Lightning Impulse Voltage Test Research Articles

Experimental Study of the Electrical and Physiochemical Properties of Different Types of Crude Palm Oils as Dielectric Insulating Fluids in Transformers

This paper gives information on the electrical and physiochemical characteristics of six different types of palm oil compared with traditional mineral oil. We found that natural processed crude palm oil (PO-C) had a higher resistance to AC breakdown voltage than other types of palm oil, including traditional mineral oil. The results of the positive lightning impulse voltage test for PO-C were still the highest compared to other types of palm oil, including traditional mineral oil, at 58.26%. The summarised dissipation factors of all tested crude palm oils were significantly higher than those of mineral oils, which will make the palm oil less insulating, especially in PO-A palm oil (36.197%), where the values were higher than those of other oils, while mineral oil has a slightly increased dispersion factor. For relative permittivity, all palm oils were compared, and it was found that PO-C had a lower relative permittivity than the other oils. In terms of physical and chemical properties, in the moisture content test on all oils, PO-C had the percentile with the highest moisture content decrease of 58.74%. In the case of testing the surface tension value, it was found that traditional mineral oil had the highest value (48.46 m/Nm) when compared to palm oil. On the other hand, the acidity in traditional mineral oil is the lowest (0.03 mg KOH/g) compared to all palm oils. Results from studies demonstrate the possibility of using natural processed crude palm oil, or PO-C, as a replacement for traditional mineral oil. This is consistent with the results of electrical properties that show PO-C is higher than other types of palm oil and includes traditional mineral oil.

Theoretical Analysis of External Insulation of Typical Electric Equipment under Impulse Voltage and Research on Design and Verification Method of Margin

Lightning impulse voltage, one of the typical characteristic working conditions for assessing high-voltage power equipment, can be divided into two forms, namely full wave, and chopped wave. The external structure size of the power equipment directly depends on the lightning impulse voltage, but how lightning impulse voltage relates to the external structure size of the power equipment needs to be carefully studied. Firstly, the theoretical analytical expressions of full wave and chopped wave lightning waveforms are given from the perspective of theoretical formulas. And the lightning waveform is considered to be a polynomial superposition of exponential functions. Based on this, the discussion is divided into two typical insulation structure scenarios: 1) the verification and analysis of the space net distance of a 10 m class super long composite insulator; 2) the analysis of the relationship between the breakdown electric field strength of 5 mm tiny oil gap and oil gap spacing. Further, the lightning impulse voltage test was conducted for large hardware structures, and the corona layer mathematical model was obtained through the precise observation of the structure with the ultraviolet imager. The research shows that the lightning impulse voltage mainly checks the external insulation performance of large insulation structures such as bushings and post-insulators. For the internal oil gap duct structure of the large converter transformer, the lightning impulse voltage mainly checks the micro gap insulation strength. Lightning impulse voltage also has a breakdown effect on SF6 insulation gas, but its internal gas decomposition mechanism needs theoretical and experimental analysis urgently. The research content of this paper has certain theoretical and practical guiding value for the optimization design of large insulation structures under lightning impulse voltage.

Experimental Study on a Compressed Arc Extinguishing Device for Power Distribution Network

A compressed arc extinguishing device for power distribution network is developed to solve the problem of existing anti‐lightning of power distribution network transmission lines. This paper first introduces the arc extinguishing principle of the arc extinguishing device, and then designs the lightning impulse voltage test around the 10 kV transmission line. The test results show that the U50% Lightning impulse voltage of the arc extinguishing device is 112.4 kV considering the atmospheric correction factor. In order to verify the feasibility of the arc extinguishing device, the power frequency continuous current interruption test is designed. The test shows that the arc extinguishing device can extinguish the arc with a peak value of 1.131 kA in 2 ms and does not reignite in the following time. Finally, the device is applied to a 10 kV power distribution network transmission line of China Southern Power Grid. The lightning trip‐out rate of the transmission line per 100 km is reduced from 12.5 (times/year) to 1.1 (times/year), which fully verifies the feasibility of the arc extinguishing device. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Investigation of High-Frequency Oscillation Discrepancies Presented by Distribution Transformers During Lightning-Impulse Voltage Tests

Investigations, through a sample evaluation, aiming to evaluate the cause of the high-frequency oscillations presented by distribution transformers in impulse tests reveal that internal imperfections, such as cavities and floating particles, cause these oscillations.

친환경 전력케이블용 PP의 백화현상에 의한 절연특성 변화에 대한 연구

Cross linked polyethylene (XLPE) has been widely used for power cables as an insulation material due to its excellent dielectric and mechanical characteristics. However, XLPE can not be recycled after the lifetime of power cables because of its thermosetting property. In many countries, power cables have been developed and installed actively since 1970. It means that the lifetime of power cables is reaching over 50 years. It is known that the lifetime of power cables is around 50 years. Therefore, many power cables in a power grid installed in 1970s should be replaced with new power cables in order to improve the reliability of a power system. In this replacement, there are many studies to change thermosetting materials, XLPE into thermoplastic materials, polypropylene (PP) which can be recycled. This paper deals with the analysis on the dielectric degradation characteristics of PP according to white rust. White rust may occur in power cables due to curvature and scratch which could be caused by bending stress and friction. Especially bending stress is inevitable because of narrow installation area and process of installation. The dielectric degradation characteristics of PP are analyzed under AC as well as lightning impulse voltage test. It is found that the dielectric degradation characteristics of PP according to white rust is more degraded at lightning impulse voltage test than AC voltage test.

Effective Simulation Approach for Lightning Impulse Voltage Tests of Reactor and Transformer Windings

In this paper, an effective simulation method for lightning impulse voltage tests of reactor and transformer windings is presented. The method is started from the determination of the realized equivalent circuit of the considered winding in the wide frequency range from 10 Hz to 10 MHz. From the determined equivalent circuit and with the use of the circuit simulator, the circuit parameters in the impulse generator circuit are adjusted to obtain the waveform parameters according to the standard requirement. The realized equivalent circuits of windings for impulse voltage tests have been identified. The identification approach starts from equivalent circuit determination based on a vector fitting algorithm. However, the vector fitting algorithm with the equivalent circuit extraction is not guaranteed to obtain the realized equivalent circuit. From the equivalent circuit, it is possible that there are some negative parameters of resistance, inductance, and capacitance. Using such circuit parameters from the vector fitting approach as the beginning circuit parameters, a genetic algorithm is employed for searching equivalent circuit parameters with the constraints of positive values. The realized equivalent circuits of the windings can be determined. The validity of the combined algorithm is confirmed by comparison of the simulated results by the determined circuit model and the experimental results, and good agreement is observed. The proposed approach is very useful in lightning impulse tests on the reactor and transformer windings.

Analysis of damage modes of glass fiber composites subjected to simulated lightning strike impulse voltage puncture and direct high voltage AC puncture

Understanding the damage mechanisms of fiber-reinforced polymer matrix composite materials under high voltage conditions is of great significance for lightning strike protection and high voltage insulation applications of composite structures. In this paper, we investigated effects of the lightning impulse (LI) voltage and high voltage alternating current (HVAC) puncture on damage modes of the electrically nonconductive glass fiber-reinforced polymer (GFRP) matrix composite materials through experimental tests and numerical simulations. The LI and HVAC tests represent the lightning strike and high voltage insulation cable puncture conditions, respectively. Our experimental examinations showed that GFRP composite specimens subjected to the LI voltage test exhibited distinct damage modes compared with those in the HVAC puncture test. The GFRP composite material suffered more charring and fiber vaporization in the HVAC puncture test, whereas less matrix charring and fiber vaporization but severe fiber breakage and delamination in response to the LI voltage tests. The findings indicate that the thermal effect dominates the damage of GFRP composites inflicted by the HVAC puncture test, whereas the mechanical impact effect governs the GFRP composite damage in the LI voltage test. In addition, the electric arc plasma formation during the puncture of the GFRP composite material was modeled through solving Maxwell’s equations and the heat generation equations using finite element analysis. Simulation results provided insights on the effects of duration and intensity of the high voltage electric discharge on the composite damage.

±100-kV HVDC SF6/N2 Gas-Insulated Transmission Line

This paper introduces an HVDC SF6/N2 gas-insulated transmission line (GIL) prototype that operates under 100-kV DC application. In the first part, the development history of GIL products over the past 50 years is reviewed. After that, the design concept of the ±100-kV HVDC SF6/N2 GIL is introduced. A GIL prototype was prepared and the DC withstanding voltage test and DC superimposed lightning impulse voltage test were performed. The results show that this GIL prototype passed all the required tests, including hydrostatic test, gas tightness test, thermal cycling test, DC withstanding test, and DC superimposed impulse test, etc. The successful preparation of this product promoted a major step in researches of the DC GIL and the prototype laid a solid foundation for the development of subsequent industrial products.

Effect of lightning impulse prestress on AC partial discharge characteristics in cast resin transformer

AbstractIt is crucial for cast resin transformer to detect void and delamination existing in the cast resin because these defects reduce insulation performance of cast resin equipment. Furthermore, defects are likely to be mixed in cast resin because of several surface boundaries between resin and conductor. It is considered that detecting partial discharge (PD) is effective to diagnose equipment of power equipment. However, it is reported that withstand lightning impulse voltage test may give influence on AC partial discharge test, especially partial discharge inception voltage. This paper deals with accumulated charge in a void under AC voltage to investigate the effect of the impulse voltage prestress on subsequent AC PD characteristics in cast resin transformer. AC PD characteristics were compared before and after impulse application. In addition, recovery of PD characteristics was measured to investigate charge decay. As a result, the mechanism of prestress effect is revealed and surface charge density is estimated quantitatively in cast resin transformer.

Application of PI/MWCNT Nanocomposite for AC Corona Discharge Reduction

Electric-field (E-field) and radio influence voltage (RIV) from high voltage transmission line occur due to corona discharge. In this proposed work, the effect of corona discharge on aluminum conductor steel reinforced (ACSR) conductor was deeply investigated and mitigated by coating the polyimide/multiwalled carbon nanotubes (PI/MWCNTs) on the surface of the conductor. Bare and coated ACSR panther conductors were tested for E-field on the surface of the conductor, corona inception voltage (CIV), corona extinction voltage (CEV), and RIV. PI/MWCNT was synthesized and characterized by X-ray diffraction, thermogravimetric analysis, and permittivity and conductivity tests. The impact of MWCNT on corona discharge parameters were realized by loading it in different weight percentages as 1 wt%, 3 wt%, and 10 wt% in PI. It has been observed from the measured value (as per the standard IEC 60060-2) that the E-field was reduced by 8%-23%, RIV was suppressed with a difference of 7-18 dB, and CIV/CEV was improved for nanocomposite coated conductor. To comprehend the reason for the E-field and RIV suppression, permittivity and conductivity tests were performed as per the standard ASTM D5568. Lightning impulse voltage test was also conducted on plane-plane electrode configuration and affirmed that PI/MWCNT material has the capability to withstand lightning discharge plasma.

Superhydrophobic modified-polyurethane coatings for bushing of power transformers: From material to fabrication, mechanical and electrical properties

The presence of dust and coastal salt water found in a high moisture content environment can affect the stability of bushings in power transformers. This problem leads to transformer shut down and in severe conditions it can cause transformer expulsion. To eliminate this problem, a superhydrophobic modified-polyurethane coating was fabricated that contained easy cleaning property. Final coating had contact angles higher than 150° and CAH (Contact Angle Hysteresis) less than 10 degrees. The coating indicated good UV durability and mechanical robustness which was investigated by an abrasion test. It could pass acid rain and dry lightning impulse voltage tests. The best results were attained with the formulation containing 8 wt% nanosilica.

A fully enclosed, compact standard lightning impulse generator for testing ultra-high-voltage-class gas-insulated switchgears with high capacitance.

At present, conducting standard lightning impulse (LI) tests in the field for gas-insulated switchgear (GIS) equipment is difficult because of the high capacitance of the test equipment and large circuit inductance of traditional impulse devices, which leads to a wavefront time T(f) ≥ 2.5 μs. A novel fully enclosed, compact standard LI generator for testing ultra-high-voltage-class GIS equipment with high capacitance is presented to solve the problem of T(f) exceeding the standard during LI voltage tests for actual large-sized equipment. The impulse generator is installed in a metal vessel filled with SF6 or SF6/N2 gas mixture at a pressure of 0.3-0.5 MPa, providing a more compact structure and a lower series inductance. A newly developed conical voltage sensor is used to accurately measure the output voltage waveform. Two test modes (via bushing docking and direct docking) for the GIS test based on the impulse generator are introduced. Calculation results show that the impulse generator can generate an LI test waveform following the present IEC standard for the test of equipment with capacitance >10,000 pF.

Uncertainty in K-factor measurement for lightning impulse voltage test

The IEC-60060-1 "High-voltage test technique" was revised in 2010 and an evaluation method using the k-factor function (test voltage function) was introduced for the lightning impulse voltage test, which involved the overshoot waveform being converted into the test voltage waveform. Test standards for UHV-class equipment are also presently being studied. To date, authors have measured k-factor values with the largest possible model, assuming actual UHV-class power equipment and proposed a new k-factor function for large-scale equipment. The present paper reports on the evaluation results of expanded uncertainty when measuring a k-factor value on the basis of the ISO/IEC Guide 98-3. As a result, it was clarified that significant uncertainty in measuring a k-factor value is brought about by the definition of the kfactor itself because small differences in the 50% breakdown voltage between the smooth lightning impulse and overshoot waveforms with respective uncertainties at a certain level were calculated. The expanded uncertainty when measuring the k-factor value in the present study for an overshoot rate of 10%, which is particularly important for practical use, was a large value of ±0.23 to ±0.36, or 0.46 to 0.72 in topto- bottom width compared to k-factor values themselves ranging from 0.2 to 1.0. However, this is actually considered a minimum level of uncertainty, which is inevitable in principle, and the best achievable measurement result rather than something caused by any problem in measurement itself.

Computer simulation as an aid in the rating of a transformer on-load tap changer

The on-load tap changer (OLTC) is a crucial and expensive component of every power transformer. Its primary function is the dynamic adjustment of load voltage to specified levels without interrupting load current. Apart from thermal limitations at each tap position, the main concern in correctly rating an OLTC is the basic impulse insulation level (BIL). The reason for this is that during the standard BIL test on a power transformer, damaging overvoltage may develop on the regulating winding, which is transferred to the OLTC, which may damage the OLTC, and as a result, fail the BIL test on the power transformer. In regard to the lightning impulse voltage test, since the overvoltage that develops is a consequence of the distributed capacitance and inductance of the regulating winding, and as there are several types of regulating windings, each with different values of distributed capacitance and inductance, the correct rating of the OLTC will depend on the type of regulating winding used. Therefore, the transformer designer must be able to estimate the transferred impulse overvoltage, for the purpose of not only choosing the right type of regulating winding, but also to limit the overvoltage through passive or active means, for example incorporating electrostatic shielding in the regulating winding or installing a MOV surge arrestor. With this aim, computational tools are a powerful aid for assisting the transformer designer, since they allow the computation of distributed capacitance and inductance of the regulating winding, as well as estimating the distributed impulsive voltage. On the other hand, in regard to voltage withstand at power frequency, the induced AC voltage test is also important for determining the BIL of an OLTC. This article presents a practical case as to the steps that must be taken by a transformer designer in rating a power transformer OLTC.

Insulation characteristics of gas insulated switchgear under lightning impulse and ac superimposed voltage

The insulation performance of gas insulated switchgear (GIS) against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated in an actual system is superimposed over the ac operating voltage to form a waveform that differs from the LI voltage waveform. For UHV- and 500 kV-class GIS in particular, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are important factors to study the insulation design. Accordingly, this paper researched the insulation characteristics for the superimposed voltage of element models simulating the SF6 gas gap, creepage surface of the insulating spacer and inside of the insulating spacer, all of which are the main insulating parts of GIS. The ac component ratio of the superimposed voltage ranged from 30 to 70% of that of breakdown (BD) voltage when ac voltage was applied alone to each model, wherein conditions for superimposed phase or similar served as parameters. Subsequently, these results were evaluated from perspectives of an insulation coordination and GIS insulation test. As a result, the BD voltage showed results almost equivalent to those obtained in the case where the LI voltage was applied alone; even when the ac component of superimposed voltage was varied at three different insulation parts. Accordingly, within the 30 to 70% range of superimposed ac voltages, it emerged that the BD voltage was almost determined by the peak value of the applied voltage and was unaffected by the ac component. When comparing with GIS test voltages in the 66 kV to UHV-class, this range of 30 to 70% covers all the ratios of power-frequency and LI test voltages for operating voltages in an actual system. Consequently, the LI voltage test was considered adequate to verify the insulation performance of GIS against surge overvoltage generated during operation whereas superimposition of ac voltage need not be considered. These results are meaningful findings for rationalizing GIS while maintaining its insulation reliability.

K-factor value of SF<sub>6</sub> gas gap under non-homogeneous electric field for UHV-class gas-insulated switchgear

The lightning impulse voltage test waveform of power equipment is prescribed in the IEC 60060-1 High-voltage test techniques revised in 2010. This revision introduced an evaluation method by k-factor function (test voltage function), which converts an overshoot waveform to a test voltage waveform. At present, the test standard for UHVclass equipment is being studied and the authors have experimentally acquired k-factor values for gas-insulated switchgears, oil-immersed transformers and air insulation using the largest possible model assuming an actual UHV-class equipment to date. This paper reports the experimental result on the breakdown characteristics of lightning impulse voltage test waveforms for the SF6 gas insulation model under a nonhomogeneous electric field. The k-factor value was calculated by measuring the breakdown voltage and time using the superimposed oscillation frequency and the overshoot rate as parameters. The change with respect to the superimposed frequency of this experiment was similar to that of the existing k-factor function, and the effectiveness of evaluation by the k-factor function was confirmed.

Investigations of Copper Chrome Coatings on Vacuum Circuit Breaker Ceramics by Electron Probe Microanalysis and Electric Field Simulation

Vacuum interrupters (VIs) are designed for a long operating life. Even after a large number of switching operations, they should still provide good arc extinction and be able to withstand high voltages. Switching operations produce substantial masses of metal vapor that condense on the inner component parts of the VI. The VIs are equipped with shields to protect the ceramic surfaces from being coated by metal vapor. Since the first VI was put into service, a large number of shield designs have been developed. If the shield design consists of a floating shield with two end shields, there are two gaps between these shields. These gaps are necessary for insulation when the VI is in the open position, but during the switching operation metal vapor can flow through these gaps and coat the ceramics. If the deposited area on the alumina ceramics reaches a critical value, the dielectric performance of the VI can become reduced. Standard VIs have been tested with a floating shield and two end shields that did not overlap the floating shield. After performing switching operations, the coated ceramics were examined. Electron probe microanalysis (EPMA) was used here to obtain a very detailed view of the thickness of the deposited layers. With EPMA, it is possible to detect very thin layers in the range of nanometers. To provide a better understanding of how strongly different deposited layers can affect the dielectric performance of the VI, surface resistance measurements were carried out. These were performed with a high-resistance meter, which is able to measure up to $10^{16}~\Omega$ using a 1-kV voltage source. Electric field simulation was carried out to know how the coatings do affect the electric field inside the VI and why they can cause breakdowns during lightning impulse voltage tests.

K-Factor Value and Front-Time-Related Characteristics in Negative Polarity Lightning Impulse Test for UHV-Class Air Insulation

In IEC-60060-1 as revised in 2010, a conversion method for lightning impulse voltage test waveforms using the test voltage function (K-factor function) was adopted. Presently, studies are being conducted on the test standards for ultra-high voltage (UHV)-class equipment. Since the existing K-factor function was established based on the experimental results for more small models compared with the insulation structure of UHV-class equipment, the issue has arisen as to whether this K-factor function can also be applied for UHV-class equipment. The authors experimentally obtained K-factor values using the largest possible model assuming UHV-class equipment in the previous studies. This paper reports the K-factor values and the insulation characteristics with respect to the front time obtained for air insulation under a negative-polarity lightning impulse voltage. According to the results of this experiment, even though the K-factor values for an air gap were likely to be slightly lower than the existing K-factor function in the negative polarity, the tendency of the change with respect to the superimposed oscillation frequency was similar. Furthermore, it was found that even though the breakdown voltage for waveforms with an extended front time was likely to decrease further than that for the standard lightning impulse waveforms, the decline was about 3.1% even for a front time of 4.8 nμ s. Consequently, it was indicated that a lightning impulse voltage test waveform with an extended front time may help properly verify the insulation performance of a UHV-class air bushing.

Study on the K-factor function in the lightning impulse test for UHV-class electric power equipment

Ultra high voltage (UHV) systems are increasingly being planned and constructed, hence studies are promoted on the standard for high-voltage test techniques for UHV-class equipment. For the lightning impulse voltage test, a study is being conducted on the application of a method of evaluating the test waveform through conversion using the test voltage function (K-factor function) that was adopted in IEC 60060-1. The existing K-factor function was established based on the experimental results for more compact models, as compared with the insulating structure of UHV-class equipment, mainly with a breakdown voltage of about 100 kV. To determine whether this K-factor function can also be used for the test of UHV-class equipment, the experimental results for large-sized models were needed. To address this issue, the authors experimentally obtained K-factor values using the largest possible model (UHV model) assuming UHV-class equipment in the previous studies. In the present paper, a study was conducted on a new K-factor function based on these experimental results using UHV models. Initially in the study, several ideas for the K-factor function were shown and applied to various waveforms to clarify their advantages and disadvantages. Subsequently, in addition to these results, a study was conducted on a K-factor function suitable for UHV-class equipment with considering the actual UHV facilities. Consequently, it was concluded that the form of the function should be the same as that of the existing one but that it would be reasonable to adopt a relatively lower K-factor function for UHV-class equipment by revising the constant.

Comprehensive evaluation of the K-factor values in the lightning impulse voltage test techniques for UHV-class electric power equipment

In the 2010 revision of IEC-60060-1 "High-voltage test techniques", the conversion of waveforms using the test voltage function (K-factor) was introduced as one of the major revisions. At present, in IEC TC42 "High-voltage and high-current test techniques," a study is conducted on the standards for the lightning impulse voltage test at WG 19 "Adaptation of TC 42 standards to UHV test requirements." The present paper comprehensively evaluates the experimental results of the K-factor values using the largest possible model assuming UHV-class electric power equipment, as well as the past experimental results of the K-factor values. Initially, these experimental results were evaluated from a view point of insulation properties to clarify the characteristics of the Kfactor values. The adaptation itself of the K-factor evaluation method to also cover UHVclass equipment is considered significant. However, for high-voltage class equipment, since the formative time lag of breakdown is longer due to the long insulation length, resulting in a longer breakdown time, the K-factor values are considered lower than the existing K-factor function. Subsequently, the experimental results of the K-factor values were comprehensively evaluated and compared with the existing K-factor function. The K-factor function may possibly have to be reviewed for UHV-class electric power equipment to take lower values than the existing K-factor function.