378 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 2, MARCH/APRIL 2007

Evaluation of Cascade-Multilevel-Converter-BasedSTATCOM for Arc Furnace Flicker Mitigation

Chong Han, Member, IEEE, Zhanoning Yang, Bin Chen, Alex Q. Huang, Fellow, IEEE,Bin Zhang, Student Member, IEEE, Michael R. Ingram, Senior Member, IEEE, and

Abdel-Aty Edris, Senior Member, IEEE

AbstractAs an industry customer of electric power, an electri-cal arc furnace (EAF) is a major flicker source that causes majorpower quality problems. For a 40-MVA EAF in Tennessee, USA,a cascade-multilevel converter (CMC)-based STATic synchronousCOMpensator (STATCOM) with high bandwidth is proposed forEAF flicker mitigation. In this paper, flicker mitigation techniquesby using a CMC-based STATCOM are presented and verifiedthrough a transient network analyzer (TNA) system. The requiredSTATCOM capacity is first studied through a generalizedsteady-state analysis. Second, the STATCOM control strategy forflicker mitigation is introduced, and simulation results are given.Finally, a TNA system of the STATCOM and an EAF systemare designed and implemented. Experimental results from theTNA test show that the proposed CMC-based STATCOM and itscontroller can efficiently and rapidly mitigate the EAF flicker.

Index TermsCascade-multilevel converter (CMC), electric arcfurnace, flicker, STATic synchronous COMpensator (STATCOM),transient network analyzer (TNA).

I. INTRODUCTION

R ECENTLY, with the growth of industry manufacturersand population, electric power quality has become moreand more important. As one of the most common power qualityissues, flicker, which is caused by feeder voltage fluctuation,influences domestic lighting and sensitive apparatus of nearbytransmission and distribution system. An electrical arc furnace(EAF), as a major industry customer of a utility, consumesconsiderable real power and reactive power with time-varying,

Paper PID-06-26, presented at the 2005 Industry Applications Society An-nual Meeting, Hong Kong, October 26, and approved for publication in theIEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Metal IndustryCommittee of the IEEE Industry Applications Society. Manuscript submittedfor review October 15, 2005 and released for publication October 30, 2006.This work was supported in part by the U.S. Electric Power Research Institute,in part by the Tennessee Valley Authority, and in part by the U.S. Departmentof Energy, Sandia National Laboratory.

C. Han is with ABB Inc., Norwalk, CT 06851 USA (e-mail: chhan@ieee.org).

Z. Yang was with the Semiconductor Power Electronics Center, NorthCarolina State University, Raleigh, NC 27695 USA.

B. Chen and A. Q. Huang are with the Semiconductor Power ElectronicsCenter, North Carolina State University, Raleigh, NC 27695 USA. (e-mail:aqhuang@ncsu.edu).

B. Zhang is with Linear Technology Corporation, Raleigh, NC 27513 USA.M. R. Ingram is with the Tennessee Valley Authority, Chattanooga, TN

37402 USA.A.-A. Edris is with the Electric Power Research Institute, Palo Alto, CA

94304 USA.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TIA.2006.889896

stochastic, and even chaotic characteristics during its meltingand refining process and, therefore, generates severe flicker tothe grid [1].

How to economically and efficiently mitigate EAF flicker isconsistently a tough issue for utility professionals. The basicmethodology for flicker mitigation can be categorized into threetypes. 1) The first is regulating the EAF passive components,such as source impedance [2]. Although, to some extent, theincreasing series reactance can mitigate the flicker, it reducesthe power supply and therefore decreases EAF productivity.Moreover, it is also expensive and laborious to control up-stream transformer reactance or series reactor in the firmerderegulation power system. 2) The second is compensationthrough the combination of thyristor and passive components,such as the well-known static var compensator (SVC). SVCcannot only improve the power quality of the nearby systembut can also increase EAF productivity and bring additionaleconomic benefits. However, it cannot catch up the fast-varying flicker (120 Hz) very well with the inherent limitof relatively low bandwidth; hence, its dynamic performancefor flicker mitigation is limited. 3) The state-of-the-art solu-tion is the synchronous STATic synchronous COMpensator(STATCOM) based on a high-frequency voltage-source con-verter (VSC) [3].

While the SVC performs as a controlled reactive admittance,the STATCOM functions as a synchronous voltage source, asits name suggests. With currently available high-power semi-conductor devices, such as insulated gate bipolar transistors(IGBTs), insulated gate commutated thyristors (IGCTs), andemitter turn-off (ETOs) thyristors, a STATCOM can switch atseveral kilohertz and achieve a closed-loop bandwidth at severalhundred hertz; hence, the response time is much less than onecycle. The STATCOM can also provide real power compen-sation if interfaced with an energy storage unit, all of whichare unattainable for SVC. With these benefits, the STATCOMperforms significantly better than SVC [4]. At present, theSTATCOM is considered the best flexible ac transmission sys-tem (FACTS) device for flicker mitigation.

For a shunt-link FACTS device such as the STATCOM, thecascade-multilevel converter (CMC), which is constructed withidentical H-bridge building blocks [5], is the most feasibletopology because of its compact structure, modularity, fastresponse, and clean power quality [5][8]. Therefore, for a40-MVA EAF in Tennessee, USA, a CMC-based STATCOMis proposed. In this paper, the control strategy and perfor-mance of the CMC-based STATCOM for flicker mitigation is

0093-9994/$25.00 2007 IEEE

HAN et al.: EVALUATION OF CMC-BASED STATCOM FOR ARC FURNACE FLICKER MITIGATION 379

Fig. 1. Worst case flicker model of electric arc furnace.

presented and evaluated through a real-time transient networkanalyzer (TNA).

This paper is organized as follows. First, an EAF flickermodel based on a worst case approach is introduced, and theSTATCOM capacity for flicker mitigation is stated throughthe generalized steady-state analysis. Second, the STATCOMcontrol strategy is presented. Decoupling control gives an in-dependent freedom to regulate reactive and real power. Satis-factory stability margins and control bandwidth are achievedthrough careful design in the s-domain. The design is simulatedin ElectroMagnetic Transients including DC/Power SystemsComputer Aided Design (EMTDC/PSCAD). Finally, a TNAsystem of the CMC-based STATCOM and an EAF flickersystem are described. The TNA experiment results are given.

II. EAF FLICKER MODEL AND STEADY-STATE ANALYSIS

Arc furnace operation is a complicated dynamic arcingprocess. Historically, there are various methods to model arcfurnace, such as arcing resistance model, harmonics accumu-lation model, and frequency-domain method [9][11]. Thesemethods can match the nonlinear V I curve and the stochasticand even chaotic characteristics of EAF; therefore, they aresatisfactory for the purpose of power quality analysis. How-ever, from the flicker mitigation control design and system-stability-study point of view, a deterministic model needs to bedeveloped.

As shown in Fig. 1, a flicker model consisting of switchingpassive loads is proposed to model EAF flicker under worst casescenario. For any operating conditions, this behavior model canrepresent similar impedance as real-world EAF and thereforeproduces similar flicker at the point of common coupling(PCC). In addition, because of its definite impedance, it issuitable for stability analysis and s-domain control design.

From the real-time recorded waveform of EAF at Tennessee,the flicker can be summarized as follows. 1) The flicker fre-quency is about 5 Hz. 2) The flicker magnitude V/V is about1%. 3) Source XS/RS is about 3 [12].

Since the 1% flicker is beyond the IEEE irritability thresh-old curve of the IEEE standard shown in Fig. 2 [13], themitigation devices have to be applied to decrease flicker intoan acceptable range.

For the reactive power compensation, the system steady-statediagram is shown in Fig. 3. The system voltage and impedanceare VS and XS , respectively. When the arc furnace consumes

power PL and QL, the voltage of PCC is decreased from theoriginal value to VPCC. To compensate the voltage of the PCCback to V PCC, the injected reactive power at the PCC has tobe QC . Based on the assumption that the angle between VSand VPCC is small, the relationship among QC , XS , VPCC, andV PCC can be derived as follows:

Qc V

PCC

(V

PCC VPCC

)

XS. (1)

With (1), the normalized relationship among flicker magni-tude (VPCC/VPCC), STATCOM capability (QC/SEAF), andsystem X/R ratio (XS/RS) are plotted in Fig. 4. As shown inFig. 4, a 4.5- and 30-MVA STATCOM can mitigate flicker by20% and 80%, respectively, which means that the flicker can befully mitigated into the acceptable range if 30 MVA is utilized.

III. STATCOM CONTROL FOR FLICKER MITIGATION

The one-line diagram of the STATCOM application forflicker mitigation is shown in Fig. 5. A CMC-based STATCOMis controlled through a two-loop structure. The internal controldiagram is shown in Fig. 6. Two current loops Id loop and Iqloop are decoupling controlled and designed with a bandwidthof hundreds of hertz and adequate stability margins. The Idreference is responsible for charging/discharging the dc capac-itor and therefore is regulated by a voltage loop to maintainthe dc-bus voltages. The Iq reference is the reactive currentcommand, which is connected with the external controller forflicker mitigation. The external control, as shown in Fig. 7,is designed for specific EAF flicker mitigation to support thevoltage VPCC. Through measuring the voltages at PCC, gen-erating error signals, and proportionalintegral (PI) regulator,the external control gives the Iq reference for internal controlso that STATCOM provides reactive power compensation andmitigates the voltage fluctuation at PCC.

Offline simulation in PSCAD/EMTDC environment demon-strates the compensation performance for different STATCOMratings, and results are shown in Fig. 8. Moreover, the simula-tion results for different XS/RS are also summarized in Fig. 9.As seen from Figs. 4 and 9, the analysis results and simulationresults match each other very well.

IV. EAF FLICKER AND STATCOM TNA SYSTEM

To evaluate the CMC-based STATCOM control strategy andflicker mitigation performance, a real-time TNA system ofSTATCOM and EAF, as shown in Fig. 10, is developed. Theleft cabinet houses the EAF flicker TNA composed of solid-state ac-switches, passive loads, and coupling transformer; theSTATCOM power stage with three-level IGBT-based CMCs,dc capacitors, ac reactors, and precharge circuits is housed inthe center cabinet; and the digital signal processor (DSP)/field-programmable gate array (FPGA)-based central controller ismounted into the cabinet on the right. The parameters of theTNA system are listed in Table I.

For the EAF flicker TNA, a three-phase ac switch is designedusing a back-to-back IGBT (SKM300GB 1240) phase leg with

380 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 2, MARCH/APRIL 2007

Fig. 2. Maximum permissible flicker in the IEEE standard.

Fig. 3. Reactive power compensation.

Fig. 4. NormalizedVPCC/VPCC (QC/SEAF, XS/RS ).

resistor capacitor diode (RCD) snubber to achieve the variableflicker frequency. Its principle diagrams and physical picturesare shown in Fig. 11.

Known from the experimental results shown in Fig. 12, theflicker TNA can be well controlled to generate the requiredflicker and meet all the requirements of flicker frequency,magnitude, power factor, and source impedance.

The CMC-based STATCOM, whose power stage is shownin Fig. 13, is shunted-connected with the EAF flicker TNAat PCC. The ac circuit breaker provides the overvoltage and

Fig. 5. CMC-based STATCOM for EAF flicker mitigation diagram.

Fig. 6. Internal control strategy of CMC-based STATCOM.

Fig. 7. External control strategy.

overcurrent protection for STATCOM TNA. The prechargecircuit energizes the STATCOM for the ac side, and the acreactors mainly filter the current harmonics. Three identical

HAN et al.: EVALUATION OF CMC-BASED STATCOM FOR ARC FURNACE FLICKER MITIGATION 381

Fig. 8. PSCAD/EMTDC simulation results.

Fig. 9. Calculation and simulation results.

Fig. 10. CMC-based STATCOM and EAF flicker TNA system.

H-bridge VSCs achieve synchronous voltage source and herebyprovide the reactive power through ac reactors.

The central controller includes sensors, interface boards,operator panel, an FPGA (Xilink XCV50), a 64-bit DSP(TMS320C6701), and a personal computer. The DSP, as thecentral process unit of the whole controller, computes the main

TABLE IBREAKDOWN OF STATCOM AND EAF FLICKER TNA

feedback control algorithm, monitors the system operation, andexecutes the protection command if necessary. The FPGA,performing as the DSP buffer, communicates bidirectionallywith the DSP to transfer in the feedback signals and out theswitch duty command and also calculates the sinusoidal pulsewidth modulation algorithm to translate the duty signal into thepractical IGBT switching signals through optical fibers.

With the central controller, all STATCOM normal operationmodes from initialization, precharge, online flicker mitigation,stop, ac disconnect, and dc discharge to, finally, shutdowncan be executed through the operator panel, and the necessaryprotection schemes are tripped when faults are detected. Theflow diagram of the STATCOM operation modes is shownin Fig. 14.

V. EXPERIMENT RESULTS

Scaling down the practical arc furnace at Tennessee, thesource XS/RS ratio and flicker frequency of flicker TNA are

382 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 2, MARCH/APRIL 2007

Fig. 11. High-speed solid-state ac switch.

Fig. 12. TNA flicker waveforms.

Fig. 13. Power stage of STATCOM TNA.

adjusted to 3.7 and 5 Hz, respectively, by regulating the seriesimpedances and ac-switch switching frequency. In addition, theSTATCOM TNA rating is set to 30% of the EAF TNA rating.

The experimental results recorded by the oscilloscope areshown in Fig. 15. Compared with the results before compen-sation, the flicker is obviously mitigated after compensation.The waveform at the right side shows that the STATCOMprovides dynamic reactive currents to compensate the powerconsumption by arc furnace and hereby mitigates the voltagefluctuation at system side. The phase shift between voltage

HAN et al.: EVALUATION OF CMC-BASED STATCOM FOR ARC FURNACE FLICKER MITIGATION 383

Fig. 14. STATCOM operation mode.

Fig. 15. Experiment results.

Fig. 16. Simulation versus experiment.

384 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 2, MARCH/APRIL 2007

and current is not exactly 90 because the Id channel needs asmall current to compensate the STATCOM internal loss and tomaintain the VSC dc-bus voltages. The waveform also verifiesthat the CMC-based STATCOM have a control bandwidth ofhundreds of hertz and hence is fast enough to compensateflickers (120 Hz).

In Fig. 16, the real-time VPCC_rms recorded by DSP iscompared with the switching-model simulation waveforms inPSCAD/EMTDC. As shown in the waveforms, the simulationresults clearly match the TNA experiment results; therefore, theproposed EAF model and CMC-based STATCOM model withits controller are all validated.

VI. CONCLUSION

In this paper, the principles and design of a CMC-basedSTATCOM for flicker mitigation have been presented andverified through a TNA system. The steady-state analysis givesa generalized relationship of VPCC/VPCC, QC/SEAF, andXS/RS and provides the steady-state evaluation of STATCOMfor flicker mitigation. The control strategy of the CMC-basedSTATCOM system is introduced and verified through the sim-ulation results. The real-time TNA system integrating CMC-based STATCOM with its digital controller and EAF flickermodel is demonstrated, and its experimental results verifythe EAF and STATCOM models, the digital controller, andthe hardware apparatus. From the experimental results, theCMC-based STATCOM system efficiently and rapidly miti-gates the EAF flickers and is feasible for future full-rating fielddemonstration.

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Chong Han (M07) received the B.S. degree (withhonors) from Huazhong University of Science andTechnology (HUST), Wuhan, China, the M.S. degreefrom Virginia Polytechnic Institute and State Univer-sity, Blacksburg, and the Ph.D. degree from NorthCarolina State University (NCSU), Raleigh, all inelectrical engineering.

From 1999 to 2001, he was with the National TNA(transient network analyzer) Laboratory and the Su-perconductivity Power R&D Center, HUST, China,where his research focused on FACTS controller,

energy storage system, and power system automation. From 2001 to 2004, hewas a Research Assistant at the Center for Power Electronics Systems, VirginiaPolytechnic Institute and State University. From 2004 to 2006, he was with theSemiconductor Power Electronics Center, NCSU. Since 2007, he has been withABB Inc., Norwalk, CT, as a Grid System Consultant. His current researchinterests include control of power electronics and power systems, real-timeTNA, energy storage systems, and renewable energy.

Zhaoning Yang was born in Yulin, China, in 1978.He received the B.S. degree from Huazhong Univer-sity of Science and Technology, Wuhan, China, in1999, and the M.S. degree from North Carolina StateUniversity, Raleigh, both in electrical engineering.

His research interests include modeling and con-trol of cascade-multilevel-converters, real-time dig-ital control of STATCOM systems, and modularcontroller architecture design.

Bin Chen received the B.S. degree from HuazhongUniversity of Science and Technology, Wuhan,China, in 1994, and the M.S. degree from TsinghuaUniversity, Beijing, China, in 2002, both in electri-cal engineering. He is currently working toward thePh.D. degree at the Semiconductor Power Electron-ics Center, North Carolina State University, Raleigh.

He was with the Center of Power ElectronicsSystems, Virginia Polytechnic Institute and StateUniversity, Blacksburg, from 2003 to 2004. His re-search interests include power semiconductor de-

vices, power converters, and modeling and control of power systems.

HAN et al.: EVALUATION OF CMC-BASED STATCOM FOR ARC FURNACE FLICKER MITIGATION 385

Alex Q. Huang (S91M94SM96F05) wasborn in Zunyi, China. He received the B.Sc. de-gree in electrical engineering from Zhejiang Univer-sity, Hangzhou, China, in 1983, the M.Sc. degreein electrical engineering from Chengdu Instituteof Radio Engineering, Chengdu, China, in 1986,and the Ph.D. degree from Cambridge University,Cambridge, U.K., in 1992.

From 1992 to 1994, he was a Research Fellowwith Magdalene College, Cambridge. From 1994 to2004, he was a Professor with the Bradley Depart-

ment of Electrical and Computer Engineering, Virginia Polytechnic Instituteand State University, Blacksburg. Since 2004, he has been the Alcoa Professorof Electrical Engineering with the Semiconductor Power Electronics Center,North Carolina State University, Raleigh. Since 1983, he has been involved inthe development of modern power semiconductor devices and power integratedcircuits. He fabricated the first IGBT power device in China in 1985. He is theinventor and key developer of the emitter turn-off thyristor technology. He haspublished more than 100 papers in international conference proceedings andjournals and holds 14 U.S. patents. His current research interests include utilitypower electronics, power management microsystems, and power semiconduc-tor devices.

Prof. Huang is a recipient of the NSF CAREER Award and the prestigiousR&D 100 Award.

Bin Zhang (S01) received the B.S. degree fromTianjin University, Tianjin, China, in 1992, the M.S.degree from the Institute of Electrical Engineer-ing, Chinese Academy of Sciences (IEE, CAS),Beijing, China, in 1999, and the Ph.D. degree fromthe Center for Power Electronics Systems (CPES),Virginia Polytechnic Institute and State University,Blacksburg, in 2005, all in electrical engineering.

From 1992 to 2000, he was an Electrical Engineerwith IEE, CAS, where he was involved in the de-velopment of ac and dc servo motor drive systems.

From 2000 to 2005, he was a Research Assistant with CPES, where he wasinvolved in research on high-power devices and power converters. In 2005, hejoined Linear Technology Corporation, Raleigh, NC, as an Analog IC DesignEngineer. He has published more than 20 IEEE conference/TRANSACTIONSpapers and holds three China patents and one U.S. patent. His research interestsinclude power semiconductor devices, analog integrated circuits, modeling andcontrol of power converters, and motor drives.

Michael R. Ingram (M91SM96) received theB.E.E. degree (with honors) from Auburn University,Auburn, AL, and the M.S. degree in engineeringmanagement from the University of Tennessee atChattanooga.

He is the Senior Manager of Transmission Tech-nologies with the Tennessee Valley Authority (TVA),Chattanooga, TN. He is responsible for research, de-velopment, and demonstration of new technologies,which improve electrical quality and reliability, in-crease power flow, and reduce the operating expense

of the TVA transmission system and interconnected distribution network. Heprovides advice to TVA executives on new technology solutions affectingthe T&D networks and sets strategy for research and development in areasof energy storage, power quality, power/transmission markets, FACTS, andsuperconductivity. He has been with TVA for 17 years, working in technicalproject management, protection and control engineering, and substation design.He has authored or coauthored more than 30 technical papers and articles withinthis area.

Mr. Ingram was a recipient of the Outstanding Young Engineer of the YearAward (2001) from the IEEE Power Engineering Society, the Engineer ofthe Year Award (2001 and 2006) from the Tennessee Valley Authority, theIEEE Millennium Medal, the Outstanding Power Engineer of the Year Award(1997), and the Chattanooga-Area Young Engineer of the Year Award (1996).He was also a top-ten finalist for the Federal Engineer of the Year Award(2001 and 2006). He has also served as Chairman or Committee Member ofseveral users/working groups of the IEEE, Electrical Power Research Institute,and CIGRE.

Abdel-Aty Edris (S74M80SM88) was born inCairo, Egypt. He received the B.S. degree (with hon-ors) from Cairo University, Cairo, the M.S. degreefrom Ain-Shams University, Cairo, and the Ph.D.degree from Chalmers University of Technology,Gothenburg, Sweden.

He spent 12 years with the ABB Company inSweden and in the USA, in the development and ap-plication of reactive power compensators and high-voltage dc transmission systems. In 1992, he joinedthe Electric Power Research Institute (EPRI), Palo

Alto, CA, as Manager of the flexible ac transmission system (FACTS) technol-ogy. He is the Technology Manager of EPRI Power Delivery and Markets.

Dr. Edris is a member of several IEEE and CIGRE working groups. Heis a recipient of the IEEE 2006 award for industry leadership and scientificcontribution to FACTS Technology, pioneering the transformation of electrictransmission systems into flexible, controllable, yet secure systems operated atthermal capacity.