Frequency converters are widely used in the variable speed transmission system industry in industrial production. Because of the power switching characteristics of the inverter rectifier circuit, a typical discrete system load is generated on its switching power supply. The frequency converter usually operates simultaneously with other devices such as computers and sensors on site. These devices are mostly installed nearby and may affect each other. Therefore, the power electronic equipment represented by frequency converter is one of the important harmonic sources in the public power grid, and the harmonic pollution generated by power electronic equipment has become the main obstacle to the development of power electronic technology itself.
1.1 What are harmonics
The root cause of harmonics is discrete system loading. When a current flows through the load, there is no linear relationship with the applied voltage, and a current other than a sine wave flows, generating higher harmonics. Harmonic frequencies are integer multiples of the fundamental frequency. According to the analysis principle of French mathematician Fourier (M.Fourier), any repetitive waveform can be decomposed into sine wave components including fundamental frequency and harmonics of a series of fundamental frequency multiples. Harmonics are sinusoidal waveforms, and each sinusoidal waveform often has a different frequency, amplitude, and phase angle. Harmonics can be divided into even and odd harmonics, the third, fifth and seventh numbers are odd harmonics, and the second, fourteenth, sixth and eighth numbers are even harmonics. For example, when the fundamental wave is 50Hz, the second harmonic is 10Hz, and the third harmonic is 150Hz. In general, odd harmonics are more damaging than even harmonics. In a balanced three-phase system, due to symmetry, even harmonics have been eliminated and only odd harmonics exist. For the three-phase rectifier load, the harmonic current is 6n 1 harmonic, such as 5, 7, 11, 13, 17, 19, etc. The soft starter key causes the 5th and 7th harmonics.
1.2 Relevant standards for harmonic control
Inverter harmonic control should pay attention to the following standards: anti-interference standards: EN50082-1, -2, EN61800-3: radiation standards: EN5008l-1, -2, EN61800-3. Especially IEC10003, IEC1800-3 (EN61800-3), IEC555 (EN60555) and IEEE519-1992.
The general anti-interference standards EN50081 and EN50082 and the frequency converter standard EN61800 (1ECl800-3) define the radiation and anti-interference levels of equipment operating in different environments. The above-mentioned standards define acceptable radiation levels under different environmental conditions: level L, no radiation limit. It is suitable for users who use soft starters in unaffected natural environments and users who solve radiation source restrictions by themselves. Class h is the limit specified by EN61800-3, first environment: limit distribution, second environment. As an option for radio frequency filter, equipped with radio frequency filter can make the soft starter meet the commercial level, which is usually used in non-industrial environment.
2 Harmonic control measures
Harmonic problems can be managed, radiation interference and power supply system interference can be suppressed, and technical measures such as shielding, isolation, grounding, and filtering can be adopted.
(1) Apply passive filter or active filter;
(2) Lift the transformer, reduce the characteristic impedance of the circuit, and disconnect the power line;
(3) Use green soft starter, no pulse current pollution.
2.1 Using passive or active filters
Passive filters are suitable for changing the characteristic impedance of switching power supplies at special frequencies, and are suitable for systems that are stable and do not change. Active filters are suitable for compensating discrete system loads.
Passive filters are suitable for traditional methods. The passive filter appeared first because of its simple and clear structure, low project investment, high operation reliability and low operation cost. They remain the key means of suppressing pulsed currents. The LC filter is a traditional passive high-order harmonic suppression device. It is an appropriate combination of filter capacitors, reactors and resistors, and is connected in parallel with the high-order harmonic source. In addition to the filtering function, it also has an invalid compensation function. Such devices have some insurmountable drawbacks. The key is very easy to be overloaded, and it will burn out when overloaded, which will cause the power factor to exceed the standard, compensation and punishment. In addition, passive filters are out of control, so over time, additional embrittlement or network load changes will change the series resonance and reduce the filter effect. More importantly, the passive filter can only filter one high-order harmonic component (if there is a filter, it can only filter the third harmonic), so that if different high-order harmonic frequencies are filtered, different filters can be used to increase equipment investment.
There are many kinds of active filters in various countries in the world, which can track and compensate pulse currents of different frequencies and amplitudes, and the compensation characteristics will not be affected by the characteristic impedance of the power grid. The basic theory of active power engineering filters was born in the 1960s, followed by the improvement of the large, medium and small output power full-control integrated circuit technology, the improvement of the pulse width modulation control system, and the harmonics based on the instantaneous speed reactive load theory. The clear proposal of the current instantaneous speed monitoring method has led to the rapid development of active power engineering filters. Its basic concept is to monitor the harmonic current originating from the compensation target, and the compensation equipment creates a frequency band of compensation current with the same size and opposite polarity as the harmonic current, so as to offset the pulse current caused by the pulse current source of the original line, and then make the current of the power network Only fundamental servings are included. The main part is the harmonic wave generator and automatic control system, that is, it works through the digital image processing technology that controls the rapid insulating layer triode.
At this stage, in the aspect of special pulse current control, passive filters and active filters have appeared in the form of complementary and mixed applications, making full use of the advantages of active filters such as simple and clear structure, easy maintenance, low cost, and good compensation performance. It gets rid of the defects of large volume and increased cost of the active filter, and combines the two together to make the entire system software obtain excellent performance.
2.2 Reduce the impedance of the loop and cut off the transmission line method
The root cause of harmonic generation is due to the use of non-linear loads, therefore, the basic solution is to separate the power lines of the harmonic-generating loads from the power lines of the harmonic-sensitive loads. The distorted current generated by the nonlinear load produces a distorted voltage drop on the impedance of the cable, and the synthesized distorted voltage waveform is applied to other loads connected to the same line, where higher harmonic currents flow. Therefore, measures to reduce pulse current damage can also be maintained by increasing the cross-sectional area of the cable and reducing the loop impedance. At present, methods such as increasing transformer capacity, increasing the cross-sectional area of cables, especially increasing the cross-sectional area of neutral cables, and selecting protective components such as circuit breakers and fuses are widely used in China. However, this method cannot fundamentally eliminate harmonics, but reduces the protection characteristics and functions, increases investment, and increases hidden dangers in the power supply system. Connect linear loads and non-linear loads from the same power supply
Points of outlet (PCCs) start to supply power to the circuit individually, so the out-of-frame voltage from discrete loads cannot be transferred to the linear load. This is an ideal solution to the current harmonic problem.
2.3 Apply emerald green inverter power without harmonic pollution
The quality standard of the green inverter is that the input and output currents are sine waves, the input power factor is controllable, the power factor can be set to 1 under any load, and the output frequency of the power frequency can be controlled arbitrarily. The built-in AC reactor of the frequency converter can well suppress the harmonics and protect the rectifier bridge from the influence of the instantaneous steep wave of the power supply voltage. Practice shows that the harmonic current without reactor is obviously higher than that with reactor. In order to reduce the interference caused by harmonic pollution, a noise filter is installed in the output circuit of the frequency converter. When the frequency converter allows, the carrier frequency of the frequency converter is reduced. In addition, in high-power frequency converters, 12-pulse or 18-pulse rectification is usually used, thereby reducing the harmonic content in the power supply by eliminating low harmonics. For example, 12 pulses, the lowest harmonics are the 11th, 13th, 23rd, and 25th harmonics. Similarly, for 18 single pulses, the few harmonics are the 17th and 19th harmonics.
The low harmonic technology used in soft starters can be summarized as follows:
(1) The series multiplication of the inverter power supply module selects 2 or about 2 series-connected inverter power supply modules, and eliminates the harmonic components according to the waveform accumulation.
(2) The rectifier circuit increases. Pulse width modulation soft starters use 121-pulse, 18-pulse or 24-pulse rectifiers to reduce pulse currents.
(3) Reuse of inverter power modules in series, by using 30 single-pulse series inverter power modules and reusing the power circuit, the pulse current can be reduced.
(4) Use a new DC frequency conversion modulation method, such as diamond modulation of the working voltage vector material. At present, many inverter manufacturers attach great importance to the harmonic problem, and technically ensure the greening of the inverter during design, and fundamentally solve the harmonic problem.
3 Conclusion
In general, we can clearly understand the cause of harmonics. In terms of actual operation, people can choose passive filters and active filters to reduce the characteristic impedance of the loop, cut off the relative path of harmonic transmission, develop and apply green soft starters without harmonic pollution, and turn the soft The harmonics generated by the starter are controlled within a small range.
Post time: Apr-13-2023