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Definition of Basic Insulation Level (BIL)

Definition of Basic Insulation Level (BIL) - photo by bob1217 @ Flickr

Introduction to BIL

Insulation levels are designed to withstand surge voltages, rather than only normal operating voltages. Since theinsulation lines and equipment is protected by surge arresters draining the surges rapidly before the insulation isdamaged, the arrester must operate below the minimum insulation 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 the components of asystem.

Figure 1a - Insulation coordination

Insulation values above this level for the lines and equipment in the system must be so coordinated that specificprotective devices operate satisfactorily below that minimum level.

In the design of lines and equipment considering the minimum level of insulation required, it is necessary to definesurge voltage in terms of its peak value and return to lower values in terms of time or duration. Although the peakvoltage may be considerably higher than normal voltage, the stress in the insulation may exist for only a very shortperiod of time.

For purposes of design, the voltage surge is defined as one that peaks in 1.5 microseconds and falls to one-half thatvalue in 40 microseconds (thousandths of a second).

It is referred to as a 1.5/40 wave, the steep rising portion is called the wave front and the receding portion the wavetail, Figure 2.

Figure 2 - Surge Voltage 1.5 by 4.0 Wave

Insulation levels recommended for a number of voltage classes are listed in Table 1. As the operating voltagesbecome higher, the effect of a surge voltage becomes less; hence, the ratio of the BIL to the voltage class decreasesas the latter increases.

Table 1 – Typical Basic Insulation Levels

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

Voltage class, kV Distribution class Power class (station, transmission 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 Safety Code.

Distribution class BIL is less than that for power class substation and transmission lines as well asconsumers’ equipment, so that should a surge result in failure, it will be on the utility’s distribution system whereinterruptions to consumers are limited and the utility better equipped to handle such failures.

The line and equipment insulation characteristics must be at a higher voltage level than that at which the protectingarrester begins to spark over to ground, and a sufficient voltage difference between the two must exist.

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

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

The impulse level of lines and equipment must be high enough for the arresters to provide protection but lowenough to be economically practical.

Surges, on occasion, may damage the insulation of the protective device; hence, insulation coordination shouldinclude that of the protective devices.

As there are a number of protective devices, mentioned earlier, each having characteristics of its own, thecharacteristics of all of these must be coordinated for proper operation and protection.

Before leaving the subject of insulation coordination, such coordination also applies within a piece of equipment itself.The insulation associated with the several parts of the equipment must not only withstand the normal operatingvoltage, but also the higher surge voltage that may find its way into the equipment.

So, while the insulation of the several parts is kept nearly equal, that of certain parts is deliberately made lower thanothers; usually this means the bushing. Since the bushing is usually protected by an air gap or arrester whoseinsulation under surge is lower than its own, flashover will occur across the bushing and the grounded tank.

The weakest insulation should be weaker by a sufficient margin than that of the principal equipment it is protecting;such coordinated arrangement restricts damage not only to the main parts of the equipment, but less so to parts moreeasily accessible for repair or replacement.

The insulation of all parts of the equipment should exceed the basic insulation level (BIL). Figure 1b.

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)

About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized fordesign of LV high power busbar trunking (<6300A) in power substations, buildings andindustry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programmingand web-design. Present on Google+

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7 Comments

1. Rating Definitions Applied to Medium Voltage Fuses | EEP

Oct 03, 2014

[…] by ANSI C37.42-1996 as having dielectric withstand (BIL) strengths at power levels, applied primarily instations and substations, with mechanical construction […]

(reply)

2. Effects of very fast transients (VFT) on HV equipment | EEP

Jun 11, 2014

[…] switching or line-to-ground faults inside a gas insulated switchgear (GIS) are below the basic insulationlevel (BIL) of substation and external […]

(reply)

3. Jan Ledochowski

Mar 27, 2014

There is a mistake below title Hot Swap Controllers PDF:….one-half that value in 40 microseconds (thousandths of a second).should be: millionth of a second.

(reply)

4. UDAYKUMAR

Feb 09, 2014

Dear Sir/Madam,

can you tell me abt earth falut details…

(reply)

5. Glossary of Power Transformer Terms | EEP

Jun 05, 2013

[...] of the winding is common to both the primary and the secondary circuits.Back to Index ↑ | A BBILBasicImpulse Level, the crest (peak) value that the insulation is required to withstand without failure.BushingAn [...]

(reply)

6. Purpose of Disconnect Switches In HV Substation | EEP

Jan 14, 2013

[...] the insulation or insulators associated with them must coordinate with the rest of the system and a BIL(Basic insulation level) capable of withstanding voltage surges.In general, they consist of a conducting blade,hinged at [...]

(reply)

7. Methods of Controlling Lightning Overvoltages in HV | EEP

Dec 08, 2012

[...] on the first three or four towers where line co-ordinating gaps are reduced in an attempt to reduce incomingvoltage surges will increase the risk of a ‘close-in’ backflashover. Location of Surge ArrestersConsidering [...]

(reply)

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