A novel wavelet transform technique for on-line partial discharge measurements. 1. WT de-noising algorithm

Medium and high voltage power cables are widely used in the electrical industry with substantial growth over the last 20-30 years ago, particular in the use of XLPE insulated systems. Ageing of the cable insulation is becoming an increasing problem that requires development of reliable methods for on-line condition assessment. For insulation condition assessment of MV and HV cables, partial discharge (PD) monitoring is one of the most effective techniques. However on-site and on-line PD measurements are affected by electromagnetic interference (EMI) that makes sensitive PD detection very difficult, if not impossible. This paper describes implementation of wavelet transform techniques to reject noise from on-line partial discharge measurements on cables. A new wavelet threshold determination method is proposed with the technique. With implementation of this novel de-noising method, PD measurement sensitivity has been greatly improved. In addition, a full AC cycle data recovery can be achieved instead of focusing only on recovering individual PD pulses. Other wavelet threshold de-noising methods are discussed and examined under a noisy environment to compare their performance with the new method proposed here. The method described here has been found to be superior to the other wavelet-based methods     

Selectivity of damped AC (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded MV cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan /spl delta/ measurements. Damped AC voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous AC voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan /spl delta/ measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded MV cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.     

The selection of the frequency range for high-voltage on-sitetesting of extruded insulation cable systems

High-voltage on-site tests of extruded insulation cables are necessary to check the quality of the installation of the system. The test voltage should simulate the stress under service conditions, and generate the same failure mechanism. With respect to an optimum design of test systems, a frequency range of 20 to 300 Hz, proposed in the latest IEC Draft for on-site testing of extruded insulation cables, seems to be reasonable. The test results on model cables can be used for real cables concerning the influence of the waveform on the breakdown voltage. The withstand voltage decreases with increasing frequency. The withstand voltage and the electrical breakdown field strength are very close together for frequencies between 20 Hz to 300 Hz. Mechanical defects, as well as water trees, reduce the breakdown voltage at 0.1 Hz more than at 20 to 300 Hz, but the absolute test voltage values are higher, and the breakdown mechanism is different compared with voltages of power frequency or adjacent frequencies. For on-site tests, frequency-tuned resonant test systems (ACRF) can be recommended because they have a very good weight-to-test power ratio, and very low power demands     

On-line partial discharge detection in cables

This paper describes an on-line PD (partial discharge) detection technique applicable to power cables. The method can be applied to both wire screen and solid shielded cables. In wire screen shielded cables, PD components with frequencies <100 MHz can be detected at distances over 150 m from the source, PD activity with levels as low as 20 pC can be detected within 100 m. However, in the case of solid shielded cables, PD levels of ⩾50 pC can be measured within 60 m in the frequency range <50 MHz. This method also can be utilized to check cable accessories such as splices and terminations. PD pulses with frequencies to ~300 MHz can be measured near the joints and terminations. The suitability and the sensitivity of the VHF technique were checked against two other alternative PD detecting techniques (pulse phase analyzer and high speed digital oscilloscope) by performing PD measurements in standard air gap and defects induced in EPR-insulated cable under laboratory-controlled conditions. The suitability and sensitivity of the VHF method were as good as the two alternative PD detection techniques     

空充变压器时直流线路沿线50 Hz分量分布的定量计算研究

空充变压器可能在邻近运行直流线路上产生电压和电流的50 Hz分量,存在引发50 Hz保护动作而导致直流降功率的风险。针对空充变压器时直流线路沿线50 Hz分量的分布展开研究,为进一步制定合理的应对措施提供指导。分析了运行直流线路产生50 Hz分量的机理,并提出基于测试信号法的直流线路入口端谐波阻抗计算方法。在此基础上,研究了在不同运行方式下,空充变压器时运行直流线路沿线50 Hz分量的定量计算方法。以牛从直流的PSCAD/EMTDC仿真模型为例,利用上述方法进行计算,并将计算结果与仿真结果进行对比。分析结果表明,二者具有较好的一致性,这验证了所提定量计算方法的准确性和有效性。     

Advanced solution for on-site diagnosis of distribution power cables

This paper describes important aspects of partial discharge (PD) diagnostics of distribution power cables. PD parameter were discussed under consideration of attenuation and dispersive effects of cable characteristics. Field experiences with PD diagnosis on paper insulated cables (PILC) and cables with cross linked polyethylen insulation (XLPE) by energizing with Damped AC Voltage demonstrate relevant demands and the effective outcome for asset management decision support. The main difficulty is to evaluate the risc of PD occurrences on the reliability of the cable system. If an sufficient amount of PD diagnostic data for the cable components is available statistical methods can be used for determing threshold levels and relevant condition indexes for the asset management.     

BPD型变频电源用于局放和耐压试验的研究与实践

介绍了某供电公司对主变局放和谐振耐压试验电源的研究和选用思路,简要介绍了局放和耐压试验现场实施情况,证实了BPD型变频电源不仅可以用于GIS及电缆等大容量设备的谐振耐压试验,而且可以用于变压器的局部放电试验。     

An expert system for XPLE cable deterioration diagnosis and life forecasting

The recent increase in the demand for power in urban areas has caused changes in the way power is supplied. Underground distribution systems have been introduced for convenience in urban areas and to prevent injuries in case of a crisis. As a result, many new types of power cables are being used. One type, XPLE cables (also called CV cables) have been rapidly taking the place of conventional cables such as OF or SL cables since the 1960s because of their relatively easy installation and maintainability. However, distribution cables used under severe environmental conditions deteriorate rapidly (due mainly to water treeing), and some cables experience insulation breakdown. Therefore, many diagnostic methods have been developed, though they have not been very effective. By combining methods, a high reliability can be achieved in comparison to the use of a single method. However, processing the data is difficult for on-site personnel, so many experts are required. But there is a lack of qualified experts, hence we have turned to the development of expert systems.     

同相供电设备有功功率差动保护

由于隔离变压器的副边额定电流很大,且难以选择电流互感器,因此隔离变压器很难设置灵敏的电流差动保护。为了给隔离变压器、隔离变压器与潮流控制器之间的连接电缆提供灵敏的保护,并给潮流控制器提供灵敏的远后备保护,文中从理论上分析了同相供电设备的基波输入有功功率与输出有功功率相等这一特性,并基于这一特性提出了同相供电设备有功功率差动保护方案,进而对有功功率差动保护在隔离变压器空载投入、潮流控制器启动、正常运行、外部故障、内部故障等各种工况下的动作行为进行了分析。文中还对同相供电设备有功功率差动保护的整定原则作了讨论,并给出了整定实例。实验室测试和工程现场运行测试结果表明了有功功率差动保护的正确性。     

A new partial discharge detection method for live UHV/EHV cable joints

Partial discharge (PD) detection on live UHVEHV power cables and accessories is one of the most important techniques to prevent cable systems from serious difficulties caused by slight errors in joining the cables. A new PD detecting method is proposed in this paper. A resonance-type partial discharge (REDI) sensor is developed for detecting PD occurring inside the joints of live XLPE insulated cables. This sensor picks up the high-frequency components of PD current pulses on the surface of live cables, hence the output signals can clearly be observed at the noiseless area (for example, 10-60 MHZ). This new method was applied to the initial ac breakdown tests with three different types of joints of UHV/EHV power cables. The PD pulse sequence was detected before their breakdown in all cases. The charge quantity and the number of pulses increased tremendously just before the breakdown. In addition, the phase of PD pulse against the applied voltage (?-q characteristics) changes with time or with the progress of deterioration. According to these experimental results, the REDI sensor can be applied to the insulation diagnosis of live power cable joints.     

变压器套管末屏对现场局部放电测量影响分析

介绍了使用变频电源进行现场局部放电测量的方法,分析了套管末屏对现场局部放电测量的影响,并提出了处理方法.     

变压器套管末屏对现场局部放电测量影响的实例与分析

介绍了使用变频电源进行现场局部放电测量的基本方法,总结了现场试验中的干扰类型及排除措施,结合2次现场测量实例,介绍了干扰排除过程及思路,分析了套管末屏接地不良对现场局部放电测量的影响,总结了此类影响的特点及处理方法。该方法对变压器现场试验、保证电力设备安全稳定运行具有一定的借鉴意义     

Ultra-wide-band PD diagnostics of power cable terminations in service

During the past decade the PD probing in the ultra-high frequency (UHF) range has successfully been proven for on-site PD diagnostics of GIS cable terminations in service. Therefore it was decided to apply this non-conventional method also for condition assessment of the insulation integrity of outdoor cable terminations. In order to evaluate the minimum detectable PD level the paper deals with comparative PD studies using the IEC and the UHF method. Moreover, experimental studies on the attenuation of PD pulses traveling through power cables were performed in order to assess the spatial selectivity of the UHF method applied.     

Discrimination between internal PD and other pulses usingdirectional coupling sensors on HV cable systems

On-site partial discharge (PD) measurement is required to ensure proper installation of extra high voltage (EHV) cable systems accessories. To achieve high sensitivity and good localization, two problems have to be overcome. First, the strong high frequency in long XLPE cables requires that the sensors be located along the cable, preferably directly at the accessories. Secondly, the detection system must be able to distinguish internal PD from other pulses. This paper describes a solution based on directional coupling sensors and a data visualization system, which displays phase-amplitude diagrams for individual PD sources which are identified by the direction of pulse propagation. It has been applied to on-site measurements, type and routine testing of HV cable joints and stress cones. Due to the reliable discrimination between internal PD from the accessory measured and from other pulses, testing can be done in unshielded rooms even using terminations with internal PD and corona. The method works independently well on line voltage, resonance sources, oscillating voltages and 0.1 Hz cosine-square voltage. It has been used to verify the cable accessories installed in the 6.3 km long 380 kV cable system in Berlin, Germany     

乙丙橡胶在我国中高压电缆中的应用

简介了乙丙橡胶在国内外中高压电缆中应用的情况,论述了中高压乙丙橡胶电缆料的设计和生产的关键,以及国内自主开发的乙丙橡胶电缆料在中压橡胶绝缘电缆上的应用。     

便携式在线局部放电测量技术经济有效的绝缘诊断方法

9年来，新西兰工业研究公司(IRL NZ)开发并广泛地在电力工业中应用了快速、非侵入、便携式在线局部放电(局放)测量技术^[1-2]。该技术为电力系统中日益增多的超龄老化设备提供了一经济有效的绝缘诊断工具，使工业界实施预防性维护，保护其重要设备。根据其绝缘诊断，合理地决定是否对价值百万的设备进行修理或更换。该技术广泛地用于电机、电缆、特别是户内开关母线及变压器绝缘诊断与监测。介绍和讨论了它的使用原理,经济效益和许多现场运用的结果。该便携式在线局放测量技术的优越性和其非侵入局放传感器用于各种设备的位置示例于这些现场实例中。该技术已由华能时代科技有限公司引进，正在电力系统推广应用。     

Cost-effective on-line partial discharge of measurements for cables

On-line partial discharge (PD) measurements are an efficient method for detecting certain insulation defects of cables. These include defects in cable joints and terminations, and defects due to mechanical impact. Noninvasive, efficient, and widely applicable, portable on-line PD measurement techniques are strongly favored by the electricity industry. Three on-site PD measurements for cables are described in this article to illustrate how the techniques achieve cost-effective insulation diagnosis to protect the main assets.     

Mitigation of transformer-energizing inrush current using grid-connected photovoltaic system

This paper presents a novel technique for suppressing the transformer-energizing inrush current. This technique is based on existing of a Photovoltaic (PV) generation system. Large magnitude inrush current can occur with certain combination of point of wave energization and residual core flux. The PV system is exploited to produce a magnetic flux in the core of the energized transformer in a negative direction of that produced by the main grid during grid-off. In this paper, the wave-energizing instant is optimally chosen and hence the amount of the residual flux existing in the core is controlled to be ready to sink the energizing effect. The impact of existing of PV system that connected to 280 kV, 60 Hz grid is studied at different energizing instances, different power ratings and different solar irradiances. The method is illustrated by simulation results and validated by harmonic analysis. The optimum energizing instances are explored at different working circumstances. The results at transient and steady states verify that the proposed technique enables the minimization of the inrush current by optimized grid-switching instance.     

Comparative analysisof grounding systems formed by MV cable lines with either uninsulated or insulated metal sheath(s)

It is well known that medium-voltage (MV) paper-insulated cables essentially reduce the potentials appearing during a ground fault in substations located in urban surroundings. This is explained by the fact that these cables usually (normally) have an uncoated metallic sheath. Due to this, MV cable lines leaving a high-voltage/middle-voltage (HV/MV) substation act as long external grounding electrodes and spontaneously form one very large and complex grounding system around this substation. Cables with an insulated metallic sheath however, are progressively being more applied in contemporary power distribution practice. Consequently, the following quite logical question arises: how will a cable with such constructive characteristic influence the general conditions for solving the grounding problem of distribution substations, especially of those working in high-voltage (HV) networks with directly grounded neutral(s)? The analysis performed in this paper shows that, when applied these cables spontaneously form a grounding system which also exhibits excellent grounding performances.     

In-service methods for distinguishing the dc component due to water treeing from noise currents in xlpe cable

This paper describes the water tree diagnoses of 6.6-kV class XLPE cables by the dc component method. The dc current component is measured by energizing the cable with ac high voltage, therefore, it would be suitable for on-line diagnoses. The dc component is considered to be caused by the rectifying effect of water trees inside the insulation. In laboratory tests, the dc component showed a good correlation with the degradation. However, almost no correlation was seen in the on-site measurement. The authors found that the cause of this bad correlation is the galvanic effect inside the jacket. This generates the noise current through the grounding circuit when the jacket resistivity is low. Based on this finding, the equivalent circuit of on-site measurement was proposed. Considering the equivalent circuit, this paper proposes nine methods for discriminating the dc component due to water tree from noise current. Also described are the analytical results of each method. Furthermore, the on-site or laboratory tests show that some of them are effective in separating the dc component due to water tree from noise current.