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Salih Mohammed Salih et al./ Elixir Renewable Energy Engg. 70 (2014) 24362-24367
Available online at www.elixirpublishers.com (Elixir International Journal)
Renewable Energy Engineering
Elixir Renewable Energy Engg. 70 (2014) 24362-24367
Practical Analysis of Harmonics Effects on Transformer
Salih Mohammed Salih, Kaleid Waleed Abid and Munther Nayef
Renewable Energy Research Center, University of Anbar, Iraq.
ARTICLE INFO
Art i cl e h i st ory :
Received: 27 March 2013;
Received in revised form:
10 May 2014;
Accepted: 23 May 2014;
Ke y w or d s
Harmonics,
Frequency converter,
k-rated Transformer,
Harmonic Analyzer,
Total Harmonic distortion.
ABSTRACT
Harmonic currents or voltages are generated from a non-linear load when it is connected
to the mains supply. The problems caused by harmonic include overheating of cables,
especially the neutral conductor, overheating and vibration in induction motors and
increased losses in transformers. In this paper, the details analysis of harmonics effect on a
1KVA transformer is analyzed. A frequency oscillator source is used for generating the
harmonics signal on the transformer. The total harmonic distortion, crest factor and Kfactor are analyzed by using a Fluke-435 power analyzer.
Introduction
Harmonics and distortion in power system current and
voltage waveforms have been present for decades. However,
today the number of harmonic producing devices is increasing
rapidly. These loads use diodes, silicon controlled rectifiers
(SCR), power transistors, etc. Due to their tremendous
advantages in efficiency and controllability, power electronic
loads are expected to become significant in the future, and can
be found at all power levels, from low-voltage appliances to
high voltage converters. One result of this is a significant
increase in the level of harmonics and distortion in power
system networks [1].
Transformers are major components in power systems. The
increased losses due to harmonic distortion can cause excessive
losses and hence abnormal temperature rise. The measurement
of iron losses and copper losses of single-phase transformers is
important in particular for transformers feeding nonlinear loads.
When the power factor capacitors are fitted, harmonic currents
can damage them and care must be taken to avoid resonance
with the supply inductance. Losses in transformers are due to
stray magnetic losses in the core, and eddy current and resistive
losses in the windings. Of these, eddy current losses are of most
concern when harmonics are present, because they increase
approximately with the square of the frequency. Before the
excess losses can be determined, the harmonic spectrum of the
load current must be known [2].
The usage of nonlinear loads on power systems increasingly
the awareness of the potential reduction of a transformer’s
operational life due to increase heat losses. The performance
analysis of transformers in harmonic environment requires
knowledge of the load mix, details of the load current harmonic
content and total harmonic distortion (THD). The additional
heating experienced by a transformer depends on the harmonic
content of the load current and the design principles of the
transformer [3].
Special transformers such as V–V, Scott and Le-Blanc
transformers are used to convert a three-phase supply into one or
two single-phase supplies. These transformers are commonly
used in the electro locomotive traction systems. The authors in
Tele:
E-mail addresses: dr_salih_moh@yahoo.com
© 2014 Elixir All rights reserved
© 2014 Elixir All rights reserved.
[4] investigated the harmonic cancellation characteristics of such
transformers. The results showed that when two harmonicproducing loads are connected to each single-phase side of the
transformers, the harmonics produced by the loads will cancel
out at the primary sides of the transformers. The amount of
cancellation is affected by transformer type and harmonic order.
A novel method for reducing harmonics in series-connected
converters is proposed in [5]. The proposed method adopted an
averaging inductor to the series-connected converters and thus
simplified the pulse multiplication process. The principle of
their method was demonstrated by a three-thyristor switching
circuit scheme, which can make a 12-pulse double bridge
converter to operate at 36 pulses. The simulation results
reinforced the theoretical work. Only slight changes are needed
for the proposed method to reach higher pulse operations. This
made this method even more attractive when high-pulse
operation is desired.
The use of companion harmonic circuit models for transient
and harmonic analysis of electrical networks where an exact
periodic steady state initialization method was proposed in [6].
The suggested companion harmonic circuit models are the result
of applying the trapezoidal integration rule to the differential
equations characterizing electric elements in the dynamic
harmonic domain. The technique was applied to a simple
network exhibiting harmonics due to transformer saturation. The
models provided a direct means of calculating both the steady
state and transient response of the harmonics in an electrical
network.
Types of Harmonics
The harmonics can have an effect on current waveform
(Current Harmonics) or voltage waveform (Voltage Harmonics).
The current harmonics can be produced by a bridge rectifier
circuits. Current harmonics have an effect on the electrical
equipment supplying harmonic current to the device
(transformers, conductors). Harmonic currents can cause
excessive heating to transformers. For electrical systems feeding
single phase loads the third harmonic has gained attention in
design consideration and transformer selection for causing the
neutral conductor to draw excessive current. Voltage harmonics
Salih Mohammed Salih et al./ Elixir Renewable Energy Engg. 70 (2014) 24362-24367
0
subplot in this figure (at row two-left side) represents the second
harmonics (2f0), while the right one represents the third
harmonics (3f0). The effect of these harmonics on the main
waveform is shown in the last subplots (subplot 5 and 6). From
these subplots (5 and 6) it can be seen that the third harmonics
can have more effect on the signal waveform shape. The third
harmonics can make the signal shape to be more fluctuated than
the effect of second harmonics. Figure (2) presents the fourth
and fifth harmonics effect on the waveform. From this figure,
the odd harmonics is still has more effect on the complex
waveform shape. So the odd harmonics (5, 7, 9, etc.) have more
effects on the waveform than even harmonics (2, 4, 6, etc.).
f undamental f requency
0
-1
0
rad
A m plitude
0
5
-1
10
Distortion f rom Second Harmonic
5
10
A m plitude
rad
0
10
5
10
Distortion f rom Third Harmonic
2
A m plitude
0
rad
rad
0
-2
5
Third Harmonic
0
rad
2
0
1
0
-1
0
-1
10
Second Harmonic
1
A m plitude
5
f undamental f requency
1
A m plitude
A m plitude
1
0
-2
0
5
10
rad
Figure (1): Second and third harmonic waveforms effect
Harmonics Reduction
The harmonics can be reduced by selecting equipments with
low THD currents, and the result will reduce the THD voltage.
If is difficult to use such equipment with a low THD current
there are other options available, such as adding line chokes or
isolations transformers to reduce the harmonic currents.
Fouth Harmonics Effect
2
Fund. Freq.
Fourth Har.
1
Amplitude
can effect sensitive equipment throughout your facility. Voltage
harmonics arise when current harmonics are able to create sags
in the voltage supply. When any device draws current it creates
a voltage dip which is required for current to flow. This voltage
dip is visible with larger loads when turning on a hair dryer or a
table saw and seeing the lights dim down. The amount of sag
depends on many factors like transformer impedance and wire
size. Current harmonics create voltage harmonics, but the
magnitude of the voltage harmonics depends on the “Stiffness”
of electrical distribution’s “System Impedance”. An example to
help understand current distortion verse voltage distortion is the
common light bulb. This low cost light bulb may have a 75%
current THD (Total Harmonic Distortion). This means that 75%
of the current drawn by the bulb is considered “Harmonic
Current”. These light bulbs usually do not affect other devices in
your home because even though the current drawn by the bridge
circuit is rich in harmonic current it creates very little sag in
home’s voltage supply and if there is a voltage analyzer , then it
can be seen the THD voltage, which will be less than 1 percent.
Figure (1) shows the harmonics effect on the waveform
shape. The upper row (left and right) shows the fundamental
frequency consists of two circle waves ( 4π f ). The third
Complex Sig.
0
-1
-2
0
2
4
2
6
rad
Fifth Harmonics Effect
8
10
12
Fund. Freq.
Fif th Har.
1
Amplitude
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Complex Sig.
0
-1
-2
0
2
4
6
rad
8
10
12
Figure (2): Fourth and fifth harmonic waveforms effect
The current distortions that can have an effect on voltage
waveform can be reduced by using a tuned capacitor. Also it can
be redesigning the systems distribution to reduce system
impedance. Many solutions are used for reducing the effects of
harmonics, some of them are:
Harmonics Current Reduction
• By adding line chokes to the equipment producing harmonics
• By adding isolation transformer to the equipment producing
harmonics
• By using a 12- Pulse or 18 pulse rectifying circuits instead of 6
pulses
Harmonics Voltage Reduction
• By adding a tuned capacitor banks to the source of current
harmonics
• By changing the transformer size and impedance
The other method by using phase-shifting transformers, which
are cost-effective, energy efficient, highly reliable passive
devices that are always on the job, treating harmonics
regardless of the level of load they are serving at a given point in
time.
Harmonic Effect on Transformer Losses
Transformer manufactures usually try to design transformer
in a way that their minimum losses occur in rated voltage and
sinusoidal current. However, by increasing the number of nonlinear load in recent years, the load current is no longer
sinusoidal. This non-sinusoidal current causes extra loss and
temperature in transformer [7]. Several methods of estimating
the harmonic load content are available which are:
1) Crest- Factor
2) Percent of THD (%THD)
3) The third method is the K-Factor which can be used to
estimate the additional heat created by non sinusoidal loads
The crest factor is a measure of the peak value of the
waveform compared to the true RMS value [4, 5].
(1)
I
CF =
max
I rms
where Imax is the peak magnitude of the current waveform
and Irms is the true RMS of the current.
The %THD is a ratio of the root-mean-square (RMS) value
of the harmonic current to the RMS value of the fundamental
frequency [7, 8].
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Salih Mohammed Salih et al./ Elixir Renewable Energy Engg. 70 (2014) 24362-24367
∞
2
∑ (I )
(2)
h
% THD =
h=2
I1
The last equation is used for measuring the additional
harmonic current to the total RMS value.
In the third method, the K-factor is used and it is defined as
the sum of the squares of the per unit harmonic current times the
harmonic number squared.
2
(3)
∞
K = ∑ (I h ( pu ) ) h 2
h =1
where Ih(pu) is the harmonic current expressed in per unit based
upon the magnitude of the fundamental current and h is the
harmonic number [8].
Simulation Results
In this section, a practical experiment is used for calculating
the THD for a single phase transformer of 1KVA. The practical
experiment is done by using the list of devices given in table (1).
The details components are shown in figure (3). The variable
frequency supplier (i.e. N700E) is used for generating the 3rd,
5th, and 7th harmonic frequency on the transformer. While the
variable resistors of 600W are used for controlling the supplied
voltage at different frequency on the transformer.
Table (1): List of used components in the experiment
Item
Variable frequency supplier
Transformer
Power quality analyzer
Storage oscilloscope
Variable resistors
harmonic percentage decreases as the applied frequency
increases. (figure (4) to figure (6). From the same subplots, the
crest factor is constant at 1.4 due to using the variable resistors
for controlling the applied currents and voltages on transformer,
which means the currents Imax and Irms are constant in equation
(1), so the values are constant.
The below-middle subplots in the same figures represent the
main signal of 230V (source voltage) in addition to the applied
harmonics voltage of 23V in neutral line. From this figure it is
clear that the signal shape is fluctuated due to the effect of
applied harmonic signal on transformer. The other subplots
(below-middle) at same figures represent the effect of harmonics
on signal shape (5th and 7th harmonics).
Finally, the last subplots (below-left) represent the
harmonics effect on the current waveform. From these subplots,
the fluctuation is higher at the 3rd harmonics than the other
values at 5th and 7th harmonics. This case is considered to be
normal, because the amplitude of harmonics decreases as the
order increases.
Model Number
N700E
1KVA
Fluke-435B
SFRAM
600W
Figure (4): Harmonic Effects at 150Hz
Figure (3): Practical experimental components
Harmonics Effect at Different Frequencies
In this section, a practical experiment is used for calculating
the THD for a single phase transformer of 1KVA. Figure (4)
gives the harmonics effects in table forms. The upper figures
represents the harmonics in Ampere, Volt, and Watt from left to
right respectively. The first table show that the THDf =266 for
line (L1) and equal to 157.9 for neutral (N) decreases as the
harmonics increases to 250Hz and 350Hz (figures (5) and (6).
The corresponding values of THDf(250Hz)=202.6 and
THDf(350)=195.5. The values of THDf were also decreased for
neutral lines at the same tables (i.e. THDf=157.9 (at 150Hz),
THDf=117.7 (at 250Hz), and THDf=116.2 (at 350Hz). The other
values at the same tables represent the values of odd harmonics
from H3 to H15. The other values of harmonics percentages in
terms of Volt and Watt are also given in the same figures. The
other subplots lie in the below-right sides illustrates the
harmonics effect as a function of Volts/Amps/Herts. The
Figure (5): Harmonic Effects at 250Hz
Figure (6): Harmonic Effects at 350Hz
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Salih Mohammed Salih et al./ Elixir Renewable Energy Engg. 70 (2014) 24362-24367
Total Harmonics Distortion and K-Factor
The THD and K-factor can also be extracted by using the
power quality analyzer (Fluke-435B). The next three figures
(1.e. figure (7), (8) and (9)) show the amplitude of harmonics in
neutral line for current waveform. From the first two figures the
THD and K-factor decrease as the frequency of simulated
harmonics increase. The THD decreases from 110.2% to 79.5%,
while the K-factor decreases from 42.2 to 18.2 in figure (9), the
THD increases relative to the corresponding value in figure (8),
and this due to taking the reading in high harmonics existing in
the lab during the testing time in addition to the injected
harmonics by the variable frequency supplier (N700E). from
figure (7) it can be seen that the amplitude of harmonics
decreases as the frequency on x-axis increases. The amplitude of
harmonics above 30% as the order of harmonics less than 13th.
The amplitude of THD less than 20% as the harmonics order be
above 17th.
Figures (10), (11) and (12) illustrate the harmonics effect in
neutral line for voltage waveform. From these figures, the THD
and K-factor decrease as the frequency increases from 150Hz to
350Hz. Also it is clear that the maximum amplitude of
harmonics lie at the odd integer of the fundamental frequency.
The THD percent in figure (10) has an amplitude above 20%
when the order of harmonics below 31th. Note that the values of
harmonics is very high in figure (10) for line voltage.
The last three figures (13), (14) and (15) present the THD
for voltage waveform in neutral line. The THD decreases from
40.9% at 150Hz to 35.3% at 250Hz and then to 31.2% at 350Hz.
Figure (9): Amplitude of Harmonics at 350Hz (neutral line
for current waveform)
Figure (10): Amplitude of Harmonics at 150Hz (Lineharmonics for current waveform)
Figure (7): Amplitude of Harmonics at 150Hz (neutral line
for current waveform)
Figure (11): Amplitude of Harmonics at 250Hz (Lineharmonics for current waveform)
Figure (8): Amplitude of Harmonics at 250Hz (neutral line
for current waveform)
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Salih Mohammed Salih et al./ Elixir Renewable Energy Engg. 70 (2014) 24362-24367
Figure (12): Amplitude of Harmonics at 350Hz (Lineharmonics for current waveform)
Figure (13): Amplitude of Harmonics at 150Hz (Neutral
Line harmonics, volt)
Figure (14): Amplitude of Harmonics at 250Hz (Neutral
Line harmonics, volt)
Figure (15): Amplitude of Harmonics at 350Hz (Neutral
Line harmonics, volt)
Conclusions
The distortion of current and voltage waveform created by
non-linear loads can cause many problems in an electrical
distribution system. The harmonics signals at 3rd, 5th, and 7th
order are simulated via variable frequency supplier (N700E) by
injecting the harmonics frequency on the load, which is a 1KVA
transformer. The results show that the harmonics has high effect
on the current and voltage waveform. The maximum harmonics
percentage occurs at 3rd order and decreases at high orders. The
crest factor and K-factors decreases as the harmonics order
increases.
Acknowledgments
This work is supported by the University of Anbar-Iraq
/Renewable Energy Research Center with Grant No. RERCTP13.
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