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The insulation of transformer coils is divided into main insulation and longitudinal insulation. Main insulation, also known as horizontal insulation, refers to the insulation between the coil and other structural parts except itself, including insulation with the oil tank, iron core, clamps, and press plates, insulation with other coils of the same phase, and insulation between coils of different phases (phase-to-phase insulation). Longitudinal insulation refers to the insulation inside the coil itself. It includes turn-to-turn insulation, layer-to-layer insulation, and insulation between line segments. In the external power frequency withstand voltage electrical test, only the main insulation of the transformer winding is tested. As the voltage level of transformers increases and capacity grows, their turn-to-turn insulation is relatively weak, but the external power frequency withstand voltage electrical test cannot test the longitudinal insulation of the transformer.
Since the induced overvoltage test uses the self-excitation method to apply voltage, if the test method is chosen reasonably, both the main insulation and longitudinal insulation of the transformer can be tested simultaneously. Considering the magnetic saturation issue of the transformer core, the inductive withstand voltage power supply often uses a frequency-doubled power supply, so the inductive withstand voltage is also called frequency-doubled inductive withstand voltage.
Fault analysis caused by insulation defects in transformers:
Compared with the main insulation of transformers, that is, the insulation between windings and between windings and the iron core, another important insulation performance indicator of transformers is longitudinal insulation. The "induced overvoltage test" specified by national standards and the International Electrotechnical Commission (IEC) standards is one of the test methods specifically used for testing transformers for longitudinal insulation performance.
The longitudinal insulation of transformers mainly depends on the insulating media within the group - the insulating varnish of the enameled wire itself, transformer oil, insulating paper, impregnating varnish, and insulating adhesive. Different types of transformers may contain one or more of these insulating media. It is difficult to guarantee 100% purity of the longitudinal insulation dielectric, and it is inevitable that solid impurities, bubbles, or moisture will be mixed in, which will be damaged to varying degrees during the production process. The highest field strength during transformer operation will concentrate on these defects. The temperature rise under long-term load operation will reduce the breakdown voltage of the insulating medium, cause partial discharge, and the power absorbed by the dielectric through the applied variable electric field will significantly increase, that is, dielectric loss. This leads to severe heating of the dielectric and increases its conductivity. The large current in this part also generates heat, causing the temperature of the dielectric to continuously rise, and the temperature rise in turn increases the conductivity of the dielectric, forming a long-term vicious cycle, ultimately leading to thermal breakdown of the dielectric and destruction of the entire transformer. This fault manifests as a significant increase in the transformer's no-load current and no-load power consumption, and the winding exhibits adverse phenomena such as burning, arcing, vibration, and howling.
Note: The above information and technical parameters are compiled and released by Yangzhou Darui Electric Co., Ltd.
