User Manual

5
■ Typical methods used to reduce
harmonics
The recommended practices of IEEE 519
provide guidelines for the design of elec-
trical systems that contain both linear
and nonlinear loads. It addresses the
responsibility that users have not to
degrade the voltage of the utility serving
other users by requiring excessive
amounts of nonlinear currents from the
utility. It also addresses the responsibility
of the utilities to provide users with close
to a sine wave of voltage. The recom-
mended practices provide guideline
limits on the amount of harmonic cur-
rent drawn from the utility at the point
of common coupling as well as limits on
the amount of voltage distortion the
harmonic currents can produce.
The design of HVAC electrical systems
using VSDs is influenced by the recom-
mended practices, and in some cases,
corrective measures must be taken to
comply with the guidelines. In the case
of VSDs, this means higher cost because
AC line reactors or DC link chokes are
often added to help reduce the amount
of 5th and 7th harmonic currents.
■ Harmonic current spectrum:
The fundamental problem
All periodic waveforms can be represen-
ted by a set of sinusoidal waveforms
consisting of the fundamental frequency
plus various other harmonic frequencies.
The AC line harmonic currents, with a
6-pulse bridge, have characteristic fre-
quencies at 6n±1 times the fundamental
frequency where n is an integer. A 6-pulse
VSD on a balanced supply will only gene-
rate odd order harmonics from the 5th
order upwards (5th, 7th, 11th, 13th,
17th, etc). Even harmonics and triplen
(multiples of 3rd) will be canceled out.
The amplitude of the harmonic currents
depends on the impedance of the
AC power system, the size of the DC link
capacitor, and the load on the induction
motor. The 5th and 7th harmonic currents
are predominantly large with standard
VSDs.
Current waveform
When measured at the input terminal of a 6-pulse VSD, Total Harmonic Destortion
can be in excess of 150 %.
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