In the previous article we read about functional relay types. So today here we will read about another most important relay which is differential relay. So let’s start.
Most of the relay we have discussed so far dependent on the excess of current for their operation. Such relays are less sensitive. Because they cannot accurately differentiate between heavy load conditions and small fault conditions. So that’s why to overcome these difficulties, differential relay is use.
What is differential relay
A differential relay is one that operates when the phasor difference of two or more identical electrical quantities exceeds a predetermined value.
So thus a current differential relay is one that will compare the current entering a section of the system with the current leaving the section. Under normal operating conditions, the two currents will be equal but as soon as a fault occurs, this condition will not apply. The difference between the incoming and outgoing current will be arranged to flow through the operating coil of the relay.
And if this differential current is equal to or greater than the pickup value, then this relay will operate and open the circuit breaker to isolate the faulty section.
It may be note that almost any type of relay can be operate as a differential relay when connected in a particular manner. In other words, it is not so much the relay construction as the way the relay is connect in a circuit that makes it a differential relay. There are two basic systems of differential or balanced protection.
- Current balance protection
- Voltage balance protection
See also this : What is induction type directional overcurrent relay?
What is current differential relay?
The picture below shows the arrangement of an overcurrent relay connected to operate as a differential relay. Firstly a pair of identical current transformers (CT) are fitted at either end of the section to be protected.
The secondary of a current transformers (CT) are connected in series in such a way that they carry the induced currents in the same direction. and The operating coil CT of the overcurrent relay is connected in the secondary circuit. Now this differential relay compares the current at the two ends of the alternator winding.
Under normal operating conditions, let’s say the alternator winding has a typical current of 1000 A. Then the currents in the two secondary of the CT are equal. These currents will circulate only between two CTs. And no current will flow through the differential relay. Hence the relay remains inactive.
Suppose if a ground fault occurs on the alternator winding as shown in figure (1) below, the two secondary currents will not be equal. And current will flow through the operating coil of the relay, which will operate the relay. The amount of current flowing through the relay will depend on how the fault is being fed.
Now let’s understand with an example :
- If some current (in this case let’s say 500 A) flows through one side while another large current (2000 A) enters the other side as seen in figure (1) below. Then the difference of CT secondary current i.e. 10 − 2.5 = 7.5 A will flow through the relay.
- If current flows in the fault from both the sides as shown in figure (2). So the sum of CT secondary current i.e. 10 + 5 =15 A will flow through the relay.
See also this : What is static relay and its advantages?- Static relay definition.
What is biased differential relay? or What is percentage differential relay
Biased differential relay is also know as percentage differential relay. This type of relay is design to respond to differential current with reference to the fractional current flowing through the protected section. You can see the image of Biased Beam Relay given below.
It is essentially an overpowered balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.
Under normal conditions the biasing force due to the restraining coil is greater than the operating force. When an internal fault occurs the operating force becomes greater than the bias force. As a result the trip contacts close to open the circuit breaker. The bias force can be adjust by changing the number of turns on the restraining wire.
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You can see the image of the equivalent circuit of the biased beam relay given above. The differential current in the operating coil is proportional to i2 – i1 and the equivalent current in the restraining coil is proportional to (i1 + i2)/2. Because the operating coil is connect to the mid point of the restringing coil.
Now it is clear that higher the value of current required in the operating winding to trip the relay, higher will be the current flowing through the restricting coil.
So under a heavy load a greater differential current through the relay operating coil is require for operation than under light load conditions. And this relay is called percentage relay. Because the operating current required to trip can be express as a percentage of load current.
See also this : Protective relays – What is relay – What is a basic relay?
Voltage balance differential relay
In this arrangement of protection two identical current transformers are connected at either end of the element to be protected (such as an alternator winding) that means of pilot wires. You can see the image of voltage balance differential relay given below.
Here the secondary of a current transformer (CT) is connect in series with a relay in such a way that under normal conditions the emfs induced there are in opposition.
Under healthy conditions equal currents (I1 = I2) flow through both the primary windings. Hence the secondary voltages of the two transformers are balanced against each other and no current will flow through the relay operating coil if a fault occurs in the protected area.
The currents in the two windings will be different from each other (means I1 ≠ I2). And there the secondary voltages will not be in equilibrium. This voltage difference will cause current to flow through the operating coil of the relay, which closes the current to trip circuit.
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