Sudden Change in Current on 35kV Busbar of Substation

A sudden change in current on a 35kV busbar is typically caused by grounding faults, internal busbar faults, or equipment failures, and requires immediate detection, isolation, and inspection to prevent damage and outages.

Possible Causes

1. Single-Phase Grounding Faults: In 35kV systems with ungrounded or arc-suppression grounded neutrals, a single-phase-to-ground fault can cause the faulted phase voltage to drop near zero while the other two phase voltages rise to √3 times nominal voltage. This results in a sudden current through the arc suppression coil and triggers busbar grounding alarms ( ).
2. Internal Busbar Faults: Short circuits or internal faults within the busbar can cause abrupt current surges. Low-impedance differential relays detect these faults by comparing currents from all connected circuits. CT saturation during external faults may produce false differential currents, but adaptive trip logic prevents unnecessary tripping ( ).
3. Equipment Failures: Failures in voltage transformers (VTs), current transformers (CTs), or circuit breakers can lead to sudden current changes. Intermittent grounding or insulation breakdown may cause fluctuating currents and voltage irregularities ( ).

Detection and Protection

• Busbar Differential Protection: Relays like GE Multilin B90 or B30 use phase-segregated, centralized protection schemes to detect internal faults and isolate only the affected bus segment, preventing unnecessary outages ( ).
• Grounding Alarms: SCADA systems monitor 3V₀ voltage, arc suppression coil current, and small-current fault line selectors to identify grounding faults ( ).
• Overcurrent and Ground Relays: Sensitive ground relays and overcurrent protection devices detect abnormal currents and trip circuit breakers to isolate the faulted section ( ).

Operational Response

1. Immediate Notification: Central control personnel should be alerted to inspect the substation and verify protection status.
2. Isolation of Faulted Equipment: Faulted lines or bus segments should be isolated promptly to prevent escalation into phase-to-phase short circuits ( ).
3. Load Transfer and Trial Switching: If the system allows, transfer loads from the faulted line before disconnection. Trial switching or remote tripping via SCADA may be used while ensuring critical loads are maintained ( ).
4. Inspection and Maintenance: Physical inspection of busbars, VTs, CTs, and arc suppression coils is necessary to locate faults, check for overheating, and repair or replace damaged components ( ).

Safety Considerations

• Avoid back-feeding power into the system during fault isolation.
• Monitor arc suppression coil temperature to prevent damage.
• Ensure selective disconnection to avoid overloading healthy lines.

Summary

A sudden current change on a 35kV busbar is a serious event often caused by grounding faults, internal busbar faults, or equipment failures. Effective detection using differential and ground relays, immediate isolation, and careful inspection are critical to maintain substation safety and reliability. Proper busbar design, protection schemes, and operational protocols minimize downtime and prevent equipment damage ( ).