Reactive Power and AC Voltage Control of LCC HVDC System With Controllable Capacitors

It is well known that traditional line-commutated converter (LCC) based high voltage direct current (HVDC) system is not able to control its reactive power and terminal AC voltages. This paper investigates the reactive power and AC voltage control at the inverter side of the LCC HVDC system with controllable capacitors. The system's ability of operating under negative extinction angle is utilized to achieve a wide range of reactive power control and, in particular, the ability of exporting reactive power. In connection with the inverter AC terminal voltage or reactive power control, among different control possibilities at the rectifier side, active power control is desirable since large variations of active power transfer is very unfavorable. Detailed theoretical analysis is carried out first to show the reactive power controllability, and the capacitor voltage level is selected based on the desired control range. In addition, a new extinction angle measurement approach is proposed for negative extinction angle measurements. The effectiveness of the reactive power/voltage control capability for the proposed system is validated through simulation results using Real-Time Digital Simulator (RTDS). To verify the effectiveness of the reactive power and voltage control, CCC HVDC and LCC HVDC with SVC are also set up in RTDS, and simulation comparisons are made. Furthermore, contribution to AC voltage control in power system using the proposed method is demonstrated through simulation results of the modified two-area four-machine AC power system.