AC Test Voltage Research Articles

Design Studies of VSC HVDC Converter According to AC Voltage Tests

Since high-voltage direct current (HVDC) systems are very expensive and operationally critical, these systems must be tested before they are put into service. Insulation and performance tests are the two main subjects of these tests. AC voltage tests, as part of the insulation tests, should be performed after system installation is complete and before commissioning. However, in this study, the objective was to perform these tests during the prototype phase of VSC HVDC. Unlike other studies, this study attempted to use COMSOL Multiphysics to determine in advance the problems that may occur in the real system. In this regard, the busbars connecting the submodules of the VSC HVDC system were first modeled in 3D, and the tests to be performed were simulated using COMSOL Multiphysics software. During the simulation, the finite element method (FEM) was used to identify critical points that could cause partial discharge. To validate the simulation results, partial discharge tests on a real system were conducted, and the design changes made in response to each test result were explained. After the improvement actions, the targeted partial discharge values were achieved.

Influence of the Cable Accessories Installing Method on the Partial Discharge Activity in Medium Voltage Cables

This article proposes a method to modify the construction of a medium voltage (MV) heat shrinkable cable termination in cases of atypical damage to the shields of cross-linked polyethylene (XLPE) insulated cables. The proposed solutions include a modified method of assembling electric field control coating. An attempt was made to check the effect of such damage to the shields of MV cables with XLPE insulation on the level of occurrence of partial discharges within the cable termination. The investigations included testing the XRUHAKXS 1 × 240/25 cable type using the electric method (ME) and high frequency (HF) method with sinusoidal AC test voltage. As a result of the measurements, the values of total charges in the period and phase-resolved partial discharge (PRPD) patterns were obtained. The presented experimental results show the influence of the damage of the semiconducting coating surface on the occurrence of a defect in the cable termination without a modified method of control mantissa pinning. We suggest new methods of assembling MV cable accessories in the case of the presented coating damage in MV cable insulation.

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.

Analysis of FXS (Foreign Exchange Subscriber) Port on IPPBX (Internet Protocol Private Branch Exchange)

PBX is a telephone communication connection device that is commonly used in buildings that require a telephone connection. However, some PBX products cannot be directly connected to analog telephones, so they require a module called an S2 module. S2 module can converts the port on the PBX to an FXS port that can be connected to an analog phone. Apart from PBX, there is ITG which can be used as telephone line communication. However, ITG already has an FXS port so it does not require additional modules. In Indonesia, there are standard provisions for the use of analog telephone communication devices which have been regulated in SNI for analog telephones. This research examines analog telephone communication devices according to SNI in Indonesia. The results of this test show the TA400 and MyPBX Standard at the impedance value when the on-hook state gets an average value of 64 K?, when the off-hook condition averages a value of 343 ?, the AC voltage test when the call comes on average gets a value of 343 ?. a value of 60 VAC, which means that it meets the SNI standard.

AC Flashover Voltage Model for Polluted Suspension Insulators and an Experimental Investigation in Salt Fog

In recent years, the electrical performance of polluted suspension insulators operating in coastal foggy regions has attracted wide attention because of the occurrence of flashover. Surface wetting by salt fog accretion increases the surface conductivity and degrades the flashover voltage of insulators. Normally, the effect of conductivity under fog condition and pre-contamination on the flashover of polluted insulators are analyzed separately. Therefore, this does not reveal the flashover characteristic in salt fog, which requires further research. This paper describes salt fog experiments with an AC test voltage conducted using suspension insulators in a 110-kV power system. The electrical strength performance of insulators with salt fog treatments is analyzed, and the effects of salt deposit density (SDD) and the fog water conductivity (FWC) on the flashover of three types of suspension insulators are studied. The concept of additional SDD is proposed to quantify analyze the effects of salt fog. The results show that there is less dependence on FWC at higher pollution levels than at lower pollution levels. That is, the effect of FWC on 50% AC flashover voltage ($U_{50}$ ) can be neglected for SDD >0.15 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , whereas FWC has a strong effect on $U_{50}$ for SDD <; 0.15 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . FWC has a greater influence on $U_{50}$ of ceramic insulators than on that of silicone rubber insulator. A model for calculating the critical flashover voltage of insulators is proposed by considering the combined effect of FWC and pre-polluting SDD.

Design and application of the automatic AC voltage test device for smart electricity meters and data acquire terminal of the special transformer

The AC voltage test is an important means to effectively detect the insulation defects of the equipment. The problems of low efficiency, poor safety, and poor compatibility of the AC voltage test for smart electricity meters and data acquire terminal of the special transformer, the fully automatic AC voltage test device are studied and designed by software programming, PLC control technology, and automatic crimping technology. The automatic test and result judgment of the AC voltage test for the smart electricity meters and data acquire terminal of the special transformer are realised, which makes many problems of the existing test equipment to be effectively solved. Thus, the efficiency, compatibility, safety and reliability of the test are improved.

Characterization of Silicone Rubber Degradation Under Salt-Fog Environment With AC Test Voltage

In recent years, some composite insulators operating in coastal foggy areas have shown different levels of degradation phenomena. This paper presents several performances and properties of silicone rubber under the salt-fog environment with AC voltage. Analysis conducted after salt-fog treatment by techniques, such as static contact angle, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), dielectric parameter, and flashover voltage show a significant degradation of samples. The absorption peak of the hydrophobic groups (Si-(CH3)2 and Si-CH3) decreased, the surface of the material became rough, and the dielectric and electrical properties were also deteriorated. Due to the thermal effects and impact of high-energy particles, the molecular chains on the surface of silicone rubber were broken, and the smooth surface structure was destroyed, resulting in the formation of hydrophilic chemical bonds and precipitation of inorganic substances. The intrusion of moisture further reduces the dielectric parameters and the electrical strength of the material. In addition, the samples in the salt-fog environment with a greater conductivity are more degraded, indicating that the salt-fog has an aggravating effect on the aging of the silicone rubber. Thus, it is recommended to pay attention to the monitoring and examining the work of composite insulators operating in a foggy and high humidity environment.

Enhancement of the Insulation Properties of Poly(2-oxazoline)-co-Polyester Networks by the Addition of Nanofillers.

Copoly(2-nonyl-2-oxazoline)-stat-poly(2-dec-9'enyl-2-oxazoline)s can be crosslinked by the thiol-ene reaction with glycol dimercaptoacetate. The copoly(2-oxazoline)-stat-copolyester is tested as dielectric for high-voltage applications, either as unfilled resin or as composite with nanoscaled fillers of silica, alumina, and hexagonal boron nitride. During AC voltage tests, all materials have an average breakdown strength of 45-50 kV mm-1 . For DC voltage tests, samples with SiO2 (hBN) have an average breakdown strength of ≈100 (80) kV mm-1 , while the unfilled copoly(2-oxazoline) has an average breakdown strength of ≈60 kV mm-1 . Permittivity measurements at 20 °C and 50 Hz reveal that all nanocomposites are dielectrics (D = 0.06-0.08), while the unfilled copoly(2-oxazoline)s has a high loss factor of D = 8.43. This phenomenon can be retraced to the phase separation in the crosslinked copolymer, the M-OH functionality of silica and alumina particles, and models of polymer-particle interactions such as the Tanaka model, revealing that the nanofillers reduce the interfacial and dipolar polarizability.

Application and Analysis on Frequency Conversion Series Resonance Technology in 220kV Cable Withstand Voltage Testing

AC withstand voltage test is the most severe, the most direct and the most effective test method of judging the XLPE Power Cable insulation Conditioning, It is of decisive significance in determining weather the Power Cable is measured up. Traditional test transformer is bulky, it requires high-capacity of power supply when it is used in AC withstand voltage test of high-capacity Power Cable. This paper adopted the Frequency Conversion Series Resonance Technology in 220kV Cable Withstand Voltage test program and accomplished the test successfully, Greatly reduced the capacity of the power test equipment, it also helps to promote the test quality and the work efficiency, which proved the feasibility and effectiveness in the long-distance high-voltage XLPE cables grade AC voltage test with the Frequency Conversion Series Resonance Technology.

Investigation on insulation material morphological structure of 110 and 220 kV XLPE retired cables for reusing

Most of the 110 and 220 kV cables are in service till the end of life, but part of them would be out of service by refurbishment. This research is focused on evaluating the cable insulation and determining whether the cable can be reused. All the cables used in this research satisfactorily passed the AC voltage and partial discharge tests. Then the fourier transform infrared spectroscopy, thermogravimetry analysis, differential scanning calorimetry, dynamic mechanical analysis, and tensile machine were used to analyze the insulation morphological structure and performance of 20 retired 110 and 220 kV crosslinked polyethylene cables obtained from the Beijing and Zhuhai districts. The results show that the structure parameters which can represent the physical change and the irreversible chemical change of cables have obvious time dependence characteristic. DSC result shows that crystallinity, melting range, and crystallization rate increase slightly with service years. DMA experiment illustrates that the temperature of mechanical loss peak, Tγ, shifts to a higher temperature while the mechanical loss increases slightly and temperature dependency of storage modulus increases with service years. The electrical treeing experiment utilized cable operated for 17 years, which is the longest operation year for the samples. Compared with new cable, the cable operated for 17 years has a slight decrease in tree initiation time and an obvious increase in tree propagation time. These experiment data show that the insulation material morphological structure is mainly stable, and the test results are also a contribution to the lifetime expectancy evaluation of the 110 and 220 kV XLPE cable.

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.

Synchronous generator for induced AC voltage test of single-phase and three-phase power transformers

Synchronous generator for induced AC voltage test of single-phase and three-phase power transformers

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.

Partial discharge criterion in AC test of oil-immersed transformer and gas-filled transformer in terms of harmful partial discharge level and signal transmission rate

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

Research and development of UHV AC transformer and shunt reactor

SUMMARYThe Jingdongnan‐Nanyang‐Jingmen UHV AC Pilot Project has already been put into service in China. In this paper, insulation level of power transformer and shunt reactor for 1000 kV power transmission project in China are introduced. UHV AC transformer and shunt reactor structures are given. Key technologies, including cooling type, regulating method for UHV AC transformer, pattern of outgoing line, and noise controlling, are discussed. Besides, due to the high voltage class, high capacity, and super size of UHV power transformer, the size of its test circuit is correspondingly enlarged, the impacts of stray inductance and capacitance will be more intense consequentially, thus in factory test including lighting impulse test, switching impulse voltage test, short‐duration induced AC voltage test at 500 kV side, and long‐duration induced AC voltage test with partial discharge measurement are presented. Copyright © 2011 John Wiley &amp; Sons, Ltd.

170[kV] GIB 내에서 건조공기(Dry Air)의 절연파괴 특성

In this paper, we changed SF? insulation gases used already in the GIB(Gas Insulated Bus, gas insulated busbar) of switching appliances of power grid 170[㎸] GIS(Gas Insulated Switchgear, Gas Insulated Switchgear) with green insulation dry air gases and studied dielectric breakdown voltage characteristics by the AC voltage withstand test. Withstand test AC voltage applied to the standard KEPCO's 170[㎸] GIB with dry air insulation and and the equivalent of dry air and SF? gas were examined. Breakdown voltage of dry air, using an expression of the experiments were calculated and AC Withstand criterion of dry air insulation for the AC voltage test was derived. Using the criterion, dry air gases can be used instead of the SF? gas was confirmed in the factory acceptance test at 170[㎸] GIB.

A comparative study of insulator materials exposed to high voltage AC and DC surface discharges

An experimental study was conducted, using the Inclined Plane Test method to determine the performance of typical power line insulator materials: RTV (Room Temperature Vulcanized) silicone rubber coated porcelain, HTV (High Temperature Vulcanized) silicone rubber and EPDM rubber. The constant voltage method was employed to evaluate the performance of the above samples when energized by ac and dc (both polarities) voltages. The relative performance of the different materials and the effect of the voltage types were investigated. The results show that the RTV silicone rubber coating exhibits the least erosion for ac voltage but shows extensive erosion in the case of negative dc voltage. The HTV silicone rubbers exhibit only minimal erosion when exposed to ac test voltage, but develop strong erosion under positive dc voltage, which can result in sample failure. EPDM experiences the least erosion for a negative dc voltage, but shows maximum erosion severity for ac voltage.

An Advanced Stator Winding Insulation Quality Assessment Technique for Inverter-Fed Machines

The lifetime of the stator winding insulation is reduced when operated with pulsewidth-modulation inverters, since insulation degradation is accelerated due to increased thermal and electrical stresses on the motor. This paper focuses on a new approach for monitoring the condition of the stator insulation for failure-prone inverter-fed machines. The main concept of the proposed technique is to apply a dc and/or variable-frequency ac test voltage to the stator insulation using the inverter and to perform standard offline insulation tests whenever the motor is not operating. This allows the stator insulation to be tested more frequently as compared to offline tests performed once every three to six years during periodic maintenance. Two methods for testing the insulation with minimal modifications to the inverter circuit are proposed and verified experimentally. The results show that the proposed technique provides a simple low-cost solution for detecting stator insulation degradation at an early stage for reliable operation of inverter-fed machine systems, since the insulation condition can be monitored frequently. This does not only helps prevent forced outages and safety risks due to insulation breakdown but also helps perform maintenance in an efficient manner, since individual motor maintenance can be prioritized and scheduled based on its present condition.

Development of 22.9 kV high-temperature superconducting cable for KEPCO

Superconducting cable is expected to be one of the most economical ways to improve both the stability and capacity of electrical transmission and distribution lines, so several studies of high-temperature superconducting (HTS) cable have been initiated around the world. SEI was recently engaged by the Korea Electric Power Corporation (KEPCO) to install a 22.9kV–1250A HTS cable system, comprising a 100m HTS cable, two terminations and a cooling system, all manufactured in Japan and shipped to the KEPCO Gochang test site in Cheolabuk-Do, ROK (Republic of Korea). After system installation and assembly, the cable was cooled with liquid nitrogen. Following several checks, including measurements of critical current and heat loss, and AC voltage and current tests, a commissioning test saw the cable carry 1250A at 22.9kV for 48h, meeting all requirements. This report outlines the KEPCO HTS cable project, including system design, manufacture, installation and testing.

AC and switching impulse performance of an ice-covered metal oxide surge arrester

Artificial ice tests were conducted for the first time with a high voltage metal oxide surge arrester (MOA) consisting of two units in series. The behaviour of the MOA under glaze ice was studied with AC voltage and switching impulse current surges. The AC voltage test results showed that an unevenly ice-coated MOA may be thermally stressed due to the leakage current transition from the ice covering of one unit to the interior of another unit not covered with so much ice. Whether the stressed unit stands up to this situation or not depends on the thermal properties of the unit and the duration of the active arcing period. With impulse current surges, the residual voltage across an ice-covered unit may easily cause a flashover. This leads to a sudden increase in the surge current. With the MOA studied, the current peak was over twice the value of normal operation, which stressed the lower unit operating normally.