Premium Membership ♕

Save 50% on all Video Courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles and guides.

Line differential relaying

The most common pilot line protection today is directional comparison by use of distance relays. However, the increased availability of digital communications channels has renewed the interest in line differential relaying. Current-only relays have many advantages.

Basic setting criteria for current differential line protection
Basic setting criteria for current differential line protection

One of the most significant advantages is the simplicity of the scheme with regards to settings. The percentage differential principle originally developed for transformer and generator protection was extended for use on short transmission lines already in the 1930s, and is still widely in use.

These Pilot Wire Relays typically use a telephone type pilot wire channel to exchange analog information between the line terminals.

The electromechanical pilot wire relays were very easy to set. They either came with factory default fixed sensitivity or had a few tap settings. What tap to use was determined by estimated fault current on the line, making sure that the relay sensitivity was sufficient for the relays to dependably trip for all types of fault on the line.

Three phase sensitivity was often limited to current above load levels in order not to trip for open pilot wires.

For digital current differential line relays, the sound and simple setting principles utilized for pilot wire relays seem to have been forgotten.

Pilot Wire Relay
Pilot Wire Relay

Commonly, the most sensitive setting (10 – 20 % of nominal current) is applied and even recommended without any consideration of actual fault current levels for the application. This practice contradicts fundamental relaying principles that balance security, sensitivity, dependability and operating speed of the protection scheme.

For sensitivity, the requirement is to set the relay to detect all faults on the line with a sufficient margin, seldom does this require the most sensitive setting.

Higher sensitivity will increase dependability, but decrease security. Too high sensitivity may cause false trips due to ct errors at external faults with high fault currents, or even false trips on load for channel delay measurement errors due to unequal transmit and receive delays.

This paper reviews the basic setting criteria for current differential line protection, and the charge comparison relay in particular. While setting parameters will differ depending on the type of current differential relay, the setting principles and system conditions to consider remain largely the same.

Operating time, sensitivity, dependability and security requirements are examined for typical applications. Considerations for weak feed conditions, ct errors and ct saturation, and charging current are discussed.

In addition, the effect of channel delay compensation errors and asymmetrical channel delays possible on digital communications networks with regards to setting considerations is examined.

Title:Current differential line protection setting considerations – S. Ward and T. Erwin; RFL Electronics Inc.
Size:723 KB
Download:Right here | Video Courses | Membership | Download Updates
Current differential line protection setting considerations
Current differential line protection setting considerations

Premium Membership

Get access to premium HV/MV/LV technical articles, advanced electrical engineering guides, papers, and much more! It will help you to shape up your technical skills in your everyday life as an electrical engineer.
50% Discount 💥 - Save 50% on all 90+ video courses with Enterprise Membership plan.

More Information

Leave a Comment

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!

  −  three  =  three

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