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Power Quality Analysis on Arc Furnace Capacitor Bank System in Thailand
To cite this article: Bancha Sreewirote and Atthapol Ngaopitakkul 2020 IOP Conf. Ser.: Earth Environ.
Sci. 541 012010
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Power Quality Analysis on Arc Furnace Capacitor Bank System in Thailand
Bancha Sreewirotea, Atthapol Ngaopitakkul b
aDepartment of Electrical Engineering, Faculty of Engineering Thonburi University, Bangkok, Thailand
bFaculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand
Email: [email protected]
Abstract: A large arc furnace plant is currently in operation in Thailand has utilize a capacitor bank system to compensate for the reactive power. However, the use of a capacitor bank, the power quality has become a problem in terms of the harmonic current owing to the switching transient that magnifies the current harmonics and causes severe damage to the capacitor bank and other nearby sensitive equipment. In this study, we investigate the power quality issue in terms of harmonics in a steel furnace factory, and its effect when switching the capacitor bank into a system. We perform the study by using data from an arc furnace plant in Phetchaburi province, Thailand for a case study. The results obtained from the power quality measurement after installing a detuning reactor are satisfactory with a significant decrease in the harmonic current. Thus, the proposed approach can be applied to an arc furnace plant in order to ensure the safety and reliability of the plant.
1. Introduction
In recent decade, Thailand has been rapidly shifting from agrarian societies toward industrial societies with significant growth in industrial sector. Steel industry is one of an important upstream industry that produce raw material to the downstream sector such as construction, automotive, machinery, which play major role in contribution to Thailand’s economic growth. Statistic data from United States Department of Commerce reveal that Thailand steel production has been increase slightly to 1.98 million metric tons in 2016 compared to previous year at 1.95 million metric tons [1]. Steel production data implied energy consumption growth in steel industry, which including steeling plant that employed electric arc furnace. Energy situation report from Energy Policy and Planning office (EPPO), Ministry of Energy, Thailand stated that in 2016, the statistical data from first 10 months shown that electricity usage in steel industry is 5,941 GWh, increase by 5.96% compare to previous year [2] and this trend still continue with energy consumption constantly increase over the period of time. This statistical data and future forecast force government to set up master plan and action plan to reduce energy consumption steel industry. The Master plan on Energy Conservation of Iron and Steel (2011- 2031) aim to reduce energy consumption in 2031 by 290 ktoe with opportunity for energy saving up to 25%. The Action plan on Energy Management for Iron and Steel Industry (2014-2019), which aim to reduce energy usage by 45 ktoe or 4.6% [3]. In additional, electric utility company charges an additional fee, if power factor falls below a threshold level set by utility due to an inefficiency in
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generation and transmission power to those plants [4]. The implementation of this plan and additional fee from electric utility company result in all steel plant including those that used electric arc furnace must implement energy saving technologies in order to compile with government policy.
For steel plant to reduce energy cost and additional fee, the methodology to improve energy efficiency in term of power factor has been continuously studies and develop. However, power-quality characteristics such as harmonics, flicker, and voltage unbalance, which result from the non-linear characteristics of electric arc furnace plants are significant factor that must take into consideration before implementing various energy efficient improvement methodologies [5]. Research and literature that focused on the power-quality issue in arc furnaces have been reviewed and discussed [6]-[10].
The electrical arc furnaces characteristics in terms of electric power, stationary and transient states, and the voltage unbalance are reported by Cano-Plata et al. [6]. The simulation model employed for arc furnace systems to evaluate harmonic problems and their relation to the load condition has been proposed [7]. Another study by Bhonsle and Kelkar [8] proposed an electrical arc furnace model for power-quality analysis using MATLAB and Simulink software. In another study, harmonics and transient analysis guidelines for arc furnaces have been discussed [9]. These power-quality issues in arc furnace plants must be addressed in order to operate the system safely and efficiently.
To improve the energy efficiency, one of the systems that can be implemented into arc furnaces is the use of a capacitor bank system. The reason for this is that arc furnaces mainly consist of a large inductance, which results in a low power factor value with a capacitor bank, and it can help the system to compensate for the reactive power that is present. Reactive power calculation for arc furnace using simple methodology has been proposed [10]. The Capacitor bank application in industrial case study to improve energy efficiency has been presented [11]. The application of capacitors in arc furnaces has been proposed and discussed by Grebe [12]. Many technical issues regarding the use of capacitor banks in arc furnaces were also discussed. The issues that require consideration include voltage magnification, the tripping of adjustable-speed drives and harmonics also been presented.
Based on the literature discussed above, the power quality in arc furnace capacitor bank systems is the main concern owing to the severity of the damage that occurs in the capacitor bank system. Therefore, many arc furnace industrial sectors have realized that frequent capacitor bank failures occur inside the main distribution board. Thus, there is a need for studies and analyses on capacitor bank systems in arc furnaces before such system are implemented in order to mitigate the issue and prevent damaged to capacitor bank units. In this paper, we propose a power-quality analysis in an arc furnace capacitor bank system. The methodology consists of using a power-quality meter to obtain electrical parameters and power-quality data in terms of the harmonic distortion from an arc furnace plant in Thailand. We then compare and analyse the obtained data with the Thailand regulation in order to identify the cause of capacitor bank damage in arc furnace plants. Results obtained from the study can be used to design a protection scheme or device to reduce the occurrence of such problems in the future.
2. Power Quality Measurement and Standard
To evaluate power-quality problems in arc furnace factories, in this study, we measured electrical parameters using a power-quality meter. A single-line diagram of the arc furnace plant used in this case study and measurement points are shown in Fig. 1. The system receives power from the Provincial Electricity Authority (PEA) through transformer and a main air circuit breaker (ACB). The equipment in the main distribution board (MDB) consists of a circuit for the arc furnace with an AC to DC converter, a load inside plant, and six steps of 60-kvar capacitor banks. The power-quality meter is used to obtain the voltage, current, power factor, total harmonic voltage distortion (THDv), and total harmonic current distortion (THDi). In this study, we also install a detune reactor in order to evaluate its performance in terms of limiting the harmonic current. With the detune reactor, the new capacitor bank has also been redesigned. We performed the measurement at three separate points in the arc furnace plant as follows. According to the diagram in Fig. 1, measurement point 1 is located after ACB in order to obtain a harmonic at the point of common coupling (PCC) between the plant and PEA distribution network. Measurement point 2 is located before the AC/DC converter and the electrical
arc furnace in order to obtain harmonics generated from the arc furnace to ACB and the capacitor unit.
Measurement point 3 is located before the magnetic contact and the capacitor bank unit to obtain a harmonic level on the capacitor bank unit. The obtained data by the power-quality meter will then compare with PEA standard before and after switching the capacitor bank in order to evaluate the power-quality problem when switching the capacitor bank into the system. According to the PEA standard.
TR
ACB MDB
Power Quality Meter
AC/DC Load Converter
Arc Furnace
Capacitor Bank 60 kVAR
Capacitor Bank 60 kVAR
Capacitor Bank 60 kVAR
Capacitor Bank 60 kVAR
Capacitor Bank 60 kVAR
Capacitor Bank 60 kVAR Measurement
Point 3 Measurement
Point 2
Measurement Point 1
Fig. 1. Single-line diagram and measurement point of arc furnace plant.
Table 1. Permitted harmonic current values at point of common coupling (PCC) Harmonic order Permitted harmonic current (A)
0.4 kV 11 and 12 kV 22, 24, and 33 kV 69 kV 115 kV and above
2 48 13 11 8.8 5
3 34 8 7 5.9 4
4 22 6 5 4.3 3
5 56 10 9 7.3 4
6 11 4 4 3.3 2
7 40 8 6 4.9 3
8 9 3 3 2.3 1
9 8 3 2 1.6 1
10 7 3 2 1.6 1
11 19 7 6 4.9 3
12 6 2 2 1.6 1
13 16 6 5 4.3 3
14 5 2 2 1.6 1
15 5 2 1 1 1
16 5 2 1 1 1
17 6 2 2 1.6 1
18 4 1 1 1 1
19 6 1 1 1 1
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Data obtain using power quality meter has been used to compare between, PEA standard, before, and after switching capacitor bank in order to evaluate power quality problem when switching capacitor bank into system. According to PEA standard, current harmonics at PCC must not exceed permitted values as shown in Table 1. These values are defined in each voltage level ranging from 400V to 115 kV and above. The voltage level at PCC in arc furnace plant considered in this case study is 400V, thus comparison must be done at appropriate voltage level.
3. Power Quality Analysis
We measured the electrical parameters using a power-quality meter from the measurement point, which is located after the main ACB, as shown in Table 2. From the table, we observe that the voltage under normal conditions is 385 V. When the switching capacitor bank six steps, each having 60 kvar into the system, the voltage level is increased to 390V, and the current is decreased by 10% from 1,990 A to 1,831A. From the result, it can be seen that the apparent power is decreased because the capacitor bank compensates the reactive power (Q) in the system; this also improves the power factor from 0.85 to 0.93. For the power-quality issue, we determined the THD values in terms of the voltage and current. The result shows that with the capacitor switching into the system, THD in both the voltage and current has been significantly increased, especially for the current, where the THDi value increased by 73% from 3.07% to 5.31% after switching the capacitor bank into the system. Meanwhile, the voltage value, THDv in every phase is increased by approximately 45% from 3% to 4.4%.
Table 2. Electrical parameters and power quality measured at main circuit breaker
Capacitor bank Off Capacitor bank On % increase
Voltage R-S (V) 382.34 388.27 2%
Voltage S-T (V) 388.47 388.47 2%
Voltage T-R (V) 385.18 385.18 2%
Current-R (A) 1,990.64 1,831.38 -8%
THDv (R-S) 2.90 % 4.20 % 45%
THDv (S-T) 3.13 % 4.41 % 41%
THDv (T-R) 2.96 % 4.38 % 48%
THDi (R) 3.07 % 5.31 % 73%
We obtained the electrical parameters and power quality from the main circuit breaker of the arc furnace using a power-quality meter. In Fig. 2, we show data obtained over the period of time in both before switching the capacitor bank indicated by the OFF-CAP period and after switching the capacitor bank into the system indicated by the ON-CAP. The electrical parameter consists of the current, voltage, and power factor, are illustrated in Fig. 2(a), 2(b), and 2(c), respectively. The actual waveform obtained from power quality meter both before and after switching capacitor bank of voltage and current is shown in Fig. 3 (a) and (b) respectively. This waveform also shown the similar trend with characteristic discussed above with both current and voltage waveforms have been distorted from sinusoidal form switching when capacitor bank into system.
(a) Current
(b) Voltage
(c) Power factor
Fig. 2. Electrical parameter in case of switching the capacitor bank on/off: (a) Current, (b) Voltage, and (c) Power factor.
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Phase A Phase B Phase C Phase A Phase B Phase C
(a)
(b)
Fig. 3. Waveform measured before and after switching capacitor bank into the system: (a) Voltage waveform, and (b) Current waveform.
The result shows that switching the capacitor bank into the system causes the power factor to increase significantly. The voltage also rises slightly compared to the value before switching. Meanwhile, capacitor bank does not affect the current characteristic. The power quality in terms of both THDv and THDi are presented in Fig. 4(a) and 4(b), respectively.
(a)
(b)
Fig. 4. Total harmonic distortion in case of switching the capacitor bank on/off: (a) THDv, (b) THDi.
Harmonic Order (n)
2 3 4 5 6 7 8 9 10 11 12 13
Current Harmonic (A)
0 20 40 60 80 100 120 140
PEA Standard Without Capacitor Bank With Capacitor Bank
Fig. 5. Current harmonic measurement values at main air circuit breaker.
It can be seen that the THD after switching the capacitor bank is increased significantly compared to the value before switching the capacitor bank into the system. This graph is in agreement with the data presented in Table 2, with drastic increase in THDv by up to 24% for case after switching the capacitor bank. Fig. 5 shows a comparison of the current harmonics obtained from the arc furnace for each order between harmonics before and after switching the capacitor bank using the PEA standard as a reference. In the case before switching the capacitor bank into the system, only the 5th current harmonic order exceeds the standard permitted value. However, after switching the capacitor bank into the system, the current harmonics in the 5th, 7th, and 11th orders are significantly higher than the standard permitted value, especially the 5th order. The result indicates that the capacitor bank can amplify the harmonic current for some orders. Hence, preventive measures must be implemented;
otherwise, severe damage may occur to the capacitor bank and other sensitive equipment.
4. Conclusion
This research proposed the power quality analysis in Thailand arc furnace that installed capacitor bank to increase its power efficiency. With the installation of a capacitor bank system in an arc furnace factory, the power factor and efficiency of the plant has been significantly improved, but the results obtained also indicate that when we switched the capacitor bank into the system, the harmonic currents from the arc furnace will be injected into the capacitor bank. This will cause resonance phenomena and increase the harmonic current. When compared with the PEA standard, we observe that current
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harmonics of some orders are significantly higher than the permitted values. This can result in severe damage to the capacitor bank system, and a protection system should therefore be implemented to ensure the safety and reliability of the arc furnace plant capacitor bank system. This research methodology and result can be applied to other arc furnace plants or other industries, which are dealing with the same technical issue. We believe that as the root of the problem has been addressed in this study, we can propose other techniques to solve the mentioned power-quality issue.
References
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[3] Energy Efficiency in the Thai Steel Sector :ISIT’S Efficiency Action Plan, Iron and Steel Institute of Thailand, 2014.
[4] Electricity Tariffs, Provincial Electricity Authority
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[7] Dugan RC, Simulation of Arc Furnace Power Systems, IEEE Transactions on Industry Applications, Nov. 1980, IA-16(6):813-818.
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[9] Mendis SR and Gonzalez DA, Harmonic and transient overvoltage analyses in arc furnace power systems, IEEE Transactions on Industry Applications, Mar.-Apr. 1992, 28(2):336-342.
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[11] Ghadimi M, Ramezani A and Bozorgi K, Energy Efficiency and Power Quality Optimization Using a Modified Capacitor Bank: An Industrial Case Study, 2009 Third UKSim European Symposium on Computer Modeling and Simulation, Athens, 2009, pp. 384-388.
[12] Grebe T, Application of distribution system capacitor banks and their impact on power quality, IEEE Transactions on Industry Applications, May-Jun. 1996, 32(3):714-719.
Acknowledgements
The authors wish to acknowledge the financial support received from the Faculty of Engineering Thonburi University Research Fund.
