Thoughts on Differential Protection Design of Series Transformer in FACTS Device

Series transformers have been widely used in power electronic devices of flexible AC transmission systems such as the unified power flow controller and the static synchronous series compensator. The series transformer under fault will cause the entire device to exit operation. Therefore, it is extremely important to discuss the relay protection of series transformers. In this paper, several typical configuration schemes of the series transformer differential protection in the project were discussed. The fault characteristics of the grid-side different positions of the series transformer were analyzed. The protection scope and the action results of the series transformer differential protection were discussed. Then, the influence of the zero-sequence was discussed. Finally, through the simulation model of the actual project, several typical internal and external faults were discussed. The simulation results verify the protection scope of several typical series transformer differential protection schemes under different fault conditions. It also verifies that the protection will not malfunction under the fault outside the series transformer protection zone, when the zero sequence current is removed from the criterion. The simulation results of the protection action are consistent with the theoretical analysis.