EFFICIENCY EVALUATION OF A DC TRANSMISSION SYSTEM BASED ON VOLTAGE SOURCE CONVERTERS

Transcription

1 EFFICIENCY EVALUATION OF A DC TRANSMISSION SYSTEM BASED ON VOLTAGE SOURCE CONVERTERS Giddani O. A (), Grain. P. Adam (), O. Anaya-Lara (3), K.L.Lo () tjb83@eee.trath.ac.uk, () grain.adam@eee.trath.ac.uk, (3) olimpo.anaya-lara@eee.trath.ac.uk Strathclyde Univerity, Electronic and Electrical Engineering Department Royal College Building 4 George Street, Glagow G XW Keyword: Converion loe, modular converter, neutral point clamped converter and pule width modulation. Abtract The efficiency of a VSC-HVDC tranmiion ytem ued to tranmit wind farm power to the network i invetigated. The loe aociated with the operation of the VSC-HVDC ytem are the converion loe (conduction and witching loe) in converter and the tranmiion loe due to I R (in the converter tranformer, moothing reactor, and DC cable). An analytical method baed on the average and root mean quare of the emiconductor current i ued to etimate the converion loe in the converter. The remaining power loe are evaluated uing I R. The reult obtained uing the analytical method are confirmed by the meaured loe obtained from imulation tudie. Introduction High-Voltage DC tranmiion baed on voltage ource converter (VSC-HVDC) preent a olution for many problem face nowaday by power network, uch a, network congetion, grid re-enforcement and aynchronou connection. The ue of forced-commutated witching device and high-frequency pule width modulation (in the order of khz), reult in a fat dynamic repone to network diturbance. The main benefit of VSC-HVDC ytem are: proving the AC fault ride-through capability. The abence of a reactive current component in the DC ytem allow active power tranmiion over long ditance at reduced power loe. Independent control of active and reactive power. Can provide voltage and frequency upport to the AC network. Active power flow i fully controlled and power reveral i achievable without the need to revere the DC link polarity. Black tart capability Currently, there are two etablihed approache for the contruction of a VSC-HVDC ytem. The firt approach ue a tandard two-level converter or a neutral-point clamped converter with forced commutated device uch IGBT. Thi approach impoe a high inulation requirement on the interfacing tranformer due to the high dv/dt that reult from witching high voltage with relatively low witching frequencie. Thi arrangement alo require fairly large filter at the output to attenuate the witching frequency component from the output voltage at the point of common coupling. The econd approach ue a two-witch modular multilevel converter with medium voltage device uch a 4.5kV IGBT. Thi approach produce lower dv/dt (allowing the ue of a tranformer with tandard inulation requirement) and ignificantly lower voltage harmonic ditortion (which may eliminate the need for the AC filter). Thi approach require however, a large number of witching device and capacitor and a relatively complex modulation trategy compared to the firt approach. Both approache reult in fat dynamic performance, independent control of active and reactive power, no commutation failure during and AC fault, better fault ridethrough capability, and the ability to provide damping and frequency upport through active or reactive power modulation. However, both approache produce higher converion loe compared to the claic HVDC ytem baed on line commutated or capacitor commutated current ource converter. Etimation of the power loe during the deign tage of the VSC-HVDC i eential, becaue it allow the deigner to optimize the overall ytem performance through a compromie of everal deign indice. It alo help in the election of heat inking equipment and cooling ytem for the converter. In reference [5, the author etimate the loe in a VSC- HVDC ytem baed on two-level converter. The reult how that the converion loe in the converter repreent more than half of the overall loe of the VSC-HVDC ytem.

2 Thi paper evaluate the efficiency of a VSC-HVDC ytem baed on the three-level neutral point clamped (NPC) converter. The converion (conduction and witching loe) and tranmiion loe (in AC and DC ide) are evaluated under variou operating condition. To thi aim the data of a 3.3kV IGBT, and inuoidal pule width modulation (witching frequency of khz) have been employed. Analyi of VSC-HVDC ytem loe Fig. how a ingle line diagram of the VSC-HVDC ytem ued in the invetigation. It conit of a 7MVA wind farm baed on fixed peed wind turbine. The converter VSC and VSC are modelled a NPC converter. The length of the DC cable connecting VSC and VSC i 5km. Fig.. VSC HVDC ytem ued in the invetigation. 3 Converion loe VSC-HVDC tranmiion ytem baed on two-level and three-level NPC converter require erie-connected IGBT, therefore, the converion loe can divided into three type a follow[: -Conduction loe: Thee loe occur due to the voltage drop acro the witching device during the conduction period, and depend on the forward reitance of the device r f, the device threhold voltage V f, the rm current I rm and the average current I av. a in equation (). P = V I + r I () cond f av f rm The device forward voltage V f i proportional to the current i(t) a: Vf = Vf + rfi() t () Where ( V f ) i the forward voltage acro the device at no load. -Switching loe The witching loe in forced-commutated device uch IGBT are dominantly turn-on and turn-off loe. The loe in the free wheeling and clamping diode are mainly recovery loe during turn-off. The witching loe depend on the witching frequency and the current and voltage at each witching intant. The mot common way to evaluate the witching loe i to expre the witching function a a firt or econd order polynomial depending on the witching characteritic of the device under conideration. However, with device ued in thi paper, the firt order approximation i ufficient to etimate the witching energy loe a a function of current with minimum error. Hence, the witching energy loe are expreed a: Ew = ki (3) Where k can be determined from the witching characteritic uually given in the manufacturer data heet. In thi paper, the two curve of witching energie (turn-on and turn-off) are added together for implicity. With a carrier frequency more than ten time the fundamental frequency, the witching loe in Watt for three level converter can be etimated uing equation (4): P w f α α = k i (4) 3-Additional loe Thee loe are produced in the gate, nubber circuit and leakage current during the off-tate and can be neglected. 4 Analytical method to calculate converion loe in three level converter According to equation () and (4) the calculation of the conduction and witching loe in any multi-level topology require the knowledge of the rm current ( I rm ), the average current ( I av ) flowing through each device, the device forward reitance ( r f ) and the forward voltage drop of the device ( V f ). The average and rm current in any pule width modulated converter with dicontinuou current can be approximated a follow: T ia = ia () t dω = δa. ia() t T (5) T irm = ia () t dω = δa. ia () t T Where T i the witching period and δ a i the duty cycle of each device. To calculate the conduction and witching loe in a three-level NPC converter, the knowledge of variou power path in the converter circuit i required. Fig. ummarie all the poible power path in the NPC converter, and Fig. 3 how the load current and voltage waveform plu the SPWM gate ignal to the converter witche.

3 Gate ignal of the witch S a4 Fig. 3: Carrier, current, voltage and gate ignal waveform. Conducting device for i a > Conducting device for i a < Fig. Power path for a 3-level converter If the load current in phae A i expreed by ia () t = I in m ( ωt ), the normalized phae voltage i va = M inωt and the duty cycle for the witche (S -S 4 ) are defined a: Min ωt ωt δ a = ωt ωt δ a = + M in ωt ωt δ = δ δ = δ a3 a a4 a Then, the average and root mean quare current in the IGBT are calculated a follow: (6) Carrier ignal, fundamental voltage and current waveform Gate ignal of the witch S a M Ia, av = Ia4, av = δaia dω = [( )co + in 4 M Ia, av = Ia4, av = δaia dω = [3 + 4co + co + Ia, av = Ia3, av = [ ia + δaia (7) = [4 + M co M in 4 + Ia, rm = Ia3, rm = [ ia + δaia = [3( M) + 4M co M co Gate ignal of the witch S a Gate ignal of the witch S a3 And the average and root mean quare current in the freewheeling diode are: M IDa, av = δa iadω = [ co in 4 IDa4, av = IDa3, av = IDa, av = IDa, av (8) M IDa, av = δa ia dω = [3 4co + co 4 I I I I Da4, rm = Da3, rm = Da, rm = Da, rm

4 The average and root mean quare current in the clamping diode are: + Ia5, av = Ia6, av = [ δa3ia + δaia M = [( )co + in 4 + (9) Ia5, rm = Ia6, rm = [ δa3ia + δaia = [3 6M M co The current given in (7), (8), and (9) are ued to calculate the conduction loe of three-level converter according to equation (). Therefore, the total conduction loe are: P = P + P + P () cond condigbt condanti diode condclampdiode. According to equation (4) the calculation of witching loe in any multi-level topology require the knowledge of the maximum load current value ( I m ), the witching frequency ( f ) and the witching reitance of the device ( k ). For a three-level NPC converter, the witching loe in the IGBT S a and S a are: f Pa, w = Ia4, w = k in( ωt ) kfi m = [ + co f + Pa, w = Ia3, w = k in( ωt ) kfi m = [ co () The witching loe aociated with free wheeling device are: f k f PDa,w = k in( ωt ) [ co = P P P P Da 4,w = Da 3,w = Da,w = Da,w P f k I in( t )d t P P + m Da 5,w = m ω ω = Da 6,w = Da 5,w k f I () Hence, the total witching loe of three-level NPC converter are: P = P + P + P (3) w wigbt wanti diode wclampdiode 5 Tranmiion loe Tranmiion loe are: The loe that occur in the converter tranformer (copper and iron loe) and the copper loe in the AC moothing reactor. The copper loe in the DC cable In thi paper, the copper loe in the interfacing reactor and tranformer are calculated uing3ri rm, where r i the reitance of the interfacing reactor or tranformer. The iron lo in the 6. interfacing tranformer due to B f and B f i taken a percent of the copper loe. Thi i due to the exitence of the witching frequency component and their ideband in the converter output voltage and current. The copper loe in the DC line connecting the two converter tation i calculated uing r dc I dc, where r dc i the DC reitance of the cable and I dc i the current magnitude in the DC cable. 6 Efficiency evaluation A the power loe vary with the operating point, the power loe of the VSC-HVDC ytem under invetigation i calculated in three different operating point. Regarding converion loe, the witche of the NPC converter mut withtand half of the total DC link voltage, 5kV. With 3 percent redundancy, the number of erie connected 3.3kV IGBT needed to withtand 5kV i 6. The parameter ued are: Number of erie IGBT=6 IGBT forward reitance=.5mω IGBT threhold voltage V T =.5V Freewheeling diode forward reitance=.8mω Freewheeling diode forward voltage=.v The clamped diode data are aumed the ame a freewheeling diode. Switching energy due to turn-on and turn-off loe i expreed a: E=ki where k=5μj/a DC cable reitance r dc =5.9mΩ/km DC cable length =5km Tranformer AC reitance=.5ω/phae Interfacing reactor reitance=.5ω/phae The reult in table, and 3 are obtained when the ytem in Fig. i imulated under three different operating condition, unity power factor,.95 lagging power factor and.95 leading power in both tation. In all thee cae the exported power from the wind farm to the grid i maintained at 33MW. Baed on thee reult, the total power loe at unity,.95 power factor lagging and.95 power factor leading are 5.3%, 5.6% and 5% repectively of the tranmitted power. It can be

5 oberved that the maximum power lo converion i obtained at.95 lagging (.86% of the tranmitted power). (A) Cae I: Unity power factor Table. Loe in a VSC-HVDC in MW (unity pf) component Calculated Meaured % loe loe VSC VSC DC cable Wind ide AC Gird ide AC (B) Cae II: Lagging power factor (.95) Table. Loe in a VSC-HVDC in MW (.95 leg) component Calculated Meaured % loe loe VSC VSC DC cable Wind ide AC Gird ide AC (C) Cae III: Leading power factor (.95) Table 3. Loe in a VSC-HVDC in MW (.95 lead) component Calculated Meaured % loe loe VSC VSC DC cable Wind ide AC Gird ide AC Concluion Thi paper preented an analytical etimation of the power loe in a VSC-HVDC ytem baed on the NPC converter. The reult obtained demontrate that the ue of a NPC converter reduce the converion loe in the converter a the effective witching frequency per device i reduced to half compared to a two-level converter. An additional benefit i the reduction of power loe in the damping element of the paive power filter required at converter output. 8 Reference [. Arrillaga.J, Liu Y.H.L and Waton N.R, Flexible Power Tranmiion-The HVDC Option, John Wiley & Son, Publication, 7 [. Bierhoff.M.H and Fuch.F.W, Semiconductor loe in voltage ource and current ource IGBT converter baed on analytical derivation, IEEE 35 TH annual Power Electronic Specialit Conference PESC [3. Brückner T and Holme D.G Optimal pule width modulation for three-level inverter in Proc. IEEE PESC 3, Acapulco, Mexico, 3,pp [4. Dahono.P.A, Sato.Y and Kataoka.T, Analyi of conduction loe in inverter, IEE Proceeding -Electric Power Application, Volume: 4, Iue: 4, July 995. [5. Hui Pang, Guangfu Tang, Zhiyuan He, Evaluation of loe in VSC-HVDC tranmiion ytem, Power and Energy Society General Meeting - Converion and Delivery of Electrical Energy in the t Century, 8 IEEE, Publication Date: -4 July 8. [6. Ikeda Y, Itumi J, Funato H., The power lo of the PWM voltage-fed inverter, Power Electronic Specialit Conference, 988. PESC '88 Record. 9th Annual IEEE Publication Date: -4 April 988, On page(): Vol.. [7. Kolar J.W, Ertl H, Zach, F.C., Influence of the modulation method on the conduction and witching loe of a PWM converter ytem, Indutry application Society Annual Meeting, 99., Conference Record of the 99 IEEE Publication Date: 7- Oct 99,On page(): 5-5 vol.. [8. Metha, L.K and Evan.P.D, Optimization of loe in PWM inverter, Third International Conference on Power Electronic and Variable-Speed Drive; July 988. [9. Rodriquez J, Bernet S, Wu B, Pontt J.O, Kouro S. Multilevel Voltage-Source-Converter Topologie for Indutrial Medium-Voltage Drive, IEEE Tran. on Ind. Electronic, Vol.54, 7, pp [. Tae-Jin Kim, Dae-Wook Kang, Yo-Han Lee and Dong-Seok Hyun, The analyi of conduction and witching loe in multi-level inverter ytem, IEEE 3 nd annual Power Electronic Specialit Conference PESC..

6 9 Appendix: Output characteritic for 33KV/A IGBT from Tohiba