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Reasons for the change in insulation value

time:2021/3/7   source:华天电力  reading:761 time

Reasons for the change of the insulation value when the insulation resistance tester performs the insulation resistance test:

The difference in insulation performance of objects of different materials will be more obvious, and the trend of changes in the value during the test will also be different.

Some insulating objects such as plastic and porcelain can reach a stable value at the moment of conduction under the action of DC voltage. However, for generators, motors, cables, etc., their insulation is composed of composite media. Under the action of DC voltage, there will be more Kind of polarization phenomenon. At the beginning of polarization, the current is very large. As time increases, the current decreases and the resistance value increases accordingly. This phenomenon is called absorption. In the absorption phenomenon, the capacitive current (polarization current) that decays quickly, the absorption current that changes slowly with time, and the conduction current (leakage current) that does not change with time. Generally, the larger the equipment capacity, the more obvious this phenomenon.

Insulation Resistance Tester (3).png

Three currents of the insulation resistance tester during the insulation test:

1. Capacitance current i1: This part of the current is caused by the capacitance effect of the medium. When the sample under test is pressurized, rapid polarization occurs inside the medium, which is equivalent to the current generated by charging the capacitor; it is instantaneous and attenuates high speed.

2. Absorption current i2: This part of the current is caused by the slow polarization inside the medium, which decays slowly over time.

3. Conductive current i3: This part of the current is caused by the conductance of the medium itself, and its current value is constant.

Therefore, when the insulated cable tester measures the insulation value of electrical equipment, with the gradual decrease of the test current, the insulation resistance will rise rapidly, and then the trend of change will slow down; generally, the value at 60s is used as a reference, which can be compared with the equipment. Comparison of the insulation requirements to determine whether the insulation of the equipment is qualified.

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