VTdrive - VARIABLE SPEED DRIVES(VSD) AND HARMONIC ABATEMENT TECHNIQUES0 pages

VTDRIVE VARIABLE SPEED DRIVES AND HARMONIC ABATEMENT TECHNIQUES
This paper will focus on the impact of harmonics when
applying Variable Frequency Drives. Its purpose is not to
provide an extensive study on this subject but rather to
offer guidance to engineers and users on the use of
harmonic mitigation solutions with VTdrive E5-H 12
Pulse and V5-H 12 Pulse drives.
IEEE 519 provides guidelines for recommended harmonic
levels in electrical distribution systems including
maximum voltage and current distortions. In general,
when harmonic mitigation is used to reduce THID to IEEE
519 limits, IEEE voltage limits are also met. Therefore this
paper focuses on the issue of current distortions (THID).
Harmonic currents are multiples of the fundamental
frequency (60Hz) that are greater than the fundamental
frequency. Continuous conduction or linear loads (e.g.,
across the line starters) contribute very little harmonic
current while discontinuous conduction situations (e.g.,
non-linear loads like VFDs) contribute an infinite number of
odd and even harmonic currents. Figure A provides an
illustration of the VFD input current waveform when no
filters or harmonic mitigation techniques are used.

Figure A

Even harmonics (e.g., 2 nd, 4 th, …) are not a concern to most
installations as they cancel each other out and add a
minor amount to total line current. In contrast, odd
harmonics (e.g., 3rd, 5th, …) add a significant amount to the
non-sinusoidal and fundamental currents and are a major
concern to most installations. The level of harmonics that
a VFD adds to the electrical line is heavily dependent on
the distribution line’s impedance – the higher the
impedance, the lower the harmonic content. Table 1
provides typical VFD harmonics depending on the input
line impedance.
Harmonic
Number

0.50%
5th
0.8
7th
0.6
11th
0.18
13th
0.1
17th
0.073
19th
0.06
% THD - I 102.5
Amps
43%
Increase

INPUT IMPEDANCE
1%
0.6
0.37
0.12
0.075
0.052
0.042
72.2

1.5%
0.5
0.3
0.1
0.06
0.04
0.03
59.6

2%
0.46
0.22
0.09
0.058
0.036
0.028
52.3

2.5%
0.42
0.2
0.08
0.05
0.032
0.025
47.6

3%
0.4
0.16
0.073
0.049
0.03
0.022
44.13

5%
0.32
0.12
0.058
0.039
0.022
0.008
34.96

23% 17% 13%

11%

9%

6%

Table 1 (1)

Several techniques can be used to mitigate harmonics.
The most commons are:
• Line Reactors
• Passive Filters
• DC Chokes
• Active Filters
• Multi Pulse drives
12 Pulse Drives

18 Pulse Drives

Line Reactors
Three phase line reactors added in series with a VFD will
reduce harmonics. This is a relatively inexpensive solution
and can lower harmonics by 50% depending on the
amount of impedance added to the line. The most
common values of AC line reactors are 3% and 5%.
DC Chokes
Instead of placing line reactors in series with the VFD, a DC
choke can be added to the drive’s DC bus reducing
approximately the same amount of harmonics as the AC
reactor. The advantage of DC chokes is they are smaller
in size and are often mounted inside the VFD. All E5-H
12 Pulse and V5-H 12 Pulse VFDs incorporate DC
chokes in the drive, providing harmonic reduction without
incurring additional material or installation cost to the
user.
Multi Pulse Drives
12 and 18 pulse drives are alternative solutions to reduce
harmonics. In 12 pulse drives, the 5th and 7th harmonics
are theoretically non-existent. Similarly, an 18 pulse drive
only injects the 17th harmonic and higher to the line. To
achieve the multiple pulse effect, additional converter
bridges and phase shifting transformers are used making
the solution expensive and increasing panel size.
Passive Filters
Passive filters consist of static components like inductors,
capacitors, and resistors arranged in predetermined
fashion to either attenuate the flow of harmonic currents
through them or to shunt the harmonic component into
them. There are several types of passive filters but the
most effective is the low pass broadband filter, which
offers great performance and versatility with low risk of
resonance with the line.
Active Filters and Front Ends
Most passive techniques aim to cure the harmonic
problems once they have been created. Active filters, or
active front ends, use dynamic switches like IGBTs and
other power components to stop harmonics from
occurring in the first place. Current flow through a switch
is manipulated to recreate a waveform that linearly
follows the applied voltage waveform. Apart from the
active front ends, there also exist active shunt filters that
introduce a current waveform into the distribution
network that when combined with the harmonic current