Multilevel cascade voltage source inverter with seperate DC sources

A multiple DC voltage source inverter for connecting to an AC power system, comprising:

• a. a plurality of full bridge inverters having a primary node and a secondary node, each of said full bridge inverters having a positive node and a negative node, each of said full bridge inverters having a voltage supporting device electrically connected in a parallel relationship between said positive node and said negative node;
• b. a plurality of cascade inverter phases, each of said cascade inverter phases having a plurality of said full bridge inverters, each of said cascade inverter phases having a consistent number of said full bridge inverters with respect to each phase, each of said full bridge inverters in each cascade inverter phase interconnected in a series relationship with said secondary node of one of said full bridge inverters connected to said primary node of another full bridge inverter, said series interconnection defining a first full bridge inverter and a last full bridge inverter, each of said phases having an input node at said primary node of said first full bridge inverter and an output node at said secondary node of said last full bridge inverter;
• c. a common node defined by the electrical interconnection of said output nodes of each of said cascade inverter phases;
• and
• d. a control means connected in an operable relationship with each of said full bridge inverters to emit a square wave signal for a prescribed period therefrom;
• whereby, a nearly sinusoidal voltage waveform approximation is generated by the controlled, alternate activation and deactivation of said full bridge inverters by said control means.

A method for inverting a plurality of DC voltage signals to approximate a sinsusiodal voltage waveform comprising the following steps:

• a. detecting the DC voltage levels of a plurality of DC voltage sources;
• b. averaging said DC voltage levels;
• c. comparing said average with a reference DC voltage;
• d. generating a first error signal from said comparison of said average with a reference DC voltage;
• e. comparing said average with said detected DC voltage levels;
• f. generating a second error signal from said comparison of said average with said detected DC voltage levels;
• g. generating a phase shift offset signal from said second error signal;
• h. generating an average phase shift signal from said first error signal;
• i. summing said phase shift offset signal and said average phase shift signal;
• j. detecting an AC line voltage having a period;
• k. generating a phase reference signal directly related to said period of said AC line voltage;
• l. generating a plurality of firing reference signals for a plurality of full bridge inverters using said phase reference signal and said sum of said phase shift offset signal and said average phase shift signal;
• m. determining a modulation index;
• n. providing a reference table for said modulation index;
• o. generating a plurality of firing angle signals for said plurality of full bridge inverters using said firing reference signal and said reference table;
• whereby, the alternate activation of a plurality of gate turn-off devices in said full bridge inverters may be controlled to construct an output voltage waveform having a sinusoidal approximation for use by an AC load.