Multilevel cascade voltage source inverter with seperate DC sources

• US 5,642,275 A
• Filed: 09/14/1995
• Issued: 06/24/1997
• Est. Priority Date: 09/14/1995
• Status: Expired

• Current Assignee: Lockheed Martin Energy Systems, Inc. (AES Corporation), The University Of Tennessee Research Corporation (University of Tennessee)
• Original Assignee: Lockheed Martin Integrated Systems Incorporated (Martin Marietta Corporation)
• Inventors: Lai, Jih-Sheng, Peng, Fang Zheng
• Primary Examiner(s): Hecker, Stuart N.
• Application Number:
  US08/527,995
• Time in Patent Office: 649 Days
• Field of Search: 363/58, 363/59, 363/129, 363/132, 363/136, 363/137, 323/271, 323/282
• US Class Current: 363/137
• CPC Class Codes: H02J 3/1857 wherein such bridge converter is a multilevel converter
• CPC Class Codes: H02M 7/4835 comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
• CPC Class Codes: H02M 7/49 Combination of the output voltage waveforms of a plurality of converters
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]
• CPC Class Codes: Y02E 40/20 Active power filtering [APF]

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.

Claims

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, a nearly sinusoidal voltage waveform approximation is generated by the controlled, alternate activation and deactivation of said full bridge inverters by said control means.

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 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.

9. 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.

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 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 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.