Comparison of thyristor-controlled reactors and voltage-source inverters for compensation of flicker caused by arc furnaces

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Aurelio García-Cerrada, Member, IEEE, Pablo García-González, Rafael Collantes, Tomás Gómez, Member, IEEE,

and Javier Anzola

Abstract—The objective of this paper is to compare the perfor-mance of thyristor-controlled reactors (TCR) and shunt-connected PWM voltage source inverters (PWM-VSI) for compensation of flicker caused by arc furnaces. First of all, arc-furnace principles are presented in order to explain the main characteristics of the problem. Secondly, traditional TCR control are analyzed. An im-proved measuring procedure is suggested to enhance TCR perfor-mance showing that it achieves faster compensation than more tra-ditional methods. Thirdly, PWM-VSI control for flicker compen-sation is described in detail using Park’s transformation. The anal-ysis shows how real and reactive power control can be decoupled. Continuous-time and discrete-time models are considered. Finally, a TCR control and a PWM-VSI control are compared by simula-tion using data and measurements from a real arc-furnace installa-tion. The analysis considers three different periods of the produc-tion cycle: a) bore-down, b) fusion, and c) refining. It is clear from the results obtained that a shunt-connected PWM-VSI is better than a TCR for flicker compensation. This can be easily justified noting that the bandwidth of the PWM-VSI control system is far better than that of the TCR control. However, the control system for a PWM-VSI inverter is more complicated than that of a TCR. Besides, the latter uses a better-established technology than the former.

Index Terms—Arc furnace, flicker mitigation, PWM, TCR con-trol, voltage source inverter control.

I. INTRODUCTION

A

RC FURNACES are medium-size stochastic nonlinear loads (see [1]–[4]). The effect of the nonlinearity is the production of harmonic currents, which produce harmonic voltages in close-by nodes of the supply system. In addition, the stochastic variations of the fundamental component of the current drawn by an arc furnace produce fluctuations of the fundamental component of the voltage in close-by nodes of the system. These fluctuations are the origin of the phenomenon known as flicker, that is, fast changes in the luminosity of incandescent lamps. Variations of voltage as low as 0.3% with frequencies around 8.8 Hz can be detected by 50% of the population [2].

Currently, the most widely-used method for flicker com-pensation is the connection of shunt static VAR compensators

Manuscript received September 14, 1998.

The authors are with the Instituto de Investigación Tecnológica and Department of Electronics and Control Engineering, Escuela Técnica Superior de Ingeniería-ICAI, Universidad Pontificia Comillas, c. Alberto Aguilera 23, 28015, Madrid, Spain (e-mail: aurelio@iit.upco.es; collantes@iit.upco.es; pable@dea.icai.upco.es).

Publisher Item Identifier S 0885-8977(00)10314-0.

Fig. 1. Thyristor-controlled reactor (TCR).

based on thyristor-controlled reactors (TCR’s). However, PWM voltage-source inverters (PWM-VSI) have been recently proposed as a flexible alternative for this purpose although they are not commercially available yet [5]. This paper will compare the performance of TCR-based and VSI-based flicker compensation using experimental records from arc-furnace installations without any of these compensations installed. A test bench installation has been simulated adding both compensation systems for comparison.

The principles of traditional TCR compensators are explained in Section II of this paper, including some im-provements suggested to minimize the current measurement delay tailored to improve the control system bandwidth. The principles of PWM-VSI control for flicker compensation are explained in Section IV giving details of the control system implementation tailored to improve flicker compensation. The test bench installation is described in Section V. Section VI compares simulation results using: a) traditional TCR com-pensation, b) improved TCR comcom-pensation, and c) PWM-VSI compensation. Finally, the main conclusions of this study are summarized in Section VII.

II. PRINCIPLES OFFLICKERCOMPENSATIONUSINGTHYRISTOR CONTROLLEDREACTORS

A TCR consists of a reactance connected in series with a pair of thyristors and it is the most-widely used practical realiza-tion of a variable reactance (see Fig. 1). Minimum reactance is obtained when the thyristors are fired at the maximum of the voltage (angle rad). Infinite reactance is obtained when the firing angle is , that is, when voltage is 0 (see Fig. 2).

The equation that relates the fundamental current and the firing angle is [6]:

(1)

(2)

Fig. 2. Voltage and currents in a TCR.

Fig. 3. Control scheme for a TCR.

where:

is the magnitude of the fundamental current, is the firing angle ,

is the value of the reactance, is the voltage across the inductance.

The control system of a TCR is tailored to keep the variations of reactive current drawn by the system “arc furnace TCR compensator” as small as possible. This will minimize voltage variations in close-by nodes of the system. Note that variations of the active current on the arc furnace will produce compar-atively small voltage variations due to the inductive nature of the line impedance. Since a TCR is fired every half cycle of the supply voltage, it can be modeled as a discrete-time system with a sampling interval equal to half a period of that voltage. The de-vice output is the current drawn by the TCR that changes every half cycle, with no dynamics [1]. The TCR control system is normally arranged with a feedforward scheme such as the one shown in Fig. 3. Note that it is necessary to determine the fun-damental current drawn by the arc furnace and some filtering or signal processing must be used. This delays the current mea-surement preventing from fast-acting, compensation. The fact that arc-furnace currents have considerable harmonic distortion worsens the problem. The designer must find a compromise be-tween speed and accuracy for the measurement system.

Strictly speaking, it is necessary at least one period of any pe-riodic function to determine its harmonic components. A DFT of one cycle of current samples could be used to calculate its harmonic content [7]. However, it will be shown that the special spectral characteristics of the arc-furnace current makes it pos-sible to obtain accurate measurements using half cycle only.

Fig. 4. Voltage–current characteristic of the furnace electric arc.

Fig. 5. Measured currents in an arc furnace.

III. IMPROVEDCONTROL FORA TCR-BASEDCOMPENSATOR

The nonlinear behavior of an arc furnace is due to the nonlinear voltage–current characteristic of the electric arc. This characteristic is given by the following equation (see [3] and [8])

(2)

where:

is the voltage across the electric arc, is the current of the electric arc,

is the voltage threshold which depends mainly on the arc length,

and are constants that determine the exact shape of the curve.

Fig. 4 shows this current–voltage characteristic with

param-eters V, kW and A as in [6].

It has been reported that since the electric arc current–voltage characteristic is an odd function, the arc furnace will produce only odd harmonics [9]. This theoretical result has been confirmed by the authors using experimental data. As an example, Fig. 5 shows data obtained from the 60 MVA AZMA arc-furnace installation close to Madrid, Spain. Clearly, even harmonics are much smaller than odd harmonics. Similar results were found in other installations. This fact can be used to measure the fundamental arc-furnace current using a modified DFT of current samples spanning only half cycle [7]. This procedure is, clearly, faster than traditional filtering (analog or digital) or DFT methods using the whole cycle. The

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Fig. 6. Schematics for PWM-VSI shunt compensation.

resulting algorithm is analyzed in detail in [10] in the context of arc-furnace analysis and control. As will be proved later, shortening the measurement delay improves significantly the TCR-compensator performance.

The measurement error using half cycle when small quanti-ties of even harmonics are present is also discussed in detailed in [10].

IV. FUNDAMENTALS OFFLICKERCOMPENSATIONUSING A SHUNTPWM-VSI

Fig. 6 shows a schematic representation of a shunt PWM-VSI. The shunt compensator consist of a PWM-VSI with a capacitor to keep the dc-link voltage almost constant. The fundamental component of the inverter output voltage is in Fig. 6. The converter is connected to the network using an inductance and, very often, a transformer. The connection impedance is modeled by in Fig. 6. Assuming that the inverter bandwidth is large enough, this behaves as an ideal voltage source which can be used to control the instantaneous current injected in the point of common coupling (PCC). The control strategy here is also tailored to keep the variations on reactive current drawn by the system “arc furnace compensator” as small as possible.

A. The Model for the Shunt Compensator

Using Park’s Transformation [11], the shunt converter can be modeled as:

(3)

where is the angular speed of the reference frame and all vari-ables can be seen as space vectors or complex numbers

. The Park’s Transformation used is power invariant. If the reference frame always keeps the axis along the PCC space vector , and using the definitions for instantaneous active and reactive power [12] one can write:

(4)

(5)

According to (4) and (5), and control is simplified to the control of and .

In addition, neglecting the inverter losses, the equation for the dc-link voltage capacitor is:

(6)

(7)

In this application, the speed of the rotating frame is almost constant and equal to the supply frequency. Note that and are the control inputs (inverter output) and can be tailored to control the active and reactive power injected by the converter in the PCC. Unfortunately, - and -axis dynamics are coupled in the state matrix . Equation (7) can be rewritten as:

(8)

(9)

with

(10)

(11)

Equations (10) and (11) are the decoupling laws that make it possible decoupled control of active and reactive power in (8) and (9).

C. DC-Link Voltage Control for the PWM-VSI

PWM-VSI operation requires a constant dc-link capacitor voltage. Equation (6) can be rewritten as:

(12)

where is the power extracted by the converter from the d.c.-link capacitor. Note that

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where is the real power used in the connection inductance to change its rms current value. This real power is necessary when changing reactive power exchange. It can be estimated and used as a feedforward term in the capacitor voltage control system as depicted in Fig. 7, .

D. Discrete-Time Control of the Shunt Converter

Section IV-B shows how real- and reactive-power control can be decoupled in a continuous-time model of the system. How-ever, the implementation of the controller requires discrete-time

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Fig. 7. Control of the d.c.-link capacitor voltage.

models. Since the system model in (7) is linear, one can write a discrete-time model which gives exact results at the sampling times [13]. This model takes the form of [14]:

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where, if a zero-order hold is placed at the input, matrices and can be calculated from matrices and in (7) and [13]:

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with the sampling interval.

Note that now there are coupling terms between the dynamics of and axis in both the state and input matrices. The system model in (14) can be rewritten as:

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with the following decoupling laws:

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The control system for the shunt converter is depicted in Fig. 8 Although the system model is now ready to design a discrete-time controller, some further comments are worth pointing out:

1) The bus voltage at the PCC has to be measured. Due to the high voltage distortion at that point, anti-aliasing fil-ters have to be used. The phase lag of these filfil-ters must

Fig. 8. Shunt PWM-VSI control system.

Fig. 9. Test bench installation for simulation.

be compensated for in the controller due to the required bandwidth for flicker compensation.

2) The PWM strategy for the VSI is designed so that the in-verter average output voltage components ( and ) are equal to the required ones at the beginning of the sampling interval. Some compensation is required to take into ac-count the movement of the rotating frame during the sam-pling interval and the time delay necessary to calculate the required inverter output. This compensation is explained in [14].

3) The converter – current components also have to be measured. These are reasonably filtered by the connec-tion impedance. Furthermore, these variables can be sam-pled when the inverter is applying zero voltage during PWM. This produces a further filtering effect making anti-aliasing filters unnecessary.

V. DESCRIPTION OF THETESTBENCHARC-FURNACE INSTALLATION

A typical arc-furnace installation is depicted in Fig. 9. The electrical network has been substituted by its Thevenin equiv-alent. The network impedance has been considered mainly in-ductive, with . The arc furnace has been substituted by the measured current during field tests in a 60 MVA arc fur-nace. The instantaneous network voltage (instantaneous value of ) was synthesized from measured currents and voltages at the PCC during field tests imposing the condition of “no flicker” in . The rated values at the PCC are: kV and

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Fig. 10. Instantaneous flicker values for the initial bore-down period. Output 5 of UIE flickermeter. TCR performance.

Fig. 11. Instantaneous flicker values for the fusion period. Output 5 of UIE flickermeter. TCR performance.

VI. SIMULATIONRESULTS ON THETESTBENCHINSTALLATION

In order to compare TCR-based and VSI-based compen-sation, simulations were carried out based on data recorded from actual arc furnaces without compensation. The data were recorded in the primary of the furnace transformer. Both, voltage and current, were recorded using a sampling frequency of 6400 Hz. Simulations were carried out for three different time intervals: a) bore-down, b) fusion, and c) refining. Figures in this Section show the output 5 of UIE flickermeter as defined and explained in [2]. This definition was accepted in the IEC norm [15]. Output 5 of a flickermeter is representative of flicker sensation. One unit of output corresponds to the visual perceptibility threshold of a flicker occurrence. Higher output values mean that flicker intensity is more than perceptible and can become annoying or intolerable. Thus the flickermeter output values must be interpreted as “per unit of perceptibility.” First of all, compensation using a TCR is considered. Results are shown in Figs. 10–12 for bore-down, fusion and refining, respectively.

Figs. 10–12 show three traces, each. Both compensation methods (DFT-based and modified DFT) show a signifi-cantly better performance than the uncompensated system. As

Fig. 12. Instantaneous flicker values for the refining period. Output 5 of UIE flickermeter. TCR performance.

Fig. 13. Instantaneous flicker values for the initial bore-down period. Output 5 of UIE flickermeter. PWM-VSI performance.

explained in Sections II and III, while the DFT-based method uses a one-cycle DFT to measure the fundamental component of the furnace current, the modified DFT method uses only half cycle to measure the fundamental component of that current. The second method imposes a smaller delay in the control system than the first one and gives better performance.

The strategy using a TCR with the best performance has been compared with the use of PWM-VSI control for flicker compen-sation. Results are shown in Figs. 13–15 for bore-down, fusion and refining, respectively. These figures also show three traces each. The uncompensated flicker value is compared to the com-pensation using TCR with modified DFT and the compensa-tion using a shunt-connected PWM-VSI. The inverter control system works with sampling frequency of 4 kHz and switching frequency of 2 kHz. In all three cases the compensation using a PWM-VSI is superior to the system with a TCR. This holds even using the best TCR control.

VII. CONCLUSION

This paper has compared the performance of thyristor-con-trolled reactors and shunt-connected PWM voltage source in-verters for compensation of flicker caused by arc furnaces. The

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Fig. 14. Instantaneous flicker values for the fusion period. Output 5 of UIE flickermeter. PWM-VSI performance.

Fig. 15. Instantaneous flicker values for the refining period. Output 5 of UIE flickermeter. PWM-VSI performance.

comparison has been carried out using a test bench installa-tion and actual records taken from field tests to simulate the arc-furnace performance. Three different periods of the melting process have been investigated and compared. It is clear from the results obtained that a shunt-connected PWM-VSI is better than a TCR for flicker compensation. This can be easily justified noting that the bandwidth of the PWM-VSI control system is far better than that of the TCR control. It is worth pointing out that the TCR controller command can only be updated every half cycle of the system frequency (50 Hz in Europe, for example) and the transient performance of the device is not considered. On the other hand, the controller command of a PWM-VSI can be updated with frequencies of, at least, 1 kHz looking at real-istic devices. However, the control system of the PWM-VSI is more complicated than that of a TCR. In addition, the latter uses better-established technology in power systems than the former. The flicker attenuation achieved by a TCR installation provided with a suitable control system (see Figs.10–12) is already re-markable and may be sufficient in many applications.

The implementation of a parallel VSI in high-power applica-tions may make PWM impossible. Multi-pulse inverter config-urations have been suggested to overcome this problem. This

would result in a far more complex power circuit than that of a TCR and the bandwidth of the resulting control system still has to be investigated thoroughly.

Measuring the arc-furnace current demand is always a problem when compensating flicker due to its large harmonic content. Simple analog or digital filters can be used for a PWM-VSI. The speed of the measurement procedure has to be taken to the limit in the case of a TCR to avoid further phase lag in the control loop. This paper has shown that a modified DFT using half cycle of the system frequency can be used to improve the performance of a TCR-based compensator. Even then, the PWM-VSI shows better performance.

This paper has looked only at flicker compensation. Har-monics produced by both power-electronic devices should also be evaluated when necessary. Note that flicker compensation re-quires minimization of the variations of the fundamental reac-tive current component, mainly and harmonics are not consid-ered.

Finally, it has also been shown that flicker is more severe during bore-down period. A shunt PWM-VSI works remarkably well during this period, too.

REFERENCES

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[7] A. V. Oppenheim and R. W. Schafer, Discrete-Time Signal Pro-cessing. Englewood Cliffs, NJ: Prentice-Hall International, 1989. [8] A. Cavanilli, G. C. Montanari, L. Pitti, and D. Zaninelli, “ATP

simula-tion for arc-furnace flicker investigasimula-tion,” European Trans. on Electric Power, vol. 5, no. 3, pp. 165–172, May–June 1995.

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compensatiors comprising switching devices without energy storage components,” IEEE Transactions on Industry Applications, vol. IA-20, no. 3, pp. 625–630, May/June 1984.

[13] K. J. Astrom and B. Wittenmark, Computer Controlled Sys-tems. Englewood Cliffs, NJ: Prentice-Hall International, 1990. [14] P. García-González and A. García-Cerrada, “Control system for a

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Pablo García-González (M.Sc. 1992, Universidad Pontificia Comillas). From

Oct. 1993 to March 1995 he was with Valenciana de Cementos where he was a Project Engineer involved in control applications. He is currently working toward his Ph.D. at the Department of Electronics and Control Engineering of Universidad Pontificia Comillas (UPCO). His research interests include power electronics applications in electric energy systems.

(Ph.D. 1989, Universidad Politécnica, Madrid, Spain). He joined Instituto de In-vestigatión Tecnológica at Universidad Pontificia Comillas (IIT-UPCO) in 1984 where he is currently a Research Fellow. He is also the Director of IIT since 1994. His areas of interest are planning and operation of transmission and dis-tribution systems, power quality and economic and regulatory issues.

Javier Anzola obtained his electrical engineering degree from Universidad

Pon-tificia Comillas, Madrid, Spain in June 1998. During last year he has collabo-rated at the Department of Electronics and Control Engineering of this Univer-sity in simulation of PWM-VSI for flicker compensation.

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