PV inverters use semiconductor devices to transform the DC power into controlled AC power by using Pulse Width Modulation (PWM) switching. PWM switching is the most efficient way to generate AC power, allowing for flexible control of the output magnitude and frequency. However, all PWM methods inherently generate harmonics and noise originating in the high dv/dt and di/dt semiconductor switching transients. Intensive efforts have been made by IEETek to eliminate or reduce harmonics distortions generated due to output of this conversion.
1. What is harmonic distortion
For simplify purpose, here use the single phase inverter for explaination. There are several output type of inverter, i.e. square wave, modified sine wave and sine wave output inverter base on different topology and control strategy.
1.1 Square wave inverter
Fig.1 show the AC output wave of a square wave inverter, this kind of inverter need very big LC filter that almost impossible to manufacture. And there are abundant of harmonic components. Not other solution but let the output wave inverter still be square.
Fig.1 Output of Square Wave Inverter
Harmonic distortion of square wave inverter is the worst one of that three kind of inverter. Fig.2 shows the harmonic pattern of square wave and Fig.3 shows components of output wave using FFT analysis.
Fig.2 Harmonics of square wave
Fig.3 FFT analysis of square wave
1.2 Modified sine wave inverter
Modified sine wave inverter improve harmonic distortion from square wave inverter. The output waveform is a little bit to close to sine wave, which is shown in Fig.4. Also, the total harmonic distortion of modified sine wave is about 30%.
Fig.4 output of modified square wave inverter
Fig.5 FFT analysis of modified square wave
1.3 Pure sine wave inverter
Contribute to sine PWM technology, we can build up a pure sine wave inverter which the harmonic distortion is almost zero after filtering by LC filter. But actually, in real circuit the distortion will be about 5% because of control strategy.
Fig.6 Output of of sine modified PWM wave
2. Limiting of harmonic emission
According to EN61000-3-2, Limits for harmonic current emissions, the total harmonic distortion must be less than 5%, and each harmonic order also specified as below.
3. What will IEE do to reduce harmonic emission of the inverter?
Fig.7 shows the block diagram of a IEE Tech one MPPT single phase inverter, including the filters used for attenuating the high frequency noise on the inverter output voltages and currents. There is a main sources of high frequency noise generated by the PWM inverters as the PWM modulation frequency (2 ~ 20kHz). This component is mainly attenuated by the LC filter.
Fig.7 inverter topology of IEE Tech
When using sine PWM modulation and LC filter, the harmonic distortion of output can be control within 4-8%. In order to reduce the total harmonic distortion, IEETech develop a strategy which will strictly reduce the total harmonic distortion of output current.
Fig.7 shows a 6kw single phase inverter operate in different DC input voltage and AC output voltage, which total harmonic distortion is less than .5%.