Why Numerical Relay over Electromechanical Relay

Advantages of Numerical relays

1.      Compact Size

Electromechanical Relay makes use of mechanical comparison devices, which cause the main reason for the bulky size of relays. It uses a flag system for the indication purpose whether the relay has been activated or not.

While numerical relay is in compact size and use indication on LCD for relay activation.

Digital protection can be physically smaller, and almost always requires less panel wiring than equivalent functions implemented using analog technology.

2.      Flexibility

A variety of protection functions can be accomplished with suitable modifications in the software only either with the same hardware or with slight modifications in the hardware.

3.      Reliability

A significant improvement in the relay reliability is obtained because the use of fewer components results in less interconnections and reduced component failures.

4.      Multi Function Capability

Traditional electromechanical and static protection relays offers single-function and single characteristics. Range of operation of electromechanical relays is narrow as compared to numerical relay.

5.      Different types of relay characteristics

It is possible to provide better matching of protection characteristics since these characteristics are stored in the memory of the microprocessor.

6.      Digital communication capabilities

The microprocessor based relay furnishes easy interface with digital communication equipment. Fibre optical communication with substation LAN.

7.      Modular frame

The relay hardware consists of standard modules resulting in ease of service.

8.      Low burden

The microprocessor based relays have minimum burden on the instrument transformers.

9.      Sensitivity

Greater sensitivity and high pickup ratio.

10.  Speed

With static relays, tripping time of ½ cycle or even less can be obtained.

11.  Fast Resetting

Resetting is less.

12.  Data History

Availability of fault data and disturbance record. Helps analysis of faults by recording details of:

  1. Nature of fault,
  2. Magnitude of fault level,
  3. Breaker problem,
  4. T. saturation,
  5. Duration of fault.


13.  Auto Resetting and Self Diagnosis

Electromechanical relay do not have the ability to detect whether the normal condition has been attained once it is activated thus auto resetting is not possible and it has to be done by the operating personnel, while in numerical relay auto resetting is possible.

14.  Other Advantages

  • By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays
  • Separate connection is not required, zero sequence voltages and currents can be derived inside the processor
  • Basic hardware is shared between multiple functions, the cost of individual protection functions can be reduced significantly.
  • Loss of voltage feature helps block the relay in case of momentary/permanent loss of voltage.

At System Protection we do Retrofitting of Electromechanical Relays with Numerical relays with complete wiring and Testing.

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