04-02-2013, 03:08 PM
EARTHING TRANSFORMERS FOR POWERSYSTEM
EARTHING TRANSFORMERS.PPTX (Size: 421.06 KB / Downloads: 35)
ABSTRACT:-
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 everystage-from planning to maintainance. The grounding of a circuit reduces potentialstresses under fault condition. Power feeding from delta delta or if there is noaccessibility for star connected transformers occasionally shorted to ground is verycommon-un-intentional grounding occurs any where from the feeding system toutilization equipment The main objective of grounding neutral is to make a short circuitcurrent sufficient in magnitude for the relay action. This article restricted to zig-zag typewith oil filled transformers. The neutral point is usually available at every voltage levelfrom generator to transformers. In the absence of a power transformer of suitablecapacity, connection and design a separate grounding transformer can be used .They are inductive devices intended primarily to provide a neutral point for groundingpurpose. The main purpose of this paper is to discuss about the difference betweenEarthing transformer and Power transformer and to discuss about the designing of earthtransformers.
COMPARISION BETWEEN EARTHING AND POWERTRANSFORMERS:-
The selection any transformer mainly by two factors-namely, primary voltageand load .For an earthing transformer load is short time or short duty( only at the time of fault) ,normally from a few seconds to one minute
Significance of zero sequence parameters
In symmetrical components the positive sequence is one having normal phase sequence; the second is of negative sequence with reversed phase sequence; and thethird that has no sequence and is called zero sequence - are important parameters. Anearth fault is one that part of the current in the faulty phase returns to the supplythrough the earth. This is due to a symmetrical component consisting of three singlephase currents - one in each line, and all being in phase.
Why inter-connected star winding is preferred?
The purpose to establish a suitable ground path can be achieved with the help ofthree pairs of concentric coils connected to oppose ampere turns. As the fluxesoppose, the transformer takes a very small magnetizing current during normalcondition. Lines of force enclosing both coils on one limb are therefore impossible.The earth fault current finds little impedance. This connection has uniquecharacteristics, and is preferred to other types of neutral deriving transformers.