Study Of Control And Protection Strategies For VSC Based HVDC System

VSC-HVDC with some particular advantages over the conventional HVDC is considered as a major development in HVDC technology. However, many technical problems in VSC-HVDC area are still unsolved or solved not well. This dissertation will study those unsolved problems and propose some effective strategies to solve them. Those novel strategies proposed are as follows:(1) The steady-state interaction between VSC and its connecting AC system are analyzed and two steady-state mathematical models are developed, one of which suits the VSC-HVDC connecting two active AC systems, the other suits the VSC-HVDC supplying passive loads. Those two models describe the relationship between the controlled variables such as the active power, reactive power, voltage, current and the controlling variables of the VSC. On the basis of those two models, two control strategies are proposed. In order to testify these two strategies, two VSC-HVDC control systems are created according to each strategy. Some important parts for these two VSC-HVDC control systems are designed in detail, especially the structures and parameters of the VSC controllers, as well as the principles for selecting the equivalent inductance of the convertor transformer and the capacitance of the capacitor on the DC side of the VSC. Digital simulation using NETOMAC is done to verify the validity of those proposed control strategies.(2) The operation performance of the VSC-HVDC under some characteristic faulted conditions without any protection measures are studied, and the over-current and over-voltage of the VSC-HVDC during those faults is revealed. Some protection strategies are proposed to prevent the VSC-HVDC from over-voltage and over-current damage in most fault conditions. Digital simulation is done to verify the validity of the proposed protection strategy.(3) A control strategy for a diode clamped three-level VSC-HVDC System is proposed and is testified by digital simulation using NETOMAC. Some practical method to lifting the ratedcapacity of VSC-HVDC is discussed.(4) The subsynchronous oscillation (SSO) characteristics of a synchronous machine connected to a static VAR compensator is studied with the complex torque coefficient method realized by time domain simulation, and some important conclusions are obtained which can guide the design and operation of SVCs to avoid SSO problem.