Peerawut Yutthagowith

This paper improves a previously developed approach for accurately modeling the dielectric equivalent circuit of high-voltage equipment. While preserving its simplicity, the enhanced method improves precision, enabling the accurate fitting of depolarization current, which is then used to determine the dielectric loss factor with higher accuracy. To assess its effectiveness, extensive test cases were conducted, and the results were systematically compared with those obtained using both the previously proposed approach and commercial software solutions. The comparative analysis confirms that the enhanced method delivers high accuracy, achieving a similar relative root mean square error while yielding a lower root mean square relative error for the depolarization current. These results emphasize the effectiveness and practicality of the proposed technique in modeling the equivalent circuit for dielectric analysis. By offering a reliable and robust approach, this method enhances the evaluation of insulation performance in high-voltage applications.

This paper presents an extended approach based on a recently developed method, designed to enhance the accuracy of dielectric loss factor (loss tangent) estimation in damped alternating current voltage (DACV) tests. The estimation process leverages a precise equivalent circuit of the DACV testing system, along with an accurately determined attenuation factor and angular frequency of the measured test voltage, to improve the reliability of loss tangent calculations. The extended approach introduces a new expression for loss tangent, derived from peak-to-peak testing voltages, and incorporates a moving average filter to mitigate the effects of DC offset, amplitude and time quantization errors, and noise. A comparative analysis of simulation and experimental results demonstrates that the conventional method lacks precision in estimating the loss tangent, especially when measuring values as low as 0.1%—a typical range for both new and aged underground power cables. However, the proposed extended method provides reliable loss tangent estimation within this range, ensuring acceptable accuracy and improved measurement consistency.

A metal oxide varistor (MOV), manufactured from a blend of zinc oxide and other metal oxides, undergoes changes in resistance or resistivity depending on changes in applied voltage or electric field strength. Typically, the nonlinear resistive characteristics of MOV elements are determined through experimental measurements of voltage and current. This article introduces a mathematical expression comprising two power functions and a constant term with five adjustable coefficients. It is utilized to describe the voltage-current or electric field-current density characteristics across a wide range of current or current density, spanning from microamperes to several tens of kiloamperes or from several hundred A/m² to a few hundred of kA/m², respectively. This expression accurately reproduces the observed nonlinear resistive behaviors of several low-voltage MOV elements. A noniterative fitting method is developed to determine the expression's five coefficients, using integration to linearize nonlinear characteristics without requiring iterations or uniform data sampling. Through comparison with a previously proposed expression and experimental data, the proposed technique demonstrates remarkably high accuracy. Furthermore, the proposed technique is applied to accurately estimate the parasitic inductance of an MOV and its lead wires during a residual voltage test conducted with an 8/20-microsecond impulse current.

Accurate simulations of transient phenomena in electric power systems with metal oxide varistors (MOVs) or lightning arresters (LAs) using the finite-difference time-domain (FDTD) method for solving Maxwell's equations require simple and accurate representations of MOVs or LAs. By representing a small cell within a MOV or LA with resistivity (ρ) dependent on electric field (E), these components can be modeled in three dimensions and seamlessly integrated into FDTD simulations. Achieving computational efficiency in FDTD simulation necessitates avoiding iterative computations for ρ from E. Hence, there is a significant need for a simple and accurate mathematical expression of ρ in terms of E. This study presents a methodology for deriving a three-coefficient exponential function from experimental data. By using integration properties, this method transforms nonlinear characteristics into linear ones without iterative processes or uniform data sampling. It also incorporates data weighting and outlier discrimination for enhanced accuracy. Comparative analysis with previous methods based on the ordinary least squares method and experimental data, using an applied current with a rise time of approximately 8 μs, confirms high accuracy and effectiveness in computing residual voltages resulting from impulse current injection using the FDTD method.

A double-real-exponential function is extensively utilized in high-power electromagnetics, encompassing research on high-altitude electromagnetic and ultrawide-band pulses, lightning, and high-voltage tests. There is a substantial demand for converting pulse waveform parameters such as rise time, full width at half maximum, fall time, front time, time to peak, time to half peak, and peak value into the first- and second-time constants and a peak factor of a mathematical function. The article introduces an accurate method named the time normalization technique for mathematically generating impulse waveforms with a set of waveform parameters, including: the rise time, the full width at half maximum, and the peak value and the front time, the time to half peak, and the peak value. The validation of the proposed method reveals that estimation errors below 10 ⁻⁶ % can be achieved. Using the results obtained from this method, expressions for mathematical parameter estimation in a wide range of time parameters are developed, which yields estimation errors below 0.02% for converting the rise time, the full width at half maximum, and the peak value to the first- and second-time constants and the peak factor, and below 0.01% for converting the front time, the time to half peak, and the peak value to the first- and second-time constants and the peak factor. Additionally, the utilization of these expressions for waveform generation to evaluate the measurement uncertainty of a voltage measuring system is demonstrated.

Power transformers are essential apparatuses used to transfer electrical energy from one voltage-level circuit to another. For reliable systems, preventive maintenance of the transformers is required to ensure good services of all mechanical, electrical, and insulation parts. Oil-immersed paper is most often used for transformer insulation. To ensure such good insulation performance and for assessing insulation conditions, advanced transformer sensing, monitoring, and effective assessment techniques are required. This paper introduces an effective technique for assessing the insulation conditions in power transformers, which are crucial for ensuring reliable energy transfer. The method utilizes advanced transformer sensing and monitoring, focusing on oil-immersed paper insulation commonly used in transformers. The technique employs dielectric response sensing, obtained from frequency-domain spectroscopy tests, to estimate degrees of polymerization (DP) and percentages of moisture content (PMCs) in the oil-immersed paper insulation. These parameters are well-known indicators of insulation performance. The approach is based on the weighted k-nearest neighbor regression, using a database of dielectric loss factors at low frequency and oil conductivities. To overcome limited data availability, linear interpolation and extrapolation techniques are applied to enlarge the database. Experimental verification and comparison with a previously developed method demonstrate the proposed technique’s superiority in accuracy and complexity. The maximum deviations of DP and PMC in the validation cases are 6.2% and 18.7%, respectively. In addition, to evaluate the validity of our proposed method in the case of a real power transformer, a comparative analysis of the DP and PMC values determined by the proposed method with those obtained through a previously developed and complicated approach was performed. The predicted results indicate that the DP and PMC values of the oil-immersed insulation fall within the ranges of 800 to 1000 and 1.5 to 2.0, respectively, which agree with the results determined by the complicated approach and closely align with real conditions. By offering a reliable and advanced means of assessing insulation conditions, this technique contributes to the preventive maintenance and overall efficiency of power transformers.

This work presents a novel numerical inverse Fourier transform for electromagnetic transient analysis emphasizing the lightning phenomena. The inverse Fourier integral is separated into nonuniform frequency intervals, and exponential interpolation is employed for each interval before applying analytical integration to acquire a continuous transient function in the time domain. In addition, a nonuniform frequency sampling approach was proposed to reduce the number of frequency samples. The performance of the proposed technique was verified using some example cases and compared with the previously developed methods, i.e., conventional inversed fast Fourier and linearizing Fourier integral techniques. In terms of computational efficiency and accuracy, the proposed technique is an alternative and effective tool for frequency-domain-based transient analyses.

In high-voltage (HV) transmission and distribution systems, HV apparatuses are subjected to electrical, thermal, and mechanical stresses that deteriorate the insulation performance. The polarization and depolarization current (PDC) measurement is an effective tool used for evaluating insulation performances. The depolarization current represented by the summation of the discharge currents with the different time constants can be utilized for the development of the dielectric model based on the extended Debye’s model (EDM). This paper presents effective techniques for determining the dielectric model. Iterative approaches with predetermination of the time constants and least squares methods (either linear ordinary or percentage ones) were utilized to fit the depolarization current in the form of multi-exponential functions. The fitting parameters determined by the proposed method with the linear ordinary least squares (OLLS) method and provided by commercial software agree very well only in the high current and beginning range. Application of the linear percentage least squares (PLLS) method shows better accuracy than that of the OLLS method, and the deviation from the measured one in the low current range and the late measuring time were reduced significantly. The fitted current by this proposed technique with the PLLS method agrees well with the measured current throughout the whole recording time, even in the low current and late time range. From the accurately fitted currents, the dielectric model and the dielectric loss factors can be determined precisely, and the insulation condition of HV equipment can be evaluated properly.

For high-voltage (HV) and partial discharge (PD) tests on high-voltage equipment, a resonant power frequency converter has recently been developed. A single-phase power frequency converter with a resonant tuning and filter circuit and an HV testing transformer comprise the developed system. A difficulty in the tuning and filter circuit design is the unknown testing system circuit parameters, including unknown parasitic inductance, capacitance, and internal resistance. In this paper, a system with a voltage rating of 75 kV_rms, apparent power of 40 kVA, and an operating frequency from 50 Hz to 200 Hz is considered for determination of the equivalent circuit parameters. From the determined circuit parameters, the appropriate resonant tuning and filter circuit was designed effectively. The transfer functions of the input and output testing voltages, along with the transfer impedance of the input voltage and signal voltage of the PD measuring port, were analyzed. The system design was verified by experiments with a voltage transformer. The gain of the transfer impedance was about 15 and 4 at the testing frequencies of 50 Hz and 200 Hz, respectively. With the proper design, it is possible to generate an output voltage waveform that is almost entirely sinusoidal and has a background noise level of under 1 pC. According to the experimental results, the system design of the resonant converter and the method for determining the equivalent circuit are very helpful for the HV and PD tests of voltage transformers in actual practice.