# Differential Relay & Its Types - Current Differential & Percentage Differential Relay

The overcurrent relays are used for the protection of electrical equipment against fault currents. But this type of relay is not very sensitive as they cannot distinguish between overloads and minor fault conditions. This can be overcome by using differential relays.

A differential relay is a relay that operates when the vector difference of two or more similar electrical quantities exceeds a predetermined value. The protection schemes using a differential relay is a type of unit protection, which operates only when the fault occurs within its protected zone. It will not respond to the fault outside the protected zone. The scheme of differential relay protection can be achieved by the suitable connection of CTs on both sides of the apparatus to be protected.

## Types of Differential Relays :

For the operation of the differential relay, it should have, two or more similar electrical quantities and these quantities should have phase displacement (normally approx 180°). There are two types of differential relays, namely,

## Current Differential Relay :

Circulating current differential relay protection is also called Merz-price differential protection. It works on the principle that, when there is a fault within the protected zone, then there will be a difference in the current entering and current leaving of that protected zone. Thus by comparing the entering and leaving currents of the protected zone either in magnitude or in phase or both we can detect the fault in the protected zone.

The relay compares the two currents and sends a trip signal to the circuit breaker if the difference exceeds a predetermined set value. The circuit connections of differential relay protection for external fault or normal condition and during internal fault are shown in the below figures respectively.

Here two CTs are used at each end of the section to be protected. The relay coil is connected in between the two CTs at the equipotential point so that no current flows through the relay coil under normal conditions. Hence, the malfunctioning of the relay can be avoided.

From the above figure during normal and external fault conditions, the current entering the protected zone is equal to the current leaving the protected zone (i.e., I1 - I2 = 0). Thus no current will flow through the relay coil and hence it remains inoperative.

During an internal fault, the current entering the protected zone is different from the current leaving it (i.e., I1 - I2 ≠ 0). The difference of these currents called the circulating current is fed to the relay operating coil and the relay operates if the operating torque is greater than the restraining torque.

## Voltage Balance Differential Relay :

In this type of protection, two CTs are connected at either end of the element to be protected i.e., alternator winding as shown in the below figure. This relay compares the two voltages, either in magnitude or in phase or both, and trips the relay circuit if the difference exceeds a predetermined set value.

The primary windings of CTs having the same current ratios are connected in series with the pilot wire. Pilot wires are always connected by joining two ends of the circuit as shown in the figure and secondary winding of CTs to the relay operating coil.

At normal operating conditions the same amount of current will flow in both the primary windings of CTs. Since the current being the same, the voltage in the secondary winding remains the same. Therefore, zero current flows in the relay operating coil.

Under the faulty conditions, there exists a phasor difference in the currents of the primary coil. Therefore, there is an imbalance in voltage at the secondary winding.

Now there exists a phasor difference in voltage of secondary coil and this voltage is fed to the relay operating coil, connected in series with the secondary winding. Due to this current flows through the relay operating coil and the relay operates.

## Percentage Differential Relay or Biased Beam Relay :

The schematic arrangement of percentage or biased differential relay is shown below. It consists of two coils, a restraining coil, and an operating coil. The operating coil is connected to the midpoint of the restraining coil. The operating coil produces the operating torque which makes the relay operate, while the restraining coil produces a restraining torque (bias force) which is opposite to the operating torque.

The relay is designed in such a way that it operates to the differential current in terms of its fractional current flowing through the protected zone. When there is no fault in the protected zone (alternator winding in the above case) or there is a fault outside the protected zone the restraining torque will be greater than the operating torque. This makes the trip circuit open and hence relay will be inoperative.

But when there is a fault in the protected zone the operating torque will be greater than the restraining torque. Due to this, the beam closes the trip circuit thereby initiating a trip signal by the relay to the circuit breaker.

The above shows the equivalent circuit of the percentage differential relay. The differential current in the operating coil is (i2 - i1), while the current in the restraining coil is (i1 + i2)/2, due to the mid-point connection of the operating coil.

Thus the ratio of differential operating current (i2 - i1) to the restraining current (i1 + i2)/2 is always a fixed percentage. Hence the relay is called a percentage differential relay and for the relay to operate, the differential current must be greater than this fixed percentage.

The above shows the operating characteristics of the percentage differential relay. It can be seen that the characteristics are a straight line except at low currents.

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