VOLTAGE SOURCE CONVERTER BASED HVDC ' TRANSMISSION SYSTEM

Abstract

The advancement in high voltage and high power, fully controlled semiconductor technology has led to the development of voltage source converters. The voltage source converter based HVDC is a recent technology having fast dynamic response, independent control of active and reactive power consumed or generated by the converter and the ability to connect with "weak" or even with passive AC network. HVDC technology based on VSC technology has been an area of growing interest recently because of its suitability in forming a transmission link for transmitting bulk amount of wind power and supply to island system. This thesis deals with the control of VSC-HVDC. The objective of the work is to understand the control structure of the VSC-HVDC system. A steady state model of a two terminal voltage source converter based HVDC system has been developed. The model has been implemented using PSCAD/EMTDC. The voltage source converters (VSC) are implemented based on power electronic switches, namely insulated gate bipolar transistor (IGBT) and sinusoidal, pulse width modulation SPWM. The phase locked-loop (PLL) used to generate the carrier wave and synchronized reference wave for implementing the sine-triangle modulation. A mathematical model of the control system based on the relationships between voltage and current is described for the VSC. A control system is implemented for direct control using proportional plus integral controller. The vector control strategy is studied. The performance under system fault is investigated in PSCAD/EMTDC simulation package. The simulation results verify that the model has fast response, control and that the system has good steady state performance. A small distribution network is developed with constant PQ load and induction motor load or industrial load and it is observed that the system is under control after AC instantaneous single phase ground fault and three phase to ground fault, testify that the control strategy has fast response and strong stability.