Development and application of creepage distance measurement system for zinc oxide arrester

Arrester is used to protect various electrical equipment in the power system from lightning overvoltage, working overvoltage, power frequency transient overvoltage impact and damage. Zinc oxide arrester is widely used in power system. Creepage distance is a necessary item of arrester performance test. It is a necessary project for lightning arrester factory testing and centralized network access testing. At present, the measurement of arrester creepage distance mainly adopts the artificial method of paper tape measurement or wire measurement. In this paper, a measurement system for arrester creepage distance is developed by using three-dimensional reconstruction and laser vision detection, which realizes one-click and fully automatic detection of creepage distance. The field application results show that the system not only solves the problems of heavy workload and low measurement accuracy of manual measurement methods, but also displays the measurement results in real time, which significantly improves the reliability and flexibility of measurement work, shortens the measurement time and improves the work efficiency.


Introduction
Zinc oxide arrester is the most advanced impulse overvoltage protector in the world [1][2][3].The core component resistor is made of zinc oxide and various oxides.Compared with the traditional silicon carbide arrester, the voltammetric characteristics and through-current capacity of the resistor are greatly improved, with high protection reliability, large flow capacity and good steep wave response [4][5].
Creepage distance is the shortest path between two conductive parts measured along the insulation surface or between the conductive parts and the protective interface of equipment.It is one of the important indexes to evaluate the insulation performance of arrester.It is a necessary project for lightning arrester factory testing and centralized network access testing.According to the provisions of the national standard GB11032-2020 "Metal-oxide surge arresters without gaps for a.c.systems", the creepage distance of the arrester need to be measured.In this paper, a measurement system for arrester creepage distance is developed by using three-dimensional reconstruction and laser vision detection.The paper is organized as follows.Firstly, the traditional method of measuring arrester creepage distance is introduced in Section 2.Then, the laser measurement and three-dimensional reconstruction are analyzed in Section 3 and the system measurement scheme is proposed in Section 4. Finally, the results from practical measurement application show effectiveness and practicability of the proposed method in Section 5. Section 6 summarizes this paper.

Traditional measurement method
At present, the main measurement methods of arrester creepage distance are paper tape measurement and wire measurement, as shown in Figure 1.The paper tape measurement method is to paste adhesive paper tape along the surface of the arrester, and the wire measurement method is to measure the surface of the umbrella skirt with the wire axially.Between the two electrodes of the sample, the shortest distance measured along the surface of the insulator is obtained, and the length of the paper tape or wire is measured after removing the paper tape or the wire.
Manual measurement cannot guarantee that the measuring tape or wire is carried out completely along the arrester axis.When the surface is fitted for measurement, the measurement path often deviates, decreasing measurement accuracy.This method is of low accuracy and poor repeatability, which can easily lead to misjudgment of measurement results and cause unnecessary disputes.On the other hand, this method is tedious and time-consuming.Therefore, it is urgent to develop a fast and accurate algorithm to measure the arrester creepage distance.

Measuring method
The insulation part of arrester is mainly composite material.After the study and analysis of the arrester appearance and shape, the key point and difficulty of this study is the accurate measurement of the umbrella skirt.After investigation and theoretical analysis, the following two measurement methods are proposed.

Laser measurement
Laser measurement technology is relatively mature.Laser rangefinder is the most common instrument.The principle of laser measurement is that by shooting a very fine laser beam at the target, the photoelectric element can receive the laser beam reflected by the target.The timer measures the time from the laser transmission to reception, and calculates the distance from the observer to the target.The simple operation, fast speed and high precision are the advantages of the laser measurement.However, as the umbrella skirt of lightning arrester is soft material, it is prone to deformation.In addition, the umbrella skirt is not completely horizontal and there is a certain degree of radian.Therefore, accurate measurements cannot be achieved by relying only on laser measurement technology.

Three-dimensional reconstruction
Three-dimensional reconstruction refers to the establishment of mathematical models suitable for computer representation and processing of three-dimensional objects [6][7].It is the basis for processing, manipulating, and analyzing its properties in the computer environment, and it is the key technology for building virtual reality in computers to express the objective world.The camera should be calibrated firstly, that is, the relationship between the camera's image coordinate system and the world coordinate system is calculated.Then, the three-dimensional information is reconstructed from the information of multiple two-dimensional images.
The steps of three-dimensional reconstruction include image acquisition, camera calibration, feature extraction, stereo matching, and three-dimensional reconstruction.The three-dimensional reconstruction effect of arrester is shown in Figure 2. Since the accuracy of three-dimensional reconstruction is closely related to the price of the hardware.To meet the standard accuracy requirements, the system cost will be greatly increased, and the degree of application will be very low.Therefore, under the premise of cost control, it is impossible to achieve accurate measurements only by relying on three-dimensional reconstruction methods.
Aiming at the advantages and disadvantages of laser measurement and three-dimensional reconstruction technology, after analysis and research, a feasible scheme for combining the two technologies and adding professional algorithm software is proposed.In addition, on this basis, it is necessary to add the mechanical structure that rotates the arrester under test during the measurement process, so that the three-dimensional reconstruction is more comprehensive and accurate.

Measuring plan
In order to solve the problem of creepage distance measurement of arrester and ensure the effective implementation of sampling inspection, a structural size measurement system of arrester is designed to achieve the simple operation, accurate measurement and convenient observation.The research ideas are mainly carried out according to the steps of theoretical research, appearance structure modeling, circuit design, software programming, physical processing, assembly and debugging, and finished product finalization, as shown in Figure 3.The system will be based on the combination of three-dimensional reconstruction, machine vision, laser measurement, image processing and motion control technology, through the lightning rod threedimensional image reconstruction, with laser and visual detection, and then using Fast Point Feature Histogram (FPFH) descriptor operator and Random Sample Consensus (RANSAC) algorithm for point cloud location estimation, to achieve rough registration of point cloud [8][9].Finally, levenbergmarquardt algorithm and linear Iterative Closet Point (ICP) algorithm combined point cloud precision registration algorithm is used to obtain the size parameters.In order to test and control the stability of the software, a long-time verification and a lot of basic tests have been carried out.The measurement accuracy can reach 1mm.
The system also includes special fixture and mechanical motion mechanism for fixing and rotating arrester, which can meet various arrester of different sizes and voltage levels, and realize the synchronization of mechanical structure and test structure.

System application
In order to verify the design rationality and practicality of the measurement algorithm, the measurement system is developed and produced, and the physical diagram and software operation interface are shown in Figure 4 and Figure 5.The test sample is a 10kV zinc oxide arrester, and the system is used to measure creepage distance.At the same time, the wire measurement method is used to compare the test time.The test results are shown in Table 1.
Table 1 shows that it only takes 40 seconds to use the new measurement system for creepage distance measurement, while it takes 600 seconds to use the wire measurement method.Considering that the system is in the trial operation stage, the measurement time will be further reduced when the system is perfected.Table 1.Time statistics of 10kV zinc oxide arrester creepage distance measurement.

Measurement method
Time/s the wire measurement method 600 the proposed method 40 The advantages of using this set of measurement tools can be seen in practice.The use of laser measurement, three-dimensional reconstruction and other technologies to mark the measurement path avoids the workload increase and the measurement accuracy decrease caused by the deviation of the measurement path, it also improves the measurement controllability.One-touch from sample placement to test completion improves the convenience of testing.What's more, the real-time data display of the display terminal can effectively guide the operation of the measurement work, and its pause, power failure memory, and two-way measurement functions improve the reliability of the operation.

Conclusions
An intelligent creepage distance measurement system for arresters is developed, and it works well in the application of actual arrester creepage distance measurement.The system can realize the automatic detection of the creepage distance, with fast speed, high precision, strong repeatability and convenient operation.
(1) The creepage distance measurement system developed in this paper solves the problems of manual measurement method with long time and low precision, and ensures the reliability of the test work.
(2) Combining three-dimensional reconstruction, machine vision, laser measurement, image processing and motion control technologies, the measurement system ensures the measurement accuracy through a large number of basic tests and experimental verification.
(3) In practical applications such as factory testing and centralized network access testing, the measurement system is easy to operate and the results are reliable, significantly improving the work efficiency of arrester creepage distance measurement.