Many end users, manufacturers and service organizations now routinely employ some form of on-line monitoring technology, e.g., partial discharge (PD), as a significant component of a predictive or condition-based maintenance pro-gram. The widespread and increasing use of such methods has led to some in the industry questioning the value of performing off-line diag-nostic testing to assess stator winding insulation condition if they have on-line monitoring. Tradi-tionally, many utilities would take the opportuni-ty during major outages to do one or more of these off-line tests, however, due to economic pressures, the interval between such outages has significantly increased. In the past, the fre-quency of outages in which the rotor was re-moved was about five to six years; however, currently intervals of 10 – 12 years have become common. Further, many plant managers are reluctant to permit electrical testing because of concerns that such testing may damage the winding even though the applied voltage used in the vast majority of tests is limited to the nomi-nal line-to-ground operating voltage. Below, we shall discuss the role that off-line tests have to play, the commonly used tests and their appro-priate application. Only those tests that are covered by IEEE or IEC standards are discussed and ac and dc hipots are precluded because these are go/no-go tests with relatively little diagnostic value.
Insulation Resistance & Polarization Index
An insulation resistance measurement is one of the most basic and commonly employed tests used in the industry. The test involves applying prescribed dc voltage across the groundwall insulation and, on the basis largely of the leakage current, the resistance value after one minute of voltage application is derived. The polarization index is obtained by taking a further insulation resistance measurement at 10 minutes and di-viding this value by that measured after 60 sec-onds. These tests are governed by IEEE 43 that specifies, among many factors, the appropriate applied voltage (dependent on the rated voltage of the machine) as well as acceptance criteria for insulation resistance and polarization index. Until recently, there was no equivalent IEC standard for this type of testing, however, this situation will change in 2016 or 2017 with the impending publication of IEC 60034-27-4. Typi-cally, these tests are used either to determine that the stator winding is fit to undergo further diagnostic testing involving high voltages or to verify a ground fault in the event of an alarm or trip. While the diagnostic content of an insula-tion resistance test has been considered limited due to sensitivity to surface leakage currents, the latest version of IEEE 43 does include guidance on more sophisticated methods of interpreta-tion that may provide insight into the bulk con-dition of the insulation. If the machine is shut down for maintenance, this test is strongly rec-ommended.
Capacitance & Dissipation Factor
Capacitance and dissipation factor measure-ments have been routinely used by manufactur-ers for decades as a means to assess the quality and uniformity of individual stator bars and coils. Dissipation factor testing belongs to the broad range of measurements of dielectric loss and is also commonly referred to as the tan delta or power factor test. Power factor is the cotangent of the loss angle (delta) whereas dis-sipation factor represents the tangent. At low values of loss angle, the tangent and cotangent are virtually the same. The higher the dielectric loss in an insulating material, the higher will be the dissipation factor. Defects in an insulation system, such as voids and delamination, result in PD which is a loss mechanism. Thus, dissipa-tion factor measurements may be used to deter-mine the void content of an insulation system. Unlike a PD test, dissipation factor also incorpo-rates information about the bulk insulation sys-tem. Thus, there may not be an exact correla-tion between the results obtained from PD and dissipation factor tests. Often the dissipation factor is obtained at two different voltages, e.g., at 25% and 100% of the nominal line-to-ground operating voltage, to derive the dissipation fac-tor tip-up. At the lower voltage the insulation system is assumed to not be subject to PD. Thus, the tip-up is used as a means to differenti-ate between effects due to the bulk and defects such as voids. This testing is governed by IEEE 286 and the recently published IEC 60034-27-3. Both documents provide guidance on perfor-mance of the test; however, the IEC standard also includes acceptance criteria, for individual stator bars and coils, which to some are contro-versial. Due to the complications caused by the stress grading components in machines rated 6.6 kV and above, no such criteria are available for measurements performed on complete sta-tor windings. Thus, with the widespread availability of either on-line or off-line PD testing, this test is becoming less popular as a maintenance test.
Partial Discharge
Off-line partial discharge measurements are employed to provide information on the void content of the insulation system. Unlike a dissi-pation factor measurement which spatially aver-ages the test result, a PD test is sensitive to those voids with the largest dimensions (which are those of most concern). Where an on-line or off-line PD test indicates anomalously high PD magnitudes, corona (or TVA) probe testing may be deployed to aid in identifying the location of this activity. Partial discharge testing is also use-ful to uncover other defects such as surface con-tamination and inadequate clearances between phases. The identification of such issues which occur in the endwinding regions of machines are significantly aided by employing additional tech-niques such as corona (ultra-violet) cameras, corona probes and ultrasonic probes. Extensive guidance on off-line PD test methods is given in IEEE 1434 and IEC 60034-27. Comparing on-line PD testing to its’ off-line counterpart, there are many advantages to performing the test with the machine operating. Among these are:
- the voltage distribution is correct,
- the stator winding is at elevated temperature,
- and, the coil/bar forces are present.
In short, there are a number of defect mechanisms, e.g., loose windings, that cannot be de-tected using an off-line PD test. Further, often the results from off-line measurements have to be treated with some caution because they may be pessimistic relative to the operating condi-tion. For example, off-line PD testing of hydro-gen-cooled machines is almost invariably done in air at atmospheric pressure which produces much higher PD. However, if one takes the view that these results would be worst case, then the data thus obtained still have value. A significant advantage that off-line PD testing provides is that the test operator has control of the applied voltage. Consequently, despite the always pre-sent background electrical interference (which is a significant problem for on-line testing), the PD activity (if present) can normally be observed as the applied voltage is varied. Further, the discharge inception and extinction voltages can be measured, which provide further insight on whether the PD activity is likely to be an issue during operation.
Summary
While experience, to date, indicates that on-line condition monitoring methods such as PD are effective in providing information on stator winding insulation condition, off-line testing still has a sig-nificant role to play. In addition to verifying the results of on-line testing, off-line diagnostic tests, es-pecially when more than one technique is used, provide additional information on which to base maintenance decisions.