An evaluation of control schemes for a hvde system feeding into a weak ac system

Abstract

An evaluation of steady-state stability boundaries various control schemes at the inverter terminal of a hvdc system feeding into weak ac system has been reported here, The stability analysis has been performed using root focus technique. The influence of the sending and the receiving ac system impedance on the system stability boundaries has also been determined, It is observed that proper optimization of controller parameters may provide the same setting time or system damping.

Since it is a well established fact that the minimal reactive power demand, and the harmonics generation are the salient features of the extinction angle control. Hence, it is not desirable to operate the hvdc system with other control schemes during steady-state. However, the extinction angle control may cause voltage/power instability during transients. Hence it is required to operate the hvdc system with another control scheme during transients. This paper also deals with effects of switching among control schemes for improving system performance. It has been observed that the switching among the control schemes may be achieved successfully and the dynamic overvoltages of the system may be minimized using the ac or the de control scheme. The effectiveness of various control schemes has been determined by introducing each control scheme just after the faults for 300 ms and then reverting the system operation to the extinction angle control. It is also observed that switching among control schemes does not cause instabilities.