Normally power systems and net works are operated under variable complex stresses.In power systems the faults are not avodable even after taking utmost care at every stage-from planning to maintainance. The grounding of a circuit reduces 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-un-intentional grounding occurs any where from the feeding system to utilization equipment The main objective of grounding neutral is to make a short circuit current sufficient in magnitude for the relay action. This article restricted to zig-zag type with oil filled transformers. The neutral point is usually available at every voltage level from generator to transformers. 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.
Rating and its inter-related parameters of an earthing transformer
The earthing transformer is of short time rating (10 seconds to 1 minute). The rating of an earthing transformer is entirely different from that of a power transformer. Power transformers are designed to carry total load continuously, whilst an earthing transformer carries no load, and supplies current only if one of the lines becomes grounded. It is usual to specify the single phase earth fault current, that the earthing transformer must carry for sufficient time. Since it is almost working on no-load, dictates to have low iron losses. Because of it being a short time device, its size and cost are less than that of a continuous duty transformer of equal KVA rating. The KVA rating of a three phase earthing transformer or a bank is the product of normal line to neutral voltage (KV) and the neutral or ground amperes that the transformer is designed to carry under fault conditions for a specified time. The total earth fault current and V the line voltage, the earthing transformer short time rating is equal to v3VI.
When specifying rating of the earthing transformer the important parameters are:
• Voltage:- The line-to-line voltage of the system .
• Current:- The maximum neutral current to carry for a specified duration. In a grounded system it is based on the type of grounding. Depending on their duration, several rates of short
• Time:- Designed to carry rated current for a short time duration i.e., 10 seconds to 60 seconds .Depending upon the time setting of the protective gear on the system, And the location of the transformer .Earthing transformer time is 10 sec for protection, and for feeder it I 60 sec.
• Reactance:- this quantity is a function of the initial symmetrical three phase short circuit KVA. It is also based on the type of grounding, and type of application of lightning arrester and transient over voltages.