New Strategy of DC Electrical Arc Furnace Converter for ... · M. Hosseini-Firouz & B. Mozafari: New Strategy of DC Arc Furnace Converter 151 The traditional AC electric Arc Furnace

Corresponding author : M. Hosseini-Firouz Department of Electrical Eng, Science and Research Branch, Islamic Azad University, Tehran, Iran [email protected], B.Mozafari Department of Electrical Eng, Science and Research Branch, Islamic Azad University, Tehran, Iran [email protected], Copyright © JES 2011 on-line : journal.esrgroups.org/jes

M. Hosseini-Firouz B. Mozafari

J. Electrical Systems 7-2 (2011): 149-164

New Strategy of DC Electrical Arc

Furnace Converter for Mitigation of Voltage Fluctuations and Comparative Flicker Voltage in AC and DC Furnaces

A novel method to mitigate voltage flicker problem usually caused by DC Arc Furnaces has been presented in this paper. This method called Direct Current Ripple Re-injection is applied to a multi-step switching reactor in the output of the AC/DC converter to make the output load current as smoother as possible. This method would lessen harmonic effects properly. The proposed approach is simulated by MATLAB-SIMULINK software. Simulation are performed using dynamic arc model in which the performance of AC and DC electric Furnaces are evaluated with respect to generation of voltage flicker on power common coupling (PCC) busbar. Voltage flicker level is measured by means of a standard pseudo flickermeter whose components and block diagrams are illustrated in this paper.

Keywords: Voltage fluctuation, Electric Arc Furnaces, Power Quality, Simulation.

1. Nomenclature

pccU Voltage flicker on power common coupling (PCC)

pccI Current flicker on PCC

1X Short circuit reactance of the supply network

2X Reactance of the flexible load

scS Short circuit power at the PCC

scfS Furnace short circuit apparent power

fS Arc Furnace apparent power

k Ratio between the Furnace apparent power and the short circuit level at the PCC FI Flicker compensation ratio

reatedAFS Rating of the Arc Furnace apparent power Sscf Fault level of the Arc Furnace IFL Instantaneous flicker value Pst Short term flicker value Plt Long term flicker value I. Introduction

Recently, the growth of industries and electric power appliances have cased electric power quality issues become of crucial importance. Many load with different nominal power connecting to electric power systems, may introduce power quality problems at various voltage levels due to their unbalanced and non-linear behavior characteristics. However, major power quality problems affecting a large number of customer are related to high power consumption industrial loads. The rapid and large amount of active and reactive


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power consumptions fluctuations by such load cause rapid repetitive voltage variation at connection point. The residential and commercial customers supplying by the same network, are affected by these voltage variations. This may disturb electrical equipment and case flicker problem.

A lamp flicker associate with electric arc Furnaces has been a problem since the early days of such installations. Arcs, by nature, tend to be unstable and will draw rapidly changing currents from the main supply. These varying currents produce voltage drops in the main supply. Most of electric utilities are standardized for tolerating a certain level of flicker on their systems. The source impedances of utility feeds tend to be mostly inductive. If the load current is unity power factor, the voltage drop in the source inductance is in quadrature with the line voltage and tends to have a small effect on voltage magnitude. The subjective effect of flicker on the observer is a function of both magnitude and frequency of the disturbance. Some type of reactive power compensation is usually required to limit the propagation of disturbances, mainly voltage flicker problems, which are caused by electric Arc Furnace into the electric power network. This is usually achieved by installing a suitable var compensate device at PCC busbar. The most effective way to control voltage fluctuations and therefore to limit flicker is to compensate the reactive power variations of the fluctuating loads at medium/high voltage levels.

In the AC Arc Furnaces depending on the operating states, the Furnace load may change from a complete open circuit to a 3-phase short circuit, generating a voltage drop on the primary side of the Furnace transformer. Given the continuously changing nature of the Arc, dynamically fast reactive compensation is required to stabilize the voltage of the feeding network during the whole scrap charge.

In the DC Arc Furnaces the conventional approach to control is various arrangements of paralleled bridges. By complementary phase shifting of the firing angles of the three-pulse sections, the system could be made to operate at constant VARs over a fairly wide power range [3],[4]. However, the VAR consumption would still be large enough to require capacitor compensation. In addition, current regulator bandwidth may not be fast enough to suppress the higher flicker frequencies.

A common solution for harmonic reduction is through the connection of passive filters, which are tuned to trap a particular harmonic frequency. However, harmonics also could be eliminated using special configurations of converters [1].

In this paper, the Direct Current Ripple Re-injection (DCRR) method is proposed by using special configurations of converters. The configuration consists of two parallel converters connected to the Furnace through a multi-step reactor. The flicker can be effectively controlled without using dynamic reactive power compensator device or converter controller.

Recently, the papers are developed in DC Plasma Arc Furnace environment [12], [13].

This paper is organized as follows:

The AC and DC Arc Furnaces are given in sections II and III, respectively. The DCRR technique are developed in section IV. The flicker measurements is discussed in section V. flickermeter Model is explained in section VI. Section VII discusses the characteristics of electric Arc Furnaces. Section VIII gives the simulation results and Section IX concludes the paper.

II. AC Arc Furnace

M. Hosseini-Firouz & B. Mozafari: New Strategy of DC Arc Furnace Converter

151

The traditional AC electric Arc Furnace is a three electrode system operating from a Furnace transformer with relatively high reactance. Power levels may range to several hundred megawatts. These Furnaces are characterized by rapid changes in the Arc current, particularly as the Furnace is accommodates a new charge. Electrodes are almost continually in motion in an effort to stabilize the Arc. Short circuits are frequent. Their effect on supply circuits can be devastating since the power factor tends to be low under these conditions.

An electric Arc Furnace consists of a refractory lined shell which holds the charge, usually scrap metal. After the Furnace is charged with scrap, operation begins by lowering the electrodes to strike electric Arcs between the electrodes and the scrap. The heat generated by the three electric Arcs provides the heat for melting and refining the scrap. There are several phases in the electric Arc Furnace operation, each presenting a different impact on the power system in terms of flicker, namely the [11]:

Boring period.

Melting period.

Refining period.

III. DC Arc Furnace

The diagram of the DC electric Arc Furnace is shown in Fig.1 [7]. The transformer has two secondary windings, in delta and star connection respectively, providing 12-pulse operation. The 12-pulse rectifier is obtained by a °30 _ phase shift between the two secondary transformers. The dc and zero-sequence components can be suppressed by the means of a double secondary transformer using a delta-connected primary side.

Harmonic current compensation and fixed capacitive reactive power compensation is performed by harmonic filters. This arrangement consists of a single electrode. A comparison of AC and DC electric Arc Furnaces shows that the DC electric Arc Furnace has lower electrode consumption and more stable Arcs that lead to higher throughput. Flicker tends to be less problematic. In case of operating sequence of DC Arc Furnace are as follows:

Sequence 1: Boring period (400 V/100 kA)—During this sequence, the Arc length is reduced and the average value of the electrical power is 40 MW.

Sequence 2: Melting period (800 V/120 kA)—during this sequence, the electrical power transmitted to the Arc is 100 MW.

Fig. 1: DC Arc Furnace scheme

IV. DCRR Technique


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Theoretically possible, pulse numbers >48 are rarely justified due to the practical levels of distortion found in the supply voltage waveforms. Further, the converter topology becomes more and more complicated. An ingenious and very simple way to reach high pulse operation is shown in Fig. 2 [1]. This configuration is called dc ripple reinjection. It consists of two parallel converters connected to the Furnace through a multi-step reactor. The reactor uses a chain of thyristor-controlled taps, which are connected to symmetrical points of the reactor. By firing the thyristors located at the reactor at the right time, high-pulse operation is reached.

Fig. 2: Direct current ripple reinjection technique

In this section, the developed model of DCRR is given in this part.

Fig. 3: Developed model of Direct current ripple reinjection technique

Fig .3 shows an example of this structure with 4 thyristors. As it is shown, T1 is connected to T4 and T2 is connected to T3. The firing procedure of thyristors should employ each of converters for equal portion of the Furnace current. Further, it is necessary to have symmetry between sides. This makes the current distribution in the secondary windings of transformer smooth and uniform. If T2, T3 is turned ON, the current follows into L2 through them. In this case, no current will pass L1 unless the T1 and T4 turn to ON. After this, the current of L1 increases from 0 to current amplitude of the L2. In this time period, a portion of the stored current in the inductor between T1 and T2 and the inductor between T3 and T4 flows in L2 and freewheel diode closes. Thus, T2 and T3 turn to OFF only when this current goes to zero, i.e., in the current injection cycle. If T1 and T4 turn to OFF (thyristor gate turns off) and T2, T3 turn to ON simultaneously, the current can be stored in both the inductors between T1 and T2 and T3 and T4. With optimal selection of this operation times and the inductor values, the ripple reduction of converter output current is practicable. The firing


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times of thyristors are determined by duty cycle of their current ripples. The output current ripple depends on structure of converter considering the number of pulses. When the duty cycle of current ripple is in increasing mode, T2 and T3 should be turned ON for the operation of DCRR in the current injection cycle. Similarly, when the duty cycle of current ripple is in decreasing mode, T1 and T4 should be turned ON for the operation of DCRR in the current saving cycle.

For this purpose, with the combination shown in Fig.3, it is possible to achieve 24 pulses operation of converter.

The dc and zero-sequence components can be suppressed by the means of a double secondary transformer using a delta-connected primary side. Finally, the ranks of current harmonics for 24 pulses operation are defined by the following relationship [4]:

124 ±= kh Nk ∈

The equation above shows that for a 24 pulses converter, the harmonics in order of 22nd and lower can be omitted.

It is to be mentioned that in the DCRR method, it is possible to produce large amount of pulses in the converter duty cycle.

V. Flicker Measurements

In the beginning of the 1980th UIE (Union International d’Electrothermie) defined a model to estimate the level of flicker in a power network. The input signal to the model is voltage waveform data. The outputs of the model are the flicker parameters IFL, PST and PLT. Based on the model from UIE, the flicker standard IEC 61000-4-15 has been developed. Fig.4 shows a functional diagram of a flickermeter designed according to the IEC 61000-4-15.

Fig. 4: Functional diagram of a flickermeter according to IEC 61000-4-15

In block 1, the input signal is scaled, anti-alias filtered and sampled. Block 2 is a squaring demodulator separating the modulating signals from the carrier and the low frequency variations are moved to the base band of 0-30 Hz. Block 3 consists of two band pass filters in cascade. The first one eliminates the DC and double mains frequency ripple component. The second one has a centre frequency of 8.8 Hz and simulates the frequency response to sinusoidal voltage fluctuations of a coiled filament lamp combined with the human visual system. Block 4 is composed of a squaring multiplier and a first order low-pass filter. The human flicker sensation is simulated by the blocks 2, 3 and 4.

The output of block 4 is the instantaneous flicker value IFL. If the lamp is connected to a voltage with IFL=1.0, statistically 50 percent of the people in a reference group will


154

apprehend annoying flicker from the lamp. In block 5, a statistical analysis is performed by the data given to the flicker parameters PST and PLT. According to EN 50160 the 95% value of the PLT shall not exceed 1.

IFL-values are recorded continuously during 10 minutes and the values are sorted in a duration diagram. The PST-value is calculated from the formula:

805030

1713108

6432.2

5.117.01.0

0267.00267.00267.0056.0056.0056.0056.0

056.00219.00219.00219.00175.00175.00175.00314.0

FLFLFL

FLFLFLFL

FLFLFLFL

FLFLFLFL

ST

IIIIIII

IIIIIIII

p

+++++++

+++++++

= (1)

Where IFLn are the IFL-value exceeded for n% of the 10 min period.

VI .Flickermeter Model Using MATLAB-SIMULINK

In this section, the explained five blocks are modeled with MATLAB-SIMULINK as follows [5].

Fig. 5: Diagram of Block A

Fig. 6: Diagram of Block B

110656.1101

325 +×+ −− SS 110522.4101

325 +×+ −− SS 110178.6101

325 +×+ −− SS

Polynomial 1 Polynomial 2 Polynomial 3 Butterworth Filter

×÷

3/rmsV

2U aUaoV

Quadratic demodulator

RMS reference value

aU 11830989.31830989.3

+SS

Butterworth filter bU

High-pass filter Cutoff frequency 0.05HZ


155

Fig. 7: Diagram of Block C

Fig. 8: Diagram of Block D

Fig. 9: Diagram of Block E

This way it is possible to enable the program to distribute the instantaneous flicker perception levels, based on frequency, to weight the disturbance generated based on their duration, and to return the accumulated flicker probability curve and the index PST.

VII. Characteristics Of Electric Arc Furnaces

A simple single phase equivalent of the Furnace and its supply system, is shown in Fig. 10.

Fig. 10: Single-phase circuit for estimating Furnace characteristics

The short circuit reactance of the supply network is as follows [11]:

pcc

pcc

IUU

X−

=1 (2)

The reactance of the flexible loads, the electrodes and the Furnace transformer:

U SdU

Memorization in brain

13.01+s

8017.3308017077.515499.100

2 ++ SSS

11371526.01043921.9

1069811.024 ++×

+− SS

SdU

CU

Lamp-Eye-Brain-Chain 1 Lamp-Eye-Brain-Chain 2

2U eUdU

Non-linear eye-brain

Perception


156

pcc

pcc

IU

X =2 (3)

The short circuit power at the PCC:

1XUU

S pccsc

−= (4)

The Furnace short circuit power:

UUU

SS pccscscf

−= (5)

The Arc Furnace apparent power could be determined according to the power factor limit.

ϕcos

ff

pS = (6)

Now it is possible to estimate the ratio between the Furnace apparent power and the short circuit level at the PCC:

sc

f

SS

k = (7)

With this K factor, it is possible to obtain the maximum flicker that could be emitted by Furnace. It is possible to estimate the flicker compensation ratio:

itst

realst

pp

FIlim

= (8)

This parameter is of fundamental importance for the determination of the compensator size with weak network.

From observations of practical installations the approximate equation for estimating the rating for an EAF STATCOM is given in [6].

reatedAFSFIQ ××= 54.0 (9)

Where

reatedAFS = (0.55 to 0.65)*Sscf

VIII. Simulation Results

The simulation consists of melting and boring procedures. Actual Arc of these processes were generated by means of controlled source current block diagram of MATAB-SIMULINK through a 10s time interval. Finally, this actual arc is applied to DC and AC Arc furnaces and then operation of them is simulated and analyzed. The actual Arc voltage and current are shown in figures 11 and 12.


157

Fig. 11: Arc voltage

Fig. 12: Arc current

A.DC Furnace

In this section, the proposed method for a 150 MW DC Furnace is simulated by MATLAB-SIMULINK. This Furnace is connected to the network by means of a 50km line with the resistor of km/12.0 Ω and inductance of kmmH /93.0 . The simulation consists of melting and boring procedures. Since simulations are performed for a weak network with long transmission line, the static filter has been used to improve the power quality. The simulation is firstly done without filter, and then performed with a 20MVAR static filter. In figures 13 and 14 the values of rmsV , rmsI are calculated at the PCC.


158

Fig. 13: PCC voltage without filter

Fig. 14: PCC current without filter

As seen in Fig 4 the effective voltage of the PCC is applied to block diagram of flickermeter. As the figure indicates, the output of block 4 and block 5 is FLI and stp , respectively.

Fig. 15: FLI diagram without filter


159

Fig. 16: PST diagram without filter

Further, this case is simulated by adding a static filter to CD electric Arc Furnaces. In this condition the effective voltage and current is calculated at point of PCC and diagrams are shown in figures 17 and 18.

Fig. 17: PCC voltage with filter

Fig. 18: PCC current with filter


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By using the flickermeter the values of FLI and stp are measured at point of PCC. The diagrams are shown in figures 19 and 20.

Fig. 19: FLI diagram with filter

Fig. 20: PST diagram with filter

As you see in diagrams FLI and stp , two cases, with and without filter have reasonable resemblance. Both of them are in the safe region. In addition, the active and reactive power of DC Arc Furnaces is simulated at point of PCC in two cases, with and without filter. The result are shown in figures 21 and 22.


161

Fig. 21: PCC active and reactive power with filter

Fig. 22: PCC active and reactive power without filter

Based on the figures usage of static filter partially improves the active and reactive power oscillations. So if rather than voltage flicker, power quality is important, application of the filter is recommended, further in figures 23 and 24, it is shown that when static filter is used, power factor is 0.9 constant and without filter it varies between 93.0.73.0 ≤≤ FP . Also a variation of reactive power is smaller than active power. In addition, by controlling of firing angle of the thyristors, it is possible to push the system to operate with lower VAR variations [3], [4]. However this control process leads to a permanent power factor oscillation. Consequently, variation of output voltage and current is inevitable. Nevertheless, in those networks with lower short circuit levels or in case of longer transmission lines, installation of compensator is unavoidable.


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Fig. 23: P-Q normalized diagram with filter

Fig. 24: P-Q normalized diagram without filter

B.AC Furnace

In this section AC electric Arc Furnace is replaced with the DC Arc Furnace and then they are simulated. In addition, the rated value of system in AC simulation is samed with the DC simulation. The AC simulation is performed with applying dynamic of Arc same to Arc of CD simulation. The Arc voltage and current diagrams are shown in figures 11 and 12. In this condition the variation of effective voltage at point of PCC is simulated and shown in Fig. 25. Finally, the effective voltage is applied to flickermeter and then the values of FLI and stp is measured.


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Fig. 25: PCC voltage with AC Furnace

Fig. 26: FLI diagram with AC Furnace

Fig. 27: PST diagram with AC Furnace


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The figures 26 and 27 show that FLI and stp value increases in AC position, respectively, and it's necessary to use from STATCOM or SVC for compensation.

IX. Conclusion In this paper, a novel method for high pulse operation of AC/DC converters is proposed based on current reinjection technique. This method decreases the harmonic effects and improves the voltage flicker at PCC point. Measurements and discussions about flicker were presented by using standard block diagrams of flicker meter. Simulation results indicate that the flicker is keep at an acceptable range respect to the standards. Moreover, Arc is applied for a AC Furnace (with the dynamic model that was applied for a DC Furnace). A comparison of AC and DC Furnace in the same condition, shows that in the AC Furnace the value of flicker has been increased. Therefore by use of AC Furnace, it's necessary to use dynamic VAR compensator. For an optimal selecting of AC or DC Furnace, it's necessary to survey techno-economic problems.

References [1] MUHAMMAD H. RASHID,"POWER ELECTRONICS HANDBOOK", Copyright # 2001 by

ACADEMIC PRESS. [2] T. Miyashita, N. Ao, and Y. Mikami, “Development and operation of large DC arc Furnace,” presented

at the Electrotech’92, Montreal, QC, Canada, Jun. 1992. [3] F. Richardeau, Y. Cheron, J. Du Parc, M. Wursteisen, and C. Glinski, “New strategy of control at low

flicker level for DC electrical arc Furnace,” presented at the IEEE Int. Conf. Industrial Technology, Gangzhou, China, Dec. 1994

[4] Philippe Ladoux, Gianluca Postiglione, Henri Foch, and Jacques Nuns '' A Comparative Study of AC/DC Converters for High-Power DC Arc Furnace'' IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 52, NO. 3, JUNE 2005.

[5] A. Bertola, G. C. Lazaroiu, M. Roscia, D. Zaninelli,'' A Matlab-Simulink Flickermeter Model for Power Quality Studies'' , 2004 11th International Conference on Harmonics and Quality of Power

[6] Static Synchronous Compensator (STATCOM) for Arc Furnace and Flicker Compensation. WG B4. 19. CIGRE Publication. December 2003.

[7] P. Mattavelli, L. Fellin, P. Bordignon, M. Perna, “Analysis of Interharmonics in DC Arc Furnace Installations ”,Paper accepted for presentation at the 8u International Conference on Harmonics and Quality of Power ICHQP '98, jointly organized by IEEE/PES and NTUA, Athens, Greece, October 14-16, 1998

[8] “Power Quality Measurements and Operating Characteristics of Electric Arc Furnaces”. F. Issouribehere, P. E. Issouribehere and G. Barbera. 2005 PES General Meeting. 0-7803-9156-X/05.

[9] A. Robert and M. Couvreur, Arc Furnace Flicker Assessement and Mitigation. Amsterdam, The Netherlands: PQA, 1994.

[10] IEC 61000-4-15. Electromagnetic Compatibility (EMC). Part 4: Testing and measurement techniques. Section 15: Flickermeter. Functional and design specifications.

[11] J. L. Agüero, F. Issouribehere, P. E. Battaiotto “STATCOM Modeling for Mitigation of Voltage Fluctuations caused by Electric Arc Furnaces” 1-4244-0493-2/06/$20.00 ©2006 IEEE.

[12] Zhao, P; Meng, YD; Yu, XY, et al “Energy Balance in DC Arc Plasma Melting Furnace” PLASMA SCIENCE & TECHNOLOGY Volume: 11 Issue: 2, Pages:206-210, Published: 2009.

[13] Tzonev, T; Lucheva, B “Recovering aluminum from aluminum dross in a DC electric-arc rotary furnace” Source: JOM Volume: 59 Pages: 64-+ Published: 2007

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New Strategy of DC Electrical Arc Furnace Converter for ... · M. Hosseini-Firouz & B. Mozafari: New Strategy of DC Arc Furnace Converter 151 The traditional AC electric Arc Furnace