Achieving an accurate power balance between the two poles of a symmetrical High Voltage Direct Current (HVDC) system is important not only for HVDC system operation, but also for maintaining a minimum current flowing to the electrode system (ES) due to regulation requirements. The Bipolar Balance Control Strategy (BBCS) for a multi-terminal HVDC system becomes much more complex than the case of two-terminal HVDC systems because an additional converter station introduces a variety of combinations of rectifier and inverter. When the power order at one station is changed, a decision must be made on how to distribute this change among the rest of the stations. In addition, there is a possible mix of monopole and bipole combination in the multi-terminal HVDC systems when one converter of a station is out of service. In this situation, if the ES of a bipole is not in service or in fault, the balance of this station needs to be checked and the power orders may need to be recalculated and distributed among all the stations in service in a coordinated manner.
This paper presents a novel control strategy for balancing a bipolar system in a three-terminal HVDC system. It is mainly about an emergency and automatic pole balance control when a fault on the HVDC neutral circuit occurs. The control strategy accounts for converter operation mode, the minimum permissible operation current of the converter and the priority assigned by energy dispatch to derive a feasible solution. This method is equally applicable to the multi-terminal Voltage Sourced Converter systems, which have been becoming popular in recent years. The control strategy was successfully implemented in the Lugaozhao ±500kV, 3000MW three-terminal Line Commutated Converter (LCC) HVDC project in China. The results of the studies performed on the world’s largest RTDS, for various different operation configurations, are presented.
Q. Guo, M. Liao, Z. Song, L. Huang, L. Deng, M. Su, Presented at CIGRE Session 2022, Paris, France
KEYWORDS: Bipolar Balance Control Strategy (BBCS), three-terminal HVDC, electrode system