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Elements of protection system
Electrical System fault and system Protection with Protective Relays
Why A System Needs Protection?
There is no ‘fault free’ system.
It is neither practical nor economical to build a ‘fault free’ system.
Electrical system shall tolerate certain degree of faults.
Usually faults are caused by breakdown of insulation due to various reasons: system over current, over voltage, lighting, etc.
Element of protection system
(1)Current and Voltage Transformers
Current transformer consists at least of two secondary windings.
The first winding is usually designed for measuring, the second is used for protection.
The secondary of current transformers are almost connected in star
Voltage transformer is often consists of two windings.
The first winding is connected in star, and the stare point must be earthed.
The second winding is connected as open delta.
Advantages for Using Protective Relays
Detect system failures when they occur and isolate the faulted section from the remaining of the system.
Mitigating the effects of failures after they occur.
Minimize risk of fire, danger to personal and other high voltage systems.
Low voltage circuit breaker
Magnetic circuit breaker
Medium voltage circuit breaker
High voltage circuit breaker
Battery bank are called as backbone of protection system
Emergency use for power system
Fuses are selected to allow passage of normal current and of excessive current only for short periods.
It is used to protect the low voltage or current rating devices
A lightning arrester is a device used on electrical power system to protect the insulation damaging effect of lightning.
All lighting arrester are earthed
Protective Devices Comparison
Circuit Breaker v/s Relay
Relays are like human brain; circuit breakers are like human muscle.
Relays ‘make decisions’ based on settings.
Relays send signals to circuit breakers. Based the sending signals circuit breakers will open/close.
Generator Differential Protection
Loss of Field or Loss of Excitation Protection
Negative Sequence or Current Unbalance Protection
Over Fluxing or Over Excitation Protection
Over Current Protection
Stator Earth Fault Protection
Rotor Earth Fault Protection
Restricted Earth Fault Protection
Backup Impedance Protection
Low Forward Power Protection
Reverse Power Protection
Over Frequency Protection
Under Frequency Protection
Over Voltage Protection
Generator ProtectionDifferential and Ground Fault
Generator ProtectionPhase Unbalance
The direction of rotation of a negative sequence is opposite to what is obtained when the positive sequence are applied.
Negative Sequence Relay will constantly measure and compare the magnitude and direction of the current.
Gas and Temperature Monitoring
Differential and Ground Fault Protection
Over current protection
Transformer ProtectionGround Fault and Differential Relay
Thermal Overload Relays
Use bimetallic strips to open/close relay contacts when temperature exceeds/drops to certain level.
Require certain reaction time
Inverse time/current relationship
Transmission line protection
Earth fault protection
Over current protection
Relays control output circuits of a much higher power.
Safety is increased
Protective relays are essential for keeping faults in the system isolated and keep equipment from being damaged.
Relay is like an insurance against damage due to fault.