Protection Relays

Protection Relay Functions

Protective devices that are used to protect electrical systems & equipment usually have one or more functions. Protective relays detect fault conditions and initiate circuit breaker trips to de-energize faulted equipment or circuits before serious damage can occur.

Basic function includes:
  1. Alert operating personnel to abnormal or potentially dangerous conditions or to the fact that a trip circuit has been energized.
  2. Automatically interrupt current flow to equipment if a potentially dangerous fault occurs.
  3. Automatically energize stand-by equipment as necessary to maintain system operation.
Sample Protection Relay Scheme

Common Characteristics of Protection Relays
  1. Reliability - Must operate when they are supposed to.
  2. Speed - Must be able to respond to a fault and isolate the affected equipment before damage can occur.
  3. Simplicity - Important for economic reasons as well as for maintenance efficiency.
  4. Sensitivity - Able to detect a fault as soon as it occursSelectivity - Should isolate only the faulted area.


The relay co-ordination refers to the tripping of protecting relay in a proper sequence or order in electrical power system. This is to avoid tripping of un-faulted branch in the system. Relay co-ordination is required to isolate the faulty part with minimized relay & circuit breaker operation.

The importance of coordination in Protection Relay Scheme

Zones of Protection

Protected zones are established to protect certain components such as:
  • Generator
  • Transformers
  • Buses
  • Motors

ANSI Device Numbers

Each relay in a protection scheme has a specific functions and responds to a certain type of fault encountered in the power system. 

ANSI device numbers. In the design of electrical power systems, the ANSI standard device numbers (ANSI /IEEE Standard C37. 2 Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations ) identifies the features of a protective device such as a relay or circuit breaker.

Sample ANSI Device Designation

Basic Protection  

ANSI Device Number
The differential relay operates whenever there is a difference between the currents going into and coming out of any of the three phases.
Generator Short Circuit Protection
The result of a breakdown of the insulation in one of the phase windings
Generator Ground Fault Protection
Can be caused by tripping of a field breaker or by short circuits in the field (rotor) windings
Generator Loss of Field Excitation
It can happen when there is not enough steam flow to the turbine to drive the generator
Generator Motoring Protection
The relay trips the feeder breaker if an overcurrent condition occurs
Bus Overcurrent Protection
If an undervoltage condition occurs, the relay trips the circuit breakers to the loads on the bus that could be damaged by the undervoltage condition.
Bus Undervoltage Protection
If a ground occurs, the relay senses it and operates an alarm to alert operating personnel
Bus Ground Detection and Protection
If a fault occurs in the transformer, current flow through the current transformers becomes unbalanced.
Transformer Differential Protection Relay

Electronic Protection Relays

Nowadays, most of the protection relays used are electronic type due to its greater precision and allow closer system coordination. Also, the accuracy of solid state relay is greater than the electromechanical relays. 

One of the great features of solid state relays is it keeps the history of the operation. And when the relays are networked together, it can be synchronized to a master clock and all significant events can be recorded. 


The electronic protection relay can be connected with the use of RS232 or RS 485 connections with a local computer. These can be used for relay configuration, monitoring and troubleshooting. Most especially, the relays now provide Ethernet capability for networking. Communication between relays in the power system makes it possible to exchange input and outputs thru a communication link, thus reducing the amount of hard wiring. 

Protection Relay Communication