Page 136 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
134 Chapter Four

lowest forcing frequency (which will be equal to the rectifier pulse
number).

Effect on Electronic Devices
A local generator is likely to incorporate electronic devices such as a
tachometer and a voltage regulator which may use the zero voltage cross-
ing points as timing signals. Deep notching of the supply voltage can lead
to additional zero crossings and can affect the operation of such devices.
If problems are experienced it may be possible to obtain a clean supply by
using a small low-pass filter.
Voltage regulators may be affected by a distorted supply; if the regu-
lator is expected to control the fundamental component of voltage, the
harmonic voltages must be removed from the sensing signal and a low-
pass filter may achieve this as suggested in the previous paragraph. If
the harmonic voltages are not removed the regulator will set to the true
rms or the average value depending on the design. In cases of severe
distortion this can lead to considerable errors.

Reduction of Distortion Due to
Rectifier Loads

Increasing the Pulse Number
In the section titled “Harmonics Generated by Bridge Rectifiers” it is
stated that the current taken by a three-phase six-pulse bridge rectifier
includes only the fifth and seventh, eleventh and thirteenth, seventeenth
and nineteenth, etc., harmonics. If the rectifier pulse number is increased
to 12, the fifth and seventh, seventeenth and nineteenth, etc., are can-
celed while the magnitudes of the remaining harmonics are unaltered. To
comply with the planning levels of Engineering Recommendation G.5/4 it
is often necessary to use 12-pulse rectifiers.
For the same reason the supplies to rectifiers of multiunit UPS
installations should be phase shifted, to achieve a higher pulse number.
The supplies to two paralleled units should be shifted by 30°, three
units by 20°, etc. For a phase-shifted multiunit redundant system, the
loss of a unit will result in some additional harmonic currents being
drawn from the system.
The manner in which this harmonic cancellation occurs is not obvious
and the reader may wish to study Table 4.3 which is an attempt to explain
how it is achieved, it is not offered as a rigorous mathematical exercise.
It can be seen that for both fifth and seventh harmonics the two sec-
ondary currents have a phase difference of 180°. It follows that they
will be cancelled in the primary; similar reasoning may be applied to
the seventeenth and nineteenth harmonics.

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