The power system has put forward increasingly higher requirements for the reliability and economy of power supply, and the safe operation of high-voltage transmission and transformation equipment has become an important factor affecting the safety, stability, and economic operation of the power system. High-voltage switches play a control and protection role in the power grid. When a fault or accident occurs, it can cause grid accidents or expand accidents. The International Council on Large Electric Systems (CIGRE) has conducted two worldwide surveys on the reliability of high-voltage switches and their operating mechanisms. The results show that most switch faults (70% of major faults and 86% of minor faults) are mechanical in nature, mainly involving operating mechanisms, monitoring devices, and auxiliary devices. At present, the problems of domestic high-voltage switches are also mainly reflected in mechanical faults, such as: insulation rod falling off, breaking, breakdown, horizontal rod pin breakage, etc. Therefore, the detection of mechanical faults in high-voltage switches is of great significance.
1 Basic methods and principles of high-voltage switch mechanical fault detection
1.1 Detection of opening and closing coil current
High-voltage switches generally use electromagnets as the primary control element for operation, and most switches use DC as the control power supply. During each opening and closing process, the current of the DC electromagnetic coil changes with time, and the waveform of this change contains extremely important information. The coil current waveform can reflect the state of the iron core stroke, iron core sticking, coil state (such as whether there are short-circuited turns), the state of the latch and valve connected to the iron core push rod, and the auxiliary contact condition and conversion time of the opening and closing coils. By detecting the operating current of the opening and closing coils, operating personnel can roughly understand the working condition of the secondary control circuit of the switch and whether there is any sticking in the movement of the iron core, providing an auxiliary criterion for maintenance. The current of the opening and closing coils is an important part of switch state detection. The current of the opening and closing coils can be easily collected through Hall sensors. By comparing the measured waveform with the typical waveform, it can be determined whether there are problems such as sticking of the switch iron core.
1.2 Detection of the travel of the moving contact of high-voltage switches
The travel-time characteristic of a high-voltage switch is an important parameter that characterizes the mechanical characteristics of the switch, and it is also the basis for calculating the opening and closing speed of the switch. The opening and closing speed of a high-voltage switch, especially the speed of the moving contact before closing and after opening, has a crucial impact on the breaking performance of the switch. Therefore, the detection of the travel-time characteristic of high-voltage switches is an important part of high-voltage switch detection. Since the moving contact of the switch performs linear motion, an additional part that moves with the moving contact can be installed. When the moving contact performs opening and closing operations, the additional part moves linearly with the connecting rod. Through a photoelectric sensor, the continuously changing displacement is converted into a series of electrical pulse signals. By recording the number of pulses, the measurement of the full travel parameters of the moving contact can be achieved; at the same time, by recording the time value of each electrical pulse and dividing the displacement by the time, the maximum speed and average speed during the movement of the moving contact can be calculated. At present, the measurement of the travel-time characteristic of high-voltage switches mostly uses photoelectric displacement sensors combined with corresponding measurement circuits. Commonly used ones include incremental rotary photoelectric encoders or linear photoelectric encoders. Currently, there are many products developed based on this method on the market, which can directly print the test results. Some test instruments have RS232/485 interfaces and can upload the test results.
1.3 Detection of vibration signals
A high-voltage switch is a transient mechanical device. During its opening and closing process, a series of moving components start, brake, and impact, and these changes in motion patterns cause multiple impact vibrations on its frame. Each vibration corresponds to a specific action event during the opening and closing process of the switch. The waveforms of these impact vibrations show a rising and decaying process, and their peak points are well recognizable in time. However, there is always a certain error due to the propagation of vibration waves between the occurrence of vibration and the peak time measured by the vibration sensor. Therefore, the peak time has a certain time delay compared to the occurrence time of vibration. However, the detection system only uses vibration signals to obtain the time differences between various vibration events, and it does not necessarily need to know the exact time of occurrence. Therefore, as long as each event is simplified similarly, the calculation error of the time difference is not affected. The peak time of the vibration signal can be used as the occurrence time of each vibration event, and the time differences between various vibration events during the movement of the moving contact can be obtained by subtracting them. In addition, for a specific switch, its vibration signals during opening and closing operations in a healthy state have strong similarity. For vibration signals obtained in real-time measurement, based on offline experimental data (vibration signal waveforms and mechanical characteristic curves), and combined with the mechanical characteristic curve of this operation, the sequence and time intervals of each vibration event can be roughly determined. Then, the peak time of each interval is used as the occurrence time of that vibration event. After subtracting the times, the relative time between events is obtained. Taking the time of receiving the opening and closing electrical pulse as the reference, the occurrence time of each event is calculated, and the opening and closing moments between the moving and fixed contacts can be found. By combining the travel signal of the moving contact with the opening and closing moments and according to the corresponding definitions, the just-opening (closing) speed, travel, and overtravel can be calculated; by subtracting the opening and closing moments of the three phases, the non-simultaneity parameters of this operation can be obtained. The detection and diagnosis of the transmission mechanism system are of great significance to ensure the safe operation of high-voltage switches. The method of diagnosing the mechanical state of switches based on vibration signals, as an indirect and non-disassembly diagnostic method, has become a research hotspot at home and abroad.
For high-voltage switches, during the opening and closing operation, the movement, impact, and friction of the main internal mechanisms can cause surface vibrations. Vibration is the response excited by various internal phenomena, including mechanical operation, electrodynamic or electrostatic forces, partial discharge, and particle movement in SF6 gas. The vibration signal contains rich mechanical state information, and even subtle changes in the mechanical system structure can be detected from the vibration signal. Therefore, using the external vibration signal as a characteristic signal, the state of the high-voltage switch can be detected. The specific method is to install vibration sensors at appropriate parts of the switch, such as parts with high vibration intensity and high signal-to-noise ratio. When the switch performs opening and closing operations, the vibration signal is collected and processed as the basis for diagnosis.
The outstanding advantage of detecting vibration signals is that the collection of vibration signals does not involve electrical measurement, the vibration signal is less affected by electromagnetic interference, and the sensor is installed externally without any impact on the switch. At the same time, the vibration sensor is small in size, reliable in operation, low in cost, high in sensitivity, and good in anti-interference, especially suitable for online detection and non-disassembly maintenance of high-voltage switches with frequent operations. The difficulty of vibration detection lies in the poor consistency of the signal and how to extract characteristic quantities. At present, the main analysis methods are as follows: ① The time and amplitude of vibration signal appearance; ② Time-domain and frequency-domain composite analysis; ③ "State diagram", mean, variance, zero-crossing count, flip count, etc.; ④ Dynamic time warping method; ⑤ Deviation test method; ⑥ Exponential decay oscillation wavelet decomposition method; ⑦ Artificial neural network method. At present, the measurement research of switch vibration is a hotspot, but there is no mature product yet.
2 Main problems in the detection of mechanical faults in high-voltage switches
At present, the detection methods for mechanical faults of high-voltage switches are basically mature. There are many mechanical characteristic test instruments, most of which have the functions of opening and closing coil current and travel testing. Their effectiveness has been recognized by switch manufacturers and power supply departments. The main problem lies in the lack of effective data management and analysis diagnosis platforms. Most test instruments can print the test results, but cannot store them; for fault judgment, threshold methods are mostly used, and multiple test results of a certain circuit breaker cannot be effectively "longitudinally compared" through mathematical statistical analysis and other means, let alone "horizontally compared" with test results of the same type of circuit breaker in different places, which is not conducive to fault diagnosis. Therefore, it is very necessary to design and develop a high-voltage switch mechanical fault analysis and diagnosis platform.
