BIL = Basic Impulse Level - bascially a indicator of dielectric strength

There are two tests, lightning impulse and switching impulse tests.  The first is to simulate lightning strikes where the full wave shape is where the voltage crest reaches a maximum at 1.2 microseconds and then decays to 50% in 50 microseconds.
The switching impulse test is used to simulate man-made type transients.

Mike

So, with a higher BIL rating, that would mean a "better" dielectric strength?

Thanks also for the explanation

Tim

That is correct. Switchgear and insulators typically include a BIL rating. 15kV ANSI equipment is typically rated 95kV BIL, for example, but 110kV BIL is available as an option. The BIL ratings should be coordinated lightning/surge protection and expected transient overvoltages.

Florida Power and Light uses a BIL of about 200 KV on suburban 7,620Y13,200 volts with lightning arrestors once every 3 poles. This amounts to using 34.5 KV insulation amounting to a 260% or 210% insulation level depending on whether you are comparing the operating rating or the BIL respectively.

On rural lines FPL uses an overhead ground wire 7,620Y13,200 so that they only need lightning arrestors once every farm. The BIL on these lines is only 95 or 110 KV.

Basically, the BIL that you need depends on how much lightning you expect.

"BIL is the reference insulation level expressed as an impulse crest (or peak) voltage with a standard wave not longer than a 1.2 x 50 microsecond wave."
_{Since electrical apparatus, equipment and installation are subjected to overvoltage either by switching surges, internally or externally, such as lightning, the insulation and clearances should be sufficient to withstand the imposed electrical & thermal stresses overtime. Therefore, the BIL or SIL (for switching) is a rough measure of the dielectric withstands capability of a particular electrical system or equipment.}

See the enclose info for additional details

a) Standard transformer BIL ratings



a) BIL standard 1.2x50 μs wave (from UWI):

For a underground distribution system, would you just go with a standard BIL rating for the cable, or is there a certain percentage for underground required?

For underground distribution cable, you normally specify an insulation level of 100% or 133% which gets you different thicknesses of insulation.  The BIL comes with the insulation thickness.  133% insulation thickness provides better reliability with some margin for degradation caused by aging, water, surges, etc.

To determine what BIL you need, you have to look at what surge arresters are used and where.  The front-of-wave (F.O.W.) equivalent and discharge voltages of the arrester, combined with possible wave reflections, must be below the chopped wave withstand and BIL respectively by a sufficient margin.

CABLE INSULATION RATING:
It  is customary in the ANSI marketplace to define the cable insulation based in the fault clearing time as describe below:

100%:   t_clearing <1 min.
133%: 1 min< t_clearing <1 hr.
173%:  1 hr < t_clearing < Infinite (indefinite).
_{NOTE: This rating is available but infrequently used}

The only voltage rise that cable insulation will be capable to withstand is the power frequency overvoltages. Therefore, often cable insulation is tested using Hi-pot stressing the cable insulation for duration rage in minutes. http://www.okonite.com/engineering/high-voltage-testing.html

 Since the cable insulation will not directly capable to withstand the lightning overvoltage (100’s to 1000’s KV for 10^-6 seconds), there is not reason to rate cable in term of BIL. However, the surge arrester used to protect the cable is rated for lightning wave and other temporally overvoltage parameters.

During design of UG distribution line there are two major factors to consider for the selection of the proper insulation thickness:

1-Critical stresses over cable system:
a) Insulation wall: Average Voltage stress.
b) Shield surface (if applicable): Max. Voltage.

2- Operation Voltage Stresses (normal or abnormal):
a)    Normal Operation:  V=Vn= VLL/1.73.
b)    Abnormal /Fault   : V= VLL.

_{Notice that In case b, one of the worst fault conditions is the line-to-ground fault (LGF). As longer the duration of the fault, larger is the electrical and thermal stresses impressed in the cable insulation.  The type of protective device (fuse, relay, breaker, etc) and system configuration (Delta, wye, …) have significant influence on the duration of the fault.}

Lightning protection will only be as good as the grounding electrode. Also, on longer lines the equipment grounding path provided by a neutral can have enough impedance that the cable needs to be rated for the phase to phase voltage or almost the phase to phase voltage.

Duquesne Light Company in Pittsburgh, PA likes to use 23 KV cable on 14,400Y24,940 volts. The neutral is of pretty good cross section for the feeders and most lines are substantially less than 25 miles long.

However, Cleveland Electric Illuminating Company in Ohio likes to use 46 KV cable on 19,920Y34,500 volts. This partly because a full capacity neutral really only exists back at the supply transformer and the load transformers are connected phase to phase. Close to the supply station this amounts to a 230% insulation level but at the far ends of the circuits amounts to about 133%. Some earth faults might flow back through the overhead ground wire or the neutral of the 7,620Y13,200 suburban distribution, but there are also line segments without an overhead ground wire and the distribution is 4,400 volts 3-wire ungrounded.

So, whether you use 100%, 133%, 173%, or even 250% insulation depends on a lot of factors.