What is Basic Insulation Level ?

What is Basic Insulation Level ?

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.

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.