6.6 Surge Stress Test (SURG)
The wire used to wind a transformer may contain defects in its insulation. For example, scratches in the enamel of copper winding wire. This test may be used to highlight insulation defects between adjacent turns in a winding as opposed to insulation defects between windings. In some cases, the wire insulation defect does not immediately cause a shorted turn, but will leave a weak spot which may eventually fail in use. By applying a higher than normal voltage across the winding, any weakness in wire insulation will be identified at the test stage.
It is applicable to any transformer, but is particularly suitable for transformers with a large number of turns using very fine wire. The test voltage is generally applied to the primary of a transformer. All other windings of the transformer are simultaneously tested because of the normal action of the transformer which applies the same volts per turn to every winding.
Each SURG test can be programmed to consist of a number of impulses For each impulse, the AT3600 will charge an internal capacitor to the high voltage specified. This stored charge will then be suddenly discharged into the winding under test, and the resulting transient voltage will be analysed. The product from the discharge will be a sinusoidal wave with decaying amplitude. The transformer is characterized by the area under the wave-form, measured in voltseconds. Faulty inter turn insulation will dissipate some of the impulse energy and reduce the decay time. The AT3600 will then report a ‘Volt-second’ measurement which is smaller than that of a perfect transformer.