Read Matlab Simulation of a DSTATCOM using Hystersis current control for Electric Arc Furnace Flicker Mitigation text version

MATLAB Simulation of a DSTATCOM using Hysteresis Current Control for Electric Arc Furnace Flicker Mitigation

S.Meschi 1, E.Hashemzadeh2 1. Iran Grid Management Co. Yasemi st. Vali-e-assr Ave. IGMC , P.O. Box : 91735-1187(Tehran-Iran) Tel (+98-21) 88889096 Fax :( +98 -21)88880098 , email: [email protected] 2. Ferdowsi University of Mashhad , email: [email protected]

Abstract. Flicker, annoying light intensity fluctuation, is a power quality problem caused by large time-varying loads like arc furnaces.

The fast response of the distribution static compensator (DSTATCOM) makes it the efficient solution for improving power quality in distribution system. This paper presents a study on the modeling and control of a DSTATCOM used for electric arc furnace (EAF) flicker mitigation. In this paper a time domain model based on a piece-wise linear approximation of the V-I characteristic of the arc furnace is used for arc furnace modeling. The power circuit of DSTATCOM and EAF are modeled by MATLAB/simulink. An alternative control strategy of a DSTATCOM, based on hysteresis current control of the switches is presented. In compare with PWM and SVM techniques, this strategy has very simple implementation and near sinusoidal current waveforms with virtually no low order harmonics. As well reference current of DSTATCOM generated from synchronous reference frame method (d-q method).

mitigation can be categorized into three types [1]: (1) Regulating the EAF passive components. (2) Compensation through the combination of thyristor and passive components, such as the well-known Static Var Compensator (SVC). SVC can not only improve power quality of nearby system, but also increase EAF productivity and bring additional economic benefits. However, if cannot catch up the fast-varying flicker (1Hz~20 Hz) very well with the inherent limit of relatively low band with and hence it's dynamic performance for flicker mitigation is limited. (3) The state-of-the-art solution is the STATic synchronous COMpensator (STATCOM) based on high frequency Voltage-Source-converter. While SVC performs as a controlled reactive admittance, STATCOM can switch at several kHz and achieve a closed-loop bandwidth at several hundred Hz, hence the response time is much less than one cycle. STATCOM can also provide real power compensation if interfaced with an energy storage unit, all of which are unattainable for SVC. With these benefits, STATCOM performs significantly better than SVC does. At present, STATCOM is considered the best FACTS device for flicker mitigation. The paper is organized as follows: Firstly, an EAF flicker model based on a piece-wise linear approximation of the V-I characteristic of the arc furnace is presented. Secondly, the STATCOM control strategy for voltage flicker mitigation is illustrated. At last simulation results and conclusion are presented.

Keywords

Power Quality, Voltage Flicker, DSTATCOM, Hysteresis Current Control, Arc furnace

1. Introduction

Recently, with the growth of industry manufacturers and population, electric power quality becomes more and more important. One of the most common power quality problems is voltage flicker. Electric Arc Furnace (EAF), as a major industry customer of utility, consumes considerable real power and reactive power with the time-varying, stochastic characteristics during its melting and refining process, and therefore generates severs flicker to the grid. How to economically and efficiently mitigate EAF flicker is consistently a tough issue for utility professionals. The basic methodology for flicker

2. Description of the Proposed System

The inverter of DSTATCOM, Fig.1, is a standard three phase two level inverter. The 55MW arc furnace is modeled as a time-varying load. When this circuit is in operation, the time-varying load (EAF) caused the voltage fluctuation, or voltage flicker in the point of common coupling (PCC) [2]. The voltage amplitude modulating frequency is in the range of 5 to 15 Hz.

Figure 2 shows the utility/customer system with MATLAB/simulink for voltage flicker mitigation study.

shows the actual V-I characteristics of arc furnace and its piecewise linear model. The arc ignition voltage vig and the arc extinction voltage vex are determined by the arc length during arc furnace operation.

Fig.1.The power circuit of the three-phase STATCOM with time varying arc furnace

3. Modeling of EAF

Arc furnace operation is a complicated dynamic arcing process. Historically, there are various methods to models arc furnace, such as harmonics accumulation model, frequency domain model and time domain equivalent circuit model. In this paper a time domain model based on V-I characteristic is used [3]. Figure 3

Fig.3. Actual and piece-wise linear approximation of V-I characteristic of an Arc Furnace

Fig.2. MATLAB/simulink model of utility/customer system

By approximating the actual V-I characteristics of an arc furnace, different time domain models has been derived. In this paper a more accurate non linear approximation model can be developed by considering parts of the characteristics in more details. In this model, the arc melting process is divided into three sections. In the first section, the voltage magnitude increases from extinction voltage -vex to ignition voltage vig .In this part, the arc furnace acts as a resistor and the arc current changes its polarity from -i3 to i1 .

The second section is the beginning of the arc melting process. There is a sudden exponential voltage drop across the electrode, thus the arc voltage decreases from vig to v st , and the arc current has a little increase from i1 to i2 . The third section is the normal arc melting process. The arc voltage drops linearly and slowly and smoothly from v st to vex . The equations showing these variations are given in (1).

(1) ( - i3 i < i1 , inc) or R1 i (-i1 i < i3 , dec) v + (v - v ) exp((i - i) i ) i i < i , inc ig st 1 T 1 2 st vst + (i - i2 ) R2 i i2 , inc v= i i3 , dec vex + (i - i3 )R3 - v + (v - v ) exp((i + i) i ) - i i < -i , dec st ig 1 T 2 1 st - vst + (i + i2 )R2 i < -i2 , dec - vex + (i + i3 )R3 i < -i3 , inc Where R1 , R 2 and R3 are the corresponding slopes of each section, and vig v (2) iT = 1.5i1 i2 = 3i1 i3 = ex i1 = R1 R1 Figure 4 shows the simulated V-I characteristic of arc furnace model.

800 600

Vig exponantial

Fig.5. control reference signal generation subsystem.

One of the most important characteristics of this method is that the reference currents are derived directly from the real load currents without considering the source voltages. The generation of the reference signals is not affected by voltage unbalance or voltage distortion, therefore increasing the compensation robustness and performance.

5 x 10

4

400

Arc voltage(V)

without DSTATCOM

200 0

Vex

0

-200 -400 -600

i3 i2

iq-ac-EAFA)

-5

i1

-800 -1

-10

-0.5

0

0.5 x 10

1

5

Arc current(A)

-15

Fig.4. V-I characteristic of arc model

-20 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

4. Control Strategy

A. Reference Signal Generating Reference signals for DSTATCOM are generated by using the synchronous reference frame method. In this method, the real currents are transformed into a synchronous reference frame. The referenced frame is synchronized with the ac main voltage, and is rotating at the same frequency [4]. The current in d-q frame can be ~ composed by instantaneous active current ild = ild + ild ~ and the instantaneous reactive current ilq = ilq + ilq . The division of the dc and ac can be obtained across low-pass filter [2], [5]. Figure 5 shows the control reference signal generation subsystem. By implementing of this method in the case study system ac part of i q archived like Fig. 6. ~ Fig. 7 shows the effective value of iq . The reference current signal for flicker mitigation can be achieved by ~ the ac component of reactive current ilq through the abc-to-dq0 transformation and low-pass filter.

9 8.5 8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 x 10

4

Time(sec)

Fig .6. iq

without DSTATCOM

~

iq-ac-rm s-EAF(A)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Time(sec)

Fig.7. Effective value of iq

~

B. Hysteresis Current Control An alternative method to reduce the low-order harmonic content of the DSTATCOM output current is to use hysteresis current control. Under hysteresis control, rapid switching of each switch according to the continuous measurement of the difference between the STATCOM supply current and reference sinusoidal current. The basic principle of current hysteresis control

iq-ac-rm s-EAF(A)

technique is that the switching signals are derived from the comparison of the current error signal with a fixed width hysteresis band [6]. With simple, extreme robustness, good stability, fast dynamic, this current control technique exhibits some unsatisfactory features. For hysteresis control the phase output current is fed back to compared with the reference current I ref . An upper tolerance band and a lower tolerance band, taken as ±2% of I ref , are also assigned in order to define an acceptable current ripple level. Whenever the phase current exceeds the upper band, the upper switch of that leg will be turned ON while the lower switch will be turned OFF. If phase current falls below the lower band, the upper switch will be turned OFF whereas the lower switch will be turned ON. Fig. 8 shows the subsystem of generating switching signal.

9 8.5 8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

x 10

4

without DSTATCOM

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Time(sec)

Fig.9. Effective value of iq for source without DSTATCOM

8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 x 10

7

~

without DSTATCOM

p-rm s-source(W )

0

0.1

0.2

0.3

0.4

0.5

Time(sec)

Fig.8. Subsystem of generating switching signal

Fig.10. Effective value of active power for source without DSTATCOM

8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 x 10

7

5. Simulation Results

without DSTATCOM

mitigate the flicker, DSTATCOM which connected ~ parallel to the PCC must produce the required iq of load. ~ In the network without DSTATCOM, the whole value iq of EAF, produced by source. This current in the route between EAF and source propagate voltage flicker. Figures 9 - 16 show the network condition before and after connecting DSTATCOM and action of DSTATCOM in voltage flicker mitigation. With comparison figure 12 and 16 obtained that the value of V % reduced from 0.6 to 0.1 and voltage flicker is V V mitigated. Note that percent voltage flicker ( % ) is V criterion of flicker evaluation.[7]

q-rm source(VAR) s-

With consider the previous parts of paper obtained ~ that current caused the voltage flicker is iq . So for

0

0.1

0.2

0.3

0.4

0.5

Time(sec)

Fig.11. Effective value of reactive power for source without DSTATCOM

542 540 538

PCC voltage(V)

536 534 532 530 528 526 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Time(sec)

Fig.12. PCC voltage without DSTATCOM

3.5 3.25 3 2.75

x 10

4

with DSTATCOM

6. Conclusion

A detailed model of a DSTATCOM and EAF has been developed for use in simulink environment with MATLAB. This paper shows hysteresis current control for DSTATCOM is simple and has fast response without delay time, good stability, and extreme robustness. As well this paper shows d-q method is efficient for flicker mitigation because the reference currents are derived directly from the real load currents without considering the source voltages. The generation of the reference signals is not affected by voltage unbalance or voltage distortion, therefore increasing the compensation robustness and performance.

iq-ac-rm s-source(A)

2.5 2.25 2 1.75 1.5 1.25 1 0.75 0.5 0.25 0 0 0.1 0.2 0.3 0.4 0.5

Time(sec)

Fig.13. Effective value of iq for source with DSTATCOM

8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 x 10

7

~

with DSTATCOM

References

[1] Chong Han, Zhanonig Yang, Alex Q.Huang, "Modeling and Control of a Cascade­Multilevel Converter-Based STATCOM for Electrical Arc Furnace Flicker Mitigation", IEEE 2005, pp883-888. [2] Su Chen, Geza Joos, "Direct Power Control of DSTATCOMs for Voltage Flicker Mitigation", IEEE 2001, pp2683-2690. [3] M.A.Golkar, S.Meschi, M.Tavakoli Bina, "A Novel Method of Electrical Arc Furnace Modeling for Flicker Study", ICREPQ07 [4] Chen.D, Xie.SH, "Review of the Control Strategies Applied to Active Power Filters", IEEE International Conference on Electric utility Deregulation, Restructuring and Power Technologies (DRPT2004), 2004 [5] Oliveria.L , Germono.L, "Improving the Dynamic Response of Active Filters Based on the Synchronous Refrence Frame Method" ,IEEE Applied power Electronics Conference and Exposition, Vol. 37,No.1,pp.374-380,1990 [6] D.Sutanto, L.A.Snider, K.L.Mako ," EMTP simulation of a STATCOM using Hystersis Current Control " , IEEE International conference on power electronics and drive systems, July1999

p-rm s-source(W )

0

0.1

0.2

0.3

0.4

0.5

Time(sec)

Fig.14. Effective value of active power for source with DSTATCOM

4 3.5 x 10

7

q-rms-source(VAR)

3 2.5 2 1.5 1 0.5 0 0

0.1

0.2

0.3

0.4

0.5

Time(sec)

[7] S.R.mendis, M.T.Bishop, J.F.Witte, "Investigation of voltage flicker in electric arc furnace power system", IEEE 1994

Fig.15. Effective value of reactive power for source with DSTATCOM

with DSRTATCOM

535.5 535 534.5

PCC voltage(V)

534 533.5 533 532.5 532 531.5 531 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Time(sec)

Fig.16. PCC voltage with DSTATCOM

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Matlab Simulation of a DSTATCOM using Hystersis current control for Electric Arc Furnace Flicker Mitigation

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