Search

Premium Membership ♕

Limited Time Offer: Save 15% on PRO Plan with discount code: LRN15 and study specialized LV/MV/HV technical articles and studies.

Home / Technical Articles / Distance Relays

Distance relay applications

Distance relays respond to the voltage and current, i.e., the impedance, at the relay location. The impedance per mile is fairly constant so these relays respond to the distance between the relay location and the fault location.

As the power systems become more complex and the fault current varies with changes in generation and system configuration, directional overcurrent relays become difficult to apply and to set for all contingencies, whereas the distance relay setting is constant for a wide variety of changes external to the protected line.

There are three general distance relay types as shown in Fig. 1. Each is distinguished by its application and its operating characteristic.

Distance relay
Distance relay

Impedance Relay

The impedance relay has a circular characteristic centered at the origin of the R-X diagram. It is nondirectional and is used primarily as a fault detector.


Admittance Relay

The admittance relay is the most commonly used distance relay. It is the tripping relay in pilot schemes and as the backup relay in step distance schemes. Its characteristic passes through the origin of the R-X diagram and is therefore directional. In the electromechanical design it is circular, and in the solid state design, it can be shaped to correspond to the transmission line impedance.


Reactance Relay

The reactance relay is a straight-line characteristic that responds only to the reactance of the protected line. It is nondirectional and is used to supplement the admittance relay as a tripping relay to make the overall protection independent of resistance. It is particularly useful on short lines where the fault arc resistance is the same order of magnitude as the line length.


Figure 1 shows a three-zone step distance relaying scheme that provides instantaneous protection over 80–90% of the protected line section (Zone 1) and time-delayed protection over the remainder of the line (Zone 2) plus backup protection over the adjacent line section. Zone 3 also provides backup protection for adjacent lines sections.

In a three-phase power system, 10 types of faults are possible:

  1. Three single phase-to-ground,
  2. Three phase-to-phase,
  3. Three double phase-to-ground, and
  4. One three-phase fault.

It is essential that the relays provided have the same setting regardless of the type of fault. This is possible if the relays are connected to respond to delta voltages and currents. The delta quantities are defined as the difference between any two phase quantities, for example, Ea – Eb is the delta quantity between phases a and b. In general, for a multiphase fault between phases x and y,

Three-zone step distance relaying to protect 100% of a line and backup the neighboring line.
Three-zone step distance relaying to protect 100% of a line and backup the neighboring line.

Three-zone step distance relaying to protect 100% of a line and backup the neighboring line. (Source: Horowitz, S. H. and Phadke, A. G., Power System Relaying, 2nd ed., 1995. Research Studies Press, U.K. With permission.)

FORMULA

where x and y can be a, b, or c and Z1 is the positive sequence impedance between the relay location and the fault. For ground distance relays, the faulted phase voltage, and a compensated faulted phase current must be used.

FORMULA

where m is a constant depending on the line impedances, and I0 is the zero sequence current in the transmission line. A full complement of relays consists of three phase distance relays and three ground distance relays. This is the preferred protective scheme for high voltage and extra high voltage systems.

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

7 Comments


  1. Johnpaul
    Feb 09, 2023

    distance protection of an 33kv power distribution line around umuahia power station network using pdf


  2. Pradeep
    Oct 22, 2019

    Very good………


  3. Ramesh
    Oct 20, 2019

    Good article


  4. Jamal
    Feb 04, 2018

    Thanks, the article is simple and clear.


  5. Murali...
    Jan 18, 2017

    Good…!


  6. prateek devnani
    Feb 22, 2014

    i want all eletrical engineering demonstrations in more theoritical approach which could be easily understood i like what u posts so just for continuing that i want more articles regarding basics


  7. YESLI
    Jul 19, 2013

    Very goog work tahnk you!

Leave a Reply to Pradeep

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

seventy four  +    =  eighty one

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge