Integrating More 

Solar with Smart 

Inverters

Andy Hoke, Julieta Giraldez, Martha-Symko Davies, and 

Benjamin Kroposki

National Renewable Energy Laboratory

Earle Ifuku, Marc Asano, Reid Ueda, and Dean Arakawa

Hawaiian Electric Company

Ben Kroposki

Grand Renewable Energy 2018 

Conference

Yokohama, Japan

June 2018

A preprint of this paper can be found at:

https://www.nrel.gov/docs/fy18osti/71766.pdf
NREL | 2

• In Hawai‘i, PV is cost-effective on 

residential homes and larger 

central-station PV plants

• On some of the islands, PV has 

reached over 50% of the installed 

generation capacity base

Integrating PV in Hawai‘i

• New smart inverter functionality is 

being evaluated at significant scale 

across the islands to maintain 

stable and safe operations

• This presentation highlights 

research conducted to validate 

high PV penetration scenarios with 

smart inverters

Photo by Ken Kelly, NREL
NREL | 3

• On the most populous Hawaiian 

island of O‘ahu, the PV generating 

capacity is 502 MW. 

• This is nearly half of the annual peak 

load for the entire island of 1.1 GW. 

• Of the total PV installed, 54% is on 

private rooftops—nearly 50,000 

residences, or about one out of 

every three single-family homes. 

Integrating PV in Hawai‘i

• Hawaiian Electric, the local utility, 

does not own or control the 

residential PV systems.

• Hawai‘i has a goal of 100% 

renewable energy by 2045.

• Smart inverters are required to 

have active voltage regulation and 

frequency response to maintain 

proper grid operations.

Photo courtesy of Hawaiian Electric Companies
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The seven grid support functions that 

were evaluated included:

• Fixed power factor operation

• Volt-watt control

• Volt-var control (baseline only)

• Voltage ride-through

• Frequency ride-through

• Ramp rate control

• Soft start reconnection

Smart Inverter – Grid Support Functions

• NREL worked with HECO to 

conduct Power Hardware-inthe-Loop evaluations of 

smart inverter grid 

functionality at the Energy 

Systems Integration Facility 

(ESIF)
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• Inverters have two output parameters 

available to mitigate this: reactive 

power and active power. 

• Reducing active power can also 

mitigate overvoltage, but this directly 

reduces PV energy production and is 

therefore typically considered an 

option only when voltage is very high 

and reactive power is not solving the 

problem. 

Grid Support Function - Voltage Regulation

• Active power production from PV 

inverters tends to increase steadystate grid voltage. 

• Absorbing reactive power can 

bring down voltage with minimal 

(sometimes zero) impact on real 

power production, and hence it is 

generally preferred. 

Photo courtesy of Ben Kroposki
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Understanding Voltage Regulation

Voltage normally 

decreases away from 

the substation

DG PV raises 

voltage when 

injecting 

active power
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Grid Support Function – Voltage Regulation

Example of Smart Inverter 

Volt-Watt Response

Representation 

of PV on Circuit
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Distribution Primary Voltage Maps for Distribution Circuit with High Level of PV

Primary voltages are lower during day due to LTC settings, as seen in heat maps above, but 

secondary (customer) voltages are higher during day due to PV (triangles above)

Voltage Regulation Operational Strategies (VROS)
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Voltage Regulation Operational Strategies (VROS)

Voltage Profile for Distribution Circuit with Extremely High PV

(Legacy and Smart Inverters)
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Photo courtesy of Ben Kroposki

• This function is called frequency-watt 

control. It follows a frequency-watt 

curve to reduce/curtail real power so 

that the system frequency would be 

reduced. 

• Responding to underfrequency events 

requires the ability to increase power 

output and this may not be possible 

unless the inverter is already running 

in a curtailed mode. 

Grid Support Function – Frequency Response

• Grid frequency is an indicator of the 

balance between load and 

generation. 

• When frequency is low, more 

generation (or less load) is needed; 

and when frequency is high, less 

generation (or more load) is needed.

• Advanced inverters can reduce 

power in response to overfrequency 

events. 
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Understanding Grid Inertia and Frequency

grid AC 

waveform 

50 or 60 Hz

synchronous 

generator

Induction 

machine or 

inverter

Induction 

machine or 

inverter

synchronous 

generator

Need advanced controls 

and technologies to 

integrate wind and solar 

while maintaining grid 

stability and reliability 

Achieving a 100% Renewable Grid

http://ieeexplore.ieee.org/document/7866938/
NREL | 12

Grid Support Function - Frequency Response

Overfrequency event causes legacy inverters to trip

Underfrequency Load Sheading 

System frequency declines occurs to save system

Smart Inverters use frequency-watt control to maintain grid stability

System remains stable and does not drop any load
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Summary

• The results demonstrated that smart 

inverter functions, if properly used, can 

help maintaining grid reliability with 

significant amounts of PV without major 

impacts on customer production. 

• Impacts on energy production were 

negligible for the vast majority of 

customers, even in future PV penetrations 

cases beyond the already-high present-day 

scenario. 

• This was true even though all PV systems 

were modeled as exporting power without 

restriction, which will not be the case in 

Hawai‘i due to new DER programs that are 

designed to avoid export during high 

irradiance periods and provide operator 

controls over DER systems. 

Photo courtesy of Ben Kroposki
NREL | 14

References

A. Nelson et al., “Hawaiian Electric Advanced Inverter Grid Support Function 

Laboratory Validation and Analysis,” National Renewable Energy Lab. (NREL), 

Golden, CO (United States), NREL/TP-5D00-67485, Dec. 2016.

https://www.nrel.gov/docs/fy17osti/67485.pdf

J. Giraldez et al., “Simulation of Hawaiian Electric Companies Feeder Operations 

with Advanced Inverters and Analysis of Annual Photovoltaic Energy 

Curtailment,” National Renewable Energy Lab. (NREL), Golden, CO (United 

States), NREL/TP-5D00-68681, Jul. 2017.

https://www.nrel.gov/docs/fy17osti/68681.pdf

A. Hoke et al., “The Frequency-Watt Function: Simulation and Testing for the 

Hawaiian Electric Companies,” National Renewable Energy Laboratory, NREL/TP5D00-68884, Jul. 2017.

https://www.nrel.gov/docs/fy17osti/68884.pdf

Power Supply Improvement Plan, Books 1-4, Hawaiian Electric Companies. Dec. 

2016. 

https://www.hawaiianelectric.com/about-us/our-vision
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency 

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

This work was authored in part by Alliance for Sustainable Energy, LLC, the manager and operator of the National 

Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. 

Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy 

Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. 

Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges 

that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce 

the published form of this work, or allow others to do so, for U.S. Government purposes.

Thank You

Mahalo

www.nrel.gov/grid

Photo courtesy of Ben Kroposki

NREL/PR-5D00-71903