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Partial Discharge (PD) – Stator testing of HV machines was developed to detect and analyse the Partial Discharge (PD) signals found in HV Generators and Motors. Failures are caused by a combination of electrical, thermal, environmental and mechanical stress factors. Typical problems detected include end winding contamination, coil movement (due to loose wedges), main wall insulation damage and ageing. The system is non-intrusive allowing tests to be performed on-line in service and under normal operating conditions on HV machines above 6 kV.
In recent years, on-line Condition Monitoring of electrical machines insulation is becoming very popular due to the benefit it has.
The problems associated with the stator windings of electrical machines that may lead, if left undetected, to insulation breakdown and the subsequent machine failure, could be grouped into the following:
• Slot discharges: these are the discharges occurring in the straight portion of the insulation inside the stator. They could be internal discharges within voids and delamination, or surface discharge between the insulation and the stator main wall.
• End-winding problems, occurring in the overhang portion of the windings.
• Phase-to-phase discharges, occurring between any two phases, or even three phases.
• Insulation abrasion caused by coil movement and vibration in the slots, due to loose wedges.
• Levels of partial discharge activity
There is no perfect insulation, and discharges are expected to be present when performing dielectric tests. Experience has shown that most machines in service are found to have discharge levels in the range 1500-15000pC. Information currently available suggests that discharge levels of less than 20,000pC are unlikely to lead to a serious rate of erosion of insulation and are not a cause for any concern.
When the level of discharges is below 20000 pC, the stator windings are generally considered in good condition. The only exception is when there is a sudden increase in the discharge level in a short time. In such a case, the insulation is not condemned, but an extra test is requested to look at the discharge development and decide if the increase in continuing.
Once the discharge levels increase above 20000 pC, a detailed analysis of the test data, based on pattern recognition, is needed to identify the origin and degree of problem.
Scenic Acoustic & Vibration Engineering (SAVE) work in conjunction with subject matter experts in this field to provide the best and most appropriate service.