Relays Part 7: Differential Relays

Introduction

Too many types of relays are used in the area of power systems. Among these many relays is the protective relay that finds great significance and uses in electrical devices such as generators and transformers protection. They employ the principle of localized zones. Differential relays have a very high sensitivity to electrical faults within these protected zones but extremely limited sensitivity to faults outside the protected zone. When you look at most relays' operating principles, they use a predetermined value for the function. For example, an overcurrent relay will use a set value of current that, when it is exceeded, the relay acts. The operating principle of the differential relay has been designed to be so different. It works by using the principle that a difference between at least two or more of the same electrical quantities must exist. In this article, we shall have a wider look at what differential relays are, their types, differential relay percentages, their connection, and CT ratio, application, merits, and demerits of differential relays. Let us learn together.

What is a Differential Relay?

This type of electrical relay operates when there exists a difference between at least two electrical quantities exceeding a predetermined set value. The diagram in Figure 1 below shows the circuit of the differential relay with two currents from two different points of the power circuit. The two currents meet at a junction point connected by the relay coil.

Figure 1: Differential Relay Circuit by Simon Mugo

By using the Kirchhoff law of current, the current that flows through the coil of the relay is the total of the two electrical currents that come from the different directions of the circuit. If the amplitude and the polarity of the two electrical currents such that the output of the phasor sum of the currents becomes zero, in the relay’s normal operating environment, there will be no current flow through the connected relay. Breaking of this balance is caused by abnormalities in the power circuit, creating a phasor current that does not sum to zero; hence, non-era current flows through the relay coil operating the relay.

The differential relay has two sets of electrical current transformers connected to either side of the power system equipment that is being offered protection by the differential relay. The currents of these electrical transformers are perfectly chosen such that their secondary coil currents equal each other in magnitude. The polarities attached to the current transformers have secondary currents of the CTs connected in a way that they oppose each other.

Differential Transformers Types

In power electronics, two types of relays are classified under differential relay, which is a classification based on the operating principles of such relays. They include

• Voltage-balanced differential Relays
• Current Balanced Differential Relays

Current Differential Relay

This utilizes current transformers fitted on either side of the power systems to offer protection. Such protected equipment includes alternators, generators, and power transformers. To be precise, the transformers are of the current type. The current transformers’ secondary coils are connected through the series connection method, which implies that they carry flowing current in a similar direction. Figure 1 above is a circuit for the current differential relay. Let us say that I1 gives the secondary current of the current transformer CT1, and I2 gives the secondary current of the current transformer CT2. From the same circuit, we can note that the current that goes through the differential relay is given by I1 – I2. However, the polarity and ratio of the CT are the fault.

Figure 2: Current Differential Relay by Simon Mugo

Current differential relays undergo several limitations, as listed below:

• There is a probability that transmission cable impedance mismatches will occur, that is, the cable from the relay panel to the current transformer.
• Cables have a pilot capacitance, which leads to relays operating incorrectly when big through faults are located externally to the protected equipment.
• It is impossible to match the connected current transformer perfectly, hence the spill current that flows through the differential relay when under normal operation.

Voltage Balanced Differential Relay

In this type of differential relay, the connection of the current transformer is on both sides. Still, this time, it works on the principle where the EMF induced on the secondary coil of both transformers opposes each other. This implies that the current transformer’s secondary coils are connected through a series method but with opposite polarities. This type of relay is suitable for the protection of feeders, which is a problem that the current balanced differential relays cannot solve. The pilot wire helps the two current transformers to achieve the mechanism. Below is the electrical circuit of the voltage-balanced differential transformer.

Figure 3: Voltage Balanced Differential Transformer by Simon Mugo

If a fault has occurred in the protected area, the current transformer current goes unbalanced due to voltage disturbances on the secondary coil. The current flows through the operating coil, triggering the relay to start operating, which signals the circuit breaker to turn into operating mode.

Advantages and Disadvantages of Differential Relays

Advantages

Below are the advantages of using differential relays in power systems:

• It makes it possible to handle digital signals using a 16-bit microprocessor.
• High measuring accuracy because of the utilization of the 16-bit analog-digital converting technique.
• simplified adaptation to different substations and alarm systems.
• They respond highly because they do not differentiate between heavy loads and minor faults.
• They can prevent malfunctions within the connected network.

Disadvantages

• The pilot cable capacitance affects the system's accuracy when there is a heavy flow of currents.
• Pilot cable impedance prevents the differential relay from having similar characteristics and ratings.
• They are only suitable for shorter lines to deliver better results.
• It isn't easy to achieve a balanced voltage between the current transformers.

Applications

• Protection of generators from faults that are localized.
• Used in protecting the equipment against internal faults.
• Applied to protect the windings of the transformer.

Key Take Aways

• Differential relays are operated when a difference between two or more similar electrical elements exceeds a predetermined set value.
• Two types of differential transformers have been discussed: current-balanced and voltage-balanced differential transformers.
• The current balanced differential relay uses the current transformers with a secondary coil connected in a series where similar currents flow through them.
• The polarities of the current-balanced differential relays and the ratio of the CT are selected such that the current in the first transformer is equal to the one In the second transformer
• A voltage-balanced differential relay works on the principle where the EMF induced on both secondary coils of the CT transformers opposes each other.
• The benefits of differential transformers include the possibility of handling digital signals, high measuring accuracy, simplified adaptation, and high response.
• The limitations of the differential relays include being affected by pilot cable capacitance and impedance, not being good for longer power lines, and difficulty in achieving voltage balancing in the CTs.
• Differential relays find use in the protection of transformers and f] generators, protection against internal faults within the types of equipment, and transformer windings protection.