What is Basic Insulation Level (BIL) – A Complete Guidance

Each high-voltage equipment requires adequate insulation to operate safely, reliably, and efficiently. Rated insulation levels indicate the amount of stress the insulation can handle during lightning strikes, sudden electrical surges, or power overloads. The IEC and Indian Standards (IS: 13118) have established clear global rules for these insulation levels to ensure consistent operation.

Actually, an insulation system endures many high-voltage situations due to various natural and operational activities. Although we are fortunate enough that these high-voltage conditions are not permanent. Most of these stresses are transient in nature.

What are Rated Insulation Levels?

Often, we refer to the rated insulation level of an equipment as the basic insulation level (BIL). As we have already mentioned, these are the specific voltage levels that a piece of electrical equipment can safely withstand without insulation failure under various conditions.

Power Frequency Withstand Voltage

The ability of the insulation system to withstand sinusoidal voltages for a short period (e.g., 1 minute or 60 seconds) is an important characteristic. One minute is enough for the purpose. Particularly during disconnections of major loads, a higher voltage can momentarily occur in power systems. The electrical protection system usually clears the overvoltages within a few seconds. Therefore, we can design insulation systems to withstand such short-term overvoltage situations for a specific duration. Obviously, it is neither necessary nor cost-effective to design insulation systems to withstand these overvoltages for an extended period.

Lightning Impulse Withstand Voltage

Lightning impulses are extremely brief and sharp. Therefore, the insulation system must be capable of enduring these sudden, high-voltage spikes. However, it is not cost-effective to design an insulation system that can handle such high voltages for even a few milliseconds. These high-voltage surges typically last only for microseconds and dissipate very quickly once they occur in the system. The actual shape of the lightning pulse is almost unpredictable. We use a 1.2/50 µs waveform to represent such voltage spikes during type tests of the equipment.

Switching Impulse Withstand Voltage

This voltage rating is relevant for higher voltage systems. This rating defines the withstand capability of insulators against voltage surges due to switching operations. Actually, the amplitude of the switching pulse is smaller than that of a lightning impulse, but the time period is longer than that of a lightning impulse. As a result, a switching impulse may also impose significant energy. We stimulate the standard switching impulse waveform with 250/2500 μs. According to the Central Electricity Authority (CEA) Guidelines, the Switching Impulse Test is mandatory for equipment of 300kV and above. According to IEC 62271-1, the Switching Impulse Test is compulsory for equipment of 245kV and above.

Key Components of Rated Insulation Levels

Rated Voltage (Ur)

This is the highest voltage that equipment can safely handle continuously without damage. We set this about 10% higher than the nominal voltage for systems up to 220kV systems. We set this to account for normal voltage fluctuations. For systems at 400kV and above, we maintain a 5%, striking a balance between reliability and cost-effectiveness.

Power Frequency Withstand Voltage

This voltage rating assesses the insulation system under a power-frequency, i.e., 50Hz voltage, for one minute, under both dry and wet conditions.

Lightning Impulse Withstand Voltage

We can not predict the exact amplitude and shape of a natural lightning surge. Therefore, we stimulate 1.2/50 µs impulses as the shape of the waveform of the lightning impulses.

Switching Impulse Withstand Voltage