Grounding transformer applications and associated protection schemes

Various aspects of the purpose, protection philosophy, application and specifications of different types of grounding transformers and two governmental system case studies illustrating improperly applied grounding transformers and associated ground fault protection systems are discussed. The first case study involves the use of an incorrect value of resistance in grounded neutral wye-broken delta grounding transformers associated with a 2400 V ungrounded system. Detailed calculations are given to correctly size the grounding transformer resistance value necessary to limit the magnitudes of transient overvoltages caused by restriking ground faults. The second case involves ground fault tripping deficiencies caused by utilizing only a single zig-zag grounding transformer with an associated neutral resistor on a multibus 2400 V ungrounded system. It is stressed that a single grounding transformer is not adequate for use with a multibus configuration. A protection scheme for use on a multibus arrangement is presented.<>