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    Electric furnace technical information
        

    Special filter for induction melting furnace

    Technical field:

        The present invention relates to a filter device used to filter out harmonics in a power system, and in particular to a filter device produced by an induction melting furnace

    Filter out the high-order fractional harmonics generated.

    Background technique:

        The high-order fractional harmonics generated by the induction melting furnace cause serious pollution to the power grid.

    The main source of pollution in the power grid.

        The harmonics generated by the induction melting furnace include integer harmonics such as 5th, 7th, ... etc. and intermediate frequency inverter frequency

    2 times the harmonics. Since the intermediate frequency inverter frequency of the induction melting furnace has nothing to do with the power frequency, the intermediate frequency inverter

    Harmonics whose frequency changes twice are classified as fractional harmonics. And because the inverter frequency of the induction melting furnace is much higher than the power frequency

    Therefore, this part of harmonics is called high-order fractional harmonics. The main harmonic components generated by the induction melting furnace

    It is a high-order fractional harmonic. The impact of high-order fractional harmonics on the power grid is very serious, and induction melting furnaces are currently used

    Many small steel plants are dotted in rural areas, so they are currently the main source of pollution in Chinas rural power grids.

        Most of the existing harmonic detection instruments use the Fourier transform method for harmonic detection, and the Fourier transform

    The leaf transformation method can only be used to detect integer harmonics, and it is incapable of detecting fractional harmonics. Therefore,

    In most cases, it is not possible to accurately detect high-order fractional harmonics, which causes problems with high-order fractional harmonics.

    Not getting the attention it deserves. The existing harmonic filter design method is to design several sets of integer harmonic filters.

    Wave filter and a set of high-pass filters, and this set of high-pass filters has flat impedance characteristics in the high frequency range,

    The filtering effect of high-order fractional harmonics is extremely poor. For the above reasons, filters are often installed at the induction melting furnace.

    After the wave device, although the results of the harmonic detection meet the requirements, the pollution situation of the power grid has not improved.

    Summary of the invention:

        The purpose of the present invention is to provide a simple structure and cost-effective method for filtering out high-order fractional harmonics of induction melting furnaces.

    This lower filter can filter out most of the high-order fractional harmonics, so that the induction melting furnace pollutes the power grid.

    To basically eliminate.

        The present invention uses the following technical solutions to achieve:

    The filter includes two sets of filter components, one of which is composed of a three-phase iron core reactor.

    Parallel resistor 3 is then formed in series with capacitor 2, and the series resonance frequency of reactor 1 and capacitor 2 is selected

    Choose between 200-500Hz, and the value of resistance 3 should be between 1-3 times of the load resistance of the induction melting furnace.

    The other component consists of a three-phase air-core reactor 4 and capacitor 5 in series, the reactor 4 and capacitor 5

    The series resonant frequency is selected between 0. 7-1 times the high-order fractional harmonic frequency of the induction melting furnace.

        The filter component composed of the reactor 4 and the capacitor 5 is a series band-pass filter, and its series

    The joint resonance point is tuned to the higher fractional harmonic frequency generated by the induction melting furnace (that is, 2 times the intermediate frequency inverter frequency)

    In the vicinity, because the series impedance of the series filter near the resonance point is very low, most of the high

    The fractional harmonics are filtered out. Because most of the high-order fractional harmonic currents pass through this group of filter components, so

    The current capacity of reactor 4 and capacitor 5 should be based on the total content of high-order fractional harmonics generated by the induction melting furnace.

    Line design. When the inductance of the series filter is small and the capacitance is large, the passband of the filter is wider, so

    The inductance should be reduced as much as possible to increase the capacitance. The research results show that when the inductance of the reactor 4 is selected as

    The best effect is when the leakage inductance of the main transformer of the induction melting furnace is about 50%. Due to the flow in reactor 4

    If the harmonic current is too large, the harmonic voltage at both ends is also high, and the inductance is small, so it must

    Use air-core reactors.

        The series resonance frequency of the band-pass filter composed of reactor 4 and capacitor 5 should be slightly lower than

    High-order fractional harmonic frequency. Although the impedance of the series filter is the largest when the resonant frequency is equal to the harmonic frequency

    Small, but still need to have a proper harmonic voltage to be able to flow through the harmonic current, so the power grid

    The harmonic voltage distortion in the inevitably exists. When the resonant frequency of the series filter is slightly lower than the harmonic frequency

    At the time, the series filter oscillating current excited by the commutation process of the SCR rectifier of the induction melting furnace

    It is close to the phase of the harmonic current, so the filtering effect is better. The research results show that the series filter

    When the resonant frequency of the wave device is equal to about 85% of the harmonic frequency, the best filtering effect can be obtained.

        The function of the filtering component composed of iron core reactor 1, capacitor 2, and resistor 3 is mainly

    It is to reduce the low-order harmonic amplification and reduce the commutation process of the thyristor rectifier of the induction melting furnace to the series bandpass filter

    Disturbance of the wave device. The capacitance of capacitor 2 should be close to that of capacitor 5, and the current capacity should be

    2 times the power frequency current flowing in the design. The current capacity of reactor l is according to the current of capacitor 2

    80% capacity for design. The current capacity of resistor 3 is designed according to the current capacity of reactor 1.

    Description of the drawings

    Figure 1 is a schematic diagram of an embodiment of the filter according to the present invention

        A specific implementation of the present invention will be described below in conjunction with the accompanying drawings:

        Suppose the main transformer of an induction melting furnace is 5uOKVA, the secondary voltage is 0Zuv, the leakage inductance is 80uH, and the negative

    The charge fundamental current is 500A, and the high-order fractional harmonic current is 150A. In this embodiment: the electricity of reactor 1

    The inductance is 400pH, and the current capacity is 40A; the capacity of capacitor 2 is 300u F, and the current capacity is

    50A; the resistance of resistor 3 is 1.2Q, the current capacity is 40A; the inductance of reactor 4 is 40uH,

    The current capacity is 150A; the capacity of capacitor 5 is 300uF, and the current capacity is 150A.

     


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