Page 20 - Uninterruptible Power Supplies

Standby Power Generating Sets
18 Chapter One

■ Integral control modifies the proportional feedback signal by adding
to it a signal which has a rate of change proportional to the error, and
attempts to nullify the offset. Adding integral to proportional control
increases the accuracy at the expense of increasing the overall
response time.
■ Derivative control modifies the feedback signal by adding to it a sig-
nal which is proportional to the rate of change of the error. Adding
derivative control to proportional plus integral control decreases the
overall response time.

The initial letters P, I, and D are often used when referring to these
modes of control. The proportional mode (P) will result in a speed or
voltage droop. If isochronous running or constant voltage is required,
integral control (I) is added to form a PI mode of control. Derivative
control (D) may be added to the PI mode to form a PID mode of control
which will result in an improved transient performance.

Starting Mechanisms
Deriving the Starting Power
Engine or turbine acceleration for starting will be by one or more bat-
tery driven electric motors or, for larger diesel engines only, by com-
pressed air.
For diesel engines an electric starting system will include one or
more preengaged electric starter motors engaging with a starter ring
on the flywheel rim, a battery, and a battery charger. After a few revo-
lutions the engine reaches its firing speed and develops sufficient
power to accelerate itself to rated speed. For gas turbines a pony motor
is mounted on the gearbox. It may take up to a minute for the rotor to
reach its self-sustaining speed, and for the turbine to accelerate itself
to rated speed and to be ready to accept load.
Compressed air starting is limited to diesel engines of ratings of the
order of 1 MW or above; smaller engines do not have space in the cylin-
der heads for the air injection valves. Selected cylinders will be fitted
with air injection valves and, during starting, a rotating air distributor
valve feeds compressed air to appropriate cylinders in turn. Within the
engine room there will be a compressed air cylinder with drain valve
and pressure gauge, an electrically driven and automatically controlled
air compressor to maintain the pressure within the vessel, and for
black starts a diesel driven compressor. In the United Kingdom the
compressed air installation will be subject to the Pressure Systems and
Transportable Gas Containers Regulations 1989 as amended by S.I.
1991/2749. The regulations set standards for the installation and

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