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Table 1
Acquisition costs for different designs of
complete power supply equipment for a large
telephone exchange ol the SPC type.
The power consumption ol the exchange is
250 kW, the electronics factor 60 "h and the mean
distribution distance 45 m. The costs are
related to the cost of a 48 V booster converter
system
Fig. 5
Plant cost as a function of the electronics
factor. The plant data, apart from the
electronics factor, are given in the text of
table 1. The cost is related to a 48 V booster
converter system with 60 °/o electronics factor
A higher electronics factor reduces the additional
outlay for direct converters. However, more
expensive rack converters (greater regulation
range) means that even with an electronics factor
of 100 Vo the 140 V plant is still more expensive
than the equivalent 48 V booster converter plant
46 V booster converter system
= = 140 V system
% Relative plant cost
creased regulation range would, on
average, increase the cost of the converters
by about 15%. Regulation of
the distribution voltage with DC/DC
booster converters is carried out centrally
and using units with relatively
high power. This gives a low cost per
watt output power. To this must be
added the fact that the principle of
booster converters is particularly economical
owing to the fact that only
about 20% of the total power passes
the main circuit of the converters.
Calculations have shown that with
booster converters the regulation cost
per watt is between 5 % and 8 % of
the cost per watt for small rack converters.
If this cost is compared with the additional
cost (15%) of an increased regulation
range for the rack converters,
it is clear that central regulation of the
system voltage using DC/DC booster
converters is definitely economically
advantageous.
Higher system voltage
An analysis of the characteristics of a
power supply system with a system
voltage in the range MOV to 300V
shows that there are essentially two
kinds of advantages with such a system:
1. As a result of the higher voltage the
current in the distribution cables is
reduced for a certain distributed
power. Thus smaller distribution
cables can be used with higher
voltages.
2. It is only to a very small extent that
the system voltage can be used for
direct feeding of the power consuming
units. Thus practically all
the power must pass through (regulated)
converters before it reaches
the units that consume it. This
means that larger battery voltage
variations can be permitted than in
the case of, for example, the full
float system or the cell switching
100 % system. Thus the batteries can be
Electronics factor utilized better since they can be
discharged to a lower level when
there is a mains failure. A considerable
reduction in the size of the
battery can be achieved in this way.
The cost of the rectifiers will also
be reduced to some extent since,
among other things, the end cell
rectifier will not be required.
As can be seen, the cost reductions in
point 2 are not actually caused by the
increased system voltage, but rather
by the introduction of an efficient regulation
of the system voltage during
emergency operation. The same advantages
are of course obtained with,
for example, a booster converter system.
Cost comparison for different
power supply systems
The relative costs for some different
designs of a power supply plant for a
telephone exchange of the SPC type
are shown in table 1. As an example a
large exchange with a power consumption
of 250 kW has been chosen. The
electronics factor is 60% and the
mean length of the distribution cables
is 45 metres. These values are typical
for exchanges of this type and size.
Battery saving possible both in 48 V
and 140 V systems
It can be seen from table 1 that the battery
costs are considerably higher in
the cell switching system than in the
other alternatives. 2 There is, however,
no difference between the remaining
alternatives in this respect, which illustrates
the above mentioned argument
that, from the point of view of the battery,
it is unimportant whether the
voltage regulation of the distributed
power takes place centrally, using
booster converters or locally, using
rack converters and direct converters
for 140/48 V.
For the 140 V system direct converters
are also required
In the 140 V system the costs for boost^r
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