- Basic Insulation Level Definition: Basic Insulation Level (BIL) is the minimum voltage that electrical equipment insulation can handle before surge protection devices activate.
- Importance of Surge Protectors: Surge protectors quickly discharge overvoltage, preventing damage to equipment.
- Design Considerations: Systems are designed with BIL to handle specific overvoltage characteristics, ensuring protection without excessive insulation costs.
- Impulse Voltage Standards: Standard impulse voltages like 1.2/50 microseconds simulate lightning surges to test equipment’s dielectric strength.
- Safety Margins: Equipment must have a higher breakdown voltage than BIL, and protective devices must have a lower discharge voltage to maintain system safety.
Basic Insulation Level Definition
When lightning impulse over voltage occurs, surge protection devices discharge it to prevent damage to the system’s equipment. The equipment’s insulation must withstand a certain minimum voltage before this discharge happens. Thus, surge protection devices must operate below this minimum voltage level. This minimum voltage is defined as the Basic Insulation Level (BIL) of electrical equipment.
The voltage withstanding capacity of all equipment in an electrical substation or transmission system must match the operating system voltage. To maintain system stability during overvoltage events, the breakdown or flash-over strength of all connected equipment must exceed a certain level.
There may be different kind of over voltage tresses appeared on the system. These over voltages may differ in characteristics such as amplitude, duration, waveform and frequency etc. In the view of economy, an electrical power system must be designed for a basic insulation level or BIL depending upon the different characteristics of all possible over voltages appear on the system. Moreover there are different over voltage protecting devices installed in the system, which safely protect the system against different over voltage phenomenon. Due to these protecting devices the abnormal over voltages disappear from the system as fast as possible.
It is unnecessary to design a system with insulation to withstand all types of overvoltages indefinitely. For instance, a lightning impulse lasts only microseconds and is quickly cleared by lightning arresters. Electrical equipment insulation must be designed to avoid damage until the arrester acts. The Basic Insulation Level (BIL) determines the dielectric strength of the equipment and is expressed as the peak value of the 1/50 microsecond full wave withstand voltage.
The insulation level of equipment, especially transformers, significantly impacts costs. Standardizing bodies aim to set the Basic Insulation Level (BIL) as low as possible while ensuring safety. Lightning impulses are natural and unpredictable, making it hard to predict their surges. After extensive research, standardizing bodies have created a basic impulse wave shape for high voltage testing of equipment. This created impulse voltage, although not directly related to natural lightning surges, is used for testing purposes. Before diving into the BIL details, let’s understand the basic shape of a standard impulse voltage.
Impulse Voltage
As per American Standard the impulse wave shape is 1.5/40 microsecond. As per Indian Standard this is 1.2/50 microsecond. This representation of the wave has a special significance. Such as 1.2/50 microsecond impulse wave represents a unidirectional wave which rises to its peak value from zero in 1.2 microseconds and then falls to 50% of peak value in 50 microseconds. The represented wave shape is shown below,
The breakdown or flash-over voltage of the electrical equipment with this wave shape are required to be equal or higher than the basic insulation level fixed and the spark over voltage and discharge voltage of the protecting devices like lightning arresters, are definitely required to be lower than these values so that during lightning surges, the discharge occurs through the lightning arresters not through the equipment itself. There must be sufficient margin between the lightning arrestor and insulation level of the equipments.
Basic Insulation Level Table
| Nominal System Voltage | Indian Standards BIL | British Standards BIL |
| 11 KV | 75 KV | – |
| 33 KV | 170 KV | 200 KV |
| 66 KV | 325 KV | 450 KV |
| 132 KV | 550/650 KV | 650/750 KV |
| 220 KV | 900/1050 KV | 900/1050 KV |
