ABSTRACTSelf-commutating multilevel current reinjection is a potential alternative to conventionalHVDCthyristor technology. Animportant drawbacof themultilevel configurations is theinterdependence of the reactive power injections at both ends of the lin. !his paper describes anewconcept applicabletolargepowerconvertersconsistingoftwoseries-connectedtwelve-pulse groups. "t is based on the use of a controllable shift between the firings of the two twelve-pulsegroupsinoppositedirections# anewconcept that providesindependent reactivepowercontrol at the sending and receiving ends. !he theory is verified by $%!DC simulation.INTRODUCTION&'"()totheir structural simplicityandfour *uadrant power controllability# pulse widthmodulation +,'%- conversion has sofar beenthe preferred option for self-commutatingmediumpower HVDCtransmission. However# thistechnologyislesssuitedtolargepowerratings and long distances# due to higher switching losses and to the rating limitations of its maincomponents +namely the power transistor switch and underground cable-. !hus the interchangeof large *uantities of power between separate power systems and the transmission of power fromremotegeneratingstationsarestill basedontheprincipleofline-commutatedcurrent sourceconversion.%ultilevel VSCconfigurationshavebeenpresentedaspossiblealternativesto,'%-VSC!ransmission# but their structural comple.ity has been the main obstacle to theircommercial implementation. Arecent proposal# the multilevel current reinjection +%/C0-concept# simplifies the converter structure and permits the continued use of conventionalthyristors for the main converter bridges. !he main advantage of self over natural-commutationin HVDC transmission is the ability to control independently the reactive power at each end ofthe lin# a property that cannot be achieved by %/C0-based +or any other multilevel-configuration when using only one double-bridge converter group.However# interconnections of large power ratings will normally use two or more 12-pulseconverter groups and these can be controlled independently from each other without affecting theoutput voltage waveform.!his fact constitutes the basis of the new control scheme proposedhere. 'hen the operating condition at one end of the lin alters the reactive power balance at thisend# thefiringsofthetwogroupsat theotherendareshiftedwithrespect toeachotherinopposite directions to eep the power factor constant. !he new control concept gives the %/C0configuration described in the fle.ibility until now only available to ,'%3VSC transmission.HVDC&verlongdistancesbulpower transfer canbecarriedout byahighvoltagedirectcurrent +HVDC-connectioncheaperthanbyalongdistance ACtransmissionline. HVDCtransmission can also be used where an AC transmission scheme could not +e.g. through verylong cables or across borders where the two AC systems are not synchroni4ed or operating atthesamefre*uency-. However# inorder toachievetheselongdistancetransmissionlins#power convertor e*uipment is re*uired# which is a possible point of failure and any interruptionindeliveredpowercanbecostly. "t isthereforeofcritical importancetodesignaHVDCscheme for a given availability. !he HVDC technology is a high power electronics technology used in electric powersystems. "t is an efficient and fle.ible method to transmit large amounts of electric power overlong distances by overhead transmission lines or underground5submarine cables. "t can also beused to interconnect asynchronous power systems !he fundamental process that occurs in an HVDC system is the conversion of electricalcurrent from AC to DC +rectifier- at the transmitting end and from DC to AC +inverter- at thereceiving end. !here are three ways of achieving conversion1. (atural commutated converters2. Capacitor Commutated Converters6. 7orced Commutated Converters(atural commutated converters8 +(CC-(CC are most used in the HVDC systems as of today. !he component that enables thisconversion process is the thyristor# which is a controllable semiconductor that can carry very highcurrents +9::: A- and is able to bloc very high voltages +up to 1: V-. ;y means of connectingthe thyristors in series it is possible to build up a thyristor valve# which is able to operate at veryhigh voltages +several hundred of V-.!he thyristor valve is operated at net fre*uency +nlieconventional thyristors#theyhave no commutationcircuit# downsi4ing application systems while improving efficiency. !hey are the most suitablefor high-current# high speed switching applications# such as inverters and chopper circuits.;ipolar devices made with SiCoffer 2:-sing the notation shown in the figure# whererd represents the total resistance of the line# we get for the DC current And the power transmitted into station ; is

"n rectifier operation the firing angle I should not be decreased below a certain minimum valueImin# normally 6F-nlie the case of ,'%-VSC# due to the low fundamental-fre*uency-related switchingnature of %/C0# it is not possible in this case to e.ercise independent control of the amplitudeand power angle. !he only variable that controls the %/C0 operation is+the power factorangle-. Asimilar situationoccurredinthecaseof the%/V0-VSCconfigurationdescribedearlier# where# the phase angle difference between the converter output voltage and the ACsource voltage# was theuni*ue controlvariable. However#in thelatter caseonlya relativelysmallvariationofwasneededtoachieve four-*uadrantoperation# whereasin %/C0 thiscondition re*uires a variation of in the range of.!hus the relationships between and V,# WX are highly non-linear. !his is shown by the following e.pressions derived from thebloc diagrams of 7igures K.2L and K.6:# where Vt is the rms value of the converter terminal+phase-to-phase- voltage87or the test case +S!A!C&% operation-# only a small variation of aroundis needed tocontrol the DC current.!heamplitudeincrement ordecrement oftheDCloadbranchcurrent dependsonthepolarity of the DC voltage across the load branch# which is proportional to the cosine function ofif the convertervoltage is ept constantand thelossesin thesmoothing reactor are ignored. However# the amplitude increment or decrement of theunidirectional DCcannot besolelydeterminedbythepolarityofthepowerangleincrementaround themar# because for .'hereas for!husthepowerangleincrement around hastobecoordinatedwiththeconverteroperating condition to generate the appropriate polarity of . +a- !he generated W and Wref D+b- !he average DC current and voltageD+c- #+d-# +e- the three-phase voltages and currentsD+f- !he DC voltage and three-phase currents +with the time scale reduced-. VO'TA() SOURC) CONV)RT)RS ,VSC- A voltage-source converter is a power electronic device# which can generate a sinusoidalvoltage with any re*uired magnitude# fre*uency and phase angle. Voltage source converters arewidely used in adjustable-speed drives# but can also be used to mitigate voltage dips. !he VSC isused to either completely replace the voltage or to inject the Ymissing voltageR.!he YmissingvoltageR is the difference between the nominal voltage and the actual. !he converter is normallybased on some ind of energy storage# which will supply the converter with a DC voltage. !hesolid-state electronics inthe converter is thenswitchedtoget the desiredoutput voltage.(ormally the VSC is not only used for voltage dip mitigation# but also for other power *ualityissues# e.g. flicer and harmonics. !he voltage source rectifier operates by eeping the dc lin voltage at a desired referencevalue# using a feedbac control loop as shown in 7igure. !o accomplish this tas# the dc linvoltage is measured and compared with a reference V0$7. !he error signal generated from thiscomparison is used to switch the si. valves of the rectifier &( and &77. "n this way# power cancomeor returntotheacsourceaccordingtodclinvoltagere*uirements. VoltageVDismeasured at capacitor CD. 'hen the current "D is positive +rectifier operation-# the capacitor CDis discharged# and the error signal as the Control ;loc for more power from the ac supply. !heControl ;loc taes the power from the supply by generating the appropriate ,'% signals forthesi.valves."n thisway#more currentflowsfrom theac tothe dc side# andthecapacitorvoltage is recovered. "nversely# when "D becomes negative +inverter operation-# the capacitor CDis overcharged# and the error signal ass the control to discharge the capacitor and return powertotheacmains.!he,'%control not onlycanmanagetheactivepower#but alsoreactivepower# allowing this type of rectifier tocorrect power factor. "naddition# the ac currentwaveforms can be maintained as almost sinusoidal# which reduces harmonic contamination tothe mains supply. ,ulse width-modulation consists of switching the valves &(and&77#following a pre-established template. !his template could be a sinusoidal waveform of voltage orcurrent. 7or e.ample# the modulation of one phase could be as the one shown in 7ig. !his ,'% patternis a periodicalwaveform whose fundamentalis avoltagewith thesame fre*uency of the template. !he amplitude of this fundamental# called V%&D in 7ig# is alsoproportional to the amplitude of the template.!o mae the rectifier wor properly# the ,'%patternmust generateafundamental V%&Dwiththesamefre*uencyasthepowersource.Changing the amplitude of this fundamental&peration principle of the voltage source rectifier.7")>0$ A ,'% pattern and its fundamental V%&D. And its phase-shift with respect tothe mains# the rectifier can be controlled to operate in the four *uadrants8 leading power factorrectifier# lagging power factor rectifier# leading power factor inverter# and lagging power factorinverter. Changingthepatternof modulation# as shownin7ig# modifies themagnitudeofV%&D. Displacing the ,'% pattern changes the phase-shift. !he interaction between V%&Dand V +source voltage- can be seen through a phasor diagram. !his interaction permits understanding of the four-*uadrant capability of this rectifier. "n7ig.# the following operations are displayed8 +a- rectifier at unity power factorD +b- inverter atunity power factorD +c- capacitor +4ero power factor-D and +d- inductor +4ero power factor-. "n7ig. "s the rms value of the source current is . !his current flows through the semiconductors inthe same way as shown in 7ig. 12.9:. During the positive half cycle# the transistor !( connectedat the negative side of the dc lin is switched &(# and the current is begins to flow through !(.i!n.!hecurrent returnstothemainsandcomesbactothevalves# closingaloopwithanother phase# and passing through a diode connected at the same negative terminal of the dclin.!he currentcanalso gotothedcload+inversion-and returnthrough anothertransistorlocatedat thepositiveterminal ofthedclin.'henthetransistor !(isswitched&77#thecurrent path is interrupted# and the current begins to flow through diode D,# connected at thepositive terminalof thedclin.!hiscurrent#callediDpin 7ig#goes directlyto thedclin#helping in the generation of the current idc. !he current idc charges the capacitor CD and permitsthe rectifier to produce dc power. !he inductances /S are very important in this process# becausethey generate an induced voltage that allows conduction of the diode D,. A similar operationoccurs during the negative half cycle# but with !, and D(Changing V%&D through the ,'% pattern. 7ig. 7our-*uadrant operation of the force-commutated rectifier8 +a- the ,'% force-commutatedrectifierD +b- rectifier operation at unity power factorD +c- inverter operation at unity power factorD+d- capacitor operation at 4ero power factorD and +e- inductor operation at 4ero power factor.>nderinverteroperation# thecurrent pathsaredifferent becausethecurrentsflowingthrough the transistorscome mainly fromthedccapacitorCD. >nder rectifieroperation#thecircuit wors lie a ;oost converter# and under inverter operation it wors as a ;uc converter.!o have full control of the operation of the rectifier# their si. diodes must be polari4ed negativelyat all valuesofinstantaneousacvoltagesupply.&therwise# thediodeswillconduct# andthe,'% rectifier will behave lie a common diode rectifier bridge. !he way to eep the diodes bloced is to ensure a dc lin voltage higher than the pea dcvoltage generated by the diodes alone# as shown in 7ig. "n this way# the diodes remain polari4ednegatively# and they will conduct only when at least one transistor is switched &(# and favorableinstantaneous ac voltage conditions are given. "n 7ig. VD represents the capacitor dc voltage#which is ept higher than the normal diode-bridge rectification value n ;0"D)$. !o maintainthis condition# the rectifier must have a control loop lie the one displayed in 7ig. 7ig. Current waveforms through the mains# the valves# and the dc lin.T*ree-0*ase Volta"e Source Inverters&Single-phase VS"s cover low-range power applications and three-phase VS"s cover themedium- to high-power applications. !he main purpose of these topologies is to provide a three-phase voltage source# where the amplitude# phase# and fre*uency of the voltages should alwaysbe controllable. Although most of the applications re*uire sinusoidal voltage waveforms +e.g.#ASDs# >,Ss# 7AC!S# var compensators-# arbitrary voltages are also re*uired in some emergingapplications +e.g.# active filters# voltage compensators-. !he standard three-phase VS" topology isshown in 7ig. and the eight valid switch states are given in !able. As in single-phase VS"s# theswitches of any leg of the inverter +S1 and S9# S6 and S=# or S< and S2- cannot be switched onsimultaneously because this would result in a short circuit across the dc lin voltage supply. Similarly# in order to avoid undefined states in the VS"# and thus undefined ac output linevoltages# the switches of any leg of the inverter cannot be switched off simultaneously as thiswill result in voltages that will depend upon the respective line current polarity. &f the eight valid states# two of them produce 4ero ac line voltages. "n this case# the aclinecurrentsfreewheel througheither theupperorlowercomponents. !heremainingstatesproduce non4ero ac output voltages. "n order to generate a given voltage waveform# the invertermoves from one state to another. !hus the resulting ac output line voltages consist of discretevalues of voltages that are vi# :# and Zvi for the topology shown in 7ig. !he selection of the statesin order to generate the given waveform is done by the modulating techni*ue that should ensurethe use of only the valid states.0U'S) 3IDTH #ODU'ATION'hat is ,'%[,ulse 'idth %odulation +,'%- is the most effective means to achieve constant voltagebatterychargingbyswitchingthesolar systemcontrollerRspower devices. 'henin,'%regulation# thecurrent fromthesolar arraytapers accordingtothebatteryRsconditionandrecharging needs consider a waveform such as this8 it is a voltage switching between :v and 12v."tisfairlyobviousthat# sincethevoltageisat12vfore.actlyaslongasitisat:v#thena@suitable device@ connected to its output will see the average voltage and thin it is being fed =v -e.actly half of 12v. So by varying the width of the positive pulse - we can vary the @average@voltage.Similarly# if the switches eep the voltage at 12 for 6 times as long as at :v# the averagewill be 659 of 12v - or Lv# as shown belowAnd if the output pulse of 12v lasts only 2SCSchedule6# all relevant )eneratorswith;%>nitshaveamended AncillaryServicesAgreements to incorporate with respect to those ;%>nits the default paymentarrangements for the &bligatory 0eactive ,ower Service as more particularly describedin C>SC Schedule 6. 0elevant )enerators with ;% >nits not operational but wishing torespond to this "nvitation to !ender are re*uired to amend or conclude Ancillary ServicesAgreementsinsimilarfashioninaccordancewithC>SCSchedule6beforea%aretAgreement can be entered into.Obl!"ator% React!ve 0o5er Serv!ce&!he &bligatory 0eactive ,ower Service is an Ancillary Service with two essential components !he provision of a minimum 0eactive ,ower capability and the maing available of thatcapability to (ational )rid.!hecapabilitycomponent ofthe&bligatory0eactive,owerServiceistheminimum0eactive ,ower capability re*uired of a ;% >nit under and in accordance with the ConnectionConditions of the )rid Code# most particularly CC=.6.2. A >ser therefore does not provide the&bligatory 0eactive ,ower Service from a )enerating >nit which is compliant with )rid CodeCC=.6.2 where compliance is not obligatory for that >ser in respect of that )enerating >nit.!he second component of the &bligatory 0eactive ,ower Service - the maing availableof the capability to (ational )rid to instruct - is typically provided by ;% >nits in accordancewiththe;alancingCodes of the)ridCode. However# it maybeprovidedbyother ,lant#specifically Small "ndependent )enerating ,lant# where the >ser and (ational )rid agree termsfortheprovisionofsuitablemeteringandcommunicationfacilities# includingtheabilityfor(ational )rid to obtain relevant technical# planning and other data.!he &bligatory 0eactive ,ower Service does not include the provision of 0eactive ,owercapability from Synchronous Compensation or from static compensation e*uipment.!he &bligatory 0eactive ,ower Service is more particularly described in subparagraph 1.1 ofC>SC Schedule 6.#OD)'IN( OF CAS) STUD7INT)RD)0)ND)NC) OF TH) R)ACTIV) 0O3)R UND)R CONV)NTIONA' CONTRO'&,'% provides fully independent controllability of the converter voltages +and thereforereactive power transfers- on both sides of the lin. !his capability is not available to multilevelconfigurations under the present control strategies. 7or instance# if e.tra reactive power is neededat the receiving end to maintain the ac terminal voltage constant# the firing angleis increasedand# therefore# the dc voltage reduced. !o continue transmitting the specified power under thiscondition# the sending end station must also reduce its dc voltage. !he dc voltage reduction isimplemented by a corresponding increase in the firing angle of the two converter groupsD thisactionwill forceanunwantede.trainjectionofreactivepowerand# thus# anincreaseofacterminal voltage at this end. Such condition would not occur if some ,'% control were to beadded to the multilevel configurations. However the use of ,'% is currently limited to threelevels and is only used in voltage source conversion schemes.#U'TI(ROU0 FIRIN(-SHIFT CONC)0T&!he e.change of reactive power between the converter and ac system is determined bythe sine of the firing angle . Alteringhas an immediate effect on the dc voltage leveland# thus# to maintain the specified dc power transfer through the lin# a corresponding change offiring angle must be made at the other end# which in turn affects its reactive power e.changewith the ac system. !herefore# under conventional converter control# the reactive powers injectedat the two ends of a multilevel CSC lin are interdependent."n multilevelCSC HVDC interconnections with two twelve pulse groups per terminal+suchasshownin7ig. 1-thesamecurrent waveformisproducedbyeachofthe12-pulseconverter groups# and thus the total output current waveform remains the same if a phase-shift isintroduced between the firings of the two groups constituting the converter station.7ig.1. Simplified diagram of a dc lin connecting two ac systems.'hen a change of operating conditions at the receiving end demands more reactive power fromtheconverter# andthus reduces thedcvoltage# shiftingthefirings of thetwosendingendconverter groups in opposite directions provides the re*uired dc voltage reduction# whilemaintainingthereactivepowerconstant+duetotheoppositepolarityofthetwofiringanglecorrections-. A relatively small change of active power will be caused by the variation of thefundamental current produced by the shift# but this change can be compensated for by a smalle.tracorrectionofthetwofiringangles. 7oraconvertertooperateinthefiring-shift mode+which in the above e.ample is the sending end converter-# the firing instants of one group +saygroup A- is ept on the positive side +thus providing reactive power-# while the second group+say group ;- may act as a source or sin of reactive power +i.e.# the firing angle may be positiveor negative-.A. Steady State Operation:!osimplifythee.planationof thesteadystatecharacteristics only# thesendingendoperates under firing shift control# while the receiving end uses a common firing angle for thetwo converter groups. !he generali4ed method with firing shift at both ends will be used in thedynamic simulation.Also# as shown in 7ig. 1# the interconnected ac systems are represented as simplified !hevenine*uivalents# i.e. # and At thereceivingendthecontrol specifications aretheterminal ac voltageand the dc voltage# while those at the sending end are the dcpower transfer and the reactive power .Rece!v!n" end&As at this end no firing shift control is e.ercised# the firing angle will be the same for the two converter groups . !he dotted lines in the phasor diagram of 7ig. 2 represent the initial operating condition +with a !hevenin impedance of- and the continuousline a new operating condition with a larger !hevenin impedance .7ig.2.&perating conditions at the receiving end for two different system strengths."n both cases# the system reactive power re*uirements are e*ually shared between theconverter andtheacsourcewiththesamefiringangleusedinbothconverter groups. !hecondition is represented by the following e*uations8bbbbb. +1-&r between the double converter ac and dc currentsbbbbbb. +1a-And bbbbbbbb +2-bbbbbbb. +6-"n terms of the specified power +which is normally controlled at the sending end- the dc voltagesacross the lin are related by the e.pression&r maingthe subject and taing the positive rootbbbbbb. +9-!he solution of +1a-3+9- provides the initial values of .Send!n" end&7ig. 6 illustrates the two operating conditions in response to the change at the receiving end."nitially the sending end is set with one firing angle positive and one negative to demonstrate thephaseshift control principle# thesearerepresentedby and . 'henachangeinoperating conditions occurs# as a result of an increased !hevenin impedance and# thus# of firingangle at the receiving end# the sending end must compensate by an increase in firing angle + and in 7ig. 6- to maintain the specified active power transfer.7ig.6. 7iring shift control maintaining constant reactive power at the sending end for twodifferent system strengths.!he following relationships apply to the sending end8bbbbb. +