Relative to the main insulation of the transformer, that is, between the winding and the winding and between the winding and the iron core, the transformer has another important insulation performance index - vertical insulation. Longitudinal insulation refers to insulation of transformer windings with different potentials and insulation between different parts, mainly including the insulation properties between windings, between layers and between segments, which are specified in the national standards and the International Electrotechnical Commission (IEC) standards. "Induction voltage test" is one of the test methods specially used to test the vertical insulation performance of the transformer.
Longitudinal insulation of the transformer mainly depends on the insulating medium inside the winding—insulating paint, transformer oil, insulating paper, impregnated paint and insulating glue, etc. of the enameled wire itself (different kinds of transformers may contain one or more insulating mediums among them); Longitudinal insulation dielectric is difficult to guarantee 100% purity, it is unavoidable to contain solid impurities, bubbles or moisture, etc., and it will also be damaged in various degrees in the production process; the high field strength of the transformer work concentrated in these defects, long-term load The temperature rise in operation reduces the breakdown voltage of the insulating medium and causes partial discharge. The power absorbed by the dielectric through the applied alternating electric field, ie, the dielectric loss, will increase significantly, resulting in serious dielectric heating, increased dielectric conductance, and high current at the site. Heat is generated, which causes the temperature of the dielectric to continue to rise, which in turn increases the conductivity of the dielectric. This long-term vicious cycle continues, leading to thermal breakdown of the dielectric and destruction of the entire transformer. This fault is manifested in the characteristics of the transformer is a significant increase in no-load current and no-load power consumption, and the winding has burning, arcing, vibration and howling and other undesirable phenomena. It can be seen that it is extremely necessary to use an induced withstand voltage test to detect whether the transformer contains vertical insulation defects.
Induction pressure test principle
When the transformer was just produced, it had not been subjected to harsh environmental tests for a long time. It was tested by applying its rated voltage and frequency power supply. The voltage across the windings, between the layers and between the segments was insufficient to achieve the breakdown voltage at the dielectric defect and it was difficult to cause these. Discharge and breakdown at the insulation defect, there is no significant difference in the no-load current and no-load power consumption of transformers with hidden insulation faults and good insulation performance, so it is difficult to find these hidden dangers;
The induction withstanding voltage test applies a voltage more than 2 times the rated voltage to the transformer, which can establish a higher and more concentrated field strength at the longitudinal insulation defect. The voltage between winding turns, layers and segments reaches and exceeds the defect at the dielectric defect. Through voltage; Induction voltage test gives the transformer a frequency above twice the rated frequency, and the higher frequency can greatly reduce the breakdown voltage of the solid dielectric, making the insulation defect easier to break through; The working time of applied voltage can also ensure the breakdown of insulation defects; therefore, the induction voltage test can reliably detect the vertical insulation performance of the transformer.
The reason why the frequency of the induced voltage test applied to the transformer is more than 2 times the rated frequency is because the characteristic curve of the exciting current i - the main magnetic flux amplitude Фm of the transformer is generally designed to be close to the bending saturation at the rated frequency and rated voltage. In part, the main flux Фm is determined by the applied voltage, U, at a constant power frequency:
U - external power supply voltage,
V△ФmE - induced electromotive force of the winding,
Vf - external power frequency,
HzW - the number of turns of the power winding,
n Therefore, adding 2 times the voltage above the rated voltage to the transformer Δii will inevitably lead to severe saturation of the iron core, and the main magnetic flux Фm will increase ΔФm. Fig. 1 shows that the exciting current i will increase sharply, causing the transformer to heat and burn; If the transformer is still not saturated above 2 times the pressure, the frequency of the power supply must be increased to more than twice the frequency. Induction voltage test provides a power supply of 2 times or more and 2 times or more power to the transformer primary. The main magnetic flux of the transformer will induce both the primary and secondary edges to induce EMF E1 and E2, respectively, under their rated operating conditions. Above 2 times, so the induction voltage test can test the vertical insulation performance of the main and secondary windings at the same time. Of course, we can also perform tests from the secondary side of the transformer as required, but the applied voltage should be more than twice the no-load voltage of the transformer in its rated operating state, and the frequency is also more than twice the rated frequency.
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