Page 803 - Industrial Power Engineering and Applications Handbook

Power capacitors: behaviour, switching and improvement of power factor 231759
-
7-
:. fl = 13.26 1- 0.4 kHz
\I 129.19
= 13.26 x 0.0556 x lo3 Hz
= 738 Hz

9
Insulation '2mmJL4 mm ' which is substantially dampened with the use of additional
inductance, and is only slightly more than the switching
frequency on single capacitor switching as worked out in
Example 23.3.
(b) If we use two 25 mm2 cables as shown, then
r1 = 7.8 mm or 0.78 cm 23.1 2 Capacitor panel design
and the number of turns that will be necessary: parameters
(
42.93 x = % x N' x 7.5 log, 7.5 Since this device is also associated with the same power
10 0.7788 x 0.78 -2) system as a switchgear assembly, it should generally
meet the same specifications (Section 13.4) except for
4n
= - N' x 7.5(10ge 98.77 - 2) small variations in operating conditions and test
x
1 og requirements. Capacitors generate excessive heat when
4n in service. Installations employing large capacitor banks
= ~ x N' x 7.5 x 2.593 must therefore have a capacitor mounting structure suitable
109 to dissipate heat freely and permit circulation of fresh air
or N = 13.26 during normal operation. To achieve this, open-type
Say, 13 turns enclosures are usually preferred when it is possible to
house the panel in a separate room or mount the units on
Switching frequency a structure in an open switchyard. It may also be provided
As calculated above: with an expanded metal enclosure. Forced cooling within
the enclosure or the room where such banks are installed
is common practice to dissipate the excessive heat. For
more details refer to the following publications:
C1 eq = 5 x 9 x 120 x F
and C, = C2 = 9 x 120 x 1 O-6F IEC 6083 1 - 1 and 6093 1 - 1 for LT, and IEC 6087 1 - 1 and
IEC 60143- 1 for HT systems

23.12.1 Selecting the voltage rating of the
capacitor units
- - , 1 (5 x 9 x 120 x 10-6) + (9 x 120 x 10-6)
-
\ i"" x 10+)(5 x 9 x 120 x 10-6) must be taken that during operation the voltage across
2n When selecting the voltage of the capacitor units care
3x5
the capacitor units does not fluctuate beyond *lo%. If
- lo3 I 6x3~5~10~ this happens the voltage rating of the capacitor units
2rr d 0.4 x 6 x 5 x 9 x 120 must be chosen so that the variation across the units
under unfavourable operating conditions does not exceed
= 13.26 kHz
*lo%.
This will be the surge frequency of the switching circuit when
the sixth capacitor is switched on a circuit that has five capacitor Example 23.6
units already switched. The improved value of this switching If 3000 kVAr capacitor banks are required for a system of 33 kV
frequency, f;, after the introduction of inductance 42.93 pH + 7.5'4, -5% and the capacitors are rated for 34/?'3 kV then
in each capacitor circuit will become. the output of the capacitors at nominal voltage will reduce to
/y+L)x6
(g)' 3000 = 2826 kVAr
x
L2e.q = x 10-6H
5
which will result in an undercompensation by 174 kVAr or
5.8%. If the system voltage falls to, say, 31.5 kV during peak
load periods the rating of the capacitor banks will fall further
Cleq, = C1 eq = 5 x 9 x 120 x F and may render the system unstable,
Effective kVAr at 31.5 kV
C, = C2 = 9 x 120 x F
Therefore the only change in the previous calculation is for = [ x 3000
LZeq, which is now substituted by (0.4 + 3L), i.e. (0.4 + 3 x
42.93) or 129.19 for 0.4 in the numerator, = 2575 kVAr