Furnaces and Harmonics

EE 545 Power System Quality Usama Ahmed 2015-MS-EE-16

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ARC FURNACES

Input voltage and current waveform to an arc furnace during the melting phase of the technological cycle

of arc furnace:

The associated harmonic spectrum and the total harmonic distortion (THD):

In the phase of the electric arc stable burning that appears towards the final of the heats making, is found

that the distortion that appear in the currents and voltages wave forms (figure(a) voltage and figure(b)

current) 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:


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The associated harmonic spectrum and the total harmonic distortion (THD):

INDUCTION FURNACES

The results obtained in the harmonic distortion analysis of voltage and current at the connection point of

the induction furnace are shown below. The THD value corresponding to the current is 4.47% while the

voltage THD is 1.04%. The highest harmonics are the 11th (550 Hz) and the 13th (650 Hz) and they are repeated

at the 23rd (1150 Hz) and the 25th (1250 Hz).


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Voltage Harmonics and

THD

Current Harmonics and

THD


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APPLICATION OF POWER FACTOR CORRECTION CAPACITORS

The figures below show the time domain voltage and currents wave forms and the frequency domain

harmonic spectrums without the application of capacitors:

Without adding capacitor banks for power factor correction, there would be no harmonic resonance

problems associated with the furnace operation. The addition of capacitors may cause system

capacitor resonance, this is illustrated in the figure below:


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Thus there is the possibility of severe harmonic distortion in case of applying capacitor banks alone

without series tuned rector. Therefore using capacitors banks in harmonics environment should be

accomplished with through analysis prior to installation.

APPLICATION OF A SERIES INDUCTOR

The effect of introducing a series inductor to load current in comparison to no compensation is as follows:


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Amplitudes of some of the noticeable harmonic components are as follows:

THE EFFECT OF HARMONICS ON TRANSFORMER

The effects of harmonics on transformers are quite self-evident, the harmonics will not only cause extra

loading on the transformer but will also result in the reduction of its overall useful life.

Application of non-sinusoidal excitation voltages to transformers increases the iron losses in the magnetic

core of the transformer in much the same way as in a motor. A more serious effect of harmonic loads

served by transformers is due to an increase in winding eddy current losses. Eddy currents are circulating

currents in the conductors induced by the sweeping action of the leakage magnetic field on the

conductors. Eddy current concentrations are higher at the ends of the transformer windings due to the

crowding effect of the leakage magnetic fields at the coil extremities. The eddy current losses increase as

the square of the current in the conductor and the square of its frequency. The increase in transformer

eddy current loss due to harmonics has a significant effect on the operating temperature of the

transformer. Transformers that are required to supply power to nonlinear loads must be derated based on

the percentages of harmonic components in the load current and the rated winding eddy current loss.

One method of determining the capability of transformers to handle harmonic loads is by k factor ratings.

The k factor is equal to the sum of the square of the harmonic currents multiplied by the square of the

frequencies.


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The figures below illustrate the predicted top oil and hot spot temperatures, respectively at different

harmonic cases. In the first case study, no harmonics are assumed to be present. The harmonics assumed

for the second case include major harmonics 5th, 7th, 11th, 13th, 17th, 19th, 23th and 25th with total

harmonic distortion (THD) 10%. The third case is with THD 22% and the same order harmonics. The

increase in top oil temperature for the second and third case is 2C and 8C, respectively compared with

no harmonics. The hot spot temperature of the HV winding shows an increase of more than 20C for the

third case.

Although the hot spot temperature has increased the loss of life factor above 1 .1 pu, it returns to normal

loss of life at the end of the load cycle as shown in the last figure.


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REFRENCES

[1] Power Quality Impacts of an Electric Arc Furnace and Its Compensation Journal of Electrical

Engineering & Technology, Vol. 1, No. 2, pp. 153~160, 2006

[2] Investigation and Mitigation of Harmonics from Electric Arc Furnaces Proceedings of the 1999 IEEE

Canadian Conference on Electrical and Computer Engineering Shaw Conference Center, Edmonton,

Alberta, Canada May 9-12 1999

[3] Harmonic distortion in a steel plant with induction furnaces I. Zamora1, I. Albizu2, A. J. Mazon, K. J.

Sagastabeitia, E. Fernandez Department of Electrical Engineering University of the Basque Country

Alda. Urquijo s/n, 48013 Bilbao (Spain)

[4] Horia Andrei, Costin Cepisca and Sorin Grigorescu (2011). Power Quality and Electrical Arc

Furnaces, Power Quality, Mr Andreas Eberhard (Ed.)

[5] The Effect of Harmonic Distortion on a Three phase Transformer Losses Canadian Journal on

Electrical and Electronics Engineering Vol. 3, No. 5, May 2012

[6] Effect of Harmonics on Transformers Loss of life Conference Record of the 2006 IEEE International

Symposium on Electrical Insulation

[7] IEEE Std. 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in

Electrical Power Systems, April 1993.

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Furnaces and Harmonics