Page 165 - Uninterruptible Power Supplies

Static Uninterruptible Power Supplies
Static Uninterruptible Power Supplies 163

The use of a common battery is not considered good practice. There is
the possibility of circulating currents due to slight variations in charger
performance, but more significant is the effect on system reliability.
Development of more elaborate systems is illustrated in Fig. 5.20. In
panel a the three modules can be operated with either mains supply 1
or 2 and loads can be mechanically switched between the two alterna-
tive bus bars. In panel b there are two separate parallel systems, each
with its own alternate mains supply, and the loads can choose to oper-
ate with either system. The load transfer module is shown in Fig. 5.21.
This design ensures that there will be no break in supply to the load if
either of the two parallel redundant systems fails.

Monitoring
Monitoring of UPS units has advanced considerably and, by using tele-
phone lines and electronic mailing systems, copious amounts of infor-
mation and control are now available remotely. Systems now can be
maintained from a remote maintenance area and, indeed, if required,
controlled and in certain instances first aid repair can be implemented.
A complete log of all events is available. This will include any volt-
age/power variations to the system, usage of the battery, and usage of
a standby prime mover in the event of a prolonged outage. In multi-
module systems such full redundant systems may also be connected
and full details are made available. Some indication of the enormous
amount of information available will be gleaned by examining Figs. 5.22
and 5.23.
The availability of such monitoring techniques has had a profound
effect on operational requirements of UPS facilities, gone are the days
which I can remember when one received a telephone call during unso-
cial hours from a member of security staff requesting what action
should be taken due to flashing lights on UPS systems when the equip-
ment was correctly responding to a power outage!
Maintenance on site is now virtually eliminated and fully qualified
staff are immediately aware of any events on site. Thus, costs are reduced
and any remedial response is immediate.

Bibliography
BS 50091-1-1993 or EN 50091-1

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