Power Distribution Equipment

The best way to resistance ground a power system is to obtain the system neutral through a generator or transformer with a wye-connected winding. However, a system neutral may not be available, particularly in many older low voltage systems and a significant number of existing medium voltage systems. To avoid the high cost of replacing a source transformer, an existing delta-connected system can be grounded using a zigzag transformer to form an artificial neutral, then connecting a resistor between the newly created wye-point and ground.

A zigzag transformer has no secondary winding, and is designed to provide a low-impedance path for the zero sequence currents to flow. During a line to ground fault condition, the zero sequence currents can flow into the ground at the point of the fault, and back through the neutral of the grounding transformer. The impedance of the zigzag transformer to balanced three phase voltages is relatively high, therefore, when there is no fault on the system, only a small magnetizing current flow in the windings. A zigzag grounding transformer provides a stable neutral point which makes possible its use for grounding an otherwise isolated neutral system.

A zigzag transformer with a resistance ground is normally designed for a short time rating of 10 to 60 seconds. Consequently, the grounding transformer is much smaller in size than an ordinary continuously rated transformer with the same rating. For short time ratings, the short time temperature rise for the resistive element is 760℃.