With the rapid development of our country’s economy, especially the rapid growth of mining, smelting and casting industries in recent years, the demand for electricity is increasing. Among them, the intermediate frequency smelting furnace rectification equipment is one of the largest harmonic power generation equipment, but because most manufacturers reduce product costs and do not install harmonic suppression technology facilities, the current public power grid is severely polluted by harmonics like haze weather . Pulse current reduces the processing, transmission and utilization of electromagnetic energy, overheats electrical equipment, causes vibration and noise, ages insulation, shortens service life, and even causes failure or burns. Harmonics can cause local parallel resonance or series resonance of the power system, thereby expanding the harmonic content and causing capacitors to burn and other equipment. Harmonics can also cause misoperation of protection relays and automatic devices and confuse energy measurements. Harmonics outside the power system can seriously interfere with communication equipment and electronic equipment.
The intermediate frequency electric furnace is one of the largest harmonic sources in the grid load, because it is converted into intermediate frequency after rectification. Harmonics will seriously endanger the safe operation of the power grid. For example, harmonic current will cause additional high-frequency vortex iron loss in the transformer, which will cause the transformer to overheat, reduce the output volume of the transformer, increase the noise of the transformer, and seriously endanger the service life of the transformer. The sticking effect of harmonic currents reduces the constant cross-section of the conductor and increases the loss of the line. Harmonic voltage affects the normal operation of other electrical equipment on the grid, causing operational errors in automatic control equipment and inaccurate measurement verification. Harmonic voltage and current affect the normal operation of peripheral communication equipment; transient overvoltage and transient overvoltage caused by harmonics damage the insulation layer of machinery and equipment, resulting in three-phase short-circuit faults and damage to transformers; harmonic voltage and The amount of current will cause partial series resonance and parallel resonance in the public power grid, resulting in major accidents. In the process of adhering to constant changes, the first thing to get from DC is a square wave power supply, which is equivalent to the superposition of high-order harmonics. Although the later circuit needs to be filtered, the high-order harmonics cannot be completely filtered, which is the reason for the generation of harmonics.
We designed single-tuned filters of 5, 7, 11 and 13 times. Before filter compensation, the power factor of the melting stage of the user’s intermediate frequency electric furnace is 0.91. After the filter compensation device is put into operation, the maximum compensation is 0.98 capacitive. After running the filter compensation device, the total voltage distortion rate (THD value) is 2.02%. According to the power quality standard GB/GB/T 14549-1993, the voltage harmonic (10KV) value is less than 4.0%. After filtering the 5th, 7th, 11th and 13th harmonic current, the filtering rate is about 82∽84%, reaching the allowable value of our company’s standard. Good compensation filter effect.
Therefore, we should analyze the causes of harmonics and take measures to suppress high-order harmonics, which is crucial to ensure the safe and economical operation of power systems.
First, the cause of the harmonics of the intermediate frequency furnace
1. Harmonics are generated by non-linear loads, such as silicon controlled rectifiers, switching power supplies, etc. The harmonic frequency generated by this load is an integer multiple of the operating frequency. For example, a three-phase six-pulse rectifier mainly produces the 5th and 7th harmonics, while a three-phase 12-pulse rectifier mainly produces the 11th and 13th harmonics.
2. Due to the harmonics generated by inverter loads such as intermediate frequency furnaces and inverters, not only integral harmonics are generated, but also fractional harmonics whose frequency is twice the frequency of the inverter. For example, an intermediate frequency furnace operating at 820 Hz using a three-phase six-pulse rectifier generates not only 5th and 7th harmonics, but also fractional harmonics at 1640 Hz.
Harmonics co-exist with the grid because generators and transformers generate small amounts of harmonics.
2. Harm of harmonics in intermediate frequency furnace
In the use of intermediate frequency furnaces, a large number of harmonics are generated, which leads to serious harmonic pollution of the power grid.
1. Higher harmonics will generate surge voltage or current. Surge impact refers to the short-term over (low) voltage of the system, that is, the instantaneous pulse of voltage that does not exceed 1 millisecond. This pulse can be positive or negative, and can have a series or oscillatory nature, causing the appliance to burn.
2. Harmonics reduce the transmission and utilization of electric energy and thermoelectric equipment, generate vibration and noise, make its edges age, reduce service life, and even malfunction or burn.
3. It affects the reactive power compensation equipment of the power supply system; when there are harmonics in the power grid, the voltage of the capacitor increases after the capacitor is put in, and the current through the capacitor increases even more, which increases the power loss of the capacitor. If the pulse current content is high, the capacitor will be over-current and loaded, which will overheat the capacitor and accelerate the embrittlement of the edge material.
4. This will reduce the speed and service life of electrical equipment and increase loss; it directly affects the use capacity and utilization rate of the transformer. At the same time, it will also increase the noise of the transformer and greatly shorten the service life of the transformer.
5. In areas with many harmonic sources in the power grid, even a large number of breakdowns of internal and external electronic capacitors occurred, and the capacitors in the substation burned or tripped.
6. Harmonics can also cause relay protection and automatic device failure, resulting in confusion in energy measurement. This is the exterior of the power system. Harmonics cause serious interference to communication equipment and electronic equipment. Therefore, improving the power quality of the intermediate frequency furnace has become the main focus of the response.
Three, intermediate frequency furnace harmonic control method.
1. Improve the short-circuit capacity of the public connection point of the power grid and reduce the harmonic impedance of the system.
2. Harmonic current compensation adopts AC filter and active filter.
3. Increase the pulse number of converter equipment to reduce harmonic current.
4. Avoid the resonance of parallel capacitors and the design of system inductance.
5. The high-frequency blocking device is connected in series on the high-voltage DC transmission line to block the propagation of high-order harmonics.
7. Select the favorable transformer wiring mode.
8. The equipment is grouped for power supply, and a filtering device is installed.
Four, intermediate frequency furnace harmonic control equipment
1. Hongyan passive filter device.
Hongyan passive filter device. The protection is a capacitor series resistor, and the passive filter is composed of a capacitor and a resistor in series, and the adjustment is connected to a certain extent. At a special frequency, a low impedance loop is generated, such as 250HZ. This is a fifth harmonic filter. The method can compensate both harmonics and reactive power, and has a simple structure. However, the main disadvantage of this method is that its compensation is affected by the impedance of the grid and the working state, and it is easy to resonate in parallel with the system, resulting in harmonic amplification, overload and even damage to the liquid crystal filter. For loads that vary greatly, it is easy to cause undercompensation or overcompensation. In addition, it can only compensate fixed frequency harmonics, and the compensation effect is not ideal.
2. Hongyan active filter equipment
Active filters cause harmonic currents of equal magnitude and antiphase. Make sure that the current on the power supply side is a sine wave. The basic concept is to create a compensation current with the same strength as the load harmonic current and reverse the position, and offset the compensation current with the load harmonic current to clear the pulse current. This is a product harmonic elimination method, and the filtering effect is better than passive filters.
3. Hongyan Harmonic Protector
The harmonic protector is equal to the capacitor series reactance. Because the impedance is very low, the current will flow here. This is actually impedance separation, so the harmonic current injected into the system is basically resolved.
Harmonic protectors are usually installed in front of delicate equipment. They are high-quality harmonic control products, which can resist surge impact, absorb 2~65 times higher harmonics, and protect equipment. Harmonic control of lighting control systems, computers, televisions, motor speed control equipment, uninterruptible power supplies, CNC machine tools, rectifiers, precision instruments, and electronic control mechanisms. All of these harmonics generated by non-linear electrical equipment can cause failures in the distribution system itself or in equipment connected to the system. The harmonic protector can eliminate harmonics at the power generation source, and automatically eliminate high-order harmonics, high-frequency noise, pulse spikes, surges and other disturbances to electrical equipment. The harmonic protector can purify the power supply, protect electrical equipment and power factor compensation equipment, prevent the protector from tripping accidentally, and then maintain the safe operation of electrical equipment in high ground.
Post time: Apr-13-2023