NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Advanced Photovoltaic Inverter 

Functionality using 500 kW 

Power Hardware-in-Loop 

Complete System Laboratory 

Testing

Preprint

Barry A. Mather

National Renewable Energy Laboratory

Matthew A. Kromer and Leo Casey

Satcon Technology Corporation

To be presented at the Innovative Smart Grid Technologies 

Conference

Washington, D.C.

February 24-27, 2013

Conference Paper

NREL/CP-5500-56833

January 2013
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1 

Advanced photovoltaic inverter functionality 

verification using 500 kW Power Hardware-in-Loop 

complete system laboratory testing

Barry A. Mather, Member IEEE, Matthew A. Kromer, and Leo Casey, Member IEEE

 Barry A. Mather is with the Distributed Energy Systems Integration (DESI) Group at the National Renewable Energy Laboratory (NREL), Golden, CO 

80401 USA (e-mail: barry.mather@nrel.gov). Matthew A. Kromer and Leo Casey are with Satcon Technology Corporation, Boston, MA 02210 USA (e-mail: 

matthew.kromer@satcon.com, leo.casey@satcon.com).

Abstract-- With the increasing penetration of distribution 

connected photovoltaic (PV) systems, more and more PV 

developers and utilities are interested in easing future PV 

interconnection concerns by mitigating some of the 

impacts of PV integration using advanced PV inverter 

controls and functions. This paper describes the testing of 

a 500 kW PV inverter using Power Hardware-in-Loop 

(PHIL) testing techniques. The test setup is described and 

the results from testing the inverter in advanced 

functionality modes, not commonly used in currently 

interconnected PV systems, are presented. PV inverter 

operation under PHIL evaluation that emulated both the 

DC PV array connection and the AC distribution level 

grid connection are shown for constant power factor (PF) 

and constant reactive power (VAr) control modes. The 

evaluation of these modes was completed under varying 

degrees of modeled PV variability.

Index Terms— Power hardware-in-loop (PHIL), high power 

photovoltaic (PV) inverter testing, advanced PV inverter 

functionality, power factor control, reactive power control 

I. INTRODUCTION

The continued high rate of deployed distribution systemconnected PV systems, particularly megawatt-scale ground 

and roof-mounted systems, has led to interest by PV 

developers and utilities in the impact of PV systems 

interconnected on the distribution system. Additionally, once 

distribution system-level impacts have been determined, the 

mitigation of negative impacts is of foremost interest to the 

PV developers and distribution utilities in order to 

accommodate the interconnection of increasing penetration 

levels of PV without expensive equipment upgrades or 

reduced distribution system performance. The Southern 

California Edison (SCE) High-Penetration PV Integration 

Project [1], led by the National Renewable Energy 

Laboratory (NREL), is in the second year of a five-year 

project that aims to identify and quantify the impacts of 

distribution system-connected PV systems, and demonstrate 

potential PV impact mitigation methods. The experiment 

described in this paper was completed under the auspices of 

the SCE High-Penetration PV Integration Project. The team 

that completed the experiment included SCE HighPenetration PV Integration Project team members and the 

Center for Advanced Power Systems at Florida State 

University where the laboratory testing was completed.

Many different concepts for PV impact mitigation on the 

distribution system have been proposed. The SCE HighPenetration PV Integration Project has focused on PV impact 

mitigation via advanced control functionality integrated into 

the PV inverter. The reason this approach was adopted is that 

PV mitigation at the point of grid interconnection (often at or 

near a PV systems inverter) is likely the most effective and 

realistic mitigation approach. Additionally, the PV inverter is 

a likely candidate for providing mitigation functionality, as it 

can provide some forms of potential PV mitigation, such as 

VAr support functions, with a low incremental cost. A 

description of potential PV mitigation VAr support functions 

is given in [2]. Also, an effort is underway to standardize the 

utility communications interface to inverters and other 

equipment providing PV mitigation services [3].

This paper describes the results of a testing effort that was 

designed to examine both the ability of a PV inverter to 

provide advanced functions designed to mitigate PV impact 

issues on the distribution system, and to quantify the efficacy 

of these advanced PV inverter functions towards mitigating 

the negative impacts interconnected PV systems on the 

distribution system. While the tests described in this paper 

address both of these issues, this paper focuses only on the 

ability of a PV inverter to implement two advancedfunctionality PV inverter functions that show potential utility 

to mitigate the impacts of PV interconnection. The two 

functions tested were the operation of the PV inverter under 

constant power factor (PF) control and under constant 

reactive power (VAr) control. While these two modes of 

operation are relatively basic, they are considered “advanced 

functions” as they are not currently widely used on 

distribution system-interconnected PV systems. 
2 

PHIL testing of the PV inverter was selected for this 

evaluation to analyze the operation of the PV inverter under 

“near” real world interconnection conditions. All reasonable 

efforts were taken to make the PHIL experiment as applicable 

to the actual deployment of an advanced-functionality PV 

inverter in SCE’s distribution system, as is planned in the 

SCE High-Penetration PV Integration Project.

Section II of this paper gives a brief description of the PHIL 

experiment and presents the baseline operation of PV inverter 

without advanced functionality enabled. Sections III and IV 

present the results of the PHIL testing for the PV inverter 

operating with constant PF and constant VAr control modes 

enabled respectively. Section V concludes the paper.

PV Inverter (DUT) VVS (AC Grid 

Emulation)

VVS (PV Array 

Emulation)

480:4160

RTDS Simulation Space

PV Array Model

Idc,in

Vdc,in

Vdc,ref

Idc,ref

Distribution Sys. Model

Iac,inj

Vac,pcc

Fig. 1. Simplified diagram of the PHIL experimental setup showing the AC 

and DC interconnection of power electronic based Variable Voltage Sources 

(VVS) that are controlled through the Real-Time Digital Simulator (RTDS) 

system to emulate both the DC PV array and AC grid equipment 

interconnections. 

II. POWER HARDWARE-IN-LOOP EXPERIMENTAL SETUP 

DESCRIPTION

A brief description of the PHIL experimental setup is given in 

this section. A more detailed description of the experiment 

setup and a discussion of the testing is given in [4]. As this 

PHIL experiment aimed to simulate real-world operation, the 

experiment runs in real time and each test run was 16 minutes 

in simulation duration.

A. PHIL Experimental Setup

Fig. 1 shows a simplified diagram of the various components

used to complete the PHIL testing. The PV inverter is shown 

in the center of the diagram. The inverter used in the 

experiment was a Satcon 500 kW Power Gate Plus with a 

native AC output voltage of 200 Vac nominal. The inverter 

interfaces to two Variable Voltage Sources (VVSs). These 

VVSs consist of high-power electronic modules similar to the 

PV inverter under test. The VVS used for PV array emulation 

(the “DC VVS”) is configured to output DC power to the PV 

inverter’s DC input terminals. A small load was connected in 

parallel to the PV inverter under test to provide a minimum 

load, should the PV inverter “trip” off-line due to internal 

protection functions. This load ensured that the voltage of the 

PV array emulating VVS would not rise too high during a PV 

inverter trip and damage the PV inverter. A diode was also 

installed in the series path of PV array connection so the PV 

inverter could not backfeed power to the VVS under any 

circumstances. Again, this was done to protect the VVS being 

used for PV array emulation from high voltage transients.

A second VVS (the “AC VVS”) was configured to provide 

the emulation of the PV inverters interconnection to the 

distribution system (AC grid). In this case, a transformer 

(480V:4160V) was used to interface the PV inverter’s AC 

output to the VVS. As the PV inverter’s AC operating 

voltage was nominally 200 Vac the VVS was operated at a 

lower voltage on the primary to match the secondary 

transformer voltage of 200 Vac nominal. 

The remaining critical component to the PHIL testing was the 

use of two models in the Real-Time Digital Simulator 

(RTDS). This functionality is shown by the connection of 

measured signals within the system, namely the PV inverter’s 

DC input voltage and current and the three-phase output 

current of the PV inverter, to functional boxes in the RTDS 

Simulation Space. These signals were processed through 

models of both the PV array model and the interconnected 

distribution system model to produce operational references 

(or set-points) that were used to control the two VVSs.

B. PV Array Emulation

Fig. 2 shows the two relative PV array power output profiles 

used to drive the PV array model shown in Fig. 1. The 

profiles were generated based on measured 1-second PV 

irradiance profiles from NREL’s Measurement and 

Instrumentation Data Center [5]. A 5-second moving average 

was applied to the data to approximate the spatial variability 

reduction of a 500 kW PV array as compared to the single 

point irradiance measurement. Additional details on the 

implementation of the PV array model realized in the RTDS 

are given in [4].

Fig. 2. Relative PV array power output profiles used during PHIL testing 

based on normalized moderate and high PV array irradiance profiles.

C. AC Grid Interconnection Emulation

A simplified RSCAD model of the Fontana, CA HighPenetration PV Study Circuit, described in [2], was 

developed for this experiment. The distribution system model 

included time varying loads and emulated controllers for the 
3 

multiple switched capacitors (for voltage regulation) located 

on the circuit. During PHIL test runs, the three-phase output 

current of the PV inverter was sampled at a high frequency. 

The currents were added to the distribution system model and 

the point of PV inverter interconnection three-phase voltage 

was determined. This three-phase voltage was used as the 

reference (with the appropriate scaling) for the AC VVS. In 

this manner, the effective impedance at the PV inverter’s 

point of interconnection as modeled in the Fontana, CA Study 

Circuit was varied as conditions on the distribution circuit 

and the available PV array power changed during the PHIL 

simulation.

(a) DC and AC power balance through the PV inverter over the 

moderate irradiance variability test case

(b) DC and AC power balance through the PV inverter over the high 

irradiance variability test case

Fig. 3. Plots of the PV Reference (power available from the emulated 

PV array), DC Power (power delivered from the DC VVS), DC Power 

Input (power input in the DC connections of the inverter under test), 

and AC Power (power output from the PV inverter under test) for both 

the moderate and highly variable PV array irradiance profiles (16minute 

/ 960 second test runs)

D. Baseline Operation of the PV Inverter without Advanced 

PV Inverter Functionality

Prior to the evaluation of advanced PV inverter functions, the 

general PHIL implementation—including both the PV array 

and distribution system interconnection emulation—was 

extensively tested to insure that the PHIL simulation was 

sufficiently stable and produced the expected operational 

characteristics. Fig. 3(a) shows the DC and AC power 

balance through the PV inverter under test. The figure shows 

the PV power reference profile, the DC input power to the PV 

inverter, the AC output power of the PV inverter, and the DC 

power output from the DC VVS for the moderate PV 

variability case. Inspection of the plot shows that the DC 

power output of the VVS tracks the PV reference as the PV 

array power varies. The DC power input to the inverter is 

slightly lower than the available power from the DC VVS due 

to the small resistive load connected in parallel with the 

inverter input for equipment protection reasons. Additionally, 

the AC output power of the PV inverter is lower than the DC 

input power to the inverter due to non-unity inverter 

efficiency. Fig. 3(b) shows a similar set of curves to those 

shown in Fig. 3(a), but corresponds to the high variability

case. The data in the high variability shows similar 

characteristics to those in the moderate case, with the 

exception that DC VSS tracking is not perfect during periods 

of very high PV array output (due to PV inverter DC input 

saturation).

III. PV INVERTER OPERATION IN POWER FACTOR CONTROL 

MODE

The PV inverter was operated in constant power factor 

control mode at four separate PF settings: 1.0, 0.95, 0.9, and 

0.85. All non-unity PF settings were inductive (absorbing 

VArs) so that the voltage rise seen at the inverter’s point of 

interconnection due to real power injection (caused by a 

reduction in the I2

R losses incurred to deliver power to loads 

at points on or near the PV inverter injecting real power) was 

counteracted by a voltage drop caused by increased losses 

due to the increased reactive power flow. The PV inverter 

controlled the PF so that it remained constant regardless of 

how much real power was injected into the modeled 

distribution circuit.

Fig. 4(a) shows a time-independent X-Y plot of the PV 

inverter’s real and reactive power salient operating points for 

the various constant PF settings for the PHIL simulation, 

which was run using the moderate PV array power variability 

profile. The straight lines of varying slope show the expected 

operation of the PV inverter under constant PF operation. The 

curved dashed line on the plot shows the rated MVA limit of 

the PV inverter tested. Fig. 4(b) presents the same 

information as Fig. 4(a), but for the high PV array power 

variability profile. Larger intermittent PF measurement 

excursions from the expected constant PF operation of the PV 

inverter are seen, particularly at higher real power operating 

points. 
4 

Figs. 5(a) and 5(b) show the same information as that plotted 

in Figs. 4(a) and 4(b), but are plotted as a function of PHIL 

simulation time. Fig 5(a) presents the PV inverter operating 

power factor for the PHIL experiment utilizing the moderatevariability PV array power profile and 5(b) presents the same 

but for the high variability PV array power profile. These 

figures also clearly show that the PV inverter had to curtail 

some production of real power during periods of high PV 

array irradiance for non-unity PF settings. This is due to the 

finite current limit and thermal management limit of the PV 

inverter being tested. VArs can be supplied to or absorbed 

from the distribution system, but at a cost of either reduced 

real power production at certain times or increased PV 

inverter power processing capability.

(a) X-Y plot of the real and reactive power during 16 minute realtime PHIL simulations for constant PF control mode set-points of 1.0, 

0.95, 0.90, and 0.85 for the moderate PV array power varaibility case

(b) X-Y plot of the real and reactive power during 16 minute real-time 

PHIL simulations for constant PF control mode set-points of 1.0, 0.95, 

0.90, and 0.85 for the high PV array power varaibility case

Fig. 4. Plots of the salient operating points (real and reactive power) of 

the PV inverter under test for moderate and high variability PV array 

power profiles with the constant PF mode enabled

(a) PV inverter real power output for PF set-points of 1.0, 0.95, 0.9, and 

0.85 for the moderate PV array power ouput variability case

(b) PV inverter real power output for PF set-points of 1.0, 0.95, 0.9, 

and 0.85 for the high PV array power ouput variability case

Fig. 5. Plots of the PV inverter real power output, showing real power 

curtailment due to the PV inverters power processing limit (VA), during 

PHIL simulation runs for moderate and high PV array power variability

IV. PV INVERTER OPERATION IN CONSTANT REACTIVE POWER 

CONTROL MODE

The PV inverter was also tested for its ability to provide a 

constant amount of reactive power support for the distribution 

grid regardless of the amount of real power the PV inverter 

was processing. This mode of operation would emulate the 

way that some distribution utilities currently maintain voltage 

regulation along their distribution circuits by using switched 

capacitors (supplying VArs to increase the voltage and reduce 

voltage losses). The PV inverter was tested under the 

condition that VArs were being absorbed at the point of the 

PV inverter interconnection. As in the constant PF operating 

test described above, VArs were absorbed at the point of the 

PV inverter interconnection to effectively lower the voltage at
5 

that point (or reduce that amount of voltage rise seen at that 

point). However, in this case the amount of reactive power 

absorbed was not a function of how much real power the PV 

inverter supplied to the distribution circuit. The amount of 

reactive power requested to be absorbed by the PV inverter 

was kept constant during the entire PHIL simulation run. 

(a) X-Y plot of the real and reactive power during 16 minute real-time 

PHIL simulations for constant reactive power control mode set-points 

of 0, 150 and 300kVAr for the moderate PV array power varaibility 

case 

(a)X-Y plot of the real and reactive power during 16 minute real-time 

PHIL simulations for constant reactive power control mode set-points 

of 0, 150 and 300kVAr for the high PV array power varaibility case

Fig. 6. Plots of the salient operating points (real and reactive power) of 

the PV inverter under test for moderate and high variability PV array 

power profiles with the constant reactive power mode enabled

Figs. 6(a) and 6(b) show the time-independent plot of the PV 

inverters salient operating real and reactive power during the 

16 minute PHIL simulation test runs with constant reactive 

power settings of 0, 150, and 300 kVAr. The straight vertical 

lines indicate that the reactive power absorbed by the PV

inverter is constant regardless of the real power operating 

level. Small deviations in the amount of reactive power 

absorbed compared to the requested amount are seen near the 

operating kVA limit of the PV inverter and are particularly 

visible for the case when 300 kVAr was requested from the 

PV inverter. Such deviations are small, and the ability of the 

PV inverter to absorb a nearly constant amount of reactive 

power during the PHIL experiment duration is confirmed by 

the collected data.

Fig. 7. Plots of the PV inverter real power output (top) and reactive 

power output (bottom) during PHIL simulation runs for moderate and 

high PV array power variability

Fig. 7 shows the real power provided to the distribution 

circuit and the reactive power absorbed from the distribution 

circuit over the period of the simulation for each constant 

reactive power set-point. Again, the real power produced is 

limited during periods of high PV array irradiance, 

particularly when 300 kVAr is constantly absorbed by the PV 

inverter. This is again due to the physical capacity constraints 

(current and thermal) of the PV inverter and is not due to the 

specific control implemented in the PV inverter. The reactive 

power plot shows that the amount of reactive power absorbed 

is nearly constant during the entire test run, providing 

evidence that such operation is feasible.

V. CONCLUSIONS

This paper presents the analysis of data collected during a 

rigorous PHIL experiment which tested the ability of a PV 

inverter to provide advanced volt/VAr based services for the 

mitigation of the distribution system level impacts of PV 

system integration. The PV inverter’s ability to operate under 

both a constant PF and constant reactive power set-point 

mode were evaluated. Furthermore, the ability of the PV 

inverter to provide these functions was tested over two 

different PV array power variability profiles (moderate and 

high variability). In all cases, the PV inverter performed these 

functions well. It was also shown that real power curtailment 

does occur due to the finite current and thermal limits of the 

PV inverter during periods of high PV array irradiance, and is 

particularly noticeable when large amounts of reactive power 
6 

(a low PF or high kVAr set-point) are requested of the PV 

inverter. The balance between the loss of captured PV energy 

and the reduction of the distribution system level impacts 

encountered by high-penetration PV integration need to be 

carefully considered, and are a topic for further research. 

VI. ACKNOWLEDGMENT

This work was supported by the U.S. Department of Energy, 

under Contract No. DOE-AC36-08-GO28308 with NREL

and the California Public Utility Commission under a 

California Solar Initiative Research, Development, 

Demonstration, and Deployment Program grant. 

VII. REFERENCES

[1] DOE SunShot Initiative High Penetration Solar Portal – NREL: 

Analysis of High Penetration Levels of Photovoltaics Into the 

Distribution Grid in California – SCE High-Penetration PV Integration 

Project, online resource: 

https://solarhighpen.energy.gov/project/nrel_analysis_of_high

_penetration_levels_of_photovoltaic_into_the_distribution_grid_in_cal

if, accessed Oct., 2012.

[2] B. Mather, B. Kroposki, R. Neal, F. Katiraei, A. Yazdani, J. R. Aguero, 

T. E. Hoff, B. L. Norris, A. Parkins, R. Seguin, C. Schauder, “Southern 

California Edison High-Penetration Photovoltaic Project – Year 1 

Report,” NREL Technical Report TP5500-50875, 2011.

[3] B. Seal, Presentation – Smart Inverter Communication Initiative – 

Project Overview and Status,: online resource: 

http://www.4thintegrationconference.com/ downloads/Inverter%20

Seal.pdf, accessed Oct. 2012.

[4] J. Langston, K. Schoder, M. Steurer, O. Faruque, J. Hauer, F. Bogdan, 

R. Bravo, B. Mather, F. Katiraei, “Power Hardware-in-the-Loop 

Testing of a 500kW Photovoltaic Array Inverter,” in proc. of IEEE Ind. 

Elect. Conf., Oct., 2012

[5] NREL Measurement and Instrumentation Data Center (MIDC), online 

resource: http://midc.nrel.gov, accessed Oct., 2012.

VIII. BIOGRAPHY

Barry A. Mather (S’2003, M’2010) received the B.S. and M.S. degrees in 

electrical engineering from the University of Wyoming, Laramie, WY, in 

2001 and 2004 respectively and the Ph.D. degree also in electrical 

engineering from the University of Colorado, Boulder, CO, in 2010. In 

March 2010 he joined the Distributed Energy Systems Integration (DESI) 

Group at the National Renewable Energy Laboratory (NREL), Golden, CO, 

where he is currently researching distributed photovoltaic energy system 

integration issues. His research interests include the analysis and mitigation 

of high-penetration PV integration into the distribution system as well as 

advanced power electronic applications in renewable energy systems.

Matthew A. Kromer received BS and MS degrees in electrical engineering 

from Brown University in 1999 and 2000, respectively, and an MS in 

Technology and Policy from the Massachusetts Institute of Technology 

(MIT) in 2007. He joined Satcon, an advanced electronics company focused 

on Grid Support and Alternative Energy, in 2011 as the R&D Program 

Manager, where he is responsible for managing Satcon’s advanced inverter 

development activities. Prior to joining Satcon, Matthew worked at TIAX, a 

clean energy technology firm, as a technical consultant and analyst. His 

research interests include integration of renewables and energy storage with 

the electric grid.

Leo Casey is Satcon’s Chief Technology Officer and a Corporate EVP. He 

has over 30 years of experience in power electronics and power engineering, 

including ultimate responsibility for the design and commercialization of 

numerous utility scale power conversion products. He serves on NREL’s 

solar advisory board, is a member of the NIST/DOE Hi-MW Leadership 

Committee (Chairman), and served as an editor of the IEEE Transactions on 

Energy Conversion for 12 years. Dr. Casey is a frequent presenter at industry 

workshops and conferences and has published over 50 papers on Power 

Conversion.