Active filter topology for cascaded inverters
A method for generating a combined output waveform (V OUT ) in a power inverter system ( 100 ) is disclosed. The power inverter system comprises a voltage equaliser ( 120 ) having an energy storage element adapted to be charged by a combined output voltage from a plurality of switching units and to add an adjustable voltage to the combined output waveform. A voltage of the combined output waveform is measured, a difference (Δ) between the measured voltage of the output waveform and a voltage of a target waveform (U T ) is determined, and a voltage (U CORR ) corresponding to the determined difference is added, by the voltage equaliser, to the combined output waveform so as to improve the matching of the combined output power waveform and the target power waveform.
1. A method for generating a combined output power (UOUT) waveform in a power inverter system comprising
a plurality of switching units, wherein each one of the plurality of switching units is adapted to receive a respective input power (UDC) and wherein the plurality of switching units are interconnected to produce the combined output waveform transmitted in a common AC power path, and
a voltage equalizer comprising an energy storage element adapted to: be connected to the common AC power path to receive the combined output waveform, be charged by the combined output power waveform, and add voltage to the combined output power waveform,
the method comprising:
measuring a voltage of the combined output power waveform,
determining a difference between the measured voltage and a voltage of a target power waveform (UT), and
adding, to the combined output power waveform, a voltage corresponding to the determined difference.
2. The method according to claim 1 , wherein the step of adding the voltage corresponding to the determined difference comprises:
charging the energy storage element in case the measured voltage exceeds the voltage of the target power waveform, and
discharging the energy storage element in case the measured voltage is below the voltage of the target power waveform.
3. The method according to claim 1 , wherein the energy storage element is a capacitive energy storage element.
4. The method according to claim 1 , wherein the energy storage element is an inductive energy storage element.
5. The method according to claim 1 , wherein the voltage equalizer is connected in series with the plurality of switching units.
6. The method according to claim 1 , wherein the plurality of switching units are arranged in cascade configuration.
7. The method according to claim 1 , wherein at least some of the switching units are connected to a photovoltaic panel, respectively.
8. The method according to claim 1 , wherein at least some of the switching units are connected to a battery, respectively.
9. The method according to claim 1 , further comprising transforming a voltage and a current output from the voltage equaliser to the combined output of the AC power path.
10. The method according to claim 1 , wherein the target power waveform is a grid power waveform.
11. A power inverter system comprising:
a plurality of switching units, wherein each one of the plurality of switching units is adapted to receive a respective input power (UDC) and wherein the plurality of switching units are interconnected to produce a combined output power waveform (UOUT) transmitted in a common AC power path; and
a voltage equalizer comprising an energy storage element adapted to: be connected to the common AC power path to receive the combined output waveform, be charged by the combined output power, and add voltage to the combined output power,
the voltage equalizer further comprising:
a measuring unit adapted to measure a voltage of the combined output power waveform,
a calculating unit adapted to determine a difference between the measured voltage and a voltage of a target power waveform, and
control circuitry adapted to add, to the combined output power waveform, a voltage corresponding to the determined difference.
12. The power inverter system according to claim 11 , wherein the control circuitry is further adapted to charge the energy storage element in case the measured voltage exceeds the voltage of the target power waveform, and to input voltage to the combined output power waveform in case the measured voltage is below the voltage of the target power waveform.
13. The power inverter system according to claim 11 , wherein the energy storage element is a capacitor.
14. The power inverter system according to claim 11 , wherein the voltage equalizer is connected in series with the plurality of switching units.
15. The power inverter system according to claim 11 , wherein the plurality of switching units are arranged in cascade configuration.
16. The power inverter system according to claim 11 , wherein each one of the switching units are connected to a photovoltaic panel, respectively.
17. The power inverter system according to claim 11 , further comprising a transformer connected between the voltage equalizer and the switching units so as to transform a voltage and a current output from the voltage equalizer to the combined output.