On-site and positioning principles based on oscillating wave detection technology

Summary:

With the advancement of urbanization and industrialization, the number of power cables in distribution networks is increasing, so the reliability of power cables has attracted more and more attention. As an effective means of detecting and locating cable insulation defects, oscillating wave detection technology is favored by many power companies and large power-consuming enterprises. In recent years, as the field application of oscillatory wave detection technology has become more and more widespread, research on its effectiveness has attracted more and more attention. Now by analyzing the principles of cable oscillating wave detection technology, studying the relevant standards of oscillating wave detection technology, statistics and research on oscillating wave test data, and combining with the current hot spots in oscillating wave testing, we summarize some problems encountered in field testing of cable oscillating waves. and the causes of the problems, and also gave suggestions for solving on-site test problems.

0 Preface

In the 1990s, Professor Edward Gulski of Delft University of Technology in the Netherlands invented the oscillating wave cable detection technology. Later, the Swiss Seitz Company developed a mature product (Onsite) for on-site detection and location of cable insulation defects. The voltage level includes 6 kV and above, the current highest test capability is the oscillatory wave test on 220 kV cables. The oscillating wave detection technology was first called OWTS (Oscillating Wave Test System), and now it is mostly called DAC (Damped Alternating Current Voltage). In 2006, oscillatory wave detection technology was first applied in Beijing Electric Power Company [1]. Since then, it has experienced nearly 15 years of development in China [2-15]. From the initial major power guarantee, pilot and Electric Power Research Institute tests, to 2016 The power industry standard "6 kV ~ 35 kV cable oscillatory wave partial discharge test method" (DL/T 1576-2016) [16] was formed, and in 2018, the State Grid Corporation of China used oscillatory wave partial discharge detection technology as cable insulation defect detection The method is included in the "Distribution Cable Line Test Procedures" (Q/GDW 11838-2018) [17], and the role of oscillating wave partial discharge detection technology in the detection and location of power cable insulation defects is increasingly recognized.

At present, oscillating wave technology is receiving increasing attention in China, especially in the State Grid Corporation of China, where it has been vigorously promoted and applied. As a result, a large amount of on-site testing data and cases have been accumulated, relatively detailed on-site testing procedures have been formed, and a large number of oscillating wave technology has been cultivated. Partial discharge detection technical talents.

This article mainly analyzes the field application characteristics of oscillatory wave partial discharge detection technology based on the field testing of oscillating wave detection technology. It also discusses solutions to the problems encountered on site and puts forward some suggestions for field testing.

1 Oscillating wave detection technology and its positioning principle

Insulation defects in power cables will produce partial discharges under the action of electric field stress. The voltage traveling waves excited by the partial discharges propagate along the cable to both ends. By measuring this pulse, the partial discharge signals in the cable can be measured and the partial discharges can be evaluated. The strength and statistical characteristics of the signal help determine the location and severity of power cable insulation defects.

Cable oscillating wave detection technology, that is, DC (or AC) charging is performed at one end of the cable. After reaching the test voltage (for the voltage charging process, please refer to DL/T 1576-2016 or Q/GDW 11838-2018), oscillation is achieved by quickly closing the switch. The resonance between the internal inductance and cable capacitance of the wave test system. During this resonance process, the pulse voltage signal generated by the partial discharge is obtained through the coupling capacitance of the high-voltage test terminal. According to the propagation principle of the voltage traveling wave in the cable, the propagation principle of the voltage traveling wave is used. The time difference between the incident wave and the reflected wave realizes the location of cable insulation defects, as shown in Equation (1):

Through the above process, multiple insulation defects in the cable can be discovered and located at the same time during one pressurization process (Figure 1), and then partial discharges are detected according to relevant standards, such as DL/T 1576-2016 or Q/GDW 11838-2018 The basis for judging the severity determines whether to repair and replace the cable defect, test again, or continue operation.

The typical time domain diagram and positioning principle of the cable oscillating wave test system are shown in Figures 2 and 3.

2 Oscillating wave detection technology in field application

Problems encountered

Cable oscillatory wave partial discharge detection technology can locate multiple insulation defects in the cable in one test, and based on the obtained partial discharge signal characteristics, combined with standards and personal experience, determine whether the cable defects need to be repaired, tested again, or continued to be used. .

In practical applications, the detection effect and positioning accuracy of cable oscillating wave partial discharge testing technology are affected by some factors. This section lists these factors and roughly analyzes their reasons. According to the survey results, the problems encountered during the field test roughly related to the application scope of oscillating wave detection technology, positioning accuracy and sensitivity, and effective test distance.

2.1 Application scope of oscillating wave detection technology

At present, oscillating wave detection technology is mainly used for cable partial discharge detection in China, but in fact, according to the introduction of IEEE Std 400.4TM-2015 [18], cable oscillating wave detection technology can also be used for cable withstand voltage test, which needs to be at the highest test voltage. Carry out 50 times of stimulation. The advantage of the oscillating wave withstand voltage is that it does not require high power supply, because the oscillating wave voltage boosting is DC (or AC) charging of the cable, that is, under the highest test load of the test system, the time for boosting to the specified voltage is related to the length of the cable. , and the required power power is not high. Since there is no relevant test of oscillatory wave withstand voltage in China and there is a lack of corresponding data support and cases, the cable withstand voltage test based on oscillatory waves has not been adopted in China.

2.2 Positioning accuracy and sensitivity

The positioning accuracy of the oscillatory wave test system can theoretically reach 1% of the cable length or ±1 m. However, in actual testing, the positioning accuracy is affected by factors such as the accuracy of the cable length value, partial discharge pulse propagation speed, and reflected wave waveform distortion. There is a certain deviation in accuracy, and some may have a deviation of more than 10 m.

Sensitivity refers to the smallest partial discharge that can be measured. The sensitivity of field tests is affected by background noise, calibration and the characteristics of the cable itself.

2.3 Effective test distance

The test distance of the oscillatory wave test system, taking single-ended test as an example, is mostly nominally 10 km, but in fact it often does not reach 10 km, sometimes even only 2 to 3 km. This is mainly due to the influence of cable characteristics on voltage traveling waves. effects, background noise, etc.

3 Suggestions for solving field testing problems

This section combines the problems encountered in the field test of oscillatory waves in the previous section to discuss ideas for solving the problems.

3.1 Positioning accuracy and sensitivity

The positioning accuracy depends on the cable length and the accuracy of the oscillating wave propagation speed. However, the exact length of the cable is often not known on site, and the wave speed is often an empirical value (168~172 m/μs), so when a defect is located, it is Look for the intermediate joint near this location as soon as possible, because the failure rate of the intermediate joint is relatively high, and the discharge amount of the intermediate joint is generally large. At this time, it can basically be determined that the problem is with the intermediate joint.

When there is no intermediate joint, there may be a cable body defect, and this local defect is often caused by external stress during construction or later operation (note: the cause is a defect. If it is a fault, it is not within the scope of this article). This kind of local defect is often caused by external stress during construction or later operation. If the situation requires a careful inspection of the external integrity of the cable, the defect can be found based on the relative position of the defect point to the middle joint.

3.2 Effective test distance

Effective testing distance is generally understood as the maximum cable length that can locate insulation defects. From the perspective of oscillatory wave test system indicators, there are two main parameters related to the maximum distance: the maximum test distance (that is, the maximum load, in μF) and the maximum positioning distance.

The maximum test distance is the pressurizing capacity of the oscillating wave system. The maximum test voltage required by the standard can be applied to the length of cable. It is related to the power supply capacity of the oscillating wave system and the capacitance of the cable. This parameter is clearly defined in the oscillating wave test system. Mark out.