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 diagnostic testing to assess stator winding insulation condition if they have on-line monitoring. Traditionally, many utilities would take the opportunity 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 nominal line-to-ground operating voltage. Below, we shall discuss the role that off-line tests have to play, the commonly used tests and their appropriate 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 ground wall 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 dividing this value by that measured after 60 seconds. 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. Typically, 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 insulation 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 interpretation that may provide insight into the bulk condition of the insulation. If the machine is shut down for maintenance, this test is strongly recommended.
Capacitance & Dissipation Factor
Capacitance and dissipation factor measurements have been routinely used by manufacturers 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 dissipation 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 partial discharge which is a loss mechanism. Thus, dissipation factor measurements may be used to deter-mine the void content of an insulation system. Unlike a partial discharge test, dissipation factor also incorporates information about the bulk insulation sys-tem. Thus, there may not be an exact correlation 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 factor tip-up. At the lower voltage the insulation system is assumed to not be subject to partial discharge . Thus, the tip-up is used as a means to differentiate 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 performance of the test; however, the IEC standard also includes acceptance criteria, for individual stator bars and coils, which to some are controversial. 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 stator windings. Thus, with the widespread availability of either on-line or offline partial discharge testing, this test is becoming less popular as a maintenance test.
Off-line partial discharge measurements are employed to provide information on the void content of the insulation system. Unlike a dissipation factor measurement which spatially aver-ages the test result, a partial discharge test is sensitive to those voids with the largest dimensions (which are those of most concern). Where an on-line or off-line partial discharge test indicates anomalously high partial discharge magnitudes, corona (or TVA) probe testing may be deployed to aid in identifying the location of this activity. Partial discharge testing is also useful to uncover other defects such as surface contamination and inadequate clearances between phases. The identification of such issues which occur in the end winding regions of machines are significantly aided by employing additional techniques such as corona (ultra-violet) cameras, corona probes and ultrasonic probes. Extensive guidance on offline partial discharge test methods is given in IEEE 1434 and IEC 60034-27. Comparing on-line partial discharge 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
- The coil/bar forces are present
In short, there are a number of defect mechanisms, e.g., loose windings, that cannot be detected 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 condition. 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 partial discharge 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 partial discharge activity is likely to be an issue during operation.
While experience, to date, indicates that on-line condition monitoring methods such as partial discharge are effective in providing information on stator winding insulation condition, off-line testing still has a significant role to play. In addition to verifying the results of on-line testing, off-line diagnostic tests, especially when more than one technique is used, provide additional information on which to base maintenance decisions.