RELAY
Relay is protective device which sense the fault & given information to circuit breaker which isolates the system.
Plug setting multiplier(PSM): = Fault current reff to secondary side of CT/Pick up current.
So, Pickup current= relay setting X rated ROC current.
Example.- If=2000A, RELAY setting 50% & the CT ratio 400/5.
then PSM=?
PSM= (2000X5/400)/0.5X5=10.
If PSM>1 relay operate.
PSM<1 relay not operate.
PSM=1 relay at usage of operation.
DISTANCE RELAY:-
Distance relay is used to protection of transmission lines universal torque eq T = ki2 + K2v2 + K3VI
IMPEDANCE RELAY:-
T= K1i2(operating force) - K2v2(restraiming force)It It is voltage restrained over-current relay
NON DIRECTIONAL RELAY
Used for medium transmission lines.
REACTANCE RELAY:-
T=K1i2 - K3VI
Directional restraint over current relay.
It is also non-directional relay.
Used for short transmission lines.
Protection against earth faults.
MHO RELAY:-
-Voltage resfrained direction relay.-It is a directional relay.
-Used for long transmission lines.
R- short
I- medium
M- long transmission, directional.
GENERATOR PROTECTION Or ALTERNATOR PROTECTION :-
-To protect a) p-p and b) p-g faults[Merz price protection, % differential protection or biased diff., % bias - 5 to 10%.].
-Stator over heating -- thermal relays.
-For loss of excitation i.e If=0 => MHO relay is Med.
-Rotor earth fault => reactance relay.
-Rotor over heating => ('-ve') sequence relay.
-Prime mover failure.--
It Prime Mover fails -alternators act as a motor
i.e. Direction changes so direction relay or reverse power relay is used .
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