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Mitigation of AC Arc Furnace Voltage Flicker
Using The Unified Power Quality Conditioner
A. Elnady, W. El-khattam, and M. M. A. Salama
Electrical & Computer Engineering, Waterloo University, Ontario, Canada
Abstract: The application of deregulation policy in power
sector has emphasized the need for new tools, which are
capable of tracking and mitigating the voltage disturbances
caused by non-linear loads. This paper introduces a new
strategy to track and mitigate the voltage flicker and the
unbalance produced by large AC arc furnaces. The mitigation
strategy depends on an innovative technique for voltage
disturbance extraction, which uses symmetrical components.
This paper proves that The Unified Power Quality Conditioner
(UPQC) is capable of suppressing the entire voltage
disturbance in the industrial system. Results of digital
simulation are presented to validate and verify the control
strategy and to assess the performance of UPQC to mitigate
the voltage flicker and the unbalance produced by AC arc
furnace.
Key Terms: UPQC, Voltage Flicker, AC Arc Furnace.
I. INTRODUCTION
Voltage fluctuation is considered as one of the most
severe power quality problems and more attention has
been paid to it lately [1]. Voltage fluctuations are
systematic variations of the voltage envelope or a series
of random voltage changes. The magnitude of these
fluctuations is between ± 10 %. One of the voltage
fluctuation effects is to cause the light to flicker. Voltage
fluctuation, being an electromagnetic phenomenon, is
always referred to as voltage flicker [2]. Beside its effect
on light, other flicker effects reduce the life of electronic,
incandescent, fluorescent and cathode ray tubes [3]. The
malfunction of phase locked–loops PLLs, misoperation
of the electronic controllers and protection devices are
other samples of flicker effects.
The mitigating devices were first based on SVC such
as Thyristor Switched Capacitor (TSC) [4], Thyristor
Controlled Reactor (TCR) [5], and Fixed Capacitor
Thyristor Controlled Reactor (FCTCR) [6], [7]. All of
these techniques achieved an acceptable level of
mitigation but their operation depends on complicated
control algorithms because the injected current from the
mitigating devices should be related somehow to the
reactive component of the arc furnace currents, as well
as all of these mitigation devices inject a large amount of
current harmonics to the system so the mitigating
devices should be accompanied with a group of tuned
and detuned filters like in [7].
Due to the drawbacks of the previous mitigating
devices and their relative high cost, an inexpensive
technique utilizing shunt capacitors are applied to
mitigate the voltage flicker [8], but this technique does
not respond to the instantaneous variation of the voltage
(like arc furnaces), besides it has the resonance problem.
The shunt capacitors are replaced by series capacitors
[8], which have a better performance for voltage
mitigation because its control principal depends on
compensation of the feeder reactance. This technique
suffers from a Ferro resonance problem due to the
existence of the series transformer, which might lead to
flash over across the series capacitors.
The power converters based mitigating devices for
voltage flicker are employed [9] in which UPQC is used
to isolate the load harmonics and mitigate the
propagation of voltage flicker to the downstream
network. The disturbance extraction depends on d-q
orthogonal coordinates, which achieved satisfying
mitigation results. In 1998 DSTATCOM was installed to
the distribution system in Vancouver-Canada [10], and it
achieved satisfying mitigation performance since it
reduced the voltage flicker from (8-5)% to (4-2.25)%.
The authors mentioned some of the difficulties that they
faced during the operation of the DSTATCOM such as
the interfaced equipment which were located outside the
DSTATCOM trail.
One of the main advantages of using the hybrid
power conditioner is to reduce the fixed and running
expenses of the mitigating devices [11]. This hybrid
power conditioner (series active filter and shunt passive
filter) utilizes Synchronous Reference Frame for the
voltage flicker and sag suppression. This technique lacks
to mitigate non-cyclic voltage flicker and the unbalance
components, which are associated with the common
kinds of AC and DC arc furnaces.
This paper presents an effective mitigating device,
which is based on a novel control algorithm to extract
the voltage disturbance to suppress the voltage flicker
without causing any problem of the aforementioned
techniques. This paper consists of five sections. First the
system configuration is demonstrated with the UPQC
structure in section II. The control strategy is explained
in section III. The mitigation results are illustrated in
section IV.
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The influence of UPQC on the power factor
improvement is investigated in section V. Finally,
section VI concludes the paper.
II . SYSTEM CONFIGURATION
A. Mitigating Device Structure
UPQC is the most effective power conditioner to
suppress and compensate the power quality problems. It
consists of four voltage source converters connected
back to back through a coupling capacitor as shown in
Figure (1). The shunt converter (shunt part) is used to
supply the required power to the other converters, in
some cases it is used to stabilize the voltage at the point
of its installation. The series part works as a series
generator, which is used to cancel out the voltage
disturbances.
Fig. (1): Internal structure of UPQC
B. System Model
The model of the system with an AC arc
furnace is depicted in Figure (2). The arc furnace is
connected to the bus with the voltage level of 4.16 kV.
Fig. (2): 13-bus industrial distribution system
The system and the distribution feeder parameters are
shown in table 1, 2 respectively;
Table 1:Parameters of step-down transformers
Parameter
Value
Leakage reactance 0.10 [pu]
Magnetizing current 0.40 [%]
Air core reactance 0.50 [pu]
Inrush decay time constant 0.15 [sec]
Knee Voltage 1.25 [p.u.]
Time to release flux
clipping
0.10 [sec]
Table. 2: Parameters of the distribution feeders
Parameter Value
Zero-sequence impedance R=0.4252 [ohm/km]
L=2.837 [mH/km]
Positive-sequence
impedance
R=0.2024 [ohm/km]
L=0.9710 [mH/km]
Line length 10 km
The specifications of the proposed system are as
follows;
G1= 69 kV, 1500 kVA.
G2= 13.8 kV, 1500 kVA.
T1 : 69 to 13.8 kV, 1500kVA.
T8 : 13.8 to 4.16 kV, 1500kVA.
T9 : 13.8 to 0.48 kV, 1500kVA.
Rest of distribution transformers: 13.8:0.48 kV,750 kVA
III. CONTROL TECHNIQUE
The control algorithm depends on extracting the
symmetrical components of the phase voltages. This
technique is effective because AC arc furnaces produce
flicker in the positive sequence voltage and unbalances
in the three phase voltages due to unbalance and non-
linear loads which influence the performance of the end-
user equipment. The block diagram of the control
algorithm is shown in Figure (3).
Fig. (3): Block diagram of control algorithm
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The control algorithm extracts the symmetrical
components; positive, negative, and zero sequence for
each phase voltage. The matrix conversion is given in
equation (1) of phase (a). The reduction in the positive
sequence which expresses as
)()()(
measuredvalueset
vvv +−+=+∆
, while negative
sequence, and zero sequence voltage drops are
mathematically expressed as shown in Figure (3). These
voltage disturbance signals are used to generate the
reference signal of the total voltage disturbance. If the
reference signal is increased above a certain threshold
then the reference signal is used to generate the control
signal of SPWM technique. SPWM technique is used to
operate the series part of UPQC.
=
−
+
c
b
a
a
a
ao
V
V
V
aa
aa
V
V
V
2
2
1
1
111
3
1
(1)
The symmetrical components of the other phases (b,
c) are generated from the symmetrical components of
phase (a) based on the equations (2), and (3).
=
−
+
2
1
2
1
a
a
ao
b
b
bo
V
V
V
a
a
V
V
V
(2)
=
−
+
2
1
2
1
a
a
ao
c
c
co
V
V
V
a
a
V
V
V
(3)
Where,
020
24011201 ∠=∠= aa ,
IV. MITIGATING TECHNIQUE AND RESULTS
The current and voltage waveforms of phase (a) are
shown in Figure (4), and (5). These waveforms are the
results of AC arc furnace operation, which has the
following parameters;
Rating= 5mW
Type of flicker = Cyclic voltage oscillation
The amplitude of oscillation =0.7 pu
Frequency of voltage flicker=10 Hz
Terminal voltage at furnace =4.16kV
The control circuit could extract the total distorted
voltage at PCC, which results from the reduction and
amplitude modulation in the positive sequence voltage
plus
the
unbalance component due to non-linear loads.
The reduction of the voltage signal is modulated by
certain magnitude and frequency, which depend on the
type of the arc furnace.
Fig. (4): Current feeding furnace (Ia)
Fig. (5): Voltage waveform at furnace (Va)
The waveforms, shown in Figure (6), (7), show the
total voltage disturbance reference signal and the
injected voltage by the mitigating device installed in
phase (a). The UPQC is activated at 0.8 sec.
Fig. (6): Disturbance signal at PCC
Fig. (7): Injected Voltage at PCC
One of the privileges of this control algorithm is its
capability of mitigating any disturbance in the positive
sequence voltage (like flicker and sag). It also has the
ability to compensate the negative sequence voltage as
G2
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shown in Figure (8). The unbalance index which is
defined in equation (4) is reduced from 0.072 to 0.036.
This curve shows the negative sequence voltage drop
before and after the compensation. The UPQC is
activated at 0.8 sec.
+
−
=
v
v
IndexUnbalance
(4)
−
v
is the negative sequence component.
+
v
is the positive sequence component.
Fig. (8): Negative-sequence voltage drop before and
after the compensation
The high frequency components in the above
waveform resulted from the switching ripples of PWM
technique, which is used to operate the series part of
UPQC.
The zero-sequence voltage drop rarely propagates in
the primary distribution systems because most of the
distribution substations and the distribution transformers
are connected in delta or isolated star, so there is no real
concern from propagation of the zero-sequence voltage
drop upstream or downstream through the distribution
systems.
The performance of the UPQC is demonstrated in
Figure (9), and (10). The first waveform depicts voltage
waveform which is distorted by the reduction of the
voltage level by 1.5 kV in addition to voltage flicker
with the flicker index of about 0.045 the flicker index is
defined by equations (5), (6);
VvIndexerFlick
/∆=
(5)
21 PP
VVv −=∆
(6)
1P
V
is the maximum positive peak.
2p
V
is the minimum positive peak.
The second waveform shows the compensated
voltage since the voltage flicker is mitigated, the
reduction in the positive-sequence is boosted up, and the
negative-sequence voltage drop is suppressed.
Fig. (9): Voltage flicker at PCC
Fig. (10): Compensated Voltage at PCC
V. INFLUENCE OF UPQC ON THE SUPPLY POWER
FACTOR
UPQC is also used to enhance the system
performance during the operation of the AC arc furnace.
UPQC is employed to improve the reactive power
oscillation profile. Figure (11) shows the reactive power
waveform before and after the compensation. The
mitigating device is activated at t=0.8 sec.
Fig. (11): Reactive power drawn by arc furnace
The previous waveform indicates how UPQC could
improve the voltage variation at PCC by suppressing the
oscillation of the reactive power drawn by arc furnace, it
also improves the power factor of the supply by injecting
a reactive power since the UPQC works as a series
variable capacitor.
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VI. CONCLUSIONS
This paper proves that the UPQC is an effective
mitigating device to suppress the voltage flicker since it
could reduce the flicker index from 0.018 to 0.0045 at
PCC. The unbalance in the primary distribution could
also be compensated since the unbalance is reduced from
0.072 to 0.036, which is so important especially for the
drive systems. Finally, UPQC is employed to improve
the supply power factor since the UPQC works as a
variable serious capacitor.
VII. REFERENCES
[1] M. Walker, "Electric Utility flicker Limitations," IEEE
Transactions on Industry applications, Vol.IA-15, No. 6, November
/December 1979, pp. 644-655.
[2] W . B . Jervis ,"An Assessment of Power System Voltage
Disturbances in Terms of Lamp Flicker Perception," Central Electricity
Generating Board, U.K.
[3] A.A. Girgis, J.W. Stephens, E.B. Makram, “Measurement and
Prediction of Voltage Flicker Magnitude and Frequency”, IEEE
Transactions on Power Delivery, Volume: 10 Issue: 3, July 1995
Page(s): 1600 –1605.
[4] L. Gyugi, A. A. Otto, “Static Shunt Compensation for Voltage
Flicker Reduction and Power Factor Correction”. American Power
Conference 1976, pp. 1271-1286.
[5] Y. Hamachi, M. Takeda, “Voltage Fluctuation Suppressing System
Using Thyristor Controlled Capacitors”. 8
th
. U.I.E. Cngress, 1976.
[6] F. Frank, S. Ivner, “TYCAP, Power-factor correction equipment
using thyristor-controlled capacitor for arc furnaces”. ASEA Journal 46
(1973): 6, pp. 147-152.
[7] I. Hosono, M. Yano, M. Takeda, S. Yuya, S. Sueda, “Suppression
and Measurement of Arc Furnace Flicker With a Large Static VAR
Compensator”, IEEE Transaction on Power Apparatus and Systems,
Vol. PAS-98, No. 6, Nov./Dec. 1979, pp. 2276-2282.
[8] M W. Marshall, PE, “Using Series Capacitors to Mitigate Voltage
Flicker Problems”, IEEE Transaction on Power Delivery, 1998.
[9] H. Fujita, H. Akagi,”The Unified Power Quality Conditioner: The
Integration of Series- and Shunt-Active Filters”, ”, IEEE Transaction
on Power Electronics, Vol .13,No. 2, March 19986, pp. 315-322.
[10] J. R. Clouston, J. H. Gurney, “Filed Demonestration of a
Distribution Static Compensator Used to Mitigate Voltage Flicker”,
IEEE Transaction on Power Delivery, 1998.
[11] K. Karthik, J.E. Quaicoe, “Voltage compensation and harmonic
suppression using series active and shunt passive filters”, Electrical and
Computer Engineering, 2000 Canadian Conference ON, Volume: 1,
2000 Page(s): 582 -586 vol.1
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