Suppression of higher harmonics of inverter - Schneider

1. About the management of higher harmonics on the output side of the inverter

The magnetic noise skills brought about by the communication harmonic components contained in the output waveform controlled by PWM reduction have been increasingly used in various inverters, such as the selection of higher frequency switching components, the installation of filter equipment at the output end of the inverter, and the use of random method to adjust the switching frequency and closed-loop control to improve the higher harmonics.

2. About the management of higher harmonics on the input side of the inverter

(1) A communication reactor is set on the communication input side of the inverter to increase the rectifier impedance, so that the rectifier overlap angle increases and the higher harmonics are reduced.

(2) The communication filter equipment is used in parallel in the power circuit, which can divert the weight of the higher harmonics from the inverter and the power supply system.

(3) For the occasion of installing multiple inverters, each can be equipped with a special transformer, and the method of staggering the phase of the input transformer can be used to press the higher harmonics.

Suppression of higher harmonics of inverter - Schneider

Suppression of higher harmonics of inverter - Schneider

3. Use passive filters or active filters

The use of passive filters is mainly to change the impedance of the power supply at a special frequency, which is suitable for a stable and unchanged system. The use of active filters is primarily used to compensate for nonlinear loads.

The traditional method mostly uses passive filters, and passive filters are the earliest, because of their simple structure, low investment, high operation reliability and low operating costs, they are still the primary method of harmonic pressing. LC filter is a traditional passive harmonic pressing equipment, which is properly composed of filter capacitors, reactors and resistors, and is connected in parallel with the harmonic source, in addition to the filtering effect, there is also the role of reactive power compensation. This kind of equipment has some shortcomings that are difficult to overcome, first of all, it is easy to overload, and it will be burned when overloaded, which may constitute a power factor over-introduction and compensation and be fined; In addition, passive filters cannot be controlled, so over time, the resonant frequency will change due to the aging of accessories or changes in the load of the grid, and the filtering effect will decrease. More importantly, the passive filter can only filter one harmonic component (if there is a filter that can only filter out the third harmonic), if the filter is different harmonic frequency, it is necessary to use different filters and add equipment investment.

There are a variety of active filters at home and abroad, which can track and compensate for harmonics with varying frequency and amplitude, and the compensation characteristics are not affected by the impedance of the power grid. The active power filter (APF) theory was formed in the 60s of the 20th century, and later with the maturity of large and medium power fully controlled semiconductor devices, the progress of pulse width modulation (PWM) control skills and the proposal of the instantaneous detection method of harmonic current according to the instantaneous reactive power theory, the active power filter has developed rapidly. The basic principle is to detect the harmonic current from the compensation target, and the compensation equipment generates a compensation current spectrum that is equal to the size of the harmonic current and has the opposite polarity, so as to cancel the harmonics generated by the harmonic source of the original line, so that the grid current only contains the fundamental weight. THE CENTRAL PART IS THE HARMONIC CURRENT GENERATOR AND CONTROL SYSTEM, THAT IS, ITS OPERATION IS COMPLETED BY DIGITAL SIGNAL PROCESSING (DSP) SKILLS TO CONTROL THE FAST INSULATED BIPOLAR TRANSISTOR (1GBT).

Now, in terms of detailed harmonic management, the method of passive filter (LC filter) and active filter complementary and mixed use is presented, giving full play to the advantages of LC filter simple structure, easy to complete, low cost, good compensation performance of active power filter, and overcoming the shortcomings of large capacity and high cost of active power filter, and the combination of the two is used, so that the whole system can obtain outstanding performance.

4. Reduce the impedance of the loop and block the transmission line

The basic cause of harmonic generation is due to the use of nonlinear loads, so the basic solution is to separate the power supply line of the load where harmonics occur and the power supply line of the load that is sensitive to harmonics (as shown in Figure 2). The distortion current caused by the nonlinear load causes a distortion voltage drop on the impedance of the cable, and the resulting distortion voltage waveform is applied to other loads connected to the same line, causing the harmonic current to flow over it (as shown in Figure 3). Therefore, the method of reducing harmonic damage can also be completed by increasing the cross-sectional area of the cable and reducing the impedance of the loop. Now, the domestic more selection to improve the capacity of the transformer, increase the cable cross-section, especially to increase the neutral cable cross-section, as well as the selection of the setting value of the larger circuit breaker, fuse and other maintenance elements and other methods, but this method can not eliminate harmonics from the bottom, but reduce the maintenance characteristics and functions, and increase the investment, add the hidden dangers of the power supply system. As can be seen in Figure 2, it is possible to transfer both linear and non-linear loads from the same source

The interface point (PCC) is supplied to the discrete circuit, so that the distorted voltage caused by the non-linear load is not transmitted to the linear load. This is now the more ambitious solution to manage harmonic problems.

5. Use green inverter without harmonic pollution

The quality standard of the green inverter is: the input and output current are sine waves, the input power factor is controllable, and the power factor can be 1 when any load is used, and the output frequency can be controlled by any power frequency. The communication reactor built in the inverter, it can press the harmonics well, and together can maintain the rectifier bridge from the influence of the instantaneous sharp wave of the power supply voltage, and the practice shows that the harmonic current without the reactor is significantly higher than the harmonic current generated by the reactor. In order to reduce the disturbance caused by harmonic pollution, a noise filter is installed in the output circuit of the inverter. And if the inverter allows, the carrier frequency of the inverter is reduced. In addition, in high-power inverters, 12-pulse or 18-pulse rectification is generally used, so that in the power supply, the harmonic content is reduced by eliminating the lowest harmonics. For example, with 12 pulses, the lowest harmonics are the 11th, 13th, 23rd, and 25th harmonics. By analogy, about 18 pulses, the lowest harmonics are the 17th and 19th harmonics.

The low harmonic skills used in the inverter can be summarized as follows: (1) The parallel multiplication of the inverter unit, two or more inverter units are selected to be connected in parallel, and the harmonic weight is offset by the superposition of waveforms. (2) For the multiplexing of the rectifier circuit, 121 pulses, 18 pulses or 24 pulses are selected in the PWM inverter to reduce harmonics. (3) The series multiplexing of the inverter unit, the 30-pulse series inverter unit multiplexing line is selected, and the harmonics can be reduced to very small. (4) Choose new frequency conversion modulation methods, such as diamond modulation of voltage vectors. At present, many inverter manufacturers have attached great importance to the harmonic problem, and have ensured the greening of the inverter from the technical approach when designing, so as to solve the harmonic problem at the foundation.

6. No matter what method is chosen, it is impossible to completely solve the problem of higher harmonic management, in order to eliminate the disturbance of the higher harmonics of the inverter to the electrical equipment in actual industrial production, it is mainly solved from three aspects: conduction, radiation and coupling. The general principle is to press and block the source of disturbance, intercept the coupling channel of the disturbance to the system, and reduce the sensitivity of the disturbance signal. To solve the conduction disturbance is to filter out or block the conduction high-frequency current in the circuit, to solve the radiation disturbance is to shield the radiation source or the disturbed line, and to solve the coupling disturbance is to reasonably place the interval and direction of the disturbance source and the disturbed line to avoid coupling.

In addition to the selection of skills such as blocking, shielding, grounding, reasonable wiring, etc., it can also take the skills of evasion and guidance, such as filtering, absorption and bypass, etc., these evasion and guidance skills are simple and wonderful, and sometimes can replace the expensive and large quality of the hardware method, and receive twice the result with half the effort.

Suppression of higher harmonics of inverter - Schneider

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