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International Journal of Enhanced Research Publications, ISSN: XXXX-XXXXVol. 2 Issue 4, April-2013, pp: (1-4), Available online at: www.erpublications.com

Power Quality Improvement Analysis of System Comprised of Electric Arc Furnaces

Haseeb Anwar, Muhammad Kamran, Bilal Anwar and Farhan Ahmed ButtDepartment of Electrical Engineering,

University of Engineering and Technology, Lahore, Pakistan

Abstract: With the increase in power electronic devices on load side of the system, power quality problems are becoming more critical. One of the most critical which may cause deterioration in Electrical System with respect to loss and power factor is the industrial area using furnaces. The voltage fluctuation & harmonics caused by rapid load changes is becoming a gigantic power quality issue for both utilities and customers. Frequent switching operation of arc electrodes in furnace produces serious harmonics which moves back to the system and cause harmonic distortion at PCC (Point of Common Coupling). These harmonics deteriorate the life of equipment by producing heat which may cause thermo chemical breakdown of equipment. It is utmost essential to stop these harmonics to enter the system. This research will emphasize and highlight the harmonics generation in Arc furnaces and their solution to keep efficiency of the system intact. Moreover, after real time data analysis, simulations are run in order to provide solution of such problems which are very useful for utility and consumer. Performance and economic comparison of these techniques is carried out for the feasibility of proposed solutionKeywords: Electric Arc Furnace (EAF), Harmonics, Point of Common Coupling (PCC), Power Factor, Static VAR Compensator SVC), Total Harmonic Distortion, ,

Introduction

The use of Electric Arc Furnaces (EAF) has been increased since they are replacing conventional furnaces. The major causes of this replacement are ease of operation, environment friendly, cost effectiveness and lesser waste of material. Electric arc furnace component wise is nonlinear in nature and produces power quality problems which have adverse effects on utilities as well as on end customers.

Every utility wants to deliver clean and uninterruptable power supply to its consumers. Power quality problems created by power electronic devices and furnaces are polluting the power. Poor power factor affects the production of furnace as less current flows due to power factor. Harmonics affect the equipment installed with the furnace on point of common coupling (PCC) i.e., transformers.

A. Electric arc furnace & its behaviorElectric Arc Furnace (EAF) uses heat of electric arc for steel production. The use of EAF has grown dramatically

over the past few decades. Now a day, EAFs account for more than 50% of steel produced across the globe. The reason to opt EAF over conventional furnace is ease of its use, less initial cost and less production cost. EAF size is normally from a few tons to several hundred Tons and there can be multiple of EAFs in the premises of one plant. EAFs can be AC or DC powered. We will discuss AC powered EAF for our work. A typical EAF takes 360 to 400 kWh of electricity to melt one ton of steel [1].

Furnace Transformer is mostly a two winding transformer. Its HT winding is connected with Medium voltage supply provided by utility while LT winding is directly connected to EAF. Supply voltage to furnace transformer ranges from 11 to 33 kV while furnace operates from 400 to 1100 V. Furnace Transformer is normally connected in Delta to trap the zero sequence harmonics produced by the nonlinear arc. Transformer Capacity required for EAF is calculated by kVA requirement per ton of furnace capacity [2].

Electric Arc is fourth state of matter called Plasma formed between two electrodes having potential difference causing ionization of space between contacts. Electric arc largely depends on potential difference between the contacts, gap length between the electrodes, current in arc channel, Air velocity and physical properties of the medium [3]. Behavior of electric arc is highly non-linear in nature. Due to its non-linearity, it is a major source of harmonics in power system. Electric arc also produces problems of unbalancing, voltage fluctuations and notching phenomenon. Power factor of electric arc is very low and it also reduces steady state voltage at point of common coupling (PCC).

B. Arc models & arc model block setArc models are used for simulations or network studies. They involve one or two differential equations involving

different parameters of electric arc & its surroundings like arc voltage, arc current, arc conductance etc. It is not possible up to now to provide exact mathematical interpretation of an electric arc. However, different mathematicians have given arc

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models based on some assumptions in above arc equation. Due to these assumptions, each arc model has its significance in different application. Some famous arc models are Cassie, Mayr, Browne, Kema, and Schwarz.

Cassie Arc Model is suitable to be used for simulation of EAF as it is designed for high current regions and its arc cylinder diameter does not decays to zero at current zero [4].

Many arc models have already been simulated and from different observations Cassie arc model gives closest pattern for arc furnace under observation [5].

Effects of EAF on Power System

Due to highly non-linear nature of arc furnace operation, Electric arc produces many adverse effects on power system including harmonics, inter-harmonics, unbalancing, voltage fluctuations, flicker and switching transients. The parameter for which evaluation of Electrical pollution is carried out is total harmonic distortion (THD) in power system. THD is ratio of power of all harmonic components to power of fundamental component [6].

Harmonics can cause excessive heating, puncturing of insulation of equipment, degradation in communication system due to interference, undesirable shutting down of sensitive equipment and low power factor. Voltage fluctuations seriously affect the life of electronic devices. Unbalancing results in failure of three phase balance loads while transient can cause failure of utility as well as end user equipment [6].

Power Quality Improvement Techniques for EAFs

There are various existing techniques for the improvement of power quality problems depending upon application. Their brief detail is given below. Series Reactor or Choke: Series reactor or choke used in series with furnace transformer stabilizes the arc, ensures maximum power transfer to arc furnace and acts as low pass filter for harmonics produced by EAF. It handles harmonics which may cause excessive heating by reducing the impact of voltage fluctuations & flicker. On the other hand, these series inductive circuits will deteriorate system power factor.

Shunt Capacitor Banks: Another solution of keeping THD within allowable limits is the use of capacitors banks in shunt at PCC to improve power factor by providing leading VARs. They also stops high frequency current component to enter the system by offering them low impedance shunt circuit. Sizing, placement & switching sequence of capacitor bank demands special care otherwise they can be harmful for equipment. In areas with high THD, detuned capacitor banks are used as simple capacitor banks have higher tendency of resonance at some harmonic frequency, ferro-resonance and switching transient [7] [8].

Passive Filter or LC Filter: Often detuned capacitors do not perform well in areas where THD exceeds the prescribed limit. Such areas require rigorous harmonic studies to be performed. Real time values of harmonics are measured at PCC as well as near installation. LC filter provide leading VARs in addition to handling of harmonics. Common type of LC filter are single tuned, double tuned & high pass filters.

Static VAR Compensator (SVC): SVC is combination of thyristor controlled reactors, thyristor switched capacitors, fixed capacitors etc. controlled by sophisticated control system. SVC provides required MVAR to the system. SVC dynamically stabilizes the system, increase power transfer capability, improve voltage stability & power factor and decrease flicker & unbalancing [9] [10].

Active Filter: Active filter is controlled by a numeric controller and it monitors the system voltage and current. It injects current in system to reshape sinusoidal waveforms. An active filter behaves such that harmonics are suppressed and load always acts as a resistive to source.

STATCOM: STATCOM provides dynamic voltage support in response to system disturbances and balance reactive power demand of large fluctuating furnace loads. A STATCOM is able to both generate and absorb reactive power continuously varying as opposed to discrete values of fixed shunt capacitors or reactors Switched.

Solution Strategy

Most steel industries globally do not have their own power generation. They usually purchase electrical power from the utility. Therefore, there is direct impact of arc furnace on voltage profile, harmonic distortion, unbalancing and voltage fluctuations at the utility bus bars as well as at point of common coupling. There are some allowable harmonic distortion limits, flicker level and voltage drop imposed by utility and Arc Furnace user is penalized for any deviation from these threshold defined levels.

A real time case is considered by taking data from Agha steel, Karachi, Pakistan. Capacity of plant is 30 tons. Simulation of the system is done based on real data in MATLAB Simulink. Simulation of arc is done by using arc block set

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from Delft University. The data was straight away considered initially without involving Power Quality improvement arrangement as shown in Figure. This observation helps to gather data for knowing the severity of problem at point at common coupling as well at furnace end. To overcome this abnormal and lossy situation with very low system efficiency, following techniques are applied one by one to suggest best solution of the problem;

Use of Series reactor or choke Detuned shunt capacitor at PCC with choke Three phase harmonic filter at PCC with choke Static VAR compensator at 132 kV bus bar with choke

Performance & economic comparison, limits & measures for utilities to control power quality, improvement techniques for furnace owners based on size of furnace are carried out in this research.

Simulation Results

Real time Data of furnace & transformer is taken from Aga Steel as shown in Table 1 and 2. Furnace is rated for 38 ton but 7~8 ton material remain in furnace after each heating. Effective rating of furnace is 30 tons. Furnace Transformer secondary is connected with electrodes of furnace through huge copper bus bars. Furnace Transformer is rated as 33 / 0.415 kV, 16 MVA. Transformer is designed to take an overload of 20 %. Transformer is also water cooled.

Aga steel is taking independent feeder from local utility Karachi Electric Supply Corporation (KESC). They have installed a 132kV substation & stepped down the voltage to 33 kV level using 132/33 kV power transformer. On 33 kV bus, plant load as well as furnace transformer is connected which is considered as Point of Common Coupling for calculations and Analysis.

132kV interconnected network on the back is modeled as three phase voltage source with RL Branch. For modeling of line, data is used by short circuit study performed by an independent consultant for KESC Network as shown in table 3. This study covers nine grid stations in different areas of Karachi. Short Circuit MVA is between 4000 to 6500 MVA. We have taken short circuit MVA of 6000 MVA in our simulation with an X/R ration of 10.

Simulation of network and FFT Analysis Window for Base Case

A. without any power quality improvement (Base Case)

SIMULINK Model of System without any power quality improvement device is shown in fig 1. This exercise is to show the current scenario & this will act as our base case.

Fig. 1 System Model without installing any PQ improvement device

Power Transformer is rated for 26 MVA, 132 / 33 kV with vector group of Dyn11. On 33 kV Bus, 5 MW plant load is connected at PF of 0.8 lagging. MATLAB GUI will be used for FFT analysis & impedance vs. frequency curves.

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Total simulation time is taken as 0.1 sec. Plant load is energized at 1/600 sec while furnace transformer at 1/60 sec. Arc will be produced at 0.04 sec which lasts until end of simulation. Voltage waveform at PCC as well as furnace transformer is shown in fig 2. in which Distortion after arc production is quite evident. Fig 3 explains FFT analysis performed to check THD at PCC. Maximum frequency is 2000 Hz means 40 harmonics. THD value is 8.80 % which is very high. Dominating harmonics are even harmonics including 2nd and 4th.

Fig. 2 Waveform at PCC and at EAF in Base Case

Fig.3 FFT Analysis Window for Base Case

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B. Simulation of network with choke in series with furnace transformer (Case 1)Fig 4 shows model with choke in series with furnace transformer. Choke of 40 % impedance is used in series with

furnace transformer. Choke will not only stabilize the arc but also improve THD at PCC. Fig 5 represents the voltage waveform at PCC as well as furnace. Waveform is much stable than fig 2. Fig 6 shows

the FFT window showing THD at PCC. THD value is 4.51 % which is in limits but voltage at PCC is dropped. Highly inductive choke has made power factor worsen.

Fig. 4 System Model with Choke in series with Furnace Transformer

Fig. 5 Waveform at PCC and EAF with Series Reactor

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Fig. 6 FFT Analysis Window with Series Reactor

C. Simulation of network with choke & detuned shunt capacitors (Case 2)Detuned shunt capacitor banks are installed on PCC as depicted in fig 7. Shunt capacitor provide leading VARs to system improving overall power factor. Detuned Capacitor values came out to be 10 MVAR to make power factor close to unity keeping in view peak load of furnace. Detuned reactor of 10% is used. Waveform of voltage at PCC as well as furnace transformer in fig 8 shows improved voltage profile & fluctuations. It also depicts an important fact of overvoltage produced by excess leading VARs in low load conditions at PCC. Fig 9 shows FFT window with THD at PCC. Voltage profile is improved but THD is dramatically high. System has resonated on 2nd and 3rd harmonics. Capacitors are not recommended in areas with high harmonics.

Fig. 7 System Model with Choke and Shunt Capacitor Bank

.

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Fig. 8 Waveform at PCC and EAF with Choke and Detuned Capacitors

Fig 9 FFT Analysis Window with Choke and Detuned Capacitors

D. Simulation of network with choke & LC filter (Case 3)

LC Filter is cost effective and technically better solution for areas with high THD. LC Filter is connected at point of common coupling via breaker in the same way as detuned capacitors as shown in fig 10.

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Fig. 10 System Model with Choke and LC Filter

Fig. 11 Waveform at PCC and EAF with Choke and LC Filter

If Harmonic Distortion Graph of case 1 is carefully investigated, it is found dominant harmonics are present in initial frequencies. Further it is observed that Third Harmonic is most dominant. If Tuned Filter is used to deal with single frequency, other harmonics will remain the part of system.

For this reason, High Pass Filter tuned at 150 Hz is used with a Quality factor of 8. It will offer Low impedance to all the higher harmonics from fundamental frequency & tries to reduce their impact. Due to High Quality Factor it will offer low impedance to neighboring harmonics as well i.e. 2nd & 4th and reduce their bad impact of signal deterioration. For this research and analysis, LC Filter is aimed to provide 10 MVAR Capacitive in addition to filtration in same way as in case 2.

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Fig 11 shows waveform at PCC which looks quite smooth. Waveform distortion at low load conditions due to excess VARs is visible. fig 12 shows THD of cycle of arc production which shows a THD of 6.14 % with 2 nd & 3rd harmonics still dominant. Fig 13 shows steady state THD graph when furnace is in operation with an acceptable value of 3.69%. Impedance vs. frequency graph of filter is depicted in fig 14. It shows filter is offering lowest impedance at 150 Hz (designed frequency).

Fig. 12 FFT Analysis Window with Choke and LC Filter at switching

Fig. 13 FFT Analysis Window at steady state with LC Filter

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Fig. 14 Impedance vs. Frequency Graph of LC Filter

E. Simulation of network with choke & SVC (Case 4)

Static VAR Compensator is very expensive option for power quality improvement. It aims to improve voltage profile, reduce voltage fluctuations & Imbalance, improve power factor to overall improve system power quality. SVC Proposed in this case is 21 MVAR Capacitive to 10 MVAR Inductive. Inductance is controlled by Thyristor Controlled reactor. Current through reactor is controlled by changing the firing angle of Thyristor. Reactors always remain in circuit while capacitors of 7MVAR banks are switched based on requirement. For SVC, another 132 kV bay is used with 25 MVA power transformer. As shown in fig 15. SVC is rated at voltage of 16 kV. Working principle of SVC is to sense the voltage at Transformer primary side & compare it with reference voltage programmed in the controller. Based on the voltage, distortion & imbalance, it injects leading or lagging VARs in the system. Fig 16 shows waveform of SVC controller. It has five windows showing Transformer Primary and Current (Voltage Yellow & Current in Pink), Reactive Power flow in the System by SVC, reference voltage and measured voltage in per unit (Measured in Yellow and Reference in Pink), Firing Angle of Thyristor Controlled Reactor, number of Thyristor Switched Capacitors Switched (Out of three). Fig 17 shows Waveforms at 132 kV Side, Voltage at 33 kV Bus, Current at 33 kV Bus, Voltage at Furnace and Current fed to furnace from System. Voltage drop is minimal & transients are less severe. Voltage waveform is smooth. Fig18 shows THD at PCC at time of switching with just 1.6 % and Fig 19 shows THD at 132kV level with just 0.03 %.

Fig. 15 System Model with Choke and SVC

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Fig. 16 Waveforms of SVC Controller

Fig. 17 Waveform at PCC and EAF with Choke and SVC

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Fig. 18 FFT Analysis Window with Choke and SVC

Fig. 19 FFT Analysis Window for THD at 132 kV

Economic ComparisonTable 1 represents the data of Electric Arc Furnace under research project. Economic Comparison of all techniques is given in Table 2. All the percentage impact is calculated as percentage of value of SVC (highest value) and value is in Kilo US Dollars. The values given below are specific to rating of equipment calculated for our specific system. Percentage impact can vary a bit if calculated for some different EAF.

Table 1: Table 1 Data of Electric Arc Furnace under Research

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S. No. Name Unit Data Remark

1) Furnace capacity

Nominal capacity t 30

Average tapping quantity t 38 5-7t volume remained each heat

Max. tapping volume t 45

2) Average tap-tap cycle min ≤ 125

3) Annual output of one furnace t ≥145000

4) Yearly utilization % 85 310 days

5) Daily work capacity heat > 11

6) Centre distance of tilting mm 5100 Raised tapping car rail

7) Slagging method slag splashing

8) Furnace body installation(with whole water cooled, closed packed pipe type)

a Furnace shell inside diameter mm Ø4600

b Bath diameter mm Ø3700

c Bath depth mm 830 slag thickness 100-150mm included

d Bath volume m3 7.8

e Refractory thickness at the furnace bottom mm 650f Molten steel volume mm 5.7g Molten steel weight t 38t specific weight 7t/ m3, new lining

h Total inner volume of the furnace m3

33i Total height of the shell mm 3500

j Water cooled furnace wall area m2

20.5 Close packed pipe type

k Furnace door size (W×H) mm 1000×760l EBT tapping hole diameter mm Ø130m Start-up mode of EBT tapping Hydraulic and manualn Water cooled furnace wall type close packed water cooled pipe type for whole wallo Furnace shell changing method Integral hoisting

As per above table, only series reactor of 16 MVA with 40 % impedance along with its control will cost owner around 300 kUSD. Detuned Shunt Capacitor Banks will cost additional 200 kUSD. If LC filter is opted, cost for system studies and controllers associated with filter increase this value to 300 kUSD. In case of SVC, special capacitors and inductors are used and have very sophisticated control system. SVC for this rating will cost around 3 MUSD with series reactor.

Table 2: Cost Comparison

Engineering & Control of SVC are main factors controlling its cost. It is suggested if SVC is to be installed within a small steel industry, it might be possible that cost of SVC becomes more than Furnace setup.

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Table 3 is the observation of data for 132kV transformer attached with Furnace. Table 4 gives the calculated short circuit levels at various grid stations associated with furnace under observations.

Table3: 132 kV Transformer Ratings Operating FurnaceNo. Item Index Remarks

1 rated capacity 16000KVA be capable of overload 20％

2 cooling mode 0FWF 0FWF

3 primary voltage 33KV power frequency

4 primary current 279.93A

5 secondary voltage 415-360-280V

level 13 on-load voltage regulation

415-360 constant power output

360-280constant current output

secondary rated current 25660A

secondary max current 30792A

isolation level L1200AC85

impedance voltage 6－7％

primary side equip CT variable ratio 800A/5A triangle within seal

structure type wire side out

voltage regulator domestic products

Table 4: List of Short circuit Levels at Various Grids

S. No Name of Grid St. 3 ph Short Power -Sk"

(MVA)

Short Circuit LevelsMax. - Ik" (kA) Min.- Ik" (kA)

3 ph 1 ph 3 ph 1 ph

1 PRL 6633.061 29.01 22.13 26.4 19.82

2 Gulshan Maymar 5215.23 22.81 15.56 20.65 13.91

3 K.South 5396.06 23.6 19.59 21.37 17.55

4 Jail Road 4582.27 20.04 14.18 18.13 12.68

5 Airport II 6670.34 29.18 25.32 26.45 22.71

6 Azizabad 6379.38 27.9 21.08 25.29 18.91

7 Memon Goth 4552.27 19.91 13.97 18.02 12.49

8 Tipu Sultan 3894.02 17.03 12.22 15.4 10.919 FTC 5368.88 23.48 17.45 21.27 15.61

Future WorkAfter careful investigation of this research, it is suggested that SVC can be installed on same 33 kV bus bar and can

check its comparison with performed analysis. Active filter can be implemented using the same system model which monitors the reactive power need and injects current as per VAR requirement. Also, We can extended this model by using STATCOM with it and compare its benefits with SVC. In addition, Similar studies in other platforms like EMTP and PSCAD can also be carried out and comparison regarding their merits and demerits can be made with SIMULINK power system Block set.

Acknowledgment

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Authors are thankful to Agha Steel Mills who have provided all facilities to collect data of Arc furnace under normal and heavy load conditions. Moreover, tests were performed on given data and conclusions were drawn to accomplish the research results. Moreover, authors are indebted to Water and Power Development Authority, Pakistan who gave permission to visit various grid stations at different locations to measure Electrical stresses on transformers associated with Arc furnace.

Conclusion/Results

It has been concluded that power loss and power factor deterioration is not only improved by using capacitors but SVC and other fixed adaptive filtering is carried out to overcome these shortcomings in power system quality. This all depends upon the type of load connected in the system. Improper arrangement to handle power quality issues may result in equipment failure and financial crunch as far as industrial product concerned. This research has put especial emphasis on the Arc furnaces which are suggested not to be connected with other loads but a separate transformer is appreciated to fulfill its working requirements.

References

[1]. The EPRI Centre for Materials Production, “Commentary Understanding Electric Arc Furnace Operations”, EPRI, 1997, pp. 1-2. J. Clerk Maxwell, “A Treatise on Electricity and Magnetism”, 3rd ed., vol. 2. Oxford: Clarendon, 1892, pp.68–73.

[2]. F.A. Oyawale, and D.O. Olawale, “Design and Prototype Development of a Mini-Electric Arc Furnace”, Pacific Journal of Science and Technology, Volume 8, Number 1, 2007, pp. 13-14.

[3]. Luigi Vanfretti, “Circuit Breakers - The Switching Arc and Arc Modeling”, Final Exam (Presentation), Surge Phenomena in EPE, Rensselaer Polytechnic Institute, 2007, pp. 4-11.

[4]. Ruben D. Garzon, “High Voltage Circuit Breakers Design and Applications”, Marcel Dekker Inc, 2002. PP 35-37[5]. P.H. Schavemaker and L. Van Der Sluis, “Arc Model Block set”, Second IASTED International Conference, Power and Energy

Systems (EuroPES), 2002, pp.1-4.[6]. Roger C. Dugan, Mark F. Mcgranaghan, Surya Santese and H. Wayne Beaty, “Electrical Power System Quality”, Second

Edition, McGraw Hill Companies, 2004, pp. 170-226.[7]. Mohd. Najib Bin Mohd. Yusof, “Optimal Allocation of Fixed Capacitor for Distribution System”, Kolej Universiti Teknikal

Kebangsaan Malaysia, 2005, pp. 3-12.[8]. Hugh McLaren Ryan, “High Voltage Engineering and Testing”, IEE., 2001, pp. 160-161.[9]. Daniel J. Sulivan, “Improvements in voltage control and dynamic performance of power transmission systems using Static VAR

Compensators (SVC)”, BSEET Pennsylvania State University, 1995, pp. 14-15[10].Tariq Masood Ch., “Propose FACTS Technology Implementation in Pakistan to Improve Underlying Power Quality Issues”,

Proceedings of IEEE ICEE 2006, University of Engineering and Technology Lahore, Pakistan, 2006, pp. 152-155

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