What are the purposes of conducting induced voltage withstand tests on transformers

For transformers, electromagnetic voltage transformers, etc., the main insulation of the tested object (referring to the insulation between windings to ground, between phases, and between windings of different voltage levels) is often checked by using the test method of obtaining high voltage from secondary pressurization. In addition, the insulation strength of the longitudinal insulation of transformers, voltage transformers, and windings between turns, layers, and sections, as well as between phases, is also tested. For power transformers and test transformers with graded insulation, both the main insulation of the winding segments and the longitudinal insulation of the transformer itself are often subjected to induction voltage withstand tests using induction voltage withstand testing devices for simultaneous assessment.

Compared to the main insulation of transformers, which refers to the insulation between windings and between windings and iron cores, there is another important insulation performance indicator of transformers – longitudinal insulation. Longitudinal insulation refers to the insulation between different points and parts of a transformer winding with different potentials, mainly including the insulation performance between turns, layers, and sections of the winding. The “induced voltage withstand test” specified in national standards and IEC standards is one of the testing methods specifically used to inspect the longitudinal insulation performance of transformers.

So what is the specific purpose of using an induction withstand voltage test device for induction withstand voltage testing?

The longitudinal insulation of the transformer mainly depends on the insulation medium in the winding — the insulation paint of the enameled wire itself, transformer oil, insulation paper, impregnating varnish and insulation glue, etc. (different types of transformers may include one or more insulation mediums); It is difficult to ensure 100% purity of vertically insulated dielectric, which inevitably contains solid impurities, bubbles, or water, and can also be damaged to varying degrees during the production process.

The high field strength of the transformer during operation is concentrated at these defects. The temperature rise during long-term load operation reduces the breakdown voltage of the insulation medium, resulting in partial discharge. The power absorbed by the dielectric through the applied alternating electric field, i.e. the dielectric loss, will increase significantly, resulting in serious heating of the dielectric and increased dielectric conductance. The large current in this part will also generate heat, which will continue to increase the temperature of the dielectric, The increase in temperature in turn increases the conductivity of the dielectric. This long-term vicious cycle has led to thermal breakdown of the dielectric and the destruction of the entire transformer. This fault is manifested in the characteristics of the transformer, which include a significant increase in no-load current and power consumption, as well as adverse phenomena such as scorching, arcing, vibration, and whistling in the winding. It can be seen that it is extremely necessary to use an induction voltage withstand test device to detect whether the transformer contains longitudinal insulation defects.

The function and purpose of induction withstand voltage for power transformers are described as follows:

1. Effectively check the longitudinal insulation of fully insulated power transformers;

2. Can test the partial main insulation and longitudinal insulation of graded insulation transformers;

3. Can effectively test the voltage resistance of the transformer’s longitudinal insulation;

4. For graded insulation transformers with different insulation levels at the beginning and end, the use of induction voltage withstand test can effectively reflect the insulation strength;

5. By using the non measured phase winding to support the tested winding and raise the neutral point position, both the main insulation and longitudinal insulation tests can be met simultaneously.

The induction voltage withstand test of transformers on site is generally measured using the self coupling method, using lower windings or non tested windings of the same voltage level to support the tested high-voltage winding. The voltage is both easy to reach the test voltage and easy to control, which is currently a commonly used measurement method in the power sector.

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