Application of the EL CID Test with Circulating Currents in Stator Windings

John Sutton,  David Bertenshaw

Summary

EL CID tests are carried out to determine the integrity of the inter-lamination insulation in large stator cores. This parameter is critical as not only does degradation result in increased losses, but the resultant heating can damage conductor insulation and in severe cases melt the stator iron.

It is generally assumed that currents do not flow in the stator winding during an EL CID test.  This assumption is based on the expectation that an EL CID excitation generates a truly circumferential flux around the stator core so that no magnetic flux links the stator winding and no currents are induced in it.  However occasionally test results on generators have produced large fluctuations in Phase and (to a lesser extent) Quad values, for which there has been no satisfactory answer.

Recently EL CID tests were carried out on a large hydro-generator with a split stator core, both before and after the stator windings (9 parallels) had been fitted. With the windings in place the Phase readings varied greatly, even becoming negative, but had been acceptably stable without the winding. It was discovered that circulating currents were flowing in the parallel circuits, both by direct measurement and deduction from the results and the winding diagram. Once these currents were taken account of, the test results were normalised and showed the machine’s true condition.

An earlier test on a turbo-generator had similarly produced very variable and also negative Phase readings when tested with a very asymmetric excitation winding. The problem had originally been resolved by using the correct axial excitation. Whilst there had been supposition that currents were flowing in the windings, there were no parallels. However with a revised analysis, circulating currents were clearly flowing between the earth bonded phases.

From these results, test analysis methods that allow correct interpretation are given, showing that the standard approach can continue to be used with the new understanding.

John Sutton, Consultant

The Willows,

Mid Holmwood Lane,

Mid Holmwood

Dorking, Surrey.  RH5 4HA.

UK

David Bertenshaw,    UK Operations Director

ADWEL International Ltd

Park House

Greenhill Crescent

Watford Business Park

Watford, Herts.  WD18 8PH

UK

tel: +44 1923 254433,   fax:  +44 1923 218278,   

email:  dbertenshaw@adwel.co.uk   web site:  www.adwel.com