Monday, 24 February 2020

Differential Relay- Working, Classification, Advantages.

Differential Relay

  • Differential relay operates when the phase difference in two or more than two identical quantities crosses the predetermined value.
  • Differential relay is another type of relay which is used for protection against faults related to current and voltage in the device. This is the most prevalent and successful method to protect the circuit.
  • This type of relay used for the protection of transformers, generators, bus bars, feeders and large motors for localized faults etc.

Working operation of differential relay

  • It compares the phase angle and magnitude of the same quantities in particular device. Arranged the relay to compare the current entering and leaving the circuit which should be same at normal condition and during external fault.
  • There are two current  comes from two part of  electric power circuit. It means two inputs. These two currents meets at junction point where the relay coil is connected.
  • If the polarity and amplitude of both the currents are to adjusted that the phasor sum of these two currents is zero at normal condition. At that time there is no current flowing through the relay coil. 
  • And if any abnormality occurs in the circuit this balance broken, it means the phasor sum of these two currents no longer zero then non zero current flowing through relay coil relay being  operated.
  • So there is two current inputs one is the reference and actual input to comparing to inputs if any difference in the input then output coil will be energized and relay will be  turn off.

Differential relay at Normal Condition
Differential relay at Normal Condition

Differential relay is classified in four categories

1. Current differential relay
2. Voltage differential relay
3. Biased or percentage differential relay
4. Voltage balance differential relay

1. Current differential relay

Current Differential Relay
Current Differential Relay

  • This relay is operated only for the current. It sense and then operates the phase difference between the current entering in the circuit and current leaving the circuit.
  • At the both end of the protected zone the current transformers are connected.With the help of pilot wire  the secondary of current transformer is connected in series. And the operating coil of relay connected to secondary of CT's.
  • If any fault occurs outside the zone and the two CT's both having same ratios are connected properly, then the no current will flow through the relay coil that why it will not operates.
  • But if the fault occurs in withing the zone and the current entering in zone is not equal to current leaving the zone. This fault current flows through relay coil and it gets operated.
  • This type of relays used in current transformers, generators, motor protection etc

2. Voltage balance differential relay
  • Current differential relay is not suitable for the feeders so the voltage biased differential relay is used for feeder protections.
  • When the fault occurs in the protected zone the phasor difference of both lines will not be zero.
  • So the voltage induced in the secondaries of CT's will different and the calculating current will flow through the operating coil of relay. Because of this relay operates and gives command to trip the circuit and the circuit breaker will be open.

3. Biased or percentage differential relay

Percentage Differential Relay
Percentage Differential Relay

  • There is one additional coil is connecting in the biased percentage differential  relay. That coil is called restrained or biased coil. Other arrangement of this type of relay is same like the current differential relay.
  • At the mid point of biased relay the relay coil is connected. 
  • Due to this the we can overcome the difficulty arising out of difference in transformer ratio for high values of short circuit current.

Advantages of Differential Relay

  •  In percentage differential relays, Individual CT inputs are given to the relays. So the relay can process these CT inputs to calculate the differential and bias. 
  • Percentage 2nd and 5th harmonic restraining can be provided in differential relay.
  • To prevent operation of relay on through fault, slope characteristics can be provided in such relays.

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