Slot discharge characteristics of high voltage machines.

The work presented in this dissertation is concerned with the fundamental mechanisms of discharge generation in the slot section of a high voltage stator winding and the correlations established between these processes and the discharge signal patterns detected at the winding termination by condition monitoring instrumentation. A better understanding of slot discharge activity has the potential to enhance the diagnostic capability of established on-line partial discharge monitoring techniques, enabling winding problems to be identified at an earlier stage. Stator winding insulation structures and degradation mechanisms are reviewed, together with discharge pulse propagation and rotating machine insulation diagnostic measurement techniques. In particular, movement of stator coils in the slots of large machines under the influence of naturally occurring electromagnetic forces leads to physical abrasion of the corona screen, and thereby the initiation of slot discharge. At least three different modes of slot discharge are possible following fragmentation of the corona screen, each generating discharge signal patterns with well defined pulse characteristics and polarity effects. A purpose built model stator slot with variable dimensions was constructed to house coil sides with known surface degradation, controlled variations in the width of airgap between the coil side and slot wall being made to establish a better understanding of the relationship between the degree of surface damage to coil sides in the slot section and the consequential discharge signatures. The use of a fully wound high voltage induction motor stator ensured that the basic requirement of a pulse propagation medium resembling a typical high voltage rotating electrical machine was met satisfactorily. A computer controlled discharge signal pattern acquisition and analysis system was developed which provided graphical and statistical analysis of the signatures in the form of ‘fingerprints’. Statistical operators which may be sensitive to each mode of slot discharge were investigated, with results obtained in the laboratory being compared to discharge signatures derived from operating machines. Finite element (F.E.) modelling of the fragmented corona screen from coils with known surface degradation was carried out, enabling the resistance and capacitance elements for an equivalent R-C network to be established. Discharge current, charge, and signal pattern simulations employing state space models of the residual corona screen produced good correlations with laboratory screen-core discharge pulse amplitudes and signature patterns. Slot discharge inception voltages were also estimated using F.E. modelling, and compared with experimental measurements. An accelerated ageing chamber was constructed in which coil sides were degraded at an accelerated rate by several modes of slot discharge. Possible explanations of the extent of correlation between detected discharge signatures and the various slot discharge mechanisms are suggested, together with the physical and chemical processes involved in the erosion and degradation of a coil by slot discharge.

Grantor dc:publisher.institution

Robert Gordon University

Year dc:date.issued

1998

Author dc:creator

• Murray, Alister C.

Advisor dc:contributor.advisor

• D.G. Edwards

• Partial discharge (PD)
• Stator winding insulation
• Slot discharge mechanisms
• Corona screen degradation
• Finite element modelling

Language dc:language

en

Identifier

oai:rgu-repository.worktribe.com:2807501
https://doi.org/10.48526/rgu-wt-2807501

OAI identifier oai:identifier

oai:rgu-repository.worktribe.com:2807501