Modulation and control methods for quasi-Z-source cascade multilevel inverters

1. A modular multilevel space vector modulation method for a quasi-Z-source cascade multilevel inverter, comprising the steps of:

generating a first switching signal for an upper left set of switches of each quasi-Z-source H-bridge inverter cell of a quasi-Z-source cascade multilevel inverter at a time T 0 /4−T sh /4 within a control cycle T s of the quasi-Z-source H-bridge inverter cell, where T 0 is an unmodified switching time interval of a zero state of the quasi-Z-source H-bridge inverter cell and T sh is a time of shoot-through zero states of the quasi-Z-source H-bridge inverter cell;

comparing the first switching signal with a triangular carrier signal over the control cycle T s and turning the upper left set of switches on if the triangular carrier signal is higher than the first switching signal and turning the upper left set of switches off if the triangular carrier signal is lower than the first switching signal;

generating a second switching signal for an upper right set of switches of the quasi-Z-source H-bridge inverter cell at a time T s /2−T 0 /4 within the control cycle T s of the quasi-Z-source H-bridge inverter cell;

comparing the second switching signal with the triangular carrier signal over the control cycle T s and turning the upper right set of switches on if the triangular carrier signal is higher than the second switching signal and turning the upper right set of switches off if the triangular carrier signal is lower than the second switching signal;

generating a third switching signal for a lower left set of switches of the quasi-Z-source H-bridge inverter cell at a time T 0 /4 within the control cycle T s of the quasi-Z-source H-bridge inverter cell;

comparing the third switching signal with the triangular carrier signal over the control cycle T s and turning the lower left set of switches on if the triangular carrier signal is higher than the third switching signal and turning the lower left set of switches off if the triangular carrier signal is lower than the third switching signal;

generating a fourth switching signal for a lower right set of switches of the quasi-Z-source H-bridge inverter cell at a time T s /2−T 0 /4+T sh /4 within the control cycle T s of the quasi-Z-source H-bridge inverter cell; and

comparing the fourth switching signal with the triangular carrier signal over the control cycle T s and turning the lower right set of switches on if the triangular carrier signal is higher than the fourth switching signal and turning the lower right set of switches off if the triangular carrier signal is lower than the fourth switching signal.

2. The modular multilevel space vector modulation method for a quasi-Z-source cascade multilevel inverter as recited in claim 1 , wherein the quasi-Z-source cascade multilevel inverter comprises a photovoltaic quasi-Z-source cascade multilevel inverter.

3. A pulse-width-amplitude modulation method for multilevel inverters, comprising the steps of:

varying carrier amplitudes for a quasi-Z-source cascade of multilevel inverters (qZS-CMI) between top and bottom amplitudes of three-phase modulating signals for a left inverter H-bridge leg and a right inverter H-bridge leg, respectively, the left and right inverter H-bridge legs each having an upper power switch and a lower power switch;

implementing boost control for a pulse-width-amplitude (PWAM) modulating signal if shoot-through references exceed predetermined minimum and maximum threshold values;

using the shoot-through references and the threshold values to determine a shoot-through duty ratio; and

for each of the power switches, determining a phase sector alternately presenting no switching action, shoot-through only, and shoot-through with active modulation, the phase sector determination depending on the shoot-through duty ratio.

4. The pulse-width-amplitude modulation method for multilevel inverters as recited in claim 3 , further comprising the steps of, for each of the inverters:

maintaining a fixed switching frequency of the inverter; and

modulating the pulse width of the PWAM modulating signal to use as a switching control signal to selectively turn the power switches of the inverter on and off.

5. The pulse-width-amplitude modulation method for a quasi-Z-source cascade multilevel inverter as recited in claim 3 , further comprising the steps of, for each of the inverters:

maintaining a fixed switching frequency of the inverter; and

modulating the pulse amplitude of the PWAM modulating signal to use as a switching control signal to selectively turn the power switches of the inverter on and off.

6. The pulse-width-amplitude modulation method for a quasi-Z-source cascade multilevel inverter as recited in claim 3 , wherein the multilevel inverter comprises a photovoltaic quasi-Z-source cascade multilevel inverter.

7. A grid-connected control method for quasi-Z-source cascade multilevel inverter-based photovoltaic power generator producing single-phase output, the generator having a cascade of photovoltaic (PV) array modules connected to a single-phase power grid, the method comprising the steps of:

for each quasi-Z-source cascade multilevel inverter (qZS-CMI) PV array module of the cascade, providing a phase lock of grid voltage, v g , as a first input to a proportional integral controller of the module;

for a first qZS-CMI PV array module of the cascade, providing a complex conjugate of a time derivative of grid current as a second input to the first PV array module's controller;

feeding an output of the first PV array module's controller to an inner grid-tie current loop to provide a reference value thereof and a total voltage loop modulation reference, v mt ;

for a second through last qZS-CMI PV array module of the cascade, providing that module's voltage loop as a second input to the corresponding module's controller;

for the second through a next to the last qZS-CMI PV array module of the cascade, summing an output of the corresponding PV array module's controller with the inner grid-tie current reference value to provide space vector modulation signals v m2 through v m(n-1) ;

for the last qZS-CMI PV array module of the cascade, summing an output of the last PV array module's controller with the grid voltage v g to provide a last space vector modulation signal v mn ;

for the first qZS-CMI PV array module of the cascade, subtracting a sum of the space vector modulation signals v m2 through v mn from the total voltage loop modulation reference V mt to provide a first space vector modulation signal v m1 ; and

applying the first through last space vector modulation signals v m1 , v m2 through v m(n-1) , and v mn to H-bridge switches of the respective qZS-CMI PV array modules, whereby shoot-through duty ratios of the H-bridge switches are controlled to extract maximum power from each qZS-CMI PV array module, transfer substantially all captured PV power to the power grid at unity power factor, assure a constant DC-link peak voltage for each qZS-CMI PV array module, and assure a balanced DC-link peak voltage among the qZS-CMI PV array modules of the cascade.