89. Transformer Earthing

Normally power systems and net works are operating under variable complex stresses in service. In power systems the faults are not avoidable even after taking utmost care at every stage – from planning to maintenance. It is a well known fact that the grounding of a circuit educes potential stresses under fault condition. Power feeding from Delta-Delta or if there is no accessibility for star connected transformers, occasionally shorted to ground is very common – unintentional grounding occurs anywhere from the feeding system to utilization equipment. The main objective of grounding the neutral is to make a short circuit current sufficient in magnitude for the relay action. Equipments connected to power systems and networks are operating under variable complex stresses in service. In power systems the faults are not avoidable even after taking utmost care at every stage of planning, designing, installing and commissioning. It is a well known fact that the grounding of a circuit reduces potential stresses under conditions of faults. For a switch gear and protection engineer, neutral grounding is very important because of the following: The earth protection is based on the method of neutral earthing. The system voltage during earth fault depends on neutral earthing. The neutral point (starpoint) is usually available at every voltage level from generator or transformer. In the absence of a power transformer of suitable capacity, connection and design, a separate grounding transformer can be used. They are inductive devices intended primarily to provide a neutral point for grounding purpose (32.1.15 in AIEE Std No. 32). They are interconnected star (zig-zag), star-delta and T connected. Factors like single winding, smaller capacity and economy factors, preferred a zig-zag transformer compared to the other two types. This article is restricted to zig-zag type with oil filled transformers and focus or various problems which confront engineers in the operation of earthing transformers in power systems.