Online Electrical Engineering
External and Internal Faults in Transformer
Backup Protection of Transformer | Over Current and Earth Fault
Protection System in Power System
Types of Electrical Protection Relays or Protective Relays
Code of practice for Power System Protection
Electrical Fault Calculation | Positive Negative Zero Sequence Impedance
Electromagnetic Relay Working | Types of Electromagnetic Relays
Transformer Protection and Transformer Fault
Small and Large Motor Protection Scheme
Motor Thermal Overload Protection
Trip Circuit Supervision
Busbar Protection | Busbar Differential Protection Scheme
Pick Up Current | Current Setting | Plug Setting Multiplier and Time Setting Multiplier of Relay
Induction Cup Relay Working Principle Construction and Types
Over Current Relay Working Principle Types
Thermal Relay Working Principle Construction of Thermal Overload Relay
Distance Relay or Impedance Relay Working Principle Types
Protection of Lines or Feeder
Inverse Time Relay | Definite Time Lag Relay
Protection of Capacitor Bank
Annunciation System Alarm Annunciator
Differential Protection of Generator or Alternator
Inter Turn Fault Protection of Stator Winding of Generator
Rotor Earth Fault Protection of Alternator or Generator
Stator Earth Fault Protection of Alternator
Loss of Field or Excitation Protection of Alternator or Generator
Feeder Protection Relays
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.