(BIL) Basic Impulse Level 38 kV

[shocker23]

a substation's has a 3-step 21.6 MVAr capacitor bank where 38 kV vacuum switches are used to switch the bank in and out. When solidly grounded Y system has a line to ground BIL of 150? if you take the rms value of 38 kv = 27 kV and the BIL at 27 kV= 150. The system is 34.5 kV and the corresponding BIL is 200.

A) when do you need consider the RMS value of voltage. why do you need it?
B) By using a switch rated 34.5 kV for a solidly grounded Y system and it rated BIL is 150 it seems the system was under designed?
C) Where, besides IEEE; can you find an industry respected guide or standard describing BIL ratings for each voltage class.

 

[magoo2]

a) The rms value of 34.5 kV is 19.92 kV on a phase to neutral basis. You need this value as a starting point to select the appropriate surge arrester rating. Normally we apply a 1.25 factor and multiply it by the line-to-neutral voltage to get the minimum arrester duty cycle rating for an effectively grounded system.

b) not sure how you reached your conclusion in b. The arrester will clamp voltage for switching surges at about 80 or 90 kV. If the equipment (switch) is rated 150 kV BIL, then it is protected by the arrester with sufficient protective margin. So it is not underdesigned even if you factor in lead length and rate of rise issues.

c) I'm sure the IEC has similar information although, in the IEC world, the lines are typically 3 phase with no neutral (which requires a higher rated arrester) and the BIL is higher (200?).

 

[tem1234]

34.5 kV is already a RMS value between 2 phase.

In the cooper EDSP book, the standard BIL at 34.5 kV for a recloser (or switch) is 150 kV, and for a transformer it's 200 kV. So the system seems to be ok.

 

[Kiribanda]

shocker22,

a)If I understand your question properly,27 kV is used to select the proper rated voltage for the arresters on your 34.5 kV (rms)system.

b)Since your system nominal voltage is 34.5 kV rms, the vacuum switch should have BIL=200 kV. But if you use a bushing or switch for your solidly grounded neutral terminal, then it can have a BIL=110 kV even.

3)Please refer IEC60071-1 for such a table which gives the BIL levels for various IEC voltages.