HVDC control development for isolated small AC system with real time digital simulator

This paper reports the control strategy and its simulator studies for HVDC transmission system feeding power to an isolated weak AC network. Authors investigated the control strategy in which the DC current control is done at the receiving-end. The combination of the receiving-end current control and the weak AC system is one of fields not yet studied in detail. The authors proposed a prototype control and evaluated it through studies with the real time digital simulator. The prototype control showed instabilities in dynamic simulation cases. Then, authors discussed and investigated the instability through the simulation and found the causes of the instability. The first point to be improved was the coordination between the current order and the actual current. The second point was the gamma control considering the large voltage phase shift at the power change in the weak AC system. The improved control was installed again in the real time digital simulator and finally verified to show good performances at step response and at the recovery from AC faults.