Voltage Source Converter Topology for Hvdc Grid Connection of Offshore Wind Farms
ABSTRACT
Large offshore wind farms are recently emerging aspromisingalternative power Sources. Long distances betweenoffshore generation and onshore distribution grid demand newsolutions for their connection to the AC network. HVDC systemsbased on voltage source converters (VSC) are a promisingalternative to conventional AC transmission above a certain cablelength. This paper presents a new VSC transmission topology forHVDC grid connection of offshore wind farms.
I.INTRODUCTION
The recent emergence of larger and more efficient windturbines establishes bright prospects in wind powergeneration. The first 5 MW wind turbine was erected inBrunsbüttel near Hamburg in summer 2004 by REpowerSystems AG. These latest developments extend thepotential of large-scale offshore wind generation, whichbecomes a rapidly growing worldwide alternative powersource.
Today, especially large offshore wind farms in the powerrange of several hundred megawatts are getting into focus.Limited availability of onshore sites and better offshore windconditions are driving the wind turbines offshore.Environmental requirements regarding noise pollution andthe visible impact as well as colliding interests in the nearshoreareas (recreation, military, coastal shipping, fishingetc.) lead to increasing distances between offshore windfarms and onshore distribution grids. Remote locations,however, often imply deep water depths, complicating thefoundation of the wind turbines in the seabed. Recentimprovements in submarine foundations (i.e. tripod,quadropod or lattice structures) allow deeper water depths,whereas the current economic limit of such installations liesin the range of 30 to 35 m. Another important factor thatcauses prolonged transmission distances is the necessity of astrong grid connection point with a significant short-circuitscapacity. Reaching a suitable AC network connection pointrequires often a long onshore transmission line. In exchange,expensive grid enforcement measures can be avoided. As aconsequence of the ongoing trend, the generated power fromthe wind farms has to be transported over longer distances in order to make a connection to the AC network for onwardtransmission and distribution. For longer transmissiondistances, HVDC transmission is a feasible solutioncompared to traditional AC transmission. AC cablesinherently generate reactive power that limits the maximumpermissible AC cable length. This is known as the criticalAC cable length.
The critical cable length for AC transmission cannot bedetermined generally. It varies for every individual projectand is given by economical and technical constraints.Nowadays, it lies in the range of approximately 100 km.As mentioned before, the AC cable length is limited due tocapacitive charging currents. Above a certain cable length, acompensation unit is required. This is particularly costly andtroublesome for submarine cables.
HVDC transmission systems based on VSCs, also calledVSC transmission, have lately emerged as a competitivealternative to conventional AC transmission. Their operationprinciple is shown in Fig. 1. VSC transmission provides anefficient and reliable solution overcoming the major technicaldifficulties facing traditional AC solutions. Mainly, DCcables are not affected by cable charging currents and thuscan have any length required. The DC cable capacitanceoffers instead the advantage of energy storage. Only the costof the complex onshore and offshore converter stations andthe losses of the semiconductor devices limit an augmentedapplication of VSC transmission. Especially the highfrequency PWM switching leads to excessive switchinglosses. Thus, an improvement in efficiency and lower initialconverter costs are highly attractive and help to decrease theminimum length at which VSC transmission can competewith traditional AC transmission.
VSC transmission offers a couple of additional advantagescompared to AC transmission. If there are environmentalconstraints on the quantity of submarine cables, DC cableswith a higher capacity are the preferable choice. Furthermore,VSC transmission allows frequency control of the local windfarm grid, as the frequencies on both ends of the DC link areindependent. Both the active power flow and reactive powerat both ends of the DC link are fully defined and controllable.This allows the control of voltage fluctuations and increasesthe exchange limits of reactive power, thus stabilizing the ACgrid at the connection point. The electrical impact of the VSCtransmission on the existing AC network, as i.e. the harmonicdistortion must be studied.
In this paper, a new soft-switched VSC topology is proposedthat promises both lower initial costs and a higher efficiency.As a consequence, the minimum cable length at which VSCtransmission gets interesting can be further decreased and theHVDC grid connection of large offshore wind farms gets farmore attractive. The operation principle of this new VSCtopology is presented with respect to the commutation of thesemiconductor switches. The advantages and challenges ofthe proposed topology are also outlined. Furthermore, acomparison with a state-of-the-art variable-speed windturbine reveals the potential of this new technology. Thereby,this paper concentrates on the losses and ratings of theconverters.
II. DESCRIPTION OF PROPOSED TOPOLOGY
The topology of the proposed AC/DC converter is shown inFig. 2. It incorporates a VSC and cycloconverters (directconverters) connected via a medium frequency (MF) AC bus.
Every wind turbine is equipped with a passive line filter,a 3-by-2 cycloconverter, an MF transformer and a circuit breaker.This enables the individual wind turbine to operate as anadjustable-speed generator (ASG), offering multipleadvantages compared to fixed-speed operation, as i.e.increased efficiency. The valves of the cycloconverter donot need any turn-off capability and can be realized by fastthyristors connected in anti-parallel. The MF transformerincreases the generator voltage from 690 V to 33 kV. Thehigh-voltage side of the transformer is connected to the MFAC bus via a circuit breaker allowing the disconnection ofthe wind turbine.
This MF AC bus connects the wind turbines to a single-phaseVSC via the main circuit breaker and the main MFtransformer. This transformer increases the bus voltage from33 kV to half the DC link voltage (150 kV). The high-voltageside of the transformer is connected to a single-phase VSC,whereas one of the transformer terminals is connected to themidpoint in the DC link created by bus-splitting capacitors.These DC capacitors provide the DC voltage sourcenecessary for the dynamics of the system and govern thevoltage ripple on the DC line. Series-connected IGBTs withantiparallel diodes form the valves of the VSC. Additionally,the VSC is equipped with snubber capacitors connected inparallel to each of the semiconductor switches. Thecapacitors should be sufficiently large to allow zero-voltageturn-off of the IGBTs. The ground reference of the VSC ismade at the midpoint in the DC link.
A. Principle of operation
By alternately commutating the cycloconverters and the VSCit is possible to achieve soft commutations for all thesemiconductor valves. The cycloconverter can be solelyoperated by line commutation (natural commutation) whereassnubbered or zero-voltage commutation is always enabled forthe VSC. The operation principle during a commutationsequence is described below.
The VSC is commutated at fixed time instants with constantintervals (switching frequency mf = 500 Hz), thus generatingan MF square-wave voltage on the AC bus. When the maintransformer current and voltage have the same sign(instantaneous power flow is directed from the DC-side to theAC-side), the conditions are set for a snubbered commutationof the VSC. The process is initiated by turning off theconducting valve at zero-voltage conditions. The current isthereby diverted to the snubber capacitors. The antiparalleldiodes of the incoming valve take over the current once thepotential of the phase terminal has fully swung to theopposite. At this stage, the IGBTs that are antiparallel to thediodes can be gated on at zero-voltage zero-currentconditions. Reversing the transformer voltage during theVSC commutation establishes the possibility for naturalcommutation of the cycloconverters. The commutation of acycloconverter phase leg is initiated by turning on the nonconductingvalve in the direction of the respective phasecurrent. The VSC voltage and the leakage inductances of thetransformers govern the natural commutation. Finally, theinitially conducting thyristor turns off as the current throughit goes to zero. The cycloconverter phase legs arecommutated in order to obtain the desired PWM generatorvoltages. Additionally, successive commutations of thecycloconverter phase legs eventually lead to a reversal of thecurrent through the main MF transformer, thus setting theconditions for a snubbered commutation of the VSC.
B. Advantages and challenges
The proposed AC/DC converter topology offers multipleadvantages in addition to the basic features of conventionalVSC transmission systems. In order to ensure properfunctioning of the topology, however, certain technicalchallenges have to be analyzed.
Lower initial costs are a main ambition of the new proposedtopology. Among other things, this is achieved by utilizingcheaper single-phase MF transformers. A positive side-effectis the reduction in weight and volume that simplifies theirintegration in the wind turbines. The design of the MFtransformers has to be adapted to the specific characteristicsof the proposed topology. Especially the design of thetransformer insulation needs to withstand high voltagederivatives (however limited by the VSC snubber capacitors).Another factor that decreases the initial costs is thesignificant reduction in the number of series-connected IGBT valves in the VSC. A reduction is highly desirable, as IGBTstend to be expensive and require complex gate drives andvoltage-sharing circuitries. SECTION VI presents the possiblesavings. On the other hand, the cycloconverters entailadditional costs, even though their valves consist ofcomparably cheap and well-established fast thyristors.
An increase of the converter efficiency is crucial, especiallyduring low power generation. Otherwise, the converter lossesmay consume a significant portion of the generated power.The switching losses are reduced considerably (refer to SECTION VI) by using a soft-switching commutation scheme.Moreover, the thyristors in the cycloconverters have lowlosses compared to IGBTs. An important factor determiningthe system efficiency is the influence of the non-sinusoidalsquare-wave MF AC voltage on the losses in the AC cables.Finally, the design of the control system has to be optimizedin terms of ensuring maximum overall efficiency.
III. DESCRIPTION OF REFERENCE TOPOLOGY
Today, the predominant solution for adjustable-speed windturbines is the doubly-fed induction generator (DFIG) ASG.Its configuration is shown in the lower part of Fig. 3. Thestator of the induction generator is directly connected to thewind farm grid whereas the rotor windings are connected to afrequency converter (back-to-back VSC) over slip rings. Thisallows the wind turbine to operate over a wide speed range,depending on the rating of the back-to-back VSC. Unlikethe new proposed topology, however, the solution with aDFIG requires slip rings (costly and maintenance intensive inan offshore environment) and does not enable full adjustable-speed operation. Nevertheless, DFIG is currently thepreferable solution when the wind turbines are directlyconnected to the main AC grid.
To ensure comparability, the reference topology shouldfeature the same characteristics as the proposed topology.Thus, this paper focuses on a comparison with the direct-inlineASG. Its configuration is shown in the upper part of Fig.3. Every wind turbine generator is connected to the wind parkgrid over a full-size frequency converter. This back-to-backVSC requires the same rating as the wind turbine generatorand is therefore very expensive. In return, a simple squirrelcageinduction generator can be applied. This topology offersall advantages of adjustable speed operation.
The onshore three-phase VSC linking the AC network to thesubmarine DC cable is not considered in this comparison, asit is independent of the offshore converters. On the other side,different topologies are suitable for the offshore three-phaseVSC, e.g. two-level, multilevel diode-clamped or multilevelfloating capacitor converters. In this comparison, thetopology applied is assumed to be a hard-switched two-levelconverter, which is an established solution in VSCtransmission systems. The frequency modulation ratio (PWMswitching frequency divided by fundamental frequency) isp = 39 and the fundamental frequency is f0= 50 Hz. It isassumed that the amplitude modulation ratio for the hard switched reference topology is increased to ma= 1.1 by thirdharmonic injection. On the other hand, the amplitudemodulation ratio for the proposed soft-switching topology ischosen as ma= 0.9 to provide the possibility of increasing thevoltage if necessary.
IV. IGBT LOSSES AND RATINGS
This section presents the procedures applied to determine theIGBT conduction and switching losses of the different VSCs,namely the single-phase VSC in the proposed soft-switchedtopology and the three-phase VSC as well as the back-to-backVSCs in the hard-switched reference topology. TheIGBT losses are an essential factor regarding the thermaldesign and influence the current rating of the IGBT. Thiscurrent rating along with the voltage rating of the IGBTdetermines the power rating, which is an important indicatorof the costs for the IGBTs. Generally, the converter ratingshould be increased by a certain margin that offers thepossibility for i.e. regenerative braking. In this study, thisaspect is not considered in the converter ratings.
A. Voltage rating
The rated SSOA (switching safe operating area) voltage VSSOA combined with the long-term stability against cosmicradiation defines the IGBT voltage rating. For improvedreliability and to avoid false triggering due to cosmicradiation the maximum allowed SSOA voltage VSSOA,maxisgenerally derated by approximately 40% from the maximumdevice voltage Vce,max. The margin between the maximum andthe rated SSOA voltage is due to voltage spikes caused bydiode reverse recovery currents. In a soft-switchingenvironment this margin is considerably smaller. The higher voltagecapability of the soft-switched IGBT can be explained by itssnubbered commutation. The limited di/dt capability and theconsequently longer turn-off time allow a higher blockingvoltage capability in trade-off, at an acceptably low on-statevoltage drop.
The number of series-connected IGBTs per VSC valve(NIGBT) depends on the DC link voltage that has to besupported. It is 112 for the soft-switched topology and 240for the hard-switched topology. In connection with series connectedsemiconductor devices, it is important that a singlecomponent failure does not lead to a malfunction of thewhole device. Therefore it is important that a short circuitfailure mode (SCFM) is guaranteed. In this case, it isdesirable to add some additional IGBTs to get a redundancy,which however is not considered in this comparison.
B. Current rating and IGBT losses in the VSCs
The required active silicon area Asiand the maximum turn-offcurrent density JSSOAdefine the current rating (CR) of theIGBT:
CR = Asi· JSSOA. (1)
The required active silicon area is determined by two factors.It must be sufficient to meet the thermal stress and the SSOArequirements of the IGBT. For a hard-switching topology, theIGBTs are usually thermally limited whereas for a soft switchingtopology the SSOA is the limiting factor due to thelower switching losses. The active area meeting the SSOAcriteria (ASSOA) is calculated from the peak device current Îceand the maximum turn-off current density including a safetymargin k:
ASSOA = (1+ k) · Îce/ JSSOA. (2)
On the other side, the conduction losses Pcond, the switchinglosses Psw and the rated loss power density pδdetermine thesilicon area required for thermal design (Ath):
Ath = (Pcond + Psw) / pδ (3)
1) Current rating and losses of the proposed topology: Theconduction losses depend on the on-state voltage drop acrossthe device and the current through it. They can be calculatedfrom the on-state threshold voltage Vce0, the on-state sloperesistance rce0 and the device current Ice as in (4). Both the on-stateslope resistance and the threshold voltage depend on thedevice temperature.
1/fo
Pcond=f0 ∫ [Vce0 . Ice(t) + rce0 . Ice2(t)]dt (4)
t=0
The switching losses consist of turn-on and turn-off losses.Thereby, the turn-on losses are neglected, as the IGBTs turnon at zero-voltage zero-current conditions. Turn-on appearswhen the IGBTs take over the current from the main diodesin the opposite VSC leg, governed by successivecommutations of the cycloconverter phase legs. Thus, theswitching losses are assumed to consist only of turn-offlosses and can be calculated from the turn-off energy Eoff.The turn-off energy is dependent on the turn-off current andthe device temperature. Fig. 4 shows the simulatedwaveforms of the main transformer voltage and current atrated load. It can be seen that the IGBTs have to switch-offthe maximum device current Ice,maxduring each cycle.
mf/ fo
Psw = f0 ∑ Eoff (Ice, max) (5)
x=1
From the IGBT losses and the maximum device current, therequired active silicon area can be calculated. As expected fora soft-switched application, the IGBT is limited by the SSOArequirement.
2) Current rating and losses of the reference topology: The switching losses are calculated from available switchingloss data of the Cross Sound Cable project. The conductionlosses are calculated according to (4). As expected for a hard-switched topology, the IGBTs are thermally limited. Thus,the current rating is determined by the conduction andswitching losses. The required active silicon area is about13 % smaller than the area required for soft-switching. Fig. 5shows the simulated voltage and current waveform of a VSCphase leg at rated load.
The IGBT rating and losses of the back-to-back VSCs arecalculated in the same way as for the main VSC. Thereby, theloss distribution between the rectifier and the inverter partwithin the back-to-back VSC is different. The IGBTconduction losses in the inverter are approximately five times higher than in the rectifier, whereas the switching losses areequal. This is due to the fact that the IGBTs are conductingunder a major part in the inverter mode. This leads to higherIGBT losses in the inverter part of the back-to-back VSC andthus to a higher accumulated IGBT power rating (7.53 GWcompared to 6.56 GW for the rectifier part). Fig. 6 shows thesimulated voltage and current waveforms (including thecurrent ripple) of an arbitrary back-to-back VSC phase legunder rated conditions.