Normally two kinds of overvoltage may appear in an electrical system.
Power Frequency withstand Voltage
One type is power-frequency overvoltage. In a three-phase system, a fault in one phase can increase the voltage in the healthy phases to an elevated value. This condition can continue until the clearance of the fault. Also, due to sudden tripping or switching off of a laorge load, the voltage of the system may rise to an abnormal over volatge. It is also temporary. However, this overvoltage condition can last for a short duration due to inharent normalizing time of the system. Normally, any such overvoltage condition does not exceed more than few seconds. Because protective and automated systems clear and normalize the abnoramalities within this time.
Therefore, the insulation system must withstand a certain overvoltage level at power frequency for up to certain seconds. This level is the power-frequency withstand voltage. Since it is a power-frequency voltage, we express it in RMS value. Standards normally recommend the duration of withstanding overvoltage up to 60 seconds.
Impulse Voltage
Other type is impulse transient overvoltage. This type of overvoltages is very sudden and short in duration. Its magnitude can be much higher than the power-frequency withstand voltage. However, it lasts only for a few microseconds or for a few hundred microseconds. Lightning or switching operations usually cause this type of overvoltages.
Impulse due to Direct Lingthning Strokes
Transmission systems may be exposed to the open atmosphere. Therefore, a lightning stroke can directly hit the electrical system. A direct stroke can cause destructive over voltageson the system.
Impulse due to Indirect Lingthning Strokes
Lightning can also hit a nearby structure or object. In both cases, the system experiences a large transient voltage disturbance. Beside the direct strokes, when lightning strikes nearby, a very large current flows between earth and claudes. This large current creates strong electromagnetic effects. Due to induction, it can cause a transient overvoltage in the nearby power system. We also treat this as a lightning overvoltage.
Impulse due to Switching Operation
During switching operations, overvoltages can also appear. Sometimes current chopping occurs during inproper switching. When current chopping happens, the sudden interruption of current produces a switching overvoltage.
A switching impulse lasts longer than a lightning impulse. However, it is still very short and steep. Its duration is usually a few hundred to a few thousand microseconds. The amplitude of a switching impulse is generally lower than that of a lightning impulse but energy is significant due to its larger duration.
Basic Insulation Level (BIL)
Therefore, an insulator must have to to withstand both lightning impulses and switching impulses along with the one minute power frequency over voltage.
We design insulators based on three main criteria:
- Power-frequency withstand capability
- Lightning impulse withstand capability
- Switching impulse withstand capability
Together, these define the insulation strength of equipment. This combined rating is called the Basic Insulation Level (BIL) of the insulator.