Study of HVDC Controls Through Efficient Dynamic Digital Simulation of Converters

As the applications of HVdc transmission grow, the need for a detailed investigation of the potential problems increases. As an example one could consider the problems of system operation with DC links feeding weak AC systems. There are various aspects of the problem such as voltage dependent current limit, var compensation etc. But it is well recognized that a suitable converter control is the most significant factor in the optimum and secure operation of the system. While HVdc simulators have been traditionally used for the design and optimization of converter control, digital simulation is becoming attractive due to less cost and increased flexibility. In this context, a novel topological model of a converter was presented in [1]. This paper describes the development of the converter model for ??/ 12 pulse operation and presents its application for the study of the performance of digital converter control system. A six pulse converter system shown in Fig. 1 is modelled as a variable voltage source behind a variable impedance. This is based on a graph theory approach which enables to formulate the converter equations for all possible modes of operation assuming continuous DC link current. Converter equations are derived using cutset matrix and the basic circuit equations. The equivalent circuit of the six pulse converter is shown in Fig. 2. The voltage source (eeq) is a function of the converter ac bus voltage.