ABSTRACT
Transformers should be designed to withstand various possible faults, such as a short to ground of one or more phases. This chapter considers three-phase units and the following fault types: three-phase line to ground, single-phase line to ground, line-to-line, and double line to ground. Faults on a single-phase system can be considered three-phase faults on a three-phase system so that these are automatically included in the analysis of faults on three-phase systems. The chapter considers a general electrical system that is viewed from the fault point or terminal, and is modeled using Thevenin's theorem. It presents some examples of zero-sequence circuits for three-terminal transformers. Zero-sequence circuits require special consideration because certain transformer three-phase connections, such as the delta connection, block the flow of zero-sequence currents at the terminals and hence provide an essentially infinite impedance to their passage; this is also true of the ungrounded Y connection.
