| With the development of science and technology around the world, the number, type and consumption of non-linear loads increase. These non-linear loads will produce harmonics and make the voltage distortion rate increase, result in adverse effects on transmission lines, electrical equipment, communications equipment, and even cause damage to electrical equipment and grid security incidents. Therefore, the Study of power system harmonic is very urgent and necessary. In recent years, the steel industry and smelting technology develop rapidly. At the same time, a growing number of electric arc furnace equipment is applied in the metal smelting industry and the capacity is becoming bigger. Gradually, electric arc furnace equipment becomes an important source of harmonic in power grid, so the harmonic analysis of electric arc furnace has practical significance.Before analyzing the arc furnace harmonics, we should fully understand its working principle and characteristics, and establish a suitable theoretical model. This article first briefly describes the development of electric arc furnace, then analyzes the working characteristics of melting, oxidation and reduction period combining with its steelmaking principle, and states some problems such as harmonics, voltage flicker, phase imbalance, low power factor it brings to power system, finally introduces several kinds of harmonic analysis method. On the basis of the above work, the simplify circuit of arc furnace electric system and the physical properties of ac arc are analyzed in detail, then the equation of state of arc furnace electric system and the resistive model of ac arc are deduced. Finally we present the feasible estimation based on each parameter of ac arc model.The study is given in two ways. A time domain controlled voltage source model of the arc furnace is given. In the process of implementing the model, at the first, the non-linear VI characteristic curve of the arc furnace is simplified to three straight lines,.And then describe the three straight sections using equations. In addition, we calculate the value of arc resistance using the real-time active power consumed by the arc furnace. The static model is established by the above equations. The dynamic model of arc furnace takes into account the periodic variation and the random variation. Dynamic changes in the arc resistance may be achieved by respectively loading sinusoidal signal and band-limited white noise signal on the arc resistance. In order to establish dynamic models, we need replace the arc resistance of the static model by the dynamic resistance.The static arc models are useful for harmonic studies, and the dynamic models are useful for power quality studies. Finally, the accuracy of the model is proved by comparing the simulation results with the actual voltage measuring from the local power company.Then, we use PSCAD (a kind of power system simulation software) to achieve the harmonic analysis of arc furnace. We think that there is background harmonic and the whole system is three-phase symmetrical. Two methods (one is to use the arc furnace coming with PSCAD, and the other is thinking arc furnace is the superposition of many harmonic current sources) are used to establish the model of arc furnace, then we will measure the voltage of two points, such as the exit of background harmonic and the point where the arc furnace is connected to the grid. After three-phase Fourier decomposition of voltage is completed, we can start harmonic analysis. Through the simulation results, we can conclude that the arc furnace harmonic injection can lead to power grid voltage distortion, and the distortion is usually asymmetric three-phase. |
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