked up by the cable runs between the measurement location in the inverter or 

shunt enclosure and the data logger. This EMI could have a relatively large effect on the millivolt shunt readings. 

Use of the RTUs also resulted in less needed cables running back to the data loggers.

Various DC measurements are also performed by the inverters, as listed in Table 3-2. The inverters communicate 

these measurements as well as the inverter operating state and any fault codes over one wire pair using the RS485 

serial Modbus RTU protocol at 9600 bps. Note that the PV integrator did not install any DC measuring sensors, but 

does communicate with the inverter.

Table 3-2 Inverter DC Measurements

Measurement Canopy and 

Ground Inverters

Roof 

Inverter

PV voltage1 X X

Input voltage2 X X

Input current X X

Input power X X

Ground current X

Ground impedance X

1 The voltage on the array side of the DC contactor

2 The voltage on the load side of the DC contactor

3.6.2. Calibrations

The voltage dividers and isolators were calibrated in the laboratory and verified to be within specifications. The 

current shunts were calibrated at the factory to an accuracy of ±0.10 %.

3.6.3. Locations and Orientations

The measurement components at the Canopy and Ground Arrays are located in the inverter DC combiner 

compartment, as shown in Figure 3-13. The voltage divider and isolators are together at the top of the compartment 

and the current shunts are horizontally oriented between the DC output circuit wires and the DC busbar.
24

Figure 3-13 The inverter DC combiner compartment at the Ground Array with the installed DC measurement components, 

including the current shunts, and left-to-right on the upper DIN rail: the ambient temperature sensor, shunt RTU, 

analog transmitter, voltage divider, and fuse holder

The sensors at the Roof Array are located exterior to the inverter in an outdoor enclosure, as shown in Figure 3-14, 

approximately three-quarters of the wiring distance from the array to the inverter. The voltage divider and isolators 

are together at the top of the enclosure and the current shunts are horizontally oriented in-line with the DC output 

circuit wires. The measurements at the Roof Array were not made in the inverter because the manufacturer of that 

inverter had recently ceased operations and therefore the installation design could not be reviewed.

Figure 3-14 The outdoor DC measurement enclosure at the Roof Array, which includes the current shunts, and left-to-right on 

the upper DIN rail: the fuse holder, voltage divider, analog transmitter, and shunt RTU

3.6.4. Mounting

At the Canopy and Ground Arrays, the voltage divider and isolators are mounted to a DIN rail fastened to existing 

threaded holes in the back panel in the DC combiner compartment. Also mounted on the DIN rail are the inverter 

ambient temperature sensor in its enclosure (far left) and a fuse in a touch-safe holder (far right) for the DC voltage 

tap. The current shunts are fastened to the front of the negative busbar using the same hardware, torque 
25

specifications, and orientation for normally fastening the output circuit cables. The installation of the shunts does not 

cause the separation between the negative and positive busbars to be reduced. 

At the Roof Array, the voltage divider and isolators are mounted to a DIN rail fastened with screws to the enclosure 

back panel. Also mounted on the DIN rail is a fuse in a touch-safe holder (far left) for the DC voltage tap (far right 

under DIN rail). The shunts are also fastened with screws directly to the enclosure back panel.

3.6.5. Wiring

At the Canopy and Ground Arrays, high temperature, 600 V rated wires are connected to both the positive and 

negative busbars using solder-filled ring terminals and the same hardware and torque specifications as that for the 

output circuit cables, or in this case the current shunts. The positive wire is connected to the 0.5 A fast-blow fuse in 

the touch-safe holder on the DIN rail, which is for disconnecting the voltage divider from the array if it fails and 

becomes shorted. The other side of the fuse holder and the wire from the negative busbar are connected to the 

voltage divider, which in turn is wired to its respective isolator. For the current measurements, the output circuit 

cables are connected to the other side of the shunts in a similar manner as fastening the shunts to the busbar, with the 

cables supporting all of their own weight due to their rigidity. High temperature, 600 V rated cables are connected to 

the sensing terminals of the current shunts using solder-filled ring terminals and wired to their respective channels 

on the isolating RTU.

At the Roof Array, a high-amp distribution block is connected in-line with one of the positive output circuit wires to 

provide a positive voltage sensing terminal; a sensing terminal on one of the shunts provides the negative sensing 

point. High temperature, 600 V rated wires are connected to both the positive and negative sensing terminals using 

solder-filled ring terminals. The positive wire is connected to the 0.5 A fast-blow fuse in the touch-safe holder on the 

DIN rail, which is also for disconnecting the voltage divider from the array if it fails and becomes shorted. The other 

side of the fuse holder and the wire from the negative terminal (on a shunt) are connected to the voltage divider, 

which in turn is wired to its respective isolator. For the current measurements, the negative current carrying wires 

are cut and fastened to both sides of the respective shunts using compression lugs. High temperature, 600 V rated 

cables are connected to the sensing terminals of the current shunts using solder-filled ring terminals and wired to 

their respective channels on the isolating RTU.

At each array, the voltage divider transmitter and isolating RTU are wired directly to a data logger multiplexer and a 

serial communication module, respectively. The inverter communication card is wired using Cat5 cable to a 

communication hub in the closest building. To reduce power consumption, termination resistors are not wired in 

parallel at the ends of the RS485 communication wires for the isolating RTUs and inverter cards because they are 

not needed to reduce signal reflections, as the cable lengths are less than 600 m (2000 ft) and the data rates are 

relatively low (19200 bps and 9600 bps, respectively). The cables are UV and moisture protected and run through 

metal dedicated data conduit, separating them from power conductors that may generate EMI. The cables are also 

shielded twisted-pair and grounded at one end on the data logger or indoor side to reduce and allow the EMI to be 

later rejected when differentially measured.

3.7. AC Electrical

3.7.1. Sensors

The alternating current (AC) voltage, current, and other metrics are measured at each array using one Schneider 

Electric Powerlogic ION8600 AC power meter. The meters use fused voltage taps (no potential transformers) and

revenue grade bar-type CTs with a 200:5 ratio at the Canopy and Ground Arrays and a 100:5 ratio at the Roof Array, 

both of otherwise unknown specifications. Both the meter and the CTs were selected and installed by the PV 

integrator. Various AC measurements are also performed by the inverters, with both the inverter and power meter 

AC measurements listed in Table 3-3. The inverters communicate these measurements as well as the inverter 

operating state and any fault codes over one wire pair using the RS485 serial Modbus RTU protocol, as described 
26

before, and the AC power meters communicate over Ethernet using the Modbus TCP/IP protocol. Note that the PV 

integrator also communicates with the inverter and AC power meter at each array.

Table 3-3 Inverter and Power Meter AC Measurements

Measurement Canopy and 

Ground Inverters

Roof 

Inverter

Power 

Meter

Line voltage A,B,C X X X

Line current A,B,C X X X

Neutral current X

Output real power X X X

Output reactive power X X

Output apparent power X X

Power factor X X

Total real energy X X X

1

Total reactive energy X

1

Line frequency X X X

2

Phase reversal X

Total harmonic distortion X

3

1

includes separate received and delivered

2

includes minimum and maximum

3 maximum on each line

3.7.2. Locations

The AC power meters are installed in the closest building to each array near the building’s switchgear, with the 

meter at the Ground Array shown in Figure 3-15. The CTs are housed in a junction box near the switchgear, and the

fused voltage taps are in the same junction box at the Roof Array and in one of the switchgear’s power distribution 

boards at the Canopy and Ground Arrays, with the fuses housed in a separate dedicated enclosure near the AC 

power meter.

Figure 3-15 The AC power meter at the Ground Array (formerly branded by Square D), also showing the separate dedicated 

enclosure for the voltage tap fuses
27

3.7.3. Wiring

The inverters are connected to the 480 VAC, three-phase campus grid via a four-wire Y connection with the neutral 

grounded. The power meters are connected to meter sockets wired to the buildings’ switchgear. The voltage taps are 

connected through 2 A fuses and the CTs are wired to the meters using approximately 6 m to 9 m (20 ft to 30 ft) 

leads. These leads are relatively long and their effect on the CT burdens and subsequent accuracy should be 

considered. Each power meter is connected to the NIST network with a Cat5e cable, and each inverter

communication card, as explained before, is wired using Cat5 cable (twisted-pair) to a communication hub in the 

closest building. The inverter communication cables are UV and moisture protected and run through metal dedicated 

data conduit, separating them from power conductors that may generate EMI. The inverter cables are also shielded 

and grounded at one end on the indoor side to reduce EMI.

4. Data Acquisition and Control

4.1. Data loggers

4.1.1. Components

The data logger at each array is a Campbell Scientific CR1000-ST, shown in Figure 4-1 in the upper left of the 

Canopy Array outdoor data acquisition enclosure. The data acquisition enclosures at the Ground and Roof Arrays 

are very similar and have identical components.

Figure 4-1 The data acquisition enclosure at the Canopy Array showing the data logger, measurement and communication 

peripherals, battery backup, and power converters and distribution blocks
28

This data logger was chosen because it has:

 good measurement accuracy

 adequate processing power

 high reliability for extended periods in outdoor environmental conditions

 the ability to interface with a large variety of peripherals

 a small form factor

 relatively low power draws and therefore needs only a small battery backup for extended power outages

The analog voltage and current signals are multiplexed using one Campbell Scientific AM25T solid-state 

multiplexer, with the current carrying wires first connected to Campbell Scientific CURS100 100 ohm current 

shunts to convert the current signals to voltage signals. The RTDs are multiplexed using one Campbell Scientific 

AM16/32B-ST relay multiplexer. The serial devices are each connected to separate Campbell Scientific SDM-SIO1 

serial communication modules.

Solid-state multiplexers are used because of their faster switching times and greater longevity than relay 

multiplexers, which are needed for measuring numerous channels at a one-second sample rate over an extended 

duration. Relay multiplexers are used for the RTD measurements instead of solid-state multiplexers because the 

solid-state multiplexers cannot pass the currents needed to excite the RTDs. This restriction does limit the RTD 

sample rate to greater than one-second, but this rate is acceptable because the module temperatures change at a 

slower rate than the irradiance and array DC voltage. The separate serial communication modules are used instead of 

the serial communication channels on the data logger because there were not enough available channels.

The relay multiplexer is connected to the data logger through a Campbell Scientific 4WPB100 100 Ω four-wire halfbridge terminal input module. The bridge employs a high-accuracy (±0.01 %), low temperature dependence (< 8 

ppm/°C) shunt to measure the current through the RTD, generated by the excitation voltage supplied by the data 

logger. A current limiting resistor is used in series with the shunt to allow for higher excitation voltages at the data 

logger, minimizing the relative noise in the excitation, while limiting the current and subsequent self-heating in the 

sensing resistor (RTD). Self-heating, which results in a measurement bias error, is caused by the excitation current 

passing through the sensor resistor.

A Campbell Scientific NL115 is connected to the data logger’s peripheral port and provides 10BASE-T Ethernet 

communications and a CompactFlash card slot interface for storing measurements. An industrial quality 2 GB 

Synchrotech CFV-2GB-TSI-3524 card is used, which like the data logger, can operate with high reliability for 

extended periods in outdoor environmental conditions.

The integrator’s RTU at each array is an Advantech ADAM-4015 RTD input module, shown in Figure 4-2 with a 

terminal block in its outdoor data acquisition enclosure at the Ground Array. This RTU is specialized for measuring 

RTDs, but the ones employed by the integrator are customized by the manufacturer to also measure analog voltages, 

including the one from the integrator’s pyranometer. The RTU samples readings at 10 Hz, integrating each for 

50 ms (three 60 Hz cycles), and does not average the readings. The RTU at each array is indirectly connected to the 

integrator’s indoor data logger, which is an industrial computer of unknown specifications, communicating over one 

wire pair using the RS485 serial Modbus RTU protocol at 9600 bps.
29

Figure 4-2 The integrator’s RTU and a terminal block in their outdoor enclosure at the Ground Array

4.1.2. Locations and Housing

Each data acquisition enclosure is located near the respective array in an easily accessible location that does not 

shade the array. Each enclosure is NEMA 4 rated and locked to control access, with an additional sensor installed to 

monitor when the door is open (at which time the data logger sends out an email alert). The humidity in the 

enclosure is regulated by sealing the conduit openings with duct seal compound and replacing desiccant packets on 

an as needed basis, with the humidity level monitored using a humidity indicator card. The enclosure at the Roof 

Array is in a horizontal orientation to not shade the array, and is inside of another larger enclosure to better protect 

against water ingress. The integrator’s outdoor RTU enclosures are also weather resistant and located near the array; 

however, the enclosure at the Canopy array is located at the south end of the center canopy near the top of the 

canopy and is only accessible with a ladder or lift.

4.1.3. Wiring and Power

All wiring inside of the enclosures is done with low capacitance, high temperature PTFE insulated stranded wires 

with insulated ferrules crimped on their ends. Ferrule-terminated wires make troubleshooting and reworking easier 

and provide a more secure, lower contact resistance connection to the terminals. Jumper wires are connected 

between the low sides of the pyranometers’ differential inputs and the respective ground terminals to keep the 

pyranometers, the only electrically floating sensors in these DASs, from floating above the ground potential and out 

of the data logger’s common mode range, which would decrease the accuracy of the measurements. The jumpers 

also keep the pyranometers from potentially floating out of the measurable range entirely.

The Ethernet communication module on the data logger connects via a Cat6 Ethernet patch cable to an Etherwan 

EL1141 hardened 10/100BASE-TX Ethernet to 100BASE-FX fiber media converter. The media converter is 

connected to a fiber light interface unit (LIU) using a fiber patch cable, where the LIU is a protective enclosure for 

splitting and adding connectors to individual optical fibers. Fiber optic communication was chosen for the data 

loggers to eliminate any EMI and reduce the potential for ground loops and power surges back to the data logger. 

The distance between the data loggers and the network access point in the nearby buildings is also near to or over 

the distance limit for Cat5/6 cable (100 m [330 ft]) which could have caused slow or unreliable communications.

The data logger is battery-backed using a Genesis NP24-12 12 V, 24 A∙h (288 W∙h) sealed rechargeable lead acid 

battery connected to a Campbell Scientific CH200 charging regulator. This regulator charges the battery and 

switches between line and battery power, providing an uninterrupted power supply (UPS) to the data logger and all 

other peripherals in the enclosure except the Ethernet to fiber media converter. The UPS at each array will provide at 
30

least 2.8 days of power to the DAS when there is no external power feeding the enclosure, even during high power 

draw conditions (e.g., while heating the wind sensor in winter). A breakdown of the DAS energy draws is shown in 

Figure 4-3.

Figure 4-3 Breakdown of the DAS energy draws and UPS backups

Power from the UPS is distributed to the data logger, peripherals, and an array of terminal blocks for the different 

sensors via a Campbell Scientific A105 terminal expansion adapter on the charging regulator. The power from the 

non-battery-backed power supply is distributed to a different array of terminal blocks, for currently just the Ethernet 

to fiber media converter. The line inputs of the charging regulators for the UPSs at the Canopy and Roof arrays are 

each connected to 120 VAC to 24 VDC power converters, while the terminal blocks for the non-battery-backed 

Ethernet to fiber media converters are connected to 120 VAC to 12 VDC power converters. These two power 

converters are connected to 120 VAC feeding those enclosures via terminal blocks and a single AC circuit breaker. 

The charging regulator for the UPS at the Ground Array and the media converter are connected via terminal blocks 

and a single AC circuit breaker to 24 VDC feeding that enclosure.

The data acquisition enclosure components at each array are grounded to a central point on the enclosure. Grounding 

in a “star-point” configuration minimizes the potential differences between the devices and therefore reduces any 

currents through the devices during a power surge event. The central point of the “star” is connected via the 

enclosure to the conduit which is locally grounded. The conduit was chosen instead of the ground wires fed from the 

nearby buildings because it provides a lower resistance path to ground and will not have an electrical choke effect 

when there is a sudden, high current surge from a nearby lightning strike, which occurs for a surge carrying wire 

inside of a ferrous conduit. The importance of proper grounding in the interest of equipment longevity cannot be 

overemphasized.

Three conduits connect to each data acquisition enclosure, one carrying the AC or DC power wires or cable, one 

carrying the fiber communication cable, and one carrying the sensor data cables. Excess lengths of the data cables

for potential future reworks (approximately 3 m [10 ft]) are stored in a nearby pull box connected to the data conduit 

at the Canopy and Ground Arrays and in the outer enclosure at the Roof Array.

The integrator’s RTU is wired to the nearest building using Cat5 cable, communicating over one twisted-pair and 

powered using two other twisted-pairs. At the Canopy Array, however, the RTU is powered by a NIST non-batterybacked power supply in the outdoor NIST data acquisition enclosure. The data wires for this RTU are also 

connected to a Moxa ISD-1230 RS485 surge protector next to the RTU in the integrator’s outdoor enclosure because 

it has been very susceptible to failure, especially during storms. To reduce power consumption, termination resistors 
31

are not wired in parallel at the ends of the RS485 communication wires because they are not needed to reduce signal 

reflections, as the cable lengths are less than 600 m (2000 ft) and the data rates are relatively low (9600 bps).

4.1.4. Communications

The communication hubs for the data acquisition systems are at an indoor location in the nearest building to each 

array. Two locked enclosures, one for the NIST equipment and one for the integrator’s data logger and RTU power 

supply, are installed in the switchgear mechanical room near the AC power meter, as shown in Figure 4-4 at the 

Ground Array.

Figure 4-4 The indoor communication hubs at the Ground Array for the NIST (top) and integrator (bottom) equipment

Near or inside the NIST enclosures are NIST network jacks with connections to the internal private network for the 

NIST data loggers and to the public firewalled network for the integrator’s data loggers and all other equipment 

accessed by both NIST and the integrator, including the AC power meters. When located outside of the NIST 

enclosure, the network patch cables are locked to the jacks to prevent unauthorized disconnection. A complete 

anonymized diagram of the communications network is shown in Appendix F, with a simplified diagram of the local

network at each array shown in Figure 4-5.
32

Figure 4-5 Diagram of local network at each array

The fiber optic cable from the outdoor data logger terminates in the NIST enclosure at a fiber LIU, which connects 

to an Etherwan EL100 10/100BASE-TX Ethernet to 100BASE-FX fiber media converter. (At the Canopy Array, 

these devices are located in a network closet on a different floor). The media converter connects to a network jack 

on the internal private NIST network with a Cat6 patch cable. The Cat5 cables from the inverter and the integrator’s 

RTU at each array terminate at separate Moxa ISD-1230 RS485 surge protectors in the NIST enclosure. These, 

along with the other equipment in the enclosure, are grounded using 10 AWG (5.26 mm2) wire in another star-point 

configuration to a central point in the enclosure that is connected to the building ground.

The outputs from the surge protectors are wired to a CommFront HUB-485-4 RS485 hub. The single input on the 

hub is wired along with the serial communication port on the integrator’s data logger to separate serial ports on a 

Moxa MGate MB3270 serial-to-Ethernet Modbus gateway, both through additional, smaller SerialComm ISO-485-P 

surge protectors. This gateway translates and coordinates communications between the inverter and integrator’s 

RTU and both the NIST data logger (Ethernet connection) and the integrator’s data logger (serial connection). The 

gateway connects to a network jack on the public firewalled NIST network with a Cat6 patch cable. The NIST data 

loggers communicate over the NIST network with the inverters and RTUs through the gateways, using the Modbus 

TCP/IP protocol over Ethernet, which is translated by the gateways into the RS485 serial Modbus RTU protocol, 

and vice versa.

The NIST indoor communication enclosure at the Ground Array also contains a 24 VDC power supply for the 

outdoor data logger and a Power over Ethernet (PoE) midspan for the Ground Array camera. The NIST enclosure at 

the Roof Array contains an extra smaller surge protector for connecting the RS485 hub to RTUs measuring module 

temperatures sensors on the older horizontal PV array.
33

4.1.5. Configuration

4.1.5.1. Sampling Rates and Parallel Processing

The data loggers are programmed using the Campbell Scientific language CRBASIC. The programs measure all of 

the sensors every 1 s, except for the RTDs which are measured every 10 s. The slower RTD sampling rate is due to 

the slower relay multiplexers to which the RTDs are connected, but is not a restriction nonetheless because the 

module temperatures the RTDs are measuring are not changing appreciably faster than every 10 s. It takes longer 

than 1 s to measure, and poll and receive data from the sensors and instruments if done sequentially, so the program 

is run in “pipeline” mode which effectively executes some of the measurement steps in parallel.

4.1.5.2. Time Synchronization

In order to maintain time synchronization within 1 s between data loggers, the data loggers are instructed every 

morning just after midnight to synchronize with a local internet time server. This synchronization keeps the data 

loggers within 0.5 s of each other at all times. The data loggers are configured to always use local standard time

(LST) to eliminate issues arising from the switch to and from daylight saving time (DST) and to allow easier 

comparisons and calculations.

4.1.5.3. Channel Ranges

The input voltage ranges for each measurement channel are explicitly chosen and fixed; auto-range is not used to 

eliminate the initial measurement needed to set the range, resulting in a shorter sampling time. The range determined 

by auto-range may also not be accurate for the irradiance and electrical measurements as they can fluctuate rapidly.

4.1.5.4. Self-Calibration

The data logger is instructed to periodically self-calibrate to compensate for aging-induced and operating 

temperature-induced (Seebeck effect) drift. Voltage offsets are measured on each channel when the signal source is 

internally disconnected, using an internal voltage standard as a reference for all channels.

4.1.5.5. Measurement Techniques

Various techniques are employed for every measurement to more accurately measure the sensed value. The voltage 

signals, both direct from the sensor and those from a shunt when measuring current signals, are measured 

differentially instead of single-ended with respect to the data logger ground. This approach eliminates any voltage 

reference offsets and rejects capacitively coupled common-mode noise.

Although considered, the thermopile pyranometers, the only electrically floating sensors in these DASs, are not 

pulled to ground before each measurement to null any common-mode voltage. Rather, jumper wires are connected 

between the low sides of the differential inputs for the pyranometers and the respective ground terminals to keep the 

pyranometers grounded, as previously described. These data loggers combine nulling the common-mode voltage 

with open input detection, commonly used for thermocouples, into one function and neither can be individually 

implemented. The open input detection, which biases the high side of the differential input for the pyranometer 

above the channel measurement range while grounding the low side, would not allow the pyranometer output 

enough time to stabilize before the measurement. No measurement channels in the DASs employ the common-mode 

nulling and open input detection function.

The settling times, the delay after the channel terminals are internally connected to the input amplifier and before the

measurement is started, were chosen to be 10 ms for the pyranometers and 3 ms for other analog signals. The longer 

settling time for the pyranometers allows further reduction of voltage transients caused by channel switching, as they

would have a larger relative magnitude compared to the pyranometers’ much smaller voltage signals. Due to the 

larger number of sensors at the Roof Array, only the essential pyranometers in the southeast corner of the array and

in the center of the older horizontal array are allowed to settle for 10 ms, with the rest settling for 3 ms, to allow for 

all sensors to be sampled every 1 s.
34

The voltage signals are integrated for exactly 16.67 ms. This integration time, the period of a 60 Hz waveform, 

rejects 60 Hz frequency components in the signal by cancelling them out. This noise is induced from nearby AC 

power sources, which operate at 60 Hz in the USA. The signals for the nonessential pyranometers at the Roof Array 

are integrated for 250 μs instead of 16.67 ms, also to allow for all sensors to be sampled every 1 s.

Each differential voltage measurement is performed twice, with the second time after the channel terminals are 

internally reverse-connected to the input amplifier. The two measurements are subtracted and divided by two to 

cancel any voltage offsets in the measurement circuitry, as well as any common-mode gain errors. Note that before 

each measurement the signal is allowed time to settle, which doubles the total settling time per measurement.

There is not enough available time in the 1 s scan period to employ all of the aforementioned techniques when 

measuring the four-wire RTDs, so tests were performed to identify those most beneficial in this application. The 

tests determined that a settling time of 3 ms was adequate and revealed that 60 Hz integration reduces significantly 

more uncertainty than reversing the inputs and reversing the excitation, combined. Reversing the excitation is 

similar to reversing the inputs, but does so for the excitation and works to cancel any voltage offsets before and in

the measurement circuitry (reversing the input cancels the offset in the measurement circuitry as well as the 

common-mode gain errors). Note that these two techniques each double the total settling time and combining both 

quadruples it. If there was a longer sampling period then all three techniques would have been employed, even 

though the benefits of the two reversals are relatively small in this application.

The excitation for the RTDs is optimized by maximizing the measurement resolution of the voltage across the 

sensing resistor (RTD) while keeping the self-heating error, caused by the excitation current passing through the 

sensing resistor, below an acceptable level. Self-heating errors for 100 Ω RTDs like the ones in these DASs are less 

than 0.001 °C (0.0018 °F) in still air when the voltage drop across the sensing resistor is less than approximately 25 

mV (6 μW) [14]. The optimal excitation voltage is therefore what results in a maximum sensor voltage of 25 mV, 

which occurs at the highest expected temperature, chosen conservatively to be 90 °C (194 °F). This corresponds to a 

maximum resistance of 135 Ω. The optimal excitation voltage is calculated for the half-bridge and current-limiting 

resistor in Figure 4-6 using Equation (1), neglecting the negligible lead resistances. The equation is calculated using 

values of the measurement bridge resistors at the low end of their respective tolerance ranges, resulting in a higher 

current and subsequently higher RTD voltage. If the tolerances are not accounted for, this potentially higher RTD 

voltage could be greater than the selected measurement range.
35

Figure 4-6 Half-bridge for measuring RTDs

 

,max

,min ,min ,max

,max

s

x cl sh s

s

V

V R R R

R

  

(1)

where:

Vx = excitation voltage

Vs = sensing voltage

Rs = sensing resistor (RTD) 

Rcl = current limiting resistor

Rsh = completion (shunt) resistor

Vsh = shunt voltage

Ix = excitation current

The optimal excitation, calculated to be 1.8 V, is only kept on during the measurement to further reduce the selfheating errors. The measurement equation for the half-bridge, when reduced, becomes the ratiometric relation

R R V V s sh s sh  . The temperature of the RTD is then determined from the resistance using the inverse of the 

Callendar-Van Dusen equation according to the ASTM E1137-04 [15] standard.

Timeouts for the serial and Ethernet communicating instruments, the time after polling the instrument waiting for a 

reply, were carefully chosen to be the maximum time the instrument normally takes to respond to a request. A flag 

in the data logger programs can be manually set while the program is running to stop polling individual instruments 

in the case where they are temporarily unresponsive, which has occurred for the integrator’s RTUs and the inverters. 

Setting this flag can speed up the program and avoid missed measurements while the data logger would normally be 

waiting the full timeout period for a response.

4.1.5.6. Control

The data loggers regulate the power to the inverter shunt RTUs and the wind sensor heaters when external power to 

the data acquisition enclosures is off and the data loggers are operating on battery power. These two devices 

consume the most power and are regulated to allow the DASs to operate for approximately three days on battery 
36

power. Power to the inverter shunt RTUs is turned off at night between 20:30 and 04:00 LST. Power to the wind

sensor heater is turned on during the first five minutes of each hour and turned off at all other times.

The data logger at the Roof Array, due to its high utilization, drops its communications every few days and needs a 

power reset to restore it. One of the data loggers that is part of the weather station remotely toggles the power to the 

Roof data logger via a relay every other day at 01:00 LST or when the data logger is unresponsive. The relay is 

controlled over the public network through the RS485 hub at the Roof Array.

4.1.5.7. Data storage and processing

Measurements are stored on the memory card as either 16-bit floating point, 32-bit floating point, or 32-bit signed 

integer data types according to the needed range and precision of the individual measurements. This selective 

allocation conserves storage space and increases processing speeds in the data logger and server. All measurements 

are saved every one-second, including the RTD measurements, resulting in nine duplicate RTD measurements saved 

every 10-second sample period. All measurements are also either averaged or the minimum or maximum calculated

for each minute, whichever more appropriate, and saved. The one-minute values are calculated from the samples 

taken between 1 s and 60 s, inclusive, preceding the recorded minute time. The wind measurements are processed 

according to EPA guidelines [16] to produce the mean horizontal wind speed, the unit vector mean wind direction, 

and the standard deviation of wind direction calculated using the Yamartino algorithm.

The 2 GB of storage can store 82 days to 131 days of measurements depending on the array for times when the 

communications are down. The storage is configured as ring memory, where the oldest data is overwritten when the 

memory card is full.

4.2. Reference Modules

Reference modules, one for each orientation at each array, are installed in the same orientations at the weather 

station, as shown in Figure 4-7. These reference modules provide baseline measurements without the significant 

wiring, mismatch, soiling, snow, shading, and other losses of the modules in the arrays. These reference modules

also provide data on their aging, and their true average cell temperature and absorbed/effective irradiance from their 

measured I-V curves, which can be correlated with the measured backsheet surface temperature and POA irradiance, 

respectively. 

Figure 4-7 The reference modules installed at the weather station

There are two modules for the Canopy Array (facing left and right), one module each for the Ground Array (back 

left) and Roof Array (second from front), one module for the older horizontal array near the Roof Array (front), and 
37

one module for the array on NIST’s Net-Zero Energy Residential Test Facility (NZERTF) (back right). These

reference modules (except the one for the older horizontal array) are from the same production batches as those 

installed at each array. The modules are arranged according to a CAD model made in SketchUp [17], which

calculated less than 45 minutes of any mutual or external shading on the modules at sunrise and sunset on the 

summer solstice and less than 5 minutes on the winter solstice, the two extremes of the year. Flashing is installed 

around the modules for the older horizontal array, Roof Array, and NZERTF array to simulate the effect of the 

respective installations on the module temperatures; the modules in the Canopy and Ground Arrays have open air 

circulation on their back sides and therefore have no flashing installed around their reference modules. 

The backsheet surface temperatures of each module are measured using one Omega CO1-T-72 thermocouple 

mounted behind a center cell and either four Omega SA1-RTD-4W-80 RTDs for the Canopy, Ground, Roof Array 

modules or four additional Omega CO1-T-72 thermocouples for the older horizontal and NZERTF array modules, 

matching the sensor type at each array. The set of four temperature sensors are mounted behind one center and three 

peripheral cells, as described in section 3.4. The POA irradiance is measured for each module using an IMT Solar 

Si-420TC flat-plate silicon irradiance sensor, as described in section 3.2.

The modules, thermocouples, and irradiance sensors are connected to a Daystar Multi-Tracer 5 (MT5) I-V curve 

tracer with maximum power tracker and load system, as shown in Figure 4-8. (The set of four temperature sensors 

on each module are connected to a data logger multiplexer for the weather station, sampled every 10 s and saved 

every 1 s, and averaged and saved every 1 min.) The modules are wired directly to the system using 10 AWG (5.26 

mm2) UV and moisture protected PV cable run through metal dedicated conduit. The sensors are wired as described 

in sections 3.2.5 and 3.4.5. 

Figure 4-8 The I-V curve tracer with maximum power tracker and load
38

The I-V tracer and maximum power tracking system performs an I-V trace for each module every minute, 

integrating every current and voltage reading for 100 μs. The curves are saved along with measurements of the 

respective thermocouple and irradiance sensor before and after each trace. At all other times, the system maximum 

power tracks each module, measuring the maximum power voltage and current and the corresponding thermocouple

and irradiance sensor, sampling every one second and averaging and saving every minute. All measurements are 

saved to internal memory. The MT5 system does not adjust its clock for DST, which allows for easier comparison 

with the measured data from the data loggers, which also do not adjust for DST. The system was calibrated at the 

factory and verified to be within specifications.

4.3. Cameras

Images of each array are taken using an Axis Q6032-E pan-tilt-zoom (PTZ) network camera located at each array, 

with the camera for the Canopy Array shown in Figure 4-9. The images taken by the cameras are used to capture the 

shading, snow, rain, and frost, and any other obstructions or damage to the arrays.

Figure 4-9 The PTZ camera that records images of the Canopy Array

The cameras are configured for constant white balance, auto exposure, and to use an IR cut filter during the day but 

not at night to increase sensitivity. The cameras, like the data loggers, are configured to not adjust their clocks for

DST for easier comparison with the measured data. The cameras take 704 x 480 resolution images, which are saved 

as uncompressed JPEG images on an internal 32 GB SanDisk Extreme SD card. Each camera is programmed to take 

images of the entire array and of close-up sections at a constant zoom for detailed analysis and stitching together

into a single higher resolution image. The cameras take images every five minutes during the day (04:00 to 20:00 

LST) and every hour all day. 
39

The array cameras communicate over the NIST private network and are powered using Power over Ethernet (PoE).

A surge protector is housed in an outdoor enclosure, and the midspan, for combining the power and Ethernet, and 

any necessary network switch and power supply are housed in an indoor enclosure, with the enclosure for the 

Canopy and Roof Array cameras shown in Figure 4-10.

Figure 4-10 The enclosure for the Canopy and Roof Array camera midspans and network switch

There is also a Campbell Scientific CC5MPX camera at the weather station that takes images of the reference 

modules. This camera does not adjust for DST and takes 2592 x 1944 resolution images saved as uncompressed 

JPEG images on an internal 32 GB SanDisk Extreme SD card. No other settings are configurable. The camera is 

programmed to take a fixed zoom image every five minutes during the day (04:00 to 20:00 LST) and every hour all 

day. This camera communicates over the NIST private network and is powered using a separate cable.

5. Data Archiving and Monitoring

Campbell Scientific LoggerNet Admin is used to manage the network of data loggers. It is installed on a network 

server running Windows Server 2008 R2. Loggernet Admin runs as a service, running whether or not a user is 

logged in and starts immediately after the server restarts. Loggernet is configured to import data from the data 

loggers every minute, leaving a copy on the data loggers as a backup to be later overwritten. The data are saved on 

the server as Campbell Scientific TOA5 formatted plain text files, one for each of the 12 defined data logger tables 

from the Canopy, Ground, and Roof Array data loggers, including the one-minute aggregated tables but not 

including the file of the temperature measurements of the reference modules from the weather station data logger.
40

The data files are imported to a MySQL database every minute by Vista Data Vision (VDV) Enterprise by Vista 

Engineering, also installed on the server. VDV is configured to send email alerts when data values are outside of 

defined ranges and when the data files are not being updated. Custom dashboards for each array with plots of select 

measurements are updated by VDV every minute and provide an overview of the system health. These dashboards 

are displayed in a web interface hosted by the server, with the main data dashboard for the Ground Array shown in

Figure 5-1. In addition to the dashboards, custom plots of various types, tables, logs of alarms, completeness of the 

database, and a viewer for the hourly camera images of each array are also provided in the web interface. This web 

interface is currently only available inside the NIST firewall, but a public data portal with the ability to download 

data will be made available. In the meantime, interested modelers and analysts who would like to use these data in 

collaboration on new research can contact the author.

Figure 5-1 The main data dashboard for the Ground Array

Data from the I-V curve tracer and maximum power tracking system and images from the cameras are copied to the 

server every 30 minutes by an FTP client on the server run as a scheduled task. A custom script, also run as a 

scheduled task, monitors and sends alerts when data files from the I-V curve tracer and maximum power tracking 

system are not being transferred to the server. VDV monitors and sends alerts when new images from each camera 

are not being transferred. All data and images on the server, including data from the data loggers, are continually 

backed up to offsite cloud storage.
41

6. Uncertainty

Uncertainties for the collected data are not listed because a complete uncertainty analysis is outside the scope of this 

report, but one will be performed for each measurement and presented in a forthcoming paper. Sensor and 

instrument-level uncertainties are not given as they are only part of the system-level values, and therefore can be 

misleading. Furthermore, they do not account for the error between the measurand, the quantity of interest, and the 

realized quantity, the quantity measurable by the sensor.

7. Maintenance

The data dashboards for each array are checked every weekday for any apparent problems in the DASs. The

irradiance sensors at the Ground and Roof Arrays are cleaned every other day, minus weekends, using cleanroom 

wipes moistened with a 6 % isopropyl/94 % deionized water mixture and dried with optical tissue. Any ice buildup 

is first melted using an ethanol solution of the same concentration. One of the flat-plate irradiance sensors in the pair 

measuring the soiling at the Roof Array is never cleaned. The irradiance sensors at the Canopy Array are cleaned 

only two to three times a year due to difficulty in gaining access to the elevated instruments, but a solution is being 

implemented that will allow regular cleaning. A log is kept of the dates and times when the sensors at the different 

arrays are cleaned.

The data files on the server grow indefinitely, so once a week a custom script is run that pauses the data importation

from the data loggers and runs the Campbell Scientific program File Format Convert that splits the files into 

separate daily files for easier archiving and use.

Thermopile pyranometers are replaced every 12 to 18 months with newly calibrated pyranometers from NREL. A 

log is kept of these instrument replacements, which also contains all other maintenance including when sensors, 

instruments, and devices fail and are replaced and when the arrays are offline.

Acknowledgements

The author wishes to thank all those who contributed time and effort planning, constructing, and implementing the 

infrastructure needed for the data acquisition systems described in this report. They include Bill Robson and John 

Selby of Legatus 6, Christoph Heinzer of Advanced Energy, Carrie Kelly of NIST’s Office of Facilities and 

Property Management, and Mike Zelinsky of SunEdison. Special thanks to Brian Dougherty of NIST’s Engineering 

Laboratory’s (EL’s) Energy and Environment Division for logistical, construction, and maintenance support. Thanks 

to Luis Luyo, also of EL’s Energy and Environment Division, for constructing the infrastructure for the reference 

modules, and to Jeffrey Sze for early testing of the systems. Thank you also to Ryan Smith of Pordis, LLC, then at 

NREL, for early instrument selection advice and design resources.

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(Accessed on Sept. 11, 2015)

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National Renewable Energy Laboratory, (2011). 

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on Sep. 11, 2015)

[3] J. Newmiller, Validation information for PV power system project review, (2011). 

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(Accessed on Sep. 9, 2014)

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A-1

Appendix A. Oblique Images of the Arrays

Figure A-1 Oblique Aerial View of the Canopy Array Site
A-2

Appendix A. Oblique Images of the Arrays

Figure A-2 Oblique Aerial View of the Ground Array Site
A-3

Appendix A. Oblique Images of the Arrays

Figure A-3 Oblique Aerial View of the Roof Array site
B-1

Appendix B. Manufacturer Data Sheets

Figure B-1 Manufacturer Datasheet for the PV Module at All Three Arrays
B-2

Appendix B. Manufacturer Data Sheets

Figure B-2 Manufacturer Datasheet for the Inverter at the Canopy and Ground Arrays
B-3

Appendix B. Manufacturer Data Sheets

Figure B-3 Manufacturer Datasheet for the Inverter at the Roof Array
C-1

Appendix C. Copper Wire Gauges and Lengths

Table C-1 Copper Wire Gauges and Lengths

Canopy Ground Roof

Modules

Lead Cross-Sectional Area [mm2] 3.31 3.31 3.31

Individual Lead Length [m] 1.1 1.1 1.1

Modules to Combiner Box

Conductor Cross-Sectional Area [mm2] 5.26 5.26 5.26

Approximate Average Distance, One-Way [m] 85 95 52

Combiner Box to Inverter DC Combiner 

Compartment

Conductor Cross-Sectional Area [mm2] 142* 127 53.5

Approximate Average Distance, One-Way [m] 60 79 167

Inverter DC Combiner Compartment to 

Inverter Power Module Assembly

Compartment

Conductor Cross-Sectional Area [mm2] 613 613 242

Approximate Average Distance, One-Way [m] 4 4 4

Inverter to Building Switchgear

Conductor Cross-Sectional Area [mm2] 253 253 42.4

Number of Wires per Phase 1 2 1

Total Conductor Cross-Sectional Area [mm2] 253 507 42.4

Approximate Average Distance, One-Way [m] 172 79 14

* Weighted average of 127 mm2and 177 mm2conductors
D-1

Appendix D. Diagrams of the Arrays

Figure D-1 Diagram of the Canopy Array Strings and Pertinent Array Dimensions
D-2

Appendix D. Diagrams of the Arrays

Figure D-2 Diagram of the Ground Array Strings and Pertinent Array and Fence Dimensions
D-3

Appendix D. Diagrams of the Arrays

Figure D-3 Diagram of the Roof Array Strings and Pertinent Array Dimensions
E-1

Appendix E. The Saved Data Values From Each Array

Table E-1 The Saved Data Values From Each Array

Variable Name Units Description

One-Minute 

Average/

Min/Max

Canopy (C)/ 

Ground (G)/ 

Roof (R)

AmbTemp_C C Outdoor ambient temperature Average C/G/R

Battery_A A Current to DAS battery, neg. = discharging Average C/G/R

Battery_V V Charge voltage of DAS battery Min C/G/R

ChgSource -

Charging regulator power source

0 = none

1 = solar terminals

2 = continuous terminals

Min C/G/R

ChgState -

State of charging regulator;

-1 = regulator fault

0 = no charge

1 = current limited charging

2 = cycle charging

3 = float charging

4 = battery test

Min C/G/R

CkBatt -

Status of DAS battery;

0 = normal

1 = check battery

Max C/G/R

CR1000Temp_C C

Temperature of the data logger front 

terminal panel Average C/G/R

DoorOpen -

Door sensor for DAS enclosure;

-1 = open, 0 =closed Min C/G/R

DPCBFaults -

Inverter digital power control board fault 

code Max R

GeneralFaults1 - Inverter general fault code, group 1 Max R

GeneralFaults2 - Inverter general fault code, group 2 Max R

HardwareFaults - Inverter hardware fault code Max R

InvDriveFault - Inverter drive fault code Max C/G

InverterFaults - Inverter inverter fault code Max R

InvEtoday_kWh kWh AC total real energy from current day, 

meas. by inverter Max R

InvEtot_kWh kWh AC total accumulated real energy, meas. by 

inverter Max C/G/R

InvEtot_kWh kWh AC total accumulated real energy, meas. by 

inverter Max R

InvEyesterday_kWh kWh AC total real energy from previous day, 

meas. by inverter Max R

InvFreq Hz AC frequency, meas. by inverter Average C/G/R

InvGndImped_kOhm kΩ Inverter ground impedance Average R
E-2

InvGridFault - Inverter grid fault code Max C/G

InvIa A Line current, phase A, meas. by inverter Average C/G/R

InvIabcAvg A Average AC line current, meas. by inverter Average R

InvIb A Line current, phase B, meas. by inverter Average C/G/R

InvIc A Line current, phase C, meas. by inverter Average C/G/R

InvIDCin A Inverter DC input current, meas. by inverter Average C/G/R

InvIGnd A Inverter DC ground current Average R

InvINeutral A AC neutral current, meas. by inverter Average R

InvIUnbal %

AC line current unbalance, meas. by 

inverter Average R

InvMainFault - Inverter main fault code Max C/G

InvOpState - Inverter operating status/state Average R

InvOpStatus - Inverter operating status/state Average C/G

InvPAC_kW kW AC real power, meas. by inverter Average C/G/R

InvPDC_kW kW Inverter DC input power, meas. by inverter Average C/G/R

InvPF - AC power factor, meas. by inverter Average R

InvPVMStatus - Inverter PV monitoring status code Max C/G

InvSystemFault - Inverter system fault code Max C/G

InvSystemWarn - Inverter system warning code Max C/G

InvTempFault - Inverter temperature fault code Max C/G

InvTempHeatsink_C C Inverter heatsink temperature Average R

InvTempInternalAir_C C

Ambient temperature inside inverter general 

enclosure Average R

InvTempInverterAir_C C

Ambient temperature inside inverter 

inverter enclosure Average R

InvVa V Line voltage, phase A, meas. by inverter Average C/G/R

InvVA_kVA kVA AC apparent power, meas. by inverter Average R

InvVabcAvg V Average AC line voltage, meas. by inverter Average R

InvVAR_kVAR kVAR AC reactive power, meas. by inverter Average R

InvVb V Line voltage, phase B, meas. by inverter Average C/G/R

InvVc V Line voltage, phase C, meas. by inverter Average C/G/R

InvVDCin V

Inverter DC input voltage on load side of 

DC contactor, meas. by inverter Average C/G/R

InvVDVoltage_V V

Array DC voltage on DC busbars, measured 

using voltage divider Average C/G/R

InvVoltageFault - Inverter voltage fault code Max C/G

InvVPVin V

Array DC voltage on line (PV) side of DC 

contactor, meas. by inverter Average C/G/R
E-3

InvVUnbal %

AC line voltage unbalance, meas. by 

inverter Average R

Load_A A

Current supplied to load (DAS) from 

charging regulator Average C/G/R

NumActiveFaults - Inverter number of active faults Max R

PwrMtrEdel_kVARh kVARh AC total accumulated reactive energy to 

(used by) inverter, meas. by AC meter Max C/G/R

PwrMtrEdel_kWh kWh AC total accumulated real energy to (used 

by) inverter, meas. by AC meter Max C/G/R

PwrMtrErec_kVARh kVARh AC total accumulated reactive energy from 

(generated by) inverter, meas. by AC meter Max C/G/R

PwrMtrErec_kWh kWh AC total accumulated real energy from 

(generated by) inverter, meas. by AC meter Max C/G/R

PwrMtrFreq Hz AC frequency, meas. by AC meter Average C/G/R

PwrMtrFreqMax Hz Max. AC frequency, meas. by AC meter Max C/G/R

PwrMtrFreqMin Hz Min. AC frequency, meas. by AC meter Min C/G/R

PwrMtrIa A Line current, phase A, meas. by AC meter Average C/G/R

PwrMtrIb A Line current, phase B, meas. by AC meter Average C/G/R

PwrMtrIc A Line current, phase C, meas. by AC meter Average C/G/R

PwrMtrP_kVA kVA AC apparent power, meas. by AC meter Average C/G/R

PwrMtrP_kVAR kVAR AC reactive power, meas. by AC meter Average C/G/R

PwrMtrP_kW kW AC real power, meas. by AC meter Average C/G/R

PwrMtrPF - AC power factor, meas. by AC meter Average C/G/R

PwrMtrPhaseRev Hz AC phase reversal, meas. by AC meter Average C/G/R

PwrMtrVa V Line voltage, phase A, meas. by AC meter Average C/G/R

PwrMtrVaTHDMax %

AC max. total harmonic distortion in phase 

A, meas. by AC meter Max C/G/R

PwrMtrVb V Line voltage, phase B, meas. by AC meter Average C/G/R

PwrMtrVbTHDMax %

AC max. total harmonic distortion in phase 

B, meas. by AC meter Max C/G/R

PwrMtrVc V Line voltage, phase C, meas. by AC meter Average C/G/R

PwrMtrVcTHDMax %

AC max. total harmonic distortion in phase 

C, meas. by AC meter Max C/G/R

Pyra1_mV mV

Millivolt output from pyranometer;

Canopy: POA, west-tilted

Ground: GHI

Roof: GHI, southwest

Average C/G/R

Pyra2_mV mV

Millivolt output from pyranometer;

Canopy: GHI

Ground: POA

Roof: GHI, southeast

Average C/G/R
E-4

Pyra3_mV mV

Millivolt output from pyranometer;

Canopy: POA, east-tilted

Roof: POA, southeast

Average C/R

Pyra4_mV mV Millivolt output from pyranometer;

Roof: GHI, northwest Average R

Pyra5_mV mV Millivolt output from pyranometer;

Roof: GHI, northeast Average R

Pyra6_mV mV Millivolt output from pyranometer;

Roof: GHI, center of older horiz. array Average R

Pyra7_mV mV

Millivolt output from pyranometer;

Roof: GHI, northwest of older horiz. 

array

Average R

Pyra8_mV mV

Millivolt output from pyranometer;

Roof: GHI, northeast of older horiz. 

array

Average R

Qloss_Ah A·h Charge deficit of DAS battery Max C/G/R

RefCell1_Wm2 W/m2

Irradiance from flat-plate silicon sensor;

Canopy: POA, west-tilted

Ground: POA

Roof: POA, southeast

Average C/G/R

RefCell2_Wm2 W/m2

Irradiance from flat-plate silicon sensor;

Canopy: POA, east-tilted

Roof: GHI, center of older horiz. array, 

east, cleaned

Average C/R

RefCell3_Wm2 W/m2

Irradiance from flat-plate silicon sensor;

Roof: GHI, center of older horiz. array, 

west, uncleaned

Average R

RTD_C(1) C

Temperature of module backsheet;

Canopy: east-tilted southwest module

Ground: center shed, northwest module

Roof: south sub-array, south-center 

module

Average C/G/R

RTD_C(10) C Ambient temperature inside inverter Average C/G/

RTD_C(2) C

Temperature of module backsheet;

Canopy: east-tilted southeast module

Ground: center shed, center module of

west half

Roof: center-east sub array, center-east 

module

Average C/G/R

RTD_C(3) C

Temperature of module backsheet;

Canopy: west-tilted center-east module

Ground: center shed, center-bottom 

module

Roof: center-east sub array, center 

module, center

Average C/G/R
E-5

RTD_C(4) C

Temperature of module backsheet;

Canopy: east-tilted center-west module, 

center

Ground: center shed, center module, center

Roof: center-east sub array, center 

module, northwest

Average C/G/R

RTD_C(5) C

Temperature of module backsheet;

Canopy: east-tilted center-west module, 

southwest

Ground: center shed, center module, 

southwest

Roof: center-east sub array, center 

module, south-center

Average C/G/R

RTD_C(6) C

Temperature of module backsheet;

Canopy: east-tilted center-west module, 

center-east

Ground: center shed, center module, 

center-east

Roof: center-east sub array, center 

module, center-east

Average C/G/R

RTD_C(7) C

Temperature of module backsheet;

Canopy: east-tilted center-west module, 

north-center

Ground: center shed, center module, northcenter

Roof: northwest sub-array, northwest 

module

Average C/G/R

RTD_C(8) C

Temperature of module backsheet;

Canopy: east-tilted center-east module

Ground: center shed, center-top module

Average C/G/

RTD_C(9) C

Temperature of module backsheet;

Canopy: east-tilted northwest module

Ground: north shed, center module

Average C/G/

SEWSAmbientTemp_C C

Outdoor ambient temperature from 

integrator’s sensor Average C/G/R

SEWSModuleTemp_C C

Module temperature from integrator’s 

sensor

Average C/G/R

SEWSPOAIrrad_Wm2 W/m2 POA irradiance from integrator’s domed 

diffused silicon-cell pyranometer Average C/G/R

ShuntCurrent_A(1) A Output circuit current from combiner box 1 Average C/G/R

ShuntCurrent_A(2) A Output circuit current from combiner box 2 Average C/G/R

ShuntCurrent_A(3) A Output circuit current from combiner box 3 Average C/G/R

ShuntCurrent_A(4) A Output circuit current from combiner box 4 Average C/G/R

ShuntCurrent_A(5) A Output circuit current from combiner box 5 Average C/G

ShuntCurrent_A(6) A Output circuit current from combiner box 6 Average C/G

ShuntCurrent_A(7) A Output circuit current from combiner box 7 Average C/G
E-6

ShuntEtot_kWh_Avg(1) kWh

Output circuit total accumulated energy 

from combiner box 1, meas. using shunts

and voltage divider*

- C/G/R

ShuntEtot_kWh_Avg(2) kWh

Output circuit total accumulated energy 

from combiner box 2, meas. using shunts 

and voltage divider*

- C/G/R

ShuntEtot_kWh_Avg(3) kWh

Output circuit total accumulated energy 

from combiner box 3, meas. using shunts 

and voltage divider*

- C/G/R

ShuntEtot_kWh_Avg(4) kWh

Output circuit total accumulated energy 

from combiner box 4, meas. using shunts 

and voltage divider*

- C/G/R

ShuntEtot_kWh_Avg(5) kWh

Output circuit total accumulated energy 

from combiner box 5, meas. using shunts 

and voltage divider*

- C/G

ShuntEtot_kWh_Avg(6) kWh

Output circuit total accumulated energy 

from combiner box 6, meas. using shunts 

and voltage divider*

- C/G

ShuntEtot_kWh_Avg(7) kWh

Output circuit total accumulated energy 

from combiner box 7, meas. using shunts 

and voltage divider*

- C/G

ShuntPDC_kW_Avg(1) kW Output circuit power from combiner box 1, 

meas. using shunts and voltage divider* - C/G/R

ShuntPDC_kW_Avg(2) kW Output circuit power from combiner box 2, 

meas. using shunts and voltage divider* - C/G/R

ShuntPDC_kW_Avg(3) kW Output circuit power from combiner box 3, 

meas. using shunts and voltage divider* - C/G/R

ShuntPDC_kW_Avg(4) kW Output circuit power from combiner box 4, 

meas. using shunts and voltage divider* - C/G/R

ShuntPDC_kW_Avg(5) kW Output circuit power from combiner box 5, 

meas. using shunts and voltage divider* - C/G

ShuntPDC_kW_Avg(6) kW Output circuit power from combiner box 6, 

meas. using shunts and voltage divider* - C/G

ShuntPDC_kW_Avg(7) kW Output circuit power from combiner box 7, 

meas. using shunts and voltage divider* - C/G

TemperatureFaults - Inverter temperature fault code Max R

TIMESTAMP LST Date and time Max C/G/R

Warnings - Inverter system warning code Max R

WindDir_deg ° Wind direction, clockwise from north - C/G/R

WindDirAve_deg ° Unit vector mean wind direction* - C/G/R

WindDirStdDev_deg ° Standard deviation of the wind direction* - C/G/R
E-7

WindHeatStateID -

1 = heating disabled

2 = enabled, not heating

3 = 50 % heating

4 = 100 % heating

5 = error

Average C/G/R

WindRef_V V Internal reference voltage in wind sensor Min C/G/R

WindSpeed_ms m/s Wind speed Max C/G/R

WindSpeedAve_ms m/s Mean horizontal wind speed* - C/G/R

WindValid -

Wind sensor data validity

-1 = wind data is valid

0 = wind data is invalid

Average C/G/R

Note: All measurements are sampled every 1 s except the RTDs, which are sampled every 10 seconds. All 

measurements are saved every 1 s, except those marked with a “*”.

* Calculated from saved 1 s samples; only the 1 min aggregate data are saved
F-1

Appendix F. Anonymized Diagram of the Data Acquisition Systems’ Communications Network

Figure F-1 Anonymized Diagram of the Data Acquisition Systems’ Communications Network