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experience with partial discharge testing - Iris Power Engineering

experience with partial discharge testing - Iris Power Engineering

experience with partial discharge testing - Iris Power

EXPERIENCE WITH PARTIAL DISCHARGE TESTING TO PLAN STATOR WINDING MAINTENANCE G.C. Stone P. Kantardziski T.E. Goodeve Iris Power Engineering 6380 Tomken Rd. Mississauga, ON, Canada, L5T 1Y4 INTRODUCTION The high voltage electrical insulation used in hydrogenerator stator windings normally deteriorates gradually over time, and eventually leads to machine failure. With proper maintenance and operation, stator windings can be expected to last 30 to 50 years before a rewind is needed. To achieve such long life, a utility must detect when deterioration is occurring, and correct it. Over the past 15 years, one of the most popular and effective means of detecting stator winding insulation problems has been the partial discharge test, performed during normal hydrogenerator operation. Partial discharges are small sparks which occur as the insulation deteriorates. Sensors needed to perform the on-line partial discharge test have been installed on over 1500 hydrogenerators around the world. Experience shows that good windings have a partial discharge activity which is as much as 20 times lower than that of windings close to failure. Using this as a guide, utility personnel can more easily determine which hydrogenerators will require long outages for extensive tests, inspections and repairs. In contrast, machines with low partial discharge activity require less frequent and shorter outages. Recent developments have made it easier to install the test, make the measurements, and interpret the results. BACKGROUND One of the main causes of hydrogenerator plant outages is failure of the stator winding high voltage insulation. The electrical insulation is primarily composed of mica and an organic binder - typically epoxy in modern hydrogenerators, and asphalt (bitumen) in older windings. In comparison to the other main components of a stator (the magnetic steel and the copper), the electrical insulation has a significantly lower ability to resist high temperatures and large mechanical forces. The result is that most stator failures are initiated by deterioration of the electrical insulation. Thus, to achieve reliable hydrogenerator operation, any deterioration of the stator winding insulation must be detected and repaired. There are three basic strategies for planning maintenance of any equipment: • breakdown maintenance, where no testing or maintenance is performed prior to failure • preventive (or time-based) maintenance, where tests, inspections and required maintenance are determined according to the time or number of operating hours since the last set of tests and inspections • predictive (or condition-based) maintenance, where any expensive tests, inspections or repairs are done only when a monitoring system has indicted that such procedures are necessary. Each type of strategy has its place. The type of strategy selected for a particular hydrogenerator depends on testing and inspection costs, the cost of replacement power in the event of a failure, the likelihood that severe damage to the stator will occur on failure (e.g. from a phase-to-phase fault with high currents), the availability of alternative energy sources, and the inherent reliability of the hydrogenerator. Testing costs play an important role in selecting the maintenance strategy. In general, for very small hydrogenerators (i.e. less than 1 or 2 MVA), breakdown maintenance is often the most viable option, since testing costs are relatively high compared to rewind costs. For larger machines, the testing costs become a smaller fraction of major repair costs, and thus preventive or predictive maintenance is more viable. For machines greater than about 10 MW, the predictive maintenance strategy has been shown to yield the lowest overall cost [1] because the failure probability of any particular stator winding at a particular time is very low (normal service life is typically 30 years). When failure rates are low, preventive maintenance often involves performing unnecessary tests and inspection because the winding is normally in good condition. With predictive maintenance, any expensive tests, inspections and repairs are only performed on the relatively few hydrogenerators requiring it, and not on all the machines, thus, reducing costs and outages. In addition, with

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