Evaluation of Alternative Backup Protection Schemes on a 66kv Distribution Network

Directional and non directional overcurrent protection are well established methods of providing either primary or backup protection on distribution networks. Compared to unit protection which only protects a specific piece of equipment, such as a transformer or feeder, overcurrent protection has the advantage of being able to provide protection to large sections of network. To do this the overcurrent protection on a network must be coordinated. On radial distribution systems this can be easily achieved, however, on a meshed or looped system it is not always possible to achieve the required coordination. For this reason overcurrent protection is sometimes unsuitable for certain applications. This paper looks at a section of 66kV distribution network for which overcurrent protection is unsuitable. The inadequacy of the standard protection scheme for the network in question is explained and possible alternatives discussed. The paper then goes through the rationale for selecting the most suitable alternative protection scheme for the section of network in question.