Online Electrical Engineering
Types of Electrical Protection Relays or Protective Relays
Code of practice for Power System Protection
Trip Circuit Supervision
Pick Up Current | Current Setting | Plug Setting Multiplier and Time Setting Multiplier of Relay
Annunciation System Alarm Annunciator
Electromagnetic Relay Working | Types of Electromagnetic Relays
Induction Cup Relay Working Principle Construction and Types
Over Current Relay Working Principle Types
Protection System in Power System
Differential Protection of Generator or Alternator
Rotor Earth Fault Protection of Alternator or Generator
Stator Earth Fault Protection of Alternator
Loss of Field or Excitation Protection of Alternator or Generator
Distance Relay or Impedance Relay Working Principle Types
Inverse Time Relay | Definite Time Lag Relay
Thermal Relay Working Principle Construction of Thermal Overload Relay
External and Internal Faults in Transformer
Backup Protection of Transformer | Over Current and Earth Fault
Transformer Protection and Transformer Fault
Protection of Lines or Feeder
Inter Turn Fault Protection of Stator Winding of Generator
Feeder Protection Relays
Busbar Protection | Busbar Differential Protection Scheme
Protection of Capacitor Bank
Motor Thermal Overload Protection
Electrical Fault Calculation | Positive Negative Zero Sequence Impedance
Small and Large Motor Protection Scheme
Motor Protection Circuit Breaker
Motor Protection Relay for High Voltage Induction Motor
Protection System in Power System
Under Electrical Protection
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 explain.
Objective of Power System ProtectionThe objective of power system protection is to isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.
Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system. For satisfying this quick disconnection the protection relays should have following functional requirements.
Protection System in Power SystemLet’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
ReliabilityThe 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.
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.
The relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.