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

VVV

aaaa

VVV

2

2

11

111

31 (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

12

1

a

a

ao

b

b

bo

VVV

aa

VVV

(2)

=

−

+

2

12

1

a

a

ao

c

c

co

VVV

aa

VVV

(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.

+

−=vvIndexUnbalance (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)

1PV is the maximum positive peak.

2pV 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”. 8th. 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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