This Electrical Engineering article explains Top 5 Relay Types and Functions. Discover the significance of Instantaneous Overcurrent, Time Overcurrent Protection, Differential Current Protection, Directional Overcurrent Protection, and Distance Protection. Uncover the intricate roles these relay types play in safeguarding and optimizing electrical systems.
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Contents
Instantaneous overcurrent protection
Instantaneous overcurrent protection is where a protective relay initiates a breaker trip based on current exceeding a pre-programmed “pickup” value for any length of time. This is the simplest form of overcurrent protection, both in concept and in implementation (relay design). In small, self- tripping circuit breakers, this type of protection is best modeled by “magnetic” breakers where the tripping mechanism is actuated by the magnetic field strength of the line conductors: any amount of current greater than the tripping threshold will cause the mechanism to unlatch and open the breaker. In protective relay-based systems, the instantaneous overcurrent protection function is designated by the ANSI/IEEE number code 50. Click here to Download complete handbook on Instantaneous overcurrent protection.
Time overcurrent protection
Time overcurrent protection is where a protective relay initiates a breaker trip based on the combination of overcurrent magnitude and overcurrent duration, the relay tripping sooner with greater current magnitude. This is a more sophisticated form of overcurrent protection than instantaneous, expressed as a “time curve” relating overcurrent magnitude to trip time. In small, self-tripping circuit breakers, this type of protection is best modeled by “thermal” breakers where the tripping mechanism is actuated by the force of a bimetallic strip heated by line current: excessive current heats the metal strip, which then forces the mechanism to unlatch and open the breaker. In protective relay-based systems, the time overcurrent protection function is designated by the ANSI/IEEE number code 51. Time overcurrent protection allows for significant overcurrent magnitudes, so long as these overcurrent events are brief enough that the power equipment avoids heat damage. Click here to Download complete handbook on Time overcurrent protection.
Differential current protection
The working principle of Differential current protection is based on Kirchhoff ’s Current Law, which is one of basic laws in field of electrical engineering. Kirchhoff’s current law states the algebraic sum of all currents at a circuit node (junction) must be zero. A simpler way of stating this is to say “what goes in must come out.” We may exploit this principle to provide another form of protection against certain faults in electric circuits, by measuring the amount of current entering and exiting a circuit component, then tripping a circuit breaker if those two currents ever fail to match.
Download complete PDF handbook on differential current protection here.
Directional overcurrent protection
While the nstantaneous and time overcurrent relay functions monitor line current magnitude and guard against excesses, there are applications where the direction of line current is just as relevant as the magnitude. In such cases, we need a protective relay function able to discriminate between current in one direction versus current in the other direction. The directional overcurrent relay serves this purpose of discriminating current in one direction vs in other.
Download complete handbook on directional overcurrent protection here.
Distance protection
A form of protection against faults on long-distance power lines is called distance relaying, so named because it is actually able to estimate the physical distance between the relay’s sensing transformers (PTs and CTs) and the location of the fault. In this way, it is a more sophisticated form of fault detection than simple overcurrent.
References and Further reading:
- Instantaneous overcurrent protection
- Time overcurrent protection
- Differential current protection here
- Directional overcurrent protection here
- Basics of Protective Relays