08-16-2017, 10:47 PM
Earthling transformers For Power systems
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Definition
Normally power systems and net works are operated under variable complex stresses. In power systems the faults are not avoidable even after taking utmost care at every stage-from planning to maintenance. 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 zigzag 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.
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 .Earthling 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.