Withstand Voltage Test Research Articles

Advanced insulating materials contributing to “carbon neutrality”: Opportunities, issues and challenges

Advanced insulating materials contributing to “carbon neutrality”: Opportunities, issues and challenges

Designing HVDC GIS/GIL spacer to suppress charge accumulation

To suppress the charge accumulation on the spacer surface in direct current (DC) gas insulated transmission line (GIL) is an important and emergent issue for the development of a clean, safe and economic smart grid. A design method of the DC spacer is proposed, and a spacer prototype is prepared and evaluated both by simulation and type test. The DC withstand voltage test and polarity reversal test are performed using the new DC spacer compared with a commercialised 220 kV AC spacer. The simulation results indicate that the surface electric field and surface charge of the DC spacer are lower than those of the alternating current (AC) spacer under DC voltage. The test results verify that the surface flashover voltage of this DC spacer is higher than that of the AC spacer. The potential feasibility of the spacer design for HVDC is discussed. It is hoped that the content of this paper can bring new ideas in the development of HVDC gas insulated equipment.

Design of 66 kV dry-type step-up transformers for large capacity wind turbines

<p indent="0mm">The development of large-scale offshore wind power has become an inevitable trend in the current renewable energy development. Offshore wind power has moved from offshore to deep-sea, and the capacity of wind turbines has reached more than <sc>8 MW.</sc> The step-up transformer increases the output voltage of the wind turbine from <sc>0.69–1.4</sc> to <sc>35 kV</sc> and above, which is the key equipment of the wind power system. The deep-sea wind power is more than <sc>50 km</sc> away, and the voltage of the step-up transformer needs to be increased to <sc>66 kV</sc> and above. At present, the <sc>66 kV</sc> step-up transformer for offshore wind power generally adopts oil-immersed transformer technology, and <sc>66 kV</sc> step-up transformer with dry-type insulation is a blank product. Oil-immersed step-up transformers have the risk of oil leakage, which may cause fire or explosion accidents, while dry-type insulation step-up transformers have low fire risk, good explosion-proof performance, and new high reliability, and have significant technical advantages in offshore wind power applications. However, limited by the design method and manufacturing process technology, the traditional epoxy casting step-up transformer cannot solve the problem of excessive partial discharge under high voltage, which is caused by high-voltage winding interface defects, restricting the application of this technology in66 kV step-up transformers. This paper breaks through the technical limitations of traditional epoxy cast transformers, innovates from the insulation design mechanism, and proposes a new design method for a <sc>66 kV</sc>/10 MVA step-up transformer based on silicone rubber insulation. By analyzing the generation mechanism of the interface defect of the epoxy casting transformer’s high-voltage winding, a design method of high-voltage insulated shielding wires incorporating semiconducting shielding layers is proposed. The outer conductor of the wire is first wrapped with the semiconducting shielding layer, and then the insulating layer is wrapped. The semiconducting shielding layer and the insulating layer use same base materials. The semiconducting shielding layer can shield the conductor interface defects, the material of the semiconducting shielding layer is designed with the same base material of the insulating layer, so the two layers can be fully compatible, which can effectively suppress the interface defects. According to a certain winding form, the high-voltage insulated shielding wire is wound into a coil structure with a certain number of turns, then integrally cast silicone rubber insulation, forming the overall structure of the high-voltage winding. The high-voltage winding is designed with sheds structure, which increases the creepage distance along the surface, and realizes the high creepage insulation design with a compact height space. The <sc>66 kV</sc>/10 MVA step-up transformer structure is designed, taking into account factors such as insulation, electric field, magnetic field, heat dissipation, and structural strength, etc. The electric field, magnetic field and thermal field of the 66 kV/10 MVA step-up transformer under steady-state operating conditions were analyzed, the electric field distribution characteristics and maximum field strength parameters of the high-voltage winding were obtained, and the magnetic field distribution and thermal field distribution characteristics of the transformer were analyzed. The rationality of the step-up transformer design is verified by multiphysics analysis. The principle prototype of 66 kV transformers was developed, and AC withstand voltage and partial discharge tests were carried out. Under the test parameters of 55 kV/<sc>30 min,</sc> the partial discharge amount was less than 5 pC. The terminal-to-ground 120 kV/<sc>1 h</sc> withstand voltage test was carried out on the high-voltage winding, during the test, partial discharge amount is less than 5 pC, the performance is good, which verifies the feasibility of the design method.

AC Voltage Withstand Test Method and Risk Suppression Measures of Energized GIS Based on Phase-frequency Control

It is essential to carry out an AC voltage withstand test after GIS extension and maintenance. At present, the common methods of GIS AC withstand test include double isolation gap, reduce the test voltage, and narrow test range. These methods generally have some defects, such as complex test processes, all bus de-energized, some devices failed to test, unreliable test results, and so on. This paper presents a distinctive GIS AC voltage withstand test method. The method controls the phase-frequency (PFC) parameters of the test voltage so that it maintains the same phase-frequency relationships as the in-service bus voltage during the process of testing. At this moment, the GIS can carry out an energized AC voltage withstand test. The article introduces the process of the PFC AC withstand voltage test method in detail and demonstrates the feasibility of the method. The phase difference margin of the PFC method is optimized through simulation and numerical calculation, and a variety of protection measures are proposed to ensure the safety of the test process. The proposed PFC AC voltage withstand test method will not affect the in-service bus and has great practical application value for the field GIS AC voltage withstand test.

Study of Liquid Nitrogen Insulation Characteristics for Superconducting Transformers

With its great advantage in weight and volume, superconducting transformer is considered as potential candidate for the portable power station. To achieve a more compact design, the insulation properties of liquid nitrogen in real operation state is crucial. This paper firstly presents an AC withstand voltage test of liquid nitrogen under different kinds of electric field. Then the bubble influence on the macroscopic insulation performance of liquid nitrogen is analyzed based on finite element method. In addition, the effect of Sumoto effect at liquid surface is also studied by experiments. The results of this paper can be used in future design of compact superconducting transformers.

Partial discharge characteristics of free moving metal particles in gas insulated systems under DC and AC voltages

AbstractFree conductive particles in a gas‐insulated metal‐enclosed system produce partial discharges during movement, resulting in insulator flashover and insulation failures. This study focuses on the partial discharge property of free conductive particles under DC and AC voltages. The relationship between the micro‐discharge property and the intrinsic properties of the particles was obtained based on experimental tests. The results show that under DC conditions, the local discharge property varies linearly with particle size and density. The discharge probability of particles in SF6 is significantly reduced compared with that in air, while the discharge magnitude rapidly increases if the lift voltage exceeds a certain value. Under AC conditions, the partial discharge generated by the particle becomes less random, and the amplitude and phase angle of the discharge are not significantly related. As the size of the electrode decreases, the partial discharge current generated by the particles on the surface of the electrode with an uneven electric field increases significantly. This research provides a basis for the optimization of withstand voltage test method and its applications.

Insulation Design of the 160 kV DC Superconducting Fault Current Limiting Winding

Superconducting fault current limiter (SFCL) is a promising device for limiting the short-circuit current of the power system. A 160-kV SFCL is developed for protecting the voltage source converter-based high-voltage direct current (VSC-HVdc) transmission system in Nanao of China. The main insulation and longitudinal insulation design of the 160-kV superconducting fault current-limiting winding are presented, where the liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is used as the cooling and insulation medium. The main insulation simulation results show that the maximum electric field strength of the SFCL winding could be restricted to 1.2 kV/mm during the 252-kV dc voltage withstand test by using optimized grading and shielding ring. The SFCL winding is composed of 96 noninductive-type superconducting bifilar coils, and the turn-to-turn insulation of the bifilar coil is a combination of LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and polytetrafluoroethylene (PTFE). The turn-to-turn distance of the bifilar coil is optimized based on the longitudinal insulation analysis under the 550-kV lightning impulse withstand voltage test, 450-kV switching impulse withstand voltage test, and 16-kA power switching test. According to the simulation and experimental results, the maximum voltage drop occurs on the first turn of the bifilar coil.

Research on the discharge characteristics of XLPE insulated DC cables

This paper focuses on the discharge of cross-linked polyethylene (XLPE) insulated DC cables, which has become a concern of power utilities when carrying out a voltage withstand test or an outage maintenance. Based on the electrical tree initiation test of XLPE block samples under grounded DC voltages, the main cause and key factors for damages of DC insulation resulted by grounding discharge were investigated, and the allowable voltage level to start the accelerated discharge operation was estimated. By the laboratory tests of two XLPE cable samples discharged under different conditions, the basic discharge rules were observed, the equivalent circuit was set up, and the equipment parameters were determined. In addition, a method to estimate the discharge time for cable lines with different lengths was proposed. Finally, the on-site measured data of decay voltage and leakage current were presented for a 200 kV DC XLPE cable line which had been discharged steppedly through a resistance grounding device after the DC voltage withstand test, and the experimental results were found to be in good agreement with the theoretical data.

Breakdown Characteristics of Long Air Gap at Different Frequency Based on Series Resonance

In today’s rapid economic development, with the development of science and technology, higher requirements have been put forward in the economic operation of the system. This requires various departments to make more reasonable use of existing energy and equipment, and to ensure quality. Meet all aspects of supply with more energy-saving and more economical operation methods. Therefore, more and more attention has been paid to the research on the method of predicting the breakdown voltage of the air gap. Although domestic and foreign scholars have used power frequency test transformers to carry out a large number of long air gap experimental researches, the research on the breakdown voltage of long air gaps based on the principle of vector machines is still blank. In this paper, a vector machine test device is used to study the breakdown discharge characteristics of a long air gap at two frequencies of 229Hz and 68Hz. Comparing the test phenomena at different frequencies, it is found that the initial discharge voltage of the streamer corona is lower when the frequency is 229Hz, and the discharge phenomenon before breakdown is more intense, and the quality factor will be significantly reduced due to the violent discharge. At the same time, under the same test conditions, the breakdown voltage at 229Hz is lower than that at 68Hz, and as the air gap increases, the difference between the two is greater. The experimental results show that the breakdown voltage of the long air gap based on the vector machine is different when the frequency is different, and the safety distance of the field withstand voltage test should be evaluated according to the experimental data under the vector machine condition to ensure the safety of the test.

Comparative study of 0.1Hz very-low frequency withstand voltage test method and traditional method

The o.1Hz very-low frequency withstand voltage test method has the advantages of small equipment size and low power demand, and is used more and more in the power grid, especially in withstand voltage test. However, the effect of this voltage waveform and the traditional test waveform on the insulation assessment is still unclear. This paper compares the difference between 0.1Hz very-low frequency and DC voltage, resonance voltage and oscillating wave voltage in the withstand voltage test. Research indicates that the 0.1Hz very-low frequency withstand voltage test has good equivalence in cables with mechanical "faults", water tree defects in cables and power frequency, and relatively poor equivalence in terms of needle plate electrode insulation defects.

Research of PD transfer ratio analysis of converter transformer IPVD test based on distributed parameters

In the process of factory test of converter transformer, the withstand voltage test with partial discharge is usually used to judge whether the internal insulation structure is in good condition or not. If there is no partial discharge, the insulation is in good condition. There are tens of thousands of insulation materials in UHV converter transformer, and the winding structure is complex. If partial discharge is detected during the withstand voltage test, it is impossible to locate the discharge position quickly and accurately. In this study, by analysing the partial discharge signal transmission relationship in the withstand voltage test of converter transformer, combined with the partial discharge signal transmission model of converter transformer winding based on distributed parameters, the partial discharge area in the converter transformer winding is calculated and judged. This method can be applied to the IVPD test of converter transformer to assist in locating the partial discharge area in the winding, greatly shorten the time of fault diagnosis and location of converter transformer, and enhance the scientificity and effectiveness of the test.

Development of three-in-one equipment for withstand voltage, partial discharge and dielectric loss

Many major accidents in domestic and foreign power grids were caused by cable defects, causing huge economic losses and adverse social impacts. At present, the three main tests for cable testing are withstand voltage test, dielectric loss measurement and partial discharge measurement, which require different equipment for testing, which is low in efficiency and time-consuming. Among them, the very-low frequency method is suitable for the withstand voltage test, and the damped oscillatory wave method is suitable for the measurement of dielectric loss and partial discharge. This paper is based on very-low frequency cosine square wave and damped oscillatory wave technology, implements technology integration, and realizes the three-in-one detection of withstand voltage, dielectric loss, and partial discharge for cable.

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.

The Partial Discharge Field Test of 1000kV Shunt Reactors

Aiming at the insulation performance field diagnosis of 1000kV shunt reactors, the series resonant withstand voltage test equipment is developed. The test equipment mainly consists of the frequency conversion power supply, the excitation transformer, the series resonant capacitor, the wave blocking resistor and the voltage divider. The resonant frequency is between 30 and 300 Hz, the rated voltage is 1200kV, and the capacity is 1000 kW. During the test, the discharge is monitored at the high voltage bushing terminal and the 1000kV reactor insulation is analyzed. By using the developed test equipment, the 1000kV reactor in 1000kV Xuyi Station is tested, which verifies the validity of the on-site partial discharge measurement test.

Effect of wave oscillation on breakdown characteristics of oil-impregnated paper under lightning impulse voltage

Effect of the oscillation on the breakdown characteristics of the oil-impregnated paper under different lightning impulse voltage waves are studied experimentally in this work. IEC60060-3 standard and the waveform requirements of the on-site impulse withstand voltage test are considered, and four lightning impulse waves with different oscillation frequencies and 1.2/50μs standard lightning impulse wave are applied during the experiment. The experimental results show that the breakdown voltage under 1.2/50μs impulse is lower than that under the oscillating impulse. Under the oscillating waves, as the oscillating frequency increases, the number of breakdown points distributed at the peak of the non-first cycle increases, the minimum breakdown time gradually shortens, and the breakdown voltage shows a decreasing trend. The possible breakdown process of paper and the influence mechanism of different waveform parameters are discussed in detail. The explanation could be that changes on the impulse waveform parameters affect the movement, trap and accumulation of the positive charges ionized inside the oil micro-cavity, causing differences in the breakdown results. This study deepens the understanding of the breakdown mechanism of paper from the perspective of the waveform parameters effect and can provide a theoretical reference for the explanation of on-site impulse withstand voltage test results.

EFFECT OF ALUMINUM FOILS NUMBER AND ITS LENGTH IN IMPROVEMENT OF ELECTRIC FIELD DISTRIBUTION OF HIGH VOLTAGE CONDENSER BUSHING

High voltage condenser bushing is one of the important component that is widely used in the high voltage system. At high voltage levels more than 52kV the distribution of electric field in condenser bushing is irregular between the lead conductor and the grounded metallic flange. This paper studied the effects of changing in both: the number layers of aluminum foils and Oil Impregnated Paper (OIP), increasing the length of aluminum foils layers, and also increasing the thickness of OIP layer on the distribution of electric potential and electric field in condenser bushing by using Finite Element Method (FEM) and built the bushing model in ANSYS software. The harmonic analysis was performed of the bushing model at maximum value of withstand voltage test at 50Hz, from the analysis results are obtained the maximum value of electric field on the inner and outer surface of the bushing, the obtained electric field values were good and acceptable compared to the permissible electrical stress values of the dielectric insulators. This work can also aid in the design of high voltage bushing stress control, a knowledge of the electrical field distribution in bushing geometry. Moreover, the results of analysis are shown as contour plots, graphs plotted, and tables.

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.

Design and Tests of a 160-kV/1-kA DC Superconducting Fault Current Limiter

DC Superconducting fault current limiter (SFCL) can be used to reduce the fault current of voltage source converter-based high-voltage direct current (VSC-HVdc) transmission systems, thereby significantly reducing the manufacturing difficulty and cost of dc circuit breakers. The authors aim to develop a 160-kV/1-kA resistive type dc SFCL, and apply it in the VSC-HVdc transmission system in Nan'ao of China. In order to verify the feasibility of the transmission voltage dc SFCL, the research is divided into two stages, and the results of the first stage are presented. The rated voltage and current of the developed dc SFCL is 160 kV and 1 kA, respectively, and the current limiting resistance is no less than 2 Ω within 10 ms, which is 1/4 of the second stage prototype. The used superconducting material is stainless steel reinforced yttrium barium copper oxide (YBCO) conductor produced by Shanghai Superconductor. A three terminal structure is presented to meet the different operation parameters. A 160-kV dc SFCL prototype has been fabricated, and the prototype has passed the steady-state current test of 1 kA, the dc withstand voltage test of 252 kV within 1 min, the lightning impulse test of 550 kV/1.2 μs/50 μs, and the operating impulse test of 450 kV/250 μs/2500 μs. The average operating thermal loss of the device at rated current is 652 W. Power switching test results show that the developed SFCL could generate the desired limiting resistance of about 2.2 Ω in less than 10 ms, and the maximum fault current duration time could be as high as 60 ms.

Using a nickel electroplating deposition for strengthening microelectrochemical machining electrode insulation

This study focused on reducing the peeling phenomenon of electrode insulation layers during electrochemical microgroove machining. An epoxy resin insulation layer was applied to the electrode surface, and the tip of the electrode was ground to reveal bare metal for target machining. During electrochemical machining, the durability of insulation under various potentials and processing times were studied. Typically, a large number of hydrogen bubbles are generated at the exposed part of the electrode during processing. These bubbles increase the peeling speed of the insulation and reduce its service life. Therefore, this study proposed the use of electroplating deposition to form an electroplated layer on the exposed part of the electrode. The deposition rate of the electroplated layer and coating state of the electroplated layer at the tip of the electrode were studied. The withstand voltage test was performed under various electrochemical machining parameters to study the insulation and peeling effects. The surface morphology of microgrooves produced through electrochemical microgroove processing with and without an electroplated electrode was compared. The comparison revealed that the uniformity of the microgrooves processed by the electroplated electrode was better and the insulation of the electrode had lower cracked area than those produced without the electroplated electrode. The electrode was tested before and after processing to verify the protective effect of the coated layer. The result revealed that the electrode with a 0.5 mm thickness exhibited the slowest cracking rate, and the electrode with the electroplating time of 4 h exhibited the most efficient protective effect. Thus, the electroplated layer can effectively protect the insulation and prevent it from peeling off during the process, which increases its service life.

Design and performance tests of a 160 kV/1.0 kA DC superconducting fault current limiter

Superconducting fault current limiter (SFCL), which is used in voltage source converter based high voltage direct current (VSC-HVDC) transmission systems to limit fault currents and protect the grid together with DC circuit breakers, could improve system safety and stability. The first transmission voltage DC SFCL is developed in China, and it has been applied in the 160 kV VSC-HVDC transmission system in Nan’ao. The electromagnetic design as well as insulation analysis of the current limiting unit is presented, where the current limiting unit is composed of 96 bifilar coils made of stainless steel reinforced YBCO tapes. The liquid nitrogen at the temperature of 77 K is used as the cooling and insulation medium, and the liquid nitrogen is cooled by a closed-loop cryogenic system consists of 3 G-M refrigerators. The 160 kV DC SFCL prototype has been fabricated, and the prototype has passed the steady-state current test of 1 kA, the DC withstand voltage test of 252 kV within 1 min, the lightning impulse test of 550 kV/1.2 µs/50 µs and the operating impulse test of 450 kV/250 µs/2500 µs. Power switching test results show that the developed SFCL could generate the desired limiting resistance, and the maximum fault current duration time could be as high as 60 ms. The developed prototype has been installed in the Nan’ao VSC-HVDC transmission system, and put into operation on August 17, 2020.