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Contract No. DE-AC36-08GO28308

Evaluating Impacts of Battery 

Energy Storage System 

Functionalities on Distribution 

Systems Using Power Hardwarein-the-Loop Simulation

Preprint

Annabelle Pratt, Jing Wang, 

Kumaraguru Prabakar, Murali Baggu, 

and Martha Symko-Davies

National Renewable Energy Laboratory

Nathan Ainsworth

Independent Consultant

Tom Bialek

San Diego Gas & Electric

Presented at the IEEE Power and Energy Society General Meeting

Portland, Oregon

August 5–9, 2018

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Conference Paper

NREL/CP-5D00-70478

August 2018
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Evaluating Impacts of Battery Energy Storage System

Functionalities on Distribution Systems Using Power 

Hardware-in-the-Loop Simulation

Annabelle Pratt1

, Jing Wang1, Nathan Ainsworth, Kumaraguru Prabakar1, Murali Baggu1, Tom Bialek2 and Martha SymkoDavies1

1

Power Systems Engineering Center, National Renewable Energy Laboratory Golden, CO 80401, USA 2

San Diego Gas & Electric Company

annabelle.pratt@nrel.gov, murali.baggu@nrel.gov

Abstract—In this paper, we present results from a power 

hardware-in-the-loop (PHIL) simulation that was performed to 

test and demonstrate the impacts of battery energy storage system 

(BESS) functionalities on a distribution feeder. The PHIL 

platform includes a simulated distribution grid in a real-time

digital simulator (RTDS), AC and DC power amplifiers, and a 

battery inverter as the device under test. Guidelines on how to set 

up a PHIL platform to evaluate the impact of grid integrated 

systems are provided. Accelerated time-series PHIL simulations of 

peak shaving and volt-watt functionalities were performed using a 

540 kVA BESS. These experimental results illustrate how PHIL 

simulations can be used to evaluate the impact of BESS 

functionalities on the distribution grid prior to installation in the 

field.

Index Terms—Power hardware-in-the-loop; battery energy 

storage system; volt-watt; peak shaving.

I. INTRODUCTION 

A battery energy storage system (BESS) can provide various 

grid support services, including voltage regulation, peak 

shaving, and photovoltaic (PV) smoothing to transmission, 

distribution, and behind-the-meter users [1]. Therefore,

understanding how battery energy storage can provide value to 

the electric power system is extensively explored and 

investigated by utilities, regulators, industry, academia, etc. [2]. 

Prior to the installation of a BESS in the field, proper evaluation

must be performed to understand how the BESS can impact the 

grid. The most popular hardware testing approach is hardwarein-the-loop (HIL) testing which has several benefits, such as 

repeatability, relatively low cost, and reliable results. Power 

hardware-in-the-loop (PHIL) combines the advantages of a

complete numerical simulation method and actual hardware [3].

Most importantly, PHIL testing allows hardware devices to be 

evaluated in an environment that emulates various realistic field 

conditions.

A PHIL simulation framework for testing functionalities of 

grid-interactive inverter is presented in [4] with the focus on 

modeling grid dynamics and replicating IEEE 1547.1 

conformance test results. A grid interconnection system 

evaluator using PHIL technique to rapidly evaluate the 

interconnection standard is developed in [5] and the high-level 

architecture and test sequencing of the evaluation tool are 

illustrated. This work applies the framework proposed in [5] and 

an augmented interface algorithm to improve the stability and 

accuracy of the PHIL test is developed. Evaluating impacts of 

BESS functionalities on distribution systems using PHIL 

simulation is presented in this paper and specific guidelines on 

how to set up this PHIL platform are provided. Thisresearch was 

encouraged by San Diego Gas and Electric (SDG&E) in light of 

their need to comply with the California Public Utilities 

Commission’s requirement to meet a target of 165 MW of costeffective grid storage by year 2020 [6]. We based our evaluation 

on an SDG&E feeder in which a lithium-ion-based BESS has 

been installed. The proposed PHIL evaluation platform ensures

stable and accurate simulations that yield reliable and trusted

results. 

II. OVERVIEW OF THE DISTRIBUTION GRID WITH BESS

The simulated distribution feeder is a reduced-order 

representation of the full network. The full SDG&E 2,174-node 

feeder model was reduced to an equivalent 7-bus model using 

the procedure described in [7][8]. The resulting line impedances 

calculated for the reduced order model were all the same. The 

parameters of the positive sequence impedance are series 

resistance 0.0143 Ω, series inductive reactance 0.0188 Ω, and the 

shunt capacitive reactance 9.6 kΩ, and the parameters for the 

zero sequence impedance are: series resistance 0.082 Ω, series 

inductive reactance 0.02146 Ω, and the shunt capacitive 

reactance 9.6 kΩ. The distribution feeder was simulated in an

RTDS digital real-time simulator with a time step of 50 μs. 

A. Description of the Distribution Grid Model in RTDS

The distribution feeder depicted in Fig. 1 shows the 

configuration of the reduced-order network. It is a 12-kV feeder 

with a peak load of 7.5 MW. The loads are modeled as dynamic 

loads. Two capacitor banks, both rated at 1.2 MVar, are used to 

regulate the voltage. A 1.2-MW PV system is connected at the 

end of the feeder, and a 1-MW/3-MWh BESS is connected

upstream of the PV system.

Fig. 1. Overview of the distribution feeder with BESS integration.

The feeder head is modeled as a controllable voltage source

with a source impedance of 0.0287 + j 0.458 Ω for positive 

sequence and 0.0271 + j1.313 Ω for zero sequence. The voltage 

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

No. DE-AC36-08GO28308 with Alliance for Sustainable Energy, LLC, the 

Manager and Operator of the National Renewable Energy Laboratory.
2

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source is controlled to emulate various voltage profiles for 

studies.

The loads are modeled as dynamic power sinks, and they are 

controlled by multiplying a per-unit load profile by a fixed load. 

The fixed load is set equal to the load at each bus during the

annual peak load condition. The load profile is the per-unit (with 

annual peak load as base) power profile measured at the feeder 

head. An assumption of a fixed ratio of power between the nodes 

was made because time-series data were available only at the 

feeder head and only peak load was provided at all the buses. 

The PV system is modeled as a controlled current source. The 

input to the PV model specifies the output active power of the 

PV system, and the PV controller regulates the current to match 

the PV output active power to this input. 

The capacitor banks are modeled as capacitive loads. The 

capacitor bank controllersswitch the capacitor banks ON or OFF 

when the measured bus voltage drops below a minimum voltage 

or rises above a maximum voltage level. The controllers ensure 

that minimum on and off times are observed.

B. Description of the PHIL Model in RTDS

In this PHIL simulation, augmented ideal transformer PHIL 

interface method is implemented. Fig. 2 shows the signal level 

“PHIL model” implemented in RSCAD, the RTDS power 

system simulation software, and the power level hardware 

circuit. A controlled-current source is used to emulate the 

characteristics of the device under test (DUT) within the power 

system simulation. This “PHIL model” has four blocks: #1 PHIL 

interfacing compensation, #2 DUT control reference generation, 

#3 analog input scaling and conditioning, and #4 analog output 

scaling.

Fig. 2. Schematic diagram of implemented PHIL simulation.

The simulated voltage at the BESS point of common coupling 

(PCC), (vvPPPPPP(tt)), is scaled by a factor of kv to a simulated lowvoltage signal, v1(t); and the #1 PHIL interfacing compensation 

block generates an augmented three-phase control reference 

voltage, v3(t). The #4 analog output scaling block generates a 

scaled three-phase control reference voltage, v4(t), that is sent to 

an AC power amplifier (grid simulator) to reconstruct the power 

level signal and feed the DUT—the BESS—through a 

transformer. A controllable DC source is connected to the DC 

terminals of the BESS inverter in place of a battery. The DC 

voltage (vvbbbbbbbb) can be adjusted to simulate changes in the state 

of charge of the battery.

The output current of the BESS will be measured at the 

transformer primary side by a current transducer (CT), and this 

measured current, i2(t), will be sent back to the RTDS through

the #3 analog input scaling and conditioning block. This scaled 

measured current, i1(t), is scaled by a factor of kI

-1 to control the

current source in RSCAD, thus closing the loop between the 

power hardware and the simulated test feeder.

In the PHIL mode, this controlled current source injects a 

current proportional to the measured current at the DUT. It can 

also be set to a user controllable value when not in the PHIL 

mode. Note that the voltage at the transformer primary that is 

measured via a potential transducer (PT), v5(t), is scaled in the 

#3 analog input scaling and conditioning block to v2(t) and sent 

back to the #1 PHIL interface compensation block in the RTDS 

to improve the stability and accuracy of the PHIL simulation.

More details about how this PHIL block is implemented will be 

discussed in Section III.

III. DEVELOPMENT OF THE PHIL PLATFORM FOR TESTING 

BESS FUNCTIONALITIES

This section describes the functional and hardware 

configuration of the PHIL test setup, the test specification, and 

the development of the test setup.

A. PHIL Test Setup Functional Description

PHIL testing is typically used to test power hardware under 

conditions that cannot (cost-effectively) be generated in an allhardware test. It works by combining a real-time digital model 

of the power system (the “virtual” part) with analog-to-digital 

converters and a controllable voltage source. The controllable 

source is controlled in real time to follow the voltage at a point 

in the virtual power system. Test conditions are created in the 

virtual power system, and the power hardware under test 

interacts with it as if with a real power system under those test 

conditions.

We present a test setup that was implemented using a BESS 

inverter (the power hardware under test), a specified controller 

for the BESS, and a simulated test feeder. A real-time circuit 

model of the test feeder was loaded on an RTDS simulator, 

which solved the circuit model in real time based on time-series 

profiles for the feeder loads (PPLL,jj(tt) for all loads jj) and PV 

generation (PPPPPP,ii(tt) for all PV systems ii). 

The BESS inverter implements peak shaving and volt-watt

functionalities, and during the PHIL simulation, we observe its 

effects on the simulated test feeder. The device-level control 

(current controller, pulse-width modulation) is embedded in the 

battery inverter, and the system-level controller (control 

reference generation) is implemented in the RTDS. Based on 

the scaled AC measurements, the RTDS system-level control 

will implement real-time controls.

B. PHIL Test Specifications

The PHIL test conditions are defined by the voltage 

conditions at the feeder head and the profiles of all loads and 

PV generation on the feeder. In addition, real and reactive 

power references, PPBBBBBBBB,RRRRRR ∗ (tt) and QQBBBBBBBB,RRRRRR ∗ (tt), representing 

precalculated reference power inputs for the BESS controller 

must also be specified based on the particular feeder and test 

conditions. If the test feeder itself undergoes any changes during 

the test conditions, then the exact change and its timing must 

also be specified.

The virtual-to-physical scaling constants kk and kkII must be 

specified; these specify the relationship between the virtual 

BESS PCC bus voltage and current, respectively, in the RTDS 

feeder model to the physical power hardware in the PHIL setup. 

These constants should be selected to ensure that the expected 

range of voltage and current values at the BESS PCC result in 
3

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physical voltage and current values that are within the 

capabilities of the power hardware under test and the grid 

simulator.

Finally, the BESS control mode for the test, its parameters, 

and the DC voltage of the emulated battery (vvbbbbbbbb ) must be 

specified. The control mode and its associated parameters and 

test procedure are specified in the sections below.

1. Parameters

These data are constant parameters that are to be defined for 

each PHIL test: the RSCAD model of the test feeder, location 

of the BESS PCC, and the test time vector t. Other parameters 

that need to be defined are:

• The virtual-to-physical scaling constants, kk and kkII , for

the BESS PCC bus voltage and current 

• Specification of any changes to the feeder structure and 

time at which such change occurs

• Specification of the BESS control functionalities to be used 

and its parameters 

• The DC voltage of the emulated battery.

2. Test vectors

These time series define the particular conditions for the 

PHIL test:

• Feeder power profile time series (PPPPPP,ii(tt) for each PV 

system, ii, and PPLL,jj(tt) for each load (jj)

• Voltage magnitude at the feeder head time series (VVssssss(tt)). 

The BESS real and reactive power reference time series 

(PPBBBBBBBB, ∗ (tt) and QQBBBBBBBB

∗ (tt)), which are precalculated for the 

particular test conditions. These will be communicated to 

the test BESS inverter in real time.

C. Development of PHIL Test Setup

This section describes the design and construction of the test 

setup for the PHIL simulation of a BESS with peak shaving and 

volt-watt functionalities.

1. PHIL test setup hardware configuration

Table 1 specifies the hardware components that were used to 

implement the PHIL simulation platform shown in Fig. 2. The

540 kVA BESS is scaled to 1 MW in the RSCAD model.

Table 1. Hardware components in the PHIL platform

Functional 

Component

Manufacturer

And Product

Ratings

Controllable DC 

source

Anderson

AC2000P

0–2,000 VDC, 540 

kW

BESS inverter Schneider

Conext Core XC 

ESS

300 VAC, 540 kVA

BESS transformer Eaton 

EE540TQ83020

480-V Delta/ 300-V 

Wye, 540 kVA

Controllable AC 

source

Ametek

RS270 (x 2)

480 VAC, 540 kVA

Real-time digital 

simulator

RTDS

Technologies

6 PB5 cards, GTAI, 

GTAO

Current transducer LEM

LF 1010-S (x 6)

1,000 A

Potential transducer LEM

DVL 750 (x 3)

750 V

2. RTDS interface between RSCAD model and hardware

The RTDS interface design is important for PHIL simulation 

because the RTDS interface is used to combine the virtual 

simulation and the hardware equipment. This combination 

introduces time delays and inaccuracies, and appropriate 

compensation techniques are needed to guarantee stability as 

well as simulation accuracy. In this platform, the following 

interfaces between the RTDS and the hardware were 

implemented:

• Analog signals from PT and CT to RSCAD: This was 

accomplished by using the Gigabit Transceiver Analogue 

Input (GTAI) card in the RTDS. The RTDS measurements 

of voltages and currents were verified, and all 

measurements and controls were calibrated.

• Analog signals from the RSCAD to the grid simulator: This 

was accomplished by using the Gigabit Transceiver 

Analogue Output (GTAO) card in the RTDS. Calibration is 

also performed for the GTAO channels used.

• The grid simulator’s built-in protection is limited under 

analog control mode, and therefore extensive protection 

needed to be built into the RSCAD model to protect against 

voltage magnitude and frequency deviations in the voltage 

signal applied to the grid simulator. This was especially 

important in the early phases of the PHIL loop stability 

analysis because instabilities were encountered, and it 

allowed the RSCAD model to seamlessly transition to 

apply a safe voltage and frequency to the grid simulator.

• Active and reactive power commands from RSCAD to the 

BESS inverter. Initially, we planned to use MODBUS; 

however, the RTDS did not support MODBUS in real-time 

mode. So, the power references PPBBBBBBBB

∗ and QQBBBBBBBB∗ were sent 

to the inverter through the GTAO. According to the manual 

of the BESS, the power references are represented by 

current variables if analog input is used. So the translated 

current control reference block was implemented on the 

RTDS. Because the RTDS GTAO card has a voltage output 

and the Schneider inverter needs a current input, 

conversion from the current representation to the voltage 

representation is implemented; however, we encountered 

grounding issues upon connecting the analog control wires, 

leading to the necessary installation of isolating voltage-tocurrent converters.

3. PHIL interface compensation design 

We tested the PHIL operation with a simple feeder model 

using the equivalent impedance at the PCC for the BESS, as 

determined using the equivalent model in OpenDSS. The PHIL 

test setup was experimentally determined to exhibit instability 

for some operating conditions. This instability condition is 

common in PHIL simulations because of feedback delays, and

this necessitates the inclusion of feedback compensation in the 

PHIL loop [9]. Often a simple low-pass filter applied to the 

current and voltage feedback is sufficient, but in this case it was 

not, and it was therefore necessary to complete a detailed threephase loop stability analysis, as described in [9], to design an 

appropriate compensator.

This compensation is realized through an augmentation of the 

voltage reference that is reconstructed by the grid simulator and 

is implemented in a combination of the #1 PHIL interface 

compensation block and the #3 analog input scaling and 

conditioning block in the RTDS, as shown in Fig. 3. 

Based on the stability analysis, we discovered that the 

uncompensated system has both a negative phase margin and

gain margin and that the PHIL loop instability is closely related 

to a resonance at a specific frequency. This was caused by the 

interaction between the grid simulator’s output filter inductance,

LGS, and the BESS inverter’s output filter capacitance, Cinv. To 

avoid the instability caused by this resonance, a notch filter,

Gcomp, was used in the PHIL interface compensation.
4

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The objective of the PHIL simulation is to assess the effect of

the BESS’s functionalities that inject active and reactive power 

into the feeder, so any error introduced in the active and reactive 

power measurement and reconstructed voltage will result in 

PHIL simulation errors. To solve this problem, a low-pass filter 

on the voltage feedback, Hcomp, is added to create the same phase 

shift in voltage and in current, which eliminates the errors in 

active and reactive power. 

The grid simulator under analog control does not regulate the 

voltage at the output terminals—i.e., it does not compensate for 

the voltage drop across its output filter—so a voltage 

compensation loop, Greg, was designed and added to the RSCAD 

model, and its operation was verified. The voltage 

compensation, Gcomp, is a feedback control, which is a 

proportional resonance control implemented in the natural 

reference frame because the three-phase voltage of the grid 

simulator is controlled separately.

Combining all the elements to improve the performance and 

stability of the PHIL platform, a schematic diagram of the PHIL 

loop with the interface compensator is shown in Fig. 3. The 

representation of each element of this compensator and its 

control parameters are listed in Table 2. More details of this 

interface compensate can be found in [9]. Based on the analytical 

study of the open-loop transfer function, the compensated PHIL 

platform has a phase margin of 44.7° and a gain margin of 11.9 

dB. This guarantees the stability of the PHIL test setup. And the 

closed-loop transfer function shows that the gain is 

approximately 43.5 dB at the fundamental frequency, which 

ensures a good tracking performance for the grid simulator, thus 

achieving good accuracy and an authentic evaluation of the 

BESS functionalities’ impacts on the feeder.

Fig. 3. Schematic diagram of the PHIL loop with the interface compensaton.

Table 2. Elements of PHIL interface compensator

Element Representation Control parameters

HH cccccc ωωnn

2

ss2 + 2ξξωωnnss + ωωnn

2

ξξ = 0.707,ωωnn = 1,000

GG cccccc ss2 + ωωnn

2

(ss + ωωnn)2 ωωnn = 1

2ππNNTTTT �2LLGG CCII II

GGRRRRRR 2KK ωω ss

ss2 + 2ωω ss + ωω0

2

KK = 150, ωω = 2ππ0.5 , ωω0 =

2ππ60

IV. EXPERIMENTAL RESULTS

Experimental results were obtained using the PHIL test setup 

for peak shaving and volt-watt functionalities. See [10] for 

further explanation of the BESS’s peak shaving and volt-watt 

functionalities. We used two test input data sets—consisting of 

a per-unit load multiplier profile and a PV output power 

profile—for a 24-hour period of feeder operation at 15-minute 

resolution. The first data set was the peak load day, and it was 

used to evaluate the impact of peak shaving. The second data set 

was the minimum load day, and it was used to evaluate the

impact of the volt-watt functionality. A baseline test case was 

performed for both data sets in which the BESS is inactive. 

For the peak shaving functionality, the capacitors were set the 

same as in the field, i.e., to switch on at voltages of 0.992 p.u. 

and off at 1.025 p.u. For the volt-watt functionality, the 

capacitors are set to switch on and off at 0.98 p.u. and 1.02 p.u. 

respectively.

A. Peak Shaving Functionality

The peak day load and PV data sets were used, and the BESS 

was inactive for the baseline test case and it was operated to 

track a precalculated real power profile for the peak shaving test 

case. Fig. 4 shows the resulting feeder head power, PSrc and QSrc

for both test cases; PV system power, PPV; and BESS active 

power, PESS.

Fig. 4. Feeder power profile results for peak load day with peak shaving. 

Observe that in the peak shaving test case the BESS sinks real 

power (charging the battery) in the early morning near the 

minimum power and sources power (discharging) in the 

evening to reduce the peak. As a result, the peak source power 

is reduced compared to the baseline test case.

In addition, there was only a small difference between the 

reference BESS real power values and the measured values. 

Because the BESS inverter regulates real power at the battery 

DC bus and reactive power at its terminals (whereas the 

measurements are at the modeled medium-voltage bus), losses 

across the inverter and transformer appear as an error between

the reference and measured profiles. In addition, the loss is 

highly asymmetric between charging and discharging. When 

the BESS charges, these losses are significant, as much as 5% 

of real power and generating nearly 10% of the real power value 

as reactive loss. Although this effect does not significantly 

change the voltage profile, it might affect the amount of peak 

shaving compensation or its duration that the BESS could 

provide in practice.

B. Volt-Watt Functionality

1. Test Case B1: Minimum Load Day Baseline

In this test case, the minimum day load and PV data set were 

tested with the BESS inactive. The voltages at all seven buses 

are shown in Fig. 5. They never rise high enough or drop low 

enough to activate the capacitors. 

Fig. 5. Feeder voltage results for volt-watt baseline test case. 

2. Test Case B2: Minimum Load Day with Volt-watt

In this test case, the minimum day load and PV data sets were 

used, as for Test Case B1, but with the BESS active and 

performing volt-watt compensation with no deadband,

according to the volt-watt curve shown in Fig. 6.
5

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Fig. 6. The volt-watt curve without deadband used for test case B2. 

The test results are shown in Fig. 7 and Fig. 8. The BESS 

provides active power (PESS, the red trace shown in Fig. 7) to 

compensate the morning and evening voltage dips. Fig. 8 shows 

the impact of the volt-watt functionality on the voltage at Bus 

5, VB5, where the BESS is located. It can be seen that, despite 

the fairly steep slope of the volt-watt curve, which transitions 

from no active injection at 1.0 p.u. voltage to the full capacity 

of 1.0 MW at 0.98 p.u. voltage, the voltage is increased by only

0.00038 p.u. compared to the baseline in test case B1 (in which 

no volt-watt was active). The voltage impact is even less at the 

other buses, therefore neither capacitor bank was activated for 

the duration of the simulation. For this feeder, the BESS 

therefore provides voltage support, but it is not of a sufficiently 

high rating to significantly impact the voltages.

Fig. 7. Feeder power profile results for minimum load day with volt-watt.

Fig. 8. Comparison of voltage at the BESS PCC with and without volt-watt.

V. CONCLUSIONS

This paper presents results from PHIL simulations that 

evaluated and demonstrated the impact of battery energy 

storage system functionalities on a distribution feeder. It 

describes modeling the components in the distribution feeder 

(e.g., capacitor banks) and the PHIL simulation platform. In 

particular, the design of the PHIL interface compensation 

algorithm to ensure stable and accurate evaluation of the DUT

is discussed in detail. Moreover, guidelines on how to set up a 

PHIL platform to evaluate the impact of grid integrated systems 

are provided.

Two functionalities—peak shaving and volt-watt—are 

simulated and the experimental results are presented. These 

results demonstrate how a PHIL simulation platform can 

provide a valuable evaluation prior to field installation. 

Information from the PHIL simulations can be used by utility 

operators to determine how to operate their utility assets most 

effectively under different scenarios in the field.

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