AC Circuits and Power Systems in Practice

12Introduction to Power System Protection

Power system protection is a relatively complex subject and requires a knowledge of the power system fundamentals presented in the preceding chapters. A thorough description of all the various protection schemes in use would fill an entire textbook. This chapter aims to provide the reader with an introduction to the underlying philosophy of power system protection and an exposure to some of the more common techniques in use. There are many excellent texts specifically on the subject of protection, several of which appear in the sources list at the end of this chapter.

We will principally confine our attention to the MV, HV and EHV networks, in which power system equipment is of a size and importance sufficient to justify the use of complex protection schemes. In contrast, the LV network generally comprises smaller and cheaper items of equipment, serving fewer customers and does not justify such protection. It is usually only overcurrent protected using fuses or magnetic circuit breakers, commensurate with the size and value of the assets.

12.1 Fundamental Principles of Protection

The primary function of the protection system is to rapidly disconnect any faulty piece of equipment from the power system in such a way that damage to both the equipment concerned and to the wider network is minimised, while causing the least disturbance possible to the connected load.

During fault conditions, network voltages near the fault become depressed, ...