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!
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.
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.
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.
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:
Under faulty condition:
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.
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.
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.
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
Distance relay compared to overcurrent relay advantages are as follows:
The disadvantages of the distance relay are: