Grid Integration of Variable 

Renewable Generation: Reliability 

Challenges and Solutions

Vahan Gevorgian, NREL

ICEF 5th Annual Meeting, Tokyo, Japan

October 10, 2018

NREL/PR-5D00-72615
NREL | 2

Status of Wind and Solar PV Capacity in U.S.

U.S. DOE Wind Vision Study Scenarios

Source: U.S. DOE

U.S. Wind Power Cumulative Installed Capacity by State

Source: American Wind 

Energy Association

U.S. Solar Power Cumulative Installed Capacity by State

Top 10 Solar States

Total U.S. Solar 

Capacity: 58.3 GW

Total U.S. Wind 

Capacity: 90 GW

Source: Solar Energy Industries Association Source: Solar Energy Industries Association
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Grid Integration Challenges for Variable Generation
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Essential Reliability Services

• Active Power Control capabilities include:

• Ramp-rate-limiting controls

• Active power response to bulk power system

contingencies

• Inertial response

• Primary frequency response (PFR)

• Secondary frequency response, or participation in

automatic generation control (AGC)

• Ability to follow security-constrained economic

dispatch set points that are sent every 5 minutes

through its real-time economic dispatch market

software.

• Performance during and after disturbances

• Fault ride-through

• Short-circuit current contribution.

• Voltage, reactive, and power factor control and

regulation (both dynamic and steady state).

CAISO’s average daily regulation procurement costs from January–June 2016 

CAISO’s generation breakdown for April 24, 2016 
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Testing 300-MW PV Plant in CAISO’s Service Territory 
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Testing 300-MW PV Plant in CA

Example of 3% droop test (underfrequency)

Measured droop response

300-MW PV plant participating in AGC

Typical Regulation-Up Accuracy of CAISO Conventional Generation

Time Frame Solar PV Plant Test Results 

Sunrise 93.7%

Middle of the day 87.1%

Sunset 87.4%

Combined

Cycle

Gas

Turbine

Hydro Limited 

Energy 

Battery

Resource

Pump

Storage

Turbine

Steam 

Turbine

RegulationUp

Accuracy

46.88% 63.08% 46.67% 61.35% 45.31% 40%

Measured Regulation Accuracy by 300-MW PV Plant

Regulation accuracy by this 

PV plant is 24%–30% better 

than fast gas turbine 

technologies. 
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Frequency Response Metrics

• Initial rate if decline of frequency (ROCOF)

• Value of frequency nadir (Point C)

• A-to-C transition time

• Value of settling frequency (Point B)

• C-to-B transition time

• C/B ratio.

Interconnection Frequency Response 

Obligation

• Calculated using statistical

observations from many similar

events

• Depends on:

o Initial frequency

o First step of underfrequency load

shedding

o Contingency criteria

o Governor withdrawal adjustment

o C/B ratio

o Demand response credit.

• Western Interconnection

frequency response obligation = - 906 MW/0.1 Hz

• BAL-003-1 standard also sets

frequency response obligations

for all balancing authorities

within interconnections.
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Inertial Response by Large Wind Power Plant

Results of 65 inertial response tests by 1.5 MW wind turbine generator
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Aggregate Inertial Response
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80% Wind Penetration Study for 

U.S. Western Interconnection

Case Wind Controls Scenarios

15%

20% 

30% 

40% 

50%

80%

No inertia,

no PFR

Inertia 

only

PFR only (5% 

headroom; 

5% droop)

Inertia + PFR (5% 

headroom; 5% droop)

Studied Cases

Largest Western Interconnection 

N-1 contingency: loss of two Palo

Verde nuclear units (2.6 GW)

80% wind penetration Impact of wind controls

Combining inertial and primary frequency controls by wind results in nadir 

improvement with increasing penetration.

Source: WECC
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Reactive Power Capability

Measured P-Q Capability of 1-MW/1-MWh Li-ion BESS

Comparison of Reactive Power Capabilities
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Value Streams of Battery Energy Storage
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Impacts of Wind and Energy Storage Controls on 

Frequency Response
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NWTC Controllable Grid Platform

NWTC Wind Turbines

SunEdison

1 MW PV Array

Controllable Grid Interface (CGI)

for Grid and Fault Simulation

Switchgear 

Building

Xcel

Substation

1 MW / 1 MWh BESS

Controlled grid,

CGI Bus

Regular grid,

Xcel Bus

(7 MVA continuous / 40 MVA s.c.)

115 kV

13.2 kV tie-line

GE 1.5 MW

Siemens 2.3 MW

Alstom 3 MW

Gamesa 2 MW

DC

AC

DC

DC

DC

DC

AC

13.2 kV 

13.2 kV 

First Solar

430 kW PV array

GE 1.25 MW / 1.25 MWh BESS Aerial view of the site AC

AC

AC
NREL Software Tools for Grid Integration

NREL in-house modelling tools

REEDS – Regional Energy Deployment System model

RPM – Resource Planning Model tool

REPRA – Renewable Energy Probabilistic Resource Assessment tool 

FESTIV – Flexible Energy Scheduling Tool for Integrating Renewables

MAFRIT – Multi-area Frequency Response Integration Tool

Gap with the existing 

commercial software 

tools
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Thinking Beyond Traditional Variable Generation Plants 

Traditional Plants

Plants Integrated 

with Storage

Hybrid Renewable 

Energy Plants

Single Renewable Technology Players Hybrid Technology Players

Evolution of Variable Renewable Power Plants

High potential to disrupt the game for 

single-technology players

2020

~

~

=

Synchronous 

Condensers

FACTS Devices

Addtional

Hybridization

with Reliability

Enhancing 

Technologies

Reliability+ • Dispatchability

• Energy shifiting

• Flexibility

• Scalability

• All essential reliability services

• Advanced reliability services

• All forms of APC and RPC

• Short circuit currents

• Grid strength reinforcement

• Real and synthetic inertias

• Synchronizing torque

• Grid forming/black start

• Fast response, ultra-wide

dynamic ranges

• Resiliency, robustness

• Grid-connected and islanded

operation

Flexible, Dispatchable and Reliable Renewable Generation Plants
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Services by Multi-Technology 

(Hybrid) Plants – Dispatchable renewable plant operation

• Long-term and short-term production forecasts

• Capability to bid into day-ahead and real-time energy markets like conventional generation

• Flexibility services.

– Ramp limiting, variability smoothing, cloud-impact mitigation

– Provision of spinning and nonspinning reserves

– AGC functionality

– Primary frequency response (programmable droop control)

– Fast frequency response

– Inertial response:

• Programmable synthetic inertia for a wide range of H constants emulated by wind generation

• Selective inertial response strategies by wind turbines.

– Reactive power/voltage control

– Black start, resiliency services

– Advanced controls: power system oscillations damping, phasor measurement unit measurement-based controls,

wide-area stability services

– Stacked services

– Plant electric loss reduction, annual energy production increase

– Battery state-of-charge management

– Optimization model-predictive control strategies

– Revenue optimization for transmission- and distribution-level applications.

Hybridization Recipe
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Potential for Hybridizing Solar and 

Taller Wind Resources in U.S.

Bringing “taller” economic wind power to areas rich in solar resource 

CAISO’s typical wind and solar hourly production 

profiles; data for July 17, 2017 (source: NREL)
NREL | 19

Conclusions

• Modern inverter-coupled variable generation and energy storage systems are capable of

providing all types of reliability services to the grid.

• Adequate market design is essential for unleashing such capabilities as important tools in

achieving the broader objective of a resilient, reliable, low-carbon grid.

• Explore economic and/or contractual incentives to maximize production and not hold back

production to provide reliability services.

• Markets should incentivize faster and more accurate resources that provide such services.

Grid-friendly PV power plant 
www.nrel.gov

Thank you

vahan.gevorgian@nrel.gov

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, 

LLC, 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 and Wind 

Energy Technology 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.