Page 236 - Uninterruptible Power Supplies

Notes on Systems Installation
234 Chapter Nine

You should note that VRLA cells do not completely inhibit the evolution
of gas, it is simply significantly reduced and most ventilation systems
will deal with this. However, emanation of hydrogen may be a problem
for large VRLA systems and one should allow for roughly a tenth of
hydrogen gas evolution compared to a normal flooded cell.
In the case of nickel-cadmium flooded cells, ambient temperatures
for operation are between 20 to 40°C ( 4 to 104°F), with some
increasing derating below 15°C (59°F). Similar environmental condi-
tions apply. Instead of protection against acid corrosion, the protection
should be against alkaline corrosion. Again, the emission of hydrogen
is calculated by using the above guideline formula. Note that in prac-
tice the gas emission from an alkaline cell is higher—for a given dc
potential voltage across a system you need more cells than for lead
acid. (Note: The nominal voltage for lead acid is 2 V per cell compared
to nickel-cadmium 1.2 V per cell.)
The alternative to batteries is kinetic energy from flywheels. Such
systems may be considered on grounds of space saving and possibly on
the grounds of high ambient temperature. Such systems are relatively
high in initial cost and insufficient time has elapsed for the writer to
determine long-term running costs. The cost of bearings replacement
against the known high cost of removing and replacing a battery. Power
efficiency comparisons show no very marked differences, possibly a
slight advantage in lower power consumption for the battery system.
Rotary UPS systems have their own advantages and disadvantages.
If we look at the rotary transformer system then clearly we have con-
siderations for the battery as defined above. The rotary transformer
system has the advantage that the standard system has a low har-
monic distortion to the mains supply (3 percent) well within G5/4.
There are no starting surges, and harmonics induced onto the machine
from the load can reach very high levels before any effect occurs.
Dimensionally the system is little different from a static system, but
is heavier. The unit may be broken down into discreet sections for instal-
lation. Antivibration mounts are used and the units are not fixed to the
floor. Maintenance is required (lubrication every 6 months), apart from
this a yearly visit is advisable. As with static systems, remote monitor-
ing is available.
The generator will clearly have to cover all load requirements and we
would suggest that the following points are given consideration:

Discharge of exhaust gases
The warm and polluted air discharged from the engine room
Noise break out from the engine room and from the exhaust discharge

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