For transformers and electromagnetic voltage transformers, the test method of applying voltage to the secondary side to obtain high voltage on the primary side is commonly used to check the main insulation of the tested items (referring to the insulation between the windings and the ground, between phases, and between windings of different voltage levels). Moreover, the longitudinal insulation of transformers and voltage transformers – the insulation between turns, layers, and sections of one winding, as well as between phases – has also been tested. For power transformers with graded insulation and test transformers, the main insulation of the winding segments and the longitudinal insulation of the transformer itself are often tested using an induction withstand voltage test device simultaneously.
Compared with the main insulation of the transformer, which refers to the insulation between windings and between windings and the core, there is another important insulation performance indicator for the transformer – longitudinal insulation.
Transverse insulation refers to the insulation between different points and different parts of the transformer windings with different potentials. It mainly includes the insulation performance between windings’ turns, layers, and sections. The “induction withstand voltage test” stipulated in national standards and the International Electrotechnical Commission (IEC) standards is one of the specific testing methods used to inspect the longitudinal insulation performance of transformers.
So, what is the specific purpose of conducting an induction withstand voltage test using an induction withstand voltage testing device?
The longitudinal insulation of the transformer mainly relies on the insulating medium within the windings – the insulating varnish of the enameled wire, transformer oil, insulating paper, impregnating varnish, and insulating gel, etc. (Different types of transformers may contain one or more of these insulating media); the insulating medium for the longitudinal insulation is difficult to ensure 100% purity and inevitably contains solid impurities, bubbles or moisture, etc., and it may also suffer varying degrees of damage during the production process.
During the operation of the transformer, the higher field strength is concentrated at these defects. The temperature rise caused by long-term load operation further reduces the breakdown voltage of the insulating medium, resulting in local discharge. The power absorbed by the dielectric through the externally applied alternating electric field, which is the dielectric loss, will significantly increase. This leads to severe heating of the dielectric, an increase in dielectric electrical conductivity, and the large current in this area will also generate heat, causing the temperature of the dielectric to continue rising. And the increase in temperature in turn causes the dielectric’s electrical conductivity to increase.
If this vicious cycle continues for a long time, it will eventually lead to the thermal breakdown of the dielectric and the destruction of the entire transformer. This fault is manifested in the transformer’s characteristics as a significant increase in no-load current and no-load power consumption, as well as abnormal phenomena such as overheating of the windings, arcing, vibration and whistling. It is evident that it is extremely necessary to use the induction withstand voltage test device to detect whether the transformer contains longitudinal insulation defects.
Post time: Jun-05-2026