Φ Abstract-- The three - phase electric arc furnaces represent

one of the important generators of harmonic currents, reactive power and unbalanced conditions in the electrical power supply networks. This paper presents a study with experimental results concerning the parameters of quality of electrical energy at three-phase electric arc furnace.

Index Terms-- Three-phase electric arc furnaces, harmonics, unbalanced conditions, power quality.

I. INTRODUCTION HE majority of electric and electronic circuits (arc welders and furnaces, variable speed controllers, PC’s, medical equipment, etc) use switch mode techniques

which act as a non-linear load or disturbance generator which degrades the quality of the electricity supply.

In these electro energetic steady state circuits, the importance of the inconvenience caused by the non sinusoidal system of running is directly correlated to the amplitude of the harmonics. Also, it is of utmost importance to determine the variation of the apparent power at non defined node, in accordance with the presence of the current and voltage harmonics. Understanding the current harmonics and voltage harmonics is of utmost scientific importance both to the beneficiaries, who thus can prevent the undesirable effects of non sinusoidal steady state in a given network, and to the possible consumers as for as the corresponding measurement and pricing are concerned. Hence the elaboration of certain rules and prescription as regards the influence of the harmonics upon the fundamental component (first harmonic).

Such combinations of traditional and non-traditional loads, coupled with fluctuating loads, causes problems often classified as “random” or “sporadic” (problems with sensitive devices), annoying (light flickering) or as “strange” or “without apparent reason” (problems with cabling, capacitor banks, tripping, signaling etc.).

The electric arc furnace produces strong disturbing effects featured by non-symmetries of currents and voltages, harmonics, flickers, voltage drops and over-voltages, characteristic parameters of power quality [1]-[5].

There are many definitions of power quality depending on a person’s point of view. A simple definition accepted by

Costin Cepisca is with the Faculty of Electrical Engineering, Politehnica University of Bucharest, Romania (e-mail: costin@wing.ro).

Horia Andrei is with the Faculty of Electrical Engineering, Valahia University of Targoviste, Romania (e-mail: hr_andrei@yahoo.com).

Stergios Ganatsios is with the Technological Educational Institute of West Macedonia, Kozani, Greece (e-mail: ganatsios@tei.koz.gr) Sorin Dan Grigorescu is with the Faculty of Electrical Engineering, Politehnica University of Bucharest, Romania (e-mail: sgrig@electro.masuri.pub.ro)

most customers interprets power quality as good if the appliances connected to an electrical system work satisfactorily.

The term power quality refers (IEEE 1159:1995) to a wide variety of electromagnetic phenomena that characterize voltage and current at a given time and at a given location on the power system. IEC 61000-4-30 “Testing and measurements techniques - power quality measurement methods” defines power quality as “the characteristics of the electricity at a given point on an electrical system, evaluated against a set of reference technical parameters”.

This paper presents qualitative indicators of power quality, and the results of measurements with specialized equipments to a 100t electric arc furnace.

II. BASIC PRINCIPLES FOR THE POWER QUALITY ANALYSIS Power quality measurement is usually considered as a

measurement of low frequency conducted disturbance with the addition of transient phenomena. The following parameters of supply voltage are influenced by disturbances:

• frequency; • voltage level; • wave shape; • symmetry of three phase system. For the analysis of electric installation of arc furnace

it is important the measurement of the quantitative parameters [6]-[17]:

- voltages and currents are non sinusoidal quantities, and can be expressed by relations

∑=

+=N

kkk tkUtu

1)sin(2)( γω (1)

and

∑=

−+=N

kkkk tkIti

1)sin(2)( ϕγω (2)

where kU , kI are the rms of each k-harmonic of voltage, respectively current, ω is the angular frequency, kγ is the phase angle or each k-harmonic of voltage, k-harmonic of voltage, kϕ is difference of each phase angle of k-harmonic of voltage and current, t is the time.

- the active power

∑=

=N

kkkk IUP

1cosϕ (3)

Power Quality and Experimental Determinations of the Electrical Arc Furnaces Costin Cepisca, Member, IEEE, Horia Andrei, Member, IEEE, Stergios Ganatsios, Member, IEEE, and

Sorin Dan Grigorescu, Member, IEEE

T

567978-1-4244-1633-2/08/.00 ©2008 IEEE

- the reactive power

∑=

=N

kkkk IUQ

1sinϕ (4)

- the apparent power

∑∑==

=N

kk

N

kk IUS

1

2

1

2 (5)

- the power factor

222 DQP

PSPK P

++== (6)

- the reactive factor

PQ

=ρ (7)

- the deforming factor

22 QP

D

+=σ (8)

where

222 QPSD −−= (9)

is the Budeanu distortion (deforming) power. The presence of voltage and current harmonics is evaluated

through a relative quantity, the total harmonic distortion (THD). Voltage harmonics are asserted with THDU. THDU is a ratio of the rms value of the harmonic voltage to the rms value of the fundamental and is calculated by relation:

( )

∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

N

n

nU U

UTHD2

2

1

(10)

Everything presented for voltage harmonics is also valid for current harmonics and THDI, is a ratio of the rms value of the harmonic voltage to the rms value of the fundamental and is calculated by relation:

( )

∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

N

n

nI I

ITHD

2

2

1

(11)

Measurement of parameters using digital systems based on data acquisition systems [18]-[23].

It’s been used a multifunctional Power Quality Analyzer METREL, shown in Fig. 1 , one advanced instrument for measuring quality of electrical power in compliance with the EN60150, [24]-[31].

Fig. 1. Measurement equipment METREL

It incorporates a number of different measurement instruments for calculating various electrical parameters which is based on current and voltage measurements [32]-[40].

III. RESULTS OF MEASUREMENTS IN A REAL ELECTRIC INSTALLATION OF ARC FURNACE

The electrical power networks of arc furnaces are presented in Fig. 2.

Fig. 2. Electrical power supply networks for arc furnaces

A. The Real Measurements of Voltage and Current Harmonics, and of the Powers

Figure 3 presents the current (a), the voltage (b) and the powers (c) for a technological cycle of arc furnace. This cycle presents two phases: melting phase (6-8 minutes) and phase of stable arc burning (12-15 minutes). The electrical quantities are strong variation in the melting phase, with an

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Fig.3, a. The real measurements of current for a technological cycle of arc furnace

Fig.3, b. The real measurements of voltage for a technological cycle of arc

furnace

Fig.3, c. The real measurements of powers (P, Q, S) for a technological cycle

of arc furnace

important voltage fall. In the phase of stable arc burning the variation of electrical quantities are more reduced.

B. The Real Measurements of Wave Forms of Voltage and Current, and of the THDU and THDI for Melting Phase of the Technological Cycle of Arc Furnace

As regard to the wave forms of the voltages, shown in Fig.4, a, and, respectively the wave forms of the currents shown in Fig.4, b, on the 30kV voltage supply line in the melting phase is found a strong distortion of currents.

The Fig. 5 presents: (a) the total harmonic distortion calculated for voltages (THDU, 2,8…3%), and (b) the total harmonic distortion calculated for the currents (THDI, 10….11%).

a. The wave forms of voltages b. The wave forms of currents Fig. 4. The wave forms of voltages and currents in the melting phase

a. The THDU b. The THDI

Fig. 5. The total harmonic distortion calculated for voltages (THDU) and currents (THDI) in the melting phase

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C. The Real Measurements of wave forms of Voltage and Current, and of the THDU and THDI in the Phase of Arc Burning of the Technological Cycle of Arc Furnace

In the phase of the electric arc stable burning (Fig.6, a, and b), that appears towards the final of the heat’s making, is found that the distortion that appear in the currents and voltages wave forms are more reduced. In this phase, the amplitude of the three phase currents and voltages are closer as value, fact which shows that the load impedance is more balanced.

a. The wave forms of voltages b. The wave forms of currents

Fig.6. The wave forms of voltages and currents in the arc stable phase

The TDH for voltages and for currents in the arc stable phase are presented in Fig. 7, a, and b.

We observe that in the arc stable phase the THDU is reduced (1…2%) and THDU are an acceptable value (4…5%). One can reach to the conclusion that the deformation of the current and voltage waves is smaller in the stable burning phase also by the fact that the distorting power is smaller in this phase, in conditions where the apparent, active and reactive power is higher.

As regard the voltage on the 30kV line, in the melting phase one can observe the presence of the important harmonics while in the oxidation phase is found practically only the presence of the fundamental. In the current’s case, the important values of harmonics demonstrate that in this phase the current is strongly deformed.

a. The THDU b. The THDI Fig. 7. The total harmonic distortion calculated for voltages (THDU) and

currents (THDI) in the arc stable phase

The variation form of powers measured values presented on the heat time (Fig.3) presents in the first period, corresponding to the melting phase, a smaller apparent power. The electrodes are more lifted-up, in order to ensure protection against breaking and this determining a smaller value current. In the stable phase the apparent power is approximately constant and higher than in the melting phase. The variation of the voltage, as well as of the arc current, is reflected partially in the variation of active and reactive powers during the heat.

D. The Variation of the THDU and THDI ,and the variation of the Power Factor

The THDU and THDI (Fig.8) are higher in the melting phase than in the stable burning phase, bat the reactive power is higher in the stable phase than in the melting phase.

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Fig.8. The variation of THDI and THDU

The power factor value (Fig.9) is higher in the stable arc

phase and lower during the melting phase. For this reason results that on the 30 kV line the currents wave is more distorted than the voltages wave.

In different moments of technological process, following the measurements, were obtained values for THDI within 1-21% for current and 1-6% for voltage. Comparing these values with the standard [11 - 14] results that the furnace is not matched in the national and international standards.

Fig.9. The variation of power factor

IV. CONCLUSIONS The ever larger use of the non-sinusoidal receptors,

especially of the higher power rectifier installations, during the past decade, has drawn the specialists’ and researchers’ attention on problems such as; to define the powers in non-sinusoidal regime, to find a more adequate power factor, to improve the functioning of the electric systems in non-sinusoidal regime.

An optimal compensation of the reactive power, taking into account the active power variation at the reactive at the terminals of the receptor, can be achieved with a capacitor bank, corresponding to an average charge, and an automatic voltage control loop employing a static tension-converter which ensures the control of the reactive power exchange with the system when the charge is changed. Also, we can use the absorbing filters for the harmonic currents.

Following the analysis of the measurements results was obtained important conclusions: in the medium voltage supply line the current wave is more distorted than the voltage and the technological phase at the heat process has an influence on the electrical values.

Comparing the values of THDI and THDU with the standards results that the installation does not matched in the international standards. It is necessary the absorbing filters for the harmonic currents.

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VI. BIOGRAPHIES Costin Cepisca (M’95) was born in Bucharest, Romania, on May, 21, 1949. He received the degree in Electrical Engineering and Ph.D. from Bucharest Polytechnic Institute in 1983. He is currently Professor of electrical measurement at the “Politehnica” University of Bucharest and Head of Research Centre for Metrology and Measurement Systems. His presently research interest include the sensor interface systems, analog circuit

design, signal processing, power systems analysis, measurement theory and low-frequency measurement. He is a member of WSEAS, BRML, INM, SRB, SRM, ANEVAR, and CTR.

Horia Andrei (M’06) was born in Moreni, Romania, on May, 15, 1954. He received the degree in Control Systems and Computer Engineering, and the Ph.D. in Electrical Engineering from the Bucharest Polytehnic Institute, in 1996. Since 1982 he performs research and teaching as Professor at the Electrical Engineering Department of “Politehnica” University of Bucharest until 2002. Currently he is Professor at the Electrical Engineering Faculty, “Valahia” University of Targoviste,

Romania. His research activities include the analysis of analog circuits, symbolic methods for analogue circuits, applications of computer algebra in analysis and design of circuits and systems, electrical measurements and power systems analysis. He is a member of WSEAS, AMSE, AIEER, SRR, and SIEAR.

Stergios Ganatsios (M’96) is currently Professor at the Technological Educational Institute of West Macedonia, Kozani, Greece Department: Electrical Engineering. His research interest activities include data acquisition and signal processing, evaluation of a mechatronic design application, experimental determination of nuclear structure parameters, laser in the semiconductor technology, direct laser synthesis of metal and semiconductors

nitrides with appropriate properties for applications in VLSI microelectronics. He is a member of WSEAS, BRML, INM.

Sorin Dan Grigorescu (M’98) was born in Bucharest, Romania on June 8, 1958. He received the degree in electronics and telecommunications (1984) and the Ph.D. from the Bucharest Polytechnic Institute in 1996. He performs research and teaching as Professor of virtual instrumentations and distributed measurement systems at the “Politehnica” University of Bucharest. His research fields include signal processing, electronic circuit design and virtual instrumentation. He is a member of APREL,

SRB, and AIEER.

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