Power-frequency Withstand Voltage Test Research Articles

Improving Detection Effectiveness of Latent Metal Particles in GIS by Applying Mechanical Impact in Power-Frequency Withstand-Voltage Test

Improving Detection Effectiveness of Latent Metal Particles in GIS by Applying Mechanical Impact in Power-Frequency Withstand-Voltage Test

Experimental Study on Surface Electric Field Distribution and Flash-over Voltage of Bushing Composite Insulator at High Altitude

At present, the high voltage bushing composite insulator is widely used in AC/DC system substation, and its structural parameters show the nonlinear growth trend with the increase of voltage level. Therefore, in 1000kV UHVAC and 800kV UHVDC system, bushing composite insulator has the typical characteristics of long insulation distance and large radial diameter. At present, with the proposal of high-voltage power equipment compact optimization design scheme, the radial diameter of the composite insulator for bushing can be effectively reduced by reasonable design of internal capacitor core or metal shield structure optimization design scheme, so as to realize its large aspect ratio compact design. On the other hand, the insulation distance of high voltage bushing composite insulator will be further increased when it is applied in high altitude area. The innovation of this paper is how to carry out flashover voltage test to fill the current test data gap. In view of this, this paper first establishes two typical structures of the high voltage bushing composite insulators, including the three-dimensional finite element models of the composite insulators for capacitive core converter transformer bushing and the double-layer metal shield through the wall bushing, obtains the distribution characteristics of typical voltage and electric field under the operating state, and analyzes the local high field strength and corona, the electric field distribution of composite insulators for bushing. Furthermore, the actual flashover tests between large fittings air metal grounding body and large fittings composite insulator metal grounding body are carried out in the laboratory environment, and the quantitative relationship curve between flashover voltage and space net distance is obtained. Based on the above theoretical analysis and test data, the size altitude correction and umbrella skirt contour topology optimization of composite insulator for the UHV AC/DC bushing are carried out. Finally, the DC withstand voltage test and one minute power frequency withstand voltage test are passed, which proves the rationality and effectiveness of the design of composite insulator for bushing at UHV level. In this paper, starting from the typical electric field distribution characteristics of all kinds of composite insulators for bushing, the flashover law is studied for the first time, and the flashover test is carried out in the laboratory to supplement the classical test data. The research results have good theoretical and practical significance for installation, design and operation maintenance of all kinds of composite insulators.

Lightning protection of multi‐fracture arc‐quenching lightning protection gap for 35 kV distribution lines

Owing to the low insulation level of 35 kV distribution lines, lightning outage accidents and huge economic losses occur frequently. Therefore, a multi-fracture arc-quenching lightning protection gap (MFALPG) for 35 kV distribution lines is developed to ensure the safety and stability of power supply. Here, the principle of MFALPG is presented through the electromagnetic self-contraction effect and qualitative modelling theory. In order to investigate the coupling effect between high-energy compressional waves and arcs, a mathematical model of airflow coupled arc was developed to simulate and analyse the arc-quenching process. Theoretical analysis and simulation results show that the high-energy compressional wave is triggered by the impulse arc rapidly, and the arc conductivity drops abruptly in a short time. Finally, a lightning impulse discharge test and the power frequency withstand voltage test are performed to determine the 35 kV power frequency continuous current interruption test parameters. Under the action of the MFAPLG, the entire time of arc duration is less than 3 ms, and reignition phenomenon of the arc is not found after the subsequent power frequency voltage is restored.

Application of C 6 F 12 O/CO 2 mixture in 10 kV medium‐voltage switchgear

Due to the high global warming potential (GWP) of SF 6 , the use of new gases in electrical equipment has drawn increasing attention. Here, the application of environmental-friendly gas dodecafluoro-2-methylpentan-3-one (C 6 F 12 O) and CO 2 mixture in 10 kV medium-voltage switchgear is studied. First, the basic physical and chemical properties of C 6 F 12 O were discussed, and the toxicity and safety of its application in equipment was analysed. Then, the breakdown voltage was measured in the laboratory. The power-frequency breakdown test was carried out with the mixing ratio of 2, 4 and 6% at 0.1~0.2 MPa, and the influence of utilisation coefficient of electric field on the breakdown performance was analysed. Finally, the appropriate mixing ratio was determined and tested in the load switch cabinet and the circuit breaker switch cabinet, including the power frequency withstand voltage test and the lightning impulse test. The test results show that 4% C 6 F 12 O and CO 2 meet the standard value tested at a pressure of >0.14 MPa, which has the potential to be used as an insulating gas in a 10 kV switchgear.

Trial calculation of the AC test voltage using a new cumulative failure probability model

In the condition settings of a power-frequency withstand voltage test for substation equipment, an approach based on insulation reliability evaluation using Weibull distribution is available. In this method, temporary overvoltage is evaluated using an “independence model”. This “independence model” assumes no remaining influence of voltage application (i.e. the model obeys the failure probability distribution starting at time 0 each time the voltage is applied). However, the authors have conducted insulation characteristics tests by applying a voltage multiple times at intervals and clarified that the influence of voltage application remains to a certain extent (i.e. the effect assumed in the “accumulation model” appears) as well as the degree of influence depending on the conditions for oil-immersed transformer. The present paper proposes an evaluation method with the degree of influence of voltage application history taken into consideration. In the proposed method, the portions following an “independence model” and an “accumulation model” are respectively assumed and combined after multiplying the relevant coefficient representing the respective degrees of influence. According to the trial calculation based on the proposed method, the required withstand voltage decreased by several percentage points, while it was confirmed that the existing method is on the safer side as a means of evaluating the insulation reliability of temporary overvoltage.

Aging effect on insulation reliability evaluation with weibull distribution for oil-immersed transformers

In the condition settings of a power-frequency withstand voltage test for substation equipment, an approach based on insulation reliability evaluation using Weibull distribution is available. However, the subjects which should be investigated remains in this method. One of the subjects is that evaluating the insulated reliability for temporary overvoltage caused in an aged equipment by the pre-aging Weibull parameter. In the first place, for an oil-immersed transformer, the breakdown voltage-time (V-t) characteristics of the oil gap were obtained as basic characteristics in the previous experiment. In this experiment the presence or absence of the oil flow was used as a parameter affecting the history of the applied voltage. Also, the validity of the present evaluation approach to temporary overvoltage during operation was examined. In this paper, an insulation characteristic was obtained in consideration of the age-related deterioration in the field. The results were then analyzed in comparison with the insulation characteristics of a new insulating oil. As a result, there was little change affecting the result of insulation reliability evaluation in the Weibull parameters. Consequently, it is reasonable to evaluate the insulation reliability by the V-t characteristic of the new insulating oil.

Weibull parameter of oil-immersed transformer to evaluate insulation reliability on temporary overvoltage

In the condition settings of a power-frequency withstand voltage test for substation equipment, an approach based on the insulation reliability evaluation using Weibull distribution is available. However, the issue to be discussed remains in this approach. For example, temporary overvoltage during the operation is evaluated through Weibull-distribution parameters obtained from an experiment without operating voltage. For an oil-immersed transformer, the breakdown voltage-time (V-t) characteristics of the oil gap were obtained as basic characteristics in the previous experiment. It was experimented with 0 cm/s, 8cm/s and 16cm/s oil flow velocity as a parameter affecting the voltage application history. In this paper, an insulation characteristic was obtained in a pattern of voltage application assuming temporary overvoltage during operation. The results were compared with the basic characteristics in the previous experiment, and insulation reliability was calculated in trial. Consequently, the insulation reliability evaluation based on the existing V-t characteristic test parameters was found to be practically on the safe side and appropriate.

Experiment on multiple-times voltage application to evaluate insulation reliability of oil-immersed transformer

For a power-frequency withstand voltage test to be conducted on substation equipment, the conditions can be established using a technique based on insulation reliability evaluation using a Weibull distribution. This can be done by either the "independence model" or "accumulation model" which have been proposed in order to handle the coexistence of multiple voltage levels, but there remains a problem of how to use these models. This study attempted to solve this problem. In the author's preceding research, the V-t characteristics of oil gaps were obtained by focusing on the presence of oil flows in oilfilled transformers as a parameter that influences the voltage application history. This paper proposes a test method that clarifies the differences between the independence model and accumulation model. Furthermore, it shows the results of conducting the proposed test of applying the same voltage multiple times and evaluates the influences of voltage application history. These results present that when the oil flow velocity is sufficiently high, they mostly conform to the calculation with the independence model. When the oil flow velocity is slower, the test results are closer to the calculation with the accumulation model. It is also found that when a voltage is applied during intervals or the interval time is shortened, the test results are closer to the calculation with the accumulation model.

Transformer insulation reliability for moving oil with weibull analysis

To develop test conditions for a short-duration power-frequency withstand voltage test, which is one of the insulation specifications of substation equipment, the reliability can be precisely evaluated using a Weibull distribution function. This can be done by either the independence model or accumulation model, which have been proposed in order to handle the coexistence of multiple voltage levels, although the problem of how to use these models persists. This problem must be examined by comparing the results calculated by both models from the breakdown voltage-time characteristics and those involving the measurement of insulation characteristics obtained by applying voltages multiple times. The present study obtained the breakdown voltage-time characteristics of oil gaps by focusing on the presence of oil flows in oil-filled transformers as a parameter influencing the voltage application history. These test results, which will be the basic data for future study, are reported.

Design of insulation test with one-minute step-up method for substation equipment

Reliability evaluation method with Weibull distribution function is used for development of test conditions for short-duration power frequency withstand voltage test. Although the evaluations by ldquoindependent methodrdquo and ldquocumulative methodrdquo are proposed, the handling of multiple voltage application still remains as one of the subjects. Therefore, insulation test with one-minute step-up method is studied as an insulation test with coexistence of multiple voltage level. As a result, the insulation characteristics by one-minute step-up method deviated from their true values. In this paper, it was calculated and evaluated by two methods with parameters of test condition. For the one-minute step-up method, the deviations from the true values were found to be influenced by test conditions: the larger the test voltage step width, or the lower the test starting voltage, or the greater the number of voltage steps, the smaller the deviations from the true values. Furthermore, the approaches to designing appropriate conditions for the test were summarized in order to design test conditions for oil-filled transformer and gas insulated switchgear. As a result, the reasonable test conditions were evaluated as follows: for oil-filled transformers, the test starting voltage was 70% of the one-minute breakdown voltage value and the voltage step was 7% and, for gas insulated switchgear, the test starting voltage was 80% and the voltage step was 5%. This result is approximately the same as the test conditions used for the actual measurement data in the past. These examinations will help to establish appropriate test conditions in the one-minute step-up method and to evaluate insulation characteristics obtained from the test.

Influence of voltage application history on insulation test with one-minute step-up method

For the development of test conditions for a short-duration power-frequency withstand voltage test, which is one of the insulation specifications of substation equipment, there is a precise method of evaluating reliability using a Weibull distribution function. For this method, "independence method" and "accumulation method" have been proposed in order to handle the coexistence of multiple voltage levels. In this paper, an insulation test based on the one-minute step-up method as condition for the coexistence of multiple voltage levels is used to calculate and evaluate the test results using these two methods. Consequently, the insulation characteristics obtained from the one-minute step-up test are found to have differences from the true values. If the independence method is adopted, there is a certain deviation regardless of the Weibull time shape parameter "a-value" and, the smaller the Weibull voltage shape parameter "m-value", the larger the deviation. If the accumulation method is adopted, on the other hand, the larger the "a-value" or the smaller the "m-value", the larger the deviation. The 50% breakdown voltage value, the standard deviation "σ-value", and the "m-value" that can be obtained from the insulation test based on the one-minute step-up method were evaluated for their deviations from the true values. The results suggest that, when a test is conducted on an oil-filled transformer, the design may turn out to be on the dangerous side due to overestimation of the dielectric strength by around 8%.

Breakdown voltage-time characteristics and insulation testing of gas insulated switchgear

There is not sufficient data on the breakdown voltage-time characteristics for gas insulated swiitchgear (GIS) at power-frequency voltages, especially in the long-term region, for complete insulation specifications to be developed. This paper gives voltage- time breakdown characteristics for a 72-kV GIS at power-frequency voltages, both for clean conditions and with metallic particles added, from times of the order of a second to times of the order of 1000 hours. The Weibull parameters were calculated to be n = 95 and a = 0.32 to 0.47 in the clean case; and in the metallic-particle case n = 31 and a = 0.92 to 0.98 for a short-term region and n = 66 and a = 0.40 to 0.47 for a long-term region, with the boundary between these two regions located at 10 minutes. Based on the statistics for the metallic-particle case, the reliability of the power-frequency withstand voltage tests were then estimated using as examples the GIS in a 500-kV system (an effectively grounded system) and a 66-kV system (a non-effectively grounded system). The paper also studied the effect of the type of grounding and the influence of the presence of metallic particles on the test voltage level.

ワイブル分布を用いた変電機器の交流短時間過電圧に対する信頼度評価

The power-frequency withstand voltage tests are regulated on electric power equipment in JEC by evaluating the lifetime reliability with a Weibull distribution function. The evaluation method is still controversial in terms of consideration of a plural number of faults and some alternative methods were proposed on this subject. The present paper first discusses the physical meanings of the various kinds of evaluating methods and secondly examines their effects on the power-frequency withstand voltage tests. Further, an appropriate method is investigated for an oil-filled transformer and a gas insulated switchgear with taking notice of dielectric breakdown or partial discharge mechanism under various insulating material and structure conditions and the tentative conclusion gives that the conventional method would be most pertinent under the present conditions.

Voltage-time Characteristics and Reliability Evaluation Method of Oil-filled Transformer for Non-effectively Grounded Voltage Class

The power frequency withstand voltage tests are specified on electric power equipment in JEC-0102 by evaluating the lifetime reliability with a Weibull distribution function. It may be applied on 66-154kV transformer for non-effectively grounded voltage class. The present paper describes the results that measured AC insulation characteristics (Voltage-time characteristics) and Weibull parameters for oil-filled transformer of 66-154kV class. These measurement parameters were compared with the conventional Weibull parameters, which were applied for oil-filled transformer of voltage class above 154kV. Furthermore the non-partial discharge test voltage was evaluated by trial calculations, using the parameters obtained this time or typical parameters.

Medium factors of electrical insulation systems in high temperature superconducting power apparatus with coil structure for equivalent ac withstand voltage test at room temperature

Equivalent insulation test voltage at room temperature of high temperature superconducting (HTS) power apparatus with coil structure for power frequency withstand voltage tests is discussed based on medium factors of insulation weak parts in electrical insulation elements of the HTS power apparatus. Uniform and non-uniform field gaps, triple junction and solid insulator surface are selected as the insulation weak parts, and their medium factors are theoretically and experimentally calculated as the ratio of partial discharge (PD) inception or direct-breakdown voltages in liquid nitrogen or cryogenic nitrogen gas to those in atmospheric air at room temperature. The results show that the type of insulation weak part affects the relationships between the medium factor and the gap length between electrodes as well as the state of cryogenic surrounding medium. These medium factors are compared with the medium factor of a coil-to-coil insulation element of HTS pancake coils that contains some of the insulation weak parts, and it is verified that the medium factor of the coil-to-coil insulation element can be obtained by using that of triple junction or solid insulator surface. Finally, a selection method of a significant unique medium factor is discussed to determine the equivalent insulation test voltage at room temperature for the confirmation of the insulation reliability of all the parts of the HTS apparatus.

Flashover Voltage Characteristics of Contaminated Bushing Shells for UHV Transmission Systems

This paper describes the power frequency withstand voltage test results of contaminated bushing shells for UHV transmission systems up to 15 m under fog condition. Described first are the facilities for testing such tall bushing shells of a large diameter. The test results show a linearity in relationship between effective length and withstand voltage both by the fog withstand method and the equivalent fog method. Assessment of 20 different shed shapes is made. The relation between average diameter and withstand voltage can be expressed in a quadratic curve.