Industry-Building
Electrical Distribution
Solutions
Harmonic distortion-voltage
versus current
The problem
In this issue,
we describe
the
differences
between
voltage
and current
distortion,
and offer
guidelines
to predict
voltage
distortion
levels on
typical power
systems
Diane Morando, a Schneider Electric sales engineer in Albuquerque, New
Mexico (USA) called one day with a knotty dilemma concerning harmonics.
She was preparing a bid package which included twenty or so adjustablespeed drives. She was stumped by a line in the bid instructions which read,
“Vendor’s equipment must not exceed 5% hamonic voltage distortion, else
harmonic filtering or other means must be provided to limit voltage
distortion to less then 5%.” Diane was understandably puzzled by this
requirement, because she knows that the harmonic voltage distortion level
is difficult to predict based on load equipment alone.
As Diane discovered, many engineers understand harmonics enough to
know that harmonics need to be limited, but not how to do it. The engineer
who wrote the specification also knew that harmonic voltage distortion
levels below 5% on low voltage systems generallly do not cause problems.
That, however, is where his understanding ended.
Diane rightly concluded that she could not predict the impact of her drives
on the plant’s voltage distortion without additional information. She knew
that the same set of drives could produce different levels of harmonic
voltage distortion at two different locations. To be safe, Diane considered
supplying line reactors on each drive. This increased her bid by an amount
she felt was unacceptable. Add to that the suspicion that other vendors
might claim that their drives do not need additional equipment like reactors
and filters to meet the 5% distortion limit.
So, what additional equipment, if any, should Diane bid?
ENMED199032EN
Electrical Distribution
Solutions
Industry-Building
Key concepts and Terms
Voltage Distortion
vs Current Distortion
Small transformer
represents high
impedance
Non-electronic loads produce undistorted
currents when energized from an undistorted
(sinusoidal) source (see figure 1).
Undistorted Current
Undistorted Voltage
Large transformer
represents low
impedance
10 kVA
4% Imped
Transformer
100 kVA
4% Imped
Transformer
10% THD
Voltage–
Excessive
10 Amps
40% THD
2% THD
Voltage–
Acceptable
Identical
Non-Linear Loads
10 Amps
40% THD
Figure 3: The same nonlinear load can produce different
levels of harmonic voltage distortion depending on the
source impedance.
Figure 1: Linear loads produce an undistorted current
waveform when energized from an undistorted voltage
source.
Electronic loads, like adjustable-speed drives,
produce distorted currents when energized
from an undistorted source (see figure 2).
Distorted Current
Undistorted Voltage
Figure 2: Nonlinear loads produce distorted currents when
energized from an undistorted voltage source.
Voltage distortion occurs when these distorted
currents flow into the source impedance. The
more source impedance, roughly, the more
voltage distortion will result.
In figure 3, the smaller transformer represents
a higher impedance to the adjustable-speed
drive than does the large transformer. An equal
amount of current distortion produces more
voltage distortion on the smaller system than
on the larger circuit. In the diagram, the
resulting voltage distortion is 10%. This level of
distortion can cause operating problems and
excessive heating, and reduce the lifetime of
equipment energized from this transformer.
ENMED199032EN
Electrical Distribution
Solutions
Industry-Building
Figure 3 indicates the level of harmonic
distortion to illustrate the differences in
impedance between the two systems.
How is the distortion level determined before
equipment is installed and energized? And, will
this distortion level be excessive? The voltage
and current THD can be predicted through
computer modeling of the source impedance
and adjustable-speed drive. The following
guidelines may also help.
Typical Harmonic Levels
Small adjustable-speed drives (in the 5-100 hp
range) are usually pulse-width modulated
(PWM) drives. These three-phase loads may
have current distortion levels over 100%, as
shown in figure 4. DC drives in this category
inject less harmonic current, usually about 30%
or so. Switch-mode power supplies serve
personal computers, fax machines, and other
electronic office equipment. These single-phase
devices inject currents with about 80% THD.
Current
Voltage
Figure 4: Pulse-width modulated adjustable-speed Ac
drives produce a characteristic double-hump current
waveform that is rich in harmonics.
Harmonic Distortion-Voltage versus Current
The solution
Voltage Distortion Rules of Thumb
Commercial
As indicated earlier, voltage distortion levels
cannot be determined solely from the current
distortion of the load. But there are guidelines
that, with a little information about the electrical
system to which the harmonic loads will be
applied, may help determine whether excessive
voltage distortion is likely to result.
Caution: if power factor correction capacitors are
or will be in service, the guidelines described
below do not apply!
Most electronic loads in a commercial building
are single phase. Typical loads are personal
computers, electronic ballasts, and other
electronic office machines. In the U.S., these
single-phase devices generally are served
at 120 V, usually by a step-down transformer
if the utility service is 480 V. Voltage distortion
on the 480 V system is usually not seriously
affected by the 120 V loads, but this guideline
has exceptions. Office buildings with a high
percentage of load in single-phase loads,
coupled with many 480 V adjustable-speed
drives (usually pulse-width modulated drives
serving chillers, air handling fans, and pumps)
can experience problems with voltage distortion.
Voltage distortion should be less than 5% if total
nonlinear loads are less than 20% of transformer
capacity.
Industrial
Most industrial plants use three-phase six-pulse
rectifiers serving dc or ac drives. The drives
produce fifth and seventh harmonic currents.
If the three-phase dc drives total less than about
30% of the transformer capacity, and there are
not significant electronic loads or power factor
correction capacitors, the resulting voltage
distortion will probably be less than 5%. For ac
pulse-width modulated drives, about 20% of
transformer capacity can be adjustable-speed
drives, without exceeding 5%.
You can get a rough estimate of this criterion by
assuming the drive horsepower approximately
equals the drive kVA requirement. Since
transformers are rated in kVA, the total drive
horsepower (kVA) should be less than 20% (ac)
or 30% (dc) of the transformer capacity. For
example, twenty 50-hp dc drives (1000 kVA)
on a 2000 kVA transformer exceeds the 30%
guideline. These 20 drives should not be applied
without a harmonic study or simulation.
ENMED199032EN
Electrical Distribution
Solutions
Industry-Building
Morando Example
The bid Diane was preparing included 16 PWM
drives on one 750 kVA transformer, and two
on another. The total horsepower was 390 in the
first case, 100 in the second. Using the estimate
that one hp equals one kVA, the first transformer
would serve 390 KVA in electronic drives,
out of a capacity of 150 kVA, or 52%.
This loading considerably exceeds the guideline
of 20%. The 16 drives should not be applied
without a harmonic system analysis. In the
second case, the electronic drives represent only
13% of the transformer capacity. This installation
will not cause excessive voltage distortion
if the drives are the only harmonic-producing
loads served by the transformer.
Conclusion
Responding to a specification which sets
requirements on voltage distortion requires
careful thought. The amount of voltage distortion
that will result from a given set of electronic
loads depends on the characteristics of the
electrical system serving the electronic loads,
not just on the loads themselves. Without power
factor correction capacitors, the amount of
electronic load can be as much as 20%
(ac drives) or 30% (dc drives) of the transformer
capacity without exceeding 5% voltage distortion.
With capacitors, excessive voltage distortion can
occur at much lower levels of harmonic currents
due to resonance.
ENMED199032EN
PowerLogic Circuit Monitors offer the benefit of
monitoring harmonics at the drive or transformer
to detect harmonic distortion before it becomes
troublesome. Circuit Monitors should be included
in any drive installation to ensure that power
system harmonic levels can be tracked and
controlled.
Schneider Electric
Industries SA
Centre Merlin Gerin
F - 38050 Grenoble cedex
France
Tel.: +33 (0)4 76 57 60 60
Fax: +33 (0)4 76 57 73 62
http://www.schneiderelectric.com
service-commc4@mail.schneider.fr
ART.28265
This paper was originally published as a part of the series PowerLogic
Solutions, copyright Square D Company 1998. Used with permission.
This document has been
printed on ecological paper.
Published by: Schneider Electric SA
Design and layout by: Insign’
Printed by:
09/1999