8.2. Breakdown or voltage endurance test compilation By R. Huber If the bars/coils pass the proof tests, there are two possible methods for determining the degree of degradation in thermally aged stator bars/coils. In Method A, the thermal aging test is followed by a voltage breakdown test. In Method B, the thermal aging test is followed by a voltage endurance test. When the results of either of these tests are compared to the results of bars/coils not subjected to the thermal aging test, an assessment of the amount of degradation can be made. The number of unaged bars/coils to be so tested shall be decided between the parties prior to testing. In Method A, thermally aged bars/coils would be subjected to a 1 minute step-by- step ac breakdown test as outlined in ASTM D149-97a (reapproved 2004). If the bars/coils fail at a breakdown voltage equal to or less than the normal operating voltage of the machine for which they were intended, they would be considered to have zero life remaining. If the bars/coils fail at a breakdown voltage at or close to the breakdown voltage of new bars/coils, they would be considered to have a life expectancy similar to new bars/coils. (Ref A) In Method B, thermally aged stator bars/coils would be subjected to a voltage endurance test as described in IEEE 1043. As with the voltage breakdown test, unaged bars/coils should be subjected to the voltage endurance test to provide a meaningful comparison. Alternatively, if the bars/coils are intended for hydrogenerators or pumped storage generators, one might use the criteria listed in IEEE 1553 as a minimum life expectancy for new bars/coils subjected to a voltage endurance test. If the thermally aged bars/coils failed in significantly less time than the unaged bars/coils or if they do not attain the minimum test life indicated in IEEE 1553, they would be considered severely degraded. If the thermally aged bars/coils failed in times similar to the unaged bars or attained the minimum test life indicated in IEEE 1553, they would be considered to have normal life expectancy. By prior agreement, shorter sections of the bars/coils may be tested per the voltage endurance procedures of IEEE 434. Using the voltage breakdown test may be convenient because it takes much less time than the voltage endurance test, but it has the disadvantage of requiring very high test voltages. For this reason bars/coils would have to be submerged in oil to avoid flashovers during the test. In both methods described the thermal and voltage stresses are applied sequentially not simultaneously as in service. (REF B, C) The use of Method A or Method B shall be decided between the parties prior to the commencement of testing.
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