Synchronous 3-Phase Partial Discharge Detection on Rotating Machines

摘要

Specific level of partial discharges (PD) is normal and an allowed occurrence in stator insulation of large high voltage rotating machines. However in certain cases PDs represent symptoms and even cause of defects of insulation or winding structure. In the stator winding these discharges may appear in the form of slot discharges from loose coils, slot discharges from deteriorating of semiconductive or grading coating, internal discharges from insulation voids or delamination, and surface discharges due to moisture or contamination of the winding [1]. PDs occur in the insulation system, where the local electrical strength is exceeded by the local electric field stress. In general, in high voltage rotating machines the insulation designers allows for that by the use of PD resistant components in the system. Nevertheless in certain cases PDs may damage the voltage grading systems of the winding, the inter-strand insulation, or the mainwall insulation or they can give a hint, that the original design of the winding has changed by ageing or other effects. A failure of the stator winding insulation is in majority of cases very costly, in term of both the costs to repair or replace a stator winding and outage time of the machine. Therefore much effort has been invested over the years in developing techniques to identify the occurrence of PD in the stator winding and to classify them in the way to distinguish between normal and harmful occurrences [2]. However, the main difficulties with the interpretation of PD measurement results are coupling with the electrical interferences that are inevitably present in industrial surrounding where the machines are mostly deployed, and recognition and separation of commonly occurring several different parallel active PD sources within the stator winding insulation. These difficulties can be eliminated through the usage of a new digital multi-terminal PD measurement system which enables synchronous measurement of PD impulses on all three phases of the machine combined with hereon based new evaluation technique that calculates the magnitude ratio of every detected PD impulse with its synchronously measured inductively and capacitively coupled impulses to the other two phase windings. The result is one evaluation diagram in which the superimposed PD sources as well as outer noise can be clearly distinguished as a cluster and separated for subsequent individual analysis. In addition efforts can be attempted to classify these PD sources toward type, intensity, risk grade and location.