Insulators are an indispensable part of high-voltage transmission lines. Its function is mainly reflected in two aspects. The first aspect is that it can connect the wire at a high potential with the tower at a low potential, and bear the mechanical force of the wire such as gravity and wind, so it needs to meet the mechanical properties. In the second aspect, it also needs to ensure the insulation between the two, which is its electrical performance requirements. The structure of the insulator is relatively simple and the manufacturing cost is relatively low, and its importance is no less than any other equipment and devices that constitute the power system. The insulators in the transmission line are operated in parallel, and any problem with the insulators in the series will cause the failure of the transmission line. In severe cases, it will cause a long-term power outage, which will cause great harm to the safe operation of the power system and people's daily life. .

At present, there are three methods used in China (for reference only) 

1. Insulation resistance method

During the on-line inspection of insulators, the measurement of insulation resistance is achieved by the measurement of leakage current. 

High-voltage transmission insulators are generally composed of disc-shaped suspension insulators with simple structure, high mechanical strength, low aging rate, and can be used on transmission lines of any voltage level after being connected in series. The equivalent circuit can be RC series-parallel circuit. Express. The problem with the insulation resistance method is not entirely the accurate measurement of the current, it also depends on the following factors: 

(1) The voltage change of the transmission line directly affects the magnitude of the leakage current, and the current change value caused by the voltage change is theoretically sufficient to be equivalent to the current change value when one or two insulators are degraded. 

(2) The leakage current of the insulator is closely related to the degree of contamination on its surface. The structure of the tower, the aging degree of the insulator, the shape of the insulator and the weather conditions, such as temperature, humidity, and even the wind speed and direction have an impact on the leakage current of the insulator, so the leakage current value is also a time-varying quantity under normal circumstances. How to correctly determine whether there are inferior insulators in the insulator string, that is, how to establish the judgment standard. 

2. Electric field measurement method

The synthetic insulator on the high-voltage line can be simplified as a continuous insulating material sandwiched between two metal electrodes, and the shed of the insulator has no effect on the electric field distribution. In this simplified model, the electric field strength and electric potential change curve A along the axial direction of the insulator calculated according to the electric field theory is smooth under normal conditions; when the insulator has a continuity defect, the electric potential becomes a constant at the corresponding position. There is distortion, the middle sinks and the ends rise. Therefore, measuring the axial electric field distribution of the composite insulator string can find out the internal insulation continuity fault of the insulator. 

3. Pulse current method

The so-called pulse current method is to judge the insulation status of the insulator by measuring the corona pulse current of the insulator. is also smaller, so the withstand voltage of other normal insulators on the insulator string must be significantly greater than the withstand voltage under normal conditions, and because the loop impedance becomes smaller, the corona phenomenon of the insulator is aggravated, and the corona pulse current will definitely become larger. According to the phenomenon that the number of corona pulses increases and the amplitude increases when there are inferior insulators on the line, the broadband corona pulse current sensor is inserted into the grounding lead of the tower to take out the corona pulse current signal. The purpose of detecting bad insulators at the low-voltage side.