# Document Library

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

 Let V2 = V2L-L = (32)

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    To allow VL-L (38)

Factoring We Get

 IMAX = (39)

 IMAX = 1747 (40)