Impulse Strength of Transformer Insulation With Nonstandard Waveshapes

While observing the impulse voltage distribution within a transformer winding, it can be seen that parts of the windings are excited by unidirectional and bidirectional oscillating voltages. The inter-turn and inter-disc winding failures are attributed to these oscillating voltages. Attempts are made to study the behavior of air, transformer oil, and oil-impregnated paper (OIP) insulation under these voltages. Small gaps of the order of a few millimeters with different electrode geometries are chosen to incorporate the extreme electrical field configurations in a practical transformer. Experimental voltage-time characteristics are obtained using statistical procedures. The Disruptive Effect Method is used to evaluate the insulation strength under unidirectional nonstandard impulse voltages. The insulation strength under bidirectional oscillating impulse voltages on these small insulation gaps are estimated using Unconditionally Sequential Approach. The ranges of the positive reinforcement time $(t_{pr})$ have been determined. The polarity effect on the breakdown due to bidirectional waveshape is duly accounted.