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Validity test of the HVDC quasisteady-state model by simulation

The quasisteady-state (QSS) model of converter is adopted by most electromechanical transient simulation programs. In a QSS model, the commutating voltage is the voltage at the converter bus, and the commutation reactance is the leakage reactance of the converter transformer. Hence, in the QSS model, the converters in series in a multiple-bridge converter are independent. However in the classical converter model, the commutating voltage is the equivalent AC EMF viewed from the commutation bus, and the commutation reactance consists of the leakage reactance of the commutation transformer and the equivalent AC reactance, that is to say, the converters in series are coupled at the AC side of the system. So, differences obviously exist in these two models. This paper uses an electromagnetic transient simulation program to verify the validity of the QSS model. The simulation proved that the QSS model is valid if the AC system is well filtered and the voltage at the commutation bus is symmetrical and sinusoidal.