- Power System Protection Definition: Power system protection is defined as the methods and technologies used to detect and isolate faults in an electrical power system to prevent damage to other parts of the system.
- Circuit Breakers: These devices are crucial for automatically disconnecting the faulted part of the system, ensuring the stability and safety of the remaining electrical grid.
- Protection Relays: Protection relays monitor the electrical network and initiate the tripping of circuit breakers when they detect anomalies, critical for mitigating damage during faults.
- Functional Requirements: Essential attributes for protection relays include reliability, selectivity, sensitivity, and speed, ensuring they perform effectively under fault conditions.
- Station Battery Importance: Station batteries provide necessary DC power to operate circuit breakers during power failures, playing a vital role in the continuous protection of the power system.
This portion of our website covers almost everything related to protection system in power system including standard lead and device numbers, mode of connections at terminal strips, color codes in multi-core cables, Dos and Don’ts in execution. It also covers principles of various power system protection relays and schemes including special power system protection schemes like differential relays, restricted earth fault protection, directional relays and distance relays etc. The details of transformer protection, generator protection, transmission line protection and protection of capacitor banks are also given. It covers almost everything about protection of power system.
The switchgear testing, instrument transformers like current transformer testing, voltage or potential transformer testing and associated protection relay are explained in detail.
The close and trip, indication and alarm circuits different of circuit breakers are also included and explained.
Objective of Power System Protection
The objective of power system protection is to quickly isolate a faulty section of the electrical power system, ensuring the rest of the system operates smoothly without significant damage from fault currents.
Circuit breakers automatically isolate the faulty section from the healthy system by opening during a fault, triggered by a signal from a protection relay. The core idea of power system protection is not to stop fault current but to quickly disconnect the fault path to prevent further damage. This quick action is critical and relies on the functional requirements of protection relays.
Protection System in Power System
Let’s have a discussion on basic concept of protection system in power system and coordination of protection relays.
In the picture the basic connection of protection relay has been shown. It is quite simple. The secondary of current transformer is connected to the current coil of relay and secondary of voltage transformer is connected to the voltage coil of the relay. Whenever any fault occurs in the feeder circuit, proportionate secondary current of the CT will flow through the current coil of the relay due to which mmf of that coil is increased. This increased mmf is sufficient to mechanically close the normally open contact of the relay. This relay contact actually closes and completes the DC trip coil circuit and hence the trip coil is energized. The mmf of the trip coil initiates the mechanical movement of the tripping mechanism of the circuit breaker and ultimately the circuit breaker is tripped to isolate the fault.
Functional Requirements of Protection Relay
Reliability
The most important requisite of protective relay is reliability. They remain inoperative for a long time before a fault occurs; but if a fault occurs, the relays must respond instantly and correctly.
Selectivity
The relay must be operated in only those conditions for which relays are commissioned in the electrical power system. There may be some typical condition during fault for which some relays should not be operated or operated after some definite time delay hence protection relay must be sufficiently capable to select appropriate condition for which it would be operated.
Sensitivity
The relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.
Speed
Protective relays must operate quickly and be well-coordinated. Proper coordination ensures that a fault in one part of the system doesn’t unnecessarily affect a healthy part. Relays in the healthy area should not trip faster than those in the faulted area to avoid disrupting the stable sections. If a fault relay fails to act due to a defect, the next relay should operate to secure the system without being overly quick, which might lead to unnecessary interruptions, or too slow, risking equipment damage.
Important Elements for Power System Protection
Switchgear
Consists of mainly bulk oil circuit breaker, minimum oil circuit breaker, SF6 circuit breaker, air blast circuit breaker and vacuum circuit breaker etc. Different operating mechanisms such as solenoid, spring, pneumatic, hydraulic etc. are employed in the circuit breaker. Circuit breaker is the main part of protection system in power system and it automatically isolate the faulty portion of the system by opening its contacts.
Protective Gear
Consists of mainly power system protection relays like current relays, voltage relays, impedance relays, power relays, frequency relays, etc. based on operating parameter, definite time relays, inverse time relays, stepped relays etc. as per operating characteristic, logic wise such as differential relays, over fluxing relays etc. During fault the protection relay gives trip signal to the associated circuit breaker for opening its contacts.
Station Battery
Circuit breakers in the electrical power system operate on DC (Direct Current) from station batteries. These batteries store DC power, allowing circuit breakers to function even during a complete power failure. Referred to as the heart of the electrical substation, station batteries accumulate energy when AC power is available and provide essential power to trip the circuit breaker if AC power fails.
