Study on the influence of UPFC on pilot protective relaying of transmission line

In order to enhance the power transmission capability of the west Nanjing power grid, the unified power flow controller (UPFC) project has been put into operation. However, the presence of UPFC would adversely impact the performance of transmission line pilot protective relaying, since it changes the features of the fault voltage and current measured by the relay. Based on brief introduction of the UPFC project and its protection system, the influence of UPFC on pilot zero-sequence protection, pilot distance protection and superimposed component direction protection is analyzed. It is found that the performances of pilot zero-sequence protection and pilot distance protection would not be impacted. However, the superimposed component direction protection would fail to trip or trip incorrectly, due to the trip of UPFC protection and subsequent out-of-service of UPFC.