NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

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

Contract No. DE-AC36-08GO28308

Phase 2 Report: Oahu Wind 

Integration and Transmission 

Study (OWITS)

Hawaiian Islands Transmission 

Interconnection Project 

Dennis Woodford

Electranix Corporation

Winnipeg, Manitoba Canada

Subcontract Report

NREL/SR-5500-50414

February 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

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

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Phase 2 Report: Oahu Wind 

Integration and Transmission 

Study (OWITS)

Hawaiian Islands Transmission 

Interconnection Project 

Dennis Woodford

Electranix Corporation

Winnipeg, Manitoba Canada

NREL Technical Monitor: David Corbus

Prepared under Subcontract No. LAM-9-99436-01

Subcontract Report

NREL/SR-5500-50414

February 2011
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL.

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- 1 - 

Executive Summary

The Stage 1 report for the Oahu Wind Integration and Transmission Study (OWITS) has been 

prepared for finalization. At the Technical Review Committee (TRC) meeting held in Honolulu 

January 6th and 7th, 2010, three scenarios were developed for future Study leading to Stage 2, the 

connection to Maui. These three scenarios are the end result of a productive evolution of ideas 

and study of electric power interconnection options beginning with the R.W. Beck report [1] 

through to developing drafts of the NREL report [2]. 

This present study examines the technical challenges in implementing the Stage 2 Scenarios.

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. The major development of the Stage 2 

Scenarios is the termination of the electric interconnection from Maui at Lanai and/or Molokai 

rather than radial at 200 MW all the way to Oahu. This 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 in the 

synchronization of the wind farms on Molokai and Lanai and the power system on Maui together 

with the voltage sourced converters (VSC) at Molokai and/or Lanai. 

Stable synchronization is readily possible ifone of the VSC converters 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 the frequency of the three islands are controlled very 

tightly to 60 Hz. This has its advantages, but suffers from lack of DC current control in the 

frequency controlling VSC converter with the possibility that it might be loaded to levels of 

undesired DC over-current that might exceed its rating. The frequency controlling VSC 

converter can be a single pole cable feeder (either symmetrical monopole or one pole of a bipole) 

that acts without power control, not unlike a swing bus in an AC system. 
- 2 - 

To combat the possibility for DC over-current in the VSC converter that is synchronized with an

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

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

farms. This is to ensure the VSC transmission to Oahu that remains in service is able to remain 

loaded within controlled limits. In addition, certain control measures can be undertaken to assist 

in the prevention of DC over-currents.

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, this operates 

satisfactorily. If the VSC pole with the independent clock is removed from service, the other 

pole with the PLL control must immediately and automatically switch 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 providing care is taken in 

specifying the requirements for the VSC converter controls. The HVDC equipment suppliers will 

be required to demonstrate their control strategy will operate acceptably. The essential 

requirement of the VSC transmission control is to ensure the wind turbine generators that are 

applied to the wind farms will operated satisfactorily. 

Acknowledgements

Appreciation is expressed for the support of Dave Corbus of NREL along with the team who met 

regularly to provide suggestions and directions. This includes Leon Roose, Marc Matsuura, Dora 

Nakafuji, Dave Burlingame, Dean Arakawa and Matt Schuerger. Appreciation is also expressed 

for the support from Josh Strickler of the Department of Business, Economic Development and 

Tourism. The concepts of the three scenarios which form the basis of this Phase 2 report were 

generated by the Technical Review Committee at their meeting in Honolulu January 6th and 7th, 

2010. Garth Irwin of Electranix contributed important ideas towards the control of the Stage 2 

system.
- 3 - 

Table of Contents

EXECUTIVE SUMMARY .............................................................................................................................. - 1 -

Acknowledgements.................................................................................................................................... - 2 -

TABLE OF CONTENTS ................................................................................................................................. - 3 -

REVIEW OF STAGE 1 ................................................................................................................................... - 4 -

OBJECTIVE OF STAGE 2 ............................................................................................................................. - 6 -

DESCRIPTION OF STAGE 2 ........................................................................................................................ - 7 -

SCENARIO 1: DC CABLE OAHU TO MOLOKAI, AC CABLE MOLOKAI TO LANAI.............................................. - 7 -

VARIATIONS IN WIND FARM PERFORMANCE.................................................................................................. - 9 -

SCENARIO 2: A 200-MW MONOPOLE TO IWILEI FROM MOLOKAI AND A 200-MW MONOPOLE 

TO IWILEI FROM LANAI. 69-KV AC CABLES MOLOKAI TO LANAI, LANAI TO MAUI AND 

MOLOKAI TO MAUI................................................................................................................................... - 10 -

OPERATION WITH SYNCHRONIZATION BY INDEPENDENT CLOCK .................................................................. - 11 -

OPERATION WITH SYNCHRONIZATION BY PHASE LOCKED LOOP.................................................................. - 11 -

OPERATION WITH ONE MONOPOLE ON SYNCHRONIZED WITH AN INDEPENDENT CLOCK AND THE OTHER MONOPOLE 

WITH A PLL .................................................................................................................................................. - 12 -

SCENARIO 3: 400-MW DC BIPOLE OAHU TO MOLOKAI, 138-KV AC 3-CORE AC XLPE-CABLE 

MOLOKAI TO LANAI, A 3-CORE 69-KV AC XLPE CABLE EACH TO LANAI TO MAUI, MOLOKAI 

TO MAUI........................................................................................................................................................ - 13 -

CONTROL REQUIREMENTS .................................................................................................................... - 14 -

CONCLUSIONS............................................................................................................................................. - 15 -

REFERENCES ............................................................................................................................................... - 16 -

APPENDIX ..................................................................................................................................................... - 17 -

SIMULATION STUDIES FOR SCENARIOS 1, 2 AND 3 INTER-ISLAND DC CABLE OPTIONS ............................... - 17 -

Introduction............................................................................................................................................ - 17 -

SIMULATION STUDIES FOR SCENARIO 2 INTER-ISLAND DC CABLE OPTIONS WITH MAUI CONNECTION ...... - 25 -

Introduction............................................................................................................................................ - 25 -
- 4 - 

Review of Stage 1

In October 2008, the State of Hawaii and the Hawaiian Electric Companies entered into the 

Hawaii Clean Energy Agreement to move Hawaii off of its dependence on imported fossil fuels 

for electricity and transportation. The U.S. Department of Energy contracted R.W. Beck in 2008 

to evaluate interconnection of the transmission systems of the islands of Lanai, Molokai and 

Maui with Oahu. Phase I of this independent review included an initial evaluation of the 

technical configuration and capital costs of establishing an undersea cable system and examining 

impacts to the existing electric transmission systems as a result of interconnecting the islands. To 

move forward from the foundational work of the R.W. Beck report [1] and in support of the 

Hawaiian Clean Energy Agreement, the Department of Energy through the National Renewable 

Energy Laboratory contracted Electranix Corporation to study the undersea cable system 

necessary to transmit 400 MW of wind energy 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 effort included a technical feasibility assessment and budgetary cost estimate of the 

undersea transmission interconnect options. Inputs from the suppliers of undersea power cables 

and direct current (DC) converter station has been provided. Six DC cable options and one AC 

cable option were selected from 18 alternative configurations derived from a previous study [1]. 

Only one of the selected options included undersea cable transmission between Lanai and 

Molokai whereas all the alternatives from this 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 XLPE cable can be applied, but which is 

limited by the depth to which it can be laid because of its weight. Consequently it is precluded 

for feeding into Oahu. However, it could be used to interconnect Molokai and Lanai, and which 

was investigated for one option. 
- 5 - 

The technology for the six DC options is voltage sourced converters (VSC). VSC converters 

create a stable AC supply for wind turbine generators to connect into. They also offer a 

significant buffer to AC system faults at one end adversely impacting 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 may be possible. It is 

only when the DC cable transmission is completely taken out of service that the interconnection 

to local load would have to be transfer tripped. 

The capital costs for the six DC cable transmission options are assembled from the responses of 

the cable and equipment suppliers. The stated accuracy of these costs is ±20%, and so the 

estimated values are the lowest provided by the suppliers plus 20%. Case A1-2(a) has a 200- 

MW DC connection between Lanai and Molokai and Case A1-2(b) has a 200-MW AC 

connection between Molokai and Lanai, which is slightly lower cost. Analysis (Appendix A) 

indicates additional justification will be needed to consider a Molokai to Lanai interconnection.

An independent option was proposed by one of the equipment suppliers whose significant 

benefit is the termination of the Stage 2 transmission from Maui at Lanai or Molokai instead of 

at Oahu as in the selected options in Table 1. Capital costs are therefore lower because less 

undersea cable is required, but suffers the disadvantage that power scheduled to Oahu from Maui 

would only be at the level possible when the wind power being generated on Lanai (and/or 

Molokai) is below full rating. Power schedules from Oahu to Maui on the other hand would not 

be restricted below the capacity limits of the converters and undersea cable transmission since 

any wind power being generated on Lanai and/or Molokai would be displaced accordingly. 
- 6 - 

Table 1: Budgetary capital costs in 2009 millions of dollars as provided by suppliers of DC cable 

and DC converter equipment for the DC options. 

Option C3-2 A3-2 A1-2 B3-2 C1-2 B1-2

Description 400 MW 

Koolau 

to 

Molokai 

200 MW 

Molokai to 

Koolau, 200

MW Molokai 

to Iwilei

200 MW 

Molokai to 

Koolau, 200

MW Lanai to 

Iwilei, 200MW 

Lanai to Molokai

400 MW 

Molokai 

to Iwilei 

200 MW 

Molokai to 

Koolau, 200 

MW Lanai to 

Koolau

200 MW 

Molokai to 

Iwilei, 200 

MW Lanai to 

Iwilei

(a) (b)

Stations $M 234 288 414 342 234 288 288

Cables $M 154 180 367 424 221 216 245

Total Stage 1 

Price $M

388 468 781 766 455 504 533

Stage 2 Maui 

to Oahu 

(Approx)

Converter 

Stations $M

144 144 117 117 144 144 144

DC Cables 

$M

420 420 192 192 283 272 272

Total Stage 2 

Price $M

564 564 309 309 427 416 416

Total Stages 

1 & 2 $M

951 1,032 1,090 1,077 882 920 949

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

maximum single contingency outage possible, which for this study is 200 MW [2]. Rating the 

converters and cables above this level requires further investigation.

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 to other similar projects of similar rating such as the BorWin Alpha 

400-MW offshore wind farm in the North Sea. 

Objective of Stage 2

At the Technical Review Committee (TRC) meeting held in Honolulu January 6th and 7th, 2010, 

the following three scenarios were developed for future Study leading to Stage 2, the connection 

to Maui. 
- 7 - 

The objective of the Stage 2 study is to assess the suitability of the three Scenarios developed by 

the TRC for expanding the Stage 1 development for interconnection to Maui. The TRC agreed 

that the work should remain focused on 400 MW of wind but emphasized that the study work 

should articulate that the designs that are modular and have flexibility for expansion. 

Description of Stage 2

A significant factor in the Stage 2 development is the application of AC cable between Molokai, 

Lanai, and Maui, with the undersea cables from Molokai and Lanai to Oahu as HVDC cables 

with voltage sourced converters (VSC). The three Scenarios developed are presented as follows: 

Scenario 1: DC Cable Oahu to Molokai, AC cable Molokai to Lanai

400 MW DC Oahu to Molokai 

 230 kV AC Molokai to Lanai

Figure 1: Scenario 1 where the bipole cables can terminate at Koolau or Iwilei on Oahu 

Scenario 1 is developed out of the Stage 1 report and does not include any interconnection to 

Maui. The AC cable between Lanai and Molokai is a three-core, XLPE cable, with a 630 mm2

copper conductor on each of the three phases as shown in Figure 2. Its weight is 78 kg/m and its 

diameter is 215 mm (8.5”). This cable has over 200-MW capacity, but it must be 100% 

compensated with fixed shunt reactors at each end. This is a very heavy cable and as a result can 
- 8 - 

only be laid in relatively shallow water. This precludes its use to Oahu because of the seabed 

depth is too great for its use. However, it can be applied between Molokai, Lanai and Maui, 

saving the cost of the VSC converters. 

Figure 2: Three-core XLPE undersea cable (courtesy of ABB). 

The 100% compensation of the AC cable means that there is minimum reactive power swing 

since cable charging is fully compensated. It is therefore only the relatively low series impedance 

of the cable that causes reactive power variations with load. With the AC cable energized only 

from one end, the 100% compensation also allows for use of pre-insertion resistance using a 

resistor and two circuit breakers so that the energization is virtually voltage transient free on the 

interconnection busbar. This alleviates the need for an SVC or STATCOM at this busbar. 
- 9 - 

Another issue that requires managing is harmonic resonances on the cable. The application of an 

SVC or STATCOM will not solve these issues and so have little impact on the recommendation 

to not require them. Further work is needed in this area. 

The results for studies with Scenario 1 are summarized in Appendix 2A. With the independent 

clock synchronizing control on the Molokai converter, there is suitable operational performance 

so long as the thyristor-controlled resistors are operational to protect the poles from current 

overload while the nearby wind farm is cut back to the capacity level of the remaining DC 

transmission if one pole is removed from service. Over-current protection is also available from 

the VSC controls which act in addition to the thyristor-controlled resistor. 

In addition, the overvoltage protection for the dc cable must be protected with a DC chopper at 

the Oahu (receiving) end of the VSC cable transmission as shown in Figure 3. 

Figure 3: Application of a DC chopper on each pole of the VSC transmission to protect the cable 

against overvoltage. 

Variations in Wind Farm Performance

Generic but detailed Type 3 (DFIG) and Type 4 (Full Converter) wind farm models are 

represented in the PSCAD models. Each demonstrates different dynamic performance at the low 

short circuit capacity operating conditions present in these cases. 

Circuit breaker opens 

to clear fault

Sending End DC Chopper

Inverter Blocks

Fault

Receiving End

Circuit breaker opens on 

transfer trip from 

receiving end breaker
- 10 - 

Scenario 2: A 200-MW Monopole to Iwilei from Molokai and a 

200-MW Monopole to Iwilei from Lanai. 69-kV AC Cables

Molokai to Lanai, Lanai to Maui and Molokai to Maui

With this scenario the connection is made for the Stage 2 interconnection to Maui as shown in 

Figure 4. 

2 x 200 MW DC Oahu to Molokai; Oahu to Lanai 

 3 x 69 kV AC Molokai to Lanai; Lanai to Maui; Molokai to Maui 

Figure 4: Scenario 2 interconnection of Phase 2 to Maui from Lanai and Molokai 

69-kV AC, three-core XLPE cables are applied between the three islands of Lanai, Molokai and 

Maui as shown in Figure 4. In this instance the longest connection is made to avoid reefs but also 

to investigate a more severe condition to observe performance. The cables are 100% shunt 

compensated with fixed shunt reactors at each termination. 1000 mm2

 copper conductors are 

selected with an 800-A capacity.
- 11 - 

The connection to Maui is made to 69-kV Bus 84 Lahainaluna. The Maui power system was 

translated from PSS/E to PSCAD with all dynamic generators represented. 

The detailed simulation studies for the key Scenario cases are in the Appendix.

Operation with Synchronization by independent clock

These studies were originally undertaken with both monopoles at their sending end converters 

synchronized with an independent clock, connected in phase with each other. Under such 

controlled conditions, recovery from the faults was effective, the ac frequency on Maui was 

stable, and all systems stayed synchronized. 

Operation with Synchronization by Phase Locked Loop

An attempt was made to synchronize the controls of both VSC monopoles with individual phase 

locked loops (PLL). This was not possible as the power system on Lanai is not stiff enough to 

hold the conventional PLLs in steady operation. 

In an attempt to ensure the PLL was as robust as possible, two units in series were applied. The 

resultant synchronizing signal was of good quality, but lacked the phase angle stabilization that 

is usually provided by inertia to support the Molokai monopole in an acceptable level of 

performance. This is shown in Figure 5: 

Figure 5: Double PLL for synchronizing the VSC monopole controls 

Va

Vb

Vc

PLL

Six

Pulse 6

thetaY

1

2

3

VRec

VaPLL

1

3

5

Sin

Sin

Sin

Va

Vb

Vc

PLL

Six

Pulse 6

thetaY
- 12 - 

Evidence that the double PLL operated effectively against an extremely poor ac wave-shape is 

shown in Figure 6. A PLL generates a ramp signal that is synchronized to the 60 Hz wave-shape 

of the busbar the VSC converter is connected to. This signal ramps from 0 degrees to 360 

degrees once per cycle as shown in Figure 6. From this ramp signal, the controls are locked onto 

to ensure the valves fire in correct sequence to the ac wave-shape. Even though the phase voltage 

is going through a severe distortion, the two PLLs generate a stable ramp, indicating its 

robustness. 

Figure 6: Functional performance of the PLLs on the Molokai monopole where the output of the 

second stage PLL is plotted as acceptable 60-Hz ramp signals.

Operation with One Monopole on Synchronized with an Independent 

Clock and the other Monopole with a PLL

Having one monopole synchronized with an independent clock and the other with a PLL is 

possible although the performance is not as robust as when both monopoles operated from phase 

synchronized independent clocks. The cases with this hybrid control configuration are tested in 

the Appendix as Scenarios 2b-1, 2b-2, 2b-3, 2b-4, 2b-5, 2b-6 and 2b-7. 
- 13 - 

Scenario 3: 400-MW DC Bipole Oahu to Molokai, 138-kV AC 3-

Core AC XLPE-Cable Molokai to Lanai, a 3-Core 69-kV AC 

XLPE Cable each to Lanai to Maui, Molokai to Maui

400 MW DC Oahu to Molokai 

 138 kV AC Molokai to Lanai

 2 x 69 kV AC Lanai to Maui; Molokai to Maui 

Figure 7: Scenario 3 interconnection of Stage 2 to Maui from Lanai and Molokai 

This Scenario 3 operates technically as does Scenario 2 with the same challenges in 

synchronization. It functions satisfactorily if the VSC converter on Molokai is synchronized with 

an independent clock that can support the frequency of the islands of Molokai, Lanai and Maui. 

Scenario 3 is unable to function with synchronization through a PLL depending on the strength 

of the Maui system to solidly connect onto. 
- 14 - 

Control Requirements

A control requirement for both Scenario 2 and Scenario 3 is the synchronization. The method 

that is proposed is as follows: 

1. Of the two VSC cable feeder poles, one pole must be on an independent clock. This 

provides the frequency control for the interconnected islands of Molokai, Lanai and 

Maui. This applies if it is configured with symmetrical monopoles as in Scenario 2. If 

configured as a bipole as in Scenario 3, both poles could be operated as an independent 

clock providing they are derived from the same clock. 

2. The other VSC cable feeder pole is synchronized through a conventional phase locked 

loop, which it is able to do through the ac voltage at its terminal connecting busbar at its 

sending end. The frequency is defined by the other VSC cable feeder with the 

independent clock. 

3. The VSC feeder with the PLL control has ability to control power. As such it can be 

automatically scheduled to transmit power equal to the power on the other VSC feeder 

with the independent clock. In this way they two VSC feeders will equally control the 

power to Oahu. Alternatively, the power order to the VSC feeder with the PLL controls 

can be derived from the power from the wind farms and Maui in such a way that losses 

are minimized and the VSC feeder with the independent clock does not overload. 

4. If the VSC feeder pole with an independent clock is faulted and removed from service, 

then a control mechanism must be in place to ensure the other pole if operating with a 

phase locked loop, immediately switches to an independent clock. This is not a trivial 

control action and must be implemented with care and reliability.

When this control strategy is implemented, the power systems and wind farms on Molokai, Lanai 

and Maui will be synchronized to each other but not to Oahu. The pole with the independent 

clock is essentially equivalent to a “swing bus”, which in power system terms is able to absorb 

and generate the power that the AC system requires to maintain its frequency. 
- 15 - 

There is a possibility that the frequency that is maintained by the pole with the independent clock 

can be adjusted as needed to best accommodate the AC power system, either on the islands of 

Molokai, Lanai, and Maui, or on Oahu. Further work is required to define the frequency control 

system so that this pertinent information can be included in a technical specification for the Stage 

1 feeders. This is to ensure these feeders will be constructed to accommodate the controls 

needed for the Stage 2 synchronization. 

Conclusions 

From this study, the following conclusions can be drawn: 

1. The application of AC three-core XLPE cables to interconnect the islands of Molokai, 

Lanai and Maui is a suitable construction method to apply. However, some more 

engineering is required on the impact of AC harmonic impedances. 

2. With VSC converters at Molokai and Lanai at around 200 MW, they can connect to the 

three islands including Maui with synchronization achieved with independent clocks. 

These independent clocks must be phased with respect to each other in order to achieve 

acceptable power exchanges on the AC cables between Molokai, Lanai and Maui.

 

3. The more conventional method of synchronizing the converter stations using phase 

locked loops will not work with the wind farms and the small (200 MW) power system of 

Maui. However, if one pole or monopole at Lanai or Molokai is synchronized with an 

independent clock, and the other pole or monopole is synchronized with the more 

conventional phase locked loop technology, then the Stage 2 system can be made to 

operate satisfactorily. 

4. Further study will result in a compromised and acceptable means of synchronizing the 

voltage sourced converters at Molokai and/or Lanai to ensure best operational 

performance of the wind farms on Molokai and Lanai, and with the Maui system. In 

particular additional work is required to ensure the synchronization and frequency control 

strategy is clearly defined for a technical specification for the Stage 1 VSC feeders to 

Oahu. 
- 16 - 

References

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

May 15, 2009. 

2. NREL Report: “Oahu Wind Integration and Transmission Study (OWITS) Hawaiian 

Islands Transmission Interconnection Project”, NREL Subcontract No. LAM-9-99436-

01, by Dennis Woodford, Electranix Corporation, May 8, 2010. 
- 17 - 

APPENDIX 

Simulation Studies for Scenarios 1, 2 and 3 Inter-Island DC Cable 

Options 

Introduction

The detailed PSCAD model of the inter-island DC cable options was developed for testing the 

technical performance of the options proposed for incorporating the future connection to Maui. 

At the Technical Review Committee (TRC) meeting held in Honolulu January 6th and 7th, 2010, 

the following three scenarios were developed:

Additional cases will be run (each with 200 MW of wind on Molokai and 200 MW of wind on 

Lanai) which include an AC loop (69 kV or 138 kV) connecting Maui to Molokai and Lanai: 

1. 400 MW DC Oahu to Molokai 

 230 kV AC Molokai to Lanai

2. 2 x 200 MW DC Oahu to Molokai; Oahu to Lanai 

 3 x 69 kV AC Molokai to Lanai; Lanai to Maui; Molokai to Maui 

3. 400 MW DC Oahu to Molokai 

 138 kV AC Molokai to Lanai

 2 x 69 kV AC Lanai to Maui; Molokai to Maui 

The TRC supported the evaluation of these three cases as the next steps for the HVDC study 

work. The TRC agreed that the work should remain focused on 400 MW of wind but 

emphasized that the study work should articulate that the designs that are modular and have 

flexibility for expansion. 
- 18 - 

The TRC requested that the various studies be integrated into a single life cycle cost benefit 

analysis for the cable project.

Scenario 1 – Base Case

The requirement for 400-MW HVDC undersea cable transmission from Molokai to Oahu was 

achieved with a bipole configuration into Koolau as shown in Figure A-1. 

Figure A-1: Scenario 1 configuration

The controls of the HVDC converter at Oahu end include a DC crowbar chopper to protect the 

DC cable from overvoltage. 

Figure A-2: DC crowbar chopper for DC cable overvoltage protection 

Circuit breaker opens 

to clear fault

Sending End DC Chopper

Inverter Blocks

Fault

Receiving End

Circuit breaker opens on 

transfer trip from 

receiving end breaker

2x200 MW DC Bipole

40 mi

200 MW 

Molokai

400 MW 

Koolau

200 MW 

Lanai Maui

Scenario 1 

Base Case

50 mi

40 mi

230 kV 

AC Cable

200 MW 

Molokai

200 MW 

200 MW

Koolau 400 MW Bipole – Stage 1

230 kV AC 

Cable

200 MW 

Lanai
- 19 - 

List of Scenario 1 Cases:

1: 3 phase fault, 200 msec at Iwilei on Oahu. Both symmetrical monopoles remain in service.

2: 3 phase fault, 200 msec at Iwilei on Oahu. Lanai to Oahu symmetrical monopole removed 

from service.

3: 3 phase fault, 200 msec at Iwilei on Oahu. Molokai to Oahu symmetrical monopole removed 

from service.

4: 3 phase fault at Molokai for 200 msec. The Lanai to Molokai 69-kV cable removed from 

service.

5: 3 phase fault at Lanai on the 69-kV AC cable between Lanai and Maui, which is cleared and 

removed from service.

6: 3 phase fault at Molokai on the 69-kV AC cable between Molokai and Maui, which is cleared 

and removed from service.

7: 3 phase fault at Bus 971 in the 33-kV AC system on Maui with the 300-MW KPWI generator 

cleared.
- 20 - 

In this case, the inverter is blocked when the crowbars at the Oahu end of the DC cables are 

operating. This is because the sending end dc current includes the crowbar current plus the 

inverter current. By blocking the inverter when the crowbar is operating, the sending end current 

does not exceed its rated value. 

Scenario 1: 3 Phase Fault at Koolau, System recovers

 Secs 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 

0.00 

0.20 

0.40 

0.60 

0.80 

1.00 

1.20 

AC Volts

VKoolau

0.00 

0.25 

0.50 

0.75 

1.00 

1.25 

1.50 

V at Molokai (pu)

VMolokai VLanai

58.00 

58.50 

59.00 

59.50 

60.00 

60.50 

61.00 

61.50 

62.00 

Frequency (Hz)

FMolokai FLanai

0 

50 

100 

150 

200 

250 

Power (MW)

Power Pole1 Power Pole2

-2.00 

-1.50 

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

2.00 

DC Current (pu)

dcCur P dcCur N

-1.50 

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

Cable Volts (pu)

VcableP VcableN
- 21 - 

When the inverter remains operational during the period of time the crowbar is blocked, the DC

current at the rectifier is overloading as shown in this case. In order for the sending end (wind 

farm) ac voltage from becoming too excessive, the sending end converters must not block. Then 

the sending end ac voltage can be controlled. 

Scenario 1: 3 Phase Fault at Koolau, System recovers

 Secs 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 

0.00 

0.20 

0.40 

0.60 

0.80 

1.00 

1.20 

AC Volts

VKoolau

0.00 

0.20 

0.40 

0.60 

0.80 

1.00 

1.20 

V at Molokai (pu)

VMolokai VLanai

58.00 

58.50 

59.00 

59.50 

60.00 

60.50 

61.00 

61.50 

62.00 

Frequency (Hz)

FMolokai FLanai

0 

50 

100 

150 

200 

250 

Power (MW)

Power Pole1 Power Pole2

-2.00 

-1.50 

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

2.00 

DC Current (pu)

dcCur P dcCur N

-1.50 

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

Cable Volts (pu)

VcableP VcableN
- 22 - 

With a severe AC system fault at the wind farms at the sending end, the system recovers without 

any dc current overload. 

Scenario 1: 3 Phase Fault at Molokai, System recovers

 Secs 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 

0.00 

0.20 

0.40 

0.60 

0.80 

1.00 

1.20 

AC Volts

VKoolau

0.00 

0.20 

0.40 

0.60 

0.80 

1.00 

1.20 

V at Molokai (pu)

VMolokai VLanai

58.00 

58.50 

59.00 

59.50 

60.00 

60.50 

61.00 

61.50 

62.00 

Frequency (Hz)

FMolokai FLanai

0 

50 

100 

150 

200 

250 

Power (MW)

Power Pole1 Power Pole2

-2.00 

-1.50 

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

2.00 

DC Current (pu)

dcCur P dcCur N

-1.50 

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

Cable Volts (pu)

VcableP VcableN
- 23 - 

The most severe overload condition comes when one DC pole blocks, such as might occur with a 

control or protection fault. There is not enough time to transfer trip a wind farm. The options 

are:

1. The wind turbine generators would be equipped with a fast power reduction feature that 

will reduce power to 50% of full-load power within a cycle. This can be accomplished 

with full converter wind turbine generators and also DFIGs to perhaps a lesser extent. 

They would have to respond to a signal received from the HVDC pole that suffers the 

inadvertent block. The back-up operation would be the shut-down of the remaining 

HVDC operating pole, resulting in a 400 MW total outage. 

2. Provide an AC thyristor-controlled resistor as an extra facility on the sending end ac 

system. When the pole is blocked, it kicks in for a period of time until a suitable number 

of wind turbine generators can be taken off-line by transfer tripping or other means. 

The thyristor-controlled resistor could be directly connected to the 33-kV busbar of the wind 

farm at its main substation. It could be sized in 100-MW blocks. No cooling, filters, or special 

controls are required. An air cored reactor might be applied as a commutating reactor.

Figure A-3: Simplified thyristor switched resistor configuration 

 

T T TT T T

Rsw

+

KC KC

KC

KC KC KC
- 24 - 

The thyristor-controlled resistor rated at 200 MW kicks in when the one pole inadvertently 

blocks. It stays in until all the excess wind turbine generators can be switched out. 

Scenario 1: 1 Pole block on Bipole to Oahu, Thyristor switched resistor operating

 Secs 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 

0.00 

0.20 

0.40 

0.60 

0.80 

1.00 

1.20 

AC Volts

VKoolau

0.00 

0.20 

0.40 

0.60 

0.80 

1.00 

1.20 

1.40 

V at Molokai (pu)

VMolokai VLanai

59.80 

60.00 

60.20 

60.40 

60.60 

60.80 

61.00 

Frequency (Hz)

FMolokai FLanai

0 

50 

100 

150 

200 

250 

300 

350 

400 

Power (MW)

Power Pole1 Power Pole2

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

2.00 

DC Current (pu)

dcCur P dcCur N

-1.50 

-1.00 

-0.50 

0.00 

0.50 

1.00 

1.50 

Cable Volts (pu)

VcableP VcableN
- 25 - 

Simulation Studies for Scenario 2 Inter-Island DC Cable Options with 

Maui Connection

Introduction

The detailed PSCAD model of the inter-island DC cable options was developed for testing the 

technical performance of the three scenarios proposed for incorporating the future connection to 

Maui. At the Technical Review Committee (TRC) meeting held in Honolulu January 6th and 7th, 

2010, Scenario 2 was developed. 

Scenario 2. 

200 MW DC ±150 kV Oahu to Molokai 

200 MW DC ±150 kV Oahu to Lanai 

 69 kV, 3 Phase AC Molokai to Lanai; Lanai to Maui; Molokai to Maui 

Scenario 2 – Base Case

The requirement for the two 200-MW HVDC undersea cable transmission from Oahu to 

Molokai and from Oahu to Lanai was achieved with a symmetrical monopole configuration from 

Iwilei as shown in Figure A-4. 

Also shown in Figure A-4 is a thyristor-controlled resistor at both the Molokai and Lanai wind 

farms. Whenever one of the DC cables or converters fail, it is required that the wind farm 

feeding that failed DC cable of converter be taken out of service so that its power generated does 

not pass through the remaining DC cable and converter left in service causing it to overload. In 

order to provide time to trip out the wind farm, allowing for back up protection to operate, the 

thyristor-controlled resistors temporarily switch into service preventing main VSC converter 

overloads and allowing time for the wind farm to be transfer tripped off line. 
- 26 - 

Figure A-4: Scenario 2 cables for interconnecting to Maui in Phase 2. 

This first disturbance to Scenario 2 is with all cables and converters in place, with full wind on 

both Molokai and Lanai. There is no scheduled power interchange with Maui. The disturbance is 

a 3-phase-to-ground fault at Iwilei, from which all converters and cables recover. The shake-up 

is most severe to HECO’s system on Oahu, but it is also felt right through to Maui. AC 

frequency is controlled by independent clocks on the Molokai and Lanai 200-MW VSC 

converters. The ac system on Maui is a heavy load condition so that the VSC converters on Lanai 

and Molokai are the reference synchronizing frequency source.

The reference frequency source of the VSC converters on both Molokai and Lanai are strong 

enough to hold the Maui system in synchronism. The wind turbine generators on Molokai are 

modeled as generic Type 3 (DFIG) and on Lanai as generic Type 4 (Full Converter). They 

exhibit different responses as a consequence.
- 27 - 

In this case, the inverter is blocked when the crowbars at the Oahu end of the DC cables are 

operating. This is because the sending end DC current includes the crowbar current plus the 

inverter current. By blocking the inverter when the crowbar is operating, the sending end current 

does not exceed its rated value. However, when the crowbar is relieved and the VSC inverters 

resume their operation, a DC current surge at near 1.0 second is observed. 
- 28 - 

In this case, the severe three-phase-to-ground fault at Iwilei causes the Lanai to Iwilei VSC 200-

MW monopole to trip out of service. The 200-MW Lanai wind farm is transfer tripped to ensure 

that the remaining HVDC cable transmission from Molokai is able to accommodate the 

remaining power of its wind farm and power swings from the Maui system. At 1.0 seconds, the 

thyristor-controlled resistor operates for several hundred milliseconds at the same time that there 

is a power swing from Maui but it stays synchronized to Molokai. 
- 29 - 

This is the same fault as Scenario 2-2, where there is a severe three phase to ground fault at 

Iwilei, except this time it is the Molokai 200-MW monopole that comes down. Consequently the 

Molokai 200-MW wind farm is transfer tripped so the operating 200-MW monopole from Lanai 

Iwilei is not overloaded. Maui stays synchronized to Lanai. 
- 30 - 

For this AC cable fault there are no real control issues. The two VSC cable transmission systems 

recover although the thyristor-controlled resistor at Molokai switches on once when the fault is 

cleared.
- 31 - 

There are no real issues with this AC cable fault. The DC cables and monopoles that remain in 

service recover nicely and resume operation. The Maui system stays synchronized with the VSC 

converters at Molokai and Lanai. 

 
- 32 - 

There are no real issues with this AC cable fault. The DC cables and monopoles that remain in 

service recover nicely and resume operation. The Maui system stays synchronized with the VSC 

converters at Molokai and Lanai. 
- 33 - 

The loss of 30 MW of generation on Maui, amounting to one of the largest units on line resulted 

in little impact. Power was automatically supplied from Oahu where wind generation from Lanai 

and Molokai and frequency generated by the independent clocks on the VSC converters kept the 

Maui system quite solid. 
- 34 - 

The recovery is not as clean as when both monopoles are operating with independent clocks as in 

Scenario 2-1. The thyristor-controlled resistor is operational between 0.7 seconds to 1.7 seconds. 

The dc overvoltage protection for the cables is operational at approximately 0.3 seconds and 1.8 

seconds.
- 35 - 

Synchronizing the Operation of the Molokai and Lanai Monopoles 

A critical control factor for the operation of the two monopole converters is the means of 

synchronizing their operation with the wind farms on Molokai and Lanai and with the 

established power system on Maui. Each monopole at 200-MW capacity can operate with the 

wind farms on Molokai and Lanai and the 200-MW power system on Maui if they apply an 

independent clock. The disadvantage is there is less over-current control on each monopole, 

requiring the thyristor-controlled resistor to switch on if the monopoles approach their overcurrent limit. In addition, the phasing of the independent clocks on each monopole must be 

synchronized through a high speed fiber telecommunication link. 

Tests show that if a phase locked loop (PLL) is applied at each monopole based on the ac voltage 

at their Molokai and Lanai terminals, their combined strength together with the strength of the 

Maui system is inadequate to hold their controls of the two 200-MW monopoles in synchronism 

by normal means. An intermediate step was applied where the Molokai monopole was 

synchronized through a PLL to its local AC voltage while the Lanai monopole retained its 

independent clock, phased to the Maui power system. A control was applied to the Molokai PLL 

controls to measure and maintain the power into and out of Maui to an operator controlled level. 

The result of this control was that the variability of the wind power on Molokai and Lanai was 

buffered from impacting Maui and all variability was transferred to Oahu through the Molokai 

monopole. 

The application of the independent clock on the Lanai monopole provided stabilization to the 

PLL controls on the Molokai monopole. When the disturbance from a three phase fault at Iwilei 

on Oahu precipitated a failure in the Lanai monopole so that it tripped out of service, the 

remaining monopole with its PLL synchronized to the ac system bus was unable to maintain 

suitable synchronism. This disturbance is shown as Scenario 2b-2: 3 phase fault at Iwilei. The 

monopole to Lanai trips out of service. PLL on Molokai monopole only. When results of this 

case are reviewed and compared to Scenario 2-2 where the Molokai monopole has an 

independent clock, then the performance is clearly unacceptable. The wind farm on Lanai is 

transfer tripped when the Lanai monopole is blocked and shut down. The power from the 

Molokai wind farm should be able to be transferred on the Molokai monopole to Oahu. 
- 36 - 

The single Molokai monopole with its PLL synchronization demonstrates unacceptable 

performance when the Lanai monopole with its independent clock is taken out of service. 

Compare with Scenario 2-2. 
- 37 - 

In an attempt to ensure the PLL was as robust as possible, two units in series were applied. The 

resultant synchronizing signal was of good quality, but lacked the phase angle stabilization that 

is usually provided by inertia to support the Mololai monopole in an acceptable level of 

performance. This is shown in Figure A-5: 

Figure A-5: Double PLL for synchronizing the VSC monopole controls 

Evidence that the double PLL operated effectively against an extremely poor AC waveshape is 

shown in Figure A-6. A PLL generates a ramp signal that is synchronized to the 60-Hz 

waveshape of the busbar the VSC converter is connected to. This signal ramps from 0 degrees to 

360 degrees once per cycle as shown in Figure A-6. From this ramp signal, the controls are 

locked onto to ensure the valves fire in correct sequence to the AC waveshape. Even though the 

phase voltage is going through a severe distortion, the two PLLs generate a stable ramp, 

indicating its robustness.

Figure A-6: Functional performance of the PLLs on the Molokai monopole where the output of 

the second stage PLL is plotted as acceptable 60-Hz ramp signals.

Va

Vb

Vc

PLL

Six

Pulse 6

thetaY

1

2

3

VRec

VaPLL

1

3

5

Sin

Sin

Sin

Va

Vb

Vc

PLL

Six

Pulse 6

thetaY
- 38 - 

Other factors impact the stability of the ac waveshape including the apparent short circuit 

capacity that the wind farm is exposed to when still operational on Malokai, as well as harmonic 

impedance resonant frequencies on the ac cables to Maui. Another factor that reduces the 

stability of the VSC converter controls with a PLL is the application of the conventional vector 

control method. This has the advantage of controlling dc current but requires reasonably good ac 

waveshape stability to be effective. Consequently, significant design will be required to ensure 

the VSC converters will operate acceptably with the wind farms and the ac system at Maui. The 

cases run as Scenarios 2-1 to 2-7 in this Appendix indicate that if the VSC converters are 

synchronized through independent clocks, then satisfactory performance is obtained. 

It is possible that a hybrid synchronization control could be applied to the VSC converters on 

Molokai and Lanai. If stable vector control is not possible with the PLL, then it could be 

replaced with a more stable control but rely on the thyristor-controlled resistors and DC 

overvoltage crowbars and transfer tripping to ensure protection from over-currents and overvoltages on the VSC converters. 

So long as one of the 200-MW VSC poles is operating off an independent clock, then 

performance of the VSC transmission to Oahu, the wind farms and synchronization to the Maui 

system is acceptable. 

Simulation Studies for Scenario 3 Inter-Island DC Cable Options with Maui 

Connection

400 MW DC Oahu to Molokai 

 138 kV AC Molokai to Lanai

 2 x 69 kV AC Lanai to Maui; Molokai to Maui 
- 39 - 

Figure A-7: Scenario 3 with a bipole configuration to Oahu 

Control Strategy

The technical performance of Scenario 3 is essentially the same as for Scenario 2. The key factor 

that is determined from both Scenario 2 and Scenario 3 is the synchronization. The method that 

is proposed is as follows: 

5. Of the two VSC cable feeder poles, one pole must be on an independent clock. This 

provides the frequency control for the interconnected islands of Molokai, Lanai and 

Maui. This applies if it is configured with symmetrical monopoles as in Scenario 2. If 

configured as a bipole as in Scenario 3, both poles could be operated as an independent 

clock providing they are derived from the same clock. 

6. The other VSC cable feeder pole is synchronized through a conventional phase locked 

loop, which it is able to do through the ac voltage at its terminal connecting busbar at its 
- 40 - 

sending end. The frequency is defined by the other VSC cable feeder with the 

independent clock. 

7. The VSC feeder with the PLL control has ability to control power. As such it can be 

automatically scheduled to transmit power equal to the power on the other VSC feeder 

with the independent clock. In this way they two VSC feeders will equally control the 

power to Oahu. Alternatively, the power order to the VSC feeder with the PLL controls 

can be derived from the power from the wind farms and Maui in such a way that losses 

are minimized and the VSC feeder with the independent clock does not overload. 

8. If the VSC feeder pole with an independent clock is faulted and removed from service, 

then a control mechanism must be in place to ensure the other pole if operating with a 

phase locked loop, immediately switches to an independent clock. This is not a trivial 

control action and must be implemented with care and reliability.

When this control strategy is implemented, the power systems and wind farms on Molokai, Lanai 

and Maui will be synchronized to each other but not to Oahu. 
F1146-E(10/2008) 

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

February 2011

2. REPORT TYPE

Subcontract Report

3. DATES COVERED (From - To)

2008 - 2011

4. TITLE AND SUBTITLE

Phase 2 Report: Oahu Wind Integration and Transmission Study 

(OWITS); Hawaiian Islands Transmission Interconnection Project

5a. CONTRACT NUMBER

DE-AC36-08GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Dennis Woodford

5d. PROJECT NUMBER

NREL/SR-5500-50414 

5e. TASK NUMBER

SS101362

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Electranix Corporation

12 – 75 Scurfield Blvd

Winnipeg, Manitoba R3Y 1G4 Canada

8. PERFORMING ORGANIZATION

REPORT NUMBER

LAM-9-99436-01

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-5500-50414

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: David Corbus

14. ABSTRACT (Maximum 200 Words)

This report provides an independent review included an initial evaluation of the technical configuration and capital 

costs of establishing an undersea cable system and examining impacts to the existing electric transmission systems 

as a result of interconnecting the islands.

15. SUBJECT TERMS

undersea cable; Maui; Lanai; Molokai; Hawaii Clean Energy Agreement; grid integration; operational impacts, 

transmission planning; Electranix; XLPE cable; voltage sourced converters

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UL

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