The ±500 kV Xiluodu-Guangdong HVDC transmission project is the first double-circuit project putting into operation in China and its rated DC power is 6400 MW. This project is designed in a scheme that two DC circuits use common earth electrode, share primary equipments in AC yard and four DC transmission lines are erected on the same tower. In contrast to traditional bipole HVDC project, the double-circuit project need more complicated DC control system and control strategies. This paper focuses on the research of the control strategies for this project.
First, a well-designed hierarchical structure of the double-circuit DC control system is presented, as well as the control function configuration. Also a complete backup control scheme is introduced which increases the reliability of the control system.
Second, the double-circuit coordinate control strategies are researched carefully and the effective principles are presented in this paper. The main contents include the following aspects:
The total DC power coordinate strategy can ensure that DC power is assigned reasonably between two DC circuits under various operation modes. In case of emergencies such as pole trip, DC power can transfer rapidly between two DC circuits and the DC power loss can be reduced to minimum.
Power modulation can improve the performance of the whole AC/DC system. In double-circuit DC system, power modulation is designed as a double-circuit level function and it also need a coordinate control strategy between two DC circuits.
In this project, two DC circuits share the AC filters in AC yard to improve the utilization ratio of the equipments. The reactive power control strategy uses a kind of unified control way to ensure that the harmonic performance requirements are met and AC filters operate safely.
In this project, the current limit level of common earth electrode is designed according to traditional bipole HVDC project. The earth electrode current may reach almost twice the design level in some extreme cases, so the earth electrode current limitation function is required and it can reduce the common earth electrode current to the safety value quickly by an elaborate strategy.
Then, the island operation mode at the rectifier side is taken into account in this project and some special control strategies are required. It provides a close loop frequency control function to ensure frequency stability of the island AC system and an AC overvoltage control function to ensure the safety operation of primary equipments.
Research of this paper can provide some references for the design and implementation of similar projects such as ±800kV double-circuit UHVDC project.
Double Circuit HVDC, Coordinate Control Strategy, Common Earth Electrode, Island Operation
