Test code for insulation resistance, absorption ratio, and polarization index

Insulation resistance, absorption ratio, and polarization index tests:

Generally, manual, electric, or digital trameggers are used, while the basic principle of a megohmmeter, whether it is a pointer type or a digital type, is based on the same principle, that is, the current flowing through two sets of coils generates rotational torque in different directions in the same magnetic field. The indication of the pointer or number is related to the following equation: a=f (I1/I2), where the corresponding voltage coil has a fixed circuit resistance, which requires a constant rotational speed during the shaking measurement to ensure stable output voltage.

Its wiring terminals include L high-voltage terminal, E grounding terminal, and G shielding terminal. The shielding terminal G is directly connected to the negative polarity, and the surface leakage current flows back through it without passing through the measuring mechanism, so it is called "shielding"

As previously described, when a DC voltage is applied to an insulating medium, the current passing through it includes Ic, Ia, and Ig. Ic is the capacitive current, which quickly completes charging. Ia is the absorption current, and the polarization phenomenon is relatively long. Therefore, the absorption ratio is used to confirm the situation of large capacity and complex multi-layer insulation, which is K=R60/R15. In recent years, with the continuous growth of transformer capacity, the misjudgment of the insulation resistance value of its winding has increased, such as the absorption ratio<1.3, But it operates well because its absorption current decay time is too long, so it is specified to measure it with polarization index, which is R10min/R1min.

For Ig, there are differences between volume and surface, so when the equipment surface is dirty and the insulation is damp or cracked, its conduction current increases sharply. Therefore, measuring insulation and its absorption ratio can easily determine the overall moisture condition, such as the insulation testing process during the withstand voltage test of the Zhanjiang Orioil generator and the leakage current testing of the lightning arrester, which can clearly explain the problem.

The selection of equipment for insulation testing is generally based on the voltage level of the tested equipment and the selection of digital megohmmeters with different voltage levels. The handover procedures include:

250V motor temperature measuring element, 500V generator rotor winding insulation test, 1000V motor bearing insulation resistance, 2500V general motor, transformer, 5000V large capacity transformer, water cooled shaking table - insulation test of water cooled generator. But there are also exceptions, such as the AC withstand voltage test of the rotor of the hidden pole generator, which can be replaced by a 2500V megger or digital megohmmeter.
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