Page 137 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 135

TABLE 4.3 The Manner in Which Harmonic Current
Cancellation Is Achieved in a 12-Pulse Rectifier

Adding an Input Filter
It is possible to reduce the harmonic currents flowing through the sup-
ply system by providing a low-impedance path across the rectifier
power input terminals. The harmonic currents generated by the recti-
fier will then be shared between the supply system and the shunt path
in a manner dependent on the inverse ratio of their impedances.
The low-impedance path may comprise one or more shunt connected
filter circuits tuned to particular harmonics, or a simple untuned capac-
itor. In either case a series inductor may be included on the supply side,
which effectively increases the system impedance, renders the shunt
path more effective, and reduces the harmonic current flow through the
supply system.
The series inductor may also serve another purpose; if properly cho-
sen it can prevent any likelihood of the capacitor (or the net capac-
itance of the filter) resonating at a harmonic frequency with, for
instance, the leakage reactance of a local transformer. This subject is

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