Page 15 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 13

The high zero sequence impedance of the interstar secondary winding
is then an advantage in that it limits earth fault currents. If the
standby supply is being added to an existing installation, the effect of
the high zero sequence impedance of the interstar secondary winding,
on the operation of any earth leakage protection relays in the high volt-
age distribution system, should be considered.
When a generator transformer is fitted, the voltage regulator should,
instead of setting the generator voltage, be arranged to set the trans-
former secondary voltage. This is best achieved by sensing the genera-
tor voltage and incorporating a volt drop compensation circuit within
the voltage regulator. The compensation circuit includes an analogue of
the transformer impedance and is supplied with an analogue of the
load current. It is therefore able to calculate the voltage drop across the
transformer and modify the generator voltage to maintain the correct
voltage at the transformer secondary terminals. The alternative arrange-
ment of sensing the transformer secondary voltage is not favored for
two reasons. Firstly, a potential transformer may be necessary and sec-
ondly, the voltage signal will probably be carried over a long length of
cable between the transformer and the voltage regulator. Such a cable
is vulnerable to damage, and if the voltage signal is lost the generator
has to be shut down.

Standard Reference Conditions
The standard conditions for ac generators are specified in ISO 8528 and
BS EN 60034-1 as:

Altitude above sea level 1000 m
Cooling air temperature 40°C
Coolant temperature at inlet 25°C

Voltage Regulators

The voltage regulator maintains the output voltage of the generator
within the specified limits. It has developed from a variety of ingenious
electro-mechanical devices to the versatile electronic devices of today.
The accurate and skilled assembly that the electro-mechanical regula-
tors demanded is not compatible with modern manufacturing tech-
niques, and friction and the inertia of moving parts set a limit to the
accuracy and speed of response.
All regulators are now electronic, the final control being by transistors
instead of variable resistors, which has led to improved accuracy and
speed of response. Typical performance figures appear in ISO 8528 and are
included in the section titled “Starting Mechanisms.”

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