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Where do the harmonics of the frequency converter come from? How should the harmonics of the frequency converter be calculated?
Release time:
2021-01-29
Harmonics of the inverter are high-order harmonics generated in the input-output circuit during the operation of the inverter, which requires high-power diodes to rectify (or transistors/inverter modules) the input power supply. During the inversion process, high-order harmonics are produced. Inverter harmonics can interfere with the power supply system, loads, and other nearby electrical devices. Moreover, the analysis of harmonics through Fourier series indicates that any periodic waveform can be decomposed into a sine wave component containing the fundamental frequency and a series of harmonics that are integer multiples of the fundamental frequency.
Inverter harmonics are sine wave components of periodic quantities, with frequencies that are integer multiples of the fundamental frequency. The amplitude and phase relationship of the harmonics relative to the fundamental frequency are important factors affecting this periodic quantity. In simple terms, if the fundamental frequency is 50Hz, then the harmonics are sine waves with frequencies of 100Hz, 150Hz, 200Hz...N*50Hz.
Where do inverter harmonics come from?
1. Mechanism of harmonic generation at the input side of the inverter
The main circuit of the inverter generally consists of a rectifier and inverter. The external input of 380V/50Hz power supply is rectified into DC voltage through a three-phase bridge uncontrolled rectifier, and then inverted into variable frequency AC voltage through capacitor filtering and high-power transistor switching components. In the rectification circuit, the waveform of the input current is an irregular rectangular wave, which can be decomposed into the fundamental wave and various harmonics according to Fourier series. The harmonic order is usually 6n±1 for high-order harmonics, and these high-order harmonics will interfere with the input power supply system. If the reactance on the power supply side is sufficiently small and the commutation overlap angle can be ignored, then the nth high-order harmonic is 1/n of the fundamental current.
2. Mechanism of harmonic generation at the output side of the inverter
In the inverter output circuit, the output current signal is a pulse waveform modulated by the PWM carrier signal. For GTR high-power inverter components, the PWM carrier frequency is 2-3kHz, while the PWM maximum carrier frequency for IGBT high-power inverter components can reach 15kHz. Similarly, the output circuit current signal can also be decomposed into components containing only the fundamental wave and other harmonics.
How to calculate inverter harmonics?
1. Method 1
Method 1: Use Fourier transform to obtain the amplitude and phase of each harmonic of voltage and current. Calculate the active power of each harmonic using P=√3UIcosφ, and sum all the active powers of the harmonics to obtain the total harmonic power.
Method 2: Measure the total active power, use Fourier transform to obtain the amplitude and phase of the fundamental frequency of voltage and current, calculate the active power of the fundamental frequency using P=√3UIcosφ, and subtract the active power of the fundamental frequency from the total active power to obtain the harmonic power. The measurement accuracy of harmonic power is relatively low; generally, the higher the harmonic frequency, the lower the accuracy. The second method is recommended.
2. Method 2
1) Inverter primary side
The possibility of distortion in the power supply voltage is extremely low, while the possibility of distortion in the current harmonics is high. Power calculations can be based on
P=U*I1*cosa1;
S=U*I;
PF=P/S=(I1/I)*cosa1;
This formula can also be simplified based on the definitions of THDi and DPF; many textbooks have written about this.
2) Inverter secondary side:
According to Zhou Shouchang's "Circuit Principles" and "Analysis of Non-Sinusoidal Periodic Current Circuits," it has been proven that different order harmonic voltages and currents cannot produce power, and their product is zero. Therefore, apart from the fundamental power, harmonic power can be easily calculated. It can be expressed as
Summation symbol (h=1 to ∞)(Uh*Ih*cosah).
How to suppress inverter harmonics?
1. Install appropriate reactors
By connecting suitable reactors in series on the input and output sides of the inverter, harmonics can be absorbed, and the impedance of the power supply or load can be increased to suppress harmonics, thereby reducing electromagnetic radiation during transmission. By suppressing harmonic currents, the power factor can be improved from the original (0.5-0.6) to (0.75-0.85);
2. Power isolation or installation of isolation transformers
Isolate the power supply of the inverter system from the power supply of other devices, or install isolation transformers on the input side of the inverter and other electrical devices to cut off harmonic currents;
3. Correct grounding of the inverter
Proper grounding can effectively suppress external interference and reduce the device's interference with the outside world. The inverter should use a dedicated grounding wire, and the grounding should be done with a thick and short wire. The grounding wire near other electrical devices must be separated from the inverter wiring and use short wires, which can effectively suppress the radiation interference of current harmonics to nearby devices;
4. Shorten line lengths
Shorten line lengths, and lay power lines and signal lines separately to avoid crossing. If crossing cannot be avoided, they must cross perpendicularly and must not be laid parallel. The shielding layer of the signal line should not be connected to the ground of the motor or inverter but should be connected to the common terminal of the control circuit;
5. Install EMI power filters
An important trend in suppressing inverter harmonics is the use of EMI power filters, which are connected in series or parallel to the main circuit to detect the inverter in real-time from the compensation object.
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