Understanding Basic Insulation Level (BIL): A 101 Comprehensive Guide

BASIC INSULATION LEVEL (BIL)

Basic insulation level, BIL refers to the voltage level that a system of insulation can withstand without breaking down or experiencing a dielectric failure. Dielectric failure is the sudden breakdown of insulation properties that occurs at a certain level of voltage at which the insulator no longer keeps up insulating but starts behaving like a conductor. BIL is crucial because electrical equipment needs to operate safely under various voltage stresses and environmental conditions.

The basic insulation level, BIL is specified in kilovolts (kV) and it signifies that the particular insulation can handle transient voltage spikes, switching surges, and similar electrical stresses within the specified limit, without compromising the safety or performance. BIL ratings to determine the appropriate insulation levels needed for reliable and safe operation of the electrical equipment.

Power system components, including reactors, transformers, circuit breakers, and power cables, are engineered not only to function at their designated voltage ratings but also to endure voltage surges or transients that can arise in the power network, such as those from lightning strikes or switching operations. For example, a power transformer designed for a primary voltage of 132 kV and equipped with a BIL of 650 kV can withstand voltage surges up to 650 kV without encountering insulation failure. This capability ensures reliable operation and protects against repeated potential damage that can be caused by electrical stress in the system.

Moreover, it’s essential to recognize that BIL requirements differ based on the type of electrical equipment and must be adhered to for dependable operation. Non-compliance with BIL specifications can cause significant damage to both the equipment and the overall electrical system. Therefore, BIL is a very important factor to consider during the design, selection, and maintenance phases of electrical equipment to safeguard against potential failures and ensure the reliability of the power system.

IMPORTANCE OF BASIC INSULATION LEVEL BIL IN POWER SYSTEM

Safety:

BIL plays an important role in ensuring the safety of electrical systems. Inadequate insulation can lead to failure, fire, and electrical shock. Proper insulation however helps prevent electrical faults, safeguard personnel, and ensure safety.

Reliability and Continuity of Service:

Basic insulation level directly impacts the reliability and continuity of power systems. Adequate BIL ensures that equipment can consistently operate under normal conditions and withstand transient voltage surges. This reliability is essential for uninterrupted service delivery.

Equipment Lifetime:

Electrical equipment represents a significant investment. Adequate insulation not only ensures reliable operation but also extends the equipment’s lifespan. Equipment capable of withstanding voltage stresses over time requires less frequent maintenance and replacement, reducing the associated costs.

Compliance with Standards and Codes:

BIL requirements are specified in international and local standards that vary from country to country (e.g., IEC, IEEE) governing the design and operation of electrical systems. Compliance with these standards is essential for legal adherence and ensures the safety and reliability of the electrical infrastructure.

FACTORS INFLUENCING BIL

Voltage Levels:

The operating voltage of the electrical system determines the required BIL. Higher system voltages necessitate higher BIL ratings to withstand higher electrical stresses. 

Environmental Conditions:

Factors such as humidity, temperature variations, and pollution levels affect insulation performance. Equipment in harsh environments will require higher BIL to maintain reliability and safety. Pollutants that get deposited over the insulator can create an ionization path leading to a flashover and causing significant damage.

Voltage Transients:

Events like lightning strikes or switching operations generate voltage transients that stress insulation. Equipment exposed to such conditions needs higher BIL to prevent dielectric failure and ensure operational integrity.

Specific Equipment Requirements:

Different types of electrical equipment have unique BIL needs based on their operational characteristics. For instance, power transformers require high impulse voltage withstand capability, while power cables need sufficient power frequency to withstand voltage to operate effectively. The surge arrester will have comparatively lower BIL. This coordination of insulation helps to engineer a failure in equipment less costly and easy to maintain rather than causing a failure to costly and important equipment which if failed can create severe power outages.

Understanding and applying appropriate Basic insulation level, BIL considerations is essential for designing, selecting, and maintaining electrical equipment to ensure optimal performance, safety, and longevity within power systems.

LIGHTNING IMPULSE VOLTAGE

The impulse voltage refers to a deliberately applied non-periodic transient voltage characterized by a rapid rise to peak followed by a slower decline to zero. Specifically, a lightning-impulse voltage is a unidirectional voltage that swiftly reaches its maximum level and then decreases slowly, with a front time of less than 20 µs. The full lightning-impulse voltage denotes the uninterrupted lightning-impulse voltage without a disruptive discharge. According to IEC 60060 standard specifications, a standard lightning impulse has a defined duration of 1.2 µs ±30% rise time to peak and 50 µs ±20% total duration.

SWITCHING IMPULSE VOLTAGE

Switching impulse surges are short-duration, irregular or impulse form, and highly damped. A typical switching impulse voltage range form is the 250/2,500 microsecond, time-to-crest/time-to-half value wave. It means it takes 250 microseconds to reach the peak voltage and 2500 microseconds to decline to half of the peak value.

POWER FREQUENCY WITHSTAND VOLTAGE

Power frequency withstands voltage is the highest level of voltage that insulation can withstand for a predetermined time mostly 1 minute. For a 132 KV system the power frequency withstand voltage is 1.1 * 132 = 145KV for 1 minute.

The breakdown or flash-over voltage of electrical equipment exposed to such waveforms must equal or exceed the fixed Basic Insulation Level (BIL). Additionally, the sparkover and discharge voltages of protective devices, such as lightning arresters, must be lower than these values. For the selection of a 132 KV Lightening arrester, the maximum phase-phase voltage is considered to be 110% of the nominal voltage which is 1.1*132 = 145 KV and for an effectively earthed system, the maximum phase-to-earth voltage is 80% of the phase-to-phase voltage. Hence the rated arrester’s voltage will be = .80* 145 = 120KV.

This will ensure that during lightning surges, any discharge happens through the lightning arresters rather than through the equipment itself. Maintaining a sufficient margin between the lightning arrester’s capabilities and the insulation levels of the equipment is critical to protecting the equipment from damage during transient events like lightning strikes.

BASIC INSULATION LEVEL TABLE