2.Hvdc TX Using Vsc-Design and Application

8/3/2019 2.Hvdc TX Using Vsc-Design and Application

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HVDC Transmission Systems usingVoltage Sourced Converters -Design and ApplicationsF. Schettler H. Huang N. hristl

Siemens AG91050Erlangen, GermanyAbstract- Rapid development in the field of power electronicdevices with tum off capability like Insulated Gate BipolarTransistors (IGBT) and Gate Tum-Off Thyristors(GTO), akes theVoltage Sourced Converters (VSC) getting more and moreattractive for High Voltage Direct Current Transmission (HVDC).This new innovative technology provides substantial technical andeconomical advantages for different applications compared toconventional HVDC transmission systems based on thyristor

technology. This paper focuses on VSC application for HVDCsystems of high power ratings up to 200 h4W). which are currentlyin discussion for several projects. M e r a brief description of themain circuit components an overview on basic design aspects ofVSC based HVDC systems is given.Keywords: HVDC, VSC, Main Data, Harmonic Performance,Insulation Coordination, Transient Stress

1. INTRODUCTION

A conventional HVDC can only operate in AC systems withappropriate short circuit power. Under weak systemconditions, rotating synchronous phase shifters, that areexpensive both in investment and maintenance, have beenused to achieve reliable operation.A HVDCPIUs onverter is equipped with semico nductordevices such as IGBTs that can be turned on and off in acontrolled manner. Therefore, the converter is able tofunction in a system where there is no local generation andcan control the ac system voltage and frequency.Thyristor converters as used in conventional HVDC systemsalways require reactive power. The reactive power demandvaries according to the active power transfend. Additionalpower components such as switched capacitor banks or StaticVar Compensators (SVC) have been used to supply thereactive power demand of the converter station.

In this paper, we use H VDCpLu"s a terminology for a HVDCtransmission system which is based on VSC technology. Theextension 'plus' stands for Eower !ink Xniversal ystems andrepresents economical solutions to the most challengingrequirements on power transmission and distribution. Someimportant highlights a re:

In an HVDCPlUSystem, each station can control active andreactive power flow independ ently from each other w ithin thetotal MVA power ratings. Thus, the scheme not only cantransmit active power from one AC network to another but italso offers the possibility of controlling the AC bus voltageand improving the AC system stab ility.Feeding AC systems with low sho rt circuit power o reven passive networks with no loca l power generation.STATCOM func tionality, i.e. continuously adjustablereactive power suppo rt to the AC system to control ACbus voltage and im prove system stability. Active andreactive power exchange can be controlledindependently from each other within the total powerrating of a station.

HVDCpl" is seen today a sophisticated alternative toconventional HVDC system s in the fields of0

0 Multi-Terminal HVDC svstems

Interconnecting weak or isolated AC systemsInterconnecting AC systems in the lower and middlepower range.Connecting remote isolated loads like off-shore Oil- andGas-platformsConnecting Windparks (on-shore or off-shore)These features make HVDCP'"~ an interesting alternative toconventional thyristor based HV DC systems. Ongoing development on the sector of pow er semiconductorsfor high power applications are expected to widen the rangeof commercial applications in the near future.

2. H V D C ~ ~ *RANSMISSION SYSTEMSHVDCP'"' systems are available today for power ra tings up to200MW for one 12pulse bipolar unit. This power transfer isachieved with DC voltages up to & 150 kV and DC currentsup to more than 700A.

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Fig. 1 shows the main circuit diagram of a HVDCdUStransmission schem e as an example. T he transmission systemconsists of two stations, Station A and Station B, connectedby a DC cable or an overhead line. The major powercomponents are:0 High voltage DC circuit0 Voltage Sourced Converters (VSC)0 TransformersThe system shown in Fig. 1 is of bipolar type. One stationcomprises two equally designed VSCs, each providing a DCvoltage Vdc.The central point of the DC circuit is grounded in one of thestations via a grounding impedance and a parallel connectedZnO arrester. The central point of the other station isgrounded only by an arrester to prevent current flowingthrough the ground path.Similar to a conventional HV DC, a HVDCp'" scheme canalso be built as a monopolar system or as a Back to Backstation.The major power components are described in more detail inthe following chapte rs.2.1. High Voltage DC CircuitThe DC circuit is formed by the storage capacitors and theDC cable or overhead line respectively. A storage capacitorprovides the corresponding VSC with a smooth DC voltageof a fixed polarity. To achieve maximum use of the powersemiconductors of the VSC, the capacitor needs to beconnected to the converter by a low inductive path. The sizeof the capacity is chosen according to the maximum DCvoltage ripple tolerated.The DC cable can take significant advantage of the fixedpolarity of the DC voltage. In conventional thyristor basedtransmission systems, power reversal is always associatedwith changing the polarity of the DC voltage. Transientphenomena in the cable required spe cial design measures andhigher insulation capab ility. A HVDCp'"' system insteadcannot change voltage polarity. Power rev ersal is achieved by

L.Station A

changing the direction of DC current instead. This allows touse new extruded DC cables, which are an attractivealternative to self contained oil filled or mass impregnatedpaper insulated cables as used for conventional thyristorbased HVDC systems.The cable length is not limited as it would be in case of ACtransmission systems. A major concern with AC cabletransmission is balancing the reactive power demand of thecable, which is due to the large working capacitance.HVDCP'"~ transmission is therefore an alternative to long ACcable transmission.In case a HVDCPIuSs built as a Back to Back station, the DCcapacitors of the corresponding VSCs can be connecteddirectly in parallel.2.2. Voltage Sourced Converters for HVDCp'USThe world of converters might be divided into two groupsthat are to be distinguished by their operation principle. Onegroup needs an AC system to operate. The AC systemvoltage forces the current to commutate from one phase toanother. Using controlled semiconductors like thyristors, thepoint-on-wave for commutation can be chosen and thus, thepower exchanged between AC and DC system via theconverter can be controlled. Converters that rely on an ACsystem to operate are called Line Commutated Converters.Conventional HVDC systems employ Line CommutatedConverters.The second group of converters does not need an AC systemto operate and is therefore called Self CommutatedConverters. Depending on the design of the DC circuit thisgroup can further on be devided into Current SourceConverters and Voltage Sourced Converters (VSC). ACurrent Source Converter operates with a smooth DC currentprovided by a reactor, while a VSC operates with a smoothDC voltage provided by the storage capacitor.Among the Self Commutated Converters it is especially theVSC that has a long history in the lower power range forindustrial drive applications. Operational experience and abroad market of semiconductors might be two important

& Station BT

Fig. 1: Main circuit diagram of a bipolarHVDCp'" transmission system0-7803-6420-1/00/$10.00 (c) 2000 IEEE 71 6


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I T -r i 1 1 I1T

Fig. 2: Equivalent circuit of a ipulse IGBT based2level VSCfactors promoting VSC to be used in high voltageapplications. Static Compensators (STATCOM) like theflOO Mvar "V A Statcon [l ] or the 320 MVA U nified PowerFlow Controller (UPFC) [2] are examples for a new group ofso-called Flexible AC Transmission Systems (FACTS)devices that have been successfully operating in high voltagepower systems for years.Introducing VSC techn ology into HVD C transmission has ledto the innovative HVDCP'" transmission system [3].Fig. 2 shows an equivalent diagram of a VSC as to be usedfor HVDCP'"s transmission system s. It con sists of a 6pulsebridge equipped with IGBTs and a anti-parallel connected6pulse bridge equipped with free wheeling diodes. Basically,either GTOs or IGBTs can be used as semiconductorswitches. To ach ieve the required power rating, a number ofdevices is connected in series. GTO valves allow highercurrents but less DC voltage than IGBT valves.For HVDCP'"' long distance transmission systems hightransmission voltages are advantageous to keep transmissionlosses low. This favors the use of IGBT valves. With thepresent design and technology, IGBT valves can block up to150kV. A VSC equipped with these valves can cany up to800A (RMS) AC line current. This results in a power ratingof approximately 140MVA of one VSC.The basic function of a VSC is to convert the DC voltage of

1 1 1

Fig. 4: Equivalent circuit of a ipulse IGBT based3level VSC

Fig. 3: Basic principle of a 2level VSC

the storage capacitor into AC voltages. Fig. 3 illustrates thebasic operating principle. Th e polarity of the DC voltage ofthe converter is defined by the polarity of the diode rectifier.The IGBTs can be switched on at any time by appropriategate voltages. However, if one IG BT of a branch is switchedon, the other IGBT must have been switched off before toprevent a short circuit of the storage capacitor. Reliableconverter interlock functions will preclude unwantedswitching of IGBTs. A lternating switching the IGBTs of onephase module as shown in Fig. 3 successively connects theAC Terminal of the VSC to the plus tapping and the minustapping of the DC capacitor. This results in a stair steppedAC voltage comprising two voltage levels: +Vdc/2 and -VdcD. A VSC as shown in Fig. 3 is therefore called a 2levelconverterDue to a switching frequency, that is conside rably higher thanthe AC system power frequency, the wave shape of theconverter AC current will be controlled to be very sinusoidal.This is achieved by special Pulse Width Modulation'(PWM )methods.Besides the 2level converter, so-called 3 level converters havebeen used for high power applications. Fig. 4 shows aprinciple equivalent of such a converter.The 3level converter comprises 4 valves in one phase leg asshown in Fig. 5 . The switching rule is that only two valvesthat are directly connected in series can be switched on at atime, i.e. S1 and S2 or S2 and S3 or S3 and S4 . Switching on

1

Fig. 5: Basic principle of a 3level VSC

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S1 and S2 connects to the plus tapping, S2 and S3 connectsvia the clamping diodes to the mid-point tapping (MP), 53and S4 connects to the minus tapping of the DC circuit to theAC terminal of the phase leg. Thus, he resulting voltage onthe AC terminal comprises three levels instead of two in caseof a 2level converter.A three level VSC provides significant better performanceregarding the total harmonic voltage distortion 0).However, the more complex converter layout resulting inlarger footprint and higher investment costs make 2levelconverter technology the preferred solution for HVDCp'"from today's point of view.2.3. TransformersNormally, the converters are connected to the AC system viatransformers. The most important functions of thetransformers are:1. to provide a reactance between converter AC terminalsand AC system2. to transform the voltage of the AC system to a valueoptimized to the converter voltages and currents3. to connect two single 6pulse converters to form a12pulse group4. to connect two single converters with different DCpotentials to groundHowever, in some cases functions 2 to 4 might not be anissue for the design concept of a HVDCp'uJ system. If, forexample, functions 2, 3 and 4 do not apply, because thepresent AC system voltage matches the converter voltagesand currents, and there is just one converter per stationnecessary to transmit the required power, then function 1 canbe taken by a reactor coil leading to a more simple stationdesign and considerable cost savings.Transformer or reactor are exposed to the converter ACvoltage which has a rectang ular wave shape as shown in Fig.3. A 2level converter will frequently change the potentialbetween the phases from zero to V,, within a couple ofmicroseconds. Steep voltage steps like these result in highstresses of the insulation materials. A low-pass filterconnected to the AC terminals of the converter might beused, to lower dv/dt values contributing to an economicdesign of the converter station.

ideal AY

Fig. 6: Equivalent circuit to calculate fundamental loadflow between VSC an d AC system

3. DESIGNCONSIDERATIONS3.1. Steady State CharacteristicsFig. 6 shows a simplified single line diagram to calculate thefundamental load flow between AC system and one of theHVDCP'"' VSCs. The AC system as well as the converter areconsidered voltage sources. These voltage sources areconnected via the transformer, represented as an idealtransformer and a leakage impedance. While magnitude andphase angle of the AC system voltage or& espectivelyis determined by the load flow of the AC system, magnitudeand phase angle of the co nverter voltage& an be adoptedby the converter control. According to Kirchhoffs law,controlling relatively to& esults in a correspondingvoltage drop across the transformer leakage impedanceXT. eglecting losses, the current I through X, lags by90"el.. As a consequen ce, controlling determines thecurrent I.Fig. 7 hows the relations between voltages and currents foran operating point as an example. The current 1 can be splitup regarding& nto one com pon ent that is in parallel andinto another component that is in quadrature.The active and reactive power exchange as seen from the ACsystem terminals of the transformer can be calculatedaccording to the following formulas

with

VN: present magnitude of the AC system voltageVN,,: agnitude of the AC system nominal voltageVa,,:present magnitude of the converter AC voltagePN: present active power exchange as seen from the ACsystemQN: present reactive power exchange as seen from the ACsystemSbT: rated apperent power of the transformerConsidering a certain AC system voltage vN, the operatingrange of a HVDCp'" station regarding active and reactive

Fig. 7: Vector diagram (Example)

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power ca n be visualized using a PQ -diagram as shown in Fig.8. Keeping the angle a onstant while varying h results instraight lines as shown for& nd G. Keeping h constantwhile varying a esults in concentric circles as shown forA,-, k1.0 and L.he power rating of the station ismarked by the circle IpN+jqNl=l.Opu. Adjusting theparameters h and a ccordingly, the VSC c an operate at anyoperating point on the circle area continuously. It cantherefore control active and reactive power exchangeindependently from each other. If there is no active powertransfer required, a station still can serve as a STATCOMproviding capacitive or inductive reactive power supp ort tothe AC system it is connected to.3.2. HarmonicsLike all power electronic converters, VSCs generateharmonic voltages and currents in the ac and dc systemsconnected. In a simplified manner, from the ac system. aVSC an be considered a h armonic voltage source behind theimpedance of the converter transformer. In the same way, aVSC can be considered a harmonic current source connectedin parallel to the storage capacitor. This behavior is justopposite to those of conventional line commutatedconverters.Harmonics generated depend on:00

0 the pulse pattern appliedUsing 12pulse configuration instead of 6pulse will improveharmonic conditions both on AC and DC side. CharacteristicAC side harmonics will have the ordinal numbersv A ~ 1 2 x t 1 ;=1,2, ...,characteristic DC harmonics will have the ordinal numbersvD p12n ; n=1,2, ...All other harmonics will be cancelled out under idealconditions.A high switching frequency is advantageous to reduceharmonic generation. Switching frequencies of some lrHzallow to use simple Pulse Width Modulation (PWM)methods like delta-sinus modulation. This modulationmethod typically results in harmonics with ordinal numbersin the range of the switching frequency and multiples thereof.However, it has to be considered, that operation losses of theconverters increase in direct proportion to the switchingfrequency applied. High operation losses not only reduce thepower available on the inverter side of the W D Ctransmission, they also warm up the converter powerelectronics and thus might reduce the fundamental powertransfer capability.Sophisticated PWM methods allow to eliminate criticalharmonics employing special optimized pulse patterns. Inmost cases, such special designed pulse patterns allow theconverters to b e operated at lower sw itching frequencies thanwould be necessary in case of delta-sinus modulation to meet

the station topolog y (e.g. 6 pulse or 1 2pulse)the switching frequency of the IG BTs

Fig. 8: PQ-diagram of a HVDCp'"'stationharmonic performance requirements. Switching frequency,pulse patterns and operation losses are therefore seen as keyissues of the design optimization process of a HVDCP'"transmission scheme.Due to its inherent harmonic elimination capability theharmonic interference of VSC converter is rather small incomparison to the conventional line commutated converters.However, harmonic filters might be necessary on the ac anddc sides depending on the harmonic performancerequirements both for ac and dc side, ac system harmonicimpedance, dc lindcable imped ance and loss evaluation.3.3. Insulation CoordinationSurge arresters are used to protect the HVDCp'"' powercompon ents against transien t over-voltages such as lightningand switching surges. The following arresters shall benormally installed on AC- and D C-side respectively:a) AC-side arresters:- AC bus arrester- AC filter arrester if applicableb) DC-side arresters:- DC line arrester- Neutral bus arrester- ,DC filter arrester if app licableCalculation and selection of the Maximum ContinuousOperating Voltage (MCOV) of the arresters is based on themaximum voltage which can appear during continuousoperation. Some extra margin is added to cover temporaryoperating conditions, measuring tolerances etc.The arrester ratings are derived as maximum values fromseveral fault cases. Th e insulation strength of equipment forlightning and switching stresses are chosen on basis ofcalculated over-voltages. The minimum margins used fordetermination of withstand levels for each equipment arechosen in accordance to practice on conventional HVDC andto IEC 60071-5.

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4. TRANSIENTTRESSESSimilar to any other equipment in power systems,componen ts of an HVDC p'us system m ight be exp osed totransient voltage and current stresses as a result of severalfault scenarios. Fig. 9 shows some characteristic fault casesthat are regarded for the design of a WD CP '" system. Faultsare to be distinguished regarding the place they could happenbetween external faults (faults outside the transmissionscheme, within the AC system) and internal faults (faultsinside the transmission sch eme).Regarding external faults, the concept is not only to protectthe equipment but also to provide best support to the faultyAC system. Depending on the fault clearing strategy, thissupport might include dynamic voltage control and poweroscillation damping after voltage recovery. A HVDCp'"sstation can take advantage of the fast reactive power controlcapability combined with the possibility to feed or absorbactive power.Internal faults will be precluded to the largest extent possibleusing appropriate design margins for any component and asafe station layout. However, in the event of an intemal fault,all components will be protected by fast acting protectionsystems.

5. SUMMARYThe terminology HVDCPlw represents the latest technologyon high power transmission and distribution applications.Innovative VSC allow high DC vo ltages of up to 150kV perVSC and AC currents up to 800A. This results in a veryeconomic design of a f 1 5 0 kV bipolar transmission systemfor power ratings up to 20 0 MW .A HVDCP'" system not only transmits active power from onesystem to the other but also comprises STATCOMfunctionality. Limited only by the power rating of a stationthe system can freely control the reactive power exchangewith the AC systems. That saves extra investment oncapacitor banks or SVC, that would be necessary in case ofconventional HVDC systems to compensate the reactivepower demand of the thyristor controlled converters.The VSCs are equipped with modem IGBTs. An IGBTcombines the advantages of a GTO in terms of high powerswitching capability with those of a transistor regardingcontrollability of voltages and currents both in On- nd Off-state. This results in a robust con verter design and con tributesto a high reliability of the overall transmission scheme.The described HV DCPLU S ransmission system pro videsattractive technical advantages for applications likeinterconnecting (point to point or multi-terminal) very weakac systems in the lower and middle power range.

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Fig. 9: Decisive fault scenarios6. BIOGRAPHY

Norbert Christl, born 1954 in Vienna, Austria, received hisDiploma in Electrical Power Engineering from University ofVienna. Since 1980 system engineering and studies ofvarious HVDC schemes with BBC Germany andSwitzerland. In 1988 he joined Siemens AG in Erlangen,Germany and is currently responsible for the marketing,business development and sales of IGBT based HVDCP'"systems.Dr. Hartmut Huanq, born 1963, received his Dip1.-Ing.degree and PhD in Electrical E ngineering from University ofBraunschweig, Germany, in 1 986 and 1992, respectively. Hejoined Siemens AG in Erlangen, Germany, in 1 992. Currentlyhe is responsible for the basic design and system studies forHVDC systems.Frank Schettler, born 1964, received his Dip1.-Ing. degree inElectrical Engineering from the University Ilmenau,Germany, in 1992. He joined Siemens AG in Erlangen,Germany, in the same ear. Currently, he is working on thebasic design of HVDCp s transmission systems.

7. REFERENCES[l ] C. Schauder et al.: "Operation of fl00Mvar TVAStatcon", IEEE Transactions on Power Delivery,Vol. 12, No. 4, October 199 7, Pages 1805-1811

AS. Mehraban et al.: "Application of the World'sfirst UPFC under practical Constraints" RE, ESWinter Power Meeting, 1997, Paper No. PE-511-

A. Koch et al.: "Advanced Technologies for PowerTransmission and Distribution - Benefits and Impactof Innovations", Beijing, October 18-21 1999,Electric Power International Conference onTransmission and Distribution, Proceedings, PaperNo. 24

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PWRD-0-11-1996[3]

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2.Hvdc TX Using Vsc-Design and Application