Large Scale Offshore Wind Power Energy Evacuation by HVDC Light

8/3/2019 Large Scale Offshore Wind Power Energy Evacuation by HVDC Light

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Large scale Offshore Wind Power Energy evacuation by HVDC Light ®

Peter Sandeberg, Lars StendiusABB ABPSG/DCSE-771 80 LudvikaTel: +46 240 78 20 00

Abstract

With several very large offshore wind farms planned to be built off the coasts of severalEuropean countries, new engineering challenges are now being identified. The design,construction and operation of large scale power plant, positioned far out at sea in hostileenvironments, will require significant skills in the design and construction. ABB has developed a

detailed design concept for the completion of the worlds first offshore HVDC Light® Power Plantfor integration of large scale wind power production. This paper will highlight the challenges andthe solutions met by ABB in the development of this pioneering project.

Introduction

For well over 100 years HVAC, HighVoltage Alternating Current, has beenregarded as the natural choice for electricalpower transmission. HVDC, High VoltageDirect Current, has been commercial

available since the mid fiftieths but hasmainly been used for large amount of bulkpower point-to-point transmission links overlong distances or interconnection ofasynchronous grids. However, HVDC, andespecially the VSC (Voltage-sourceconverter) based HVDC, is now emergingas a robust and economical feasiblealternative offering a superior solution for anumber of reliability and stability issuesassociated with connection of sustainableenergy schemes in harsh environments,e.g. offshore wind power applications.

VSC based HVDC transmissions (withinABB called HVDC Light

® ) are attractive for

connecting remotely located (e.g., offshore)wind farms to the main grid. This is partlybecause the capacitance per length unitmakes an ac cable impractical for cablelengths above 50–100 km: a significantamount of reactive power is generated, andlow frequency resonances may result ininstability phenomena. Moreover, classicalthyristor-based HVDC transmissions areless attractive, since a synchronouscompensator or a static synchronouscompensator (STATCOM) may be required

at the wind farm in order to maintain asmooth line voltage for the thyristors tocommutate against. This problem howeverdoes not exist for VSC-HVDCtransmissions, which use pulse widthmodulated transistor VSCs with inherentvoltage controlling capability.

VSC HVDC - rectifying, inverting andcontrolling

With VSC based HVDC, the use of series-connected power transistors has allowedthe connecting of voltage source convertersto networks at voltage levels hithertobeyond reach. This can be used for powertransmission, for reactive powercompensation and for harmonic/flickercompensation. With fast vector control, this

converter offers the ability to control activeand reactive power independently whileimposing low levels of harmonics, even inweak grids.

Paper presented at EWEC 2008, March 31st - April 3rd 2008, Brussels, Belgium



Figure 1 PWM, Pulse With Modulation

In VSC based HVDC, Pulse WidthModulation (PWM) is used for thegeneration of the fundamental voltage.Using PWM, both the magnitude and phase

of the voltage can be controlled freely andalmost instantaneously within certain limits.This allows independent and very fastcontrol of active and reactive power flows.Pulse Width Modulation based VSC istherefore a close to ideal component in thetransmission network. From a system pointof view, it acts as a zero-inertia motor orgenerator that can control active andreactive power almost instantaneously.Furthermore, it does not contribute to theshort-circuit power, as the AC current canbe controlled.

Figure 2 HVDC Light principles

The ABB provided HVDC Light ®

converterdesign is based on a two-level bridgegrounded via a midpoint capacitor. Thisdesign philosophy ensures operation, bothsteady state and dynamic, with extremelylow levels of induced ground currents. Thisfeature is one of the critical factors forimplementing an HVDC system in an

offshore environment. There is no need for

any cathode protection in conjunction withthe installation.

The HVDC Light system allows fullyindependent control of both the active andthe reactive power flow within the operatingrange of the design. Normally each stationcontrols its reactive power contribution(both inductive and capacitive)independently of the other station. Theactive power can continuously and almostinstantaneously be controlled from fullpower export to full power import. However,the flow of active power in the DC cables amust be balanced, which means that theactive power entering the HVDC systemmust be equal to the active power leaving it.A difference in power would imply that the

DC voltage in the system would rapidlyincrease or decrease, as the dccapacitance increases its voltage withincreased charge (and vice versa). With anormal design the stored energy isequivalent to around 2 ms powertransmission on the system.

To attain this power balance, one of thestations has to control the DC voltage. Thismeans that the other station can adjustarbitrarily the transmitted power within thepower capability limits for the HVDC Lightsystem design, whereby the station that

controls the DC voltage will adjust its powerto ensure that the balance (i.e. constant DCvoltage) is maintained. The balance isattained without telecommunicationbetween the stations, quite simply based onmeasurement of the DC voltage.

Variable frequency

An HVDC Light converter station normallyfollows the AC voltage of the connectedgrids. The voltage magnitude and

frequency are determined by the controlsystems of the generating stations. For awind power application however, theoffshore converter station could control thegrid frequency and voltage to a referencevalue set by an overall wind farm controlsystem in order to optimize the wind powerproduction should such a solution bepreferred.

Operation with variable frequency in oneend and fixed grid frequency in the otherdoes not require main circuit equipment thatdiffers from the normal design. In general,the design principles adopted for normal



transmission system applications alsoapplies for wind farm applications.

Islanded operation

In case of a voltage collapse, a “black-out”,the HVDC Light

® converter can

instantaneously switch over to its owninternal voltage and frequency referenceand disconnect itself from the grid. Theconverter can then operate as an idling“static” generator, ready to be connected toa “black” network. For more information seealso section below regarding start up ofislanded offshore networks.

VSC HVDC Cable - transporting the

power

The HVDC Light ®

concept includes also theextruded polymer HVDC Light

® cables. It is

flexible and cost effective cables that are animportant part of the HVDC Light

® concept.

The cable is designed with a copper oraluminum conductor surrounded by apolymeric insulating material, which is verystrong and robust. The water sealing of thecable is designed with a seamless layer ofextruded lead and finally two layers ofarmoring steel wire in counter helix for the

mechanical properties of the cable. Thestrength and the flexibility make the HVDCLight

® cables well suited for severe

installation conditions and deep waters likein the North Sea for example.

Going for Wind Offshore - Challengesand Solutions

It is obvious that there will be a number ofmore or less tough challenges trying toevacuate large amount of offshore wind

power. The aim is to design a very robustelectrical transmission with high availabilityand minimized maintenance meeting notonly the strict national grid codes but also torelieve stresses from the wind turbines byisolating electrical transients from themainland grid. Another, not less important,aspect is of course to design a system thatcan withstand the harsh and sometimesvery hostile offshore climate in the NorthSea.

The following sections will discuss some of

the challenges met and the correspondingsolutions.

Offshore environment - OffshorePlatform

Space and weight are scarce resources onoffshore installations. Particularly in the lightof these constraints, the VSC based HVDCconcept offers important advantages; sincethe filters are small, VSC based HVDC canbe made compact and lightweightcompared to other solutions. Apart from theobvious needs to make the converterstation compact and lightweight, the harshoffshore environment and the remotelocation places a number of other demandson the converter station and equipment.Examples include:

− Safety for personnel as well as forequipment.

− Salt and humid air imposes severerequirements on the choice of materialsand surface treatment.

− Minimized maintenance requirements

− Extensive monitoring facilities

Except for the main transformers all highvoltage equipment will be installed insidecompact modules at the offshore platform.The ventilation system in the modules hasbeen designed to protect the high-voltageequipment and the electronics from salt

laden and humid air. The main circuitequipment is therefore exposed to lowerenvironmental requirements than a normaloutdoor installation which allows for a morecompact design. The ventilation has also toconsider the airborne losses. An advantageof being offshore in the North Sea is ofcourse that cold (5-11 °C) water for coolingis readily available.

The design of HVDC Light permits theConverter Stations to be operated remoteor unmanned. The HVDC Light

maintenance concept is based upon theobjective to keep a very high performanceof the link throughout the whole operationallifetime. The estimated annual maximumenergy unavailability due to scheduledmaintenance is about 0,4% orapproximately 35 hours. The annualmaintenance can be performed at the mostconvenient time for the Owner.

The MACH 2 control system and itsauxiliary systems have an inherentextensive internal monitoring system builtinto it. This means that the status of the



system can be continuously monitored alsofrom a remote location.

Meeting strict Grid Codes

As wind penetration increases the Gridcodes requirements have become stricter.Ride-through of voltage dips down to 15%of the nominal voltage—or even zerovoltage—for up to 150 ms is today oftenrequired. It is also anticipated thatrequirement for frequency response, i.e.,that the wind farm output power should beincreased as the grid frequency decreasesand vice versa, will be imposed. Frequencyresponse can be introduced in a wind farmconnected via an HVDC Light

®

transmission by maintaining atelecommunication link between the main-grid-side (onshore) and wind-farm-side(offshore), where, among other variables,the instantaneous main-grid frequency istransmitted. Since the voltage at the wind-farm bus is fully controllable (amplitude,frequency, and phase) by the rectifyingVSC, the grid frequency can be “mirrored”to the wind-farm grid without significantdelay.

Neither is ride-through of voltage dips dueto faults at the rectifying VSC, i.e., in the

wind-farm grid, difficult. By closed loopcontrol, the converter current is kept withinits prescribed limits, which allows the VSC-HVDC transmission to remain on-line untilthe fault is cleared.

Ride-through of voltage dips at the invertingVSC, i.e., in the main grid, is, on the otherhand, more challenging. A reduced main-grid voltage implies that the powertransmission capability is reduced by asimilar proportion, due to the current limit ofthe inverting VSC. For example, for a dip

down to 15% of the nominal voltage, only15% of the transmission capability remains.In a “standard” HVDC Light

® transmission

connecting two utility grids, a similarscenario is solved by instantaneouslyreducing the input power of the rectifyingVSC through closed-loop current control. Ina strong grid with an amount of generationmuch greater than the rated transmissioncapability, this will occur without asignificant change in the voltage. Thecharacteristics of a wind farm are, however,different. The wind-farm network is much

smaller than the typical utility grid, and

consequently weaker. Also, its ratedgeneration normally matches the ratedHVDC transmission capability. A fastreduction of the input power of the rectifyingVSC may therefore lead to a significant

increase of the wind farm bus voltageresulting in an over voltage tripping of theVSC and/or the wind turbines. In principle,there are two methods to overcome thisproblem:

1. Signal to the WTGs via the wind-farmgrid voltage that their output powershould be reduced as quickly aspossible.

2. Use a chopper solution to dissipate theexcess energy that cannot betransmitted by the inverting VSC.

Complicating the first solution are two facts.Firstly, the total dc capacitance (sum of thecapacitances installed in the VSCs and thecable capacitance) is normally small; ifpower transmission is interrupted, the dcvoltage may reach an unacceptably highlevel (typically, the protection action level isset at an over voltage of about 30%) in aperiod of only 5 to 10 ms. The WTGs musttherefore be able to both detect that apower reduction should be made and reduce the output power (possibly to zero)

within this time frame, which may be quitedemanding. Secondly, the response ofWTGs to a varying voltage is generally notthe same for the main WTG types: fixed-speed induction generators (FSIGs),doubly-fed induction generators (DFIGs),and full-converter generators (FCGs).However, since it is possible that differentWTG types may be used within a certainwind farm, it is desirable that the ride-through strategy selected should be generalfor all WTG types.

Figure 3 HVDC Light® with chopper solution

HVDC OffshorePlatform

HVDC Onshore

~=

+ DC

- DC

BR

BR

~=

P,Q(dot

Ua,Ub,U

P,Q(dot

Ua,Ub,Uc



The latter solution is very robust, andleaves the wind farm unaffected duringmain-grid faults. The dc chopper is aresistor in the dc-circuit with high energycapability. The dc chopper evacuates thesurplus of energy during network faults,when a power transmission is not possible.Therefore, there will be no abrupt change inthe output power from the wind turbinesand the disturbance seen by the windturbines will be minimized.

One can also anticipate a lot of positiveside effects with such a solution as theHVDC link together with the chopper willmake the wind farm “immune” against

electrical transients thereby eliminating themechanical stresses originating from theelectrical side that may arise on theequipment in the nacelle, e.g. gearboxes.

Starting up of and islanded offshorenetwork – Black Start

The ability of the HVDC Light® convertersto generate a voltage that can be changedvery quickly in amplitude and phase, offersthe possibility of energizing a network aftera blackout. This is especially useful when it

comes to energization of an offshorenetwork. The converter transformer will beequipped with a special auxiliary powerwinding for self-supply of the converterstation, and the control system will havespecial schemes for detecting a networkblackout. If such an event occurs, theconverter will automatically trip theconnection to the grid, and continue tooperate in “house-load” operation, suppliedthrough the DC cables from the mainlandgrid. The converter can also be startedmanually in Black-start mode, if needed.The network restoration sequence startswith the offshore station running withoutload. The voltage and frequency aredecided by the converter, which in this caseoperates in frequency control mode as agenerator. The AC-voltage can be smoothlyramped up by the VSC thereby preventingtransient over-voltages and inrush currents.The WTG:s can be automatically connectedto the offshore network after seeing thecorrect AC-voltage for a certain time.

Figure 4 AC voltage at start-up of an isolated network (measurements from the Hällsjö project)

Offshore Commissioning

Considering HSE (Health, Safety andEnvironmental) aspects for the personneland also from an economical point of viewthe number of man-hours offshore shall tryto be minimized. For this reason anextensive commissioning program of theoffshore station will take place at a nearbyharbor before shipping the modules to theoffshore site. This will not only be moreconvenient and safer for the personnel butwill also mean increased quality and ensurefast and effective commissioning at theoffshore platform out at sea.

Future Power Transmission for OffshoreWind

With several gigawatts of offshore windgeneration in Europe now in the advancedstages of planning, the demand for reliableand robust power transmission to shore isnow a fact.

The design, construction and operation oflarge scale power plant, positioned far out

at sea in harsh environments, will requiresignificant skills in the design andconstruction. ABB has developed a detaileddesign concept for the completion of theworlds first offshore HVDC Light

® Power

Plant for integration of large scale windpower production. The valuable lessonslearned by engineers and briefly highlightedin this paper may help to reduce thetechnical and hence financial risks faced byoffshore wind farm developers currentlyconsidering alternative designs for theconnection of future very large offshore

renewable energy projects.



References

[1] D Wensky et al.: “FACTS and HVDC forgrid connection of large offshore windfarms”, EWEC conference 2006.

[2] A.-K. Skytt, P. Holmberg, L.-E. Juhlin:"HVDC Light for connection of wind farms",2nd international workshop on transmission networks for offshore wind farms, Royal Institute of Technology, Stockholm, 2001.

[3] L Harnefors et al.: “Ride-throughmethods for wind farms connected to gridvia a VSC HVDC transmission”

[4] P Jones, Bo Westman, “FromGeneration to grid”, renewable energy focus

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Large Scale Offshore Wind Power Energy Evacuation by HVDC Light