Toshiyuki Saida

Development of high frequency circuit model for oil-immersed power transformers and its application for lightning surge analysis

The lightning impulse withstand voltage for an oil-immersed power transformer is determined by the value of the lightning surge overvoltage generated at the transformer terminal. This overvoltage value has been conventionally obtained through lightning surge analysis using the electromagnetic transients program (EMTP), where the transformer is often simulated by a single lumped capacitance. However, since high frequency surge overvoltages ranging from several kHz to several MHz are generated in an actual system, a transformer circuit model capable of simulating the range up to this high frequency must be developed for further accurate analysis. In this paper, a high frequency circuit model for an oil-immersed transformer was developed and its validity was verified through comparison with the measurement results on the model winding actually produced. Consequently, it emerged that a high frequency model with three serially connected LC parallel circuits could adequately simulate the impedance characteristics of the winding up to a high frequency range of several MHz. Following lightning surge analysis for a 500 kV substation using this high frequency model, the peak value of the waveform was evaluated as lower than that simulated by conventional lumped capacitance even though the front rising was steeper. This phenomenon can be explained by the charging process of the capacitance circuit inside the transformer. Furthermore, the waveform analyzed by each model was converted into an equivalent standard lightning impulse waveform and the respective peak values were compared. As a result, the peak value obtained by the lumped capacitance simulation was evaluated as relatively higher under the present analysis conditions.

An electric model of gas-insulated shunt reactor and analysis of re-ignition surge voltages

The authors proposed a simple electric model to simulate the windings used in gas-insulated shunt reactors up to a high frequency range. Its applicability was evaluated by comparison with measurements of the full-scale apparatus model. The impedance of the high frequency electric model is well simulated over a frequency range of several kHz to 10 MHz. In this paper, the proposed high frequency electric model was used for analysis of the re-ignition surges at interruptions of the reactor of a model substation. The analysis waveform gave a faster rise and lower peak value than those of the low frequency electric model, and the cause was clarified.

Application of controlled switching to 500-kV shunt reactor current interruption

To suppress reignition overvoltages caused when a 500-kV shunt reactor current is interrupted by a 550-kV one-break circuit breaker (CB), a study was carried out on controlled switching. Using a full-scale test circuit, reactor current interruption tests were carried out to obtain the relation between opening phase angle and generation of reignition. The results showed that even with the dispersion of CB operations taken into account, there were contact separation points free from high reignition overvoltages. It was also proved that no voltage escalations were caused by reignition and high-frequency arc extinction, and that overvoltages due to current chopping were at a safe level in terms of equipment insulation.

A high‐frequency model of an oil‐immersed transformer, and its use in lightning surge analysis

A circuit model of an oil-immersed transformer for use in surge analyses in the high-frequency region is proposed, and its effectiveness is evaluated by comparison with measurements on a model winding. The circuit model is used for lightning surge analyses of a 500-kV substation, and the effect of transformer modeling on the lightning surge voltage at the transformer terminals is investigated. When the new circuit model is used for transformer modeling, the peak value of the surge voltage is lower but the rate of voltage rise is higher than in conventional transformer modeling by a lumped capacitance. This difference can be explained in terms of the charging of capacitances in the transformer model.

Transformer Equivalent Model for Switching Transients Computation

AbstractWe measured transient recovery voltage (TRV) in a 1500 MVA-525 kV/276 kV/63 kV transformer of the three-phase-in-one-tank type. We proved that TRV can be measured by interrupting the current generated by discharging a capacitor of low voltage through the transformer. The current was interrupted at its zero point by a diode at a low voltage. To calculate TRV via a computation, we developed a transformer equivalent model composed of a L-C multi-mesh circuit. The TRV waveform computed using the transformer equivalent model we developed agreed well with the measured TRV waveform.

Steep fronts at transient recovery voltages appearing with the interruption of inrush currents of transformers

High lightning overvoltages do not appear in underground substations connected to transmission cables. Consequently, it is very important to thoroughly investigate switching overvoltages and to achieve rational insulation coordination for apparatus installed in such underground substations. This paper discusses the occurrence of steep fronts at transient recovery voltages (TRV) appearing at circuit breakers when the inrush currents of transformers are interrupted. Caused by a steep front at the TRV, reignitions occur at circuit breakers, resulting in the generation of high overvoltages with high frequencies. The overvoltages are among the highest switching overvoltages appearing at the terminal of a transformer. The authors clarified the mechanism of the generation of steep fronts at TRV by means of EMTP analysis, as well as by carrying out tests in a high-power laboratory.

Study on decomposition gas for diagnostics of gas‐insulated transformers

Concentrations of population and business activities result in high energy demand in urban areas. This requires the construction of underground substations. Oil-free, nonflammable, nonexplosive equipment is recommended for underground substations. For this reason, gas-insulated transformers have been developed. A diagnostic method for gas-insulated transformers is thus required. This paper provides an experimental survey of the main components of decomposition gas generated by various faults in gas-insulated transformers carried out through simplified model tests. The phenomena of overheating and partial discharges are modeled, taking the actual materials related to each fault into account. For example, CO, CO2, and aldehydes are produced by overheating of pressboards and PET films. The amount of gas produced increases with rising temperature. While various gases are produced from a partial discharge, the principal components are SO2 and SOF2. These results will be used to develop a diagnostic method for gas-insulated transformers.

Measurement and computation of transient recovery voltage of transformer limited fault in 525kV–1500MVA three-phase transformer

Transient recovery voltage (TRV) in a 525kV-1500 MVA transformer of the three-phase-in-one-tank type was measured. It was proved that TRV can be measured by interrupting the current generated by discharging a capacitor of low voltage through the transformer. The current was interrupted at its zero point by a semiconductor diode at a low voltage. To compute the TRV, a transformer equivalent model composed of an L-C multi-mesh circuit was developed. The TRV waveform computed using the transformer equivalent model agreed well with the measured TRV waveform. The difference among the TRV of the first, second and third pole interruption was explained by the computation with the circuits of symmetrical component method. The transformer inductance that determines the TRV did not have frequency dependency up to the rage of about 20 kHz of the TRV frequency.