ABSTRACT
The key elements of a protection philosophy are speed, sensitivity, selectivity, reliability and security. When developing a philosophy, these key elements have to be considered in conjunction with protection objectives such as safety, minimizing fire risk and protecting equipment. In this chapter, these elements are used to explain how to protect a medium voltage network (create a philosophy) that uses overcurrent protection. Elements that are addressed include network layout, different protective devices, relay technology, protection operating curves and auto-reclosing. Emphasis is placed on equipment let-through energy exposure, how to determine conductor limits and then the application of this concept to both radial and interconnected (or multi-source) networks. This ensures that the protection is actually protecting the conductors. The above concepts are then used in three different grading methods, namely, top-down, bottom-up and course grading. Examples are provided to show how to implement the grading methods and to illustrate the advantages and disadvantages of each method. Flow diagrams are provided to facilitate computer implementation of these methods. Finally, the effect of traditional overcurrent protection in an interconnected or multi-source network is discussed. This discussion includes the effect on overcurrent relays when current is changing direction and how to incorporate this when determining operating times for grading and protection purposes.
