Frequency Response Analysis (FRA) has been in use since the last decade as a sensitive method for detecting transformer internal defects. This study is focused on the influence of temperature and moisture migration on the FRA response of transformer winding. It also discusses the feasibility of FRA capability in moisture diffusion recognition in transformer paper insulation. To conduct this study, a single phase model transformer involving concentric LV and HV windings and a 20/0.4 kV, 1.6 MVA threephase two windings transformer are taken as test objects. Experiments are carried out at different temperature and moisture conditions. FRA spectra are then recorded and analyzed. FRA spectra deviations as well as total capacitance variations due to the temperature and moisture changes in the test objects are calculated. Karl-Fischer Titration (KFT) is utilized to monitor the moisture migration within oil and paper insulations. Furthermore, a mathematical model is used to simulate one of the test object windings and verify the experimental result, and also clarify the main reason of FRA spectrum deviation in this circumstance. Finally, statistical indices in FRA evaluation are calculated to explore their capability in FRA spectrum interpretation once the moisture content of paper insulation is changed.

Influence of temperature and moisture content on frequency response analysis of transformer winding

Frequency Response Analysis (FRA) has been utilized as an off-line diagnosis test since last decade to investigate mechanical integrity of transformer. FRA data typically reports as a spectrum in Bode diagram over determined frequency band. To evaluate FRA data, determined frequency band may be conveniently divided into three bands, namely low-, medium- and high- frequency bands. It is well-known low-frequency band dominated by core, mid-frequency region dominated by winding structure and high-frequency band influenced by measurement connection leads. This study has concentrated on midfrequency oscillations of transformer winding frequency response spectrum presented in Bode diagram. Mathematical approach using travelling wave theory is employed to explore frequency response trace behavior. Practical studies on a prepared glassy transformer as well as two 66 kV, 25 MVA continuous and interleaved disc windings have been performed to validate mathematical calculation. In addition, two 245 kV, 45 MVA and 66 MVA power transformers have been examined to study mid-frequency oscillations and compare the result with mathematical evaluation.

Transformer frequency response analysis: mathematical and practical approach to interpret mid-frequency oscillations

Evolution and operation of modern grids and networks necessitate reconsideration of several characteristics. For example transformers operating in power grid or in industrial environment are subjected nowadays also to transients or stimulus with different waveforms. Traditionally, in case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such system disturbances are stressed due to the non-linear transient voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non-uniform distribution of voltage results in concentred stress in certain parts of the winding. Winding resonance might occur, if the frequency spectrum of incoming surge matches the winding natural frequencies. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model windings with different constructions subjected to voltages with different rise time. The experimental setup allows for investigation of influence of the dielectric (air/oil) and temperature on the stress distribution along the winding. Also the impact of oil and temperature on frequency characteristics of internal voltages are shown. The presented results might be used both for design and optimization of transformers windings.

Analysis of fast transient voltage distributions in transformer windings under different insulation conditions

The premature failure phenomenon that occurs in solid insulation under the application of repetitive impulse voltages is known as the accumulative effect of impulse voltages on a solid dielectric. The accumulative failure of winding insulation is considered to be a core problem resulting from this phenomenon. The main problems include the accumulative failure of motor winding insulation under pulse-width modulation impulses and the accumulative failure of transformer winding insulation under the application of overvoltages, and these problems represent new challenges in the safe and stable operation of power grids. With regards to the above problems, this study reviews the results of recent research on the accumulative failure characteristics and mechanisms of winding insulation when subjected to repetitive impulse voltages. The challenges faced are discussed at the end of this study.

Review of accumulative failure of winding insulation subjected to repetitive impulse voltages

In this article, a new testing method for the diagnosis of winding faults based on the resonant frequency of the transformer is proposed. The system that consists of the high-voltage dc power and high-voltage insulated gate bipolar transistor (IGBT) could charge the transformer periodically. The response measured from the bushing of the winding is the oscillating wave that can be used for evaluating the status. Carrying out the new test on a 220-kV three-winding, three-phase transformer in the field, the tests of insulation and the winding have been completed in one test. The calculated frequency verifies the feasibility and applicability of the method in theory and the voltage does not affect the resonant frequency in the test. The three-winding, three-phase model transformer was built to simulate the various parameters of the power supply, which proves that the features of the oscillating waves are brought about by the properties of the transformer. Also, the new testing method provides a new opportunity to do parameter identification of axial displacement and short-circuit (SC) fault. The abovementioned research shows that the proposed method can conduct the test of winding and insulation in one test for a short time, which proves a reference in the field.

A New Testing Method for the Diagnosis of Winding Faults in Transformer

During operation transformers are typically subjected to overvoltages with different wave shapes. Even though overvoltage protection systems, such as metal oxide surge arresters, are often employed, the overvoltages generated in power systems can still have a significant impact on the insulation systems of the windings of a power transformer. There are numerous examples of transformer failures as a result of overvoltages occurring during operation, even in cases that standard impulse voltage tests show no indication of potential problems and the transformers are protected by surge arresters. Comparisons of the internal overvoltages generated in transformers during voltage tests as well as those generated during standard operation of electrical networks provides fundamental insights into these overvoltages; insights which in turn can be used to improve the construction of insulation systems and overvoltage protection approaches and ultimately increase the reliability of transformers. In this paper, internal overvoltages occurring in the windings of selected transformer during standard voltage tests are compared with those which occur during typical operation of a transformer in an electrical power system. Overvoltages measured in the windings of the transformer form the basis of this comparison. Measurements were conducted with the transformer being subjected to voltage impulse tests, as well as transient voltages with waveforms simulating overvoltages generated in electrical power systems. In order to investigate the response of the windings to voltage strokes, full and chopped lightning voltage impulses were applied to the transformer. The waveforms of overvoltages generated in the electrical power systems were modelled as a rectangular voltage stroke, sinusoidal voltage, voltage with shape of “sweep sinus” also often referred to as a chirp signal and voltage with shape of “sweep ramp”.

Comparison of transformer winding responses to standard lightning impulses and operational overvoltages

Evolution and operation of modern grids and networks leads to reconsideration of several aspects. For example, transformers operating in a power grid or in industrial environment are nowadays also subjected to transients or stimuli with ultrafast wavefronts. Traditionally, in the case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such effects are stressed due to the voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non uniform character of stress, results in focus on certain parts of the winding. The impact of oil and temperature on initial distribution attributes will be presented in this paper. Winding resonance might occur, if the spectrum of incoming surge voltage matched the winding one, and the corresponding resonance was excited. Therefore external transients occurring in power systems might trigger out internal overvoltages with high maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model winding subjected to ultra fast stresses. On a special setup the influence of oil and temperature on the stress distribution along the winding could be investigated. The distribution of stress as a function of surge rise time and winding position will be presented, while oil impact and temperature will be additional parameters. Special focus is paid on the analysis of the surge rise time and steepness on the winding stress. The presented results might be used both for designing and optimization of transformers windings.

Impact of oil and temperature on initial voltage distributions in transformer windings at ultra fast stresses

The insulation systems of power transformers are subjected to many transient overvoltages. They are usually generated in power systems during lightning and switching operations. If the spectrum of incoming surge voltage matches the winding one, the corresponding resonance might be excited. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. Overvoltages having such character have been the root cause of many power transformer failures. This paper presents an approach to the identification of sensitive zones in the transformer windings based on the measurements of overvoltages inside windings and frequency dependences of admittance of the transformer. The presented approach might be used both for design optimization and diagnostics of distribution and power transformers.

Frequency method for identification of resonance overvoltages in transformer windings

Voltage tests of power transformers are performed with usage of full and chopped voltage impulses. Due to the normalized procedures the extension of the tests with lightning voltage impulse have to consider voltage impulses chopped after different time. This specific tests are suggested for transformers which are directly connected to GIS substations by means of oil to $SF_{6}$ bushings or when the power transformer is protected by rod gaps. Due to actual standardized tests for transformers with nominal voltages below 170 kV chopped wave lightning voltages impulses are described as special tests. For Power Transformers with nominal voltages higher than 170 kV chopped wave lightning voltages impulse are performed as an routine tests. While chopped voltage impulse tests, applied wave of lightning impulse should be chopped after time which is between $3 \mu \mathrm{s}$ and $6 \mu \mathrm{s}$. The plan of analysis presented in this article cover measurements of overvoltages appearing inside transformer windings while normalized voltage tests conducted with full wave lightning voltage impulse and chopped impulses after time to chopping in the range from $1 \mu \mathrm{s}$ up to $50 \mu \mathrm{s}$. Comparative analysis of internal overvoltages in tested transformer winding are presented on the basis of measured overvoltages waveforms at chosen points inside windings.

Piotr Pajak

Papers

Analysis of fast transient voltage distributions in transformer windings under different insulation conditions

Comparison of transformer winding responses to standard lightning impulses and operational overvoltages

Impact of oil and temperature on initial voltage distributions in transformer windings at ultra fast stresses

Frequency method for identification of resonance overvoltages in transformer windings

Investigations of transformer winding responses to standard full and chopped lightning impulses