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PARTIAL DISCHARGE ANALYSIS

CASE STUDY 1

PD trending is the key factor in identifying severe insulation problems and increased likelihood of stator insulation failure.

PDA testing was performed on two 6.9 kV, 7000 HP induction motors of the same age and design.

Initially Motor A had higher PD activity but its PD readings were stable over time. The first PDA tests on Motor B showed lower levels of PD initially but there was a steady increase in PD activity over six months. Consequently, Motor B failed in service due to a puncture in its ground wall insulation, as shown in the photograph below.

This case demonstrates that a significant increase in PD activity within a certain period of time is the best way to identify severe stator insulation deterioration.

Trending graph showing stable +NQN and -NQN values for Motor A.
 

Trending graph of +NQN and -NQN values of motor B.


Visual inspection verifies the insulation failure

 

CASE STUDY 2

Using PDA monitoring to extend the lifetime of a generator with severe stator winding insulation damage.

During a start-up in 1993, a 730 MW, 20 kV steam turbine generator was extremely overheated due to a failure of the automatic hydrogen cooling system. The stator winding temperature reached 165 C° before the cooling failure was discovered and the generator was immediately shut down. The wedges, filler blocks and bracing ties of both the stator and rotor were melted or burned black beyond repair. The class B thermoplastic stator insulation, however, did not fail. The opinion of many experts at the time was that, if restarted, the stator winding would most likely fail and should be rewound immediately. The confidence to continue running the generator was lost.

A PDA system from ADWEL International was installed in 1993. The first on-line PDA readings were taken in July 1993, as shown in the following figure <2>. The PD levels were not alarmingly high. This suggested that the insulation system was still in good shape, even though the generator had been overheated. The utility then had confidence to continue running the generator.

They have performed periodic PDA testing on the generator to monitor any progress of insulation deterioration since 1993. The PDA trend was steady from 1993 to 1999. A recent PDA reading taken in August 1999 is shown in the following figure <1>. There is negative pulse predominance in the curves. This is a sign of internal PD occurring closer to the interface between the copper conductor and the ground-wall insulation. The generator is still running today, maximizing the lifetime of the stator winding. The avoided cost of the emergency rewind was millions of dollars.

Comparison of the PDA readings taken in 1999 <1> and in 1993 <2> shows very little change, an indication of stable insulation condition and low likelihood of failure.

CASE STUDY 3

Maximize the lifetime of rotating machines.

A 13.2 kV, 13,000 HP motor operating in a petrochemical plant was tested. The insulation of the motor is epoxy mica. The first PDA test was carried out in 1993. There was high PD activity in phase C, as shown in the following figure. The predominance of negative pulses indicates that partial discharges and de-lamination were occurring at the interface of the copper conductor and the ground-wall insulation.

This type of partial discharge attacks both the ground-wall insulation and turn-to-turn insulation, causing electrical treeing and turn-to-turn shorts. There is no practical means of repairing such defects. Regular PDA monitoring can detect any progress in PD activity and give an indication of further insulation deterioration and increased likelihood of failure.

PDA test results for the motor in 1993 <1> and in 2000 <2> coincide, indicating stable insulation condition and small likelihood of failure.

 

Although the PD activity in this motor was relatively high, a decision was made to continue running the motor with regular PDA tests to monitor the condition of the stator winding insulation rather than to do an immediate rewind. The regular PDA test results for seven years show that, although it has been high, the PD activity in phase C is stable. This indicates that further severe insulation deterioration has not occurred. The test result of phase C in the year 2000 is also shown in the following figure. Regular PDA monitoring has given the maintenance engineer the confidence to keep the motor running for seven years with some insulation degradation. The lifetime of the stator winding insulation has been extended. This motor is still in service today. It will continue to run until a significant change in the PD activity is detected or an appropriate time for rewind is reached, then a repair action may be taken. The lifetime of the motor has been maximized.

 

CASE STUDY 4

Higher sensitivity of 500 pF capacitors gives better PD detection than 80 pF capacitors.

A 6.9 kV, 6000 HP motor was tested under the same operation conditions to compare the detection sensitivity of the 80 pF capacitor and the 500 pF capacitor.

Partial discharges detected from the 80 pF capacitor Partial discharges detected from the 500 pF capacitor

The 80 pF capacitor detected almost no partial discharges, while the 500 pF capacitor detected a number of partial discharges on the same motor.

Higher sensitivity capacitors bring the following benefits:

  • Ability to detect small partial discharges enables user to see the onset of stator insulation damage earlier. Both a small number of large PD and a great number of small PDs may indicate that damage has occurred to the stator winding insulation. Therefore a great number of small PDs should not be ignored for insulation diagnosis. The 500 pF capacitor can detect some PD pulses, which the 80 pF capacitor is not sensitive enough to detect.
  • Ability to identify and monitor stator insulation problems at the beginning stage to give an early warning instead of detecting no PD signals. To be detected by 80 pF couplers, much higher PD activity must occur. Therefore, the time between detection of higher PD activity and possible failure is dangerously shortened
  • Ability to detect more PD activity to provide better information for analysis and diagnosis of insulation problems.
 

 

 
 
 
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