Presented By:
Dr. Diego M. Robalino
Megger North America
TechCon 2018
Abstract
Line-frequency power factor / dissipation factor testing technique has been used throughout the electrical industry and has triggered the attention of those who are directly involved in electrical assets operation, maintenance and management. The power factor/dissipation factor testing technique, as a tool for insulation diagnostics, has evolved beyond the initial line-frequency analysis and this evolution should be well understood for field implementation.
In this paper, the author describes advanced features incorporated into the power factor testing technique for field insulation diagnostics. The Individual Temperature Correction (ITC) algorithm to normalize power factor measurements based on insulation condition assessment and not generic tables; and, the voltage dependence detection (VDD) algorithm as an evaluation tool to identify non-linearity of the dielectric system.
This paper provides practical guidelines to facilitate the decision making process. The advanced features described herein, will allow prioritization of maintenance activities and better understanding of results.
Introduction
The process to deliver electric energy from generation to transmission and distribution systems involves the use and application of complex engineering design and operation strategies. The path for electric energy involves a number of components with dedicated functionality and specific technical characteristics. In general, rotating machines, cables, power, distribution and instrument transformers, protection and control devices are designed, built and commissioned to perform in a reliable and safe manner during the entire expected service life.
Low voltage (LV), medium voltage (MV), high voltage (HV) and extra-high voltage (EHV) components must pass a series of testing procedures before reaching the field. The test should resemble normal operation condition and potential abnormal conditions to simulate the mechanical, dielectric, electric and thermal stress expected during the service life of a specific component.
Factory and field-testing procedures have been developed to assure standardized practices around the entire technical community to deliver a high quality final product to the end-user and minimize the risk of failure during its service life.
Undoubtedly, a good understanding of the condition of the insulation in the electrical system is paramount. The risk of failure of electrical equipment in the field due to dielectric breakdown increases as the aging of the assets in the field increases. Aging is not the only factor, for power and distribution fluid-filled transformers, moisture, oxygen, oil-aging byproducts and particles of different origin are agents of degradation, shortening transformer service life under thermal, electric, electromagnetic and electrodynamic stresses [1].
The electrical power industry in North America has relied on single-frequency (line-frequency) power factor/ dissipation factor and capacitance testing to assess the insulation condition of power and distribution transformers for almost a century. Industry specialists, researchers and instrument manufacturers have worked together to provide advanced features to improve, expand and better interpret the information obtained from line-frequency power factor testing on fluid-filled power and distribution transformers.
I. LINE-FREQUENCY POWER FACTOR
A perfect or ideal insulation system (represented by a capacitor) would have no other component of current except the capacitive component. However, no dielectric materials, not even vacuums, are perfect. When an AC voltage is applied across an insulation system, while most of the resulting current that flows through the insulation is capacitive (representing the energy being stored by the insulation), loss (resistive) current results as well. This loss component, IR, which is in phase with the applied AC voltage, is associated with the insulation dielectric losses. The total resulting current that flows through the insulation, IT, is the vector sum of the capacitive current, IC, and loss current, IR, as given in Figure 1.
Dielectric loss is the energy lost or released as heat when an electrostatic field is present across an insulation system. Losses can be broadly classified as conductive losses, arising from leakage current, or polarization losses. The total losses (IR ∙ V) in a dielectric are equal to the combined polarization losses and conductive losses present.
Dielectrics perform best when they are clean, dry, relatively void-free, and utilized within a certain temperature range. Adversaries to a dielectric’s continued good health are heat, moisture and oxygen. Continuous degradation of the insulation system is observed when power factor/dissipation factor test reports higher dielectric loses. Normal transformer service or aging show a slow increase of these dielectric losses and that is expected to see during normal and/or routine maintenance testing. Nevertheless, a rapid increase of losses is an indication of an active failure condition that may lead to dielectric breakdown.
Power Factor (PF) as expressed in (1) is the cosine of (the complementary angle of the “loss angle”) while Dissipation Factor (DF) as expressed in (2) is the tangent of . As described in [2], the normal in-service and new PF limit for mineral-oil-filled power transformers < 230 kV is 0.5% PF at 20 °C, and the normal and new limit for transformers 230 kV is 0.4%. To help reduce the risk of catastrophic failure, the limit for serviceability of all mineral-oil-filled transformers is 1.0% PF at 20 °C. PF values between 0.5% and 1.0% at 20 °C require additional testing and investigation to confirm that a problem is not worsening.
II. SPECIFICS OF THE LINE-FREQUENCY POWER FACTOR
A. Line-frequency reference only
Line-frequency power factor is commonly used to detect potential aging and degradation of insulation due to thermal, chemical, mechanical or electrical stress. Trending analysis for interpretation of power factor/dissipation factor results relies on measurements at one specific voltage and one single frequency and “assumes” proper temperature correction of this measured value.
Increasing levels of some contaminants might not provide notable change in line-frequency power factor and depending on the conductivity of the material, the effect of contamination may be observed at a frequency different from 50/60Hz.
To overcome this limitation, a dielectric frequency sweep in the range between 1 – 500 Hz is required.
B. Thermal dependence
On this point and from IEEE Std. C57.12.90-2015, the following is extracted as published: “Table 4. NOTE 3— b) Experience has shown that the variation in power factor with temperature is substantial and erratic so that no single correction curve will fit all cases…”
The standards clearly indicate that measurements of losses in the dielectric material are sensitive to temperature variation as well as dielectric condition changes. Therefore, changes in the dielectric condition should imply changes in the thermal behavior of dielectric parameters and table correction factors selected based on nameplate information are merely an average reference not specific for the asset under test.
To overcome this limitation, an algorithm to estimate the individual temperature correction is required.
C. Voltage Dependence
The voltage dependence phenomenon in solid insulation during line-frequency PF/DF test is well described in [3] and [4]. The losses measured in the insulation system when a line-frequency (50/60Hz) signal is applied, is a composite of dielectric losses which are constant with voltage and power loss due to discharges. Mathematically, this can be expressed as the total conductance of the system in (3)
Based on (3), if no discharges occur, the amount of charge increase on the electrodes or conductors as a result of internal discharges (Qi) equals zero. To visualize this effect, a dielectric frequency sweep between 500Hz and 1Hz is performed at different voltage levels on epoxy type MV equipment and results are presented in Figure 2.
It is clear from Figure 2 that the voltage dependence of an insulation system is detected by measurement at different voltage levels. In the specific case of oil-impregnated insulation, the expected condition is to measure PF/DF at line frequency at a single voltage and that the value under ideal conditions will not change. A deviation from this statement will indicate that the ideal condition has been altered and potential degradation is taking place within the insulation system of the transformer. Figure 3 shows an ideal condition of the insulation inside the transformer in a tip-up test.
Historically, power factor test at line frequency has been carried out at one test voltage value. To overcome this limitation, a technical approach to determine voltage dependency is required.
III. TRANSFORMER INSULATION DIAGNOSTICS BEYOND LINE-FREQUENCY POWER FACTOR
A. Narrow band dielectric frequency response
It is not a big surprise to find, on different objects, similar line-frequency power factor values. This finding should trigger the “curiosity” of the testing specialist to further investigate this coincidence. The approach provided nowadays to simply visualize the dielectric response in a short period of time (~3 minutes) is to perform a narrow band dielectric frequency response (NBDFR) sweep. NBDFR is typically carried out in the range between 500Hz and 1Hz and for the most part, the sweep is obtained by applying a low voltage AC signal to the insulation system.
The benefit of performing NBDFR in a complex insulation system is difficult to rank. First, NBDFR provides direct visualization of the dielectric material condition and, in the case of bushings; it provides a comparative analysis signature. Figure 4 describes six different examples of insulation conditions where line-frequency power factor is very similar amongst different objects but the NBDFR sweeps show important differentiation.
The research presented in [5] shows the importance of the narrow band dielectric response. The specific application on oil-impregnated paper (OIP) HV bushings allows a better assessment of the insulation at 1Hz. It is recommended to plot the narrow band dielectric response in a logarithmic scale (as shown in Figure 4), otherwise the smallest decade (1 – 10Hz) is not clearly visualized and important information will be omitted.
NBDFR from 1 to 500Hz has already been implemented in state-of-the-art power factor test sets and its use has more applications as discussed in the next section.
B. Individual Temperature Correction (ITC)
As mentioned in the previous section, NBDFR has an additional application when used for advanced insulation diagnostics.
The implementation of mathematical algorithms allows moving from the dielectric response in the frequency domain into the dielectric response in the thermal domain [6]. Figure 5 shows how the dielectric response in the frequency domain and in the thermal domain change based on the condition of the insulation system and, therefore, the use of tables and average correction factor values might be misleading and not accurate.
The obtained NBDFR from the inter-winding insulation is also used to determine the individual temperature correction (ITC) of the line-frequency power factor measured at temperatures different from 20°C.
The mathematical approach used to correlate line-frequency power factor with temperature is an Arrhenius-based equation (4). This approach describes the ‘frequency shift’ factor, which is dependent on the temperature difference between the temperature of the measurement T2 and the reference temperature T1 (expressed in Kelvin).
The equation considers an exponential function related to temperature (T), Boltzmann constant (kB) and the activation energy value (Ex,y) of the material. It has been found that the shape of the dielectric response (PF/DF versus frequency) does not change very drastically with temperature for quite a large group of solid dielectric materials; rather, as temperature changes, the response (a spectral shape) shifts with respect to frequency while remaining intact. This means that a PF/DF value measured at line-frequency and at one specific temperature, is the exact same PF/DF that would be measured at the reference temperature (typically 20°C) at a frequency different from line-frequency.
Therefore, using NBDFR, the individual temperature correction (ITC) factor can be estimated for the line-frequency power factor based on the real insulation condition [6].
C. Voltage Dependence Detection (VDD)
The effect of voltage over an insulation system is not observed by a single-voltage, single-frequency power factor test. The voltage dependency effect has been observed and studied by different researchers and end-users who faced power factor voltage dependency on insulation systems which should not be voltage dependent as it is the case of oil-paper insulation in power and distribution transformers. The developed feature to identify voltage dependency on the insulation system is the percentage voltage dependence factor (%VDF).
To perform a line-frequency power factor test, the instrument applies a perfect sinusoidal voltage to the insulation system. Thus, it is expected (ideally) that a perfect sinusoidal current, representing IT will be measured. In fact, this is not always the case. As an example, the distortion in the measured current signal is represented in Figure 7.
The deviation of the measured signal from its fundamental implies that the insulation system has lost its linearity [7]. Therefore, the %VDF can be calculated in a similar way as the total harmonic distortion (THD) of the measured current signal as presented in (5):
Voltage dependence detection is easily demonstrated while testing solid insulation specimens when performing a tip-up test as shown in Figure 7. The ideal oil-paper insulation presented in Figure 3 compared to the plot in Figure 7 clearly support the need to have implemented voltage dependence detection algorithms in the power factor testing instrument.
Figure 7 shows voltage dependency on a solid insulation specimen. The %VDF follows the PF voltage dependency. If the insulation shows deterioration, PD activity or contamination, %VDF provides a warning flag to the end user “suggesting” to add a tip-up test to the series of tests carried out on that specific transformer.
%VDF (percentage voltage dependence factor) is a very small value, typically below 0.1% for linear %PF response.
IV. ADVANCED DIAGNOSTICS BY MEANS OF DIELECTRIC FREQUENCY RESPONSE
Frequency Domain Spectroscopy (FDS) also known as Dielectric Frequency Response (DFR) is an advanced application of the power factor test to determine the condition of the insulation system in power and distribution transformers.
In the frequency domain, the dielectric spectrum is obtained applying an AC excitation voltage between 140Vrms and 1400Vrms to the insulation system. The frequency spectrum is typically obtained between 1 kHz and 1 mHz as shown in Figure 8.
The response is analyzed based on mathematical modeling and comparative analysis against a well-documented materials’ database to determine primarily:
- Percentage moisture concentration in the solid insulation, and;
- Conductivity of the liquid insulation
In addition, DFR provides information regarding:
- Presence of contaminants by non-typical responses, and;
- A solution to convert the frequency domain response into a thermal domain response of the insulation system as a plot of %PF or %DF vs. insulation temperature as shown in Figure 9.
The IEEE transformers committee is finalizing the work to generate the guidelines for DFR application on power and distribution transformers. The document is expected to be published in 2018 under the designation IEEE Std C57.161.
V. SUMMARY
As presented in this paper, condition assessment of transformer insulation is not a simple task. The limited information obtained by a single line-frequency power factor value may now be expanded with newly developed features that certainly ease the condition assessment of complex insulation systems in power and distribution transformers.
References
[1] V. Sokolov, V. Bulgakova, Z. Berler, “Assessment of Power Transformer Insulation Condition”. Proceedings of the IEEE Electrical Insulation Conference & Electrical Manufacturing Coil Winding Conference, 2001
[2] IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors. IEEE Std C57.152TM-2013.
[3] IEEE Std. 286-2000 (R2012). “IEEE Recommended practice for measurement of power factor tip-up of Electric machinery Stator Coil insulation”.
[4] T. W. Dakin, “The Relation of Capacitance Increase with High Voltages to Internal Electric Discharges and Discharging Void Volume”. AIEE Transactions. Part III, Issue 3, 1959
[5] D. Robalino, I. Guener, P. Werelius, “Analysis of HV Bushing Insulation by Dielectric Frequency Response”. Proceedings from the IEEE EIC Conference 2016.
[6] D. Robalino, “Individual Temperature Compensation – benefits of dielectric response measurements”. Transformers magazine, Vol. 2, Issue 3. 2015.
