What is the Transformer BIL (Basic Insulation Level)?

BIL Allows Transformers to Withstand Lightning Strikes

The Basic Insulation Level (BIL) of a transformer is a method of expressing the voltage surge the transformer is capable of withstanding without a breakdown. Any outdoor distribution system is subject to lightning surges. Even with lightning striking the line some distance from the transformer, voltage surges may travel down the line, entering the transformer.

Lightning impulse over voltage appearing in the system travels down a path of the least resistance. Usually, surge-protecting devices provide this path, discharging the impulse before it can damage the system. That implies the operating voltage level of the protecting devices must be lower than the minimum voltage withstanding capability of the equipment.

Opening and closing of high-voltage switches and circuit breakers associated with the transformer can also create similar voltage surges. The steep wave fronts in both types of surges are very damaging to electrical equipment. Lightning arresters do minimize the effects of these surges on the electrical system, but they cannot totally eliminate the surges from reaching the transformer.

Basic Insulation Level

This is where the BIL of the transformer comes in. The design of a transformer makes it capable of withstanding voltage surges to a certain level. For instance, all transformers rated 600 V and below have a BIL rating of 10 KV. Transformers operating at 2400 V and 4160 V are rated at 25 KV BIL. Basic Insulation Level of a transformer is also called its lightning-impulse withstand voltage level.

Therefore, different types of electrical equipment in a system have different insulation or withstand voltage capabilities. As the various equipment in a system come from different manufacturers, a minor degree of difference in ratings is inevitable. However, there is another major reason for the difference in ratings. This relates to the actual role of the equipment in the system. This is the primary reason surge protecting devices must have lower operating voltage levels than the lowest voltage withstanding capability of the equipment they are protecting.

Transformer Specs

Standard Transformer Specifications with Listed BIL Rating

Withstanding Lightning Strikes

When lightning strikes, it is not just the voltage peak causing the damage, but also the abruptly rapid voltage rise. In several cases, the voltage transients rise far more steeply than in those situations that common standards typically cover—they can surge by megavolts within microseconds. As the design of the insulation covering the windings of transformers and other equipment are not normally capable of withstanding such transients, there can be permanent damage, unless additional protection is present.

Worldwide, despite equipment being capable of withstanding typical surges, such surges cause nearly 35% of total dielectric failure in power equipment such as transformers. Investigations explain this as due to the high-frequency components in the spectrum of a voltage surge, resulting in a highly non-uniform voltage distribution. This causes local stresses on the insulation system going far beyond those encountered during normal operating conditions.

Additionally, internal structures of transformers are complicated and act as multi-resonant circuits. This can result in high frequencies being amplified locally, stressing the insulation system significantly, compromising the lifetime, and often leading to internal short circuits within the transformer.

New Types of Transformers with higher BIL

Distribution systems where transformers are exposed to frequent atmospheric discharges are increasingly demanding higher withstand levels. Manufacturers are meeting this demand with non-conventional design of windings, leading to an increase in both design and manufacturing costs. For instance, for complying with the Finnish Standard SFS 2646, transformers are tested against a voltage rise of 2 MV/µs. Usually, such a transformer is protected with a spark gap in its proximity.

However, as the spark gap is relatively slow to react, the voltage at the transformer levels may rise to levels in excess of the BIL ratings. Moreover, the high rise-time of the waveform results in a highly non-linear initial voltage distribution if the winding is conventional, leading to overstressing of the insulation system.

To overcome the above situation, manufacturers apply a special winding design. This comprises additional elements such as electrostatic screens for equalizing the initial potential distribution. Although this adds to design complexity, the solution helps to avoid overstressing the local insulation from the high rise-times of the applied transient voltage.

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