HYDROGENERATOR POLES DAMAGE BY ELECTRICAL EROSION - PATTERN, CAUSE, DIAGNOSTICS AND FAILURE PREVENTION

摘要

During the last decades some cases of hydrogenerator poles damage by electrical erosion were found. In spite of significantly different design of these machines all the damages were of the same type: as the result of electrical erosion rather extended zone of poles steel neighbouring to the dampher winding rod was burnt out, the slot became opened and at last the cooper rod failed in the middle of destructed area. Centrifugical forces bent the cantilever parts of the damaged rod into the air gap and caused their abrasion against the inner surface of stator. The detection of destruction products presence (copper particles) served as the symptom of pole's fault. The nature of such faults remained not clear for long times, but it was determined that cases of electrical erosion of the poles: are met only with generators that have rather significant ratio of stator slot width to air gap, become to arise after rather long-duration term of generator service. Although classic theory was unable to explain why potential difference sufficient to initiate sparking may arise between the neighbouring rods of dampher winding, for practical engineers it was quite obvious that in these dangerous areas the rods should be either insulated against the pole steel or vice versa - tightly coupled. The procedure of "caulking" was introduced in accordance with the last alternative. The key of this phenomenon origin was found during the investigation of damaged poles from hydrogenerator of HEPP "Djerdap-1", where in service were 9 generators of "Elektrosila" (Russia) production and 3 others of local manufacturing. Injuries were observed only with the poles of latter machines, though the difference in design lied only in the stator, that had more wide slots. Elaborated analysis showed that more wide stator slot increases the risk of electrical erosion site of origin, but quite another factor provokes the defect initiation - it is air gap axial nonuniformity. Namely there where this nonuniformity runs up to maximum value the damage originates and develops. The date of hydrogenerator survey on HEPP "Dneproges-2", "Kievskaja" and "Djerdap-1" where the cases of poles damage due electrocution corrosion were observed are presented in the rapport. Theoretical equations are given to enlight the nature of discussed phenomenon. On this basis there are given recommendation on maintenance and diagnostic development as well as on hydrogenerator design in the modern direction of "low-slot-number" machine creation as one of the effective ways to decrease the expenses of large generators manufacturing and maintenance.