Analysis of 115kv Line Differential Protection Trip After The Shunt Capacitor Bank Energization

Ensuring prompt fault clearance and minimizing power outages are top priorities for electric utility companies. Protection schemes must function effectively to quickly isolate faults and reduce their impact on the transmission system. This paper investigates the misoperation of line differential protection systems following the energization of line shunt capacitor banks. The fault records captured by the relay and Digital Fault Recorder (DFR) provided valuable insights into the nature of this disturbance. The analog and digital data from these fault records enable efficient investigation and accurate analysis of the events.

The super short 115kV T3L line connects stations M2 and M5. A 115kV 63MVAR shunt capacitor bank is tapped to the line at station M5, and the current from the bushing current transformers (CTs) on the line side of the cap bank dead tank breaker is subtracted from the T3L line CTs at M5 to ensure that the differential relays measure the 'pure' line current. On March 5, 2025, the T3L line was energized without the capacitor bank to verify the circuits and differential relays, and everything functioned normally. However, when the shunt capacitor bank was put into service, the line differential relays at both terminals tripped immediately after the capacitor bank breaker was closed, despite the cap bank current being subtracted for the line differential protection. A review of the relay and Digital Fault Recorder (DFR) records revealed that the capacitor bank breaker had been installed in the incorrect orientation. This misalignment caused the A and C phase secondary currents from the bushing current transformers (CTs) to be swapped and additive in differential circuit due to the polarity of the CT connection on the wrong side of the breaker. This misconfiguration clarified the operation of the differential protection. Additional protection issues were also identified due to the incorrect orientation of the breaker. Corrections were implemented by rewiring the bushing CTs.

After reading this paper, the reader will understand how to identify the root cause of unwanted protection trips and initiate corrections to the secondary system resulting from improper installation of primary equipment, ultimately minimizing costs and outage time.