Simulation Analysis of Lightning Impulse Withstand Voltage of Epoxy Dry-Type Transformer

Author： GOZ Electric Time：2024-09-15 09:44:16 Read：19

The transformer was modeled with a 1:1 finite element analysis model (the head end was located at the top of the winding, and the neutral point end was located at the bottom of the winding), and a 480 kV standard lightning impulse full wave was input to calculate the electric field distribution at different times.

After the shock wave entered, the extremely high electric field intensity was first concentrated near the high-voltage winding inlet end (head end), and the air on the outer surface of the winding head end and the first three airways were broken down in turn, and the electric field intensity of the upper part of the first airway reached 5.4kV/mm. When the shock wave was about 12μs, the maximum field intensity in the high-voltage winding airway had dropped to within 3.5kV/mm, but at this time the field intensity on the outer surface of the low-voltage winding began to increase (up to 3.9kV/mm), and the electric field intensity showed a trend of decreasing diffusion from the main airway to the high-voltage winding, that is, the air in the main airway had been broken down. As the shock wave continues to develop, the position of the maximum field strength oscillates and shifts. Although the frequency of fluctuations decreases during the oscillation process, the maximum field strength in the high-voltage winding airway exceeds 3.0 kV/irnn many times. Finally, the oscillation gradually stabilizes, and the field strength in the airway drops below 3.0 kV/mm.

Through the test and simulation-assisted analysis of the actual winding, the following conclusions can be drawn:

(1) The relevant literature's discussion on the BIL value of 250 kV has a certain factual basis, that is, the dry-type transformer has a bottleneck limitation of lightning impulse withstand voltage. Although the conventional segmented layer winding has a large interlayer capacitance and better lightning impulse characteristics than the pancake winding, it stops at about 300 kV.

(2) When the lightning impulse voltage is 350 kV or above, the air in the winding airway and the main airway is broken down, causing waveform deformation. The air in the winding airway is broken down many times, but only the air is broken down, and the winding itself is not damaged, so a recoverable phenomenon is formed. "Power Transformer Part 11: Dry-Type Transformer" (GB 1094.11-2007) also points out that "dry-type transformers may experience capacitive partial discharge in the air during lightning impulse tests... The slight distortion of the damage current waveform cannot be used as a reason to reject the product." However, the discharge phenomenon at this time is obviously more serious and is no longer a "slight distortion" phenomenon. It is still a long way from being recognized as qualified or accepted by users.

(3) The field strength of the outer surface air around the high-voltage winding near the incoming line end (head end) is relatively high in the early stage of the lightning impulse wave, which may cause air breakdown discharge. Therefore, the starting point for improving the lightning impulse withstand voltage of 110kV dry-type transformers is to reduce the electric field strength of the air around the high-voltage winding during the lightning impulse process so as not to be ionized and broken down.