Analysis and Solution for The Failure of 35kV Dry-Type Transformer Lightning Impulse Test

As an important part of transformer type testing, lightning impulse plays a critical role in the verification of new product specifications and the finalization of insulation structure testing. Due to the high voltage amplitude applied during the lightning impulse test, combined with the steep impulse waveform, a complex voltage gradient distribution and wave process is generated within the transformer windings and spatial structure, caused by the distributed parameters. In addition, the discharge mechanisms of gas discharge, oil particle discharge, and solid insulation discharge each have their own specific characteristics. Therefore, the discharge patterns and mechanisms differ accordingly.

We will begin by discussing a case from a factory setting.

The case involves a three-phase 50Hz 4600kVA 33000V/400V epoxy-cast dry-type transformer, with a winding configuration of Dyn11 and all-aluminum windings, and a lightning impulse level of 170kV BIL. The product testing process was as follows:

Initial Lightning Impulse Failure:

1. During the lightning full-wave process, a steep voltage drop in the waveform was observed, and surface flashover discharge was noted on the coil. Discharge marks extended along the lower part of the B-phase high-voltage terminal toward the clamping piece and lower yoke. The situation is illustrated in the following image:

2. Improvement Measures:

1）Add a 1.8mm (=2x0.9) NMN layer between the lower spacer and the lower clamp to cut off the creepage path along the surface by creating a thin and wide sectional barrier.

2）Add a "skirt" to the lower spacer, based on the same principle as mentioned above. However, this measure is only applicable to composite spacers. For integrated spacers, it is likely not suitable.

Second Lightning Impulse Failure:

After implementing the improvement measures, a second lightning impulse test was conducted. However, a puzzling situation occurred: while the lower discharge issue was successfully resolved, the signs of lightning creeping appeared at the upper end. The creeping traces followed the upper terminal, winding toward the upper yoke, and eventually disappeared near the coil end surface.

Measures after the second failure:

You have probably seen the specific measure applied—correct, it involved wrapping heat shrink tape around the exposed terminal. A simple analysis reveals that under the impact of lightning impulse, covering an exposed electrode in gas or liquid with a thin insulation layer has little to no effect in improving the surrounding electric field. Therefore, for this purpose, it is largely ineffective.

Third Lightning Impulse Failure:

During the third lightning impulse test, the surface discharge path was still very clear. This time, it took a shortcut, directly going from the upper terminal toward the upper clamp.

Measures after the third lightning impulse failure:

Additional protection was added to the core column, and the height of the central insulating cylinder was increased, creating a stepped barrier that made the creepage path more difficult to follow.

The Fourth Lightning Impulse Test Passed Successfully!

Some Reflections and Summary:

1. Reevaluate the current skirt/slotted structure of the spacer for its rationality. In many cases, satisfying the calculated physical creepage distance in engineering does not necessarily mean surface flashover discharge will not occur in real scenarios. There are many factors that determine surface discharge, and creepage distance is only one of them. If there are factors in the environment that facilitate the generation of the first discharge particle, the conditions for an avalanche are set. Future discussions will further explore gas discharge.

2. The rounding of surrounding electrodes is especially important and critical. The essential purpose of rounding the electrodes is to reduce electric field concentration and to nip the first discharge particle in the bud.

3. In the structural design of insulation, it is crucial to pay attention to the coordination of insulation structures at different parts, and when conditions allow, to leave enough creepage distance as much as possible.

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