Research On Large Power Transformer Core Remanence Demagnetization Method

Remanence of power transformer core is a common problem in current power system.With the development of large power system,the cap acity of the power transformer is larger and larger,and voltage grade is higher and higher.Remanence problem in large power transformer core exposes becomes more and more serious,especially for the step-up transformers used in nuclear power plants,converter transformers used in HVDC and contact transformers used in power plant.It can increase the active loss of net running transformer,the vibration of the transformer core and the noise level result in noise pollution;When remanence of power transformer core is serious it can lead to a serious impact on the no-load switching-in test,no-load switching-in inrush current of transformer excitation is too large,the protecting relay trip,at the same time,the switching impulse voltage will be higher than the rated voltage of the transformer,possiblly causes insulation damages of the transformer;At the same time,as the core highly saturated,the excitation inrush current makes the core local overheating,cause damage to core;It can also increase the leakage flux,cause structure and tank local overheating,accelerate insulation aging,transformer oil decomposition,thus affect the transformer’s running life.So judging whether there is remanence of the core and how to eliminate it has increasingly become a key issue of concern and a problem urgently to be solved.This paper analyzes the working principle and structure characteristics of the transformer.Current large power transformer is all oil-immersed of which magnetic circuit is a closed loop,with its core and winding all blocked up by the transformer tank,So we can use the conventional magnetic flux measurement method to measure the transformer core flux.In this paper the core flux measurement method is to use the winding itself as the measuring coil,according to the characteristics of core excitation curve(the characteristics and residual flux),by measuring the integral of the fore and aft ends of transformer winding ’s voltage to accurately measure the flux in the core,measure the saturation remanence of the core and calculate the amount of the initial residual flux;The eliminating method is to use a DC power which apply a flux equal in magnitude and opposite in direction of the saturation magnetic flux.Through the experiments on different types of power transformer,eliminating remanence by DC power is very effective and has a very big advantage compared with the method currently used.