What is the function of a distance relay?

Short Answer:

A distance relay is a type of protection relay used mainly for transmission line protection. Its main function is to measure the impedance (which depends on voltage and current) between the relay location and the fault point. If the measured impedance drops below a set value, it means the fault is within a protected zone, and the relay trips the circuit breaker.

Unlike overcurrent relays, distance relays work based on how far the fault is from the relay location. This makes them suitable for long transmission lines, where fault current levels can vary with distance or source strength.

Detailed Explanation:

Function of a distance relay

The distance relay is a critical protective device in power systems, especially for high-voltage transmission lines. It operates by calculating the apparent impedance from the relay location to the fault location. This impedance is proportional to the distance from the relay to the fault, hence the name distance relay.

Impedance (Z) is calculated using the formula:

Z=VIZ = \frac{V}{I}Z=IV​

Where:

  • V is the voltage at the relay location
  • I is the current flowing during the fault

If the impedance value falls below a set threshold (known as the relay’s zone setting), the relay interprets this as a fault within its protection zone and sends a signal to the circuit breaker to trip and isolate the fault.

How it works

  1. Voltage and current measurement:
    • The relay takes inputs from voltage transformers (VTs) and current transformers (CTs).
  2. Impedance calculation:
    • It continuously calculates the V/I ratio to determine the apparent impedance.
  3. Zone comparison:
    • The measured impedance is compared with preset zone settings.
    • If impedance is less than the setting for a particular zone, it means the fault lies within that zone.
  4. Tripping decision:
    • If the fault lies within the zone, the relay issues a trip command to open the circuit breaker.
  5. Time delay coordination:
    • Multiple zones (Zone 1, Zone 2, Zone 3) are defined with increasing reach and associated time delays to ensure proper backup coordination.

Zones of protection

  • Zone 1:
    • Covers 80–90% of the transmission line from the relay location.
    • Instantaneous operation with no intentional delay.
  • Zone 2:
    • Extends beyond the end of the line to protect part of the next line section.
    • Has a time delay to coordinate with downstream protection.
  • Zone 3:
    • Acts as a backup for the second next section.
    • Has a longer time delay and wider reach.

Advantages of distance relays

  • Independent of fault current magnitude:
    Operates based on impedance, not just current levels.
  • Good selectivity and speed:
    Can detect and clear faults quickly within defined zones.
  • Ideal for long lines:
    Especially effective where current-based relays may not perform accurately.
  • Coordination and backup:
    Zone settings allow time-graded protection across multiple line sections.

Applications

  • Transmission line protection in HV and EHV systems
  • Backup protection for other relays
  • Multi-terminal line protection where directional decisions are important
  • Protection for parallel lines and long radial feeders
Conclusion:

The function of a distance relay is to measure the apparent impedance to a fault and determine whether the fault lies within a predefined zone. It provides fast, accurate, and distance-based protection for transmission lines. Its ability to work independently of fault current magnitude makes it a reliable and essential tool in high-voltage power system protection.