Page 847 - Industrial Power Engineering and Applications Handbook

System voltage regulation 241801

It is difficult to operate such a system on full load. It = 19.05 + 1.024 (0.98 - J 0,199) + 2.761 (0.199
is bound to have wide voltage and load fluctuations, + J 0.98)
more so on a line disturbance. Such a system may have
to be operated well below its rated capacity to retain its = 19.05 + 1 .00 - J 0.02 + 0.55 + J 2.71
stability. even when the voltages on the primary and the = 20.6 + J 2.51
secondary transformers are adjusted so that the required
rated voltage is available at the receiving-end. A voltage = 20.75 tan- I 2.51
suing of 158 between a full load to a load rejection ~ 20.6
condition is too wide and may lead to outage of the = 20.75 L 6.95"
system on a line disturbance. Since there is no further
scope to improve the above situation with the help of :. voltage drop = 20.75 ~ 19.05 = 1.7 kV
shunt capacitors (the p.f. is already at 0.98). let us do so or 8.2% of E.
with the help of series compensation. Since the far-end
p.f. is being maintained at a high level, the system can This is also the regulation of the system. See the phasor
achieve some stability. A series compensation to the extent representation shown in Figure 24.27(b).
of, say 60% of the total line impedance should not be
excessive, as the line already has some series resistance. 1njeftiuric.e.r
Moreover, the transformers will have some resistance 1 Raising the compensation from 60% to, say, 70%
too which has been ignored in the above analysis and may further improve the above situation but this may
hence the chances of a sub-synchronous or ferro-resonance not he advisable to maintain a high level of stability
occurring will be remote. during line disturbances. Moreover. the p.f. of the
system has already reached a high of cos 6.95". i.e.
:. Series compensation = 0.6 (I .c)S + J 13.26) 0.99, which also is not advisable. Tu be more safe. the
= 8.04 R (in absolute terms) level of shunt compensation should be slightly reduced.
Say. x,. = 8 R
Note
and 5ize of series capacitors. Since the load variation on an HT dictriburion network will be
only nominal. and the network will also have enough reAance.
52s' x 8 it should be possible to compensate the systeni up to 70% 01- so.
- to further improve the regulation of the network. hay up to 5%
-
IO00 oI'E,. withom jeopardizing the level ofstnbility. Thc application
= 3305 kVAr per phase engineer can take a more judiciou\ decijion. knowing the
condition of the network to he compensated.
For 10% load variation, to be on the safe side. the
capacitors must be rated for: 2 The voltage variation with the series compensation,
= ( 1 1 )I x 220s although high, at about 8.2%, is still manageable by
adjusting the tappings on the transmitting-end
= 3668 kVA1 transformer, for which a tramformer with higher
tappings may be selected or a transformer with a higher
and voltage across the capacitors, secondary voltage may be chosen. say, at 36 kV or
v, = I, . x, so. For minor adjustments, the tappings on the
receiving-end transformer may be used. With this,
= 52s x 1.1 x 8 the above system can be utilized to its optimum
= 4.62 kV capacity.
3 The phasor displacement between the transmitting
If we consider three units in series and nine in parallel and the receiving ends, with the use of series
(Figure 24.27(a)) then the size of each unit compensation, is reduced and the receiving-end voltage
has moved closer to the transmitting-end voltage. which
-- 2668 = 98.8 say. 100 kVAr will provide more stability to the system during a line
-
3x9
disturbance.
and the voltage rating of each unit 4 Even a higher cross-section of line conductors would
be able to improve the above situation by reducing
- 4.62 = 1.54 kV
-
3 the line resistance and hence the voltage drop.
Thc improved line impedance S It will be pertinent to note that series compensation
on HT lines will be more effective when the line
= I.Y5+J 1326-JSO inductive reactance itself is high, as when the line is
individually feeding highly inductive loads, such as
= 1.95 + J 5 26
an induction or an arc furnace or other similar loads.
and the improved transmitting-end voltage, the load Nevertheless, it can also be effectively applied on
remaining same. overloaded distribution networks similar to the one
we have considered above. to raise the line capacity
E\= 1905+(0525L- I148")(1.9S+JS.26) and reduce the voltage dip at the receiving end.