Calculation of valve damping circuit losses in 12-pulse HVDC converters

The use of published equations for calculating damping circuit losses in a 12-pulse HVDC power converter relies on the assumption that the inductive coupling between the pair of 30° phase-displaced 6-pulse converter groups is negligably small. However, there are instances where this coupling cannot be ignored. An analytical method is presented for the calculation of valve damping circuit losses of 12-pulse converters, taking the effects of inductive coupling into account. The extra commutation notches introduced to the valve voltage waveform are expressed in terms of a notch factor m, and a general equation for the valve damping circuit loss is derived. The equation is divided into two parts so that the effects of valve stray capacitance and multiple valve grading circuits can be accommodated in a logical manner. An additional component of loss arising from the thyristor stored charge phenomenon is also discussed