• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
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    • 专利摘要:
    The invention is concerned with a voltage source converter and converter station comprising a voltage source converter as well as to a method and computer program product for controlling current on an alternating current side of the voltage source converter, The Voltage Source Converter (14) converts between alternating current, AC, and direct current, DC, has a galvanic link (L) to an AC system (ACN) and comprises a number of phase legs, each comprising a set of converter valves connected to a phase of the AC system via a corresponding galvanic connection of the galvanic link, and a control unit operative to control the converter valves to generate AC waveforms and to reduce at least one component of a zero uence current on the galvanic link (L).
    公开号:SE1650854A1
    申请号:SE1650854
    申请日:2016-06-16
    公开日:2017-12-17
    发明作者:Ramya KALLURI Sri；Kasal Gaurav-Kumar
    申请人:Abb Schweiz Ag；
    IPC主号:
    专利说明:

    lO HANDLING OF ZERO SEQUENCE CURRENTS IN A VOLTAGE SOURCECONVERTER FIELD OF INVENTION The present invention relates to a voltage source converter and converterstation comprising a voltage source converter as well as to a method andcomputer program product for controlling current on an alternating current side of the voltage source converter.
    BACKGROUN D Voltage source converters are often used for converting betweenalternating current (AC) and Direct Current (DC), such as between three-phase AC and DC. A converter is then typically provided in a converterstation and made up of a number of phase legs, one for each phase, whereeach phase leg comprises two phase arms, an upper and a lower phase HIITI.
    It is in some power transmission situations of interest to remove thetransformer with which the voltage source converter is connected to an ACpower network. There may exist many reasons for this. One reason may bethat the converter already converts to the voltage used by the AC powernetwork. There may thus not be any need for any further adaption. Anadditional transformation may therefore be unnecessary from a voltagelevel adaption point of view. Furthermore, a transformer may in thesecases also be big, bulky and expensive and it would therefore also be ofinterest to remove it for this reason. Another reason for removing the transformer is for limiting losses.
    There may thus exist several reasons for removing a transformer. lO A converter station comprising a converter that is in this way connected toan AC system is often denoted a transformerless converter station. Oneexample of a transformerless converter station is given in WO 2013/044940- However, the removal of the transformer may lead to other problems. As iswell known an AC voltage is made up of a fundamental frequencycomponent and a number of harmonics components. Due to theinteraction of a second harmonic component and the fundamentalcomponent inside each phase arm, a third harmonic differential modevoltage is generated in the converter arms for a transformerless converterstation. This voltage appears as third harmonic zero sequence currents inthe converter phase currents. In a transformer based converter station, thetransformer blocks this current from entering into the AC power network,hence it creates no problem. However, in a transformerless converterstation, this current has to be controlled through some other means toavoid unwanted harmonic injection into the grid. Similarly, Zero SequenceCurrents of fundamental frequency can occur in few other cases such as due to a single-line-to ground (SLG) fault in the AC network.
    The present invention is directed towards reduction of such zero sequence currents.
    SUMMARY OF THE INVENTION The present invention is directed towards reducing zero sequence currents.
    This object is according to a first aspect of the present invention achievedthrough a Voltage Source Converter converting between alternatingcurrent, AC, and direct current, DC, and having a galvanic link to an AC system the voltage source converter comprising: lO a number of phase legs, each comprising a set of converter valvesconnected to a phase of the AC system via a corresponding galvanicconnection of the galvanic link, and a control unit configured to control the converter valves to generate (22)AC waveforms and to reduce at least one component of a zero sequence current on the galvanic link.
    This object is according to a second aspect achieved through a converterstation comprising a galvanic link and a voltage source converter according to the first aspect.
    The object is according to a third aspect achieved through a method ofcontrolling current on an AC side of a voltage source converter, the voltagesource converter converting between alternating current, AC, and directcurrent, DC, and having a galvanic link to an AC system, the convertercomprising a number of phase legs, each comprising a set of convertervalves connected to a phase of the AC system via a corresponding galvanicconnection of the galvanic link, the method being performed in a controlunit of the voltage source converter and comprising: controlling the converter valves to generate AC waveforms and to reduce at least one component of a zero sequence current on the galvanic link.
    The object is according to a fourth aspect of the present invention achievedthrough a computer program product for controlling current on an AC sideof a voltage source converter, the voltage source converter convertingbetween alternating current, AC, and direct current, DC, and having agalvanic link to an AC system, the converter comprising a number of phaselegs, each comprising a set of converter valves connected to a phase of theAC system via a corresponding galvanic connection of the galvanic link, thecomputer program product comprising a data carrier with computerprogram code configured to cause a control unit of the voltage source converter to lO control the converter valves to generate AC waveforms and to reduce at least one component of a zero sequence current on the galvanic link.
    The present invention has a number of advantages. It enables reduction ofundesirable components of a zero sequence current. It can be incorporatedinto existing control with a limited amount of modifications. It can beimplemented without the addition of further hardware such physical filters, common mode chokes etc. Also the control strategy may be simple.
    BRIEF DESCRIPTION OF THE DRAWINGS The present invention will in the following be described with reference being made to the accompanying drawings, where fig. 1 schematically shows a DC power transmission system comprising aDC power line between two converter stations both being transformerlessand each connected to a corresponding AC power network, fig. 2 schematically shows a first converter of a first converter station, fig. 3 schematically shows a control unit for controlling the first converter,fig. 4 schematically shows a first realization of a first zero sequence currentcontrol module in the control unit, fig. 5 schematically shows a zero sequence current forming module in thecontrol unit, fig. 6 schematically shows an alpha transforming section of a DQtransforming block in the zero sequence current control module of fig. 4,fig. 7 schematically shows a first DQ transforming section of the DQtransforming block in fig. 4, fig,. 8 schematically shows a second DQ transforming section of the DQtransforming block in fig. 4, fig. 9 schematically shows a first DQ control section of a DQ control blockin fig. 4, fig. 10 schematically shows a second DQ control section of the DQ control block in fig. 4, lO fig. 11 schematically shows a wave forming control module of the controlunit together with a second variation of the first zero sequence controlblock and a second variation of the second zero sequence control block,and fig. 12 schematically shows a computer program product in the form of adata carrier comprising computer program code for implementing the control unit; DETAILED DESCRIPTION OF THE INVENTION In the following, a detailed description of preferred embodiments of the invention will be given.
    Fig. 1 shows a single line diagram of a simplified Direct Current (DC)power transmission network DCN comprising a first converter 14, a secondconverter 16 and a DC power line 18. The first converter 14 may here beconnected to a first alternating current (AC) power network ACN1, forinstance a network where energy is generated, and the second converter 16may be connected to a second AC power network ACN2, for instance anetwork where energy is consumed. The AC power networks may be three- phase networks.
    The DC power line 18 may here be a power line covering a long distance for transferring power.
    The first converter 14 may more particularly be provided in a firstconverter station 1o and the second converter in second converter station12, where the first converter station 1o also comprises a first link L1 to thefirst AC power network ACN1 and the second converter station 12comprises a second link L2 to the second AC power network ACN2. Eachlink furthermore comprises connections, one for each phase, to thecorresponding AC power network. Thereby the first converter 14 has a first link L1 to the first AC power network ACN1 and the second converter lO 16 has a second link to the second AC power network ACN2. The first andsecond links L1 and L2 are furthermore galvanic links and thereby theconnections of the links are galvanic connections. A galvanic link is a linkin which, at least during steady state operation, all elements are electricallyand physically in contact with each other. Each element also has a physicaland electrical path within itself. Thereby there are no gaps in the electricalpath, such as magnetic or capacitive gaps. However, the galvanic link maycomprise a circuit breaker. This may be opened in case of faults in forinstance the DC system and the first converter. However, during steady state operation such a circuit breaker is closed.
    Thereby there is no transformer in any of the first and second galvanic links L1 and L2. The converters are thereby both transformerless.
    In the following the first converter 14 and the first galvanic link L1 will bedescribed. It should however be realized that this description may just aswell be provided for the second converter 16 and the second galvanic linkL2.
    Fig, 2 shows one way of realizing the first converter 14. The first converter14 is a three-phase voltage source converter for converting between AC andDC and furthermore has a DC side facing the DC power network DCN andan AC side facing the first AC power network ACN1, where the AC side isconnected to the first galvanic link L1. The first converter 14 thereforecomprises three phase legs PL1, PL2 and PL3, for instance connected inparallel between a first and a second DC terminal DC1 and DC2, where thefirst DC terminal DC1 may be connected to a first pole of the DCtransmission network and the second DC terminal may be connected to asecond pole of the second DC transmission network or to ground. Eachphase leg furthermore comprises a pair or set of converter valves. The firstphase leg PL1 therefore comprises a first and a second converter valveCVA1 and CVA2, the second phase leg comprises a first and a second converter valve CVB1 and CVB2 and the third phase leg PL3 comprises a lO first and a second converter valve CVC1 and CVC2. The mid points of thephase legs are connected to corresponding AC terminals AC1, AC2, AC3,where each AC terminal is connected to a corresponding galvanicconnection or phase of the galvanic link. A phase leg is in this exampledivided into two halves, a flrst upper half and a second lower half, wheresuch a half is also termed a phase arm. Thereby the phase legs areconnected to the phases of the first AC power network via corresponding galvanic connections of the first galvanic link.
    The first DC pole furthermore has a first potential that may be positive.The first pole may therefore also be termed a positive pole. A phase armbetween the first DC terminal DC1 and a first, second or third AC terminalAC1, AC2 and AC3 may be termed a first phase arm or an upper phasearm, while a phase arm between the first, second or third AC terminal AC1,AC2 and AC3 and the second DC terminal DC2 may be termed a secondphase arm or a lower phase arm. The phase arm mid points arefurthermore connected to the AC terminals via phase reactors LAC1, LAC2and LAC3.
    Moreover, the upper phase arms are joined to the first DC terminal DC1via a corresponding first or upper arm reactor LA1, LB1 and LC1, while thelower phase arms are joined to the second DC terminal DC2 via a second or lower arm reactor LA2, LB2 and LC2.
    The first voltage source converter 14 may be a two-level converter, whereeach converter valve is made up of a number of series connected switchingunits. Alternatively the converter may be a modular multilevel converterwhere each converter valve is formed through a series-connection of anumber of cells, where a cell may be a half-bridge cell or a full-bridge cell.A cell then comprises one or two strings of series connected switchingunits in parallel with an energy storage element like a capacitor. A switching unit may be realized in the form a transistor with anti-parallel lO diode. However, it is also known to be realized using other types of semiconducting units.
    It should here also be realized that there exist countless variations ofvoltage source converters, where a converter may for instance be an n-levelconverter, such as a neutral point clamped three-level converter. Also amodular multilevel converter may be made up of a number of differenttypes of cells. There may also exits hybrid converters that use cells in an n- level environment.
    There is finally a control unit 20, Which controls the operation of theconverter 14 and more particularly controls each converter valve. In thefigure only the control of the first convert valve CVA1 of the first phase legPL1 is indicated. However, the control unit 20 is provided for controllingall the phase arms of the converter. Therefore, in order to simplify thefigure only the control of the upper phase arm of the first phase leg PL isindicated. It should thus be realized that all converter valves are controlledby the control unit 20. The control unit 20 may be implemented through acomputer or a processor with associated program memory. The controlunit controls the converter valves to generate AC waveforms and to reduceat least one component of a zero sequence current on the first galvanic linkL1.
    Fig. 3 shows a block schematic of one way of realizing the control unit 20.The control unit 20 comprises a waveform control module WFC 22, a firstzero sequence control module ZSC1 24, a second zero sequence controlmodule ZSC2 26 and a zero sequence current forming module ZSCF 28.Here it may be realized that it is possible to omit one zero sequence control module.
    As mentioned earlier the converter 14 is transformerless. There is thus no transformer between the first AC system ACN1 and the converter 14. This lO has a number of advantages. However, it may also give rise to some problems.
    The phase arms of a phase leg generates a waveform on the correspondingAC terminal, which waveform is to provide an AC voltage that is suppliedto the first AC network ACN1.
    Due to the interaction of second harmonic component and fundamentalcomponent of the voltage generated inside each arm, a third harmonicdifferential mode voltage is generated in the converter arms. This voltageappears as third harmonic zero sequence currents in the converter phasecurrents, i.e. in the phases or galvanic connections of the first galvanic linkL1. In a transformer based converter station, the transformer blocks thiscurrent from entering into the first AC network ACN1, hence it creates noproblem. However, in a transformerless converter station, this current hasto be controlled through some other means to avoid unwanted harmonicinjection into the grid. Similarly, Zero Sequence Currents of fundamentalfrequency can occur in few other cases such as if there is a single-line toground (SLG) fault in the AC network.
    Aspects of the invention are therefore directed towards controlling theconverter so that such zero sequence currents are reduced or even eliminated, How this may be done will now be described.
    The waveform control module 22 of the control unit 20 controls theconverter valves so that an AC waveform is generated on each AC terminalAC1, AC2, AC3, where the waveform on an AC terminal may in a knownfashion be separated by 120 degrees from the waveforms on the other ACterminals. In this control it is possible that the waveform control module22 performs pulse width modulation control, such as Sinusoidal PulseWidth Modulation (SPWM) or 3PWM. In doing this a converter valve may lO lO receive a control signal representing a voltage level that it is desired to output by the converter valve.
    However, as was mentioned above, the control performed by the waveformcontrol module 22 may also cause the generation of zero sequence currentson the galvanic link L1 that may be desirable to cancel out. It may forinstance be desirable to cancel out certain components of the zerosequence current, such as the fundamental frequency component and/ orthe third harmonic frequency component. The first zero sequence controlmodule 24 is provided for reducing one such component, as an examplethe fundamental frequency component, while the second zero sequencecontrol module 26 is provided for reducing another component of the zerosequence current, for example the third harmonic component. It is thus possible to reduce the fundamental and/ or the third harmonic component.
    The waveform control module 22 thus controls the set of converter valvesof each phase leg to generate an AC waveform on the galvanic connectionsof the galvanic link, while at least one and with advantage both of the zerosequence control modules 24 and 26 reduces at least one component of azero sequence current appearing on the same galvanic link. If both zerosequence control modules 24 and 26 are used then two frequencycomponents are removed, where as was mentioned above two components may be the fundamental and third harmonic components.
    Now a first way of reducing the fundamental frequency component will be described with reference being made to fig. 4 - 1o.
    Fig. 4 schematically shows a first realization 24A of the first zero sequencecurrent control module 24. This first realization 24A of the first zerosequence control module comprises a DQ transforming block DQT 30, aDQ control block DQTC 32 and a Zero Sequence Voltage transformingblock ZSVT 34. It should be realized that a first variation of the second zero sequence control module may be implemented in essentially the same lO ll way. The difference between the modules is essentially only the settings made in relation to frequency due to which component is being reduced.
    Fig. 5 schematically shows the zero sequence current forming module 28,fig. 6 schematically shows an alpha transforming section 38 of the DQtransforming block 30 in the zero sequence current control module, fig. 7schematically shows a first DQ transforming section 44 of the DQtransforming block 30, fig. 8 schematically shows a second DQtransforming section 52 of the DQ transforming block 30, fig. 9schematically shows a first DQ control section 60 of the DQ control block32 and fig. 10 schematically shows a second DQ control section 68 of theDQ control block 32.
    The operation starts by measuring the currents in the first galvanic link L1,such as in the individual phases or galvanic connections of the galvaniclink L1, thereby a first phase current iva1, a second phase current ivb and athird phase current ivc are being obtained. Measurements may be madeusing any suitable current sensor, such as a current transformer. Thesecurrents are then added together by a first adding element 36 of the zerosequence current forming module 28 in order to form a zero sequence current ivzero.
    Thereafter the zero sequence current ivzero is transformed from an abc ora static frame into a dq frame using single phase dq transformations,where d denotes direct and q quadrature. This transforming is performedby the DQ transforming block 30. The transforming first involves creatingan imaginary signal by shifting the original zero sequence current signalivzero by 90 degrees and this together with the original signal form alpha-beta waveforms. Thereafter the two alpha-beta waveforms are furthertransformed into the dq frame. In order to achieve this the zero sequencecurrent ivzero is supplied to the alpha forming section 38, where it isamplified with unity by a first amplifying element 40 in order to obtain an alpha component ivzeroa of the current. The current ivzero is also supplied lO l2 to a phase shifting element 42, which shifts the phase of the current byninety degrees, i.e. amplifies the current with exp (-j sft/ 2) in order toobtain the beta component ivzeroß. The alfa and beta components ivzeroaand ivzeroß are then both supplied to the first and second DQtransforming sections 44 and 52, where the first DQ transforming section44 provides the d component of the zero sequence current, while the DQ transforming section 52 provides the q component or representation.
    The alpha component ivzeroa is in the first DQ transforming section 44received by a first multiplying element 46, while the beta componentivzeroß is received by a second multiplying element 48. In the firstmultiplying element 46 the alpha component is multiplied with cos9, whilein the second multiplying element 48 the beta component is multipliedwith sinö. The two products are then added together in a second addingelement 50 in order to provide the d component ivzerod or representation of the zero sequence current.
    In a similar manner, the alpha component ivzeroq is in the second DQtransforming section 52 received by a third multiplying element 54, whilethe beta component ivzeroß is received by a fourth multiplying element 48.In the third multiplying element 54 the alpha component is multipliedwith sin9, while in the fourth multiplying element 56 the beta componentis multiplied with cos9. The product produced by the third multiplyingelement 54 is then subtracted from the product produced by the fourthadding element 56 in a first subtracting element 58 in order to provide the q component or representation ivzeroq of the zero sequence current.
    The angle 9 depends on which zero sequence component that istransformed. In case the component is the fundamental component thenthe angle 6 is the phase angle of the fundamental voltage of the phase atthe galvanic link. In case the component is the third harmonic component,then the angle would be the fundamental voltage phase angle multiplied bythree. lO 13 It can thereby be seen that each component of the zero sequence currentthat it is desirable to reduce is transformed from an abc frame to a direct and quadrature frame.
    After the d and q currents ivzerod and ivzeroq have been obtained theseare then provided to the first and second DQ control sections 6o and 68 ofthe DQ control block 32.
    The d component ivzerod is here provided to an optional first low passfilter 64, which low pass filters the d component and provides the low passfiltered d component to a negative input terminal of a second subtractingelement 62. The second subtracting element 62 also has a positiveterminal on which a reference current ivzerod* is received. The secondsubtracting element 62 subtracts the filtered current ivzerod from thereference current ivzerod* and provides the difference to a first controller66, which may be a PI controller performing proportional and integratingcontrol with respect to the difference in order to provide a d control signalrd.
    In a similar manner the q component ivzeroq is provided to an optionalsecond low pass filter 72, which low pass filters the q component andprovides the low pass filtered q component to a negative input terminal ofa third subtracting element 7o. The third subtracting element 70 also has apositive terminal on which a reference current ivzeroq* is received. Thethird subtracting element 70 subtracts the filtered current ivzeroq from thereference current ivzeroq* and provides the difference to a secondcontroller 74, which may also a PI controller performing proportional andintegrating control with respect to the difference in order to provide a q control signal rq.
    It can thereby be seen that at least proportional and with advantage both proportional and integrating control is performed on the zero sequence lO 14 current component that is to be reduced. It can furthermore be seen thatthe control is performed on both the q and d representations of the zero sequence current component.
    In this embodiment, the ivzerod* and ivzeroq* reference currents are set tozero in order to eliminate the zero sequence current component. This inturn means that the signals input to the first and second controller 66 and 74 are the negative d and q components -ivezerod and -ivezeroq.
    The control signals rd and rq may with advantage be combined with thewaveform control of the phase arms and therefore the control signals maybe in the form of voltages. However, before such combination can be done,it may be necessary to convert the control signals rd and rq. The controlsignals rd, rq may more particularly be converted back to the originallyused abc frame in the zero sequence voltage transforming block 34through inversely transforming the dq components. It is thus the reverseprocesses compared to the activities performed by the DQ transformingblock 30. It may thus involve transformation from the dq to the alfa-betaframe and from the alfabeta frame to the abc frame. Through this reverseprocess one control signal in the abc plane is obtained, which controlsignal may then be combined with a control signal used by the waveformcontrol module for controlling a phase arm. The control signal may thusbe combined With the arm modulation indices, i.e. the different waveformcontrol signals used by the waveform control module 22. The controlsignal may more particularly be combined with all such control signals. Itmay as an example be added to the upper arm control signals andsubtracted from the lower arm control signals used by the waveform control module 22 It can in this way be seen that the zero sequence current component is controlled to zero using PI control. lO The second zero sequence control module may be controlled in the same way in order to reduce the other component of the zero sequence current.
    It is in this way thus possible to remove harmonics generated by theconversion process as well as harmonics caused by faults such as the zerosequence fundamental currents which appear in case of an SLG fault. Thecomponents that are of interest to be reduced may thus be reduced to zero, i.e. removed.
    - The invention provides a method to control the zero sequence components in the converter phase currents of an HVDC system.- The control can be applied to ZS currents of any frequency.
    - The zero sequence current is converted into dq frame of reference and then a controller is implemented.
    - Single phase dq transformation is used - It can be incorporated into existing control without manymodifications.
    - No additional components are required (physical filters, common mode chokes etc.) - A simple first order PI controller is sufficient to control the current The transformation into the dq frame is one Way of realizing the control.In the first embodiment a PI control into the dq frame was used. As analternative it is possible use PR control without transformation into the DQ frame.
    A second embodiment where this is done will now be described. lO 16 In this second embodiment there is also formed a zero sequence current. Azero sequence current forming block may thus be used, which may becommon for both zero sequence current forming modules. The zerosequence forming block may be realized in the same way as in the firstembodiment. Therefore, as a zero sequence forming block has alreadybeen described, it is here omitted from the description of the second embodiment.
    One example of a control unit where a zero sequence current formingblock has been omitted is shown in fig. 11. Here there is a first forwardcontrol path 22A, which is a first control branch of the waveform controlmodule 22 and a first feedback control path 22B, which is a second controlbranch of the waveform control module 22. The first forward control path22A and first feedback control path 22B are thus parts of the conventional waveform control.
    There is also a second feedback branch 24B which is a second realizationof the first zero sequence control module 24 and a third feedback path26B which is a second realization of the second zero sequence control module 26.
    The feedforward path receives the zero sequence voltage UFzero of the reference PWM waveform of the phase arms, performs proportional andresonance control in a third controller 88, which is thus a PR controller,where the resonance control is performed at the fundamental frequency.The third controller 88 then provides the result to the positive input of a third subtracting element 90.
    The zero sequence current ivzero is here, in the first feedback path 22B,provided to a fifth multiplying element 84, where it is multiplied with aconstant A1pa_C_Mul_L and the product supplied to a third addingelement 86. The constant Alpha_CMul_L corresponds to the bandwidth of lO l7 the controller and is set based on what frequencies the controller responds to.
    The zero sequence current ivzero is furthermore supplied, in the secondfeedback path 24B, to a fourth controller 80, which may be a PR controllerperforming proportional and resonance control at the desired frequency,which in this case is the fundamental frequency of 50 Hz. The result is alsoin this case multiplied With the constant Alpa_C_Mul_L in a sixthmultiplying element 82 and supplied to the third adding element 86. It canthus be seen that at least proportional and with advantage proportionaland resonance control is performed on the zero sequence current component to be reduced.
    The zero sequence current ivzero is also supplied, in the third feedbackpath 26B, to a fifth controller 76, Which may be a PR controller performingproportional and resonance control at the desired frequency, which in thiscase is the frequency of the third harmonic, i.e. 150 Hz, where the results ismultiplied in a seventh multiplying element 78 With the constant10*Alpa_C_mul_L and supplied to the third adding element 86. Since,the third feedback path 26B is provided controlling the 3rd harmoniccomponent, the constant is set it to a higher value which is approximately10 times the bandwidth for fundamental frequency. It can also in this casebe seen that at least proportional and with advantage proportional andresonance control is performed on the zero sequence current component to be reduced.
    The outputs of the three feedback paths 22B, 24B and 26B are therebysummed in the third adding element 86 and this sum is then supplied to anegative terminal of the fourth subtracting element 90, Where the sum issubtracted from the output of the proportional controller 88. Thedifference is then provided as a zero sequence voltage referenceuv_zero_ref, Which is then used for forming an arm reference for controlling a converter valve of a phase arm. lO 18 As the feedforward and first feedback paths together provide theconventional control signal used for waveshaping, it can be seen that theresults of the proportional and resonance control performed on each zerosequence current component to be reduced is combined with a zero sequence control signal used for waveform shaping.
    It can in this way be seen that it is possible to implement the controlwithout transformation to the dq frame and back, which increases thespeed of the control. The control also provides a substantial reduction of the zero sequence current.
    The control units may be realized in the form of discrete components.However, it may also be implemented in the form of a processor withaccompanying program memory comprising computer program code thatperforms the desired control functionality when being run on theprocessor. A computer program product carrying this code can beprovided as a data carrier such as one or more CD ROM discs or one ormore memory sticks carrying the computer program code, which performsthe above-described control functionality when being loaded into a controlunit of a voltage source converter. One such data carrier in the form of a CD Rom disk 92 carrying computer program code 94 is shown in fig. 12.
    From the foregoing discussion it is evident that the present invention canbe varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.
    权利要求:
    Claims (15)
    [1] 1. AVoltage Source Converter (14) converting between alternatingcurrent, AC, and direct current, DC, and having a galvanic link (L1) to anAC system (ACN1), the voltage source converter comprising: a number of phase legs (PL1, PL2, PL3), each comprising a set of convertervalves (CVA1, CVA2, CVB1, CVB2, CVC1, CVC2) connected to a phase ofthe AC system via a corresponding galvanic connection of the galvanic link,and a control unit (20) operative to control the converter valves (CVA1, CVA2,CVB1, CVB2, CVC1, CVC2) to generate (22) AC waveforms and to reduce at least one component of a zero sequence current on the galvanic link.
    [2] 2. The voltage source converter according to claim 1, wherein thecontrol unit (20) is operative to form the zero sequence current through adding (36) the currents of the galvanic connections to each other.
    [3] 3. The voltage source converter according to claim 1 or 2,wherein the at least one component comprises the fundamental and/ or the third harmonic component.
    [4] 4. The voltage source converter according to any previous claim,wherein the control unit (20) when controlling the converter valves toreduce at least one component of the zero sequence current is operative toapply at least proportional control on each of the components of the zerosequence current to be reduced in order to obtain a control signal (rd, rq) for the converter valves.
    [5] 5. The voltage source converter according to claim 4, wherein thecontrol unit (20) is further operative to transform, for each component tobe reduced, the zero sequence current from an abc frame to a direct andquadrature frame for obtaining a direct and quadrature representation of the zero sequence current, to apply said proportional control on both lO representations for obtaining a direct and quadrature control signal (rd,rq) and transform the direct and quadrature control signals to a zerosequence control signal for use in the valve control to reduce said at least one component of the zero sequence current.
    [6] 6. The voltage source converter according to claim 5, wherein thecontrol unit is further operative to perform integrating control together with the proportional control.
    [7] 7. The voltage source converter according to claim 5 or 6,wherein the control unit is operative to low pass filter the direct andquadrature representations of the zero sequence current components before performing proportional control.
    [8] 8. The voltage source converter according to claim 4, wherein thecontrol unit comprises a proportional resonant controller and is furtheroperative to perform, for each component to be reduced, resonant controlat the frequency of the component together with the proportional controlfor obtaining a zero sequence control signal (uv_zero_ref) to be used for reducing said at least one zero sequence current component.
    [9] 9. A converter station (10) comprising a galvanic link (L1) to analternating current power network (ACN1) and a voltage source converter (14) according to any previous claim. 1o.
    [10] 10. A method of controlling current on an AC side of a voltagesource converter (14), said voltage source converter converting betweenalternating current, AC, and direct current, DC, and having a galvanic link(L1) to an AC system (ACN1), the converter comprising a number ofphase legs (PL1, PL2, PL3), each comprising a set of converter valves(CVA1, CVA2, CVB1, CVB2, CVC1, CV C2) connected to a phase of the AC system via a corresponding galvanic connection of the galvanic link (L1), lO 2l the method being performed in a control unit (20) of the voltage sourceconverter and comprising: controlling the converter valves (CVA1, CVA2, CVB1, CVB2, CVC1, CVC2)to generate AC waveforms and to reduce at least one component of a zero sequence current on the galvanic link.
    [11] 11. The method according to claim 10, wherein the control of theset of converter valves to remove at least one component of the zerosequence current comprises applying (60, 68; 76; 80) at least proportionalcontrol on each of the components of the zero sequence current to bereduced in order to obtain a control signal (rd, rq, uv_zero_ref) for the converter valves.
    [12] 12. The method according to claim 11, further comprisingtransforming, for each component to be reduced, the zero sequencecurrent to a direct and quadrature frame for obtaining a direct andquadrature representation of the zero sequence current, applying (60, 68)said proportional control on both representations for obtaining a directand quadrature control signal (rd, rq) and transforming the direct andquadrature control signals back to a zero sequence control signal for use inthe valve control to reduce said at least one component of the zero sequence current.
    [13] 13. The method according to claim 12, further comprising performing integrating control together With the proportional control.
    [14] 14. The method according to claim 11, further comprisingperforming, for each component to be reduced, resonant control at thefrequency of the component together with the proportional control forobtaining a zero sequence control signal (uv_zero_ref) to be used for reducing said at least one zero sequence current component. lO 22
    [15] 15. A computer program product for controlling current on an ACside of a voltage source converter (14), said voltage source converter (14)converting between alternating current, AC, and direct current, DC, andhaving a galvanic link (L1) to an AC system (ACN1), the convertercomprising a number of phase legs (PL1, PL2, PL3), each comprising a setof converter valves (CVA1, CVA2, CVB1, CVB2, CVC1, CV C2) connected toa phase of the AC system via a corresponding galvanic connection of thegalvanic link (L1), the computer program product comprising a datacarrier (92) with computer program code (94) configured to cause acontrol unit (20) of the voltage source converter (14) to control the converter valves (CVA1, CVA2, CVB1, CVB2, CVC1, CVC2) togenerate AC waveforms and to reduce at least one component of a zero sequence current on the galvanic link (L1).
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    公开号 | 公开日
    SE539945C2|2018-02-06|
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    SE1650854A|SE539945C2|2016-06-16|2016-06-16|Handling of zero sequence currents in a voltage source converter|SE1650854A| SE539945C2|2016-06-16|2016-06-16|Handling of zero sequence currents in a voltage source converter|
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