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

Contract No. DE-AC36-08GO28308

Integrating High Penetrations of 

PV into Southern California: 

Year 2 Project Update

Preprint

Barry Mather

National Renewable Energy Laboratory

Russell Neal

Southern California Edison

Presented at the 2012 IEEE Photovoltaic Specialists Conference

Austin, Texas

June 3–8, 2012

Conference Paper

NREL/CP-5500-54131

August 2012
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1

Integrating High Penetrations of PV into Southern California: Year 2 

Project Update

Barry Mather1 and Russell Neal2

1

National Renewable Energy Laboratory, Golden, CO 80401, USA, 2Southern California Edison, 

Westminster, CA, 92683

Abstract — Southern California Edison (SCE) is well into a 

five-year project to install a total of 500 MW of distributed 

photovoltaic (PV) energy within its utility service territory. 

Typical installations to date are 1–3 MWpeak rooftop PV systems 

that interconnect to medium-voltage urban distribution circuits 

or larger (5 MWpeak) ground-mounted systems that connect to 

medium-voltage rural distribution circuits. Some of the PV 

system interconnections have resulted in distribution circuits that 

have a significant amount of PV generation compared to 

customer load, resulting in high-penetration PV integration 

scenarios. The National Renewable Energy Laboratory (NREL) 

and SCE have assembled a team of distribution modeling, 

resource assessment, and PV inverter technology experts in order 

to investigate a few of the high-penetration PV distribution 

circuits. Currently, the distribution circuits being studied include 

an urban circuit with a PV penetration of approximately 46% 

and a rural circuit with a PV penetration of approximately 60%. 

In both cases, power flow on the circuit reverses direction, 

compared to traditional circuit operation, during periods of high 

PV power production and low circuit loading. Research efforts 

during year two of the five-year project were focused on 

modeling the distribution system level impacts of highpenetration PV integrations, the development and installation of 

distribution circuit data acquisition equipment appropriate for 

quantifying the impacts of high-penetration PV integrations, and 

investigating high-penetration PV impact mitigation strategies. 

This paper outlines these research efforts and discusses the 

following activities in more detail: the development of a quasistatic time-series test feeder for evaluating high-penetration PV 

integration modeling tools; the advanced inverter functions being 

investigated for deployment in the project’s field demonstration

and a power hardware-in-loop test of a 500-kW PV inverter 

implementing a limited set of advanced inverter functions.

Index Terms — high-penetration PV integration, PV impact, 

distribution system, power hardware-in-loop testing, PHIL

I. INTRODUCTION

The Southern California Edison (SCE) High-Penetration PV 

(HPPV) Integration Project is a five-year study aiming to 

develop distribution planning best practices for integrating 

large commercial/utility-scale PV systems into the distribution 

system with minimal operational and cost impact to the 

distribution system operator and the PV system developer. The 

National Renewable Energy Laboratory (NREL), Southern 

California Edison (SCE), Quanta Technology, Satcon 

Technology Corporation, Electrical Distribution Design 

(EDD), and Clean Power Research (CPR) have teamed 

together on the project to analyze and quantify the impacts of 

high-penetration levels of photovoltaic (PV) resources 

interconnected with the SCE distribution system. Specifically, 

SCE will be interconnecting a total of 500 MW of 

commercial-scale PV by 2015 within their service territory 

through a program approved in 2010 by the California Public 

Utility Commission (CPUC). More details are given on the 

status of SCE’s PV deployment in Section II.

A report that covers the project’s first-year efforts and 

findings is available [1]. Following the findings of this report, 

specific tasks were planned to begin to identify and quantify 

the impacts of interconnected PV systems on the distribution 

system level. The tasks undertaken are divided into the 

following three categories:

• Modeling and simulation of distribution circuits to 

identify and quantify the impacts of high-penetration 

PV integration

• Development of advanced functionality inverters with 

functions useful in mitigating the impacts of highpenetration PV integration

• Laboratory testing of advanced functionality inverters 

to determine the effectiveness of PV impact mitigation 

strategies

Under the modeling and simulation task heading, multiple 

efforts were commenced under the project. The development 

of PV resource datasets of the appropriate temporal and spatial 

resolution are under development. Additionally, methods for 

completing high-penetration PV integration studies are being 

developed for the current state-of-the-art PV inverter 

technologies. A report, to be completed later this year, will 

investigate the usefulness of quasi-static time-series 

simulation for accurately modeling some low-bandwidth 

dynamics of the distribution system such as capacitor bank 

switching, load variability etc. Finally, a quasi-static timeseries test feeder based was developed as a testing and 

evaluation platform for high-penetration PV integration 

analysis. A report on the developed test case will be published 

at the upcoming IEEE Power and Energy Society (PES) 

General Meeting in July 2012 [2] and is described in Section 

III.

The development of advanced-functionality inverters is

ongoing. Current development efforts include refining the 

Volt/VAr characteristics that may be effective in mitigating 

some of the PV impacts on an interconnected distribution 

system. Details on this effort are briefly covered in Section IV.

Laboratory testing of a 500-kW PV inverter was undertaken 

as a joint effort between the SCE High-Penetration PV 

Integration Project, Southern California Edison, and the 

Florida State University (FSU) Center for Advanced Power 

Systems (CAPS), specifically the Sunshine State Solar Grid 
2

Initiative (SUNGRIN). The laboratory testing of the large PV 

inverter is described in Section V.

II. STATUS OF HIGH-PENETRATION PV DEPLOYMENT AT SCE

Interest in the integration of PV on the distribution grid 

continues to increase as the amount of deployed distribution 

interconnected PV within utility service territories increases. 

Predictably, when considering the first few commercial/utilityscale PV system interconnections, the locations within a utility 

service territory that afford the easiest interconnections (most 

likely causing the least impact to the local distribution system) 

are exercised first. As PV deployment continues, there is everincreasing pressure to integrate PV onto the distribution 

system at locations that are of higher impact as the locations of 

easier interconnection are exhausted. In some service 

territories and at some level of distribution system impact, a 

limit on the amount of PV that can be integrated into a local 

distribution system may be reached, effectively barring 

additional PV system interconnections in that area.

SCE has currently deployed 71 MW of the total planned 500

MW of distribution system interconnected PV within its 

service territory [3] under the Solar PV Project (SPVP). 

Additionally, SCE had originally proposed installing 250 MW 

of PV itself and purchasing the power output of an additional 

250 MW of PV from independent power producers (IPP). SCE 

requested, from the California Public Utility Commission 

(CPUC), that the utility-owned PV generation portion of the 

total 500-MW build-out be lowered to 125 MW. In this case 

the IPP share would be increased to 375 MW, leaving the 

overall PV build-out rating of 500 MW the same but with a 

different ownership model. SCE has also requested that a 

greater share of the deployed PV systems be ground-mounted 

types as opposed to roof-mounted PV systems.

Overall, the PV deployment under the SPVP is progressing 

smoothly, with the greatest challenges being management of 

the large interconnection request queue and managing the 

interconnection study process when multiple interconnections 

are requested on the same or adjacent distribution feeders. In 

these cases it becomes difficult to complete an accurate PV 

impact study of the system as the deployed PV on the system 

is largely unknown due to the fact that many PV 

interconnections are not exercised even when approved due to 

changing economic and financial circumstances of the PV 

developers. In short, the engineers completing the 

interconnection studies are trying to analyze a system whose 

basic system level attributes are in flux.

SCE’s experience with high penetration is considerable for a 

North American utility. Other utilities around the world as 

reported in [4] are also gaining experience with high

penetrations of PV in their systems.

Fig. 1. One-line diagram of the IEEE 34 node test feeder 

highlighting the actively controlled voltage regulators (orange), the 

fixed capacitors (blue), and the point of common coupling for the 

analysis presented at the end of the circuit (green).

Fig. 2. Phase A per unit voltage at the end of the circuit (node 840) 

for operation during an entire year (2010) with no PV interconnected 

onto the circuit.
3

Fig. 3. Phase A per unit voltage at the end of the circuit (node 840) 

for operation during an entire year (2010) with a 2-MW PV plant 

interconnected at the end of the circuit (node 840).

Fig. 4. Diagram showing a piecewise linear operating mode for 

VAr output based on point of common coupling voltage that may be 

implemented to mitigate the effects of high-penetration PV 

integration.

Fig. 5. Diagram showing the basic setup of the power hardware-inloop (PHIL) PV inverter test. The thyristor switches and circuit 

breaker CB1 were not used during the PHIL tests but were used for 

previously conducted steady-state testing.

III. DEVELOPMENT OF A QUASI-STATIC TIME-SERIES TEST 

CASE FOR EVALUATING THE IMPACT OF HIGH-PENETRATION 

PV INTEGRATION AND THE EFFECTIVENESS OF IMPACT 

MITIGATION MEASURES

Quasi-static time-series simulation of distribution circuits 

has traditionally been used to determine the low-frequency 

dynamic operational characteristics of a distribution circuit 

containing low bandwidth voltage control equipment such as 

switched capacitor banks, load tap changers (LTCs), and 

inline voltage regulators (VRs). These pieces of equipment 

operate at very low bandwidths compared to the time it takes

for a perturbation of the distribution system to effectively 

settle out within the distribution system. Due to this fact, 

quasi-static time-series simulation at discrete time intervals 

around the effective control bandwidth of the fastest automatic 

voltage regulation equipment is an effective way of evaluating 

the operation of a distribution circuit over time. The quasistatic time-series test feeder was developed to be used as a tool 

and platform for benchmarking PV interconnection studies 

and the resulting PV impact mitigation solutions. The test 

feeder case is based on the IEEE 34 node test feeder [5] that 

includes two inline voltage regulators. The IEEE 34 node test 

feeder is shown in Fig. 1. While the systems under 

investigation within the SCE service territory do not employ 

inline voltage regulators, many utilities do use these devices 

regularly. This is particularly true on long rural circuits where 

the impedance of the line is significant due to the length of the 

circuit. The test feeder case uses data inputs for the circuit 

loading and PV resource that are both publically available. A 

detailed paper describing the quasi-static time-series test 

feeder will be published in the proceedings of the IEEE PES

General Meeting in July 2012 [2].

Using the test feeder developed under the SCE HPPV

Integration Project, it is easy to investigate a wide range of PV 

impacts caused by large PV systems. In Fig. 1, node 840 is 

circled in green and has been used as the point of 

interconnection for much analysis related to PV impacts and 

mitigation. For instance, the impact of the point of 

interconnection voltage for a 2-MW PV system interconnected 

at the end of the circuit (node 840) can be evaluated by 

inspection of Figs. 2 and 3. Fig. 2 shows the phase A per unit 

voltage at node 840 for every minute of the test year (2010) 

for the base case, i.e. when no PV systems are interconnected 

on the circuit. Note: per unit voltages are referred to the 

primary, so all the voltages shown are above 1 p.u. to 

compensate for voltage drop in the secondary circuit. The perunit voltage magnitude is plotted by color (blue denoting 

lower voltage, red denoting higher voltage). Fig. 2 shows that 

the voltage at node 840 normally stays within a relatively tight 

range. The voltage regulators are actively operating as the load 

on the circuit shifts due to changes in load during the day. 

Fig. 3 shows the same phase A voltage at node 840, but for the 

case when a 2-MW PV system is interconnected. The 

expected voltage rise at the point of interconnection is clearly 

shown as a football-shaped region centered on the middle of 

the day and widest during the summer when sun angles result 
4

in the most PV power production. The added voltage 

fluctuations can also be seen as the color changes during a day 

(on a vertical line trajectory), sometimes showing dark red 

colors denoting high voltage and lighter yellow or blue 

voltages during the next instance.

IV. VOLT/VAR PV INVERTER CAPABILITY

As part of the project, an advanced-capability PV inverter 

will be installed in the SCE distribution system on one of the 

study feeders. In order to prepare for this field demonstration,

multiple advanced inverter functions are being developed. All 

the advanced inverter functions are designed to mitigate or 

reduce the effects of high-penetration PV integration on the 

distribution circuits. Many advanced inverter functions have 

been proposed by utilities, consultants, and developers and are 

described in [6]. In this project we are currently focusing on a 

limited set of Volt/VAr control methods that do not require 

active utility control. Such control requires robust 

communications channels as well as an active operator. 

Largely independent mitigation methods that require only set 

points from the operating utility, perhaps only seasonally or 

perhaps only when a major circuit modification occurs, require 

a less robust communications channel and do not burden the 

operating utility with making regular control decisions. Along 

with constant power factor (PF) control and constant VAr set 

point control, Fig. 4 shows one of the desired Volt/VAr 

control methods. Given the set points Q1 through Q6 and V1 

though V6, nearly any desired piecewise linear relationship 

between the point of common coupling voltage seen by the PV 

inverter and its VAr output set point can be implemented for 

Volt/VAr mitigation strategies.

Additionally, further refinements have been made to 

determine Volt/VAr modes of operation for situations when 

the PV inverter could produce more real power output but the 

VAr or PF settings limit such output due to the current rating 

of the inverter. In these cases, control modes that “fold-back” 

the PF or VAr set point are often desirable in order to provide 

as much PV impact mitigation as possible but also not 

curtail—even for short periods of time—any realizable real 

power generation. From a PV impact mitigation point of 

view, these modes may not make sense as the VArs being 

supplied are seen as the required level of VArs in order to 

mitigate the real power generation impact. Studies and tests 

have been undertaken to investigate both the “fold-back” 

modes and the “non-fold-back” modes to determine the 

differences in operation on the impacts of PV interconnection 

on the distribution system.

Fig. 6. Sixteen-minute-long PV power profiles used during PHIL 

tests. Profiles had a temporal resolution of 2 seconds.

Fig. 7. Example strip-chart like output from the PHIL tests for the 

interconnection of a 500-kW PV inverter on the Fontana, California,

study feeder operating at a power factor of 0.85 lagging.

V. POWER HARDWARE-IN-LOOP (PHIL) TESTING OF A 500-

KW PV INVERTER

The laboratory testing undertaken at the Florida State 

University (FSU) Center for Advanced Power Systems 

(CAPS) as part of the SCE HPPV Integration Project had the 

following three primary objectives:

• Demonstrate and evaluate the ability of a large PV 

inverter to implement a limited set of advanced inverter 

functions

• Analyze how effective certain advanced inverter 

functions are at mitigating the impacts of highpenetration PV integration 

• Advance the state-of-the-art practice in power 

hardware-in-loop (PHIL) PV inverter testing

All three objectives were met following a week of testing in 

March 2012. The PHIL PV inverter testing included the

complete emulation of both the DC and AC connections to a 

500-kW PV inverter. The DC PV array was emulated using a 

Real-Time Digital Simulator (RTDS) and allowed the PV 
5

inverter to operate in a fully operation maximum peak power 

tracking mode for all the tests completed. The I-V 

characteristics of the emulated PV array could be adjusted to 

effectively adjust the available DC power (peak power point) 

to the inverter DC terminals. The three-phase grid connection 

to the PV inverter was also fully emulated via RTDS and a 

three-phase variable voltage supply. Fig. 5 shows the 

connections of the PV inverter under test with the DC and AC 

variable voltage sources. The role of the RTDS is also shown. 

The RTDS was used for DC and AC connection emulation, 

data acquisition, and safety controls. This is the first test of its 

kind that includes full DC and AC grid connection emulation. 

The goal of testing in this manner was to effectively test the 

PV inverter in a real-world system. For this case the emulated 

grid to which the inverter was “connected” via PHIL testing 

was the Fontana study feeder described in [2]. A full RSCAD 

model of the feeder was implemented within the RTDS 

framework. By implementing the study feeder in the RTDS it 

was possible to collect data from all feeder nodes during the 

tests completed, allowing for the investigation of the complete 

PV impacts on the entire circuit caused by a single PV system 

interconnection.

The tests conducted consisted of sixteen-minute-long 

periods of emulated circuit operation. Three different PV 

profiles with varying variability were used during the tests. 

These profiles are shown in Fig. 6. The feeder loading profile 

for all tests was derived from measured loading data for the 

Fontana, California, study feeder.

Fig. 7 shows a graph of the testing output resulting from one 

of the sixteen-minute test runs. In this case the 500-kW 

inverter was interconnected to the Fontana, California, study 

feeder at the same location that a PV system presently exists in 

the actual circuit. The power factor (PF) set point for the test 

run shown was 0.85 inductive. Upon examination one can see 

that the real and reactive power move in concert but in 

opposite directions as the available DC power from the 

emulated PV array varies. This is the expected behavior for 

constant power factor operation. Small variations in the PV 

inverter output voltage (200 Vac,rms base) can also be seen as 

the available PV array power varies. These changes are small 

as the PV inverter is interconnected fairly close to the 

substation in this simulation so that the circuit voltage at the 

point of interconnection is fairly stiff. Nonetheless, the data 

has been collected and will be analyzed to quantify the 

impacts of high-penetration PV integration on the simulated 

circuit. The findings of the described PHIL testing will be 

compared with actual data collected on the study feeder to see 

if the PHIL PV inverter testing correctly emulated the PV 

impacts.

VI. CONCLUSION

The National Renewable Energy Laboratory (NREL), 

Southern California Edison (SCE), Quanta Technology,

Satcon Technology Corporation, Electrical Distribution

Design (EDD), and Clean Power Research (CPR) continue to 

make progress toward developing best practices for integrating 

commercial/utility-scale PV systems on the distribution 

system. A number of efforts under this project were discussed,

including the development of a quasi-static time-series test 

feeder for use in evaluating PV impact study tools and 

mitigation measures. The advanced PV inverter functions 

being investigated and implemented in real PV inverter 

hardware were briefly described, and a recent testing effort 

including the first of its kind power hardware-in-loop (PHIL) 

of a PV inverter, effectively emulating the complete 

interconnection of the PV inverter in one of the study feeder 

circuits included in the study, was discussed.

ACKNOWLEDGEMENT

The SCE High-Penetration PV Integration Project is 

supported by the U.S. Dept. of Energy (DOE) Office of 

Energy Efficiency and Renewable Energy (EERE), Solar

Energy Technologies Program (SETP), under the Analysis of 

High Penetration Levels of PV into the Distribution Grid in 

California Project, award no. DE-EE0002061, with match 

funding from the California Public Utility Commission 

(CPUC) California Solar Initiative (CSI) Research, 

Development, Demonstration and Deployment (RD&D). SCE 

also supports the project as the host utility.

REFERENCES

[1] B. Mather et al., “Southern California Edison High-Penetration 

Photovoltaic Project – Year 1,” NREL Technical Report: TP5500-50875, 2011.

[2] B. Mather, “Quasi-static time-series test feeder for PV 

integration analysis on distribution systems,” accepted to the

IEEE Power and Energy Society General Meeting, Austin, TX, 

2012.

[3] G. D. Rodriguez, “SCE Experience with PV Integration,” proc. 

of SEPA/EPRI/DOE/SNL/NREL High-Penetration PV Grid 

Integration Workshop, April 28th, 2012, available online at: 

http://www.solarelectricpower.org/events/utility-solarconference/usc-home.aspx#tabWorkshop.

[4] B. Braun et al., “Is the distribution grid ready to accept large 

scale photovoltaic deployment? – State of the art, progress and 

future prospects,” Prog. Photovolt: Res. Appl., Nov. 2011. 

[5] Distribution System Analysis Subcommittee of the IEEE Power 

Engineering Society, IEEE 34 Node Test Feeder, online 

resource: http://www.ewh.ieee.org/soc/pes/dsacom/testfeeders/

index.html.

[6] J.W. Smith, W. Sunderman, R. Dugan and B. Seal, “Smart 

inverter Volt/VAr control functions for high penetration of PV 

on distribution systems,” in proc. of the IEEE/PES Power 

Systems Conference and Exposition, 2011.