A SIMULINK Study of Electric Arc Furnace Inactive Power Compensation by Using STATCOM

Journal of Applied Mathematics, Islamic Azad University of Lahijan

Vol.3, NO.10, Autumn 2006

A SIMULINK Study of Electric Arc Furnace Inactive Power Compensation by Using STATCOM STATCOM a

b

Abdolreza Tavakoli , Mehdi Ehsan , Seyed Mohammad Tagie Batahiee

c

a

Islamic Azad University-Sci University-Science ence and Research Research Branch, Branch, Tehran, Iran b Sharif University of technology, Tehran, Iran c Khaje Nasir Toosi Toosi University of technology, technology, Tehran, Tehran, Iran

Abstract

D I S f o e v i h c r A

The nonlinear and time varying nature of electric arc furnaces (EAFs) create flicker, harmonics, and voltage/current unbalances. Nowadays high-speed compensators like STATCOM can improve the performance of EAFs and the power quality problems of electrical systems around them. This paper uses a time domain model for electric arc furnaces and a new model of STATCOM respectively for creating mentioned power quality problems and inactive power compensation. In this paper, we investigate and simulate electric arc furnace and STATCOM using SIMULINK/PSB. Then, we simulate a network including them completely. The proposed model takes into account arcing conditions and some power quality effects improvement. Keywords: Keywords: Electric Arc Furnace, STOTCOM, SIMULINK, Power Quality.

1. Introduction

Study of power quality problems caused by EAFs requires a general and accurate model for EAF. Different arc furnace modeling, both in time and frequency domain, is as following: 1- The V-I characteristic method. 2- Time-domain equivalent nonlinear circuit model. 3- Harmonic voltage source model. 4- Harmonic domain solution of nonlinear differential equation. 5- Random and stochastic process method. met hod. 6- Use of actual recorded terminal quantities use in current injection models. 7- Use of electric arc furnace power balance equation. 8- Use of combined Cassie/Mayr model. In this paper combined Cassie/Mayr model, a three-phase arc furnace model, which can simulate all the mentioned power quality indices, is used. This model enables us to investigate different design alternatives for the secondary current conductors, study the effects of EAFs circuit parameters on electric states, and rationalize furnace conditions in service [1, 2 and 3]. Also in this modeling, the furnace transformers tap effect and the mutual inductance between the flexible cables of the secondary side of the furnace transformer has been considered.

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This model is in the time domain and in addition to generating power quality parameters, can investigate the effect of different feed systems designs on performance of furnace. The basic methodology for flicker mitigation can be categorized into three types: 1- Regulating the EAF passive components, such as source impedance [4]. Although, to some extent, the increasing series reactance can mitigate the flicker, it reduces power supply and therefore decreases EAF productivity. Moreover, it is also expensive and laborious to control upstream transformer reactance or series reactor in the firmer deregulation power system [5]. 2- Compensation through the combination of thyristor and passive components, such as the well-known SVC1. SVC can not only improve power quality of nearby system, but also increase EAF productivity and bring additional economic benefits. However, it cannot catch up the fast-varying flicker (1Hz~20Hz) very well with the inherent limit of relatively low bandwidth and hence its dynamic performance for flicker mitigation is limited. 3- The state-of-the-art solution is the STATCOM 2 based on high frequency VSC 3 [5, 6].


2. Simulation of the Electrical arc furnace

In simulating the following electrical circuit, phase parameters and arc equation are used [4]: Fig 1 A simplified electrical circuit for EAF simulation

R L1=0.0004  ,R L2=0.0004  ,R L3=0.0004  , Rs=0.1m  L1=15.97 mH, L 2=15.97 mH, L 3=15.33 mH, Ls=1mH M 12=4.58 mH, M 13=3.89 mH, M23=4.58mH,

g

=

gmin + (1- exp(-

i2

)) 2

vi

io E02

+

(exp(-

i2

i2

))( 2 i0 p0

−

θ

dg ) dt

In order to determine the arc static model the arc has the following parameters: ( θ 0 =0.000110 or θ 0 =0.000510), Eo=200V, α =0.0005, θ1 =0.000100; po=100; io=10; gmin=0.008

The above-mentioned mathematical model has been simulated in the form of a program in the MATLAB /SIMULINK Software, and is presented in Fig. 2. The selection of different time constants for the arc has taken place because the information processing related to the melting processes shows that the arcs time constant increases from 80 to 100 microseconds in the initial stages of melting to 500 to 550 microseconds in the final and refining stages.

1

2 3

Static VAR Compensation Static synchronous Compensator Voltage-Source-Converter

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The arc voltage versus current has been seen in Fig. 3. The corresponding arc conductance, voltage, and the comparative voltage/current for phase an are shown in figures (4) to (6). In continuation of the topic, investigation of the arcs dynamic characteristics is done. As was explained, for creating the flicker effect, it is necessary to modulate the arc voltage-current characteristic with sinusoidal and random signals. With the assumption for balanced working conditions for the electric arc furnace and the sinusoidal variation law for the arc effective voltage, the different quantities wave shape in the furnace system are shown in figures in figures (7) to (10).

With the assumption of balanced working conditions for the furnace and random time variations, for E (for a, b, c ), different quantities have been drawn in figures (11) to (14). For the unbalanced performance of electric arc furnace, the voltage values at different phases have been chosen as follows: θ 0 =0.000510, Eoa=200V, Eob=180V, Eoc=250 α =0.0005


Figures (15) to (18) show static characteristics of different quantities at the furnace different point in its unbalanced performance. Fig 2 The Electric Arc Furnace Simulated in SIMULINK

Fig 3 Voltage/current characteristic seen at phase a

Fig 4 Electric Arc Conductance Related to Phase a

Fig 5 Phase a Arc voltage Curve

Fig 6 Phase a voltage/current comparative curve

Fig 7 the phase current curve with the 10Hz sinusoidal balanced flicker

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Fig 8 Electric arc conductance with the balanced 10Hz flicker

Fig 10 Phase a s voltage and current comparative curve

Fig 9 Electrical arc voltage-current characteristic seen at phase a

Fig 11 Arc voltage


Fig 12 Electric Arc Conductance

Fig 14 Phase as voltage and Current Comparative Curve

Fig 16 Voltage/Current Characteristic Seen at phase a

Fig 13 Voltage characteristic and the Electric Arc current Seen at phase a

Fig 15 Phase A’s Current in Unbalanced conditions

Fig 17 the phase a Arc Voltage Curve

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Fig 18 Arc voltage and current comparative curve

3. Distribution Static Synchronous Compensator (DSTATCOM)


The power circuit of the three-phase DSTATCOM with time varying arc furnace load has shown in Fig. DSTATCOM comprised of a voltage source inverter that has a capacitor at its DC side. A transformer usually does the inverter outlet connection to the network. In the noload state, the DSTATCOM output voltage and the networks voltage are the same and as a result, no current is injected to the network. In the capacitive state, the DSTATCOM voltage amplitude is more than the network voltage. As a result, in facts it is a capacitive Inactive current to the network. In other words, DSTATCOM acts like a capacitor, and generates reactive power. In the inductive state, DSTATCOM’s voltage amplitude is less than the network voltage and as a result receives an inductive current from the network. In other words DSTATCOM acts like an inductor and consumes reactive power. In this way by changing the DSTATCOM output voltage amplitude that’s done by controlling the implemented pulses to the gate switches of the semiconductor, we can control the consumed or generated reactive power by it [7, 8]. The different applied flicker reducing methods in DSTATCOM can briefly be listed below: 1-Direct methods of controlling the current and direct control of power have a good performance in compensating voltage flicker but because they are based on measuring the loads current, they are only usable when only compensating the flicker duo to a specific load is intended. 2- Voltage positive sequence component control method and positive and negative sequence Components simultaneous control method. Fig 19 The power circuit of the three-phase DSTATCOM with time varying arc furnace load

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Table 1 COMPARISON OF DIFFERENT CONTROLLING METHODS OF DSTATCOM IN ORDER TO COMPENSATE VOLTAGE FLICKER


4. Power system Description

Modeling the sample power system with STATCOM & its controller and the electric arc furnace EAF by using MATLAB/SIMULINK has given in Fig. 20. In this paper a completely new model of STATCOM for improving electric arc furnace power quality in the time domain & a power system is simulated. This simulation is done by using the MATLAB/SIMULINK and the PSB 4. In this simulation, STATCOM has made equivalent to an instantaneous inactive power compensator with an active consumer (for showing the switching loss). Also the suggested control method is based on the generalized instantaneous power theory that was expressed the first time in the year 1996 is valid for the three phase systems in the sinusoidal, non-sinusoidal, balanced and unbalanced conditions and has generality compared to the previous methods [9]. Fig.20 shows a STATCOM at the connection to a 138kv network. The feeding network has shown with its equivalent there in network. While the dimension of the voltage is 1.06 × 138KV and the short circuit of the network is 1000MVA with the ratio

X R

=

2 .5 ,

and is connected to B2

bus bar. The systems complete information has been mentioned in table (2) [10]. The power is applied to the dynamic compensator by noting the necessary signal controlling strategy. So by considering B2 bus bar as the consuming bus bar and PCC bus bar as the feeding bus bar the vector relations have been applied according to Fig.20 in the SIMULINK environment. In which Q L(t) and PL(t) are the furnaces instantaneous active and inactive power and QS(t) and PS(t) are the feeder network instantaneous power and Q L and PL are the furnaces average power and Q S and PS are the feeder network average powers. Also V abc and I abc are the instantaneous voltages and currents at the STATCOM connection to the utility and the load according to Fig 20.

4

Power System Blackest

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By defining an adequate function of f(q) we can define the STATCOM responsibilities for improving power quality. For example whenever f(q) = q is defined then the injected power by STATCOM will cause the furnace inactive power compensation. It is clear that in this case the applied equipments in the STATCOM structure shall be able to tolerate a large value of the current. If f(q)=q –qave5ohz is defined all of the nonnative power oscillating values except components below the 50 Hz frequency will also be compensated. In figures (22) to (28), the outputs resulting from this simulation is shown. Fig 20 Schematic of the simulated circuit in SIMULINK


Table 2 THE COMPLETE SPECIFICATIONS OF THE SIMULATED SYSTEM

Fig 21 Simulated vector Relations in the SIMULINK Environment.

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Fig 22 Furnace voltage

Fig 23 Furnace Current

Fig 24 Load, supply and STATCOM Instantaneous Inactive power


Fig 25 Load, supply and STATCOM Average Inactive power

Fig 28 Supply Inactive and Active Average power

Fig 26 Supply Inactive and Active Instantaneous power

Fig 27 Load, supply and STATCOM current

Also if f (q) is defined as the suitable function, of inactive power oscillating values are also compensated at flickering frequencies. In other words Flicker will be eliminated from the electric arc furnaces inactive power (with the least power from the elements forming STATCOM), And because the amount of voltage value variations have direct relationship with the variations of inactive power values, One can say a major portion of voltage Flicker will be eliminated. Figures (29) to (31) are the outputs of simulation.

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Fig 29 Load Inactive and Active Instantaneous power


Fig 30 Load Inactive and Active power


Fig 31 Supply Inactive and Active Instantaneous power

If STATCOM controlling function acts according to compensating q(t) and f(p) then compensation reactive power and a lot of active power oscillations are also supplied by STATCOM, And as a result the amount of passing current decreases on the supply side (feeding) and a lot of non-sinusoidal components and generating flicker will be supplied by this compensator. 5. Conclusion

In this paper , a three phase model of the electric arc furnace by depending on the Cassei/Mayr model by using the MATLAB /SIMULINK toolboxes was introduced that has the real time modeling capability of the different status of the furnace. Investigation of the results obtained by simulating is briefly as follows. 1- Investigating the results obtained from simulation show that the noticeable frequencies in the Flicker effect are usable by the cassie/Mayr model for the electric arc furnace. 2- In order to compensate the destructive effects resulting from the furnaces specially the flicker one can use elements such as DSTATCOM and STATCOM. These elements can completely compensate the loads reactive power variations at each frequency and their response speed is very fast. 3- Here the equations of instantaneous active and inactive power have been used and have been controlled with the necessary purpose fullness of STATCOM. In order to be clear regarding this matter each of the intended goals of improving the network by STATCOM was addressed and the obtained results show the success of STATCOM. By using the adequate control strategy one can compensate the load inactive power, stabilize the voltage profile, reduce the voltage flicker and improve the power factor.

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4- In this paper, a new model of STATCOM without depending on the elements of power electronics and by using load dynamic models has been simulated. It shows the obtained results of the very adequate model presented in compensating the electric parameters at the power coupling point. 5- When improving the different parameters with a particular weighting factor is intended, the intended improvement function is applicable to the Model. It is observed that using STATCOM provides the connection capability of the arc furnace to the weak networks. Acknowledgment


This work is sponsored by Islamic Azade University Science and Research branch and the main author is a graduated PH.D student from it. References

[1] S.R. Mendis, M.T.Bishop, J.F.Witte, “Investigations of voltage flicker in electric arc furnace power systems”, IEEE Industry Applications Magazine, vol. 2, no. 1, Jan.-Feb.1996, pp. 28 – 34. [2] L. Zhang, Y. Liu, M.R. Ingram, D.T. Bradshaw, S. Eckroad, and M.L. Crow, “EAF Voltage Flicker Mitigation By FACTS/ ESS,” Power System Conference & Exposition, vol. 1, pp. 372-378, Oct. 2004. [3] T. Zheng, E.B. Makram, A.A. Girgis “Effect of different arc furnace models on voltage distortion”. Harmonics and Quality of power proceeding , Vol. 2, pp. 1079-1085, 1998. [4] H .Mokhtari, M. Hejri, “A new three phase time-domain model for electric arc furnaces using MATLAB”, Transmission and Distribution Conference and Exhibition 2002: Asia Pacific. IEEE/PES Vol. 3, pp .2078- 2083, Oct. 2002. [5] C. Han, Z. Yang, B. Chen “Evaluation of Cascade-Multilevel Converter based STATCOM for Arc Furnace Flicker Mitigation”, IEEE IAS’2005, October 2-6, pp. 67-71, 2005. [6] C. Qian, M.L. Crow, “Cascaded converter-based STATCOM with energy storage”, IEEE PES winter meeting , vol. 1, pp. 544 – 549, Jan. 2002. [7] A. Garcia-Cerrada, P. Garcia-Gonzalez, R. Collantes, T. Gomez, J. Anzola, “Comparison of thyristor-controlled reactors and voltage-source inverters for compensation of flicker caused by arc furnaces”, IEEE trans on Power Delivery, vol. 15, no. 4, pp. 1225 – 1231, Oct. 2000. [8] R.C. Dugan, S. Santoso, H.W. Beaty, F.McGranaghan,“Electrical Power Systems Quality”, McGraw-Hill publications, 2002.

[9] M.T.Bina, “Inactive Power and Harmonics Control”, Khaje Nasir Toosi University of technology publications, pp. 40-45, 2003. [10] A.Tavakoli, M.Marzband, “A Time Domain Time-varying Nonlinear Loads Modeling for Individually Power Quality Effects Studies”, Accepted for oral presentation and publishing at The 27 th IASTED International conference on Modeling, Identification, and Control- MIC2008- ,February 11-13,2008.

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A SIMULINK Study of Electric Arc Furnace Inactive Power Compensation by Using STATCOM

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