A boost inverter includes a first capacitor for connection in parallel with a dc supply voltage; a second capacitor connected in series with the first capacitor; an energy transfer path including a transfer capacitor and switches connected to the transfer capacitor for cyclically transferring energy from the first capacitor to the transfer capacitor and then from the transfer capacitor to the second capacitor; a multilevel inverter circuit connected in parallel with the series combination of the first and second capacitors, the inverter having at least one phase output for connection to an electric motor; and a PWM controller that activates the energy transfer path when a desired peak-to-peak output voltage exceeds the dc supply voltage and deactivates the energy transfer means when the desired peak-to-peak output voltage is less than the dc supply voltage.
The invention discloses a system and a method for controlling neutral point voltage balance with a low frequency oscillation suppression function. The system comprises a multi-level inverter and a digital processing control module, wherein the digital processing control module comprises a sampling unit, a closed-loop control unit and a digital pulse width modulation (DPWM) unit; the output end of the DPWM unit is connected to each switching tube of the bridge arm of each phase of the multi-level inverter. The method comprises the following steps: in each switching period, the sampling unit acquires a direct-current busvoltagesignal of the multi-level inverter, and a three-phase voltagesignal and a three-phase current signal both output by the multi-level inverter, respectively; the acquired signals are processed by the closed-loop control unit to obtain three-phase modulating waves; the three-phase modulating waves are processed by the DPWM unit to obtain a pulse width modulationcontrol signal which is used for controlling the working state of the switching tube of each phase of the multi-level inverter by virtue of a drive circuit. The system and the method are capable of suppressing the low frequency oscillation of the neutral point voltage of the direct-current bus of the multi-level inverter and realizing the neutral point voltage balance.
A multilevel inverter using a cascade configuration with a device for equally controlling a power factor for each unit cell includes: a master controller, a plurality of power cells connected in series for each of three AC (Alternating Current) phases, a phase shift transformer for supplying a power cell input voltage, a CAN (Controller Area Network) communication network to provide a communication path between the master controller and each power cell, and cell controllers to control output voltages and output frequencies of the respective power cells according to a voltage command and a frequency command from the master controller, each cell controller having a pulse width modulator to generate a pulse width modulationcontrol signal of a variable voltage and a variable frequency to the connected corresponding power cell, wherein each cell controller is configured to calculate a phase difference value of the corresponding power cell according to a preset total number of connected power cells and the floor number (i.e., the serial connection sequence) of the corresponding power cell to thereafter phase-shift a chopping wave by the calculated phase difference value, thereby compensating for the phase differences between each power cell.