Dynamic simulation of ac/dc systems with reference to convertor control and unit connection

Abstract

This thesis investigates the limitations of conventional steady state formulation when applied to the unit connected generator-HVdc convertor systems and justifies the need for dynamic simulation. The merits of available dynamic simulation algorithms, namely the state variable and EMTP techniques are discussed with reference to generator-convertor modelling. The state variable is selected and an existing algorithm (TCS) is improved to permit flexible controller modelling. The TCS algorithm is verified by comparison with another dynamic simulation program, EMTDC, which is based on EMTP algorithm. Both TCS and EMTDC are shown capable of predicting the same dynamic performance following disturbances. The limitations of steady state formulation for unit connected HVdc systems are demonstrated with the help of TCS and it is shown that the characteristics must be derived using a dynamic simulation program. Comparative operational capability charts are developed using TCS for conventional and unit connected HVdc schemes, showing the limitations of the latter to provide temporary overloads. Harmonic current and voltage ratings spectra for the region of operational capability of the unit connection are also derived. The operating characteristics and harmonic problems of variable speed unit connected generator-HVdc convertor systems are later analysed. With reference to a typical hydro electric test system, it is shown that it is possible to operate the turbine generator units within a wide range of frequencies at high efficiencies and with good voltage controllability.