How on-line partial discharge testing can extend generator life20 October 1999
On-line partial discharge testing is a useful tool for assessing the condition of stator winding insulation – a key determinant of generator life. This condition-monitoring technique can give operators the confidence to run with some degree of insulation degradation, avoiding expensive rewinds and maximizing the useful life of the stator winding.
Rotating machine failures due to insulation breakdown can cause catastrophic damage to equipment, loss or de-rating of power output, lengthy forced outages and heavy costs to the utility. Therefore it is desirable to detect the onset of significant insulation deterioration and identify potential failures as early as possible. As in the medical field, regular testing and early detection of problems mean better chances of survival for the 'patient'. Faced with growing pressure to increase unit availability yet reduce maintenance costs, utilities and other electric power producers are moving away from time-based maintenance practices to condition-based maintenance techniques. One such technology is partial discharge analysis, which owners and operators of generators (and large motors) are using to perform on-line condition monitoring of the stator insulation system.
On-line partial discharge (PD) measurements have become recognized as an effective, reliable method of assessing stator insulation condition to forewarn plant personnel of possible machine failures. The measurements are obtained with the machine in normal operation and thus form an integral part of a condition-based maintenance programme. Regular condition monitoring of the stator winding insulation can give users the confidence to safely operate machines, even ones that have suffered some degree of insulation degradation.
Using PD to assess degradation
Operating generators and motors are subjected to electrical, mechanical, thermal and environmental stresses that cause ageing and degradation of the stator insulation system. Deterioration of the stator winding insulation is an important factor in determining the lifetime of high voltage generators and motors.
Partial discharges are essentially sparks occurring in voids within the electrical insulation system itself or adjacent to the insulation of high voltage stator windings. They are the major symptom of stator insulation deterioration.
Deterioration of the stator insulation system on most generators and motors is a gradual process. The breakdown of the stator winding insulation usually occurs over a period of years or decades, not hours or days. Therefore periodic on-line PD measurements taken at, say, three-month intervals are ideally suited for detecting the onset and monitoring the progression of stator winding deterioration.
Once PD activity is detected, the condition of the stator insulation can be assessed based on trends in the PD data. The machine owner can continue to operate the machine with on-line PD monitoring until a critical condition is reached and thus maximize the lifetime of the machine. Suitable maintenance actions can be taken before problems get worse, to avoid insulation failures.
Case study: overheated stator
Thermal stress, especially overheating, is a major contributor to insulation ageing and thus a principal root cause of many stator winding failures. The following example describes a case where the stator winding of a large steam turbine generator was briefly but severely overheated and how the utility decided to continue operating the machine.
The 730 MWe, 20 kV steam turbine generator in question has been in service since 1972. The unit has been plagued by end winding 120 Hz resonant vibrations due to structural looseness. Between 1972 and 1988, the generator went through many maintenance outages to repair insulation damage such as phase to ground faults, loose blocks and weld cracks in the support rings. Fortunately, none of the damage was fatal. The generator end winding structure was improved in 1989 with Westinghouse stator winding maintenance modules, which effectively stiffened the end winding structure and enhanced the integrity of the generator insulation.
However, during a start-up in 1993 the 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, either during the restart or within a very short time of it, and should instead be rewound immediately. The confidence to continue running the generator was lost.
The stator and rotor were immediately re-wedged with new slot wedges and fillers and painted with new resin. However, because of the lengthy outage needed and the high cost of an emergency rewind, the utility strove to determine just how severe the stator winding damage was before deciding upon an immediate stator rewind. Extensive tests such as power factor, DC hi-pot, core thermal scans and off-line corona/TVA probe tests were performed.
The stator winding withstood the hi-pot but showed signs of winding looseness and accelerated winding deterioration in the structural FRF (frequency response function) and power factor tests.
After examining the data from the off-line stator insulation tests, the utility decided to install an on-line condition-monitoring system for the stator insulation and continue operating the generator with the same stator winding. A PDA system from Adwel International (formerly FES International) was installed during the re-wedge outage in 1993 to enable future monitoring of the partial discharge activity in the stator winding.
The utility successfully restarted the generator and began closely monitoring the on-line PDA readings to detect any signs of severe insulation degradation. The first on-line PDA readings were taken in July 1993, as shown in Figures 1 and 2. The PD levels were not alarmingly high. This suggests that the insulation system was still in good shape, even though the generator had been overheated. Confidence to continue running the generator was regained.
Partial discharges appearing in the positive cycle (0°-180°) of the AC voltage sine wave are called negative and those on the negative cycle (180°-360°) are called positive.
The positive and negative curves in Figure 1 overlap, with no predominance of one pulse polarity over the other. This is an indication of internal PD occurring within the groundwall insulation. Internal groundwall PD often occurs within slowly deteriorating thermoplastic insulation, especially if it has been overheated. Pressurized hydrogen gas reduces PD activity by masking voids in insulation materials.
The PDA trend was steady from 1993 to 1999 and consistent with the power-factor test data. A recent PDA reading, taken in August 1999, is shown. 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 groundwall insulation. The low PD activity initially detected by the PDA system supported the utility's decision to continue operating the generator despite the overheating of the stator winding.
Consistent PDA readings from 1993 to date have given the utility confidence to keep running the unit and to maximize its lifetime. The avoided cost of the emergency rewind was millions of dollars.
Partial discharge techniques have also proved useful in the case of high voltage motors installed at power plants and other facilities. For example, in 1992 an on-line PDA system was installed on a 13.2 kV, 13 000 HP motor with epoxy mica insulation. The first PDA test was carried out in 1993. There was high PD activity in phase C. The predominance of negative pulses indicates that partial discharges and delamination were occurring at the interface of the copper conductor and the groundwall insulation. This type of partial discharge attacks both the groundwall 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.
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 over five years show that the PD activity in phase C is stable even though it has been high. The stable PD readings indicate that further severe insulation deterioration has not been occurring. The test result for phase C in 1998 is also shown in Figure 5. Regular PDA monitoring has given the maintenance engineer the confidence to keep the motor, with its insulation degradation, running for six years. The lifetime of the stator winding insulation has been extended and considerable savings achieved. The motor is still in service today and will continue to run until a significant change in the PD activity is detected or an appropriate time for rewind is reached.
Higher sensitivity. Improved capacitive couplers with increased sensitivity* are allowing partial discharges to be detected in stator winding insulation at an earlier stage and they also present more partial discharge information for data analysis. This technology promises to increase the opportunities for extending the life of stator winding insulation.