Improving Existing Motor Protection for Medium Voltage Motors

Evolutionary concepts in motor protection make possible the opportunity to retrofit earlier installations in line with modern practices. Motor failures and associated present-day costs are analyzed, and potential troubles generated within the plant and from the utility distribution system are documented. These troubles, their effects, and resultant motor failure modes have been illustrated to determine the maximum protection desired for prolonged motor life. Limitations of older forms of motor protection utilizing induction-type over current relays are presented in contrast with the desired protection. The features of several different types of current microprocessor-based relays are reviewed as eligible candidates for motor protection upgrades. An example of work performed as part of a comprehensive field retrofit is presented as well as any economic considerations. MOTOR FAILURE SUMMARY RECENT IEEE article (8) included the results of both an A IEEE survey and an EPRI survey. The data were reviewed to determine the percentage of failures associated in the following four groups: electrically related, mechanically related, environmentally and maintenance related, and other. The two sources of information proved extremely useful in this analysis, because the IEEE survey identified the "failure contributor," and the EPRI survey identified the "percentage failure by component." The IEEE survey includes an objec- tive opinion, whereas the EPRI survey includes actual failed components. The summary of the analysis of the two studies is shown in Table I, and the results indicate the following percentage of failures for the four groups considered.

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