Haruhisa Wada

Partial discharge-induced degradation characteristics of insulating structure constituting oil-immersed power transformers

The soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, only insufficient basic data backing this criterion are available. In addition, the harmful level of the insulation performance has not necessarily been perceived in such a manner as the present one, for example, only the insulation breakdown, instead of the insulation performance degradation, has been assumed to be harmful. In light of the consideration presently paid, it is necessary to determine the PD level that causes the insulation performance to be degraded in order to confirm the soundness by the test using an actual transformer. As the initial step of the study for this purpose, the previous paper investigated the PD and the insulation damage level using oil-impregnated press-boards (PBs) used in the transformer, at which PD is more likely to occur when ac voltage is applied than at any other parts, and evaluated the harmful PD level for insulating materials themselves. To investigate the harmful PD level for the actual transformer, this paper studied the harmful PD level that decreases the residual lightning impulse (LI) withstand voltage using the main insulation model, which is the insulating structure constituting the oil-immersed power transformer. As a result, black discoloration occurred on the surface of the PB and the residual LI withstand voltage also tended to decline when exposed to ac PD of 20,000 pC or more. On the other hand, when it had been exposed to ac PD of 10,000 pC, no visible damage was observed and there was no drop in the residual LI withstand voltage either. Consequently, the harmful ac PD level for the oil-immersed transformer can be determined at between 10,000 pC and 20,000 pC. This result coincided with that for insulating materials themselves, hence it was revealed that the harmful PD level be set to 10,000 pC on a conservative side.

Partial discharge-induced degradation characteristics of oil-impregnated insulating material used in oil-immersed power transformers

The soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, only insufficient basic data backing this criterion are available. In addition, the harmful level of the insulation performance has not necessarily been perceived in such a manner as the present one, for example, only the insulation breakdown, instead of the insulation performance degradation, has been assumed to be harmful. This paper was intended to determine the harmful PD level causing the insulation performance of the transformer to be degraded, and investigated the degradation level of an oil-impregnated press-board (PB) used between windings, at which PD is more likely to occur when ac voltage is applied than at any other oil-impregnated insulating paper material used in transformer. The degradation level was quantitatively evaluated through comparison of the lightning impulse (LI) breakdown voltage between the unused PB and what had been exposed to PD. As a result, if the material was exposed to PD of 20,000 pC or more when the voltage was applied for an hour, the residual LI withstand voltage for the insulating material itself tended to decrease along with the magnitude and application time of PD. On the other hand, in the case of the PD charge of 10,000 pC or less, the residual LI withstand voltage barely decreased even if the material was exposed to PD for more than 1 hour. Consequently, the conclusion was reached that the harmful PD level as the insulating material itself used in the oil-immersed power transformer is approximately 15,000 pC. It was also confirmed that PD exceeding this permissive level for several minutes caused the insulation performance to be degraded.

Partial discharge signal propagation characteristics inside the winding of oil-immersed power transformer using the equivalent circuit of winding model in the oil

The technical soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear control criteria for this PD test, it is necessary to study PD propagation characteristics inside the transformer windings. In this paper, an analysis model was constructed using constants in the oil for a 500 kV-class transformer to analytically obtain PD signal propagation characteristics. For this analysis model, a circuit model for high frequencies up to several MHz was used in order to obtain the characteristics of highfrequency PD propagating inside the winding in detail. Using this model, PD propagation characteristics were investigated in terms of the PD occurrence position, measurement position, and front time as parameters. As a result, it emerged that the propagation pattern was determined by the winding constants as viewed from the PD injection position and the terminal conditions. When the charge amount was the same, the steeper the PD signal front time, the higher the peak voltage value was likely to be because the waveform became more oscillatory. Conversely, assuming an actual test using the ERA device (PD detector developed by Electrical Research Association), it was found that the front time had little impact on the charge amount measured because the oscillatory high-frequency components were removed. As with the previous studies, it emerged that the PD signal transmission rate was lowest when a PD occurred between sections and the signal was detected as one significantly damped to a level of several percent depending on the conditions.

Partial discharge signal propagation characteristics inside the winding of gas-filled power transformer - study using the equivalent circuit of the winding model

The soundness of a gas-filled power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear control criteria for this PD test, the authors initially conducted a study on the harmful PD level of materials themselves used in a gas-filled transformer. In the actual PD test, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors. Since this damping must be taken into consideration to establish control criteria, a preceding study investigated the propagation characteristics of PD signals through actual measurement using transformer winding models. As a result, it was found that the PD having occurred inside the winding is detected as the signal significantly damped depending on the position of occurrence. This paper studies whether PD signal propagation characteristics can be analytically evaluated using the equivalent circuit of the transformer winding. This analytical evaluation, if feasible, can be applied to various winding structures and will make it possible to evaluate the propagation characteristics in further detail as well as the validity of the actual measurement results. The study was conducted on the interleaved and continuous disk windings to make a comparison between the analysis and measurement results of the voltage waveform and the propagation pattern at various locations inside the winding with the position of occurrence and measurement position of the PD as parameters. Consequently, it emerged that the analysis using the equivalent circuit could adequately simulate the propagation pattern and waveform shape of the PD signal. A detailed study using this equivalent circuit also clarified the fact that the PD signal propagation waveform was determined by the winding constant and the terminal conditions. Furthermore, the analytical and measurement results of the transmission rate were almost identical, whereby it was analytically confirmed that the transmission rate significantly decreased depending on the position of occurrence of the PD. It also emerged that, since the frequency characteristics of the PD measurement device may influence the transmission rate, the PD control criteria must be studied taking such influence into consideration.

Deterioration characteristics due to partial discharges in insulating structure constituting gas-filled power transformers

The soundness of a transformer under operating voltage is evaluated in a partial discharge (PD) test of a long-duration ac withstand voltage test. Presently, the same criteria for evaluating oil-filled transformers are adopted as those for evaluating gas-filled transformers in this PD test. However, gas-filled transformers are made of different insulating materials than those for oil-filled transformers, and may therefore have different strengths against PDs to oil-filled transformers. With this in mind, it is necessary to establish PD evaluation criteria for gas-filled transformers. A preceding paper discussed the raw materials of solid insulating ones used in gas-filled transformers in terms of the generated PD magnitudes and the degrees of insulation damage and evaluated the harmful PD levels of the insulating materials themselves. In this paper, the harmful PD levels of actual equipment were studied using the turn-to-turn and section-to-section insulation models, insulating structures that constitute gasfilled transformers, in order to examine harmful PD levels that lowered the residual impulse withstand voltages. Consequently, it was confirmed that, on the turn-to-turn insulation model at normal test voltage levels, the generated PDs were too small to damage the insulator, even with a particle present. To damage the insulator, a voltage significantly exceeding the test voltage had to be applied. Therefore there is little necessity to consider the harmful PD level on the turn-to-turn insulation. On the other hand, on the section-to-section insulation model, PDs were found to be generated if a particle completely bridged separate sections. As a result, the degradation statuses related to the magnitude and duration of PDs were identified and the harmful PD levels were estimated at 10000 pC according to the residual impulse withstand voltages.

Partial discharge-induced degradation characteristics of insulating materials of gas-filled power transformers

The soundness of a transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. At present, the same criteria for evaluating oil-filled transformers are adopted as those for evaluating gas-filled transformers in this PD test. However, gas-filled transformers are made of different insulating materials than those for oil-filled transformers, and therefore they may have different strengths against PDs from oil-filled transformers. Therefore, it is necessary to establish PD evaluation criteria for gas-filled transformers. This paper, as the first step toward this objective, discusses the study of solid insulating materials used in gas-filled transformers in terms of the generated PD magnitudes and the degrees of damage of insulation and examines the harmful PD levels that lower the residual impulse withstand voltage. As a result, it identified the PD-induced degradation patterns of some insulating materials used in gas-filled transformers, i.e., polyethylene terephthalate (PET) films and Nomex® boards and evaluated the harmful PD levels based on the residual impulse withstand voltages. It was also verified that the insulations of pressboards are not degraded even by relatively large PDs.

Partial discharge signal propagation characteristics inside the winding of gas-filled power transformer - experimental study using winding models in the air

To establish clear control criteria for a partial discharge (PD) test of long duration ac withstand voltage test of a gas-filled power transformer, the authors have conducted a study on the harmful PD level of materials themselves using the insulating material- and structural-models of an actual gas-filled power transformer. In the PD test on an actual transformer, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors, such as the position of occurrence, measurement position, and internal structure of the equipment. In particular, in the event of the PD between sections or turns, which might actually occur, significant damping of the signal is anticipated in certain cases due to the influence of the position of occurrence and the internal structure of the equipment, which is a propagation path. Therefore, this damping must be taken into consideration to establish control criteria for the PD. In this paper, to understand the characteristics of this damping, such as its magnitude, two types of winding, interleaved disk and continuous disk, were produced as transformer models to investigate the propagation characteristics of PD signals with the position of occurrence, measurement position, and wavefront duration of the PD as parameters. Consequently, it emerged that, where the PD occurs between sections or turns, the transmission rate at the measurement position was generally low because the PD signal circulates within the PD occurrence parts and does not come outside. Assuming measurement at the bushing test terminal, the lowest transmission rate was 2.8% and 2.0% between sections and turns, respectively. A study was also conducted on the occurrence of the PD against ground, despite the low anticipated potential of actual occurrence, and the lowest transmission rate was 4.0%. Accordingly, the PD having occurred inside the winding of the transformer can be detected with a magnitude of 2% or more; however, from a reverse perspective, the signal is detected as one significantly damped to a level of about 1/50.

Partial discharge signal propagation characteristics inside the winding of oil-immersed power transformer - using the three-winding transformer model in air

The soundness of a power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear control criteria for this PD test, it is necessary to study PD propagation characteristics inside the transformer windings. In this paper, a study was conducted on PD signal propagation characteristics for an oil-immersed transformer. This involved a winding model of a three- winding transformer, or the structure of an actual transformer being produced to investigate the PD signal propagation characteristics relative to the occurrence position, measurement position, and front time of the PD as parameters. Subsequently, since a preceding study on a gas-filled transformer confirmed that an equivalent circuit of the winding could be used for analytical evaluation, an analysis model was also constructed for a three-winding model of the oil-immersed transformer in the present study to investigate the propagation characteristics. Consequently, it emerged that an analysis using an equivalent circuit could also effectively simulate the propagation characteristics, waveform shape, and transmission rate for an oil- immersed transformer. As for the transmission rate, it was found that the signal was detected as significantly damped to a level of several percent depending on the occurrence and measurement positions of the PD, as was also the case for a gas-filled transformer.