Page 800 - Industrial Power Engineering and Applications Handbook

231756 Industrial Power Engineering and Applications Handbook

- 5 x 0.4 + 0.4
3x5 pH
01
and C,eq = 9 x 120 + 9x 120 + 9 x 120 + 9 x 120 + 9 x 120
= 5 x 9 x 120 pF
or 5 x 9 x 120 x F

Figure 23.25 Equivalent circuit for a 60 kVAr bank in star
made of 3 x 20 kVAr units
5 x 9 x 120 x 10-6 x 9 x 120 x 104
0.4x6 x10-6(5x9x120x10-6 +9x120x10-6)
Capacitor normal current for each 60 kVAr bank
\I' ~ ~ 6 x 9 ~ 1 2 0
2
= 415 -X 5x9x120x9x120
Switch and HRC fuse and contactor rating, as in Section 3x5
25.6, = 415 x 43750
= 1.5 x 83.5 or 25 414.6 Amps
or 125.25 A If switching is effected when the incoming capacitor is already
charged, then the inrush current, I,,,, will be nearly twice this,
Thus, for a single capacitor switching of a 60 kVAr bank, a i.e.
rating of 125 A will be required for the switch, fuse and the
contactor etc. = 25 414.6 x 2
= 50.80 kA
Step 2: Effect of parallel switching
Consider the case when all five units are already energized However, in no case will the switching current exceed the
and the sixth is switched. The equivalent circuit can now be short-circuit kA of the switching circuit.
represented as shown in Figure 23.26.
:. Le, for five switched capacitor banks Inference
This current is excessive for a normal switching device to
- 1 make frequently, even for one half of a cycle each time.
1
1
1 +- 1 +--+ - However, on an LT circuit, it would be much below this due
& + 0.4/3 0.413 0.413 0.413 to the circuit's own resistance and inductance, which have
been ignored in the above analysis. Nevertheless, it would
0.4
= -pH be advisable to limit such a switching surge to a reasonable
3x5 value to protect the system from damage during repeated
and L2 eq for the total circuit switchings as well as the switching devices (selected at 150%
of the capacitor's normal current).
Step 3: Limiting the inrush current
Consider an inductance L pH introduced into each capacitor
0.413 pH circuit as shown in Figure 23.27(a) or (b) to limit the switching
current within the making capacity of the switching device.
Then
(?+Lj
L& = 5 x 104H
9x 120
PF
[y+L)x6
L;eq = 5 x 104H
and C;eq = 5 x 9 x 120 x 104F

-1 2 3 4 5 6 The equivalent switching circuit is shown in Figure 23.27(c):
Steps Unit being
< , switched now :. Improved
Already switched units ( G) J2 5 x 9 x 120 x 10-6 x 9 x 120 x 10-6
(Lq and Cleq) I' --~415
m-6 /(e+L)x6
Figure 23.26 Equivalent switching circuit for six capacitor IC" ' x10-6x6x9x120x10-6
units of 60 kVAr each 5