What is a Distance Relay : Working & Its Applications

Distance relays are the most important distance protection elements, which depend on the distance of the source/feeder point and the point where the fault occurs. The principle of these relays differs from one form of protection to others because its performance depends on the ratio of voltage and current. These are said to be double actuator relays because one coil is energized by voltage and the other coil is energized by the current. This type of relays are most widely used where there is a need for fault protection, backup protection in transmission and distribution lines at high speeds, and also when the overcurrent relaying is very slow. This article helps to know about the distance relay and its types in detail.

What is the Distance Relay?

The distance relay is also referred to as the impedance relay or distance protection element or voltage-controlled device. It’s working mainly depends on the distance between the impedances of the points where the fault occurs and where the relay is installed (feeding point). The relay gets operated when the ratio of voltage and current is set to a predetermined value or less than the relay. This type of relay is used for backup protection, fault protection, phase protection, and main protection of transmission and distribution lines. The distance relay diagram is shown below.

The design of the distance relay is a simple over overcurrent relay. The distance relay diagram with voltage and current characteristics is shown below. The dashed line in the below diagram represents the operating condition at a constant impedance of the point or line.

Distance Relay Theory

The distance relay is a distance protection element designed to measure the faulty point. The operation of this relay depends on the value of the impedance. It trips the circuit breaker and closes the contacts when the impedance of the faulty point is less than the impedance of the relay. The voltage and current flowing through the PT and CT are continuously monitored by the relay and it starts operating only when the ratio of voltage and current (value of impedance) is less than the predetermined impedance value of the relay.

Distance Relay Principle

The distance relay working principle is very simple and it is based on the ratio of voltage and current i.e, impedance. This relay contains a potential transformer to supply voltage and current transformer for the current element, which is connected in series with the entire circuit. The secondary current of CT produces the deflecting torque whereas the potential transformer produces restoring torque. As we know that its operation depends on the ratio of voltage and current ie, the ratio of impedance value, which is also known as impedance relay.

The distance relay gets starts operating only when the voltage and current ratio, which means impedance is less than the predetermined impedance value of the relay. As the impedance of the transmission line is directly proportional to its length, then the relay starts operating if any fault occurs within the length of the transmission line or predetermined distance.

How Does Distance Relay Work?

The working of distance relay is explained in two conditions such as normal condition and faulty condition.

Normal Condition: It is said to be an operating condition because the line voltage or restoring torque is higher than the current or deflecting torque.

From the above figure, we can observe that impedance or distance relay is placed on the transmission line between the points AB Consider the impedance of the line is Z in operating condition. The distance relay starts working only when the impedance of the line is less than the impedance Z of the relay

Faulty Condition: In this condition, there is a chance of occurrence of a fault on the transmission line when the magnitude of the current rises than the voltage (less). That means the current on the line is inversely proportional to the impedance of the relay. Hence the relay starts working in this condition because the impedance on the line decreases and less than the predetermined impedance value.

If a fault F1 has occurred on the line AB, the impedance of the line is decreased below the predetermined value of the relay and it’s starts operating by sending the tripping command to the circuit breaker. The contacts of the relay would be unclosed if the fault is reached beyond the positive state.

Types of Distance Relay

As the distance relay depends on the ratio of voltage and current values, they are classified into 3 types. They are

Impedance Relay

This type of relay depends on the impedance Z suitable for phase fault protection of transmission line at a moderate length

Reactance Relay

This type of relay depends on the value of reactance X suitable for ground-fault protection of the line.

Admittance or MHO Relay

This type of relay depends on the value of admittance Y suitable for phase fault protection of long transmission line, used where severe power surges occur and also of distance measurements.

If any fault occurs, then the distance relay starts working depends on the values of impedance or admittance, or reactance.

Definite Distance Relays

This type of relay starts working when the value of reactance or admittance is below a predetermined impedance value of the relay. These are impedance, reactance, admittance, or mho type relays.

Time Distance Relays

The working of this type of relay depends on the value of impedance. That means its operation depends on the distance between the fault and the relay point. It works more efficiently and earlier when the fault is nearer to the relay point. These come under impedance, reactance, or mho type relays.

Distance Relay Testing and Its Procedure

The distance relay testing is required to check the settings for the protection relay, configuration of the relay, installation, testing, and commissioning the entire device for protection

As the distance relays are used for universal short-circuit protection, it’s operating condition depends on the electrical quantities measurement like voltage and current, impedance value evaluation for fault, that is proportional to the distance between the relay and the point of fault occurs.

Make sure that all the 3 zones of the protection relay are set properly.

Zone 1 is set for instantaneous tripping mode in forwarding direction

Zone 2 is set for over-reaching with a time delay (single)in the forward direction

Zone 3 is set for over-reaching with time-delay in the double mode for the reverse direction.

Make sure that the type of power system used for 400kV transmission line of 3-phase model, and two loads ( 3 resistive loads with two 9kV) should operate at 400V

Make sure that all the remaining protection operating modes are turned off while testing any protection mode.

Checking all the connections of PT, CT, and transmission line links are connected properly

Distance Relay Characteristics

The distance relay characteristics in the operating condition are shown below. The current flowing through CT is taken on X-axis and the voltage supplied by the PT is taken on Y-axis.

If the impedance of the transmission line is more than the impedance of the relay in a fault condition, then the positive torque is produced above the operating characteristic line. In the same way, if the impedance of the line is less than the impedance of the relay in the fault condition, then the negative torque is produced.

Also, the operating characteristics of the distance relay can be explained by using the R-X plane. Let the radius of the circle be the impedance of the line.

X be the phase angle and R be the vector position.

In the positive region, the impedance of the line will be less than the radius of the circle. In the negative region, the impedance of the line will be more than the radius of the circle. From these operating characteristics, we can conclude that these types of relays are suitable for high-speed transmission lines and said to be high-speed relays.

Example 

SIPROTEC 7SA522 is an example of a distance relay, which is a modern type of relay. It is used to achieve full-scheme distance protection and performs all the functions, which are necessary to protect the power line. The single line diagram of this type of relay is shown below.

From the above figure,

21/21N is the distance protection

FL is fault locator

50N/51N, 67N is directional ground-fault protection

50/51/67 is for overcurrent backup protection

50 STUB is stub-bus overcurrent stage

68/68T represents power swing (detection or tripping)

85/21 is for distance protection of teleprotection27WI is for protection of weak-infeed
85/67N is for teleportation for ground-fault protection

50HS is for switch protection

50BF is brake fail

59/27 is for protection of overvoltage

810/U is over/under the protection

25 is synchro check

79 is auto-reclose

74TC is the trip circuit

86 denotes lockout command

Advantages

The advantages of distance relay over overcurrent relay are given below

• It replaces the protection of overcurrent transmission lines
• Provides protection very fastly
• Coordination and application is very simple
• Available with permanent settings and there is no need to readjust the settings
• Effect of a generation of fault levels, fault current magnitude is less
• Permits high load lining

Disadvantages

The disadvantages of distance relay or impedance relay are shown below

• As it operates on both sides faults of a line, then it is said to be non-directional.
• It fails to recognize between internal and external faults of a line
• The resistance of the arc of a fault line affects the function of the distance relay. Since an arc exists when the fault occurs at any point.
• The power swings affect the performance of the distance relay because the area covered by the circle on the sides of the R-X plane is large
• The measurement capacity of fault resistance is limited.

Applications

The distance relay applications are

• These are most widely used to protect transmission lines and distribution lines over high AC voltages
• Provide backup protection of AC voltages against the several faults in 3-phase, phase to phase, and phase to the ground of distribution and transmission lines.
• Static distance relays are widely used because it provides distance protection for all types of line faults in transmission lines (short, medium, long, and main).

Thus, this is all about the distance relay-definition, theory, diagram, principle, working, advantages, disadvantages, applications, testing, and testing procedure. Here is a question for you, “What is an over-current relay? “