HVDC Models Used in Stability Studies

Power system ac/dc stability simulation results obtained using three types of HVDC models are compared. The difference in results is shown to be much smaller than the level of uncertainty inherently involved in stability calculations. The theoretical basis for the two commonly used types of HVDC models is discussed and a third model is introduced which uses less computer time than the most detailed model and is more accurate than the simpler model. AC system harmonics and asymmetries cannot generally be represented in power system stability programs so the effects of harmonics and asymmetries on the HVDC system also cannot be modeled. Simulation results obtained using different assumptions regarding the HVDC converters response to ac system asymmetries, different assumptions regarding commutation failure and different ac system load models are compared to estimate the level of uncertainty inherent in the normal stability calculations.