Page 43 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 41

standby supply is available the generating set may be shut down and
no circuit breaker operations are necessary.
For the shutdown sequence the first operation would be to open the
standby supply switching device, thus unloading the set, followed by
the closing of any load-shedding devices and, after a short delay, the
closing of the normal supply switching device. The delay between open-
ing one supply and closing the other is important. The two supplies are
not synchronized and if one is replaced by the other without a break
several problems can arise. Typical problems are:
■ If the load includes any large induction motors they will be recon-
nected to the supply with their rotor fluxes in random phase rela-
tionship with the new supply. If the rotor flux is far removed from its
synchronized position very large currents will flow and protective
devices may operate. The rotor flux of an induction motor decays
rapidly and the problem is avoided if there is a break of say 0.5 sec-
ond between the two supplies.
■ Any transformers or other static electromagnetic machines will expe-
rience a changeover of supplies in random phase relationship at the
time of changeover. This can result in the magnetic circuits of the
machines experiencing two successive half cycles of the same polar-
ity leading to magnetic saturation, an excessive magnetizing current,
and the operation of protective devices. The problem is avoided if
there is a break of say 0.5 second between the two supplies.
■ Any synchronous machines will experience a changeover of supplies in
random phase relationship at the time of changeover. If the two sup-
plies are far removed from synchronism very large currents will flow
and protective devices may operate. To overcome this problem it may
be necessary to allow such machines to run down to a stationary con-
dition before connecting the new supply, and to restart it. This problem
can theoretically arise when changing from normal to standby supply
as well as from standby to normal supply. If a large synchronous motor
is running light its inertia could very easily keep it running during the
10 to 15 seconds required for the standby supply to become available.
It follows that any synchronous motor should be disconnected from its
supply before any changeover of its supply is initiated.

Engine Warning and Alarm Shutdown
Sequences
To protect the engine against catastrophic damage and to avoid unnec-
essary running when disconnected from its load the installation should
be provided with devices which monitor the following parameters and
pass appropriate signals to the control module:

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