Digital-computer simulation of transients in h.v. d.c. convertors with harmonic filters

In the practical design of h.v.d.c. convertor installations, a thorough investigation should be made of all the possible component-overstressing situations that may arise for a variety of reasons. This may, for example, be attempted with a scaled-down model of the installation, although the component losses in such a model are usually excessive, and, consequently, measured peak overvoltages and overcurrents tend to err on the low side. To provide the designer with a more flexible and reliable tool for the analysis of the transient behaviour of h.v.d.c. systems, a digital-computer simulation of a typical convertor station has been developed. The paper is chiefly concerned with the accurate simulation of the main circuitry, having particular regard to the more complex modes of operation that arise during valve faults. Particular attention is also paid to the influence of the harmonic filter, which must be accurately simulated if the results are to be meaningful. The computer programs, based on the analysis presented in the paper, have been used successfully to solve the many and varied transient problems that must be considered at the design stage. An example is presented to demonstrate the results obtained with these programs. It is concerned with maintaining a sufficient margin of commutation in an invertor, despite a sudden voltage drop in the receiving a.c. network. The influence of the harmonic filter, in this respect, is clearly demonstrated. The simulation of an overall h.v.d.c. system is quite within the capabilities of the programs that have been developed, which is particularly relevant when the transient performance of a complex d.c. link is to be investigated. The present paper does not, however, concern itself with details of practical or theoretical control systems for such links.