Principles of Relay Protection Zones for Electrical Power Systems

The "protection zone" in an electrical power system is defined as the specific region within the system that is monitored and protected from faults by protective relays. This zone is established around each major piece of equipment within the power system.

When a fault occurs within any of the protection zones, the protective relays will call on the circuit breakers within that zone to be opened. This allows only the affected equipment to be isolated while keeping the rest of the system energized.

Protection schemes encompass the entire power system, meaning no piece of distribution equipment is left unprotected. How a zone of protection is defined within a power system mainly depends upon the rating of the apparatus, its location, the probability of faults and abnormal condition of the equipment.

Protection zones in a electrical power distribution system

Example of protection zones in an electrical power distribution system. Photo: EATON.

If a fault were to occur outside the zone of protection, it would be undetected by the relays and not cleared until physical equipment failure or a fault magnitude great enough to cause a disturbance in another zone. For this reason, each zone of protection is designed to overlap its neighboring zone for increased reliability in the event of relay failure.

The probability of failure in the overlapping regions is very small but it may cause additional circuit breakers within the zone to trip. Protective relays used within any protection scheme should be designed, manufactured and applied with the following criteria:

  • Reliability – Relays must have the ability to perform correctly when needed (dependability) and to avoid unnecessary operation (security).
  • Speed – Relays must operate with minimum fault duration and minimize equipment damage.
  • Selectivity – Protective relays must provide maximum service continuity with minimum system disconnection.
  • Economics – The best relays will offer maximum protection at minimum cost.
  • Simplicity – Protective relays should contain minimum equipment, circuitry and sequence of operations.

Further Reading