When a direct current voltage is applied to the dielectric, the initial instantaneous current is high, gradually decays over a certain period of time, and finally stabilizes. These three stages of current variation exhibit different physical phenomena. The initial instantaneous current is determined by the elastic polarization of the dielectric. The time for the establishment of elastic polarization is fast, and the charge moves quickly, so the current is large and lasts for a short time. This current is called capacitive current (IC). The current that slowly decays over time is caused by interlayer polarization and relaxation polarization of the dielectric. The longer they are established, the slower the current decay until relaxation polarization is completed. This process is called absorption phenomenon, and this current is called absorption current (Ic). The final stable current that does not change with time is determined by the conductivity of the dielectric, called the conductivity current (Ig), which is the leakage current during the dielectric DC test.
The absorption phenomenon is particularly evident in interlayer polarization. For example, generators and oil-paper cables are both multi-layer insulation, belonging to interlayer polarization, and both absorb current for a long time to decay. The absorption phenomenon of small and medium-sized transformers is weaker. Insulators are a single insulation structure with weak relaxation polarization, so there is basically no absorption phenomenon. Due to the significant variation of the absorption current of sandwich insulation over time, this characteristic can be used to determine the state of insulation in practical experiments. Due to the variation of absorption current over time, time must be specified when testing insulation resistance and leakage current. For example, in the current standards for electrical equipment handover and routine testing, the insulation resistance ratio (i.e. absorption ratio R60s/R15) at 60s and 15s, and the ratio of the insulation resistance value when pressurized for 10 minutes to the insulation resistance value when pressurized for 1 minute (polarization index) are used as an indicator to determine the degree of insulation moisture or dirt. After the insulation is damp or dirty, the leakage current increases, and the absorption phenomenon is not obvious.
The ZC-430D high-voltage insulation resistance tester is designed and developed to test the electrical insulation resistance characteristics of large high-voltage transformers, motor appliances, remote power cables or buried cables under strong interference in high-voltage substations and power plants. It is a common and necessary instrument for telecommunications, electricity, meteorology, machine room, oil field, electromechanical installation and maintenance, and industrial enterprise departments using electricity as industrial power or energy. It is suitable for measuring the resistance values of various insulation materials and the insulation resistance of transformers, motors, cables, and electrical equipment.
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