In order to reduce the pulse current pollution caused by the intermediate frequency furnace, China has adopted multi-pulse rectifier technology, and developed several intermediate frequency furnace equipment such as 6-pulse, 12-pulse, and 24-pulse intermediate frequency furnaces, but because the cost of the latter is relatively high High, many ironmaking companies are still melting metal materials in 6-pulse intermediate frequency furnaces, and the problem of pulse current environmental pollution cannot be ignored. At present, there are mainly two kinds of management schemes for frequency furnace harmonics: one is the management scheme of relief, which is one of the methods to get rid of the current harmonic problems, and is a preventive measure to prevent the harmonics of intermediate frequency induction furnaces. Although the second method can deal with the increasingly serious problem of harmonic environmental pollution in many ways, for the intermediate frequency induction furnaces currently used, only the first method can be used to compensate for the resulting harmonics. This paper discusses the principle of IF furnace and its harmonic control measures, and proposes an active power filter (APF) to compensate and control the harmonics in different stages of 6-pulse IF furnace.
Electrical principle of intermediate frequency furnace.
The intermediate frequency furnace is a fast and stable metal heating device, and its core equipment is an intermediate frequency power supply. The power supply of the intermediate frequency furnace usually adopts the AC-DC-AC conversion method, and the input power frequency alternating current is output as intermediate frequency alternating current, and the frequency change is not limited by the frequency of the power grid. The circuit block diagram is shown in Figure 1:
In Figure 1, the main function of a part of the inverter circuit is to convert the three-phase commercial AC current of the power transmission and distribution provider into AC current, including the power supply circuit of the power transmission and distribution provider, bridge rectifier circuit, filter circuit and rectifier control circuit . The main function of the inverter part is to convert the AC current into a single-phase high-frequency AC current (50~10000Hz), including the inverter power circuit, starting power circuit, and load power circuit. Finally, the single-phase medium-frequency alternating current in the induction coil in the furnace generates a medium-frequency alternating magnetic field, which causes the charge in the furnace to generate induction electromotive force, generates a large eddy current in the charge, and heats the charge to melt.
Harmonic Analysis
The harmonics injected into the power grid by the intermediate frequency power supply mainly occur in the rectifier device. Here we take the three-phase six-pulse full-control bridge rectifier circuit as an example to analyze the content of the harmonics. Neglecting the whole phase transfer process and current pulsation of the thyristor inverter circuit of the three-phase product-release chain, assuming that the AC side reactance is zero and the AC inductance is infinite, using the Fourier analysis method, the negative and positive half-wave currents can be The center of the circle is used as the zero point of time, and the formula is derived to calculate the a-phase voltage of the AC side.
In the formula: Id is the average value of the DC side current of the rectifier circuit.
It can be seen from the above formula that for a 6-pulse intermediate frequency furnace, it can generate a large number of 5th, 7th, 1st, 13th, 17th, 19th and other harmonics, which can be summarized as 6k ± 1 (k is positive Integer) harmonics, the effective value of each harmonic is inversely proportional to the harmonic order, and the ratio to the fundamental effective value is the reciprocal of the harmonic order.
Intermediate frequency furnace circuit structure.
According to the different DC energy storage components, intermediate frequency furnaces can generally be divided into current type intermediate frequency furnaces and voltage type intermediate frequency furnaces. The energy storage element of the current type intermediate frequency furnace is a large inductor, while the energy storage element of the voltage type intermediate frequency furnace is a large capacitor. There are other differences between the two, such as: the current-type intermediate frequency furnace is controlled by thyristor, the load resonance circuit is parallel resonance, while the voltage-type intermediate frequency furnace is controlled by IGBT, and the load resonance circuit is series resonance. Its basic structure is shown in Figure 2 and Figure 3.
harmonic generation
The so-called high-order harmonics refer to the components above the integer multiple of the fundamental frequency obtained by decomposing the periodic non-sinusoidal AC Fourier series, generally called high-order harmonics. Frequency (50Hz) The component of the same frequency. Harmonic interference is a major “public nuisance” that affects the power quality of the current power system.
Harmonics reduce the transmission and utilization of power engineering, make electrical equipment overheat, cause vibration and noise, make the insulation layer deteriorate, reduce the service life, and cause common faults and burnout. Increase the harmonic content, burn out capacitor compensation equipment and other equipment. In the case where invalidation compensation cannot be used, invalidation fines will be incurred and electricity bills will increase. High-order pulse currents will cause misoperation of relay protection devices and intelligent robots, and the precise measurement of power consumption will be confused. Outside the power supply system, harmonics have a great impact on communication equipment and electronic products. The temporary overvoltage and temporary overvoltage that generate harmonics will destroy the insulation layer of machinery and equipment, causing three-phase short-circuit faults, and the harmonic current and voltage of damaged transformers will partially produce series resonance and parallel resonance in the public power network, causing major safety accidents.
The intermediate frequency electric furnace is a kind of intermediate frequency power supply, which is converted to intermediate frequency through precision and inverter, and generates a large number of harmful high-order harmonics in the power grid. Therefore, improving the power quality of intermediate frequency furnaces has become the top priority of scientific research.
governance plan
A large number of data connections of intermediate frequency furnaces have aggravated the pulse current pollution of the power grid. The research on the harmonic control of intermediate frequency furnaces has become an urgent task, and has been widely valued by scholars. In order to make the impact of the harmonics generated by the frequency furnace on the public grid meet the requirements of the power supply and distribution system for equipment commercial land, it is necessary to actively take measures to eliminate harmonic pollution. The practical precautions are as follows.
First, the transformer uses a Y/Y/connection pattern. In the large space medium frequency induction furnace, the explosion-proof switching transformer adopts the Y/Y/△ wiring method. By changing the wiring method of the ballast to communicate with the AC side transformer, it can offset the characteristic high-order pulse current that is not high. But the cost is high.
The second is to use LC passive filter. The main structure is to use capacitors and reactors in series to form LC series rings, which are parallel in the system. This method is traditional and can compensate both harmonics and reactive loads. It has a simple structure and has been widely used. However, the compensation performance is affected by the characteristic impedance of the network and the operating environment, and it is easy to cause parallel resonance with the system. It can only compensate for fixed frequency pulse currents, and the compensation effect is not ideal.
Third, by using the APF active filter, high-order harmonic suppression is a relatively new method. APF is a dynamic pulse current compensation device, with high partition design and high-speed responsiveness, it can track and compensate pulse currents with frequency and intensity changes, has good dynamic performance, and the compensation performance will not be affected by the characteristic impedance. The effect of current compensation is good, so it is widely valued.
Active power filter is developed based on passive filtering, and its filtering effect is excellent. Within the range of its rated reactive power load, the filtering effect is 100%.
Active power filter, that is, active power filter, APF active power filter is different from the fixed compensation method of traditional LC filter, and realizes dynamic tracking compensation, which can accurately compensate harmonics and reactive power of size and frequency. The APF active filter belongs to the series-type high-order pulse current compensation equipment. It monitors the load current in real time according to the external converter, calculates the high-order pulse current component in the load current according to the internal DSP, and outputs the control data signal to the inverter power supply. , The inverter power supply is used to generate a high-order harmonic current of the same size as the load high-order harmonic current, and the reverse high-order harmonic current is introduced into the power grid to maintain the active filter function.
Working principle of APF
Hongyan active filter detects the load current in real time through the external current transformer CT, and extracts the harmonic component of the load current through internal DSP calculation, and converts it into a control signal in the digital signal processor. At the same time, the digital signal processor generates a series of PWM pulse width modulation signals and sends them to the internal IGBT power module, controlling the output phase of the inverter to be opposite to the direction of the load harmonic current, and the current with the same amplitude, the two harmonic currents are exactly opposite to each other. Offset, so as to achieve the function of filtering harmonics.
APF technical features
1. Three-phase balance
2. Reactive power compensation, providing power factor
3. With automatic current limiting function, no overload will occur
4. Harmonic compensation, can filter out 2~50th harmonic current at the same time
5. Simple design and selection, only need to measure the size of harmonic current
6. Single-phase dynamic injection current, not affected by system imbalance
7. Response to load changes within 40US, the total response time is 10ms (1/2 cycle)
Filtering effect
The harmonic control rate is as high as 97%, and the harmonic control range is as wide as 2~50 times.
Safer and more stable filtering method;
The leading disruptive control mode in the industry, the switching frequency is as high as 20KHz, which minimizes the filtering loss and greatly improves the filtering speed and output accuracy. And it presents infinite impedance to the grid system, which does not affect the grid system impedance; and the output waveform is accurate and flawless, and will not affect other equipment.
Stronger environmental adaptability
Compatible with diesel generators, improving the ability of backup power shunting;
Higher tolerance to input voltage fluctuations and distortions;
Standard C-class lightning protection device, improve the ability to withstand bad weather conditions;
The applicable range of ambient temperature is stronger, up to -20°C~70°C.
Applications
The main equipment of a foundry company is an intermediate frequency electric furnace. The intermediate frequency electric furnace is a typical harmonic source, which generates a large number of harmonics, causing the compensation capacitor to fail to operate normally. Or so, the temperature of the transformer reaches 75 degrees in summer, causing waste of electric energy and shortening its life.
The foundry workshop of the intermediate frequency furnace is powered by 0.4KV voltage, and its main load is the 6-pulse rectification intermediate frequency furnace. The rectifier equipment generates a large number of harmonics while converting AC to DC during work, which is a typical harmonic source; harmonic current is injected into the power grid , Harmonic voltage is generated on the grid impedance, causing grid voltage and current distortion, affecting power supply quality and operation safety, increasing line loss and voltage offset, and having a negative impact on the grid and the electrical equipment of the factory itself.
1. Characteristic harmonic analysis
1) The rectification device of the intermediate frequency furnace is a 6-pulse controllable rectification;
2) The harmonics generated by the rectifier are 6K+1 odd harmonics. Fourier series is used to decompose and transform the current. It can be seen that the current waveform contains 6K±1 higher harmonics. According to the test data of the intermediate frequency furnace, the harmonic The wave current content is shown in the table below:
During the working process of the intermediate frequency furnace, a large number of harmonics are generated. According to the test and calculation results of the intermediate frequency furnace, the characteristic harmonics are mainly the 5th, and the 7th, 11th, and 13th harmonic currents are relatively large, and the voltage and current distortion is serious.
2. Harmonic control scheme
According to the actual situation of the enterprise, Hongyan Electric has designed a complete set of filtering solutions for the harmonic control of intermediate frequency furnaces. Considering the load power factor, harmonic absorption needs and background harmonics, a set of active filtering devices is installed on the 0.4KV low-voltage side of the enterprise transformer. Harmonics are governed.
3. Filter effect analysis
1) The active filter device is put into operation, and automatically tracks the changes of various load equipment of the intermediate frequency furnace, so that each harmonic can be effectively filtered out. Avoid burnout caused by parallel resonance of the capacitor bank and the system circuit, and ensure the normal operation of the reactive power compensation cabinet;
2) Harmonic currents have been effectively improved after treatment. The 5th, 7th, and 11th harmonic currents that were not put into use were seriously exceeded. For example, the 5th harmonic current drops from 312A to about 16A; the 7th harmonic current drops from 153A to about 11A; the 11th harmonic current drops from 101A to about 9A; Conform to the national standard GB/T14549-93 “Power Quality Harmonics of Public Grid”;
3) After harmonic control, the temperature of the transformer is reduced from 75 degrees to 50 degrees, which saves a lot of electric energy, reduces the additional loss of the transformer, reduces noise, improves the load capacity of the transformer, and prolongs the service life of the transformer;
4) After treatment, the power supply quality of the intermediate frequency furnace is effectively improved, and the utilization rate of the intermediate frequency power supply is improved, which is conducive to the long-term safe and economic operation of the system and the improvement of economic benefits;
5) Reduce the effective value of the current flowing through the distribution line, improve the power factor, and eliminate the harmonics flowing through the distribution line, thereby greatly reducing the line loss, reducing the temperature rise of the distribution cable, and improving the load capacity of the line;
6) Reduce the misoperation or refusal of control equipment and relay protection devices, and improve the safety and reliability of power supply;
7) Compensate the three-phase current imbalance, reduce the copper loss of the transformer and the line and the neutral current, and improve the quality of power supply;
8) After the APF is connected, it can also increase the load capacity of the transformer and distribution cables, which is equivalent to an expansion of the system and reduces the investment in the expansion of the system.
Post time: Apr-13-2023