Toshihiro Tsuboi

Observation results of lightning shielding for large-scale transmission lines

Lightning strokes to transmission lines have been estimated using the electro-geometric model proposed by Armstrong and Whitehead. The observed results of the lightning to large-scale transmission lines are reported here to validate the calculated results, in which ultra high voltage (UHV) designed transmission lines and 500 kV transmission lines were selected as the subjects of the observation. Lightning observations were carried out for direct strokes to phase conductor caused by shielding failures, as well as strokes to ground wires that have rarely been previously reported. The observed results showed that total number of direct lightning strokes to phase conductor were nearly identical to the calculated results based on the conventional method, while observed strokes to upper phase were larger and strokes to lower phase were smaller than those derived from calculations. It was also revealed that lightning strokes to ground wires of UHV designed transmission lines and 500 kV transmission lines were 5.1 times and 2.7 times larger, respectively, than calculated results based on the conventional method.

Investigation of aging degradation status of insulating elements in oil-immersed transformer and its diagnostic method based on field measurement data

To ensure the reliability of oil-immersed power transformers in long-term operation, the influence of aging on various characteristics of their insulating elements must be clarified. The present paper reviewed the degradation status of insulating oil and paper of field-aged transformers, covering investigative results ranging from about 30 years ago to a relatively recent period. The field transformers treated were diverse in terms of their manufacturing year, manufacturer, structure, and voltage class. Despite the examination results of such wide-ranging transformers, there were similar changes in various characteristics due to the aging of insulating oil and paper. For example, as for the insulating oil, due to the aging for about 30 years, the characteristics of the total acid value and volume resistivity had degraded, whereas the breakdown voltage decreased only slightly. Regarding the insulating paper, mechanical characteristics such as the average polymerization degree (DP) were subject to age-related decline, whereas the breakdown voltage did not significantly decrease as with insulating oil. Therefore, based on use of about 30 years, since the life of a transformer is determined by factors such as the degradation of mechanical strength due to thermal stress, insulation degradation is considered less likely to pose a problem at an early stage. Even though it is difficult to directly measure the degree of degradation of insulating paper, this can be indirectly estimated by measuring the CO2 and CO content and furfural content in insulating oil. On the other hand, given the increasing number of transformers remaining in service over 30 years, which had been deemed the expected service life, consideration must be taken to aging degradation of the insulation performance. It emerged that, in order to identify the degree of degradation of these transformers, it was effective to manage the volume resistivity and dielectric loss tangent trends in electrical terms, the interfacial tension trend in physical terms and the total acid value trend in chemical terms. Analysis of the insulating oil components is also considered an effective means of evaluating aging degradation.

Reliability evaluation with weibull distribution on AC withstand voltage test of substation equipment

For the development of a ldquoshort-duration AC withstand voltage testrdquo, an insulation specification of substation equipment, there is a precise method of reliability evaluation using a Weibull distribution function. Regarding this method, there remains a subject of handling coexistence of multiple voltage levels. This paper first defines the two reliability evaluation methods, ldquoindependence methodrdquo; and ldquoaccumulation methodrdquo, applying to Weibull evaluation for coexistence of multiple voltage levels in relation to their physical meanings. Next, the influence of the Weibull parameter values are examined on the cumulative fault probabilities and test voltages calculated using these methods. When the time shape parameter a>1, the accumulation method gives higher values than the independence method; When a=1, the two methods give the same values; When a<1, the former gives lower values than the latter. Then, appropriate reliability evaluation methods are investigated for various insulation media and insulation structures of substation equipment from the viewpoint of inception and development mechanisms of dielectric breakdown and partial discharge. According to the result of engineering evaluation of the presently available data, the independence method may be appropriate for both gas insulated switchgear and oil-immersed transformers.

Improved method of calculating lightning stroke rate to large-sized transmission lines based on electric geometry model

The characteristics of the lightning shielding of large-sized transmission lines have been calculated based on an electric geometry model (EGM) proposed by Armstrong and Whitehead. However, the characteristics of lightning strokes to large-sized transmission lines observed in recent years differed from calculation results, hence the present study was conducted to improve the conventional calculation method. First, the relationship between the striking distance and the lightning current was reviewed. Specifically, the air gap discharge characteristics based on test data with complementing the large gap region were applied. In addition, the return stroke velocity distribution obtained by actual measurement was newly applied. Second, the distribution of lightning waveforms was reviewed; the distribution of peak values of lightning current with the distribution of front duration was used in this study. With these improvements, the calculated lightning stroke rate to power lines and ground wires differs less from actual observations than that calculated by the conventional method.

Basic study of fitting method for base curve extraction in lightning impulse test techniques

The IEC, which prescribes lightning impulse test techniques for electric power equipment, is about to introduce the k-factor method for evaluating the overshoot of impulse waveforms. Its procedure specifies that waveforms are to be fitted with a double exponential function to derive a base curve. Some derived waveforms, however, deviate upwards from the central lines of the recorded waveforms in the wavefront area. This paper examines a more rational method of extracting a base curve by considering the relationships between equivalent electric circuits and the solutions of their governing equations. A trial calculation with the new methods gives results with lower crest values and slower rises than the existing method.

Study on degradation causing components of various characteristics of transformer insulating oil

To operate power transformers long-term, as well as ensuring their insulating reliability, adequate consideration must also be paid to the age-related decline in various insulating oil characteristics. This decline is considered attributable to the generation and increase of trace components in oil over time, which were not present when the oil was new. In this paper, to identify the components causing such degradation, various components detected in actual field aged transformers were added to the base oil to measure a range of characteristics (water content, acidity, interfacial tension, breakdown voltage, dissipation factor, and volume resistivity). The influence of these additives on various characteristics of insulating oil was evaluated to investigate the components responsible for the decline. When organic acids were added, the acidity value declined, as well as a significant decline in volume resistivity for decanoic acid among organic acids. For methyl ethyl ketone, which is a kind of ketones, a degradation of the characteristics was observed in the dissipation factor and volume resistivity, showing the same trend as that in the investigation results of the actual transformer oil. It emerged that phthalate compounds, or those derived from rubber material among the aromatic series, influenced the dissipation factor and volume resistivity in the presence of water. Consequently, the influence of various components detected in actual transformer oil on various characteristics of insulating oil was clarified.

Study on a field data of secondary arc extinction time for large-sized transmission lines

Secondary arc extinction time is an important factor, which influences system stability because it directly affects the reclosing time. However, few reports exist on a field data of the secondary arc extinction time on transmission lines. This paper describes the investigation results of secondary arc extinction characteristics by analyzing voltage wave shapes during the lightning faults on 550 kV transmission lines. Recovery voltages and secondary arc currents on each transmission line during faults were calculated to consider correlation with the secondary arc extinction time. The secondary arc extinction time is estimated by the existing formula, the secondary arc current and the recovery voltage required for one second reclosing are evaluated.

Method of calculating the lightning outage rate of large-sized transmission lines

To evaluate the lightning outage risk to large-sized transmission lines, the lightning outage rate is predicted by calculating the probable lightning stroke rate. Conventionally, the increased potential caused by lightning strokes is calculated based on distributed constant circuit theory, and the calculated results are compared with the air gap withstand voltage between the arc horns of phase conductors, usually having six phases, to evaluate whether flashover will occur, and thus calculate its frequency of occurrence. However, the lightning outage rate of large-sized transmission lines calculated by this conventional method provides a poor match with reality. Consequently, the present study was conducted to improve the calculated prediction of the lightning outage rate. The lightning outage rate is a composite figure that includes back flashover caused by lightning strokes to ground wires and flashover caused by lightning strokes to phase conductors. The calculation of each of these cases has been improved. The main improvements include corrections to the withstand voltage to match the differences at larger and smaller facilities, as well as an improved method of calculating the development from a one-phase ground fault (1LG) of a phase conductor to a two-phase ground fault (2LG). Thanks to these improvements, the calculated lightning outage rate was closer to the actuality than that calculated by the conventional method. A trial calculation of the lightning outage rate with the voltage increased from 500 kV to UHV was also performed using the improved calculation method. Even though both 1LG and 2LG due to flashover caused by direct lightning strokes and back flashover increased to a certain extent, the lightning outage rate was about 30% lower than that of the present 500 kV transmission lines.

Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods

The k-factor, which evaluates an overshoot of the withstand voltage test waveform in a lightning impulse withstand voltage test, is about to be adopted in IEC 60060-1 that specifies high-voltage test techniques for electric power equipment. In its procedure, it is defined that the recorded waveforms are fitted with a double exponential function to derive a base curve. For some waveforms, however, an extracted base curve sometimes deviates upward from the central lines of the recorded waveforms in the wavefront area. In particular, the degree of such dissociation is significant in waveforms with a high overshoot rate and a low frequency, and further it can not be said that the base curve is appropriate in terms of insulating properties. Accordingly, such significant dissociation may result in an irrational calculation of the overshoot rate. To resolve these problems, a numerical equation resulting from a relationship of a solution between an equivalent electric circuit and dominant equation was used, and fitting methods were overviewed in order to extract a more reasonable base curve in previous studies. In this paper, various options from these fitting methods were narrowed down to several options, which were then applied to simulated recorded-waveform; for which the overshoot rate and frequency of superimposed oscillating served as parameters, and a fitting method for extracting a more reasonable base curve was examined. Consequently, the calculation results of the overshoot rate were brought closer to their actual values than with the existing method by using a new base curve extraction method by which the fitting can be performed with removing the oscillatory part from the recorded waveform. While there is significant dissociation in the existing method, especially in the waveform on which a relatively low frequency oscillatory wave is superimposed, evaluation results of the overshoot rate were significantly improved as a result of the use of the new base curve extraction method. In the meantime, it should be noted that the k-factor filtering scheme was implemented after the application of various fitting methods resulted in the shape parameters of the test voltage waveforms, such as crest value, wavefront and wavetail durations, emerging as virtually identical, alongside confirmation of the minor effect of the fitting methods on the test voltage waveform.

Analysis of aspects of lightning strokes to large-sized transmission lines

When designing large-sized transmission facilities, evaluating the risk of their lightning outage is very important, hence the authors have been conducting a study to determine a more precise method for calculating the lightning stroke rate and the lightning outage rate. In that study, it is essential to clarify the aspect of lightning strokes to large-sized transmission lines. In this context, the Tokyo Electric Power Company has observed lightning strokes to overhead ground wire or transmission tower (shielding success), for the first time as far as the authors are aware, and direct lightning strokes to a phase conductor (shielding failure). The present study analyzed and compared the ratios of shielding success or failure, and the (forward) flashover or back flashover outage rates based on the observation results of UHV-designed and 500 kV transmission lines. The number of lightning strokes to transmission facilities is 167.3 cases/100km/year for UHV-designed transmission lines and 130.8 cases/100km/year for 500 kV transmission lines, meaning the taller the facilities, the more lightning strokes occur. For these lightning strokes, the shielding failure rates were 1.80 and 0.98% for UHV-designed and 500 kV transmission lines, respectively, whereas the lightning outage rates due to direct lightning strokes (flashover) were almost equal at 0.37 and 0.39% for UHV-designed and 500 kV transmission lines, respectively. This is due to the balance between the shielding effect associated with the insulation clearance and the flashover voltage between arcing horns. Conversely, the lightning outage rate due to back flashover was mainly a factor of the distance between arcing horns and was 0.05% for UHV-designed transmission lines, which was very low compared with 0.59% for 500 kV transmission lines. Based on these results, since a comparatively high number of lightning outages were caused by direct lightning strokes on large-sized transmission lines, e.g. UHV class, the need for insulation design to tackle such cases was indicated.