Discussion of ±800 kV UHVDC transmission control and protection system

Abstract

To explore the similarities and differences between the control and protections of ultra-high voltage direct current systems (UHVDC) and conventional high voltage direct current systems (HVDC), the characteristics of UHVDC control and protection system is analyzed in detail. Firstly, taking the Xiangjiaba-Shanghai UHVDC project as an example, we introduced its control and protection framework, configuration features, and its similarities and differences compared with that of HVDC. Then, we analyzed the control algorithm changes in UHVDC, such as changes in the control algorithms of power compensation, valve control, online input and withdrawal strategy of converters, de-icing operation mode and so on. Finally, we discussed the converter transformer saturation protection and the final breaker protection principles which were developed based on HVDC protection, and described the specific protection functions of UHVDC. According to the analysis, the dual 12-pulse valve in series adopted in the UHVDC projects, and the bypass breakers added in the bus area of valve connecting in which UHVDC is different from HVDC in primary connections, as well as the higher protection standard for UHVDC, are the main reasons that cause the difference of UHVDC protection systems from HVDC. A protection system DCC800 is capable of controlling valves in series, enhancing the coordination of control and protection among valves, which will result in more flexibility and diversity of UHVDC operations. In the presented UHVDC protection system, a "Two out of three" protection principle and redundant eTDM buses are adopted in UHVDC, and the problem of main computer stalling is solved well, ensuring that the system has more overall reliability.