Def init ion of Basic Insulation Level (BIL) - photo by bob1217 @ Flickr

http://electrical-engineering-portal.com/definition-basic- insulation- level-bil December 3, 2012

Definition of Basic Insulation Level (BIL)

Introduction to BILInsulat ion levels are designed to withstand surge voltages, rather than only normal operat ingvoltages. Since the insulat ion lines and equipment is protected by surge arresters draining thesurges rapidly before the insulat ion is damaged, the arrester must operate below the minimuminsulat ion level that must withstand the surges.

An example is shown in Figure 1a below.

The minimum level is known as the Basic Insulation Level (BIL) that must be that of all of thecomponents of a system.

Figure 1a - Insulation coordination

Figure 2 - Surge Voltage 1.5 by 4.0 Wave

Insulat ion values above thislevel for the lines andequipment in the system mustbe so coordinated thatspecif ic protect ive devicesoperate sat isfactorily belowthat minimum level.

In the design of lines andequipment considering theminimum level of insulat ionrequired, it is necessary todef ine surge voltage in termsof its peak value and return tolower values in terms of t imeor durat ion. Although the peakvoltage may be considerablyhigher than normal voltage,the stress in the insulat ionmay exist for only a very shortperiod of t ime.

For purposes of design, thevoltage surge is def ined asone that peaks in 1.5 microseconds and falls to one-half that value in 40 microseconds(thousandths of a second).

It is referred to as a 1.5/40 wave, the steep rising port ion is called the wave front and thereceding port ion the wave tail, Figure 2.

Insulat ion levelsrecommended for a numberof voltage classes are listedin Table 1. As the operat ingvoltages become higher, theeffect of a surge voltagebecomes less; hence, therat io of the BIL to thevoltage class decreases asthe lat ter increases.

Table 1 – Typical BasicInsulation Levels

Basic insulat ion level, kV(standard 1.5- × 40-μs wave)

Voltage class, kV Distribut ion class Power class (stat ion, t ransmission lines)

1.2 30 45

2.5 45 60

5.0 60 75

8.7 75 95

15 95 110

23 110 150

34.5 150 200

46 200 250

69 250 350

*For current industry recommended values, refer to the latest revision of the National Electric SafetyCode.

Distribut ion class BIL is less than that for power class substat ion and transmission lines as well asconsumers’ equipment, so that should a surge result in failure, it will be on the ut ility’s distribut ionsystem where interrupt ions to consumers are limited and the ut ility better equipped to handle suchfailures.

The line and equipment insulat ion characterist ics must be at a higher voltage level than that atwhich the protect ing arrester begins to spark over to ground, and a suff icient voltage dif ferencebetween the two must exist .

The characterist ics of the several type arresters are shown in the curves of Figure 3.

Figure 3 - (a) Sparkover Characteristics of Distribution Value Arresters; (b) SparkoverCharacteristics of Expulsion Arresters

Theimpulselevel oflines and

equipment must be high enough for the arresters to provide protect ion but low enough to beeconomically practical.

Surges, on occasion, may damage the insulation of the protective device; hence, insulat ioncoordinat ion should include that of the protect ive devices.

As there are a number of protect ive devices, ment ioned earlier, each having characterist ics of itsown, the characterist ics of all of these must be coordinated for proper operat ion and protect ion.

Before leaving the subject of insulat ion coordinat ion, such coordinat ion also applies within a pieceof equipment itself . The insulat ion associated with the several parts of the equipment must notonly withstand the normal operat ing voltage, but also the higher surge voltage that may f ind itsway into the equipment.

So, while the insulat ion of the several parts is kept nearly equal, that of certain parts isdeliberately made lower than others; usually this means the bushing. Since the bushing is usuallyprotected by an air gap or arrester whose insulat ion under surge is lower than its own, f lashoverwill occur across the bushing and the grounded tank.

The weakest insulation should be weaker by a suff icient margin than that of the principalequipment it is protect ing; such coordinated arrangement restricts damage not only to the mainparts of the equipment, but less so to parts more easily accessible for repair or replacement.

The insulat ion of all parts of the equipment should exceed the basic insulat ion level (BIL). Figure1b.

Figure 1b - Simplistic diagram illustrating Basic insulation level (BIL) and Insulation coordination

Resource: Power Transmission and Distribution – Anthony J. Pansini (Get this book from Amazon)