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Sections: Magnitude Relays | Directional Relays | Ratio Relays | Differential Relays | Pilot Relays

Directional Relays

Definition. The zone of the relay includes all of the power systems that is situated in only one direction from the relay location.

Operation
1. Discriminate between a faultwithin its zone of protection and all other system conditions.
2. Act dependably for faults within its zone of protection
3. Provide security against fake tripping for faults outside its zone of protection.

Classes.
1. Magnitude Relays
2. Directional Relays
3. Ratio Relays
4. Differential Relays
5. Pilot Relays

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Sections: Magnitude Relays | Directional Relays | Ratio Relays | Differential Relays | Pilot Relays

Magnitude Relays

Definition. Magnitude Relays respond to the magnitude of their input current and operate to trip whether the current magnitude exceeds a certain value which is adjustable. Example of this type are the current and the overcurrent relays.

Principles.
Magnitude Relays are governed by the following:

| I_f | > | I_p |trip

| I_f | < | I_p |block

Where
I_f is the fault current magnitude in terms of the secondary winding of the current transformer
I_p is the pick-up (pull-in) value of the relay.

Pull-In Current is the value in which the coil current must rise beyond a cetain values to move the armature of the relay and switch the contacts.

Hold-In Current or the Drop-Out Current is the value in which once the relay has been pulled-in, the coil current must drop below a certain lower value of current to cause it back to normal.

Operation Circle.

T = F ( | I_f | - | I_p | ) if | I_f | > | I_p |

Where:
T is the relay operating time after |I_f | exceed | I_p |. Usually: ½, 1, 2, 3, 4, 6, 8, or 10
F is a function dependent on the fault current level. Usually asymptotic to the pick-up value and decreases as some inverse power of the current magnitude for |I_f | > | I_p |

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