Experience with Dielectric Dissipation Factor testing of complete stator windings

Dielectric dissipation factor (DDF) testing has been widely employed for many decades by OEMs, end users and service providers as a tool to assess stator winding insulation condition. The test method is governed by international standards, IEEE 286^[1] and IEC 60034-27-3^[2], with the latter standard providing acceptance limits for new stator bars and coils. However, IEC 60034-27-3 specifically precludes the use of these limits on completely assembled stator windings due to the presence of the stress grading materials employed on high voltage rotating machines. Measurement of dissipation factor and tip-up is complicated by the presence of silicon carbide stress control coatings on coils or bars rated at 6 kV or above. At low voltage, the silicon carbide is essentially a very high resistance coating, and no current flows through it. Thus, there is no power loss in the coating. However, when tested at rated line-to-ground or line-to-line voltage, by design the silicon carbide coating will have a relativity low resistance. Capacitive charging currents flow through the insulation and through this stress relief coating. The charging currents flowing through the resistance of the coating produce an I²R loss in the coating. The dissipation factor measuring device measures this loss. Since the loss is zero at low voltage and nonzero at operating voltage, the coating yields its own contribution to tip-up. This coating tip-up creates a noise floor. Very significant partial discharge (PD) must be occurring in most windings for the PD loss to be seen above the silicon carbide tip-up. The influence of the losses on DDF measurements associated with the stress grading materials may be significantly reduced by the use of guard electrodes as described in the standards. However, guard methods are not practical on complete stator windings. Despite this limitation, many organizations routinely employ DDF testing on complete stator windings (or individual phases) often in conjunction with other dielectric tests such as insulation resistance, partial discharge, high voltage dc ramp, etc. Given the lack of any acceptance criteria for DDF test results, the data is trended and/or the results from individual phases is compared to determine if any anomalies are present that may indicate degraded insulation condition. Some organizations have empirically derived internal values for what is considered acceptable DDF on complete stator windings. Dielectric dissipation factor data may also be compared to PD measurements, if made, because both tests provide a measure of void content. However, unlike PD measurements, dielectric dissipation factor measurements provide no indication of the distribution of loss within the insulation system and thus do not permit localization of weak points of the insulation system. This contribution, based on DDF data obtained over many years on a number of service-aged stator windings, will examine the effectiveness of trending the data as well as determining the prospects for applying any DDF limits, e.g., absolute DDF or tip-up values, to aid condition assessment of the stator winding insulation system.