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Client Objective

The client requested a full assessment of the insulation condition on his 13.8 kV hydro generator stators to determine the remnant insulation life and potential need for replacement of four stator windings that had been in operation for approximately 25 years.

Scope of work

A total of 6 bars were removed / replaced by the client from one of the operational generator stators which, along with an original spare bar, were shipped to Kinectrics’ facility for assessment.

Work performed

Detailed assessment at Kinectrics of the 7 bars supplied consisted of a combination of tests and inspections, including:

• Assessment of the client-supplied Partial Discharge data
• Visual examination
• Surface Resistivity tests
• Partial Discharge tests
• Capacitance & Dissipation Factor tests
• Hi-pot testing
• Voltage Endurance tests to IEEE 1043
• Dissection of the insulation

Client benefits

The results of the above tests on the serviced bars and the original spare bar indicated that although the insulation system had clearly lasted 25 years in service, there was now concern for the continued long-term operation of the insulation in its current condition. This conclusion was not based on the effects of thermal aging as expected, but was due to design and manufacturing issues.

Upon dissection of the insulation, it was established that the bars tested had failed the ac hipot and thermal endurance assessment tests due to two factors:

• The main wall insulation was found to be 2.4 mm as opposed to 3 mm thick, which is usual for 13.8kV bars, and would have resulted in a main wall stress (voltage between copper conductor to stator core (ground)) of 3.3 kV/mm (85 volts/mil) as opposed  to the usually adopted standard of 3 kV/mm (55 to 65 Volts/mil).
• There was creasing on the edges of the all the bars inspected as a result of the insulation tapes being pulled too tight during manufacture. This resulted in stressing / shearing of the mica, which resulted in a weakness within the main wall insulation.

The short-term reliability of the insulation will need to be further determined by trending various operating parameters of the machine and determining if any of the four machines are at increased risk, compared to the bars tested.

Based upon this information the client is able to determine an effective strategy for the long-term reliability and operation of the stator winding insulation and the need for rewind.

For more information

Andrew Brown, 416.207.6000x5403, andrew.brown@kinectrics.com