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  • 1. Hybrid HVDC Converters and Their Impact onPower System Dynamic PerformanceAbstract: results is also compared with a Hybrid converter conventional HVDC scheme.HVDC transmission is a newhybrid transmission system for I. NOMENCLATUREconnecting two ac systems. Voltage Sourced Converter:Because it uses different (VSC); Line Commutatedconverters, this new Converter:(LCC); Forcedconfiguration offers several Commutated Converter: (FCC);advantages over conventional Series Hybrid Converter: (SHC);HVDC Commutation Failure: (CF); Pulsesystems. This paper Width Modulation: (PWM).demonstrates the superiorperformance of hybrid II. INTRODUCTIONconverter based HVDC THE conventional HVDCtransmission systems with transmission systems that utilizerespect to increased stability Line Commutated Convertersand terminal ac voltage control. (LCC) have advantages overA control system is developed HVAC systems such as theirfor the hybrid system and its ability to connect ac networksdynamic performance is non-synchronously and theirinvestigated. The hybrid system ability to carry powerperformance with emphasis on economically over large distances.commutation failure during Unfortunately, thesesevere disturbances and its
  • 2. schemes do have certain LCC converter, at the same timedisadvantages such as a need for introducing the additional benefitsreactive power, commutation of the VSC converter. Several different topologies suitable tofailures, higher over-voltages and such combination have beenpoorer recovery especially when proposed in literature.they are connected intoweak This paper introduces aterminating ac networks. hybrid topology which includes a Unlike the LCC which series dc--side connection of anrelies on the ac voltage for LCC and VSC as shown insuccessful valve commutation, the Fig. 1. The paper studies dynamicVoltage Sourced Converter (VSC) control performance, faultuses special devices that can be recovery transient performanceturned off with and commutation failureappropriate control signals. While susceptibility of the proposedmaintaining most of the scheme and shows it to beadvantages, VSC based HVDC superior to a purely conventionalschemes also overcome a LCC based scheme.number of disadvantages inherentto conventional systems. Ratherthan consume reactive power,their ability to generate III.PROPOSED HYBRIDlagging or leading reactive power CONVERTERpermits them to operate andprovide voltage support to very The hybrid topology mayweak ac networks. Thus they are employ the LCC and VSCan ideal option for providing converters connected either inreliable power to remote locations parallel or series on the dc side.such as offshore plants. Their More complex schemes may notdisadvantages include higher be justified easily due to controlcosts, sensitivity to dc-side faults, complexities, expenses, need forhigher power losses due to the larger space, etc.high frequency of switching, and In a parallel hybridsmaller ratings in comparison to configuration the converterconventional converters. voltage rating is limited to theAppropriately sized VSC and highest voltage level permissibleLCC converters can be for the VSC converter, which isincorporated into a single much lower than that of acomposite “hybrid” converter comparable LCC andwhich combines the lower costs consequently limits the powerand robustness of the conventional rating of the topology. The
  • 3. proposed hybrid converter is of the CIGRE benchmark. At thelabeled a “Series Hybrid inverter side, the VSC used is aConverter” (SHC), as it includes modified version of [6] whichone LCC and one VSC in series. considering its optimal rating, the In contrast to some earlier LCC has been re-sized so as toapproaches in which the VSC has keep the overall ratings identicalonly been used for reactive to that of the CIGRE benchmarksupport or for active filtering the systems.proposed topology uses both This paper describes theconverters for real power transfer. principles of the proposed SHC system along with its main controlIV. BAISCS OF THE strategies including the terminalPROPOSED SHC SYSTEM voltage control, real power controlMODEL at the receiving end and inverterThe schematic for the proposed dc capacitor voltage control.SHC has been depicted in Fig. 1.The sending end (rectifier side) A. Optimizing the SHC’s Powerhas been assumed to be a LCC and Voltage Ratingconverter station and the receiving Based on the nominalend (inverter side) is a LCC-VSC power (Pnom) of the HVDCseries connection, along with its system the power / voltage ratingharmonic filters. for the hybrid-side converters may be calculated. To find the appropriate voltage level on the converters an optimization concept is employed to establish a connection between inverter-side voltage ratings, and major system components’ prices. Assume that the price for each converter is proportional to its MVA rating. Based on this assumption minimizing the total MVA of the inverter side The “First CIGRE HVDC converters (SLCC + SVSC) whichBenchmark System” [4] has been also equals to sum of theirused as the test bed for the corresponding transformers’performance of the proposed SHC ratings hassystem to be compared to. In the the same meaning of minimizingproposed SHC system the rectifier the total converter expenses.side is structurally identical to that Using an engineering-based
  • 4. estimation for filter reactive power The LCC generates voltage(Qfilt) hybrid converter’s complex harmonics. In its 12-pulsepower may be written as: configuration, the harmonics at 11, 13, 23 and 25 times the fundamental frequency (60 Hz) are present. As the VSC switches at rather high frequencies it willIn (1), the (Pnom – PLCC ) term only add high frequencyis equal to PVSC. Also under harmonics to the system. Selectingnormal working conditions the a switching frequency of 27th ofterm (0.6 * PLCC ) approximates the fundamental frequencythe QLCC ; the last expression generates harmonics at 25th andsimply equals the reactive power 29th order harmonics. To cancelthat has to be generated by VSC out the detrimental effects of these(QVSC). Differentiating (1) with harmonics and help to meet therespect to PLCC and setting that system’s harmonic requirementsequal to zero, the PVSC will be the 11th, 13th, 23rd, 25th and 29thdetermined. Based on this value order harmonic filters werethe appropriate voltage rating for installed, with total static reactivehybrid-side converters, and as the power support of around 80last step the LCC and VSC’s MVAR. These filters will providetransformer ratings, will be voltage-dependent reactive powerdetermined. The optimized supply to the inverter side andmagnitudes are given in help to meet the standards forAppendix. It has to be reminded system harmonic levels. Duringthat even at the design level there the steady state conditions theare other expenses that could be VSC has to provide the un-considered but as the converters supplied reactive power neededand transformers are the most for the conventional converter part“expensive” parts of each of inverter side. During theconverter, only these two major transient conditions it has also todevices have been included in supply the extra reactive power tooptimization. Other design provide voltage support at inverterphilosophies may bring equally terminal. The ability of VSC tovalid results for the purpose of supply voltage support depends onconverter rating design. its electrical rating and the coordination between LCC and VSC controls. The dynamic response is also a function of theB. SHC’s Filter and dc capacitor dc capacitor(s) size. A suitable dcconsiderations capacitor size has been selected to
  • 5. give an acceptable dynamicresponse; however, in this paper,no attempt has been made tooptimize thisperfectly.V. CONTROL OF THE SHCSYSTEM The system design outputted from terminal powerphilosophy has been based on two error signal.control objectives:1) Terminal voltage of the hybridconverter must be maintained at 1P.U.2) Power delivered to terminalduring normal working conditions The VSC inverter controlmust be 1 P.U. system shown in Fig. 3 has twoThe SHC control block diagram degrees of freedom. The first ishas been shown in Fig. 2. In this used by the VSC’s dc voltagefigure the upper and middle parts capacitor controller whichdepict the rectifier and inverter generates the reference realangle controls, respectively. The current (Idref) signal. The secondbottom part is used to control the terminalillustrates the VSC controls. voltage via the reference reactive The basis for SHC current (Iqref) signal. The d and qcontrollers is a coordinated current errors are used to generateversion of LCC-HVDC [4] and the corresponding voltage ordersVSC-HVDC system [5] controls. (Vd and Vq) through a decoupled In the SHC presented controller block. These arehere, the rectifier’s LCC works in converted into a modulation indexcurrent control mode while the magnitude (m) and phase (φ)inverter’s LCC works in signal. A phase locked loop (PLL)extinction angle (γ) control mode, is used to synchronize with the acusing the current control as its network voltage and generates thebackup [2]. The current order synchronizing angle signal (θ)signal that would end up to LCC’s which is used to generate theinverter angle order (αinv) is firing pulses for the IGBT devices of the VSC.
  • 6. commutation margin resulting from a sudden change in the acVI. OPERATION ISSUES IN voltage phase. Having largerHVDC SYSTEMS commutation margin in normalA. Commutation Failure operation improves the system’sPhenomena CF Commutation failure (CF) susceptibility, but this also resultsis one of the most onerous in a poorer power factor andtransient events experienced by potential over-voltage problemsHVDC systems. Its causes include on load rejection.sudden transient reductions and/or In a conventional HVDCphase shifts in the ac voltage and converter, the fault induced CFsudden transient reductions in the leads to power disruptions. Indirect current. The sensitivity of a some cases there are repeated CFHVDC inverter to CF depends on occurrences from which recoverythe main circuit design and its is not possible without a full re-control system. In conventional start. Additionally, CF also causesconverters, commutation failure over-current in the valves.likelihood is significant when The VSC in the hybridthere is a 10% or larger voltage HVDC converter cannot sufferreduction caused by an ac system CF. Thus HVDC transmissiondisturbance. systems with hybrid converters are The main reason for CF is less susceptible to CF relatedthat the excessive reduction in the power disruptions. Also, the sameextinction angle during its fault which would have resulted ininitiating system disturbance. This serious system failure in thedecrease could be caused by an conventional converter has a muchincrease in the converter’s overlap smaller impact on the hybridangle due to ac voltage reduction converter.or due to a change in the
  • 7. The disruption of the HVDC alternatives. Thenormal switching sequence parameters for the controllersfollowing a CF will lead to were selected for overallconsiderable waveform distortion performance and were notof the optimized for any particularcommutating voltage waveform disturbance event.making the problem unsuitable foranalytical formulation. Therefore A. System step responsenumerical simulation on an To investigate bothelectromagnetic transients solver systems’ responses to set pointis required to assess the behavior changes, the conventional (CIGREof the system. Here the PSCAD / benchmark) converterEMTDC software has been option was subjected to a 10%selected for simulating the system change in power order. Also as theand studying its behavior. hybrid system operates directly in power control, its controller wasB. Dynamic Response and Fault subjected to a 10% change inPerformance power order. The results are A well designed HVDC shown in Fig. 4.system should show react rapidlyto set-point changes and also showrapid recovery from In the system with thesystem faults. In order to assess conventional converter only, thethese issues, the dynamic behavior steady state terminal voltageof CIGRE benchmark and the settles to a different magnitudeproposed series hybrid converter after the change is applied, due toHVDC systems will be compared. the resulting mismatch in reactiveThe hybrid’s robustness under power. As the VSC in the hybridsmall and large dynamic control option is capable ofdisturbances will be demonstrated. reactive power generation and isNext, the two HVDC systems will tasked withbe compared by simulating their maintaining the ac voltage at ratedperformance following single and magnitude, the ac voltage3-phase to ground faults of eventually returns to its post-faultvarying severity. magnitude. As can be seen from Fig.VII. CASE STUDIES 4., both options show a quick and The following section well damped response to the set-contains simulated results for the point changes. However, the VSCdynamic and fault performance of option shows a smaller settlingthe conventional and hybrid
  • 8. time with a slightly oscillatoryresponse. capacitor limits the over voltagesB. System fault response of dc capacitor to 25% To investigate bothsystems’ responses to large Fig. 5 shows the pre and postdynamic disturbances a fault terminal voltage and powersymmetrical three phase short curves obtained based on applyingcircuit to ground at the inverter such a fault. In comparison to theterminal was applied for 0.1 conventional option, the hybridsecond (6 cycles) to each systems’ option shows significantly fasterterminals. Various fault power recovery with 90% powerimpedances were used, but the restored within 200 ms after faultcase reported below only shows clearance. Thethe response to the most severe corresponding conventional optionfault which is a solid short circuit requires approximately 400 ms.that reduces the terminal voltage However, the conventionalto essentially zero. One difference option shows a more gradualbetween the two systems is that in voltage recovery without any overthe hybrid case the arrester voltage stress on the equipment.connected across VSC’s dc The hybrid option, on account of
  • 9. its voltage control function, causes phase faults under varyingthe voltage to be rapidly regulated, inductances (not shown here)and in doing so experiences a suggests less overall hybridmodest 10% over voltage system’s sensitivity toduring recovery.C. Commutation Failureperformance Several other tests wereconducted with various differentfault impedance values (inductive)to investigate the impact of faultseverity on the performance. TheVSC showed generally superiorfault recovery times in all cases.Also for certain less severe (highimpedance) faults, theconventional converter basedsystem experienced total powerloss whereas the hybrid systemmanaged to continue operation commutation failure comparing toduring the faulted period. a only-conventional converter Fig. 6 shows the lowest ac based HVDC system, whichvoltage and power magnitudes means that the hybrid system isreached during the fault for immune to more sever faultsvarying fault inductance values. comparing to a conventionalBoth three phase-to-ground as HVDC system. Noticeably in casewell as single phase-to-ground of commutation failure in a hybridfaults were applied. As can be converter system it is probableseen the hybrid converter was able that it only experiences ato maintain current and power to commutation failure (CF) in itsabove 90% even with fault conventional converter part whichimpedances of 1 H or higher for does not lead to total terminalboth types of fault, whereas the power disruption while even less-conventional converter starts sever faults would cause CF andexperiencing similar reductions at power disruption in a conventionala much less severe fault system.inductance of 2.5 H. Examining The ranges for single phasethe power variation vs. extinction faults show similar trends withangle curves for the two options fault and converter types, but thesubjected to three phase and single fault severity required to cause failure is marginally smaller for
  • 10. each case. The above tests were conditions without resorting toconducted with the short circuit complicated control strategiesratio of the ac system set to 2.5, with the added benefit ofwhich is considered fairly low and superior system performance (lesshence expected to cause power / voltage drop, less chancechallenges for the transmission of commutation failure and shorteroptions. However, because of the recovery time using equal faultVSC’s fast dynamic response to inductances).reactive power demands theproposed hybrid converter also IX. APPENDIX : MODELhas the unique capability of DATAworking under even lower short HVDC system rating: 1000circuit ratios (SCR) where the MW; 500KV DC Rectifierconventional converter would not specifications:be able to operate at all As per First CIGRE benchmark model [4].VIII. CONCLUSIONS Inverter specifications: Using a coordinated Terminal voltage: 230 KV, Lcontroller for a SHC (Series SCR: 2.5Hybrid Converter) HVDC LCC converter voltage (DC side):transmission system results in 390 KVsuperior inverter terminal VSC DC voltage: 110 KVperformance in response to small Line parameters: As per Firstand large dynamic changes in CIGRE benchmark model [4].comparison to a conventionalconverter case (First CIGRE X. REFERENCESbenchmark model). As such, the [1] B. R. Andersen, L. Xu,proposed configuration may be “Hybrid HVDC system for powercounted as a promising transmissionconfiguration for delivering real to island networks,” IEEE Trans.power to systems that feed more on Power.sensitive loads. Using digital time [2] A. M. Gole, R. Verdolin, E.domain Kuffel, “Firing angle modulationsimulation with PSCAD / forEMTDC program and employing eliminating transformer DCconventional closed-loop currents in coupled AC-DCstructures it has been shown that systems,” IEEEstable [3] H. Jiang, A. Ekstrom,operation of the proposed “Harmonic cancellation of aconfiguration can be insured in a hybrid converter,”broad range of operating .