Basic differential relay working function

Basic differential relay working function

Defination

The differential relay is one that operates when there is a difference between two or more similar electrical quantities exceeds a predetermined value.

A relay that by its design or application is intended to respond to the difference between electrical quantities associated with the protected apparatus called differential relay.

The basis principle on which a differential relay operates in the circulating current principle.

• Differential relays are intended to compare input current and output current (transformer differential relay, generator differential relay etc).
• Differential relays that compare voltage inputs are used for EHV capacitor protection.
• Depending on the application, differential relays may be instantaneous or short time, with fixed or variable restraint, and include other features that have been developed to optimize relay applications.
• Minimum operates current and/or voltage restraint CT ratio and/or VT ratio should be specified when differential relay settings are selected. CT and VT connections and polarity should be verified.

Types of differential relay:

• Current balance differential relay
• Biased differential relay or percentage differential relay
• Voltage balance differential relay

Current balance differential relay:

In current balance differential relay, the circuit current is a comparison quantity. Two current’s magnitude and phasor comparison takes place.

Current differential relay

                    Consider a circuit in the figure, two current I1 and I2 are Two CT’s output. The operating coil is placed across the current transformer. Here during normal condition two current I1 and I2 have equal magnitude and phasor. Hence at a point A both cancels each other. Therefore, the current flow through the coil is zero and the relay become in operative. 
                  During abnormal condition, the current I1 is not equal with I2. Then there is a current flow through the coil.
                  if the current flow through the coil k is exceed the preset value, then the relay operates the associated circuit breaker.

Ground fault current differential relay

In this figure ground fault occur in the system primary side 2000 amp current flow and secondary side 1000 amp current.

so this secndary current.

           I1-l2=

          10-5=5

so this 5 amp current flowing through relay so this current enough to actuated the relay and this give signal to breaker to trip the equipment

                    In three phase circuit two kind of CT configuration are made in differential protection one is high impedance differential protection and another one is low impedance differential protection.
       High Impedance differential protection means the relay coils operates high voltage low current and low impedance differential protection the relay coil operates under high current with low output voltage.

Biased or percentage differential relay

biased or percentage differential relay

This is the most used form of differential relay. Their arrangement is same as that of the current differential relay; the only difference is that this system consists an additional restraining coil connected in the pilot wires as shown in the figure below.

This biased differential relay overcome the drawback from differential relay like 

1st drawback while we are changing the tap change that time some current difference available in the system so possibility of the relay operate and trip the system so connecting the restraining coil in the normal condition restraining coil torque mare than the operating coil so restraining coil hold the operating coil to operate.

2nd drowback magnetising inrush current available in the transformer differential relay this 7 or 8 time more this current persist in the system 5 to 6 second so possibility of the relay operate but we no need to operate through this magnetising inrush current because the is the not fault so this drawback remove through restraining coil.

Voltage Balance Differential Relay

The current differential relay is not suitable for the protection of the feeders. For the protection of the feeders, the voltage balance differential relays are used. The voltage differential relay uses two similar current transformer places across the protective zone with the help of pilot wire.

The relays are connected in series with the secondary of the current transformer. The relays are connected in such a way that no current flows through it in the normal operating condition. The voltage balance differential relay uses the air core CTs in which the voltages induces regarding current.

Voltage balance differential relay

When the fault occurs in the protection zone, the current in the CTs become unbalance because of which the voltage in the secondary of the CTs disturbs. The current starts flowing through the operating coil. Thus, the relay starts operating and gives the command to the circuit breaker to operates.