Page 42 - Uninterruptible Power Supplies

Standby Power Generating Sets
40 Chapter One

Monitoring the Status of the Normal Supply
Monitoring the voltage and frequency of the normal supply should be
at a point upstream of the normal supply changeover contactor or cir-
cuit breaker. If it is at a point downstream of the changeover device the
restoration of the supply cannot be monitored. In addition the logic
should be such as to prevent the generator attempting to supply the
load in the event of the normal supply having tripped on fault or over-
load. This can be achieved by connecting an auxiliary contact on the
protective device which passes information to the control module. It is
usual to monitor the three phase voltages, any phase unbalance, and
the frequency.

Automatic Starting and Shutdown
Sequences
The automatic start sequence should commence after the expiry of an
adjustable delay, (say 0–10 seconds), initiated by the normal supply
mains monitor. The delay avoids spurious starting during a transient
loss of the normal supply, and is particularly useful if the supply sys-
tem includes any autoreclosing switchgear.
The automatic shutdown sequence is initiated either by the control
system or by one or more of the engine safety shutdown devices. Some
engines (particularly the larger units) are arranged to continue run-
ning off load for 5 or 10 min in order to allow heat to be removed from
the cylinder block, heads, and pistons before finally shutting down.
This procedure is not applicable to alarm shutdown signals such as
overspeed, but the omission of the light load running period can lead to
boiling of the cooling water at engine hot spots.

Operating the Switching Devices
After a failure of the normal supply the generating set should be auto-
matically started, and the switching sequence to load the set com-
pleted, after which the standby supply remains in use until a shutdown
command is given.
The switching sequence may involve three or more functional devices,
which may be circuit breakers or contactors. They are the supply change-
over devices, the load-shedding device(s), and any step load limiting
devices necessary to apply the load to the set in two (or more) steps or
stages. It is important that any load-shedding and step load limiting
devices are open before the standby supply changeover device is closed.
If the switching devices are circuit breakers, the control system
should arrange for their operation to be delayed until the generating
set is up to speed and ready to accept load. This avoids unnecessary cir-
cuit breaker operation—if the normal supply is restored before the

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