25-08-2017, 09:32 PM
Abstract
This paper presents a general model of dual infeedHVDC system applying VSC-HVDC, which can be used as anelement in large multi infeed HVDC system. The model may havedifferent structure under different grid faults because of theaction of breakers. Hence power flow of the system based on thismodel is analyzed under steady state and different kinds of gridfault transient situations. Two main control methods applied onVSC-HVDC link in this dual infeed HVDC system areinvestigated, and comparative analysis of them under transientsituation is presented. A simulation model is built inPSCAD/EMTDC to verify the theoretical analysis. Simulationresults indicate that this dual infeed HVDC system can realizehigher stability than single infeed HVDC system. And differentcontrol strategies on a VSC-HVDC link may result in differentinfluence on AC voltage and active power oscillation duringtransient situation.Keywords--HVDC, VSC-HVDC, multi-infeed HVDC,Transmission system, voltage stability
I. INTRODUCTION
OVER the last decade, High Voltage Direct Current (HVDC)technology has been widely applied for long-distance,bulk power delivery. With increased utilization of windenergy especially offshore wind power, connection of largescale wind energy, especially offshore wind farms makeHVDC links play a more important role in the future grid.Moreover, the future super grid in Europe force the HVDClinks even become denser.With the increased applications of HVDC transmissionsystems, two or more HVDC links terminate into AC system.Such systems are termed as Multi-Infeed Direct Current(MIDC) systems [1]-[2]. Appearance of MIDC system canimprove the transmission ability and provide more flexibleoperation modes. However, MIDC system based on LCC(Line Commutated Converter) HVDC links may bring theproblem of voltage stability [3]-[4]. With the rapiddevelopment of power electronics, voltage source converter(VSC) based on self-commutation devices such as GTO orIGBT has been applied in HVDC links which is known asVSC-HVDC system [5]-[7]. Flexible control of voltage sourceconverter makes VSC-HVDC link good performance ofreactive power compensation. Hence, in contrast to singleinfeed conventional LCC HVDC links, multi infeed HVDCYan Liu and Zhe Chen are with the Department of Energy Technology,Aalborg University, Aalborg, 9220 Denmark (e-mail: yli[at]iet.aau.dk).system applying VSC-HVDC not only help to keep constantpower deliver under emergency situation but also stabilize thevoltage of AC buses, and consequently improve the stabilityof the system.A general model of a dual infeed HVDC system applyingVSC-HVDC is established below. Detailed analysis aboutperformance and power flow based on that model understeady state and grid fault transient situation followed.Furthermore, two main control methods including d-q controland AC voltage control are applied in VSC-HVDC. Theperformances of these control schemes for this dual infeedHVDC model under the same system situations are discussed.Finally, to verify the theoretical analysis, simulation on themodel by using PSCAD/EMTDC is performed.
II. SYSTEM CIRCUIT CONFIGURATION
Many research works on the multi-infeed HVDC systemapplying VSC-HVDC have been published in previousliterature, in which different kinds of models were developedbased on different system structures. For multi infeed systemwith two HVDC links, [8] provided a model assume that bothlinks terminate into one AC bus as depicted in Fig.1 (a). Formulti infeed system with three HVDC links, [9] present amodel assume the VSC-HVDC link connect to both other twobuses of LCC-HVDC links but not directly terminate to thegrid as presented in Fig.1
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