Why is the zero value of porcelain insulators low? Do you know the reason?
When the breakdown voltage of the working frequency of the porcelain insulator is lower than the dry flash voltage, it is an inferior insulator. Damaged porcelain insulators are called zero insulators. According to DL/T626-2015 "Quality Inspection Regulations for Hanging Porcelain Insulators":
When the voltage level is above 500kV and the insulation resistance of the insulator is less than 500mol, it is judged as a deteriorated insulator.
When the voltage level is lower than 500kV and the insulation resistance of the insulator is less than 300m, it is judged as a deteriorated insulator.
A degraded insulator with an insulation resistance greater than 10m is generally called low insulation, and a degraded insulator with an insulation resistance lower than 10m is called zero insulation.
The two main reasons for the low zero value of porcelain insulators are as follows:
First of all, the deterioration rate of some batches of insulators is relatively high, and there was no strict inspection in the early stage of production, resulting in defective products.
In addition, due to the long-term effects of electromechanical loads, sunlight, changes in cold and heat, etc. Porcelain insulators also have failures such as insulation resistance drop, cracks, and perforations.
When the porcelain insulator is zero, when the insulator is overcurrent, the probability of pollution flashover is relatively high, even under normal working voltage, pollution flashover will occur. During the flashover process of the insulator string, the short-circuit current passes through the internal gap of the zero insulator. The thermal effect of high current will cause a series of serious accidents, such as broken ceramic parts, broken head pieces, broken castings, shedding of steel legs, firing of ceramic heads into glass bodies, etc.
Compared with high-quality insulators, zero-porcelain insulators have different electrical characteristics, partial discharge and temperature distribution. According to these characteristics and differences, different measurements can be made with corresponding instruments or equipment. The voltage distribution method (spark fork method) is the current detection method for zero-valued ceramic insulators at home and abroad, which uses infrared thermal imaging technology and insulation resistance measurement method.
Voltage distribution measurement method (spark split method): When there are zero-value insulators in the insulator string, the voltage distribution of the insulator string is greatly affected. Install a fork made of wire at the end of the spark fork so that one end of the spark fork touches the insulator steel cover under the insulator. If the other end is close to the steel cover plate of the insulator under test, the gap between the steel caps can generate sparks. The higher the distribution voltage of the tested insulator is, the earlier the spark will be generated and the louder the spark sound will be. According to the discharge situation, the voltage of the insulator under test can be judged. If the measured insulator is zero, it is not affected by the voltage, so there is no spark.
Leakage current measurement method: Generally, the leakage current of porcelain insulator strings is milliamp level. When insulators exist, the leakage current value increases with time. At present, the current sensor for measuring leakage current has no problem with the measurement accuracy, but under normal circumstances, the leakage current value will change with time and the environment, so there is still a problem.
Infrared imaging technology: The principle of infrared thermal imaging technology to detect deteriorated insulators is to make them appear bright (low) or dark (zero) according to their thermal imaging characteristics. If the temperature distribution in the insulator string is discontinuous, with a bright (or dark) thermal image, it is judged as a low (or zero) insulator. The method of measuring insulation resistance: mainly composed of insulation shaker and electronic insulation resistance meter. Zero overload direct measurement insulation resistance meter to check zero insulation. It is very accurate to use an insulation resistance meter to detect zero or low value insulators. However, due to the long test time for a single test, the test time for the resistance value of each porcelain insulator is at least 1 minute, which is not suitable for large-scale insulator testing. In addition, due to the low detection voltage, it is difficult to find defects in large-tonnage insulators.