Transformer Frequency Response Research Articles

Improved Winding Mechanical Fault Type Classification Methods Based on Polar Plots and Multiple Support Vector Machines

The accurate and fast diagnosis of transformer winding deformation faults is of significance to power suppliers and utilities. An improved winding mechanical deformation fault classification method is proposed. In this study, the transformer frequency response data is used to draw polar plots, and then its texture features are extracted for fault classification. The classification model constructed by multiple support vector machines is successfully obtained and shows good classification effect. Besides, this article uses an improved genetic algorithm based on the Emperor-Selective mating scheme and catastrophic operation, to optimize the parameters of support vector machine. The feasibility and accuracy of the proposed method are verified with experimental data obtained from a model transformer, and the proposed method is demonstrated to exhibit better performance compared with the traditional method.

Electromagnetic transients modelling of split‐winding traction transformers for frequency response analysis

High-frequency electromagnetic transients modelling of voltage and current distribution due to electromagnetic transients are of great importance in transformer design. In this study, a 2D and 3D finite-element, coupled field-circuit and modified detailed model is introduced for high-frequency electromagnetic transient studies in split-winding traction transformers. The proposed coupled field-circuit approach uses a 2D finite-element model that is coupled with an external circuit to analyse the electromagnetic transient behaviour of the split-winding traction transformer. Also, a detailed model is introduced for this type of transformer and its results are compared with the introduced coupled field-circuit model and complete 3D finite-element model. After validating the introduced models, the models are employed to analyse the frequency response of the split-winding traction transformer.

Analysis of the non‐linear behaviour of the magnetisation inductance during the frequency response test of a transformer

This study focuses on the analysis of the non-linear behaviour of magnetisation inductance during a transformer frequency response test in open-circuit configuration. For this work, frequency response measurements were made on a three-phase transformer and a transformer model was used to simulate the response curve and to estimate the magnetisation inductance curve as a function of frequency. It was observed that the magnetisation inductance increases with increase of the test voltage. Also observed was an initial decrease at a very low frequency of the magnetisation inductance, caused by the displacement of the work point by the first elbow of the magnetisation curve. Additionally, the magnetisation inductance acquires a linear behaviour when the input small signal is sufficiently low. Finally, it was observed that the remanent magnetisation level in the core reduces the value of the magnetisation inductance.

Transformer Frequency Response Analysis With the Grouped Indices Method in End-to-End and Capacitive Inter-Winding Measurement Configurations

The paper presents a method for assessing the frequency response results of transformers. The frequency response analysis (FRA) method is widely used in the industrial testing of the mechanical condition of transformers, however, there are still some problems with interpreting the test results. The measurements are usually taken only in an end-to-end open measurement configuration, according to the IEC standard. The literature indicates many numerical indices that are used for the assessment of FRA curves. It is not possible to use all of them, especially as they may give different results. The objective of the presented research was to analyze the behavior of 14 of the most popular indices in the medium-frequency range under the influence of deformations. The experiments allowed them to be divided into four groups, based on similar behavior under data recorded in conditions of deformations introduced into windings. All research is performed for two measurement configurations: end-to-end open and capacitive inter-winding. The division into groups is also tested with data recorded on transformers from industry. The proposed grouped indices method needs only four indices for FRA data assessment, while their behavior covers all 14 analyzed formulas.

High frequency electric circuit modeling for transformer frequency response analysis studies

Power transformers are subject to winding and core deformations due to short circuit faults and other environmental conditions. Majority of these faults are of progressive nature and should be rectified as soon as they emerge. Frequency response analysis (FRA) has been widely accepted as a reliable diagnostic tool to detect such faults. As reliable interpretation codes for FRA signatures have not been fully developed and accepted yet, researchers have put much effort to investigate the impact of various mechanical deformations on the transformer FRA signature. Because of the intrusive nature of such faults when staged on a real transformer, most of the studies in the literatures were conducted on a transformer high frequency electric circuit model. Simplifications assumed in these models by ignoring the turn-to-turn capacitance, not taking into account the detailed winding structure and not considering all mutual inductances between coils in various phases have reduced the accuracy of the obtained results. To establish reliable FRA interpretation codes, it is essential to develop electric equivalent circuit models that can yield FRA signature as close as possible to the FRA signature trend of the real transformer. This paper proposes a detailed transformer high frequency electric circuit model which considers the winding structure, inter-turn capacitance and all mutual inductances. Calculation details of the model’s parameters are presented. The accuracy of the proposed model is assessed by comparing its FRA signature with that of an equivalent transformer hardware model during healthy, short circuit disks, radial deformation and axial disks buckling fault.

Transient characteristics verification method for DC transformer used in flexible HVDC system

Previous studies have proposed higher requirements for the transient characteristics of a DC transformer used in a flexible high-voltage direct current (HVDC) system to achieve faster sampling speed and meet wider bandwidth requirements of the control and protection signal, and to eventually suppress the large transient fault current. In this study, a transient characteristics verification method is proposed for transient characteristics verification of a DC transformer used in a flexible HVDC system based on resampling technology and LabVIEW measurement technology after analyzing the key technology for transient characteristics verification of a DC transformer. A laboratory experiment for the transient characteristics of a full-fiber electronic DC transformer is conducted, and experimental results show that such verification method can be employed for frequency response and step response verification of a DC transformer at 10% of the rated voltage and current, and can eventually improve the screening of a DC transformer.

The Influence of Capacitance and Inductance Changes on Frequency Response of Transformer Windings

Frequency Response Analysis (FRA) is a test method used for assessment of mechanical condition of transformer active parts. Its biggest problem is the interpretation of test results, namely the relationship between scale of differences between compared curves and the decision for further operation of the given transformer. Very often visible differences between two FRA curves do not mean that there is a deformation in the winding. The cause of the curve shift may come from other elements of the transformer that influence inductive or capacitive parameters. This paper takes under consideration the influence of both capacitance and inductance changes on transformer frequency response (FR). The analysis is performed with the computer model of a transformer and also some experimental results are presented, showing the influence of capacitance and inductance changes on the FR of real transformers. The results of research showed that this influence may lead to misleading effects on the shape of FR characteristics. The paper presents an analysis that can be used in the assessment of FRA measurement, especially in the case of uncertain data comparison results.

Transformer frequency response: a new technique to analyze and distinguish the low-frequency band in the frequency response analysis spectrum

Smart monitoring applications in all aspects of power systems are becoming increasingly popular because of the indirect effect on energy management and the smart grid. In this regard, high voltage equipment monitoring and real-time assessment are critical. Among these evaluations, transformer life and condition assessment has been considered for many years [1], [2]. Currently, advanced methods such as transfer function measurement are employed to determine the transformer condition [3]. Transfer function measurement is generally used for black box system identification [4]. This measurement might be implemented in the time domain as an impulse response or in the frequency domain using a swept signal; the latter approach is commonly called frequency response analysis (FRA) [5].

Investigating the Capacitive Inter-Winding Response of Power Transformer

This paper presents an investigation on the occurrence of oscillation in the frequency response of transformer due to aging process. The frequency response was obtained using frequency response analysis (FRA) test and capacitive inter-winding test configuration. It has been reported that, there is abnormal occurrence to the frequency response after some period of operating time. The response shows a small oscillation at the mid-frequency region. Although, this oscillation was not there at first measured response. Therefore, three case studies are presented to investigate this occurrence, the measurement, the transfer function mathematical equation and the developed circuit. The case studies of the transfer function and the developed circuit are simulated using MATLAB and being compared with the actual measured response. The parameters that control the mid–frequency oscillation are also studied. The oscillation occurred at mid-frequency region is related with the actual condition of the transformer experiencing aging process. The new finding provides an understanding on the factors that affecting transformer aging. It is believed that, thermal, moisture and oil degradation are the main contributors of transformer aging process. Therefore, the significance of this study is to present the relationship between occurrence of mid-frequency oscillation and transformer aging condition.

Tuning CCVT Frequency Response Data for Improvement of Numerical Distance Protection

A new method to numerically estimate the primary voltage waveform of coupling capacitor voltage transformers is presented. The outcome is a simple digital filter suitable for the frequency range from 10 to 60 Hz, enabling us to recover the transformer primary voltage waveform from its secondary voltage waveform. Unlike the techniques reported in the literature, the method does not depend directly on the transformer topology nor on the operational characteristics of the protected system, once only the coupling capacitor voltage transformer frequency response is required as input. The evaluation of the technique is performed through digital simulations using the Alternative Transients Program and transmission-line data from the Brazilian National Power System Grid. Frequency- and time-domain analysis of two coupling capacitor voltage transformers with different topologies and voltage levels are presented. It is shown that the proposed method is effective for the dynamic correction of low-frequency disturbances that may improve the speed, reliability, and security of the numerical distance relay.

Air Core Transformer Winding Disk Deformation: A Precise Study on Mutual Inductance Variation and Its Influence on Frequency Response Spectrum

It is well-known that the deformation of transformer winding can produce detectable changes to the frequency response spectrum compared with a referenced past measurement. To interpret such changes for diagnostic purposes, main causes of the trace deviation need to be recognized precisely. In addition, it is useful that the interpretation of transformer frequency response is classified in a way that IoT-based techniques can be developed in the near future to analyze the transformer mechanical integrity. This paper has specifically concentrated on the inductance and capacitance variation due to the axial and radial disk deformation of transformer winding. Analytical analyses on self- and mutual-inductance variations are discussed and capacitance variation is studied in detail for symmetrical and asymmetrical transformer disk deformations. A numerical example is provided to establish the analytical approach and compare inductance and capacitance variation. The analytical approach is finally examined through the experimental study of disk deformation in a 66 kV transformer winding.

Performance evaluation of online transformer internal fault detection based on transient overvoltage signals

Winding deformation is a critical power transformer issue that needs to be detected as soon as it emerges due to its progressive nature and severe consequences it may lead to. Frequency response analysis (FRA) is the only reliable technique currently used to detect such deformation. However, the technique is conducted offline that may cause interruption to the electricity grid when taking an operating transformer out of service for testing. Moreover, as the current FRA technique is only conducted on suspected units, a mechanical fault may emerge and progress before the utility becomes aware of. To overcome these limitations, this paper presents a practical feasibility study to detect transformer winding deformations online. The technique relies on utilizing transient overvoltage signals that a transformer is subjected to during its normal operation as a natural variable frequency excitation source. The feasibility of utilizing various transient overvoltage signals as excitation source for transformer FRA signature measurement is firstly investigated and optimum signal parameters are recommended based on signal voltage and energy spectrum analyses. Secondly, the proposed technique is verified through simulation analysis by comparing the transformer frequency response signatures when transient overvoltage and conventional sweep frequency voltage are used as excitation sources. Also, experimental testing is conducted to assess the feasibility of the proposed technique to identify various winding deformations such as radial buckling, telescoping and short circuit turns. Custom-made capacitive voltage divider is developed to accurately detect the response of transient overvoltage signals generated due to environmental conditions or power system switching operations. The proposed technique along with the developed capacitive voltage divider is implemented on a 3-phase Wye-Delta, 50 Hz, 31.5 MVA, 110/10.5 kV power transformer to measure the frequency response signatures of the high voltage winding.

Using Absolute Average Difference (DABS) in Interpreting the Frequency Response of Distribution Transformer

Different numerical indices have been proposed in different works to analyze the Frequency Response Analysis measurements, such as Standard Deviation, Spectrum Deviation, Absolute Sum of Logarithmic Error, and Correlation Coefficient. A comparative evaluation of absolute average difference (DABS) index on the same footing appears to have not yet been reported. In this paper, DABS is amongst the five numerical indices used in interpreting FRA that have been evaluated for their respective suitability, reliability and sensitivity based on the same test cases involving winding deformation of real life power transformers. It was found that DABS and ASLE are the most reliable and sensitive index for various transformer faulty conditions. On the other hand, CC is the least reliable index as it could not indicate faulty winding on most presented cases.

Frequency response and harmonic distortion testing of inductive voltage transformer used for power quality measurements

International standards related to power quality measurements define methods and accuracies for the measuring instruments, but do not specify the accuracy of instrument transformers. Therefore, it is not possible to specify an overall accuracy for such measurements. Inductive voltage transformers (IVTs) which are used for power quality measurements should be tested to determine the ratio correction factors (RCFs) and phase angle errors at higher frequencies. In this paper, harmonic distortion and frequency response tests are performed to determine the level of harmonics generated by IVT and its ability to transform harmonics from high voltage (HV) to low voltage (LV) side. Complex voltages consisting of fundamental voltage and certain amount of superimposed harmonic are used to check the frequency response of IVT. RCFs and phase angle errors of the IVT are determined and can be applied to power quality monitors for compensation of errors that occur at high harmonic frequencies. Different test circuits are proposed for generation of HV consisting of fundamental voltage and harmonic voltages with amplitudes in range 5-15 % of the applied fundamental voltage. In order to improve the testing capabilities at higher voltage levels (for equipment with 123 kV≤Um≤420 kV), a compensation for both fundamental and harmonic voltages is proposed with a special connection through blocking and pass filters.

Evaluation of numerical indices for the assessment of transformer frequency response

This study is an effort to evaluate the numerical indices for the assessment of frequency response analysis (FRA) to provide a deeper understanding of their characteristics. First, the study introduces the indices in the literature and categorises them into three groups. Then, the results of 11 case studies from an experimental setup, two transformer models, and a power transformer real case are employed to examine the indices from the monotonicity aspect. Moreover, a short review on the interpretation experiences with indices is reported for further understanding. This contribution continues with a novel uncertainty analysis on the measurement setup and, finally, evaluates the indices based on their vulnerability to these uncertainties. Although the results show the advantage of some indices over the others in the FRA interpretation, further studies, especially with large transformers in the field, are needed to reach a general conclusion.

New statistical approach to interpret power transformer frequency response analysis: non‐parametric statistical methods

The frequency response analysis (FRA) test has been recognised as one of the sensitive tools for detecting electrical and mechanical faults inside power transformers. However, there is still no universally systematic interpretation technique for these tests. Many research efforts have employed different statistical criteria in order to aid the interpretative capability of the FRA, but it is shown that the methods used so far are based on parametric statistics, which need a set of assumptions about the normality, randomness and statistical independence of FRA data. Therefore, this study aims to propose some non-parametric statistical methods which are based on explicitly weaker assumptions than such classical parametric methods. The proposed statistical methods are applied to the experimental FRA measurements obtained from two test objects: a three phase distribution transformer (35/0.4 kV, 100 kVA) to study the winding interturn fault and a two winding transformer (1.2 MVA, 10 kV) for the study of radial deformation. The non-parametric statistical methods, namely Spearman correlation coefficient, Wilcoxon signed rank test and Friedman test are used to compare FRA traces. It was found through this research work that the applied non-parametric methods can effectively reflect the differences between compared FRA data and diagnose the fault.

Impact of capacitive coupling circuit on online impulse frequency response of a power transformer

Detecting the early signs of mechanical failures of power transformer winding is necessary and is possible with online monitoring techniques. Online impulse frequency response analysis (IFRA) is a promising diagnostic method when a transformer is in service. This paper examines the unrevealed problem existing in the method, namely, the impact of bushing capacitive coupling circuit on online impulse frequency response. An equivalent electrical model of capacitive coupling circuit and transformer winding is established. The frequency response of the capacitive coupling circuit is obtained to study its influence on online impulse frequency response. The parameter variations of capacitive coupling circuit caused by coupling capacitance variation and bushing dielectric breakdown are simulated to investigate their influence on online impulse frequency response signatures. A few experiments are eventually performed to verify the theoretical analysis and simulation results. This paper contributes to the application of online IFRA.

Impulse Generator Optimum Setup for Transient Testing of Transformers Using Frequency-Response Analysis and Genetic Algorithm

A novel optimization-enabled electromagnetic transient simulation approach is proposed for calculating the resistor setting of a multistage impulse generator for insulation impulse testing of high-voltage transformers. This approach enables test engineers to overcome most of the major challenges attributed to the use of conventional trial-and-error methods for determining resistor settings, including the excessive time consumption and the potential damage to the test transformer due to a greater number of trial tests. The proposed method uses the frequency response of the test transformer to synthesize a circuit model for it. The test setup, including the test transformer and the impulse generator, is simulated using an electromagnetic-transients-type simulator. A genetic-algorithm-based approach is used to optimize the setting of the impulse generator. Optimized resistor values are then used in impulse testing of a three-phase power transformer. Different cases of impulse generator resistor arrangements are studied in this paper and simulated waveforms are compared with those obtained from measurements.

ANALYSIS ON DEGRADATION AND DEFORMATION OF TRANSFORMER INSULATION SYSTEM

Performance of insulation system of power transformer is essential to ensure better performance of power network. Most of the power and distribution transformers in service are aged and, have experienced several thermal and electrical stresses due to varying loading conditions, in addition to ageing. The rate of degradation and hence, the useful life of the insulation system mainly depends on the variation of operational stresses. Due to the progress in insulation degradation, the dielectric strength of the insulating oil and paper over transformer winding may reach to a point such that it could not withstand any small abnormal currents and voltages. In order to maintain the quality of system operation it is necessary to keep the transformer in good condition. This needs appropriate maintenance based on reliable diagnostics. In this paper experimental results on the application of frequency response analysis (FRA) for the diagnosis of power transformers. Frequency Response of Transformer, also serves for assessing the deformation process.

English

Instrument transformers are the integral components of power system protection and metering. The objective of instrument transformers is to provide scaled down replicas of high voltages and currents of the power system. In case of high voltage transmission network (≥132KV), capacitive voltage transformers are widely used as an alternative to electromagnetic voltage transformers due to their techno economic advantages. They transform higher voltages to lower values which are easily measurable and can be directly used for metering, protection and control applications. During the system transient conditions, there is a deviation in the scaled down secondary voltage signal from the actual signal which comprises of wide range of frequencies, due to energy storing elements of capacitive voltage transformer. There are several practices in the industry to mitigate the deviation in the secondary voltage signal of capacitive voltage transformer. For satisfactory operation of capacitive voltage transformer, better frequency response is desirable. The simulation study to determine the effect of various factors influencing the frequency response of capacitive voltage transformer are presented in this paper.