Analysis of Resonant Causes and Suppression Methods of Frequency Converter Harmonics
The harmonic wave has important implications for the safe and stable operation of the meter network, causing serious distortion of waveforms and causing the equipment to become soft and easy to be broken. One of the main sources of harmonics is the nonlinear load. With the widespread use of converters, the harmonic emission of converters has become the main source of system harmonics.
The basic principles of converters are related to the modulation technology of power electronic devices. Its main components are square wave inverter, rectifier and filter capacitor. They are all non-linear devices, and their output waveforms have harmonic components.
The frequency converter is a dc current conversion device in the form of an ac grid. The ac-to-ac converter is an ac-to-ac device that converts the ac grid power of one frequency and one phase into another frequency and one phase. The main three-phase rectifiers consist of a three-phase controlled AC power grid, an inverter circuit and a filter. In addition to control, the introduction of frequency converter technology makes the system more complete and efficient. The task of the frequency converter is to convert the electric energy from flexible frequency into constant frequency and fixed phase, so as to be combination of low harmonic distortion and good dynamic response.
The conversion of harmonic components between electric energy and magnetic energy causes the load equivalent leakage current harmonic to be directly injected in the neutral line. In general, a frequency converter will produce low-order harmonic (2nd, 3rd, 5th, 7th) and the harmonic order n is usually odd because the current waveform of the frequency converter is a rectangular wave. Compared with other nonlinear loads, the harmonic current produced by the frequency converter is much larger and introduces more low-order nonlinear current to the system, which results in ideal electromotive force and voltage distortion at higher harmonic orders.
In order to effectively suppress the harmonic distortion generated by frequency converters, a variety of effective harmonic suppression measures have been developed. At present, the following approaches are mainly used:
1) Active Power Filter
The active power filter is a three-phase three-wire system integrated with active devices and sensors. The power filter is connected to the non-linear load through capacitors or reactors and works in parallel with the load. It acts as an absorption device to compensate for the existing harmonic distortion to a certain extent.
2) Passive Filtering
The passive filter used in parallel with the non-linear load can absorb the harmonic corrupted current generated by the load. The filter can be provided in series or parallel with the load according to the harmonic spectrum of the system.
3) Phase Displacement Modulation
Time-domain inverter modulation, also known as space vector modulation or phase displacement modulation, greatly reduces current distortion by making the current waveform close to sinusoidal voltage.
4) Frequency Constraint
The low frequency pulsed output of the inverter may interfere with the electromagnetic compatibility of the electronic system, so the pulse frequency should be constrained within the allowed range.
In conclusion, the frequency converter has important implications for the efficient and stable operation of the system. It is also the main source of low-order harmonics which cause significant distortion of waveform and possible damage to devices. To effectively suppress the harmonic distortion generated by the frequency converter, various devices and methods have been developed. Among them, active and passive filters, phase displacement modulation and frequency constraint are commonly used.
参考文献
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[3] H. Hatami et al., “Harmonic Estimation and Suppression Method for 3-level Diode-Clamped Inverters,” IEEE Transactions on Industrial Electronics, vol. 57, no. 4, pp. 1274-1283, Apr. 2010.
[4] X. Xu et al., “Real-Time Harmonic Estimation of Inverters With High Dynamic Performance,” IEEE Transactions on Industry Applications, vol. 48, no. 2, pp. 722-732, Mar. 2012.
