Line Protection for Multi-terminal Flexible DC Distribution System Based on Control and Protection Coordination

In a multi-terminal DC distribution system, the converters will be blocked within 1~2ms due to the poor ability of the power electronic equipment to withstand the fault current. Most of the DC protections use transient signal of extremely short data to form the criterion, which is susceptible to noise, measurement error and control switching. As a result, its performance will be affected. Therefore, this paper proposed a protection for multi-terminal DC power distribution system based on the coordination with control and protection. This protection utilizes the coordinating control between local protection and converters, and then it changes the modulation frequency of the converters in the system by controlling the switch mode. Through the process above, the converter can be changed into an injection source with characteristic signals. Afterwards, this protection calculates and detects using characteristic signals to ultimately form the protection of DC lines. Compared with the traditional DC protection, this method solved the problem that the converter cannot provide effective fault information after fast-blocking. Besides, it does not need to add additional equipment, nor does it need high data sampling frequency. The principle is simple and reliable, and moreover, it can effectively realize the combination of control and protection in power electronic equipment. Finally, a multi-terminal DC power distribution system was built in PSCAD/EMTDC to verify the effect of the proposed protection. © 2020 Chin. Soc. for Elec. Eng.