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The maximum PD magnitude at line-to-ground voltage should be less than 50pC. Testing Laboratory The test requires an AC power source capable of supplying PD-free 50 or 60Hz variable voltage up to at least the rated line-to-ground voltage of the insulation system under test. You may experience issues viewing this site in Internet Explorer 9, 10 or ICMflex connected to a single stator bar under test.
Thermomechanische Bewertung von Ieec. Sedding Dissipation Factor Acceptance Criteria for Stator Winding Insulation Dielectric dissipation factor testing, also known as tangent delta or power factor testing, is a measure of the dielectric losses in an insulation system. In the field of rotating machines, this technique is widely used as an appropriate means of assessing the quality of new and also aged stator winding insulation.
The method is useful for assessing the uniform quality of manufacturing and the dielectric behavior of the insulation as a whole. For aged stator windings, the dielectric dissipation factor provides information about insulation condition. Certain deterioration processes, such as thermal aging or moisture absorption, will increase dielectric losses. Thus, trending of the dielectric loss over time may be employed as an indication of certain types of insulation problems.
The main principle is to measure the dielectric dissipation factor over a range of voltages and to derive different characteristic dielectric loss parameters as a basis for the evaluation. Typically, practitioners of dissipation factor measurements have used three basic parameters as a means to assess insulation condition.
The absolute dissipation factor value at a prescribed voltage, usually the rated phase-to-phase or phase-to-ground voltage. The incremental change in dissipation factor as the voltage is raised in prescribed increments. The change in dissipation factor as the voltage is increased from prescribed minimum to maximum voltages. This latter parameter is widely known as the dissipation factor tip-up and has been considered by many to be the key value providing some insight into insulation condition.
Empirical limits of these three parameters, verified in practice, may be used as a basis for evaluating the quality of stator winding insulation systems in manufacturing. Until recently, the limits applied to dissipation factor measurements were largely based on internal standards or procedures developed by manufacturers and end users.
Perhaps not surprisingly, these guidelines are viewed by some as too restrictive and by others as overly lenient. Consequently, efforts are underway by some organizations, e.
As part of the effort, this article reports on the results collected by Kinectrics Inc. 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. 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. 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 PD must be occurring in most windings for the PD loss to be seen above the silicon carbide tip-up. When testing individual coils and bars, the tip-up contribution due to the stress relief coating can be minimized.
Details on guarding methods are provided in the relevant standards. An example of a stator bar with measuring and guard electrodes applied is illustrated in Figure 1. Figure 1: A One of the guard electrodes is circled in this photo. Photo courtesy of Kinectrics Example Test Results The results obtained from the analysis of the data are presented in Figures 2 and 3.
There are a few points to note regarding these Figures. Process A and B refer to the two different manufacturing methods commonly employed, however, the actual processes are not identified because it is not the objective of this work to imply that one manufacturing method is superior to the other.
According to the IEC standard, a dissipation factor measurement is accomplished by recording data at 0. For a However, in this case, the North American convention was followed and the results were derived at 2 and 8 kV. The maximum value recorded was With respect to the dissipation factor tip-up limits set out in IEC , a number of stator bars or coils would not have met the requirements of the standard.
This limited data implies that the absolute dissipation factor limit at 0. In contrast, the findings from an examination of the dissipation factor tip-up values showed that a significant number of bars or coils failed to meet the limit required by the standard.
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