OAHU WIND INTEGRATION

AND TRANSMISSION STUDY:

SUMMARY REPORT

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OAHU WIND INTEGRATION

AND TRANSMISSION STUDY:

SUMMARY REPORT

—————————————————————

1 National Renewable Energy Laboratory

2 Hawaiian Electric Company

3 Hawaii Department of Business, Economic Development, and Tourism

4 Formerly with the Hawaii Natural Energy Institute

5 General Electric

6 Electric Power Systems

7 Electranix

November 2010

Dave Corbus1, Matt Schuerger1, Leon Roose2, Josh Strickler3, 

Terry Surles4, Devon Manz5, Dave Burlingame6, and Dennis Woodford7

NREL/TP-5500-48632

Cover photos: Courtesy of the Hawaiian Department of Business, Economic 

Development, and Tourism; Photovoltaic system, PIX 08879.
ACKNOWLEDGMENTS

The National Renewable Energy Laboratory (NREL) thanks the Hawaii Electric Company 

(HECO), the state of Hawaii Department of Business, Economic Development, and Tourism 

(DBEDT), the Hawaii Natural Energy Institute, and the U.S. Department of Energy (DOE) for 

supporting and sponsoring OWITS; the Technical Review Committee (TRC) for such great 

participation and input; and the study team of GE, Electranix, Electric Power Systems (EPS) 

and AWS Truepower for carrying out the work.

TECHNICAL REVIEW COMMITTEE MEMBERS

Graeme Ancell Transpower New Zealand

Dean Arakawa, Colton Ching,

Dan Giovanni, Marc Matsuura,

Dora Nakafuji, Kerstan Wong,

Robert Young, Leon Roose Hawaiian Electric Company

Mike Behnke BEW Engineering

Leslie Bryans Northern Ireland Electricity

Marcey Chang Hawaii Department of Consumer Advocate

Dave Corbus, Debbie Lew, 

Vahan Gevorgian, Matt Schuerger National Renewable Energy Laboratory

Abe Ellis, Ben Karlson Sandia National Laboratories

Jay Griffin, Terry Surles Hawaii Natural Energy Institute

Mathew McNeff Maui Electric Company

Mark McGranaghan Electric Power Research Institute

Mark O’Malley University College Dublin

Dale Osborn Midwest ISO

Steve Pullins Horizon Energy Group

Henry Shiu University of California, Davis

Charlie Smith Utility Wind Integration Group

Joshua Strickler Hawaii Department of Business, Economic 

 Development & Tourism

Rich Vandrunen State Public Utilities Commission

Bob Zavadil EnerNex

WORK TEAM MEMBERS

Devon Manz, Nick Miller, 

Sebastian Achilles, Gene Hinkle, 

Reigh Walling, Harjeet Johal General Electric

David Burlingame, Jim Cote Electric Power Systems 

Dennis Woodford, Garth Irwin Electranix

John Manobianco, Michael Brower AWS Truepower

Finally, special thanks to Bill Parks and Steve Lindenberg of DOE, Ted Peck and Ted Liu of 

DBEDT, and Leon Roose of HECO for their continued support for this work and their 

commitment to HCEI.
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EXECUTIVE SUMMARY

The Hawaii Clean Energy Initiative (HCEI), which was announced in January 2008, 

includes an aggressive mandate for the state of Hawaii to generate 40% of its energy 

from renewable resources by 2030. To reach this goal, the state will need to develop and 

utilize a diverse set of renewable energy options including solar, wind, biomass, geothermal, hydropower, and ocean technologies; the 40% goal can only be met by harnessing different renewable resources in a multi-technology approach. The HCEI agreement 

specifically identifies up to 400 MW of wind energy from Molokai and Lanai that will be 

brought to Oahu (the “Big Wind” projects) as part of this diversified portfolio of renewable energy technologies. This Big Wind element of the HCEI agreement is a significant 

part of the 40% renewable energy goal.

An important aspect of the Big Wind projects identified in the HCEI agreement is that 

they require a transmission component to carry the power generated on Molokai and 

Lanai to Oahu. The favorable wind resources on Molokai and Lanai will be used to 

generate 400 MW of wind power that is transmitted to Oahu, which has the biggest 

electrical load, by an undersea electrical cable. As a result, it is necessary to study and 

implement the Molokai and Lanai wind projects in conjunction with the undersea 

transmission cable. 

Integrating large amounts of variable renewable energy such as wind or solar into an 

electrical grid is challenging because of the variability of the power output. Previous 

“wind integration” studies have been performed over the last several years to examine 

the technical aspects of integrating large amounts of wind power into the bulk electrical 

grids in the United States and Europe. The methodologies and lessons learned from 

these studies were applied to the Oahu grid, which is even more challenging because it 

has a significantly smaller load. This study, the Oahu Wind Integration and 

Transmission Study (OWITS), was composed of several smaller studies and was sponsored jointly by the Hawaiian Electric Company, the state of Hawaii Department of 

Business, Economic Development, and Tourism (DBEDT), and the U.S. Department of 

Energy (DOE). The scope of the OWITS work includes the following:

• Identify the technical requirements and configuration for an undersea inter-island 

cable to transmit electricity from large wind plants on Molokai and Lanai to Oahu; 

• Identify the ancillary services and potential mitigation measures to offset the variable 

nature of planned wind and solar generation;

• Evaluate potential modifications to the utilities’ existing conventional generating 

units to offset the variable nature of wind and solar energy; and
4

• Changing some of the utilities operational practices and procedures, including an 

evaluation of the potential benefits of wind forecasting, required to operate the island 

grids with inter-island wind integration.

The scenarios studied in OWITS include 200 MW of wind generation on both Molokai 

and Lanai with an additional 100 MW of wind and 100 MW of solar generation on 

Oahu. Small amounts of the Molokai and Lanai wind power will be available to contribute to the small local loads on Molokai and Lanai, but the vast majority of it will be used 

for the much larger Oahu electrical load. The OWITS scenarios do not include all the 

solar energy envisioned for Oahu under the HCEI, but rather an amount that could be 

technically integrated into the study methodology within the budget and timeline of the 

OWITS project. Future PV integration studies will more specifically address integrating 

solar energy on Oahu and other Hawaiian Islands by building off the results and 

methodologies of OWITS.

In January 2009, the National Renewable Energy Laboratory (NREL) convened and 

facilitated a Technical Review Committee (TRC) for the OWITS studies. The primary 

focus of the TRC was to provide technical review of the OWITS work including methods, assumptions, and preliminary results. The TRC members were regional, national, 

and international technical experts with substantial experience in power systems, 

renewable energy, direct-current (DC) cable systems, island grids, and wind and solar 

integration. The TRC held five in-person meetings, each two days long, over the project’s 18-month period. 

OVERVIEW

The goal of the Oahu Wind Integration and Transmission Study (OWITS) is to help 

stakeholders, especially the utility and the state, to 1) understand the costs and operating impacts of significant amounts of wind power—up to 400 megawatts (MW) on 

Molokai/Lanai and 100 MW on Oahu—on their island grids, and 2) plan for future 

transmission to accommodate this wind.

The Hawaii Clean Energy Initiative (HCEI) includes the following:

• 40% renewable generation from clean energy by 2030 

(10% by 2010, 15% by 2015, and 25% by 2020),

The findings of the TRC contain many technical details that describe the complex analysis of the Big 

Wind component of the HCEI and the undersea cable, as discussed herein and in related reports. These 

studies (see references 3, 4, and 5) provide a ground-breaking analysis of high renewable penetrations in 

an island grid system. The conclusion of the TRC analysis of the Big Wind and undersea cable initiatives 

under the HCEI agreement is that bringing 400 MW of wind to Oahu with an undersea cable, although 

challenging from both an engineering and environmental permitting aspect, is technically feasible and 

should be pursued as an important part of the 40% renewable HCEI goal.
5

• Expanded distributed generation and load reduction, 

• Biofuel conversion, and 

• Up to 400 MW wind power from Molokai/Lanai to Oahu 

(the “Big Wind” projects).

A number of studies, driven by specific near-term objectives, have been done that relate 

to the Hawaiian Electric Company (HECO) Big Wind Study. The scope of 

the OWITS work includes the following:

• Identifying the technical requirements and configuration for an undersea 

inter-island cable to transmit electricity from large wind plants to Oahu; 

• Identifying the ancillary services and potential mitigation measures to offset the 

variable nature of planned wind generation;

• Evaluate potential modifications to the utilities’ existing generating units to offset the 

variable nature of wind energy; and

• Changing some of the utilities operational practices and procedures, including 

an evaluation of the potential benefits of wind forecasting, that are required to 

operate the island grids with inter-island wind integration.

The four primary projects included in the OWITS listed below are inter-related and have 

similar data needs:

1. HECO studies—A series of studies to better understand the operating characteristics 

and limitations of HECO’s generating units, the energy management system (EMS), 

the interaction between the two, and potential changes/enhancements that can be 

reasonably assumed for the forward-looking study;

2. Electric Power Systems Inc. (EPS) study—A dynamic stability modeling study of the 

existing Oahu grid;

3. General Electric (GE)/Hawaii Natural Energy Institute (HNEI)/HECO study—

Development and validation of a forward-looking modeling effort for the Oahu 

electrical grid with consideration of wind and solar integration; and

4. R.W. Beck and Electranix studies—Studies of various inter-island cable options. 

In addition, there has been analysis comparing photovoltaic (PV) and wind, as well as 

a study on financing the cable project.
6

Organization of Study and Technical Review Committee

The OWITS was sponsored jointly by the Hawaiian Electric Company, the state of 

Hawaii Department of Business, Economic Development, and Tourism (DBEDT), and 

the U.S. Department of Energy (DOE); see Figure 1.

In January 2009, the National Renewable Energy Laboratory (NREL) convened and 

facilitated a Technical Review Committee (TRC) for the OWITS studies. The primary 

focus of the TRC was technical review of the OWITS study work including methods, 

assumptions, and preliminary results.

The TRC members were regional, national, and international technical experts with 

substantial experience in power systems, renewable energy, direct-current (DC) cable 

systems, island grids, and wind integration. The TRC held five in-person meetings, each 

two days long, over the project’s 18-month period. The meetings were facilitated on a 

consensus basis and consisted of robust, active discussions of best practices and 

approaches for these technically challenging studies. 

The TRC also reviewed and provided feedback on gaps, overlaps, data needs, and 

timelines relating to the coordination of the four primary studies. Although the TRC 

reviewed the studies and made recommendations, the individual project sponsors had 

final decision-making authority for their projects.

Project

Sponsors/Managers

— HECO

— DBEDT

— DOE Steering

Committee

— DBEDT

— HECO

— DOE/NREL

— HNEI

— Other

Project Teams

— GE Energy

— EPS

— AWS Truepower

— RW Beck

— Electranix

Technical Review

Committee (TRC)

Stakeholders

Figure 1. Organization of the OWITS.
7

MAJOR TASKS

The major tasks of OWITS described below include the HCEI Big Wind scenarios, 

HECO grid background and baseline assessment, the Hawaiian Islands Transmission 

Interconnection Project, the Oahu Wind Integration Study, and additional tasks.

BIG WIND SCENARIOS FOR HCEI

Table 1 shows the initial scenario analysis matrix developed by GE and HECO to assess 

the cable and resource options. This matrix was shared with the TRC in subsequent 

discussions. Figure 2 provides a map that illustrates the wind and PV scenarios on 

Oahu, Molokai, and Lanai.

TABLE 1 – INITIAL BIG WIND SCENARIO ANALYSIS MATRIX

SCENARIO

WIND (MW) SOLAR (MW)

OAHU LANAI MOLOKAI OAHU

1. OAHU WIND 100 100

2. OFF-ISLAND WIND 100 200

3. CONCENTRATED WIND 100 400 100

4. OAHU SOLAR 100

5. HIGH RENEWABLES 100 200 200 100

Figure 2. Map of Oahu, Molokai, and Lanai showing electricity potential from wind and 

photovoltaic generation.
8

HECO Grid Background and Baseline Assessment

Table 2 provides a list of key power plants on the island of Oahu. The Oahu system has 

a daily peak of about 1200 MW and a daily minimum of about 600 MW. Total firm 

generation capacity on Oahu is 1,817 MW, with an average unit age of 43 years. 

Baseload units run 24 hours a day, seven days a week, and follow load variations. These 

generating units have annual capacity factors in the range of 50% to 60%. Cycling units 

are on during the day and off at night (scheduled a day ahead). They run about 2,000 

hours per year, with about 300 starts per year. Peaking units run in the range of 500 to 

1500 hours per year. All units are on regulation except the 46-MW H-POWER plant. 

Independent power producer (IPP) units are the most economical, so they are run 

first—HECO baseload units cycle down at night, while IPP units continue running at a 

high level. Because of the characteristics of the island power system, the reserve margin 

is about twice that of an interconnected system. The target loss of load probability is 1 

day in 4.5 years. An evaluation was done to better characterize the ramping and other 

flexibility capabilities of the HECO units.

TABLE 2. LIST OF KEY POWER PLANTS, CAPACITY AND FUEL 

RESOURCE. (SOURCE HECO FACT SHEET)

PLANT CAPACITY (MW) FUEL RESOURCE

HONOLULU 113 OIL (LSFO)

WAIAU 499 OIL (LSFO)

KAHE 651 OIL (LSFO)

CIP 120 OIL (DIESEL/BIODIESEL)

H-POWER (IPP) 46 REFUSE

KALAELOA (IPP) 208 LSFO

AES-HAWAII 180 COAL

TOTAL FIRM GENERATING CAPACITY 1,817

Generation is located primarily in the west portion of Oahu, whereas loads are largely 

in the east, which results in a primary power flow direction from west to east (see 

Figure 3). An evaluation identified a potential of 100 MW of wind resource on the island 

of Oahu, and HECO is working with the industry to investigate and reliably tap this 

on-island potential.

HECO’s study included the evaluations described below of generators, reheat-unit 

recycling, transmission infrastructure, and dynamic studies.
9

HECO GENERATOR STUDIES

HECO studied how to improve the dynamic response of their existing generators. Their 

work included developing generating units’ specific control logic to increase unit ramp 

rates, characterizing the existing governor droop response on HECO generators, tuning 

units for improved dynamic response and ensuring a uniform droop response for all 

regulating units connected to the grid. A key focus was to increase the ramp rates of the 

existing units to accommodate more variable renewable energy on the system. Testing 

was conducted for each generic design and size steam unit to prove the functional 

capability and identify equipment constraints. Control logic is being developed for each 

steam unit. These improvements to dynamic response will provide sufficient regulating 

capability to allow 500 MW of wind and 100 MW of solar PV energy to be integrated 

into the Oahu electrical system without constraining wind farm ramping and with 

minimal curtailment of the renewable resources.

Figure 3. Overivew of Oahu system showing primary electrical flow direction from the west 

side of the island, location of majority of power plants, to the east side, and primary load 

centers. 
10

Additionally, HECO contracted with KEMA to evaluate the operation and design 

configuration of HECO’s Energy Management System (EMS), including automatic 

generation control (AGC) and SCADA for higher penetrations of variable generating 

and distributed resources. Modeling and tuning of the system was completed. 

Evaluation of the AGC system identified improvements in stability and responsiveness. 

The results of the AGC tuning were inputs to the AGC models in the GE study, as was 

the dynamic response work.

Recommendations from the model study include:

• Increase the frequency of the AGC cycle from 10 seconds to 2 or 4 seconds.

• Change the load increase / decrease signal from a fixed-pulse width to set-point 

control or, alternatively, to a variable-width pulse. It is currently a 200-millisecond 

pulse width. 

• Sustain central EMS control during major frequency deviations and eliminate the use 

of the local frequency control unit at each generating unit.

HECO staff will be working with generator industry experts to evaluate the value and 

potential long term operating impacts of these recommendations on unit life, long-term 

performance and maintenance costs associated with more aggressive and “up-anddown” operations.

OPERATIONAL FLEXIBILITY STUDY

HECO completed a study of its base loaded reheat units to determine various modes of 

modified operation including daily on/off cycling, weekend on/off cycling, long-term 

on/off cycling, load cycling to lower minimum loads, and turbine full speed no load 

operation. Two of the eight existing baseload reheat units were tested for some of these 

operating modes during the course of the study. As with the other two HECO studies, 

the results of the reheat analysis provided inputs to the GE modeling study. It was 

determined that load cycling to lower minimum loads in conjunction with the faster 

ramping capability provided the most benefit. The results of these simulation studies 

need to be further investigated, but initial findings show potential gains in wind farm 

capacity factors and also show that regulating reserve improvements can contribute to 

better system frequency control that can help counter the short-term variability of wind 

and solar resources.

TRANSMISSION INFRASTRUCTURE

HECO assessed potential cable landing sites and transmission corridors. The evaluation 

included a system overview, HECO transmission assumptions, and options for cable 

landing sites and transmission corridors.
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DYNAMIC STUDIES

The EPS study evaluated the existing system and current operating practices. The work 

included evaluating transient stability issues on the existing system and contingency 

conditions on the proposed high-voltage direct-current (HVDC) cable configurations. 

Reliability criteria (e.g., allowable load shed, stability, voltage criteria) were developed 

for the HECO system with the new HVDC interconnection. The overall goal is to ensure 

that reliability is maintained.

A baseline of the current system with existing operating practices was established by 

identifying the impacts of wind generation at peak, off-peak, and shoulder periods. 

Transient stability issues were expected to be a possible critical factor as wind generation 

is increased on the existing HECO system and as faults and unit losses are introduced 

into the model. EPS focused on the current system and did not do an optimization. The 

baseline model developed by EPS served as a starting point in the GE work, with 

potential mitigation measures identified to improve the system performance.

For analysis of fault conditions, the HVDC transmission system mirrored the contribution of the wind turbine. This modeling approach was used because the fault current 

varies with each manufacturer, and it is a very conservative assumption because fault 

current is reactive and HVDC does not transmit reactive power. The EPS analysis focused 

on the HECO side of the system. The interaction between the wind turbine controls and 

HVDC controls is an important issue to be addressed in future design studies.

EPS studied peak, valley, and off-peak load cases. In many of the cases, the proposed 

improvements to the load-shedding settings were important to system reliability. The 

largest contingency studied was the loss of a 200-MW HVDC line, then the loss of the 

following generators: AES (180 MW), Kahe 5 (142 MW), and Kalaeloa CT 1 (84 MW).

The frequency plot analysis further supported the benefits of the proposed load-shedding revisions, including the addition of a kicker block. The PSS/E application was used 

to model the rate of change of angle at the buses, resulting in about a 7-cycle relay delay. 

This delay is not expected to be a problem because electrically it is a very tight system. 

Although the analysis showed that the system can be stressed further than previously 

thought, there are still limits and cost implications on the existing system. System 

performance can be significantly improved by using inertial response in the wind 

turbines. Additionally, a case was examined where the system is stable but units on 

the weak side of the system are stressed and oscillating against each other.

The power flow results from the EPS analysis are within the planning criteria. The 

alternating-current (AC) inverter bus voltage and reactive power output sensitivity 

show that although the inverter can help with some reactive power, something else 

is needed. There does not appear to be a stability constraint due to the future wind 

generation, but there could be an issue of sufficient inertia. The spinning reserve is 

adequate. Although inertial response from wind turbines has some limitations (e.g., 
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it is important not to stall the wind turbines), there can be considerable inertial response 

available from wind generators during high wind, which is when the inertial response is 

needed. Overall, the analysis incorporates many conservative assumptions and indicated no major transient stability problems on the grid. It is recommended that wind 

turbines with an option to supply inertia to the grid be considered for the project.

HAWAIIAN ISLANDS TRANSMISSION INTERCONNECTION 

PROJECT

This section describes the initial study of interconnection of transmission systems done 

by R.W. Beck, as well as the expansion of this work by Electranix.

INITIAL ASSESSMENT

In 2008, the U.S. Department of Energy contracted R.W. Beck to evaluate interconnection 

of the transmission systems of the islands of Lanai, Molokai and Maui with Oahu. This 

independent review included an initial evaluation of the technical configuration of 

establishing an undersea cable system and examining impacts to the existing electric 

transmission systems as a result of interconnecting the islands.

INTERCONNECTION STUDY

To move forward from the foundational work of the R.W. Beck report and to support the 

Hawaiian Clean Energy Agreement, DOE contracted with Electranix Corporation 

through NREL. Electranix studied the undersea cable system necessary to transmit 

400 MW of wind power from the islands of Lanai and/or Molokai to Oahu (Stage 1) and 

the future installation of a 200-MW-capacity cable system to interconnect the island of 

Maui (Stage 2).

The study included a technical feasibility assessment and budgetary cost estimate of the 

undersea transmission interconnect options. Inputs have been provided from the 

suppliers of undersea power cables and DC converter stations. Building on the R.W. 

Beck study, 18 alternative configurations were developed, and six DC cable options and 

one AC cable option were selected for further study. Only one of the selected options 

included undersea cable transmission between Lanai and Molokai, whereas all the 

alternatives from the previous study included undersea cable transmission between the 

two islands. The economic justification for undersea cable transmission between Lanai 

and Molokai could not easily be established, so the feed to Oahu from these islands was 

radial only.

The AC option was only considered viable if three-core cross-linked polyethelene 

(XLPE) cables can be used; but this approach is limited by the depth to which the cable 

can be laid because of its weight. Consequently, AC cables cannot be used for feeding 

into Oahu. However, they could be used to interconnect Molokai and Lanai—and even 

to Maui, which was investigated for one option. For long AC undersea cables, the 

three-core XLPE cable is the most viable configuration because having the three-phase 
13

conductors as close together as possible results in significant cancellation of the magnetic field generated by the phase currents. Eddy current and other induced currents in 

the shield and armor conductors of the cable are minimized, allowing low induced cable 

losses. Alternatively, three separate single-core cables will result in higher losses in the 

shields and armors because there is no cancellation of the magnetic field generated by 

the phase currents unless the three cables are lashed and laid together.

The six DC options use voltage-sourced converters (VSCs), which can create a stable AC 

supply into which wind turbine generators can connect (see Figure 4). VSCs also offer a 

significant buffer to AC system faults at one end that adversely impact the AC system at 

the other end. The wind turbine generators on Molokai and Lanai will not be affected 

significantly by severe AC system faults on Oahu.

With a suitable control strategy for the VSC transmission, it will be possible to maintain 

a steady AC voltage and frequency for the sending-end wind farms. This means that a 

direct and conventional interconnection to the small local load on Molokai or Lanai is 

possible, although the economics should be further evaluated. Only when the DC cable 

transmission is completely taken out of service would the interconnection to local load 

need to be transfer-tripped.

The level of power schedules into or out of the HECO system on Oahu is restrained to 

the maximum single-contingency outage possible—which, for this study, is 200 MW. 

Rating the converters and cables above this level is certainly possible and perhaps 

desirable, but needs to be further investigated.

From a technical perspective, the VSC cable transmission applied to these DC options is 

completely viable, with little risk of significant problems that would impede its use. The 

technology is being used in other projects of similar rating such as the BorWin Alpha 

400-MW offshore wind farm in the North Sea.

Figure 4. Schematic diagrams showing 200-MW symmetrical monopole and 400-MW bipole 

configurations.
14

Three additional scenarios were developed for study leading to Stage 2, which looked at 

the connection to Maui. Essentially, the islands of Molokai, Lanai, and Maui are interconnected by AC cables in Stage 2, and as such, must be synchronized with each other. 

In the Stage 2 scenarios, the electric interconnection from Maui is terminated at Lanai 

and/or Molokai, rather than radial at 200 MW all the way to Oahu (see Figure 5). This 

scenario avoids a long undersea cable connection all the way to Oahu and allows an AC 

interconnection between the islands of Molokai, Lanai, and Maui.

The technical challenge in implementing the Stage 2 development based on a Stage 1 

interconnection of VSC undersea cable transmission to Oahu from Molokai and Lanai is 

the following: synchronizing the wind farms on Molokai and Lanai and the power 

system on Maui together with the VSCs at Molokai and/or Lanai.

Stable synchronization is readily possible if one of the VSCs at Molokai and/or Lanai is 

operated with an independent clock that controls the frequency of the interconnected 

islands of Molokai, Lanai, and Maui. This means that the frequency of the three islands 

Figure 5. An illustrative Stage 2 scenario showing termination of electric interconnection 

from Maui at Lanai and/or Molokai, rather than radial at 200 MW all the way to Oahu. 
15

is controlled very tightly to 60 Hz. This scenario has its advantages, but suffers from lack 

of DC current control in the frequency-controlling VSC, with the possibility that it might 

be loaded to levels of undesired DC overcurrent that may exceed its rating. The frequency-controlling VSC can be a single-pole cable feeder—either a symmetrical monopole or one pole of a bipole—that acts without power control, not unlike a swing bus in 

an AC system.

To combat the possibility of DC overcurrent in the VSC that is synchronized with an 

independent clock, thyristor-controlled resistors will need to be applied near each VSC 

to absorb any power surges and provide time for transfer-tripping or power-down of 

the wind farms. This is to ensure that the VSC transmission to Oahu remaining in 

service is able to remain loaded within controlled limits. In addition, certain control 

measures can be undertaken to assist in preventing DC overcurrents.

This configuration operates satisfactorily when one of two VSC transmission poles 

or monopoles is synchronized with an independent clock and the other VSC transmission pole or monopole is synchronized with the more conventional phase-locked loop 

(PLL) operating off the AC interconnection busbar. If the VSC pole with the independent 

clock is removed from service, the other pole with the PLL control must switch immediately and automatically to independent clock operation to maintain frequency on 

Molokai, Lanai, and Maui.

The technical operation of the AC-cable-connected and synchronized Molokai, Lanai, 

and Maui systems and wind farms can work with the VSC transmission to Oahu 

if care is taken in specifying the requirements for the VSC controls. The HVDC equipment suppliers will be required to demonstrate that their control strategy will operate 

acceptably. The essential requirement of the VSC transmission control is to ensure that 

the wind turbine generators applied to the wind farms will operated satisfactorily.

OAHU WIND INTEGRATION STUDY

The key objective of GE’s Oahu Wind Integration Study is to identify operational 

strategies and new technologies that could help enable high penetrations of renewable 

energy. The study quantified and prioritized the benefits of new operational strategies 

and technologies. Many of the solutions identified in the study are already being 

implemented by HECO.

A variety of tools, shown in Figure 6, are used across several time frames, including 

models for Positive Sequence Load Flow (GE PSLF), Statistical Wind Power Variability 

Assessments, and Multi-Area Production Simulations (GE MAPS). GE’s PSLF has been 

developed and advanced to incorporate long-term dynamic simulations (AGC response 

in 2-second steps), which can be used to analyze events as they unfold over an hour. 

GE’s MAPS incorporates inter-hour renewables variability analysis (10-minute steps) 

that flags hours throughout the study year that need to be examined further in the AGC 

model, while respecting the ramp rates within each hour. The modeling was time 
16

consuming because the details of every generator unit are critically important when 

modeling a small island system such as Oahu. The system model was dispatched with 

forecasted wind generation and then run with actual wind generation—with the 

difference being the reserves requirements.

The 2014 baseline model of Oahu’s power system has a maximum hourly load of about 

1,243 MW, a minimum hourly load of about 513 MW, and annual energy of about 

7,800 gigawatt-hours (GWh). In the dynamic sub-hourly modeling in GE PSLF, all AGC 

is centrally controlled, based on the KEMA study work; there is no local frequency 

control. A conservative assumption is the change of Oahu’s quadratic frequency-dependent model to a linear dependence of real power to frequency. Overall, many of the 

changes to the model are forward looking, and most simulations were run both as the 

system is today and as it is anticipated to be in the future. HECO has been able to 

increase the ramp rate on their existing generators by a factor of about three. The Oahu 

wind plants were modeled as full-converter wind turbines.

The analysis focused on the base case and three of the scenarios: the 2014 Baseline, 

Scenario 1 (Big Wind—Oahu only), Scenario 3 (Big Wind—Oahu and Lanai only), and 

Scenario 5 (Big Wind—Oahu, Lanai, and Molokai). These three scenarios were analyzed 

Figure 6. GE’s Oahu Wind Integration Study used a variety of existing tools and developed 

new tools to assess system behavior across several time frames.
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to determine the unit commitment/dispatch, identify new operating characteristics, and 

establish a base from which to evaluate strategies to enhance operation with high 

penetrations of variable generation from renewables.

• The Scenario 5 model was developed from the 2014 Baseline model by adding 

100 MW of Oahu wind generation, 200 MW of Lanai wind generation, 200 MW 

of Molokai wind generation, and 100 MW of Oahu photovoltaic generation (see 

Table 1, Scenario 5). Cases were run first with no modification to unit commitment or 

up-reserve (5A—no wind forecast included), and then cases were run that included a 

wind power forecast (4 hour) in the unit commitment and refined the up-reserves 

(5B—based on expected wind variability). The second set of cases 

was originally run with 40 MW of effective down-reserves; 90 MW were later determined to be needed/adequate. The second set of cases (5B) resulted in significantly 

less curtailment of available annual wind energy on Molokai and Lanai (about 15%, 

compared with about 21%).

• The Scenario 1 model was developed from the 2014 Baseline model by adding 

100 MW of Oahu wind generation and 100 MW of Oahu photovoltaic generation (see 

Table 1, Scenario 1). This scenario is operated in a manner similar to scenario 5B 

(wind power forecasts in the unit commitment and refinements to the up-reserves 

based on expected wind variability). No solar forecasts are included in any of the 

cases. This scenario resulted in no wind curtailment.

• The Scenario 3 model was developed from the 2014 Baseline model by adding 

100 MW of Oahu wind generation, 400 MW of Lanai wind generation, and 100 MW 

of Oahu photovoltaic generation (see Table 1, Scenario 3). This scenario 

is also operated in a manner similar to scenario 5B, and there are similar levels 

of wind curtailment in scenarios 3B and 5B. Scenario 3B had higher sub-hourly 

variability due to the co-location of the off-island wind projects, resulting in slightly 

higher reserve requirements. The use of contingency reserves was still not required; a 

good ramp forecast will be needed to manage the reserves.

GE developed and analyzed additional mitigation strategies to enhance the integration 

of renewables. Key strategies included:

• Wind power forecasting to improve unit commitment;

• Refine the up-reserve requirements; 

• Reduce the minimum power of the baseload units; 

• Seasonally cycle off some select baseload units; 

• Reduce the reserve requirements by using fast-start units and load control; 
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• Increase the thermal unit ramp-rate capability and enhancing the droop; 

• Consider advanced wind turbine technologies such as inertia and frequency control. 

Down-regulation is included in both advanced wind turbine technologies and in 

reduced minimum power of baseload units. In addition, some aspects of storage were 

examined (e.g., for implementing ramp-rate limits on wind plants).

Scenario 5C was developed from 5B by reducing the minimum power on the HECO 

baseload units. The simulation of load-rejection events indicated that 40 MW of downreserves could put the system at risk. Scenario 5F1 was developed from 5C by increasing 

the down-regulation from about 40 MW to about 90 MW. Scenario 5F2 was developed 

from 5F1 by adding the seasonal cycling of three baseload units. An additional scenario 

was developed by adding a perfect solar forecast to 5F2. Scenario 5F3 was developed 

from 5F2 by modifying the up-reserve requirement.

An analysis and comparison was done of the overall impact of the proposed mitigation 

strategies on the delivered renewable energy. The mitigation strategies evaluated in the 

scenario analysis captured 2,001 of 2,091 GWh of total available wind and solar energy; 

the remaining 90 GWh of curtailment is mostly due to night-time minimum generation 

events.

Dynamic performance assessments validated feasible performance of the scenarios 

already analyzed and assessed technology and operational strategies that could help 

integrate wind and solar power in the sub-hourly timeframe. Additional strategies—

increased thermal ramp capability, enhanced droop, and advanced wind turbine 

technologies—were evaluated as part of the sub-hourly dynamic assessment.

Sub-hourly impacts were assessed for the following situations:

• Sustained drops in wind and solar power over an hour. The long-term (60-minute) 

analysis focused on the top-ten worst hours. Reserve requirements were adequate 

for slow sustained wind/solar drops.

• Sustained drops in wind and solar power within an hour. The targeted up-ramp 

adequacy is twice what would be just enough (i.e., twice what would be fully 

exhausted after 10 minutes). The largest drops in wind and solar power over 10-minute periods can be handled with the future improved AGC ramp rates—that is, two 

to three times the current ramp rates. Results indicate that the HECO steam units 

carry the burden of regulating system frequency as they are typically dispatched to 

lower operating loads and are positioned to respond to variable wind and solar 

resources. 

• Rises in wind and solar power when thermal units are near their minimum power. 

Wind rise events can require timely (automatic) wind plant curtailment when the 

system is constrained on down-regulation.
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• Quick variation in wind and solar power within an hour, including analysis of the 

wind and solar data, variability sharing among thermal units, frequency performance 

and power variability sharing, and impact of wind plant ramp-rate limits. The 

storage required to implement the up/down ramp-rate limits at each wind plant is 

yet to be determined; the amount of energy curtailed in the down limits is a similar 

order of magnitude as that curtailed in the up limit. Load variability was not 

included in the analysis of the 1- and 5-minute timeframes; HECO’s load has been 

relatively smooth because it is weather-based, with very few large industrial loads.

• Load-rejection events that occur when thermal units are backed down. Wind plants 

can provide fast response at a low cost for over-frequency (load-rejection) events.

• An HVDC cable trip event. Inertial response from the wind generators can significantly improve the frequency response and help to avoid under-frequency load-shedding. The cable VSC control strategy needs to include effective communication of the 

Oahu system frequency directly to the wind plants on Molokai and Lanai.

Overall, the GE study found that about 15% of wind energy available on Oahu (postcable losses) is estimated to be curtailed if today’s operating practices do not evolve to 

include wind power forecasting and refinement of the up-reserve requirements. With 

the proposed strategies, most of the available renewable energy will be accepted—about 

4% of the available energy is curtailed (see Figure 7). Wind curtailment occurred at 

night-time light-load conditions. 

The technical operational analysis found no limitations to the flexibility of either 

distributing the off-island wind generation between Molokai and Lanai or co-locating 

it on one of the two.

Figure 7. Comparison of four scenarios, showing increasing renewable energy and 

decreasing annual variable cost as operational strategies, thermal unit modifications, and 

modified reserve requirements are added to the base case.
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RELATED WORK

Related work to OWITS not included herein includes a comparison of PV and wind 

economics and an evaluation of cable finances.

1. PV/Wind Comparisons

The results indicate that it is not realistic for the proposed 400 MW of Molokai/Lanai 

wind power to be replaced with Oahu solar power, but that both an aggressive solar 

deployment on all the Hawaiian Islands and the 400 MW of Big Wind are needed to 

meet the 40% renewable by 2030 HCEI goal.

2. Cable Finances

Initial work on the economics of the Big Wind and the cable projects is under way, 

including key tasks to build a business case for Big Wind and the framework for a 

net present value model to evaluate economics. Initial financial analysis includes 

studying costs for on-island infrastructure needed to support the cable system and 

identifying key financial issues.

SUMMARY AND FUTURE WORK

The HCEI is an ambitious program to develop and implement renewable energy 

and energy efficiency for the state of Hawaii. Realizing the goals of HCEI requires 

detailed technical analysis of the electrical grid operations on the various islands and 

closely coordinated work between renewable energy analysis, transmission expansion 

studies, and analysis of the electrical grid operations by the various utilities and cooperatives in Hawaii.

The OWITS project is an important step in furthering the understanding of Big Wind 

initiatives and evaluating the different options for an undersea cable. Future work that 

builds off the results of OWITS will include the following:

• Evaluate cable financing options and determine business ownership model for cable 

and how it will be operated

• Develop detailed bid specifications that can be used to implement both undersea 

cable and wind plant projects either turnkey together or as separate bids implemented in close coordination
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• Further evaluate cable siting, environmental permitting, and EIS requirements

• Proceed with funding studies and work with Public Utilities Commission to determine impact on ratepayers

In addition to the work described above, additional renewable integration and transmission studies are being undertaken for all of the Hawaii Islands to further examine the 

path forward to meeting the HCEI goals.

REFERENCES

1. R.W. Beck, “Hawaii Inter-Island Cable Study,” Draft report to the U.S. Department of 

Energy, May 2009.

2. Hawaiian Electric Power Facts — http://www.heco.com/vcmcontent/StaticFiles/pdf/

PowerFacts_6-2010.pdf

3. Electranix, Oahu Wind Interconnection and Transmission Study (OWITS) Hawaiian 

Islands Transmission Interconnection Project, November 2010.

4. AWS Truepower, “Development of Regional Wind Resource and Wind Plant Output 

Data Sets for the Hawaiian Islands,” June 2010.

5. General Electric, Sebastian Achilles, Krishna Anaparthi, Harjeet Johal, Gene Hinkle, 

Devon Manz, Shakeer Meeran, Nicholas Miller, Wei Ren; “Oahu Wind Integration 

Study,” November 2010.

6. Hawaii Clean Energy Initiative — www. Hawaii Clean Energy Initiative.org
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National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401

303-275-3000 • www.nrel.gov

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.

NREL/TP-5500-48632 • November 2010

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 

10% post consumer waste.