Inverters are a type of power electronic device widely used in industrial automation. They adjust the speed of motors by changing the frequency and amplitude of the motor input voltage. Although inverters improve energy efficiency and control performance, they generate harmonics during operation, which have an adverse effect on power grids and equipment. Understanding the causes and principles of inverter harmonics is essential for taking appropriate suppression measures.
Ⅰ. Causes of inverter harmonics:
1. Nonlinear load characteristics: Inverters contain nonlinear components such as diodes, thyristors, IGBTs, etc., which cause current waveform distortion during operation, thereby generating harmonics.
2. Pulse Width Modulation (PWM): Inverters usually use PWM technology to adjust output voltage and frequency. The PWM working principle is to generate a series of pulses with different widths by rapidly switching power devices to obtain the desired average voltage value. This rapid switching causes high-frequency components in the current and voltage waveforms, namely harmonics.
3. Rectification process: The front stage of the inverter is usually a rectifier, which converts AC power into DC power. During the rectification process, the current can only be conducted near the peak value of the voltage, causing the current waveform to be pulse-shaped, thereby generating harmonics.
1. Time harmonics: During the operation of the inverter, due to the fast switching characteristics of PWM control, the waveforms of the output voltage and current are actually composed of a series of discrete pulses in time. These pulse waveforms contain the fundamental wave (a sine wave of the required frequency) and time harmonics of integer multiples of the fundamental frequency.
2. Spatial harmonics: In addition to time harmonics, the spatial arrangement of power electronic components in the inverter may also cause harmonics. For example, pulse waveforms of different phases in a multi-phase inverter may interfere with each other and generate spatial harmonics.
3. Electromagnetic interference (EMI): The high-order harmonics generated by the inverter will propagate in the form of electromagnetic fields and interfere with surrounding electronic equipment. This electromagnetic interference may cause malfunction or performance degradation of other equipment.
Ⅲ. The impact of harmonics:
Harmonics can cause a variety of adverse effects on power grids and equipment, including:
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Increase the loss of transformers and cables, leading to overheating.
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Cause grid voltage distortion and affect power quality.
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Interfere with the normal operation of communication systems and other sensitive electronic equipment.
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Cause premature aging or damage to protective devices such as capacitors and relays.
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Reduce motor efficiency and increase noise and vibration.
The generation of inverter harmonics is the result of the interaction of nonlinear elements, PWM technology and rectification process inside the inverter. Harmonics not only affect the stability and power quality of the grid, but may also cause damage to equipment. Therefore, taking effective harmonic suppression and filtering measures is the key to improving the overall performance and reliability of the system. Besides adding input and output reactor for VFD, the active harmonic filter is the best way for whole distribution system harmonic mitigation.
