pd criterion in transformers

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 19, No. 4; August 2012 1431

1070-9878/12/$25.00 © 2012 IEEE

Partial Discharge Criterion in AC Test of Oil-immersed Transformer and Gas-filled Transformer in Terms

of Harmful Partial Discharge Level and Signal Transmission Rate

Shigemitsu Okabe and Genyo Ueta

Tokyo Electric Power Company 4-1, Egasaki-cho, Tsurumi-ku, Yokohama, Kanagawa, 230-8510, Japan

ABSTRACT

The soundness of a power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration induced ac voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, few basic data backing this criterion are available. To establish a clear criterion for this PD test, the authors initially conducted a study on the harmful PD level of materials themselves constituting the insulation of an oil-immersed transformer and a gas-filled transformer. This PD level was evaluated based on the rate of decline in the lightning impulse breakdown voltage using the insulating materials themselves and the element models simulating the insulating structure which had been exposed to a PD. Consequently, it emerged that insulations of both types of transformers were degraded if exposed to a PD of 7,000 pC to 10,000 pC. With the safety factor for this PD value taken into account considering the long-term operation and the structural difference of an actual transformer, 5,000 pC was deemed as the harmful PD level at the PD occurrence position. Subsequently, using a winding model of a transformer, PD signal propagation characteristics inside the winding were investigated through actual measurement and analysis. As a result, it was found that the PD having occurred inside the winding is measured as the signal significantly damped depending on the position of occurrence. The transmission rate was 2.2% for an oil-immersed transformer and 2.8% for a gas-filled transformer in the respective lowest cases. What should be controlled in the PD test of an actual transformer is the value of the harmful PD level at the PD occurrence position multiplied by the transmission rate at the PD detection position. Therefore, the conclusion was reached that the acceptance criterion in the test should be set to 5,000 pC 2.2% = 110 pC or less for an oil-immersed transformer and 5,000 pC 2.8% = 140 pC or less for a gas-filled transformer, respectively.

Index Terms — Oil-immersed power transformer, gas-filled power transformer, ac test, partial discharge, insulation degradation, signal transmission rate, harmful partial discharge level, acceptance criterion.

1 INTRODUCTION

THE insulation performance of a power transformer is verified by a lightning impulse withstand voltage test and a long-duration induced ac voltage test [1, 2]. The lightning impulse withstand voltage test mainly verifies the surge overvoltage whereas the ac test verifies the normal operating voltage and temporary overvoltage during operation. Of these, in the ac test, the partial discharge (hereafter, PD) magnitude is generally measured [3]. The acceptance criterion for this PD test is 500 pC according to the IEC standard [1, 2] and 100 pC according to JEC standard [4, 5], differing from each other with

the grounds for the criterion not necessarily clarified. This criterion value should be determined based on the “PD magnitude at the PD occurrence position that damages the insulating materials” and the “signal transmission rate to the measurement position in the test” in principle; however, few data backing this criterion are available. To establish the PD acceptance criterion based on clear grounds, the PD level that degrades the insulation itself and the PD signal propagation characteristics when measurement is conducted must be clarified.

This series of research ultimately aims to study the harmful PD level for an oil-immersed power transformer and a gas-filled power transformer in the test. As the first step, a study was Manuscript received on 4 October 2011, in final form 14 May 2012.

1432 S. Okabe and G. Ueta: Partial Discharge Criterion in AC Test of Oil-immersedTransformer and Gas-filled Transformer

conducted on the PD magnitude at the PD occurrence position that damages the insulating materials for an oil-immersed transformer and a gas-filled transformer, respectively [6-9]. In the study, using an insulating material model and an insulating structure model of a part where PD might potentially occur, the relationship of the magnitude and duration of the PD with the degradation of the insulating materials was quantitatively evaluated based on the rate of decline in the lightning impulse breakdown voltage. As a result, it emerged that insulations of the oil-immersed transformer and the gas-filled transformer were degraded if a PD of about 7,000 pC to 10,000 pC had occurred.

As the second step, a study was conducted on the PD signal propagation characteristics inside a transformer through the experiment by producing the winding models for an oil-immersed transformer and a gas-filled transformer as well as EMTP (Electro-Magnetic Transients Program) analysis using their equivalent circuits [10-13]. In these studies, the transmission rate to the detection position, such as the test terminal and coupling capacitor on the high voltage side and the neutral point terminal, was investigated with the PD occurrence position, such as the position between sections or against ground, as a parameter. As a result, 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.

In this paper, the PD acceptance criterion in the test is studied based on the harmful PD level at the PD occurrence position and the transmission rate from the PD occurrence position to the detection position. Since details such as the experimental conditions were described in the previous reports [6-13], the present paper focuses on the typical results and study based on the same.

2 HARMFUL PARTIAL DISCHARGE LEVEL

AT THE POSITION OF OCCURRENCE OF

THE PARTIAL DISCHARGE This section reviews the main points of references previously

stated [6-9] concerning the PD level that degrades the insulating materials themselves used in an oil-immersed transformer and a gas-filled transformer. This PD level is evaluated based on the rate of decline in the lightning impulse breakdown voltage using the insulating materials themselves and the element models simulating the insulating structure which have been exposed to a PD.

2.1 OIL-IMMERSED POWER TRANSFORMER

The insulation elements of a power transformer can be roughly classified into, for example, the main insulation between windings, the insulation between turns and sections inside the winding, and the insulation against ground, such as that between the winding and the tank or core. Of these, a dominant insulation part is the main insulation [14, 15] in order to study the degradation caused by a PD in the ac withstand voltage test of an oil-immersed transformer. This main insulation comprises the press-board (hereafter, PB) and the oil gap. Conversely, for insulation between turns among that inside windings, since the electric field applied to the insulating materials is low under normal ac operation voltage,

the probability of occurrence of the PD that degrades the insulating materials is considered low. For insulation between sections, even though the electric field exceeds that between turns and a PD might possibly occur in the case of, for example, a manufacturing defect, the probability is lower as compared with the main insulation. For insulation against ground, PD is unlikely to occur because the electric field is much lower than that inside windings.

Consequently, for an oil-immersed transformer, a study was conducted focusing on the harmful PD level of the main insulation. The study used the oil-impregnated PB itself as the insulating material model [6] and the model simulating the structure of the main insulation as the insulating structure model [7].

2.1.1 INSULATING MATERIAL MODEL

Five 0.8 mm thick PBs were placed on a 2 mm thick PB to adjust the thickness of the insulating material model to 6 mm, equivalent to the PB of an actual transformer in thickness. This 6 mm thick sample was then immersed in oil and degraded in the PD exposure test with the PD magnitude and the voltage application duration as parameters [6].

Figure 1 illustrates the PD test setup. In the test, applied voltages were adjusted to set the initial PD charges to specified value and then the voltages were maintained for specified duration of time. Since the PD magnitude was on a downward trend as the time elapsed in this constant-voltage PD test, the initial PD charge can be regarded as the maximum PD charge.

As an example of the damage pattern of the test PB, Figure 2 shows the result of voltage application for 4 hours with a PD of 20,000 pC [6]. When the test PB had been exposed to PD exceeding 20,000 pC, significant black discoloration as shown in the figure was observed on the surface of the test PB and its range expanded over time. This black discoloration is caused by the carbonization of the PB fiber, which is electrically conductive and has no dielectric strength. As for the internal condition, following observation using transmitted light, a tree-like pattern was confirmed. This was considered to be gas generated by the PD, which remained inside the PB. Figure 3 shows the relationship of the tree-like pattern size (maximum diameter) with the PD magnitude and the voltage application duration [6]. When the size of the tree-like pattern is compared with respect to the PD magnitude, it emerges that there is little

Figure 1. Partial discharge degradation test setup for insulating material model.

=2 mm

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difference in the case of 15,000 pC or less whereas the size becomes larger in line with the PD magnitude in the case of 20,000 pC or more. When the size is compared with respect to the voltage application duration, the tree-like pattern becomes larger with the increase in voltage application duration, and such characteristics were more pronounced in the case of 20,000 pC or more.

In the lightning impulse breakdown test, a surface layer was detached from the sample degraded by the PD to obtain the breakdown voltage. Figure 4 shows the result of the lightning

impulse breakdown test [6]. In each test condition, 5 samples were tested and the average values were plotted in the figure. The vertical axis indicates the relative value to the breakdown voltage of a new sample not exposed to a PD as 100% (the same description method also applies to Figures 6, 7, and 9). For the insulating material model, when the threshold value for the harmful PD level was set to 90%, in the case of a PD of 10,000 pC or less, the breakdown voltage decreased little, even if it was exposed to a PD for 4 hours. Conversely, it emerged that, in the case of a PD of 20,000 pC or more, the breakdown voltage declined significantly if the voltage had been applied for about 10 minutes.

2.1.2 INSULATING STRUCTURE MODEL

For the insulating structure model, as illustrated in Figure 5a, PB plain plates were placed opposite to each other via a PB insulating spacer to simulate the main insulation structure. Since no PD occurs in the normal model, even if voltage equivalent to the test voltage for the actual transformer is applied, the infiltration of metallic particles in the PB insulating spacer as a manufacturing defect was simulated to generate PD as shown in Figure 5b [7].

In the PD test, similarly to the insulating material model, a constant-voltage PD test was conducted. A visual check after the test revealed hardly any visible change in the case of a PD of 10,000 pC. Similarly to the insulating material model, black discoloration was observed in several samples in the case of a PD of 20,000 pC or more and the puncture breakdown occurred in some cases while the voltage was applied.

Similarly to the material model, the lightning impulse breakdown test results were evaluated by the relative value to the breakdown voltage of a new sample. Figure 6 shows the

Figure 2. Example of damage pattern of press-board after ac voltageapplication for 4 hours with maximum partial discharge of 20,000 pC.

Length between the holes is 120 mm

(a) PB surface by external light (b) PB inside by transmitted light

Figure 3. Relationship of the voltage application duration and the partialdischarge with the size of the tree-like pattern.

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Figure 4. Residual impulse withstand voltage of press-board material modelafter ac partial discharge test.

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Figure 5. Partial discharge degradation test setup for insulating structure model.


results of the lightning impulse breakdown test [7] together with those for the material model in Section 2.1.1. The number of samples is 5 to 14 in each test condition. Since the breakdown occurred in some samples while the voltage was applied in the case of a PD exceeding 20,000 pC, an evaluation was made based on the values estimated according to the Weibull distribution. For the insulating structure model, in the case of a PD of 10,000 pC, the breakdown voltage was almost equivalent to that of a new sample. A PD of 20,000 pC was considered to be at a level that influenced the insulation performance because the puncture breakdown occurred in some cases during the PD test even though the breakdown voltage did not decrease significantly. When a PD was 50,000 pC, the breakdown voltage was obviously decreased.

2.1.3 HARMFUL PARTIAL DISCHARGE LEVEL OF OIL-IMMERSED POWER TRANSFORMER

Based on the above test results, a study was conducted on the harmful PD level at which insulating materials themselves of an oil-immersed transformer were degraded. In the case of a PD of 10,000 pC, no apparent change was observed in either the insulating material model or the insulating structure model, nor did the lightning impulse breakdown voltage decrease. When a PD exceeded 20,000 pC, black discoloration appeared due to the carbonization of the PB, and the breakdown occurred in some cases during the PD test for the insulating structure model. The lightning impulse breakdown voltage also tended to be obviously decreased.

Based on the above, the harmful PD level of an oil-immersed transformer is deemed to be between 10,000 pC and 20,000 pC, where the insulation performance of the PB is degraded, and 10,000 pC on a conservative side.

2.2 GAS-FILLED POWER TRANSFORMER

For an oil-immersed power transformer, it was deemed that the degradation due to a PD occurs at the main insulation, and the mixing-in of particles was simulated in the case of the insulating structure model because no PD occurred under a clean condition. On the other hand, in the case of the main insulation of a gas-filled power transformer, particles, if any, fall down due to its structure, meaning a PD is unlikely to

occur at the main insulation. In addition, in the event of occurrence of a PD at the main insulation, breakdown is most likely to result, hence the main insulation is not considered a dominant part in the verification of insulation by the PD test. Therefore, it is the PD in the insulation inside windings caused by, for example, particles that should be verified in the test of a gas-filled transformer.

To this end, for a gas-filled transformer, the materials used for the insulation inside windings, i.e. the insulation between turns and sections, were used. To be specific, these were polyethylene terephthalate (PET) film, used for the insulating coating of windings, and Nomex® board, used as a key spacer between sections. Since little is known of the degradation of various insulating materials caused by a PD in SF6 gas, an experiment was also conducted for a PB used for, for example, the basic cylinder between windings [8]. As insulating structure models, those simulating the insulation structures inside windings, i.e. the section-to-section and turn-to-turn insulating structures, were used [9].

2.2.1 INSULATING MATERIAL MODEL

Used for the insulating material model were PET films (50 μm 40 sheets), Nomex® boards (2 mm 1 or 2 sheets), and PBs (0.8 mm 5 sheets). These samples were installed in a tank filled with SF6 gas at 0.5 MPa (absolute pressure) and degraded in the PD exposure test with the PD magnitude and the voltage application duration as parameters [8].

As for the damage pattern caused by a PD, in the case of PET film, the white discoloration due to the loss of clarity (so called clouding) was observed when the PD was about 8,000 pC or less. When the PD exceeded about 8,000 pC, the black discoloration appeared due to the carbonization of PET film. Similarly, in the case of Nomex® board, the black discoloration appeared when the PD was about 8,000 pC or more. Conversely, in the case of a PB, no damage such as discoloration could be confirmed, even if it had been exposed to the maximum PD of 32,000 pC in the test. These characteristics differed from those of an oil-impregnated PB. It is considered attributable to the fact that the energy by a PD could escape more easily into the surrounding area in the gas rather than the oil, resulting in no damage to the material at the PD magnitude in this experiment.

In the lightning impulse breakdown test, a surface layer was detached from the sample degraded by the PD to investigate the breakdown voltage. Figure 7 shows the test results for PET film as a representative case [8]. In the case of PET film, if the sample had been discolored black, the breakdown voltage was significantly decreased. In the case of Nomex® board, the breakdown voltage was not significantly decreased if the black discoloration was only on the surface, but did drop considerably if the black discoloration had progressed into the inside. The PD level that decreased the breakdown voltage of these materials was about 7,600 pC for PET film and 7,000 pC for Nomex® board, respectively. Conversely, in the case of a PB, even if it had been exposed to a significant level of a PD, no apparent damage was observed and the breakdown voltage remained constant at the same level as that of a new PB.

Figure 6. Residual impulse withstand voltage after ac partial discharge test.

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2.2.2 INSULATING STRUCTURE MODEL

Of the insulating structure models, for the turn-to-turn model, two rectangular copper wires coated with PET film were produced, which were mutually opposed in order to construct the model. For the section-to-section model, three PET-coated rectangular copper wires were lined up to simulate a single section. Subsequently, two sections were opposed to each other via a set of Nomex® key spacers to construct the model. In the PD exposure test, each structure model included particles that simulate manufacturing defects in order to generate a large PD. These particles were provided assuming various conditions of mixing into an actual transformer. As an example, Figure 8 exhibits a section-to-section model structure with a particle bridging between sections. These samples were installed in a tank filled with SF6 gas at 0.5 MPa (absolute pressure) and degraded in the PD exposure test with the PD magnitude and the voltage application duration as parameters [9].

As for the damage pattern due to a PD, in the case of the turn-to-turn model, hardly any change was visible, even if the model had been exposed to a PD of 10,000 pC or more. The turn-to-turn voltage was low in the first place and a voltage several tens of times more than the ac test voltage had to be applied in order to generate a PD exceeding 10,000 pC, even if a particle existed. Consequently, in actual fact, for the turn-to-turn model, the harmful PD level need not essentially be considered. In the case of the section-to-section model, damage to the PET film and/or the key spacer was observed if the model had been exposed to a PD of about 10,000 pC. When the PD magnitude was further increased, both were discolored black in all cases [9].

Subsequently, the lightning impulse breakdown voltage was investigated using each insulating structure model degraded by a PD. For the turn-to-turn model, a sample degraded by a PD of 8,000 pC was used. This sample was barely degraded in the PD test and the breakdown voltage was almost the same as that of a new one. For the section-to-section model, as shown in Figure 9, even if the PET coating was damaged, the breakdown voltage barely decreased if the damage to the key spacer was minor [9]. On the other hand, even if the PET coating was undamaged, the breakdown voltage significantly decreased if the key spacer was damaged. The threshold value of a PD that caused this breakdown voltage to decrease was about 10,000 pC on a conservative side.

2.2.3 HARMFUL PARTIAL DISCHARGE LEVEL OF GAS-FILLED POWER TRANSFORMER

Based on the experimental results above, the harmful PD level that degrades the insulating materials themselves of a gas-filled transformer is studied here. For the PET film material itself used for the insulation between turns, a PD of 7,600 pC was deemed harmful; however, the harmful PD level, including the insulating structure, can be deemed as 10,000 pC or more. A PD of 10,000 pC or more is unlikely to occur between turns under normal ac test voltage and thus need not essentially be considered. For the insulation between sections,

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Figure 7. Residual impulse withstand voltage of PET films after ac partialdischarge test.

Figure 8. Section-to-section insulating structure model with particle that bridges the entire length between sections.

Figure 9. Residual impulse withstand voltage of section-to-section insulating model after ac partial discharge test.

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a PD that damages Nomex® board used for a key spacer is considered harmful. For the Nomex® board itself, a PD of 7,000 pC was deemed harmful; in addition, considering the insulation structure as well, the harmful PD level was deemed at 7,000 pC to 10,000 pC.

3 SIGNAL TRANSMISSION RATE TO THE

MEASUREMENT POSITION IN THE TEST This section focuses on, assuming an actual PD test, a

signal transmission rate of a PD having occurred inside the transformer toward each measurement position. The PD transmission rate is obtained through the experiments using the winding models actually constructed as well as EMTP analysis using their equivalent circuits [10-13].

The PD transmission rate is evaluated based on the reference value obtained in the calibration test similar to the actual PD test [1-5]. In the evaluation process, the PD signal for calibration is initially injected against ground on the high voltage terminal of the winding model to measure the output signal at various measurement terminals. The value obtained in this calibration test is reference. The actual measured value obtained with the input position of the PD signal as a parameter with respect to this reference value is evaluated as the PD signal transmission rate (measured value/reference value).


For an oil-immersed power transformer, the transmission rate was obtained in the event of occurrence of a PD between windings (main insulation), between sections, and against ground. Figure 10 illustrates the circuit configuration of a 500 kV-class transformer winding model used in the study [10, 11]. This circuit configuration is a model constructed based on the experiment. Three positions were used to inject the PD signal, namely the first injection against ground such as toward the tank and core at the series winding, the second injection

between sections in the series winding, where a relatively high voltage was applied to the inside of the winding, and the third injection between the series and shunt windings as a main insulating part. To measure the PD signals, ERA device [16], a form of general purpose equipment, was used. The PD signal was measured at three positions, namely the high voltage bushing test terminal (H-BgTT), the medium voltage bushing test terminal (M-BgTT), and the neutral point terminal (NPT).

Figure 11 shows a representative case of the relationships between the PD occurrence position and the transmission rate at each measurement position [11]. Where a PD occurred against ground, the transmission rate at each measurement terminal was relatively high. Conversely, in the event of occurrence of a PD between windings, the transmission rate was as low as 10% or so in some cases. In the event of occurrence of a PD between sections, the component circulating between the PD occurrence sections is significant, resulting in the low transmission rate at each measurement terminal.

Assuming the measurement at the H-BgTT, the minimum transmission rate was 3.6%. At some other measurement positions, the transmission rate was even lower, down to 2.2% in one case. Accordingly, it emerged that the signal was detected as one significantly damped to a level of about 1/28 to 1/45 in some cases, depending on the PD occurrence position. Based on the above, where a PD of, for example, 5,000 pC occurs inside a transformer, it may possibly be detected as a small one of 5,000 pC 3.6% (2.2%) = 180 pC (110 pC) in the test.


For a gas-filled power transformer, as mentioned in Section 2.2, it is crucial to consider a PD between sections on the high voltage winding side in terms of the position of occurrence. Therefore, a high voltage winding model of a 275 kV-class transformer shown in Figure 12 was produced to obtain the PD transmission rate through experiments and EMTP analysis using the equivalent circuit [12, 13]. Figure 13 exhibits the

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Figure 10. Circuit configuration of a 500 kV class oil-immersed transformerto measure the propagation characteristics of the PD signal.

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Figure 11. Relationship between the PD occurrence position and the transmission rate. The transmission rate is evaluated using both the peak value at the first peak and the crest value during the period up to 200 μs referred toas full wave. (Left axis: Against ground and between windings, Right axis:Between sections)


image of the PD measurement circuit. The positions of occurrence of a PD were mainly between sections but also included those between turns and against ground. The measurement positions of a PD were the bushing test terminal (BgTT), the neutral point terminal (NPT), and the coupling terminal, and the ERA device was used for measurement.

Figure 14 shows as an example of the transmission rate in the event of occurrence of a PD between sections [13]. When a PD occurs between sections, the component circulating between the PD occurrence sections is significant, as is the case for an oil-immersed transformer, resulting in the low transmission rate at each measurement terminal. Assuming the measurement at the BgTT, the minimum transmission rate was 2.8%. Accordingly, it emerged that the signal was detected as one significantly damped to a level of about 1/35 in some cases, depending on the PD occurrence position. In the event of a PD against ground, the transmission rate generally exceeded that of a PD between sections and the minimum transmission rate was 4.0%. In the event of a PD between turns, the signal component that reaches the measurement terminal diminishes because the signal immediately circulates between the PD occurrence turns. Eventually, the transmission rate was less than 1% in some cases.

4 EVALUATION OF PARTIAL DISCHARGE

ACCEPTANCE CRITERION IN THE TEST Section 2 evaluated the PD magnitude at the PD occurrence

position that damaged insulating materials used for a power transformer and determined the harmful PD level. Section 3 evaluated the transmission rate of the PD signal that occurred at various positions inside the winding relative to the measurement position and obtained the minimum transmission rate. These results are summarized in Table 1. Based on these, this section studies the PD acceptance criterion in the PD test.


For an oil-immersed transformer, consideration must be given to the PD at the main insulation in the test, and the harmful PD level as an insulating material was deemed to be 10,000 pC. Even though no experiment of a PD between sections was conducted because the main insulation in the experiment was prioritized, a PD might possibly occur between sections as discussed for a gas-filled transformer. For a PD between sections, the PD level that degrades a key spacer is considered harmful according to the experimental results of a gas-filled transformer. A PB material is generally used for a key spacer of an oil-immersed transformer and its harmful PD level is considered about 10,000 pC, which is the same as that for the main insulation. Through the evaluation of these PD levels of 10,000 pC using a safety factor of two times considering the long-term operation and the difference in the insulating design by manufacturers, it is considered reasonable to determine 5,000 pC as the harmful PD level at the PD occurrence position. The minimum transmission rate of the PD that occurred between windings and between sections to the high voltage bushing test terminal was 3.6%. It even declined to 2.2% at one point at some other measurement terminals. Therefore, the acceptance criterion in the test is considered to be set to 5,000 pC 3.6% (2.2%) = 180 pC (110 pC) or less.

Based on the results above, the acceptance criterion of 500 pC according to the IEC standard [1] might result in the degradation of the insulation performance and the acceptance criterion of 100 pC according to the JEC standard [4, 5] is considered reasonable.

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Figure 14. Transmission rate of the PD signal occurrence between sections.



For a gas-filled transformer, consideration must be given to the PD between sections in the test, and the harmful PD level as an insulating material was deemed to be 7,000 pC to 10,000 pC. By evaluating this value using a safety factor of 1.4 to 2 times considering the long-term operation and the difference in the insulating design by manufacturers, it is considered reasonable to set the harmful PD level at the PD occurrence position to 5,000 pC as is the case for an oil-immersed transformer. The minimum transmission rate of the PD that occurred between sections above to the measurement position was 2.8%. Therefore, the acceptance criterion in the test is considered to be set to 5,000 pC 2.8% = 140 pC or less.

Based on the results above, the acceptance criterion of 500 pC according to the IEC standard [2] might result in the degradation of the insulation performance and the acceptance criterion of 100 pC according to the JEC standard [4, 5] is also considered reasonable for a gas-filled transformer, as is the case for an oil-immersed transformer.

5 SUMMARY In this paper, a study was conducted on the PD acceptance

criterion in the test for an oil-immersed power transformer and a gas-filled power transformer. This acceptance criterion was evaluated based on the “PD magnitude at the PD occurrence position that damages the insulating materials” and the “signal transmission rate to the measurement position in the test”.

The examination results are summarized as follows:

(1) For an oil-immersed power transformer, the PD level that degrades the materials constituting the insulation was 10,000 pC. Using a safety factor of two times, 5,000 pC was deemed to be the harmful PD level. In an actual test, this PD signal was obtained as one damped, in the worst case, to about 2.2% to 3.6% depending on the PD occurrence position. Consequently, the PD acceptance criterion in the test is considered to be set to 110 pC to 180 pC, which is obtained by multiplying these, or less.

(2) For a gas-filled power transformer, the PD level that degrades the materials constituting the insulation was 7,000 pC to 10,000 pC. Using a safety factor of 1.4 to 2 times, 5,000 pC was deemed to be the harmful PD level. In an actual test, this PD signal is obtained as one damped, in the worst case, to about 2.8% depending on the PD occurrence position. Consequently, the PD acceptance criterion in the test is considered to be set to 140 pC, which is obtained by multiplying these, or less.

(3) Based on the results above, it is considered reasonable to set the harmful PD level to be controlled in the PD test of an oil-immersed power transformer and a gas-filled power transformer to 100 pC. This acceptance criterion of 100 pC is a valid value backed by a series of research.

REFERENCES [1] IEC 60076-3, “Power transformers Part 3: Insulation levels, dielectric

tests and external clearances in air”, 2000. [2] IEC 60076-15, “Power transformers Part 15: Gas-filled power

transformers”, 2008. [3] IEC 60270, “High-voltage test techniques - Partial discharge

measurements”, 2000. [4] JEC 2200, “Power transformers”, 1995. [5] JEC 0401, “Partial discharge measurements”, 1990. (in Japanese) [6] S. Okabe, G. Ueta, H. Wada, and H. Okubo, “Partial Discharge-induced

Degradation Characteristics of Oil-impregnated Insulating Material Used in Oil-immersed Power Transformers”, IEEE, Trans. Dielectr. Electr. Insul., Vol. 17, pp. 1225-1233, 2010.

[7] S. Okabe, G. Ueta, H. Wada, and H. Okubo, “Partial Discharge-induced Degradation Characteristics of Insulating Structure Constituting Oil-immersed Power Transformers”, IEEE, Trans. Dielectr. Electr. Insul., Vol. 17, pp. 1649-1656, 2010.

[8] S. Okabe, G. Ueta, H. Wada, and H. Okubo, “Partial Discharge-induced Degradation Characteristics of Insulating Materials of Gas-filled Power Transformers”, IEEE Trans. Dielectr. Electr. Insul., Vol. 17, pp. 1715-1723, 2010.

[9] S. Okabe, G. Ueta, H. Wada, and H. Okubo, “Deterioration Characteristics Due to Partial Discharges in Insulating Structure Constituting Gas-filled Power Transformers”, IEEE Trans. Dielectr. Electr. Insul., Vol. 17, pp. 1204-1213, 2010.

Table 1. Harmful PD level at the PD occurrence position and transmission rate of the PD signal to the measurement terminals of an oil-immersed transformer and a gas-filled transformer.

Type of power transformer Insulation part Significance in verifying the

insulation by the PD test

Harmful PD level at the PD occurrence position Transmission rate of the PD to each measurement

terminal Material model Structure model

Oil-immersed power

transformer

Main insulation High Press board: 10,000 pC 10,000 pC 10% or more

Section-to-section Medium -

Considered to be about 10,000 pC even though no direct

experiment was conducted

Minimum valuesH-BgTT: 3.6% M-BgTT: 2.2%

Turn-to-turn Low: The possibility of occurrence of a PD is low - - -

Gas-filled power

transformer

Main insulation Low: Structurally, the

possibility of occurrence of a PD is low PET film: 7,600 pC

Nomex® board: 7,000 pCPress board: 32,000 pC

- -

Section-to-section High 10,000 pC Minimum value

BgTT: 2.8%

Turn-to-turn Low: The possibility of occurrence of a PD is low 10,000 pC or more Less than 1.0% in some

cases


[10] S. Okabe, G. Ueta, and H. Wada, “Partial Discharge Signal Propagation Characteristics inside the Winding of Oil-immersed Power Transformer - Study Using the Three-Winding Transformer in the Air”, IEEE, Trans. Dielectr. Electr. Insul., Vol. 18, pp. 2024-2031, 2011.

[11] S. Okabe, G. Ueta, and H. Wada, “Partial Discharge Signal Propagation Characteristics inside the Winding of Oil-immersed Power Transformer - Using the Equivalent Circuit of Winding Model in the Oil”, IEEE, Trans. Dielectr. Electr. Insul., Vol. 19, pp. 472-480, 2012.

[12] S. Okabe, G. Ueta, and H. Wada, “Partial Discharge Signal Propagation Characteristics inside the Winding of Gas-filled Power Transformer - Experimental Study Using Winding Models in the Air”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, pp. 1658-1667, 2011.

[13] S. Okabe, G. Ueta, and H. Wada, “Partial Discharge Signal Propagation Characteristics inside the Winding of Gas-filled Power Transformer - Study Using the Equivalent Circuit of the Winding Model”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, pp. 1668-1677, 2011.

[14] M. Ikeda, T. Yanari, and H. Okubo, “PD and BD Probability Distribution and Equi-Probabilistic V-t Characteristics of Oil-Filled Transformer Insulation”, IEEE, Trans. Vol. PAS-101, pp. 2728-2737, 1982.

[15] T. Yanari, M. Honda, M. Ikeda, Y. Taniguchi, and Y. Ebisawa, “Problems of Long-Term Reliability for UHV Transformer Insulation”, IEEE. Trans. Vol. PAS-102, pp. 1693-1701, 1983.

[16] M. Hikita, S. Okabe, H. Murase, and H. Okubo, “Cross-equipment Evaluation of Partial Discharge Measurement and Diagnosis Techniques in Electric Power Apparatus for Transmission and Distribution”, IEEE, Trans. Dielectr. Electr. Insul., Vol. 15, pp. 505-518, 2008.

Shigemitsu Okabe (M’98) received B.Eng., M.Eng. and Dr. degrees in electrical engineering from the University of Tokyo in 1981, 1983 and 1986, respectively. He has been with Tokyo Electric Power Company since 1986, and presently is a group manager of the High Voltage & Insulation Group at the R & D center. He was a visiting scientist at the Technical University of Munich in 1992. He has been a guest professor at the Doshisha University since 2005, at the Nagoya University since 2006, and a

visiting lecturer at the Tokyo University. He works as a secretary/member at several WG/MT in CIGRE and IEC. He is an Associate Editor of the IEEE Dielectrics and Electrical Insulation.

Genyo Ueta received the B.S. and M.S. degrees from Doshisha University in 2000 and 2002, respectively. He joined Tokyo Electric Power Company in 2002. Currently, He is a researcher at the High Voltage & Insulation Group of R & D Center and mainly engaged in research on insulation characteristics of electric power equipment.

pd criterion in transformers

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