Relay Protection System Requirements

Some of the basic Requirements of the protection system is to provide: Reliability, Selectivity, Sensitivity and Speed of operation. Protection system is an extremely important part of the power system as it is provided to operate under abnormal conditions to prevent failure or isolate faults and limits the effects of the faults. 

Reliability of Relay Protective System: 

Reliability is the basic requirement of the protection system as it is provided to improve the reliability of the whole power system. Reliability in simple words means that the protection system must be ready to function correctly at all the times and under all conditions of the fault and abnormal conditions of the power system for which the protection system is designed,

Reliability is a quantitative term. Reliability of the power system is assessed from the statistical data. Statistical survey and records give idea about the reliability of the protection system. With the growing size of the system and use of EHV lines, interconnections and use of large generators and transformers the importance of the reliability of the protection system increased

Inherent reliability is a matter of design based on long experience. This can be achieved partly by:

  • Simplicity and robustness in construction
  • Higher contact pressure
  • Dust free enclosures
  • Good contact material
  • Good workmanship
  • Careful maintenance

Simplicity of Relay Protective System:

Simplicity is closely associated with the reliability. Simplicity of construction and good quality of the relay, correctness of the design and installation and good maintenance and supervision etc.. are the main basic factors which influence the protective reliability. As a rule of thumb, the simple the protective scheme and lesser the number of relays, circuits and contacts it contain, the greater will be the reliability. Reliability is rater closely associated with simplicity. Therefore one of the basic requirements of the protection system is simplicity and reliability

Selectivity and Discrimination of Relay Protective System: 

Selectivity is the ability of the protective system to select correctly that part of the system in trouble and disconnect and isolate the faulty part without disturbing the rest of the power system (the protective system must be able to discriminate between those conditions for which the prompt operation is required and those for which no operation or time-delay operation is required).

A well designed and efficient protective system should be selective i.e, it should be able to detect the point at which the fault occurs and operate the circuit breaker nearest to the fault with minimum or no damage to the system.. If the protective system is not selective and operates for faults beyond its protective zones, a larger part of the power system will be disconnected unnecessarily.

Sensitivity of Relay Protective System:

Any protective system must be sufficiently sensitive so that it will operate reliably when required under the actual condition that produces the least operating tendency. Sensitivity of the protective system refers to the smallest value of the actuating quantity at which the protective system starts operating in relation with the maximum value of the fault current in the protected zone

Sensitivity of a protective relay is a function of volt-amperes input to the relay coil necessary to cause its operation. The smaller the volt-amperes input required to cause the relay to operate, the more sensitive is the relay. A protection with high degree of the sensitivity is more complex and uses more equipment and circuitry and is more expensive. Such protection is only employed in that case where simple arrangement cannot be applied because of the low degree of sensitivity

Speed of Operation of Relay Protective System:

The protective system should disconnect the faulty section as quick as possible. This is desirable for may reasons such as:

  • Reduction in the damage occur
  • Improving the power system stability
  • Less interruption to the power supply to the consumers 
  • Decrease in the likely-hood of development of one type of fault into the other more severe fault
  • Permits use of rapid reclosure of the circuit breaker to restore service to the consumers

However too fast operation may result in undesired operation during the transient faults

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