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Relays Part 6: Distance Relays Important Theory

by: Jan 15,2024 1314 Views 0 Comments Posted in PCB Basic Information

Summary:       This article introduces an engineer or a technician to distance relay working, basic theory, types, and applications.


Several types of protective relays exist, and distance relays are one of them. Distance relay is a relay that is significant in the area of power electronics. It utilises the feeder point distance and the point of fault occurrence. The performance of such relays depends on the voltage-current ratio; hence, it defers from one protection form to the other. They are sometimes referred to as double actuator relays due to their two-coil activation process. One coil activation is made possible by voltage while the current does the other. They are commonly used in areas with fault protection requirements, distribution and transmission lines operating at high-speed backup protection, and finally when the system has very low overcurrent relaying. This article is significant in letting us know about distance relay in detail. Let us get into these details!

Defining Distance Relay

This type of relay is also known as an impedance relay, voltage-controlled device, or distance protection element. The working is persistently dependent on the distance between the relay location and the point where the fault occurs. The relay depends on the preset values of both current and voltage. You will come across this relay in the transmission line: paramount protection, phase protection, distribution lines, fault protection, and backup protection.

The diagram of the Distance Relay

Figure 1: Distance Relay Courtesy of Simon Mugo

Distance relay design is a simplified overcurrent relay. From the diagram, you can take note that the relay has both current and voltage characteristics. We shall discuss the characteristics using a graph later in the article.

Basic Theory of Distance Relay

Distance relays are designed to detect faulty points in electric circuits. Its operation depends on a measured impedance value. The relay will trip the electric circuit immediately if the impedance of the tested flawed point goes below the relay’s impedance. The current and the electrical voltage through the CT and PT are monitored continuously using the relay, and its operation starts once the voltage and the current ratio become less compared to the predetermined relay impedance value.

Working Principle of the Distance Relay

The relay can be operated using two operating conditions. It can be managed through normal and faulty conditions.

Figure 2: Distance Relay Connected to Transmission Line Courtesy of Simon Mugo

Under normal conditions:

  • The condition is also referred to as operating condition. This is simply because the restoring torque or line voltage goes higher than the deflecting torque or current.
  • From Figure 2 above, the distance relay is connected at a distance between point A and point B with an impedance of Z while in operating condition.
  • The relay will start functioning only if the impedance of the transmission line becomes less than the Z impedance of the distance relay.

Under faulty condition:

  • Under this condition, there exists a chance that a fault will occur on the transmission line the moment the current magnitude goes higher than that of the voltage.
  • This statement means that the line current is inversely proportional to the relay’s impedance.
  • The relay will start operating under this condition because line impedance will decrease to a value that is below the value of the impedance that is already predetermined.

Distance Relay Types

In the area of power electronics and electricity, distance relays are classified into impedance, reactive, and admittance relays. Below is a further breakdown of theories of the three types of distance relays.

Impedance Relay

The relay is dependent on the impedance Z and is suitable for transmission line fault protection at a moderately higher length.

Its diagram is shown below.

Figure 3: Impedance Relay Working Diagram Courtesy of Simon Mugo

From the diagram, we can observe that the voltage coil of the relay is connected to the main transmission line through a potential transformer PT, In contrast, the relay current coil is connected to the main transmission line through the current transformer CT. The main transmission line is the one that is to be protected by the impedance relay.

The work of the relay’s current element is to produce the operating torque, which is determined to be proportional to the current of the transmission line. The relay voltage element is the source of the restraining torque, which opposes the operating torque and is proportional to the voltage of the transmission line. Operating torque is also known as positive torque, while restraining torque is known as negative torque.

From the diagram of Figure 3 above, section AB of the electrical circuit is our protected zone, and we assume ZL as the impedance of the transmission line when the fault is absent.  The design of the relay works under the principle that when the ratio V/I goes below ZL, the relay trips.

Impedance Relay Torque Equation

From Figure 3, the positive torque produced is due to the element of the current being proportional to the square of the present, and the restraining torque is produced due to the component of the voltage being proportional to the square of the voltage.

Now, let K3 be the torque due to the relay’s control spring.

The electromagnetic impedance relay torque equation is as follows.

Reactive Distance Relays

This is a high-speed type relay. It is made up of two units: the overcurrent element, which develops positive torque, and the current-voltage element, which plays the role of aiding or opposing the overcurrent element depending on the current and voltage phase angle. This explanation is an indication that a reactive relay is an overcurrent relay that has a directional element.

The diagram of the reactive relay is shown below:

Figure 4: Reactive Distance Relay Circuit Courtesy of Simon Mugo

Let K3 indicate the control effect of the spring. The relay torque equation becomes

Where the spring constant has been neglected, too.

Admittance or MHO Distance Relays

The relay is dependent on the admittance value Y, which is suitable for the protection of long electricity transmission lines. They are applicable in areas where extreme power surges are present.

The presence of any fault causes the relay to start working depending on the value of the admittance, impedance, or reactance.

Figure 5: Admittance Distance Relay Circuit Courtesy of Simon Mugo




Advantages and Disadvantages of Distance Relay

Distance relay compared to overcurrent relay advantages are as follows:

  • Replaces the protective function of the overcurrent relays on the area of transmission lines
  •  Fastly protection provision
  • Very simple to coordinate and apply
  • Come with permanent settings that do not require readjustment.
  • Less fault current magnitude
  • High load lining permission.

The disadvantages of the distance relay are:

  • It is non-directional, as it can operate from both side’s faults
  • It cannot distinguish between external and internal faults
  • Has limited fault resistance measurement.


Key Away Points

  • Distance relays, also known as impedance relays, are relays that are persistently dependent on the distance between the relay location and the point of occurrence of the default.
  • Distance relay is a simplified overcurrent protection relay with both current and voltage characteristics.
  • Distance relays are operated under two conditions that are through both standard and faulty conditions.
  • Under normal conditions, which are also known as operating conditions, the restoring torque is higher than the deflecting torque.
  • Distance relays can be classified into three types: impedance, reactive, and admittance distance relays.
  • The electromagnetic impedance torque equation is given by

  • Reactive relays, also known as high-speed relays, are made up of two points: the overcurrent element that is good for developing positive torque and the current-voltage element that opposes the overcurrent element.
  • Admittance distance relays are dependent on the admittance value Y.
  •  Advantages of the distance relays include providing quick protection, being easy to coordinate and use, having less fault current magnitude, and having high load lining permission.
  • The limitations of the distance relay include limited fault resistance measurement, failure to distinguish between external and internal faults, and non-directional.


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