Electric Power Principles, 2nd Edition

10System Analysis and Protection

Sometimes things go wrong. Designers of electric power systems must ensure that those systems don't damage themselves during either normal operation or when something happens. Contingencies range from lightning strikes to animal contact and automobile accidents. This chapter deals with methods for analyzing current flows in power systems in abnormal situations, and methods used for protecting the system from the consequences of short circuit contingencies.

Up to this point, this book has dealt primarily with networks that are balanced, in which the three voltages (and three currents) are identical but for exact 120° phase shifts. Unbalanced conditions may arise from unequal voltage sources or loads. It is possible to analyze some simple types of unbalanced networks using straightforward solution techniques and wye–delta transformations. However, power networks can become quite complex and many situations would be very difficult to handle using ordinary network analysis. For this reason, a technique which has come to be called symmetrical components has been developed.

Symmetrical components, in addition to being a powerful analytical tool, is also conceptually useful. The symmetrical components themselves, which are obtained from a transformation of the ordinary line voltages and currents, are useful in their own right. Symmetrical components have become accepted as one way of describing the properties of many types of network elements such ...