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

http://electrical-engineering-portal.com/definition-basic- insulation- level-bil February 25, 2013

Definition of Basic Insulation Level (BIL)

Edvard

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 abovethis level for the lines andequipment in the systemmust be so coordinatedthat specif ic protect ivedevices operatesat isfactorily below thatminimum level.

In the design of lines andequipment considering theminimum level of insulat ionrequired, it is necessary todef ine surge voltage interms of its peak value andreturn to lower values interms of t ime or durat ion.Although the peak voltagemay be considerably higherthan normal voltage, thestress in the insulat ion mayexist for only a very shortperiod of t ime.

For purposes of design, the voltage surge is def ined as one that peaks in 1.5 microseconds andfalls 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 anumber of voltage classesare listed in Table 1. Asthe operat ing voltagesbecome higher, the ef fectof a surge voltagebecomes less; hence, therat io of the BIL to thevoltage class decreasesas the 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

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

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 ElectricSafety Code.

Distribut ion class BIL is less than that for power class substat ion and transmission lines as wellas consumers’ equipment, so that should a surge result in failure, it will be on the ut ility’sdistribut ion system where interrupt ions to consumers are limited and the ut ility better equippedto handle such failures.

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 voltagedif ference between the two must exist .

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

Theimpulselevel oflinesand

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

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

economically 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 ofits own, the characterist ics of all of these must be coordinated for proper operat ion andprotect ion.

Before leaving the subject of insulat ion coordinat ion, such coordinat ion also applies within apiece of equipment itself . The insulat ion associated with the several parts of the equipmentmust not only withstand the normal operat ing voltage, but also the higher surge voltage thatmay f ind its way 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 isusually protected by an air gap or arrester whose insulat ion under surge is lower than its own,f lashover will 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).Figure 1b.

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