Harmonic current detection of electric arc furnace system based on the method of eliminating zero-sequence component

Lu Xiuhe,Yang Guang Electrical and Electronic Engineering Institute

ChangChun University Of Technology ChangChun, China

e-mail: luxiuhe@mail.ccut.edu.cn,gerile-yg@163.com

Shi Wei Electrical and Electronic Engineering Institute

ChangChun University Of Technology ChangChun, China

e-mail: shiweiyzx@163.com

Abstract— Detecting harmonic current in net is a key to compensate the system. Aimed at the problem of harmonic distortion in electric arc furnace system, this paper brings forward the method of eliminating zero-sequence component and puts it into three-phase four-wire of arc furnace system. Furthermore, the paper builds mathematic model and simulates it. The simulation results show that this method can detect higher harmonic components. Compared with the traditional detecting method, this way simplifies the detecting algorithm and increases the precision of harmonic current detecting in arc furnace system.

Keywords-Electric arc furnace; The method of eliminating zero-sequence component; Harmonic current detecting

I. INTRODUCTION With the development of the steel making technology,

more and more electric arc furnace invest to the industry, and its capacity also become more bigger. Because of its own operational factor, arc furnace has created increasing harmonic pollution, for example the voltage flickering changes, the frequency shifts, the unbalance of three-phase and harmonic. The problems would affect the electrical energy quality, the transmission efficiency and the equipments severely in safe operation. Therefore, in accordance with the problem of curbing harmonic and compensating reactive power in electric arc furnace system, this paper brings forward the method of eliminating zero-sequence component, which is base on the theory of instantaneous reactive power. Using this way in arc furnace system, it has a good filtration effects for the higher harmonic.

1 furnace transformer 2 converge row 3 electric arc furnace

Figure 1. System structure flow chart

The system flow chart is shown in Fig.1. After going into system through electric net, three phase voltages carry on the voltage transformation through the transformer and then supply power to the electric arc furnace. But at that moment the voltage and current contain so much harmonic, and the harmonic detecting appliances detect the harmonic order. In the end, directed towards the detecting structure, it adopts harmonic suppression appliance to eliminate harmonic and guarantees the power supply of high quality.

Based on the theory of instantaneous reactive power the qp ii − algorithm can detect all the harmonic and reactive

current exactly in the three-phase symmetric and orthoscopy system. But in the three-phase dissymmetry and distortion situation, the detect results have big error. How to apply this theory into system except the three-phase three-wire system has become a hot issue for the domestic and foreign scholars in recent years. Because of the existence of median, the three-phase four-wire system has the generality. Therefore the research of instantaneous reactive power theory in the three-phase four-wire system has more practical application value.

II. THE METHOD OF ELIMINATING ZERO-SEQUENCE COMPONENT

The difference between the three-phase four-wire system and the three-phase three-wire system is the existence of median, so composed to the detecting method of reactive power and harmonic current in the three-phase three-wire system, the way in the three-phase four-wire system is focused on the treatment of zero-sequence component in three-phase current.

TαβCαβC

⊗

⊗⊗

sinwtcoswt−

⊗⊗

⊗

ai′

bi′

ci′

Figure 2. The schematic diagram of harmonic current detecting with the method of eliminating zero-sequence component

2011 Fourth International Conference on Intelligent Computation Technology and Automation

978-0-7695-4353-6/11 $26.00 © 2011 IEEE

DOI 10.1109/ICICTA.2011.120

457

2011 Fourth International Conference on Intelligent Computation Technology and Automation

978-0-7695-4353-6/11 $26.00 © 2011 IEEE

DOI 10.1109/ICICTA.2011.120

431

To set up the instantaneous value of three-phase currents and median current are ai , bi , ci and Ni , then

cbaN iiii ++= . (1)

ai , bi and ci contain zero-sequence components which are equal. That is

. 3/)(3/0 cbaN iiiii ++== . (2)

The method eliminates zero-sequence component from all currents and sets up:

3

' cbaaa

iiiii

++−= . (3)

3

' cbabb

iiiii

++−= . (4)

3

' cbacc

iiiii

++−= . (5)

Then 'ai , '

bi and 'ci only contain positive-sequence

components and negative-sequence components. Using the

qp ii − algorithm to detect 'ai , '

bi and 'ci , we can get

fundamental harmonic positive-sequence components, which reduce ai , bi and ci , and then the results contain harmonic current, fundamental harmonic negative-sequence component and zero-sequence component. The way simplifies the operation process greatly, and it guarantees the real-time in detecting harmonic current. The schematic diagram is shown in Fig.2. In chart:

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

−

−−=

23

230

21

211

32

αβC . (6)

⎥⎦

⎤⎢⎣

⎡−−−

=tttt

Cωωωω

sincoscossin

. (7)

The process is as followed: firstly, operating tωsin and tωcos which are produced through the phase-locked loop

with ai , bi and ci , we can get pi and qi . After the LPF

filtering, the results are pfi and qfi . Secondly, pfi and qfi which are operated through TC and TCαβ

become the three-

phase fundamental harmonic component afi , bfi and cfi .

Third, let the three components minus ai , bi and ci , we can

get the exact results: the harmonic component ahi , bhi and

chi . When the supply voltage distorts, this method can also

calculate the harmonic component accurately. Because it only takes tωsin and tωcos in the computation, the harmonic components of distorted voltage do not participate in the computation. Therefore, no matter if the supply voltage distorts or not, the method based on the qp ii − algorithm can detects the harmonic current effectively.

III. SIMULATION RESEARCH In order to confirm the accuracy of the new algorithm,

this paper builds mathematic model and simulates it with MATLAB. The simulation model of main circuit is shown in Fig.3. The PWM inverter adopts the structure of four bridge arms. The simulation model mainly includes the non-linear load, the harmonic extracting module and the PWM signal generating module. It adopts the power source of 380V and 50Hz. The load is inductive load and the cut-off frequency of low-pass filter is 20Hz. The higher the cut-off frequency is, the quicker the low-pass filter responses, but it will possibly be unable to filter some harmonic. The sampling frequency is 12.8kHz, and the series inductance in main circuit is 1.14mH, and the capacitance in direct-current is 2000uF. The PWM control mode adopts the triangle carrier-wave control mode, and it controls component's make-and-break. According to the signal of driving circuit, the canceling current circuit produces canceling current for the voltage PWM inverter.

Figure 3. Main circuit simulation model

The command current detecting module is shown in Fig.4, which adopts harmonic current detecting method based on the way of eliminating zero-sequence component . In the simulation model, non-linear load uses three-phase diode rectifier. The important modules of harmonic extracting are abc/dq0、dq0/abc、PI and PLL.

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Figure 4. The command current detecting module

IV. ANALYSING OF THE SIMULATION RESULTS This paper compares the wave of a phase and median

current making-up before with the wave making-up after.

0 2 4 6 8 10 12

x 104

-400

-200

0

200

400

600

800

1000

1200

1400

Figure 5. Comparison of a phase current making-up before and after

0 5 10 15

x 104

-300

-200

-100

0

100

200

300

Figure 6. The median current before making-up

0 5 10 15

x 104

-300

-200

-100

0

100

200

300

Figure 7. The median current after making-up

Seen from the simulation charts, before the compensation, a-phase current is dissymmetry seriously and the wave is distorted heavily. The median current contains a large number of harmonic. After the compensation, the waveform of a-phase current is symmetrical basically and it presents the sine. The amplitude of median current is fading down sharply. Therefore, this method can eliminate the current harmonics and reactive power effectively.

V. CONCLUSION In the view of increasing harmonic pollution in the arc

furnace system, this paper provides the method of eliminating zero-sequence component, which is based on the

qp ii − algorithm of instantaneous reactive power theory. This way mainly carries on the examination to the harmonic current in arc furnace system, and the paper builds a simulation system to simulate it. The result is that after compensation the waveform is symmetrical basically and the THD drops from 34.96% to 6.77%. It illustrates that the method increases the precision of harmonic current detecting in arc furnace system and it has a very good effect to harmonic compensation, which does a lot work to maintain the safe operation of arc furnace equipment.

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method of three-phase four-wire system based on pi qi algorithm,” Electronic Design Engineering, vol. 18, January 2010.

[2] Li Shengqing, Luo Fei and Zhang Changfan, “An Improved Harmonic Current Detecting Method Based on pi and qi Calculating Modes.,” Electric Drive, vol. 10, March 1999.

[3] ZHOU Lin, LI Qiu-hua and ZHANG Feng, “New Algorithm for Random Harmonic Current Detection in Three Phase Four Wire System,” High Voltage Engineering, Journal Of Electronic Measurement And Instrument, vol.33, No.11, November 2007.

[4] Luo Ping, Li Zhaoji and Chen Guangju, “Detecting and compensation of harmonic currents and reactive power in 3-Phase 4-Wire,” vol.17, No.1, March 2003.

[5] Pang Shi-Feng and Wang Li-Na, “Study on A New Filter to Cancel The Neural Harmonics in Three-Phase Four-wire System,” Application Devices, 2008 No.4.

[6] DING Fang-li, “Research of Harmonic Current Detect Methods in Three - phase Four - wire System,” Journal of Anhui Science and Technology University, vol.20, No.3, 2006, pp27~29.

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