Page 794 - Industrial Power Engineering and Applications Handbook

23/750 Industrial Power Engineering and Applications Handbook
We have ignored any leakage capacitance between line to system voltage, as shown in Tables 26.4 or 13.1 unless
line or line to ground at 33 kV. specified for still higher voltages by some users. Accordingly
equation (23.8) does not apply in this case. Some local
Tackling the harmonics electricity distribution authorities, depending upon the likely
To eliminate all the harmonic contents from the supply it is maximum voltage variation on their systems, may sometimes
essential to provide shunt-filter circuits separately for each ask for still higher voltages. But such distribution networks
harmonic disorder. As this is a costly arrangement it is not may not remain stable in the long run and must be improved
followed in practice. It is enough to drain the highest (in as far as possible with the use of reactive controls, as discussed
magnitude) harmonic disorder. This will substantially improve later.
the current and voltage waveforms and make the circuit
inductive for all higher harmonics, preventing a condition of The above configuration or size of each unit is not mandatory
resonance at any harmonic disorder during a line disturbance. and can be altered, depending upon the economics of capacitor
The line reactance and capacitance of the capacitors may voltage and size of units available. Protection for all types of
resonate at a frequency configurations is easily available through various schemes
discussed in Section 26.1.
1
rh = - (23.6)
2nm From the above it can be inferred that for an accurate
= 1. 1 analysis of a system, particularly where the loads are of
varying nature or have non-linear characteristics it is
2n 442.23 x x 18.42 x lo6 necessary to conduct a harmonic analysis. The above
= 180.5 HZ
corrective measures will provide a reasonably stable
say, at about the third or fourth harmonics. But both these network, operat-ing at high p.f. with the harmonics greatly
harmonics are considered negligible in the system hence, suppressed. The improved actual line loading, eliminating
the filter circuit will be essential for the fifth harmonic. The the fifth harmonic component, which is compensated,
series reactor required in the capacitor circuit is tuned, say,
at about 225 Hz to be on the safe side, to account for variation I
in the line parameters and the inductor itself to keep the I; = [4002 + 49 x 27.5' + 121 x 10' + 169 x 52]5
circuit inductive under all conditions of line disturbances:
= 461.9A as against 525 A
:. 27~ 225 x L' = 1
x
352
2rrx 225 x C and improved actual p.f. = -
L'= 1 461.9
or
(2n)' x (225)' x 18.42 x lo4 = 0.76
= 27.2 mH and 44 = 40.5"
or XL' = 2n x fx 27.2 x 10-3 A phasor diagram (Figure 23.18(c)) illustrates the
= 8.54 Q at fundamental frequency. reduction in the actual loading and enhanced load transfer
capacity of the network which can be achieved with
This is about 4.94% of Xc, and must be compensated by the help of harmonic suppressions. For even better
providing additional capacitor units to maintain the required utilization, the system may be tuned for higher harmonic
level of p.f. as discussed in Section 23.9.1.
disorders also.
:. Modified size of capacitor banks = 6300 x 1.0494
= 661 1.2 kVAr
23.1 0 Excessive charging currents
Let us choose a bank size of 6720 kVAr for easy selection of (switching inrush or making
each unit. To adopt a better protective scheme, let us select
the configuration of a double star (Figure 25.5(b)). c u r re n t s)
:. Each bank for each arm of a double star
The circuit of an uncharged capacitor unit, when switched,
- 6720 acts like a short-circuit due to the very high natural
-~
6 (transient) frequency& of the switching circuit, causing
= 1120 kVAr the capacitor reactance X,, to approach zero. The value
Let us arrange them two in series and seven in parallel, of the making current will depend upon the magnitude of
based on recommendations made in Section 25.5.1 (iii) and the applied voltage and the impedance of the circuit (motor
as shown in Figure 23.18(b). and cables) or of the system that will form the switching
circuit. In other words, it will depend upon the fault level
:. Size of each unit = 1120 = 80 kVAr of the circuit and the rating of the capacitor units connected
2x7
in that circuit. This switching transient current, as illustrated
36*
and voltage rating = - in Figure 23.19, as such being high, will be even higher
45x2 if the circuit already has some switched capacitor units
= 10.39 kV due to partial discharge of the already charged units into
the uncharged units, as discussed in Section 23.10.2. The
*Note behaviour of the capacitor switching circuit under various
The capacitor units are generally designed for the highest switching conditions is described below.