Impedance Relays in Distance Protection Strategies

Welcome to a field where advanced technology performs the important task of protecting our most important power transmission lines. Ensuring uninterrupted power flow is of utmost importance in power distribution, especially in high and extra-high voltage transmission lines. Enter the world of “Impedance Relays in Distance Protection Strategies”, a field where the marriage of cutting-edge relay technology and the science of electrical impedance provides uncompromising protection against potential interference. In this article, we uncover the mechanisms that make impedance relays a cornerstone of distance protection and raise standards for network reliability and stability. Join us as we explore how these ingenious devices are changing the landscape of power transmission security and ushering in an era of enhanced protection and unprecedented efficiency.

Impedance relays for protection of transmission lines

Distance protection impedance relay is a protection system widely used to protect high or high voltage lines .

The operation of conventional current relay, whether directional or non-directional, depends on the magnitude of current or power in the protected circuit. In contrast, distance protection relay works based on the principle of relationship between applied voltage and current in this circuit. This relationship is proportional to the distance along the line, and the relay This is the measure of space Distance protection relay . It is not a uniform protection system. A single scheme provides primary and backup protection.

Types of distance protection relays

The Distance Protection Relay family consists of the following types of relays:

1. impedance relay
2. reactance relay
3. MHO relay or admission relay .

Join us on a journey into the world of impedance relays, discover how they work and discover how they contribute to the safe and reliable operation of power systems.

impedance relay

The impedance relay measures the line impedance at the relay location. If a fault occurs in the protected line section, the impedance measured will be the impedance of the line section between the relay location and the weak point. It is proportional to the length of the line and therefore the distance along the line, as shown below.

OF is the distance between the relay location and the fault location; The voltage drop across OF and the current I flowing in the line are used by the relay for measurement and the relationship between the two quantities is nothing more than the impedance.

Structure of an impedance relay

The figure shows the simple arrangement of an impedance relay that works depending on the distance from the fault.

Here a balanced beam type EM relay is used as impedance relay. CT and PT are powered by the current and voltage of the circuit to be protected.

Impedance relay working principle

A simple form of EM relay with symmetric beam resistance is shown in the figure. It has a fixed beam and two electromagnets (EM). The zone voltage activates one EM through the TP, and the other EM is activated by the zone current through the CT.

Under fault-free conditions, the pull across the voltage element is greater than the pull through the current element and the trip circuit (CT) remains open.

As this type of relay is based on the circuit impedance , which in turn depends on the distance from the fault to the relay location, it is called a distance relay.

Operating characteristics compare the impedance of the circuit to the voltage at the relay location. Current creates a positive torque, called operating torque, and voltage has a negative torque, called Holding Torque .

This equation for the operating torque of an electromagnetic relay is:

T = K ₁ EU ² –K ₂ v ² –K ₃

K ₁ K ₂ and K ₃ are constants, K ₃ is the torque due to the action of the control spring.

Neglecting the effect of the spring used, which is very small, the torque equation can be written as follows:

T = K ₁ EU ² –K ₂ v ²

The following condition must be met for the relay to work.

K ₁ EU ² >K ₂ v ² or K ₂ v ² < K ₁ EU ²

v ₂ /I ² < K ₁ /K ₂

V/I < K, where K is a constant.

from V/IZ, Z

The above expression explains that the relay is about to trip when the ratio between V and I, that is, the measured value of the line impedance, is equal to a certain constant. The relay is activated when the measured value Z is less than the specified constant. This given constant is a design value that depends on the total length of the HT/EHT supply line to be protected.

A distance relay can also be called an ohmmeter because it measures the line impedance in ohms.

Operating characteristics of an impedance relay

The image above shows the Operating characteristics of the impedance relay in terms of voltage and current. Therefore, the above is called Diagram VI . The working part is slightly bent near the origin due to the action of the control spring. If the relay is static relay type the quantity would be a straight line as there is no control spring. The positive torque region is the relay operating zone (above the curve), and the negative torque region below the curve is the relay non-operating zone.

RX Chart

A different and more useful way of presenting the operating characteristic of the relay is an RX diagram, as shown below:

Z = K = Radius of the circle. If Z, i.e. the impedance of the line to the fault point, measured from the relay location, is less than K, the relay is activated, i.e. the fault point is within the circuit. If it is outside the process, the relay will not detect it; therefore it is this exclusion zone . The relay operations depend on the magnitude of Z and not the angle Φ, as Z is the radius of the circle and has the same meaning along the circumference of the circle from the center. It can also be seen that the impedance relay is a non-directional relay because it is based on the meaning of the operational quantity and not on the direction of the flow; The figure shows that the operating time of this relay is constant regardless of the distance within the protection zone.

Directional Impedance Relay

An impedance with a non-directional characteristic is triggered if there is a fault in the circuit. Regardless, the protection scheme attempts to limit the trigger zone to the forward direction only.

At each relay location there are three impedance relays and a directional unit connected in series with the impedance relays.

The impedance of Zone I is Z ₁ If an error occurs in F ₁ the impedance of Zone I is reduced to a value lower than the preset value (Z ₁ ). This increases the size of the circuit and activates the relay.

For each fault in Zone II F ₂ In the figure, the impedance of Zone I does not change and the relay does not work.

Protection zones through impedance relays

Typically, three impedance relay units are required for three protection zones at a given location. Typically, the first unit is configured to only protect 80% to 90% of the protected line. The first protection zone is 80% to 90% of the protected line. It is a high speed drive. The operation is instantaneous, about 1 to 2 cycles.

The second unit protects the remainder, 20% of the protected line and 50% of the shorter adjacent line. This protection zone is called second protection zone . The dual zone device works after a certain delay. Its operating time is 0.2 to 0.5 seconds.

The third unit serves as a backup for the adjacent line. This unit's configuration includes the first line, which is the protected line plus the second longest line plus 25% of the third line. The relay operating time is 0.4 to 1 second.

For cost reasons and due to the limited space available in the relay panel, today only one measuring unit is used for all three protection zones. The time unit determines the distance settings for zones II and III. The gradual temporal distance Characteristics of impedance relays are shown in the figure.

A _1, A _2, and A ₃ are the operating times for the relays in zones 1, 2, and 3 placed in A. Likewise, B ₁ b ₂ and B ₃ are the active times for the relays in zones 1, 2, and 3 with B rating.