Harmonic Misconceptions
Conrad St. Pierre - Electric Power Consultants, LLC
There have been many harmonic articles in trade magazines and the problems that can occur because of them.
Due to limited space and the technical level of the readers, many simplifying generalizations are made. These
generalizations become the rule, because they are repeated articles after an article, without explanation or
technical detail. These rules can be misapplied and point to harmonics as the culprit when they are not. The
following are samples of these rules, which are true for some conditions but not true for all conditions. The
reason for some of these rules will be explained.
The maximum amount of a harmonic current from a three-phase drive is 1/(harmonic order). That
is, the maximum 5th harmonic is 1/5 or 20% of the fundamental. This is a theoretical rule that applies for the
following particular set of conditions.
1.
The drive is a properly operating 6-pulse full-wave bridge drive operating at full-load with a step wave
output. The 12-pulse rectifier bridge drive will have greatly reduced harmonics.
2.
The DC bus voltage or current is constant. - meaning it has sufficient inductance or capacitance.
3.
The source impedance is low.
A good exception to this rule is a PWM (pulse width modulated) drive witha constant magnitude varying pulse
width output. These drives can have the 5th harmonic current up to 80% of the fundamental current with a
low series or internal reactance. The 5th harmonic current decreases to approximately 22% with a higher
series or internal impedance. The percent harmonic current also dependents upon the impedance being on
the input ac or dc portion of the converter. Figure 1 shows the change in harmonics on a PWM drive when
external impedance is added.
Percent Fundamental Current
100.0
80.0
5th
60.0
7th
11th
40.0
13th
17th
20.0
19th
0.0
0.1
1
10
Percent Impedance (Source plus Reactor or Transformer)
Fig. 1 - PWM Drive Harmonic Content
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Triplex harmonics do not flow through a wye-delta transformers. The fact is that triplex harmonic
currents can flow through and do flow through wye-delta transformers. The amount that flows depends on
the phase angle and magnitude of the currents in each of the three phases. For example, 100% of the third
harmonic current would pass through the transformer if they were equal in magnitude and displaced 120o from
each other. This flow of current would be just like a balanced three-phase 60 Hertz current as shown in Fig
2. On the other extreme, if the third harmonic current produced is in phase with the third harmonic currents
in the other two phases, the third harmonic current will add in the neutral of a wye connected transformer.
Most of third harmonic currents in a 60-Hz system are the latter. The third harmonic current in the wye legs
of a transformer will circulate in the delta winding to balance the amp-turns. Figure 3 shows this condition.
This misconception is caused by the generalization that all third harmonics behave similarly to fundamental
frequency zero-sequence currents. A 60-Hertz three-phase system can have positive, negative and zero
sequence currents when the phases are not balanced. Likewise, the third harmonic currents can have positive,
negative and zero sequence currents. When the third harmonic current in each phase is balanced and
displaced by 120o, the current is positive sequence. When the third harmonic currents are balanced and inphase with each other, the current is zero sequence. In most three phase systems the third harmonic current
produced are closer to being in-phase, but with a slight unbalanced in magnitude and phase angle. Therefore,
most of the third harmonic current will flow in the transformer neutral and the remainder through the
transformer.
1.0
1.0
0.0
0.0
-1.0
-1.0
TIME
1.0
1.0
0.0
0.0
-1.0
TIME
-1.0
TIME
TIME
1.0
1.0
0.0
0.0
1.0
-1.0
-1.0
TIME
TIME
0.0
-1.0
TIME
Fig. 2 - Out-of-Phase Third Harmonic Current Flow
(Displaced by 120 degrees)
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1.0
1.0
0.0
0.0
-1.0
-1.0
TIME
TIME
1.0
1.0
0.0
0.0
-1.0
-1.0
TIME
TIME
1.0
1.0
0.0
-1.0
TIME
1.0
0.0
0.0
-1.0
TIME
-1.0
TIME
Fig. 3 - In-of-Phase Third Harmonic Current Flow
(Displaced by 0 degrees)
Harmonic currents can cause an induction motor to run backwards. Besides running backwards, it is
commonly said that motors will overheat due to the additional horsepower required to overcome the harmonic
"torque fight.” - a term used to describe the backward torque on the motor. This misconception comes from
the fact that the 5th harmonic currents have negative sequence characteristics. By using the simple equivalent
circuit for an induction motor shown in Figure 4, the torque due to a 5th and 7th harmonic voltage can easily
be quantified.
S
XS
R
R S = 0.003 PU
X S = 0.08 PU
V
RM
XM
R
R
SLIP
R R = 0.01 PU
X R = 0.08 PU
R M = 20
XM = 5
SLIP = 0.01 PU
Fig. 4 - Induction Motor Equivalent Circuit
Figure 5 shows the per unit torque for an induction motor subject to 10%, 5th and 8.5%, 7th harmonic
voltages caused by adjacent SCR controlled drives. The harmonic voltage will result in 12% 5th and 6% 7th
harmonic current in the motor based on rated current or the total rms current being 101% of rated. The
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negative torque of the 5th harmonic at approximately rated speed is approximately 0.003% of the fundamental
torque, (Trq . I2*RR/s = 0.122*0.01/6 = 0.00003). Also, the 0.00001% forward torque produced by the
7th harmonic will cancel a potion of the backward torque produced by the 5th harmonic voltage. Clearly, the
motor with this very low negative torque will not run backwards. The additional fundamental current required
to supply the "torque fight" phenomenon is insignificant, but the increase in eddy currents and hysteresis losses
within the motor due to harmonics are not.
Many IEEE papers have quantitatively addressed this temperature rise and they appear to indicate a greater
possibility of motor overheating due to unbalanced voltage rather than from harmonics currents.
Per Unit Torque
4.0
-6
-5
-4
-3
-2
-1
Fundamental Torque
3.0
2.0
7th Harmonic Torque
1.0
0.0
0
1
2
3
4
5
6
7
8
Per Unit Speed (Base 60 Hz)
-1.0
5th Harmonic Torque
-2.0
-3.0
-4.0
Fig. 5 - Induction Motor Torque
Harmonic filters are always required when VSD (Variable Speed Drives) are used. Several factors
should be considered before a harmonic filter is installed. If the amount of drives is small (<15%) compared
to the system loading, it is likely that a filter will not needed to help control harmonics. Generally, as the ratio
of the short circuit kVA to drive kVA increases, the need for harmonic filters decreases further.
Bus loads and drives at a low load may require capacitors for an improved power factor at the utility metering
point to avoid demand or power factor penalties. The location of the capacitors compared to the location of
the drives is important. Increasing the impedance between them, reduces the possibility of adverse resonance
due to the drives. Therefore, impedance between the capacitor and drive reduces the need for harmonic
filters. Not all resonance conditions caused by the addition of capacitors are problems. If the system
resonance is at a harmonic not produced by drive, then filters may not be required.
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Harmonic filters trap harmonics. A properly designed filter presents a low impedance current path at its
design frequency. For example, when 5th harmonic currents are present on the same bus with a 5th harmonic
filter, the amount of 5th harmonic current in the filter is inversely proportion to all connecting impedances.
Since a 5th harmonic filter is designed to have low impedance near the 5th harmonic, most the 5th harmonic
currents will flow in the filter.
Other harmonic currents can also flow in a 5th harmonic filter. Since the filter impedance at the other
harmonics is higher, a smaller portion of the non-5th harmonic currents also flow in the 5th harmonic filter. The
filter does not trap harmonics, but it provides a low impedance return path for the harmonic current to flow.
The closer the filter is tuned to the source of the harmonics, the less disruptive the harmonics will be.
Harmonics must follow Kirchhoff's voltage and current laws and are not trapped and disappear in free space.
A harmonic load must meet harmonic standard IEEE-519. IEEE Standard 519, "IEEE Recommended
Practices and Requirements for Harmonic Control in Electrical Power System" pertains only to the point of
common coupling (PCC), generally the utility metering point. The only place where IEEE-519 is required to
be met is at the PCC.
The standard does not provide guidelines for harmonic distortion within the user facilities. What the standard
does, is to provide an acceptable level of harmonic distortion at the PCC. The user and utility share in this
responsibility. The utility must supply a relatively clean voltage to the user and the user must limit the amount
of harmonic current flowing from their loads back into the utility.
Individual drives or loads do not need to meet IEEE-519. In addition, IEEE-519 voltage distortion limits do
not apply within the users' facility. However, the user can specify harmonic current and voltage distortion limits
of equipment specified from manufactures. For equipment manufactures to meet a specified criteria, the
following information has to be provided to the manufacture.
a.
b.
c.
d.
e.
Single-line diagrams,
the operating conditions,
the location of any capacitors,
the bus short-circuit impedance
Other drives and harmonic load information.
The neutral current in a 3 phase, 4 wire system is limited to 173% third harmonic. Because of the mix
of loads in a facility this "rule" is generally true. Lets take an extreme example of 15 kVA 480/208-120 volt
transformer serving personal computers that have a measured current wave shape and harmonic content as
shown in Fig. 6. In addition, lets assume each phase has 5 kVA (41.6 A) of load connected line to neutral
and the third harmonic in-phase with each other. Of the total load, the fundamental current in Fig. 6 is
1.03/1.57 = 0.656 (27.3A) of the total current, the third 0.8615 times the fundamental or 23.5A. Because
the third harmonics are in-phase with each other, the neutral will carry three times the phase third harmonic
current or 70.5 A. This is 258% (100*70.5/27.3) of the fundamental and 169% (100*70.5/41.6) of the total
phase current. As with any percentage, the reference base needs to be provided.
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Harmonic
1
3
5
7
9
11
13
15
17
19
Percent
Fundamental
Current
100.00
86.15
63.76
38.12
15.82
3.47
6.73
6.85
3.97
2.48
RMS
Fundamental
1.57A
1.03A
Fig. 6 - Personal Computer Current Wave Shape
Pulsed Width Modulation (PWM) drives produce no harmonics problems. All equipment that uses
electronic control power circuits will produce harmonics when connected to an ac power system. PWM
drives are no exception. By themselves, they generally produce more harmonics than other types of drives.
As stated above, they can produce high percentage of harmonic currents. While the percentage of harmonic
current at light loads can be higher at full load, the current is small and therefore the actual harmonic amps are
also small. To help reduce the amount of harmonic current some manufactures add isolation transformers or
series ac or dc reactors to the drives. The added impedance causes a greater voltage drop to the motor and
to compensate for it the drive controller has to conduct for a longer period which lowers the harmonic content.
Total harmonic current distortion remains above 20%. Figure 7 is current waveform and harmonic content
of a PWM drive with and without the added impedance. While a small single drive may not cause a problem,
a large number PWM drives can.
Isolation transformers stop harmonics. Placing an isolation transformer in series with a drive does not stop
harmonics. If the drives produce harmonic currents that have the characteristics of positive and negative
currents, Kirchhoff's law says they have to flow through the transformer and they do. By the careful selection
of delta-wye and delta-delta isolation transformers, the total amount of harmonic currents in a system can be
reduced. Having an equal kVA mix of delta-wye and delta-delta transformers can provide some of the
benefits of a 12 pulse system. The 5th and 7th harmonic currents will partially be reduced by cancellation.
The triplex harmonics will be reduced because they will generally circulate the transformer delta winding and
in the transformer wye-connected neutral back to the drive. On PWM type drives, a reactor can be used in
place of the delta-delta transformer to help lower harmonics. Cost determines the selection of a reactor or
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isolation transformer. One benefit of the isolation
transformers or source side reactors is the reduction of
commutation notches. If commutation notches are not
a problem, then the delta-delta or series reactors are
not needed at all. On non-PWM drives, the same
benefits of cancellation can be obtained by having half
the drive kVA directly connected to the system and
the other half on delta-wye isolation transformers.
The above discussions presented several reasons why
some harmonic 'truths' exist. Usually they are
generally true but there are exceptions.
Fig. 7 - Full Load Current in PWM Drive
Myth1.wpd
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