Differential Relay : Circuit, Working, Types & Its Applications

A Relay is one type of switch used to turn ON or OFF a high current and high voltage-based device using a signal. Relays are classified into different types like latching, reed, solid state, automotive, timer delay, differential relay, etc. In power system protection, various types of relays are used but among them, a very frequently used relay to protect a transformer, as well as a generator from localized faults, is a differential relay. This relay is very responsive to the faults that occurred in the protection zone however they are less responsive to the faults that occurred outside of the protected zone. This article gives brief information on a differential relay – working with applications.

What is Differential Relay?

The relay which works once the phasor difference for a minimum of two or above the same electrical quantities exceeds a fixed amount is known as a differential relay. Generally, most of the relays work when any quantity goes beyond a fixed value however, this relay works based on the difference between two or more same electrical quantities.

The function of a differential relay is to provide high-speed, sensitive & naturally selective protection. These relays will not provide safety to turn-to-turn winding faults within machines and transformers due to the small growth within the generated current by those faults, which stay under the relay’s pickup sensitivity.

Differential Relay Working Principle

Differential relay works on the principle of comparison between the phase angle & two or more same electrical quantities magnitudes. Comparing these two electrical quantities within a circuit with a differential relay is very simple in application & positive in action.

For instance, in comparison to the entering current & leaving current in a line, if a huge current goes through the protected line as compared to the current leaving from it, then additional current must supply within the fault. So, the difference between the two electrical quantities can control a relay to separate the circuit.

In normal operating conditions, the entering & leaving currents are equivalent in phase & magnitude thus the relay will not work. However, if any fault occurs within the system, then these flow of currents will be no longer equivalent in phase & magnitude.

This kind of relay is used in such a way that the difference between the entering & leaving current supplies throughout the relay’s operating coils. So, the relay coil can be energized in fault conditions because of the various quantity of the current. So, this relay functions & opens the circuit breaker for tripping the circuit.

In the above differential relay circuit, there are two current transformers which are connected to any face of the power transformer like one CT is connected on the primary side and the other is connected at the secondary side of the PT (power transformer). This relay simply compares the flow of currents on both sides. If there is any unbalance in the current flow of the circuit then this relay tends to operate. These relays can be current differential, voltage balance & biased differential relays.

Types of Differential Relay

These relays are classified into three types current differential, voltage balance, and percentage differential relay or biased beam relay.

Current Balance Differential Relay

This differential relay works whenever there is a fault in the protected region then there will be a variation in the entering & leaving current of that region. So by comparing these currents either in phase or magnitude or in both, we can detect the fault within the protected region. if the difference beats a fixed value then this relay compares the two currents & transmits a trip signal to the CB (circuit breaker). The differential relay protection circuit connections for the normal condition or external fault & during internal fault are shown in the following figure correspondingly.

The two CTs in the above circuit are utilized at every end of the section to be protected. In between the two CTs, the relay coil is simply connected at the equipotential position so that there is no flow of current throughout the relay coil in normal conditions. So that malfunctioning of the relay can be avoided.

In normal & external fault conditions from the above circuit, the flow of current moving into the protected region is equivalent to the flow of current going away from the protected region (I1 – I2 = 0). Therefore no flow of current will be there throughout the relay coil. So, it remains out of service.

Similarly, in an internal fault case from the above figure, the flow of current into the protected region is dissimilar from the flow of current leaving it (I1 – I2 ≠ 0). So these current flow differences are known as the circulating current which is fed to the operating coil of the relay & the relay works if the operating torque is higher as compared to the restraining torque.

Voltage Balance Differential Relay

The two CTs in the voltage balance differential relay are simply connected at any side of the element to be protected that is alternator winding which is shown in the above figure. This type of relay simply compares two voltages either in phase or magnitude or in both & it trips the relay circuit if the difference exceeds a fixed set value.

The CT’s primary windings have similar current ratios which are connected with the pilot wire in series. These wires are connected always by simply connecting two circuit ends as shown in the above figure & CTs secondary winding is connected to the operating coil of the relay.

In the above relay circuit, the flow of current in both the main windings of CTs will be the same at normal operating conditions. So when the flow of current is the same, then the voltage within the secondary winding will be the same. So, there is no flow of current in the operating coil of a relay.

Similarly in the faulty conditions, a phasor difference will exist within the primary coil’s currents. Thus, there is a difference in voltage at the second winding. Now a phasor difference will exist in the secondary coil’s voltage which is fed to the operating coil of the relay and it is connected with the secondary winding in series. Because of this, the flow of current will be there throughout the operating coil of the relay.

Percentage Differential Relay

The schematic diagram of the percentage differential relay is shown below which is also known as a biased beam relay.

The schematic arrangement of the percentage or biased differential relay is shown below. This circuit includes mainly two coils like restraining & an operating coil. Here, the operating coil is simply connected to the restraining coil’s center point.

Here, the operating coil generates the operating torque so that the relay operates whereas the restraining coil generates a bias force or restraining torque which is quite reverse to the operating torque.
This relay operates with the differential current which is flowing throughout the protected region. Whenever there is no fault within the protected region or there is a fault outside of the protected region then restraining torque will be higher as compared to the operating torque. So this will make the trip circuit open & thus relay will be inoperative.

However, if there is a fault within the protected region then the operating torque will be higher as compared to the restraining torque. Because of this, the beam simply closes the trip circuit so which initiates a trip signal through the relay to the CB or circuit breaker.

In the above equivalent circuit, the differential current within the operating coil is i2 – i1 whereas the restraining coil is i1 + i2/2 because of the middle connection of the operating coil.

So the ratio of i2 – i1 (differential operating current) to the (i1 + i2)/2 (restraining current) has a fixed percentage always. Therefore, this relay is known as a percentage differential relay. To operate this relay, the differential current should be higher as compared to this fixed percentage.

Advantages

The advantages of differential relay include the following.

• Digital signal handling is completely possible with a 16-bit microprocessor.
• This is the most significant protection within the power system.
• Measuring accuracy is high on all the ranges of settings because of a precise 16-bit analog-to-digital conversion method.
• These are very simply adaptable to different alarm & substation systems.
• These relays are very responsive because they cannot differentiate between minor faults & heavy loads.
• These relays avoid malfunctions within a network.

Disadvantages

The disadvantages of differential relay include the following.

• The current differential relay accuracy in heavy current flow will get affected because of the pilot cable’s capacitance.
• The current transformers in this relay cannot have similar characteristics or ratings because of the pilot cable impedances & constructional errors. So this causes a relay to operate incorrectly.
• The construction of a voltage balance type relay becomes complex to achieve the perfect balance between CTs.
• The protection of this relay can be used effectively for shorter-length lines.

Applications

The applications of differential relay include the following.

• This relay is very frequently used in protecting generators & transformers from localized faults.
• Usually, these relays are mainly used for protecting the equipment from internal faults. So, Merz price protection is one type of differential relay, used to guard the alternator’s stator winding from the inner faults.
• This kind of relay protects the winding of a transformer.
• These are perfectly suitable for the protection of compact items and also power system equipment like bus bars, generators, reactors, transmission lines, transformers, feeders, etc.

Thus, this is all about an overview of a differential relay – working with applications. The differential relay should have a minimum of Two or above similar electrical quantities. These quantities should include phase displacement for the relay operation. Here is a question for you, what is the function of a relay?