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Abstract - This paper investigates the steady-state and transient performance of high-voltage DC (HVDC) transmission systems connected to passive network. The VSC HVDC tie employing PWM may well represent the ultimate FACTS device. Besides controlling the through power flow, it can supply reactive power and provide independent dynamic control at its two terminals. The control systems for rectifier and inverter are discussed in DC (HVDC) transmission systems based on three-level voltage source converters. The study involves analysis of active-reactive power capabilities (P-Q envelope) including active power flow and provision of voltage support to AC networks. The transient performance is explored by examining the VSC_HVDC response to external AC faults. Finally, the models and results are presented and tested by simulations using Matlab Simulink and its toolbox SimPowerSystems.
Keywords- HVDC, Voltage source converter (VSC), IGBT, SPWM, Control design, passive load
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Introduction
With the scale of the new energy utilization enlarging, and the power requirements of offshore drilling platforms, isolated island, and other passive load, the traditional ac/dc transmission technology becomes diseconomical and environmental pollution.
VSC-HVDC system is a new generation of HVDC technology based on pulse width modulation and voltage source converter (VSC), the modern high-power power electronic technology applied in the power system. Comparing with traditional HVDC based on phase control converter (PCC), control method of VSC-HVDC system is flexible, and has no failure of commutation, small harmonic content, without capacity requirements about terminal power system, the reactive power of system could be controlled. The economic capacity of VSC-HVDC system extends from several megawatts to hundreds of megawatts [1]. There are seven VSCHVDC system put into operation abroad[8-10].
The introduction of pulse width modulated voltage source converter technology into high-voltage DC (HVDC) transmission systems has increased their viability in many applications in terms of cost and performance [5]. The main benefits of VSC-HVDC over the classic LCC-HVDC are [4]- [12]:
* Converter inherent reactive power capability (resulting in smaller converter size and reduced filtering requirements.
* Independent control of active and reactive power (allowing the converters to provide damping, frequency and voltage support to AC networks without compromising system performance).
* Black start capability (extending the use of HVDC systems for connection of weak AC networks with no generation).