Offline Diagnostic Testing of Stator Winding Insulation

Many end users, manufacturers, and service organizations now routinely employ some form of online monitoring technology, e.g., partial discharge (PD), as a significant component of a predictive or condition-based maintenance program. 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 online 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 frequency of outages in which the rotor was removed 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 offline 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 a prescribed DC voltage across the ground wall insulation, and, based on 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, which 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 determine the void content of an insulation system. Unlike a partial discharge test, the dissipation factor also incorporates information about the bulk insulation system. 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 not to 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 guide the 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 online or offline partial discharge 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 dissipation factor measurement, which spatially averages the test result, a partial discharge test is sensitive to those voids with the largest dimensions (which are those of most concern).

Where an online or offline 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 that occur in the end winding regions of machines is significantly aided by employing additional techniques such as corona (ultraviolet) cameras, corona probes, and ultrasonic probes. Extensive guidance on offline partial discharge test methods is given in IEEE 1434 and IEC 60034-27. Comparing online partial discharge testing to its offline counterpart, there are many advantages to performing the test while the machine is operating. Among these are:

• The voltage distribution is correct
• The stator winding is at an elevated temperature
• The coil/bar forces are present

In short, there are several 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 hydrogen-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 the worst case, then the data thus obtained still has 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-present background electrical interference (which is a significant problem for online 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 into whether the partial discharge activity is likely to be an issue during operation.

Summary

While experience, to date, indicates that online condition monitoring methods such as partial discharge are effective in providing information on stator winding insulation condition, offline testing still has a significant role to play. In addition to verifying the results of online testing, offline diagnostic tests, especially when more than one technique is used, provide additional information on which to base maintenance decisions.