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Back-to-Back Capacitor Switching Formulas From IEEE C37.99-1990

There are two formulas shown in this section both have the same fundamental beginning.

On the capacitor inrush current calculations sheet (20) stated

IMAX =

When switching a capacitor step with other steps already energized, the banks, at this instant are in series therefore, we have the following:

CEQ = or, for two steps

 (29)

Formula (20) will now become

IMAX =

(30)

From (21) C =, Substituting this into (30) we get

LET (2A F)= T

(31)

 factoring under the radicand we get

(32)

Let V2 = V2L-L =

 

Factoring this (32) we get the following:

(33)

This will reduce to

IMAX = 1330

(34)

For 60 Hz and 50 Hz

IMAX = 1456.7

 (35)

The second formula from IEEE C37.99-1990 development is as follows:

Let KVAR =

(36)

Example:

A bank of 600 KVAR @ 12.47KV with 1-200KVAR per phase

=27.7 Amps

 (37)

Looking back at (34) we had

IMAX = 1330

Replacing KVAR with (36)

IMAX = 1330

(38)
To allow VL-L
 

Factoring we get

IMAX =

(39)
IMAX = 1747

(40)

 

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