A research method for lightning impulse withstand voltage of dry-type transformer

Author： GOZ Electric Time：2024-09-13 09:39:26 Read：33

It is generally believed that the benchmark impulse level (BIL) value of epoxy cast dry-type transformers is 250 kV, which is suitable for the production of 66/77 kV dry-type power transformers. Through the combination of simulation-assisted analysis and physical winding test, it is found that the air in the high-voltage winding airway and the main airway has undergone serious ionization. Based on this, a more reasonable improvement measure is proposed to increase the lightning impulse withstand voltage to 480 kV to meet the requirements of 110 kV power transformers.

Keywords: dry-type transformer; lightning impulse; epoxy resin

Lightning impulse waves are high-frequency impulse waves with the characteristics of high voltage, short time, high frequency and large wave head steepness. The equivalent circuit of the winding is a complex complex of capacitance, inductance and resistance. The potential oscillation and complex electromagnetic transient process generated during the propagation of lightning waves will cause a significant increase in local field strength and easily lead to insulation breakdown, which is extremely harmful to the transformer. People have found that even if the main and slave insulation strengths are checked separately according to the traditional test method and sufficient margin is left, they often cannot withstand the corresponding test voltage during the lightning impulse test. The quality of insulation performance plays a decisive role in whether the transformer can operate safely. Unlike the "oil-paper" insulation system of the oil-immersed transformer, the epoxy resin cast dry-type transformer is a composite insulation system composed of insulating materials and air (solid + air). Although the tolerable electric field strength of epoxy resin is as high as 6 times that of air, the ratio of the dielectric constant of air to epoxy resin is about 1:4, so the field strength acting on the air in the actual electric field distribution will be higher than that of epoxy resin. This contradiction directly affects the insulation layout of the dry-type transformer, and this problem is more prominent under higher lightning impulse voltage. The internationally recognized benchmark impulse level (B1L) value of epoxy cast dry-type transformers is 250kV, which can manufacture 66/77kV dry-type transformers. Therefore, the technical bottleneck that hinders the further improvement of the voltage level of dry-type transformers includes the improvement of lightning impulse withstand voltage, and densely populated urban substations are also unfortunately unable to use flame-retardant, environmentally friendly and safe 110 kV dry-type transformer products.

The problem of improving the lightning impulse withstand voltage of epoxy cast dry-type transformers is analyzed to seek solutions, so that the lightning impulse withstand voltage of dry-type transformers can reach 480 kV (peak value), that is, the standard value of 110 kV oil-immersed transformers.

The electric field simulation analysis under lightning impulse is carried out and a physical model is manufactured for testing. Simulation analysis method: A finite element analysis model is established at a ratio of 1:1, and the distributed capacitance, inductance and resistance parameters of each subdivided unit of the winding are calculated by the finite element method, which is faster, more practical and more accurate than the pure formula calculation method. The equivalent circuit model of the winding is constructed in MATLAB Simulink, and the full wave of lightning impulse is applied to the circuit for analysis and calculation. Then the calculated waveforms of each circuit node are imported into the finite element analysis model to calculate the voltage and electric field distribution at different times, that is, the research method of "field-circuit coupling".

According to the standard "High Voltage Test Technology Part 1: General Definitions and Test Requirements" (GB/T 16927.1-2011), the standard lightning impulse full wave is shown in Figure

Figure 1