Page 841 - Industrial Power Engineering and Applications Handbook

System voltage regulation 241795
After we have assessed the optimum power level it our purpose, parts of the curves that lie near the rated
becomes easy to decide the type and amount of reactive voltage, say, within V, f 5%. alone are relevant for study.
powcr control required to achieve this level, assuming The line will perform best at p.f.s very near unity and
that the lines can be loaded up to their thermal capacity cause the least possible voltage fluctuations by maintaining
and the optimum power derived above can be attained. a near-flat voltage profile over reasonable variations of
Our main objective will now be to arrive at the stability load. Leading p.f.s are not considered for reasons of
level of the system and the parameters that define this. capacitive overvoltages. The p.f. of the load can be
As noted above, the stability level defines the maximum improved by applying shunt capacitors near the load points,
power that can be transferred through a line without as discussed above.
causing a voltage tluctuation and angular difference
beyond acceptable limits, or a consequent outage of the
line. during a load variation, or a temporary line Irxje re n ce
disturbance. It should, in fact, maintain its continuity The voltage stability level diminishes with an increase
even during a fast clearing of a major fault. To determine in the line length. For very long line lengths, the far-end
the effects of Zo and 6 on the receiving-end voltage and voltage may swing from high to very high values during
consequently the transfer of power, P, within stable limits load variations, rendering it unsuitable for operation near
we will study the voltage equation of a lossless the maximum load transfer level. During light loads too
transmission line (considering K, = 0, for an easy the steeply rising voltage profile may cause a high-voltage
illustration), feeding a load P at a p-f. cos 4. swing on a small load variation. A load variation therefore
will cause wide to very wide voltage fluctuations and
render the system unsuitable rather than unstable for a
Radial lines power transfer near the required level. For transfer of a
load under stable conditions the line lengths of the
The transmitting-end voltage in terms of line parameters uncompensated lines will be too short and hence will not
can be represented by be economically viable. We will seek a solution to these

E, = V,. cos 0,. + JZ,, . I, sin 0, (24.12) problems with the help of these curves which will provide
an introduction to the utility of reactive power controls
where to improve the power transmission capacity of a line and
E, = phase voltage at the transmitting-end its quality through the following discussion.
V, = phase voltage at the receiving-end
0, = line length effect or Ferranti effect at the end of the Iiifluence of PF
line. in degrees
I, = load current The power transfer capability of the line rises ai. the
- P- JQ
- p.f. swings towards the leading region and diminishes
L’, as it swings towards the lagging region. Since a power
P = active load supply system is not run at leading p.f.s for reasons of
Q = reactive load dangerous overvoltages that may develop (as a result
K,, = line resistance per phase. It has been ignored and of overexcitation of the capacitors, Section 23.13) across
the line is considered lossless the terminal equipment it is advisable to run the system
Z,, = surge impedance of the line as close to unity p.f. as possible. Moreover, the field
system of the generating machines is also designed
The voltage stability of a system is the measure of for maximum operation at lagging p.f.s only, as
voltage fluctuations which must remain within permissible discussed in Section 16.4. At leading p.f.s (after a
limits during load fluctuation or rejection or other line certain limit) there is a possibility of its field system
disturbances and even temporary faults. We may therefore losing control and becoming ineffective.
solve the above equation for V, and P, to study the The receiving-end voltage rises with leading p.f.s and
behaviour of the system under varying load conditions, droops with lagging. This is illustrated with the help
P. As there are two more variables, load p.f. and the line of phasor diagrams (Figures 24.22(a) and (b).
length, which will influence P and V,, different sets of At unity p.f. the voltage variation and hence the
load curves can be drawn as illustrated in Figure 24.2 1, regulation is the least and maintains a near-flat voltage
for different line lengths at different p.f.s (at near unity, profile. This is the best condition to provide the highest
to obtain the best performance). From a study of these level of system stability from a voltage point of view.
curbes one can identify the most appropriate line
lengths which can extend the highest level of stability The power factor can be improved with the use of
to the system. For example, set ‘a’ of curves are more shunt capacitors at the load points or at the receiving
ideal compared to set ‘b’, which correspond to very end, as discussed above. It is not practical to have a
long line lengths, compared to the ideal line lengths of near-fixed loading for all hours of the day. Moreover.
set ‘a’. there may also be seasonal loads which may upset the
After identifying the likely line lengths we can then parameters considered while installing the capacitor banks.
study the most appropriate p.f. at which the load must be In such conditions the system may therefore have to be
transmitted to maintain the highest level of stability. For underutilized or run under a high risk of instability during