Line Protection

Damien THOLOMIER, AREVA T&D Automation - Canada
Simon RICHARDS, AREVA T&D Automation - UK

Distance relays have been successfully used for many years as the most common type of protection of transmission lines. The development of electromechanical and solid state relays with mho characteristics can be considered as an important factor in the wide spread acceptance of this type of protection at different voltage levels all over the world.

The implementation of distance relays requires understanding of the operating principles, as well as the factors that affect the performance of the device under different abnormal conditions. The undesired operation of numerous distance relays during the August 14 2003 North American blackout and the November 4 2006 European disturbance focuses the attention of protection engineers on the need for improvements in the application of these relays.

The setting of distance relays should ensure that they are not going to operate when not required (security) and will operate to trip when necessary (dependability).

The behavior of distance relays during several recent major blackouts combined with the significant pressure on utilities to increase the loading of their transmission systems are the reasons to look at dynamic loading of transmission lines and the effects that this has on the commonly used distance relays.
The application of Zone 3 as a backup protection function is being questioned by some engineers and was discussed in detail during last year's conference [1]. The paper discussed in detail the requirements for backup protection, resistive reach coverage and dynamic load encroachment.

Many different characteristics of modern distance relays were analyzed in order to demonstrate that they can provide better protection and at the same time are not affected by the dynamic loading conditions that occur during wide area disturbances. This results in significant improvement of the performance of distance relays during wide area disturbances by preventing the operation of distance relays under dynamic load conditions. However, blocking the operation of the distance elements if a fault occurs at this time may result in a further degradation of the system conditions.

The paper discusses advanced methods for distance protection that detect load encroachment and out-of-step conditions, as well as unbalanced or balanced faults that may occur at the same time. A new method for power swing detection based on superimposed components is also presented. It allows better detection, combined with correct operation of the relay when a fault occurs during the power swing.

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