Power protection relays employ numerous schemes to safeguard infrastructure. These schemes are chosen based on system configuration, location, and specific power applications.
In our work with high-voltage transmission substations across Nigeria, we commonly implement the following critical protection layers:
1. Distance Protection 21
This protects transmission lines by dividing the protected element (line and connected buses) into zones based on its impedance. This allows engineers to pinpoint the location of a fault and coordinate protection based on the specific zone affected.
2. Differential Protection 87
This monitors the current entering and exiting a zone. If a fault occurs within that zone, a difference is sensed, causing the breakers at both ends to trip. We typically deploy this for:
- 87T: Transformer Protection
- 87B: Busbar Protection
- 87L: Line Protection
3. Overcurrent Protection 50/51
Senses phase or neutral current. If it exceeds a threshold, it signals a trip.
• IDMT (51): Uses an inverse curve (Higher current = Quicker trip).
• Instantaneous (50): Immediate trip without time delay.
4. Earthfault Protection 50N/51N & 50G/51G
50G/51G works via a Neutral CT connecting the transformer star point to ground. 50N/51N vectorially sums the three phases to detect unbalance currents.
5. Autoreclosure 79
Crucial for overhead lines. It automatically recloses the CB after a trip to determine if a fault was transient (like a falling tree branch) or permanent. If the fault persists after specified attempts, the CB locks out.
Technical Discussion
These are just a few schemes utilized in professional transmission substations. For other engineers in the field: What additional protection schemes are you currently prioritizing in your system designs?