Page 94 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
92 Chapter Three

before excitation begins to increase. It can be seen that the generator
voltage control loop is complex and the calculation of voltage dip on
application of load is best undertaken by the machine designer. The drop
cannot be less than the product of the impact kVAr and the subtransient
reactance.
The subtransient reactance (X″) is due to the stator leakage flux, in
other words, that part of the stator flux which does not link with the
rotor windings. The leakage flux originates in the stator winding over-
hangs and in the stator slots themselves. The stator windings will also
have some resistance which is inseparable from the reactance but the
value is low compared with the reactance and it is has little effect.

The Voltage Drop Caused by Step Load
Application
For normal loads having a power factor of say 0.8, the voltage drop on
application of generator rated kVA is likely to be 14 to 16 percent for a
self-excited machine (in which the exciter field is energized from the
generator output), and 11 to 13 percent for a machine having a perma-
nent magnet pilot exciter. The limitation of engine step loading usually
precludes the application of full rated kVA to the generator but the volt-
age drop may be assumed to be proportional to the applied kVA.

The Voltage Drop Caused by Motor Starting
Starting of squirrel cage induction motors is a common cause of voltage
dips and the most common method of starting is to connect direct on
line. The direct on-line starting current (impact current) is likely to be
seven times the rated current for motors up to 200kWm, and six times
rated current for larger motors, but there is wide variation. The power
factor will be low, of the order of 0.15. The voltage drop on application
of generator rated kVA is likely to be 18 to 20 percent for a self-excited
machine, and 14 to 16 percent for a machine having a permanent magnet
pilot exciter. The impact kWe will typically be of the order of 120 percent
of the running kWe, but special motors such as those designed for high
torque starting will take a higher impact kWe.
As the motor accelerates the current taken reduces and its power fac-
tor increases. The locus of the current phasor is a semicircle as shown
in Fig. 3.1.
During acceleration the kWe demand rises to a maximum and then
reduces to the running power. Provided that the motor is not coupled to a
high inertia load the peak power will be transient and of short duration.
For low-cost installations use is sometimes made of the power which the
engine has in reserve for governing purposes, but this power is not
intended for this purpose and the practice is not recommended. The peak

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