Multilevel cascade voltage source inverter with seperate DC sources

Abstract

A multilevel cascade voltage source inverter having separate DC sources is described herein. This inverter is applicable to high voltage, high power applications such as flexible AC transmission systems (FACTS) including static VAR generation (SVG), power line conditioning, series compensation, phase shifting and voltage balancing and fuel cell and photovoltaic utility interface systems. The M-level inverter consists of at least one phase wherein each phase has a plurality of full bridge inverters equipped with an independent DC source. This inverter develops a near sinusoidal approximation voltage waveform with only one switching per cycle as the number of levels, M, is increased. The inverter may have either single-phase or multi-phase embodiments connected in either wye or delta configurations.

What is claimed is:  
     
       1. 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. at least one cascade inverter phase, each cascade inverter phase having a plurality of said full bridge inverters, each cascade inverter phase 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 phase 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 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,  detect a period and a reference signal associated with the AC power system and to alternate activation and deactivation of each of said full bridge inverters in response to the reference signal to create a nearly sinusoidal voltage waveform approximation is generated by the controlled, alternate activation and deactivation of said full bridge inverters by said control means  having substantially the same period as the AC power system and having a desired phase shift defined with respect to the reference signal.  
     
     
       2. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  1  having three cascade inverter phases. 
     
     
       3. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  2  having a plurality of phase connectors, one of said phase connectors electrically connected between said input node of the first of said cascade inverter phases and said output node of the third of said cascade inverter phases, another of said phase connectors electrically connected between said input node of the third of said cascade inverter phases and said output node of the second of said cascade inverter phases, another of said phase connectors electrically connected between said input node of the second of said cascade inverter phases and said output node of the first of said cascade inverter phases. 
     
     
       4. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  3  further comprising each of said full bridge inverters having a first switching pair and a second switching pair, each of said switching pairs having a plurality of switching means for controllably regulating electrical current flow, each of said switching means having a first end and a second end, said first switching pair having a plurality of switching means electrically connected at said first end at said positive node of said full bridge inverter, said second end of one of said switching means of said first switching pair electrically connected to said primary node, said second end of another of said switching means of said first switching pair electrically connected to said secondary node, said second switching pair having a plurality of switching means electrically connected at said second ends at said negative node of said full bridge inverter, said first end of one of said switching means of said second switching pair electrically connected to said primary node, said first end of another of said switching means of said second switching pair electrically connected to said secondary node. 
     
     
       5. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  4  wherein said switching means comprises a gate turn-off device and an anti-parallel device connected in parallel and oppositely biased with respect to one another. 
     
     
       6. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  5  wherein said gate turn-off device is a component selected from the group consisting of a gate turn-off thyristor, an insulated gate bipolar transistor, a power MOSFET, a MOSFET controlled thyristor, a bipolar junction transistor, a static induction transistor, a static induction thyristor and a MOSFET turn-off thyristor. 
     
     
       7. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  5 wherein said anti-parallel device is a diode. 
     
     
       8. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  1  wherein each of said voltage supporting devices is a component selected from the group consisting of capacitors, fuel cells, photovoltaic cells and biomass ceils  cells. 
     
     
       9. A multiple DC voltage source inverter for connecting to an AC power system having a plurality of phases, 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 corresponding to one of the phases of the AC power system and 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,  cascade inverter phase to detect a period and a phase reference signal associated with a corresponding phase of the AC power system and to alternate activation and deactivation of each of the full bridge inverters of the cascade inverter phase in response to the phase reference signal to create a nearly sinusoidal voltage waveform approximation is generated by the controlled, alternate activation and deactivation of said full bridge inverters by said control means  having substantially the same period as the corresponding phase of the AC power system and having a desired phase shift defined with respect to the phase reference signal. 
     
     
       10. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  9  further comprising each of said full bridge inverters having a first switching pair and a second switching pair, each of said switching pairs having a plurality of switching means for controllably regulating electrical current flow, each of said switching means having a first end and a second end, said first switching pair having a plurality of switching means electrically connected at said first end at said positive node of said full bridge inverter, said second end of one of said switching means of said first switching pair electrically connected to said primary node, said second end of another of said switching means of said first switching pair electrically connected to said secondary node, said second switching pair having a plurality of switching means electrically connected at said second ends at said negative node of said full bridge inverter, said first end of one of said switching means of said second switching pair electrically connected to said primary node, said first end of another of said switching means of said second switching pair electrically connected to said secondary node. 
     
     
       11. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  10  wherein said switching means comprises a gate turn-off device and an anti-parallel device connected in parallel and oppositely biased with respect to one another. 
     
     
       12. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  11  wherein said gate turn-off device is a component selected from the group consisting of a gate turn-off thyristor, an insulated gate bipolar transistor, a power MOSFET, a MOSFET controlled thyristor, a bipolar junction transistor, a static induction transistor, a static induction thyristor and a MOSFET turn-off thyristor. 
     
     
       13. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  11 wherein said anti-parallel device is a diode. 
     
     
       14. A multiple DC voltage source inverter for connecting to an AC power system as described in claim  9  wherein each of said voltage supporting devices is a component selected from the group consisting of capacitors, fuel cells, photovoltaic cells and biomass cells. 
     
     
       15. A multiple DC voltage source inverter for connecting to a AC power system as described in claim  9  having three cascade inverter phases. 
     
     
       16. A method for inverting a plurality of DC voltage signals to approximate a sinusiodal  sinusoidal 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. 
     
     
       17. The multiple DC voltage source inverter of claim  1 , wherein: 
       
         the phase shift is selected to generate a desired level of positive or negative reactive power delivered to the AC power system while generating sufficient real power to offset losses incurred within the full bridge inverters. 
       
     
     
       18. The multiple DC voltage source inverter of claim  1 , further comprising a smoothing inductor connected in series between the cascade inverter phase and the AC power system. 
     
     
       19. The multiple DC voltage source inverter of claim  1 , wherein: 
       
         the phase shift is selected to perform a flexible AC transmission operation selected from the group including static VAR generation, power line conditioning, series compensation, phase shifting, voltage balancing, and generator interfacing. 
       
     
     
       20. The multiple DC voltage source inverter of claim  2 , wherein the AC power system includes three phases corresponding to the three cascade inverter phases, further comprising three smoothing inductors, one of the smoothing inductors connected in series between each cascade inverter phase and a corresponding phase of the AC power system. 
     
     
       21. The multiple DC voltage source inverter of claim  10 , wherein: 
       
         the phase shift for each cascade inverter phase is selected to generate a desired level of positive or negative reactive power delivered to the corresponding phase of the AC power system while generating sufficient real power to offset losses incurred within the cascade inverter phase. 
       
     
     
       22. The multiple DC voltage source inverter of claim  10 , further comprising a smoothing inductor connected in series between each of the cascade inverter phases and a corresponding phase of the AC power system. 
     
     
       23. The multiple DC voltage source inverter of claim  10 , wherein: 
       
         the phase shift for each cascade inverter phase is selected to perform a flexible AC transmission operation selected from the group including static VAR generation, power line conditioning, series compensation, phase shifting, voltage balancing, and generator interfacing. 
       
     
     
       24. A multiple DC voltage source inverter for connecting to an AC power system, comprising: 
       
         at least one cascade inverter phase including a plurality of full bridge inverters connected in a series relationship;  
       
       
         a control means connected in an operable relationship with each of said full bridge inverters to detect a period and a reference signal associated with the AC power system and to alternate activation and deactivation of each of said full bridge inverters in response to the reference signal to create a nearly sinusoidal voltage waveform approximation having substantially the same period as the AC power system and having a desired phase shift defined with respect to the phase reference signal; and  
       
       
         a smoothing inductor connected in series between the cascade inverter phase and the AC power system. 
       
     
     
       25. The multiple DC voltage source inverter of claim  24 , wherein the control means further comprises: 
       
         a first control loop for controlling the power flow to the cascade inverter phase; and  
       
         a second feed - back control loop for offsetting the power flow to each of the full bridge inverters of the cascade inverter phase.   
     
     
       26. The multiple DC voltage source inverter of claim  24 , wherein the control means further comprises: 
       
         a switching pattern table containing switching timing data for generating the nearly sinusoidal voltage waveform approximation in response to a reference output voltage signal and a desired phase angle;  
       
       
         means for calculating the reference output voltage signal;  
       
       
         a phase detector for determining the reference signal; and  
       
       
         means for determining the desired phase angle in response to the reference signal and a feedback signal produced by the first and second control loops. 
       
     
     
       27. The multiple DC voltage source inverter of claim  26 , wherein: 
       
         the AC power system includes three phases;  
       
       
         the cascade inverter includes three phases, one cascade inverter phase corresponding to each phase of the AC power system;  
       
       
         the cascade inverter includes three smoothing inductors, one smoothing inductor connected in series between each cascade inverter phase and each phase of the AC power system; and  
       
       
         the control means includes for each cascade inverter phase,  
       
       
         a first control loop for controlling the power flow to the cascade inverter phase,  
       
         a second feed - back control loop for offsetting the power flow to each of the full bridge inverters of the cascade inverter phase,    
       
         a switching pattern table containing switching timing data for generating the nearly sinusoidal voltage waveform approximation in response to a reference output voltage signal and a desired phase angle for the cascade inverter phase,  
       
       
         means for calculating the reference output voltage signal for the cascade inverter phase,  
       
       
         a phase detector for determining a phase reference signal associated with a corresponding phase of the AC system, and  
       
       
         means for determining the desired phase angle for the cascade inverter phase in response to the phase reference signal and a feedback signal produced by the first and second control loops for the cascade inverter phase. 
       
     
     
       28. The multiple DC voltage source inverter of claim  27 , wherein: 
       
         each of said full bridge inverters includes a primary node and a secondary node, each of said full bridge inverters includes a positive node and a negative node, each of said full bridge inverters includes a voltage supporting device electrically connected in a parallel relationship between said positive node and said negative node; and  
       
       
         each cascade inverter phase includes a plurality of said full bridge inverters, each cascade inverter phase 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 phase 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. 
       
     
     
       29. The multiple DC voltage source inverter of claim  28 , wherein: 
       
         the phase shift for each cascade inverter phase is selected to generate a desired level of positive or negative reactive power delivered to the corresponding phase of the AC power system while generating sufficient real power to offset losses incurred within the cascade inverter phase. 
       
     
     
       30. The multiple DC voltage source inverter of claim  28 , wherein: 
       
         the phase shift for each cascade inverter phase is selected to perform a flexible AC transmission operation selected from the group including static VAR generation, power line conditioning, series compensation, phase shifting, voltage balancing, and generator interfacing. 
       
     
     
       31. The multiple DC voltage source inverter of claim  30  having a plurality of phase connectors, one of said phase connectors electrically connected between said input node of the first of said cascade inverter phases and said output node of the third of said cascade inverter phases, another of said phase connectors electrically connected between said input node of the third of said cascade inverter phases and said output node of the second of said cascade inverter phases, another of said phase connectors electrically connected between said input node of the second of said cascade inverter phases and said output node of the first of said cascade inverter phases. 
     
     
       32. The multiple DC voltage source inverter of claim  30 , further comprising each of said full bridge inverters having a first switching pair and a second switching pair, each of said switching pairs having a plurality of switching means for controllably regulating electrical current flow, each of said switching means having a first end and a second end, said first switching pair having a plurality of switching means electrically connected at said first end at said positive node of said full bridge inverter, said second end of one of said switching means of said first switching pair electrically connected to said primary node, said second end of another of said switching means of said first switching pair electrically connected to said secondary node, said second switching pair having a plurality of switching means electrically connected at said second ends at said negative node of said full bridge inverter, said first end of one of said switching means of said second switching pair electrically connected to said primary node, said first end of another of said switching means of said second switching pair electrically connected to said secondary node. 
     
     
       33. The multiple DC voltage source inverter of claim  32 , wherein said switching means comprises a gate turn- off device and an anti - parallel device connected in parallel and oppositely biased with respect to one another.   
     
     
       34. The multiple DC voltage source inverter of claim  33 , wherein said gate turn- off device is a component selected from the group consisting of a gate turn - off thyristor, an insulated gate bipolar transistor, a power MOSFET, a MOSFET controlled thyristor, a bipolar junction transistor, a static induction transistor, a static induction thyristor and a MOSFET turn - off thyristor.   
     
     
       35. The multiple DC voltage source inverter of claim  34 , wherein said anti- parallel device is a diode.   
     
     
       36. The multiple DC voltage source inverter of claim  35 , wherein each of said voltage supporting devices is a component selected from the group consisting of capacitors, fuel cells, photovoltaic cells and biomass cells.