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.
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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. The test methods are described in IEEE 286 and the old
IEC . 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. As described on page 4, some limits for DF and DF tip-up have now been published in a new document, IEC -27-3. 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.g. CIGRE Study Committee A1 (Rotating Machines), to assess, on an objective basis, for the appropriateness of the published limits in IEC -27-3, for future revisions of this document. As part of the effort, this article reports on the results collected by Kinectrics Inc. (the old Ontario Hydro Research Group) of a study of dissipation factor results involving a large number of stator bars and coils.
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. The most common way is to “guard” out the currents due to the silicon carbide by overlapping the coatings with grounded aluminum foil, or even isolating the silicon carbide coating from the semicon coating, and grounding it separately. 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 13.8 kV stator coil undergoing dissipation factor testing with guard electrodes. 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 0.2UN and 0.6UN, where UN is the nominal phase-to-phase voltage of the stator winding. For a 13.8 kV system, these voltages correspond to 2.8 and 8.3 kV respectively. However, in this case, the North American convention was followed and the results were derived at 2 and 8 kV.
The limits referred to in Figures 2 and 3 are those stipulated in IEC -27-3 and are reproduced in Table I below.
TABLE I. Dissipation Factor Limits
(source: IEC -27-3, Table 1)
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Initial value of dissipation factor at 0.2UN 20 x 10-3 (2%) ΔDissipation factor per 0.2UN up to UN 5 x 10-3 (0.5%) Dissipation factor tip-up between 0.6UN and 0.2UN 5 x 10-3 (0.5%)
Examination of Figures 2 and 3 reveals a number of features. These are,
All of the coils and bars tested were well within the prescribed absolute dissipation factor limit of 20 x 10-3 (2%) defined in IEC -27-3. The maximum value recorded was 13.54 x 10-3 (1.354%).
With respect to the dissipation factor tip-up limits set out in IEC -27-3, a number of stator bars or coils would not have met the requirements of the standard. Approximately 20% of the Process A bars or coils and 15% of those stator winding elements manufactured using Process B exhibited dissipation factor tip-up values above 5 x 10-3 (0.5%).
This limited data implies that the absolute dissipation factor limit at 0.2UN recommended by IEC -27-3 may be too conservative and that some consideration of this criterion may be advisable in any future revision of the document. 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.
What is Dissipation Factor?
Dissipation Factor is an electrical test that helps define the overall condition of insulating material.
A dielectric material is a material that is a poor conductor of electricity but an efficient supporter of an electrostatic field. When an electrical insulating material is subjected to an electrostatic field, opposing electric charges in di-electric material form di-poles.
A capacitor is an electrical device that stores an electrical charge by placing a dielectric material between to conductive plates. The Groundwall Insulation (GWI) system between the motor windings and the motor frame creates a natural capacitor. The traditional method of testing the GWI is to measure the value of the resistance to ground. This is a very valuable measurement for identifying weaknesses in the insulation but fails to define the overall condition of the entire GWI system.
The Dissipation Factor provides additional information regarding the overall condition of the GWI.
In the simplest form when a dielectric material is subjected to a DC field the diploes in dielectric are displaced and aligned such that the negative end of the dipole is attracted toward the positive plate and the positive end of the dipole is attracted toward the negative plate. Some of the current that flows from the source to the conductive plates will align the dipoles and create losses in the form of heat and some of the current will leak across the dielectric. These currents are resistive and expend energy, this is resistive current IR. The remainder of the current is stored on the plates current and will be stored discharged back into the system, this current is capacitive
current IC.
When subjected to an AC fi eld these dipoles will periodically displace as the polarity of the electrostatic field changes from positive to negative. This displacement of the dipoles creates heat and expends energy. Simplistically speaking, the currents that displace the dipoles and leaks across the dielectric is resistive IR, the current that is stored to hold the dipoles in alignment is capacitive IC.
Dissipation Factor is the ratio of the resistive current IR to the capacitive current IC, this testing is widely used on electrical equipment such as electric motors, transformers, circuit breakers, generators, and cabling which is used to determine the capacitive properties of the insulation material of the windings and conductors. When the GWI degrades over time it becomes more resistive causing the amount of IR to increase. Contamination of the insulation changes the dielectric constant of the GWI again causing the AC current to become more resistive and less capacitive, this also causes the dissipation factor to increase. The Dissipation Factor of new, clean insulation is usually 3 to 5%, a DF greater than 6% indicates a change in the condition of the equipment’s insulation.
When moisture or contaminants are present in the GWI or even the insulation surrounding the windings, this causes a change in the chemical makeup of the dielectric material used as the equipment’s insulation. These changes result in a change in the DF and capacitance to ground. An increase in the Dissipation Factor indicates a change in the overall condition of insulation, comparing DF and capacitance to ground helps determine the condition of insulation systems over time. Measuring the Dissipation Factor at too high or too low temperature can result in unbalanced results and introduce errors while calculating. IEEE standard 286- recommends testing at or around ambient temperature of 77 degrees Fahrenheit or 25 degrees Celsius.
Dissipation/Power Factor & Capacitance Measurement (Tan Delta) on Power Transformers
The condition of the insulation is essential for secure and reliable operation of your transformer. Measuring capacitance and dissipation/power factor helps you to determine insulation condition in bushings or between windings.
Changes in capacitance can, for example, indicate mechanical displacements of windings or partial breakdown in bushings. Aging and degradation of the insulation, coupled with the ingress of water, increase the amount of energy that is converted to heat in the insulation. The rate of these losses is measured as dissipation factor.
With our testing systems, you can even determine the capacitance and dissipation/power factor at variable frequency. Therefore, aging phenomena can then be detected earlier, and corresponding action such as repair, oil treatment or drying can be initiated.
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