Exploring the Power of Distance Protection and Impedance Relays

The operation of conventional over-current relays, either directional or non-directional, depends on the magnitude of current or power in the protected circuit. In contrast, the distance protection relay operates on the principle of the ratio of the applied voltage to current in that circuit. This ratio is proportional to the distance along the line, and the relay that measures the space is called the distance protection relay. It is not a unified system of protection. A single scheme provides both primary and backup protection.

Types of Distance Protection Relay

The distance protection relay family consists of the following types of Relays:

Impedance Relays
Reactance Relays
MHO Relays or Admittance Relays.

Join us on a journey to explore the realm of the Impedance Relay, unraveling its inner workings and discovering how it contributes to the safe and reliable operation of power systems.

Impedance Relay

An impedance relay measures the impedance of the line at the relay location. When a fault occurs in the protected line section, the measured impedance is the impedance of the line section between the relay location and the point of weakness. It is proportional to the length of the line and, hence, to the distance along the line, as shown below.

OF is the distance between the relay location and the fault location; the voltage drop along OF and the current I flowing in the line are taken for measurement by the relay, and the ratio of both quantities is nothing but impedance.

Construction of Impedance Relay

The figure shows the simple arrangement of an impedance relay that operates based on the distance of the fault.

Here, a balanced beam-type EM relay is used as an impedance relay. The ci rcuit’s current and voltage energize the CT and PT, which is to be protected.

Operating Principle of Impedance Relay

Fig shows a simple form of EM balanced beam impedance relay. It has a fixed beam and two electromagnets (EM). The zone’s voltage energizes one EM through PT, and the zone’s current energizes the other EM through CT.

Under no-fault conditions, the pull due to the voltage element will be more than the pull due to the current part, and the trip circuit (TC) will remain open.

The operating characteristics of the circuit’s impedance are compared with the voltage at the relay location. The current produces a positive torque, called an Operating Torque, and the voltage has a negative torque, called a Restraining Torque.

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

T = K₁ I² – K₂V² – K₃

K₁, K₂, and K₃ are constants, K₃ being the torque due to the control spring effect.

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

T = K₁ I² – K₂V²

For the operation of the relay, the following conditions should be satisfied.

K₁I² > K₂V² or K₂V² < K₁I²

V₂/I² < K₁/K₂

V/I < K where K is a constant.

as V/I is Z, Z<K.

The above expression explains that the relay is on the verge of operation when the ratio of V to I, i.e., the measured value of line impedance, is equal to a given constant. The relay operates if the measured Z is less than the given constant. This given constant is a design value depending on the total length of the HT / EHT feeder to be protected.

A distance relay can also be called an Ohmmeter, measuring the impedance of the line in ohms.

Operating Characteristics of an Impedance Relay

The above figure shows the operating characteristic of the impedance relay in terms of voltage and current. Hence, the above is termed a V-I diagram. The working part is slightly bent near the origin due to the effect of the control spring. If the relay is of a static relay type, the amount would have been a straight line, as there is no control spring. The positive torque region is the relay operating zone (above the characteristic curve), and the negative torque region below the curve is the relay non-operating zone.

R-X Diagram

Another and more useful way of representing the operating characteristic of the relay is an R-X diagram, as shown below:

Z = K = radius of the circle. When Z, i.e., the impedance of the line up to the fault location measured from the relay location, is less than K, the relay will operate, i.e., the fault location lies inside the circle. If it is outside the process, the relay will not sense it; hence, it is the Blocking zone. The relay operations depend upon the magnitude of Z and not on angle Φ, as Z is the circle’s radius, having equal importance along the circle’s circumference from the center. It is also seen that the impedance relay is a non-directional relay, as it operates on the significance of the operating quantity and not on its direction of flow; the figure indicates that the working time of this relay is constant irrespective of the distance within the protective zone.

Directional Impedance Relay

A non-directional impedance in characteristic will trip for a fault within the circle, irrespective. It will be tried in the protective scheme to restrict the tripping zone in the forward direction only.

Three impedance relays and a directional unit connected in series with the impedance relays are provided at any relay location.

Zone-I’s impedance is Z₁. If a fault occurs at F₁, the impedance of zone-I reduces to a lower value than the pre-set value (Z₁). Due to this, the circuit increases and operates the relay.

For any fault at zone-II F₂ in the figure, the impedance of zone-I will not change, and the relay will not operate.

Zones of Protection by Impedance Relay

Three units of impedance relays are normally required at a particular location for three zones of protection. It is normal practice to adjust the first unit to protect only up to 80% to 90% of the protected line. The first protection zone is 80% to 90% of the protected line. It is a high-speed unit. Its operation is instantaneous, about 1 to 2 cycles.

The second unit protects the rest, 20% of the protected line and 50% of the shortest adjoining line. This zone of protection is called the second zone of protection. The dual zone unit operates after a certain time delay. Its operating time is 0.2 to 0.5 sec.

The third unit is a backup of the adjoining line. Its setting covers the first line, i.e., the protected line, plus the longest second line, plus 25% of the third line. The time of relay operation is 0.4 to 1 sec.

Because of the cost involved and relay panel space restrictions, only one measuring unit is employed for all three protection zones nowadays. The timing unit sets the distance settings for II and III zones. The figure represents the stepped time–distance characteristics of impedance relays.

A_1, A_2, and A₃ are the operating times for the zones 1, 2, and 3 relays placed at A. Similarly, B₁, B₂, and B₃ are the active times for the zones 1, 2, and 3 relays rated at B.

Conclusion

Impedance relays play a critical role in protecting and reliability of power systems. By accurately measuring impedance to detect faults, these relays ensure quick and selective isolation of faulty sections, preventing damage and maintaining system stability. Various impedance relays, including simple impedance relays, directional impedance relays, mho relays, and quadrilateral relays, offer tailored solutions for different protection needs.

The applications of impedance relays in transmission lines, generators, and transformers highlight their versatility and importance in maintaining the integrity of power systems. Despite the complexity involved in setting and calibrating these relays, their advantages in sensitivity, directional capability, and adaptability make them indispensable tools for power system protection.

FAQs

What is an impedance relay, and how does it function?

An impedance relay is a protective relay used in power systems to detect faults. It measures the impedance (resistance and reactance) between the relay location and the fault. When a fault occurs, the impedance changes, and if it falls within a predefined range, the relay activates to isolate the faulty section, protecting the system from damage.

What are the different types of impedance relays?

There are several types of impedance relays, including:

Simple Impedance Relay: Operates when the measured impedance falls below a set value. It is straightforward but may lack selectivity.
Directional Impedance Relay: This relay adds a directional element to ensure it operates only for faults in a specific direction, improving selectivity and reducing false trips.
Mho Relay (Admittance Relay): This relay combines impedance and directional elements with a circular characteristic on the R-X diagram, providing better selectivity and stability in complex networks.
Quadrilateral Relay: This relay features a quadrilateral characteristic on the R-X diagram, offering flexibility in setting and better coverage of the impedance plane. This is useful for protecting lines with varying impedance.

What are the applications of impedance relays in power systems?

Impedance relays are commonly used in:

Transmission Line Protection: This involves detecting and isolating faults in transmission lines, ensuring minimal disruption, and preventing damage to equipment.
Distance Protection: As part of distance protection schemes, they measure the distance to the fault and provide graded protection based on the distance from the relay location.
Generator and Transformer Protection: Protect generators and transformers from internal faults by measuring impedance and detecting abnormal conditions.

How do impedance relays improve the reliability of power systems?

Impedance relays enhance the reliability of power systems by:

Quick Fault Detection: Rapidly detect faults and isolate the affected section to prevent damage and maintain system stability.
Selective Operation: Ensuring that only the faulty section is disconnected, minimizing the impact on the rest of the system.
Flexibility and Adaptability: Providing adjustable settings and characteristics to accommodate various system configurations and fault conditions.
Coordination with Other Relays: Working in coordination with other protective relays to provide comprehensive protection and improve overall system reliability.

What are the advantages and limitations of impedance relays?

Advantages:

High Sensitivity: Effective in detecting faults at varying distances.
Directional Capability: Improved selectivity by incorporating directional elements.
Versatility: Suitable for different applications, including transmission lines, generators, and transformers.

Limitations:

Complexity: Setting and calibrating impedance relays can be complex and require skilled personnel.
Limited Coverage: This may not cover all fault types and conditions without combining them with other protective devices.
Dependency on System Parameters: Performance can be affected by changes in system impedance and load conditions.

Understanding impedance relays and their functions helps design effective protection schemes for power systems, ensuring safety, reliability, and efficient fault management.

Impedance Relays in Distance Protection Strategies

Impedance Relays for Transmission Line Protection