1. Methods to suppress or weaken the effects of harmonics
(1) Why can increasing the carrier frequency suppress or weaken harmonics?
The carrier frequency of a frequency converter is adjustable, typically 2 to 16 kHz. When harmonics are high, the carrier frequency should be increased as much as possible, especially for domestically produced frequency converters. Generally, the factory-set carrier frequency is set at a lower value to reduce IGBT power consumption. For example, increasing the carrier frequency from 2 kHz to 16 kHz (an 8-fold increase) results in an approximately 2 to 2.5-fold increase in power consumption and a 4 to 6.25 -fold increase in heat generation. While increasing the carrier frequency improves the sinusoidal performance of the output current waveform, reduces glitches, and creates a smoother waveform, which is beneficial for reducing harmonics, appropriately increasing the carrier frequency is advantageous for suppressing or reducing harmonics.
(2) How to increase the output frequency of the frequency converter to reduce harmonics?
Increasing the output frequency is beneficial for reducing harmonic effects. Whenever possible, increase the output frequency as much as permitted, as detailed in Table 1 .
As the output frequency increases, the absolute value of the harmonics increases, but the relative value with respect to 50Hz decreases.
(3) Can adding an input AC reactor reduce the harmonics at the input of the frequency converter?
The filtering effect of AC reactors connected in series in a three-phase input circuit is not very good. After use, the cosine efficiency can be increased to 0.75 to 0.85 .
1) When the voltage is 380V, the transformer capacity is above 500kV·A, or more than 10 times the inverter capacity, the relationship between the distribution transformer capacity, the inverter capacity and the selected AC reactor is shown in Figure 1.
2) The three-phase imbalance rate of the power supply output voltage is greater than 3%.
3) When the distribution transformer is connected to a power factor compensation capacitor or has a thyristor (SCR) rectifier, the comparison between the 6-pulse rectifier and the presence or absence of the LAC reactor is shown in Figure 2. The relationship between the LAC reactance value and the harmonic currents is shown in Figure 3.
As shown in Figure 3, the LAC (Low Voltage Arrangement) is very effective in suppressing the 5th to 19th harmonics. Generally, when selecting it, the voltage drop across the reactor should be approximately 3% of the rated voltage. It can also be connected in series in the output circuit of the frequency converter. Its main function is to suppress the frequency converter's emission and induced interference, and to suppress the voltage fluctuation effect of the motor. For frequency converters with a rated power of 18.5kW , it is generally built-in; for frequency converters with a rated power of 22kW, it is generally external, although there are cases where additional configuration is required.
(4) How to select AC reactor parameters?
The data for the AC reactor are listed in Table 2.
(5) What is the function of a DC reactor?
A DC reactor is connected in series between the rectifier bridge and the filter capacitor. It has a simple structure, small size (single-phase), and good filtering effect, which can improve the cosine density to 0.95 . It can be installed either internally or separately. After installing the DC reactor, the input harmonic current decreases significantly, and the fundamental current increases, as shown in Figure 4(c).
(6) What are the parameters of a DC reactor?
The data for the DC reactor are listed in Table 3.
(7) What effect does the AC reactor XL have on the input current?
Figure 4 shows the input current waveform when the reactor is connected and the inverter circuit when the AC reactor and DC reactor are connected.
(8) How does using a power supply matching inductor improve the characteristics of the input circuit?
The improved power characteristics after using a power matching inductor are shown in Table 4. This device is relatively expensive but performs well and is generally installed at the beginning of the input. Table 4 shows that when the inverter power is 75kW, the input current decreases and the power factor increases, with both effects being quite significant.
(9) How to use a zero-sequence noise filter?
The zero-sequence reactor FL-Z, which reduces radio interference, can be installed at both the input and output terminals and is suitable for applications with lower requirements.
Depending on the power rating and wire diameter, an appropriate zero-phase-sequence noise filter can be selected. Utilizing the high attenuation characteristics of the zero-phase-sequence noise filter (which exhibits high attenuation across the amplitude modulation frequency range of 100kHz to 50MHz), radiated interference generated by the frequency converter can be effectively suppressed. Zero-sequence noise filters can be used on the input or output side of the frequency converter. When using them, each phase's wiring can be wound several times in the same direction; the more turns, the better the effect. If the wiring is too thick to be wound, several zero-phase-sequence noise filters can be connected in series in the same direction by directly passing the wiring through each phase.
(10) What is the function of a radio noise filter?
Radio noise filters are used to suppress the transmission of electromagnetic interference noise generated by frequency converters, and can also suppress external radio interference and transient impacts and surges that interfere with the motor.
(11) What should be noted when using a power supply high-pass filter?
A typical high-pass input CLC filter circuit is connected in series with the power supply terminals, as shown in Figure 5. A high-pass output LRC filter circuit is connected in series with the output circuit, as shown in Figure 6. Note that these two circuits must not be connected incorrectly. When the inverter capacity is greater than or equal to 22kW, a parallel power filter is generally selected. Power filters are selected when higher requirements are needed.
(12) How to select the parameters of a power supply high-pass filter?
The power supply filter selection is listed in Table 5.
