Testing Methods For Power Fittings And Insulators

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Insulators are an indispensable component in high-voltage transmission lines. Its role is mainly reflected in two aspects. It can connect the conductor at high potential with the tower at low potential, and bear the mechanical forces such as gravity and wind of the conductor, so it needs to meet the requirements of mechanical performance; secondly, it also needs to ensure the insulation between the two, which is its electrical performance requirement. The structure of the insulator is relatively simple, and the manufacturing cost is relatively low. 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. Any problem with any string of insulators will cause the failure of the transmission line. In severe cases, it will cause a long power outage, which will cause great harm to the safe operation of the power system and people's daily life.

There are currently three methods used in China (for reference only)

1. Insulation resistance method

In the process of Insulator online detection, the measurement of insulation resistance is achieved by measuring the leakage current. High-voltage transmission insulators are generally composed of disk-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. Their equivalent circuit can be represented by RC series-parallel circuit. The problem with the insulation resistance method is not entirely the accurate measurement of 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 deteriorate. (2) The leakage current of the insulator is closely related to the degree of contamination on its surface. The tower structure, the degree of insulator aging, the shape of the insulator and weather conditions, such as temperature, humidity, and even wind speed and direction, all have an impact on the magnitude of the insulator leakage current. Therefore, the leakage current value is also a time-varying quantity under normal circumstances. There is a problem of 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 composite 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 curve A of the change of the electric field strength and potential along the axial direction of the insulator calculated according to the electric field theory is smooth under normal conditions; when there is a conductivity defect in the insulator, the potential at this location becomes a constant, and there is distortion at the corresponding position, with a depression in the middle and an increase at both ends. Therefore, measuring the axial electric field distribution of the composite insulator string can find out the internal insulation conductivity fault of the insulator.

3. Pulse current method

The so-called pulse current method is to judge the insulation condition of the insulator by measuring the corona pulse current of the insulator. The principle is: in the insulator string with inferior insulators, since the insulation resistance of the degraded insulator is very low, the voltage it bears in the insulator string is also small, so the voltage borne by other normal insulators on the insulator string must be significantly greater than the voltage borne under normal conditions, and because the loop impedance becomes smaller and the insulator corona phenomenon intensifies, the corona pulse current will inevitably increase. According to the phenomenon that the number and amplitude of corona pulses increase when there are inferior insulators on the line, a wide-band corona pulse current sensor is inserted into the tower grounding lead to extract the corona pulse current signal. Through certain signal processing methods, the purpose of detecting bad insulators at the low-voltage end is achieved.

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