The “surge test” is performed to evaluate turn insulation short. Initially the test was performed on individual turn insultaion as a manufacturing control. Today we also use it to evaluate complete windings, whether new or in operation. It is a test that does not evaluate tendencies, but whether it is good or bad, pass or fail. In this test a relatively high value is applied between turns (IEEE 522 and IEC 60034-15), like a “Hipot”, and can be destructive. This test is not relevant for the “Roebel” bars of large machines because they are single turn.<\/p>\n
When we analyze the nominal voltage between turns of a machine, even for large equipment, the level is low and its operation is around 100\/150 volts\/turns. At this level, the electrical deterioration is low and the failures are due to thermal and mechanical actions on the insulating material. Because it is in direct contact with copper, the highest temperature point, it is the insulator that heats up first. Another wear factor is the movements by dilatation due to thermal cycles in operation. Electrically we can have deterioration coming from the impurities in the environment in which the machine is operating.<\/p>\n
We have several switching sources inserted in the circuit that can generate voltage surge waves with fast rise time wavefronts. These waves, when they reach the winding, distribute the voltage (the circuit is seen as a capacitive divider) in an uneven way and concentrate high values in the first windings of the machine that can cause early deterioration by the appearance of electrical discharges due to the high voltage level. Remember that this insulation is made for a low voltage level, and is basically composed of the conductor’s own insulation (enamel and\/or fiberglass) or in some projects an insulating layer is applied, but of only one turn.<\/p>\n
Hardly ever does the machine go out of operation due to a short turn insulation. What normally happens is that this short deteriorates the main insulation, causing a short to ground that causes the protection to operate.<\/p>\n
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