What is directional and non directional relay ?

Relays are devices used in power systems for protection and control purposes. Directional and non-directional relays are two categories of protective relays that differ in their operating principles and applications.

Directional Relay:

  1. Purpose: The primary purpose of a directional relay is to provide selective protection by responding to faults only in a specific direction within a power system. This selective operation helps isolate and clear faults more precisely, minimizing the impact on the rest of the system.
  2. Operating Principle: Directional relays operate based on the direction of the fault current concerning the relay location. These relays are designed to respond only when the fault current flows in the predetermined direction, allowing them to selectively trip the circuit breakers connected to the faulty section.
  3. Applications:
    • Transmission Lines: Directional relays are commonly used to protect transmission lines. By detecting the direction of fault current, these relays can isolate faults on specific line sections without affecting the entire transmission network.
    • Busbar Protection: In substation protection, directional relays are used to protect busbars by responding to faults occurring in specific directions within the substation.
  4. Methods of Directional Protection:
    • Current Comparison: Directional relays can compare the current magnitude and phase angle at different locations to determine the fault direction.
    • Voltage Comparison: By comparing voltage magnitudes and phase angles, directional relays can ascertain the direction of the fault.

Non-Directional Relay:

  1. Purpose: Non-directional relays provide protection without considering the direction of the fault current. They are simpler in design and are suitable for applications where selective protection based on fault direction is not a critical requirement.
  2. Operating Principle: Non-directional relays operate based on the magnitude of the fault current or voltage, without considering the direction. When the current or voltage exceeds a predefined threshold, the relay operates to trip the circuit breaker and isolate the faulty section.
  3. Applications:
    • Distribution Systems: Non-directional relays are commonly used in distribution systems to protect feeders and distribution transformers.
    • Backup Protection: In systems where directional protection is already provided by other relays, non-directional relays can serve as backup protection, ensuring fault clearance in case the primary protection fails.
  4. Methods of Operation:
    • Overcurrent Protection: Non-directional relays often use overcurrent protection schemes. When the current exceeds a set threshold, the relay operates to trip the circuit breaker.
    • Voltage Protection: Some non-directional relays use voltage protection to detect abnormal voltage conditions, triggering a protective response.

Comparison:

  • Selectivity: Directional relays offer higher selectivity by responding to faults only in a specific direction, minimizing unnecessary tripping. Non-directional relays are less selective and may trip for faults in any direction.
  • Complexity: Directional relays are generally more complex in design and require additional features to determine the fault direction accurately. Non-directional relays are simpler and often used in less critical applications.
  • Applications: Directional relays are preferred for critical applications like transmission lines and busbar protection where selective tripping is crucial. Non-directional relays are suitable for distribution systems and backup protection scenarios.

In summary, the choice between directional and non-directional relays depends on the specific requirements of the power system and the level of selectivity needed for effective protection. Directional relays offer precision in fault detection and isolation, while non-directional relays provide simpler and more cost-effective protection in less critical scenarios.

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