Smart and scalable off-grid mini-inverters

• US 8,994,218 B2
• Filed: 06/11/2012
• Issued: 03/31/2015
• Est. Priority Date: 06/10/2011
• Status: Active Grant

• Current Assignee: CyboEnergy, Inc.
• Original Assignee: CyboEnergy, Inc.
• Inventors: Cheng, George Shu-Xing, Mulkey, Steven L., Chow, Andrew J.
• Primary Examiner(s): Deberadinis, Robert
• Application Number:
  US13/493,622
• Publication Number:
  US 20120313443A1
• Time in Patent Office: 1,023 Days
• Field of Search: 307/82
• US Class Current: 307/82
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/46 Controlling of the sharing of output between the generators, converters, or transformers
• CPC Class Codes: H02M 1/007 Plural converter units in cascade push-pull DC/DC converters with pre-regulator H02M3/3374; DC-AC converters following a DC-DC stage including a high frequency transformer H02M7/4807; DC-AC converters following a DC-DC conversion stage generating periodically varying voltages H02M7/4826
• CPC Class Codes: H02M 7/48 using discharge tubes with control electrode or semiconductor devices with control electrode
• CPC Class Codes: Y02B 10/10 Photovoltaic [PV]
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

A method and apparatus is disclosed for intelligently inverting DC power from DC sources such as photovoltaic (PV) solar modules to single-phase or three-phase AC power to supply power for off-grid applications. A number of regular or redundant off-grid Mini-Inverters with one, two, three, or multiple input channels in a mixed variety can easily connect to one, two, three, or multiple DC power sources such as solar PV modules, invert the DC power to AC power, and daisy chain together to generate and supply AC power to electrical devices that are not connected to the power grid including motors, pumps, fans, lights, appliances, and homes.

Claims

1. A system for providing AC power to an AC load from a plurality of individual DC power sources each having a DC power output port, comprising:

a) a plurality of power inverters, each of said power inverters having one DC power input port, an AC power input port, and an AC power output port;

b) said AC power output port of each inverter being connected in a daisy chain to the AC power input port of the next inverter, except for the AC power input port of the first inverter being left open, and the AC power output port of the last inverter being connected to the AC load;

c) each of the power inverters including;

i) a DC-DC boost converter arranged to convert the voltage of a DC power source to a higher DC voltage suitable for inversion;

ii) a DC-AC inverter connected to said DC-DC boost converter and arranged to invert the DC power to AC power;

iii) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

iv) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

v) a digital microcontroller connected to said DC-DC boost converter, DC-AC inverter, and load interface circuit, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converter, perform maximum power point tracking (MDPT), perform DC-AC inversion, monitor AC current and voltage for generated power amount and status, perform powerline communications, and perform logic controls such as AC powerline switching and isolation; and

vi) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline;

d) one of the power inverters further including;

i) a load detector connected to said internal AC powerline and external AC powerline, and arranged to detect the impedance of the connected AC load;

ii) said digital microcontroller further connected to the load detector and arranged to check the impedance of the AC load to determine if it is within predetermined specifications, initially energize the internal and external AC powerline, continually deliver AC power to the internal and external AC powerline to allow the other power inverters also connected on the same external AC powerline to synchronize the AC power being produced, continually check and determine whether the AC load is too large or too small for the power generation system to handle, and turn the power off and trigger an error signal if the load is too large or too small; and

iii) a line sensing circuit connected to said internal AC powerline and said microcontroller, and arranged to detect if there is AC power on the internal AC powerline prior to the startup of the inverter, and to monitor the internal AC powerline for over voltage, under voltage, over current, or under current conditions;

e) each of the other power inverters further including;

i) said digital microcontroller arranged to perform AC power synchronization;

ii) a line sensing circuit connected to said internal AC powerline and said microcontroller for detecting the phase and zero-crossing point of the incoming AC power from the external AC powerline; and

iii) a solid state switch connected to said internal AC powerline and external AC powerline, and arranged to disconnect said internal AC powerline from the external AC powerline during the non-generation time.

2. A system for providing AC power to an AC load from a plurality of individual DC power sources each having a DC power output port, comprising:

a) a plurality of power inverters, each of said power inverters having m DC power input ports, where m is an integer greater than or equal to two, an AC power input port, and an AC power output port;

b) said AC power output port of each inverter being connected in a daisy chain to the AC power input port of the next inverter, except for the AC power input port of the first inverter being left open, and the AC power output port of the last inverter being connected to the AC load;

c) each of the power inverters including;

i) m number of DC-DC boost converters arranged to convert the voltage of a DC power source to a higher DC voltage suitable for inversion;

ii) a DC power combiner connected to said m number of DC-DC boost converters for combining the DC output from all DC-DC boost converters and allowing said DC-DC boost converters to connect in parallel so that all DC currents are added together;

iii) a DC-AC inverter connected to said DC power combiner and arranged to invert the DC power to AC power;

iv) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

v) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

vi) a digital microcontroller connected to said DC-DC boost converters, DC-AC inverter, and load interface circuit, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converter, perform maximum power point tracking (MPPT), perform DC-AC inversion, monitor AC current and voltage for generated power amount and status, perform powerline communications, and perform logic controls such as AC powerline switching and isolation; and

vii) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline;

d) one of the power inverters further including;

i) a load detector connected to said internal AC powerline and external AC powerline, and arranged to detect the impedance of the connected AC load;

ii) said digital microcontroller further connected to the load detector and arranged to check the impedance of the AC load to determine if it is within predetermined specifications, initially energize the internal and external AC powerline, continually deliver AC power to the internal and external AC powerline to allow the other power inverters also connected on the same external AC powerline to synchronize the AC power being produced, continually check and determine whether the AC load is too large or too small for the power generation system to handle, and turn the power off and trigger an error signal if the load is too large or too small; and

iii) a line sensing circuit connected to said internal AC powerline and said microcontroller, and arranged to detect if there is AC power on the internal AC powerline prior to the startup of the inverter, and to monitor the internal AC powerline for over voltage, under voltage, over current, or under current conditions;

e) each of the other power inverters, further including;

i) said digital microcontroller arranged to perform AC power synchronization;

ii) a line sensing circuit connected to said internal AC powerline and said microcontroller for detecting the phase and zero-crossing point of the incoming AC power from the external AC powerline; and

iii) a solid state switch connected to said internal AC powerline and external AC powerline, and arranged to disconnect said internal AC powerline from the external AC powerline during the non-generation time.

3. The system of claim 2, in which the output of each of said power inverters is single-phase AC or three-phase AC.

4. The system of claim 2, in which said digital microcontroller includes Model-Free Adaptive (MFA) controllers which control the DC-DC boost converter, and MFA optimizers which provide maximum power point tracking (MPPT) to allow the power inverter to achieve optimal power production.

5. A DC-to-AC off-grid AC Master power inverter, comprising:

a) one DC power input port;

b) one AC power output port arranged to supply AC power to an AC load;

c) a DC-DC boost converter arranged to convert the voltage of a DC power source to a higher DC voltage suitable for inversion;

d) a DC-AC inverter connected to said DC-DC boost converter and arranged to invert the DC power to AC power;

e) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

f) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

g) a load detector connected to said internal AC powerline and external AC powerline, and arranged to detect the impedance of the connected AC load;

h) a digital microcontroller connected to said DC-DC boost converter, DC-AC inverter, load interface circuit, and load detector, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converter, perform maximum power point tracking (MPPT), perform DC-AC inversion, monitor AC current and voltage for generated power amount and status, perform powerline communications, check the impedance of the AC load to determine if it is within predetermined specifications, initially energize the internal and external AC powerline, continually deliver AC power to the internal and external AC powerline to allow the other power inverters also connected on the same external powerline to synchronize the AC power being produced, continually check and determine whether the AC load is too large or too small for the power generation system to handle, and turn the power off and trigger an error signal if the load is too large or too small;

i) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline; and

j) a line sensing circuit connected to said internal AC powerline and said microcontroller, and arranged to detect if there is AC power on the internal AC powerline prior to the startup of the inverter, and to monitor the internal AC powerline for over voltage, under voltage, over current, or under current conditions.

6. A multiple channel off-grid AC Master DC-to-AC power inverter, comprising:

a) at least two DC power input ports;

b) one AC power output port arranged to supply AC power to an AC load;

c) for each DC power input port, a DC-DC boost converter arranged to convert the voltage of a DC power source to a higher DC voltage suitable for inversion;

d) a DC power combiner connected to said DC-DC boost converters for combining the DC output from all DC-DC boost converters and allowing the said DC-DC boost converters to connect in parallel so that all DC currents are added together;

e) a DC-AC inverter connected to said DC power combiner and arranged to invert the DC power to AC power;

f) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

g) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

h) a load detector connected to said internal AC powerline and external AC powerline, and arranged to detect the impedance of the connected AC load;

i) a digital microcontroller connected to said DC-DC boost converter, DC-AC inverter, load interface circuit, and load detector, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converter, perform maximum power point tracking (MDPT), perform DC-AC inversion, monitor AC current and voltage for generated power amount and status, perform powerline communications, check the impedance of the AC load to determine if it is within predetermined specifications, initially energize the internal and external AC powerline, continually deliver AC power to the internal and external AC powerline to allow the other power inverters also connected on the same external powerline to synchronize the AC power being produced, continually check and determine whether the AC load is too large or too small for the power generation system to handle, and turn the power off and trigger an error signal if the load is too large or too small;

j) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline;

k) a line sensing circuit connected to said internal AC powerline and said microcontroller, and arranged to detect if there is AC power on the internal AC powerline prior to the startup of the inverter, and to monitor the internal AC powerline for over voltage, under voltage, over current, or under current conditions; and

l) a power supply connected to said DC power combiner and arranged to supply DC power to the electronic components of said power inverter.

7. The inverter of claim 6, in which the output of said inverter is single-phase AC or three-phase AC.

8. The inverter of claim 6, in which said digital microcontroller includes Model-Free Adaptive (MFA) controllers which control the DC-DC boost converter, and MFA optimizers which provide maximum power point tracking (MPPT) to allow the power inverter to achieve optimal power production.

9. The inverter of claim 6, in which the said digital microcontroller is programmed with a main program to iteratively:

a) turn on and off the inverter'"'"'s generation mechanism based on the DC power source input and conditions of the inverter and AC powerline;

b) calculate the inverter'"'"'s power statistics such as the amount of power generated during a predetermined period of time;

c) perform diagnostics for the inverter'"'"'s status and operation;

d) run redundancy routine for every input channel;

e) set the inverter'"'"'s unit address;

f) perform powerline communications; and

g) respond to queries from data gathering or acquisition devices to report the power statistics.

10. The inverter of claim 6, in which said digital microcontroller is further programmed with a generation and synchronization subroutine to iteratively:

a) check if the inverter is an AC Master;

b) if a) is positive, check if it is generating power;

c) if b) is negative, check if AC is present;

d) if c) is positive, send an error signal through an inverter status LED to alert the fact that the off-grid AC Master inverter is connected to a powerline that has AC power;

e) if c) is negative, check if the AC load passes the impedance requirement tests based on predetermined specifications;

f) if e) is positive, generate AC power and digital Sinewave signals based on an internal clock;

g) if e) is negative, send an error signal through the status LED;

h) if a) and b) are positive, keep generating power and the digital Sinewave signals;

i) if a) is negative, check if AC is present;

j) if i) is negative, exit the generation and synchronization subroutine; and

k) if i) is positive, get AC zero-crossing time, synchronize internal clock with AC zero-crossing time, get present AC phase, and generate AC power based on the AC zero-crossing time and phase.

11. A multiple channel regular off-grid DC-to-AC power inverter, comprising:

a) at least two DC power input ports;

b) one AC power output port arranged to supply AC power to an AC load;

c) for each DC power input port, a DC-DC boost converter arranged to convert the voltage of a DC power source to a higher DC voltage suitable for inversion;

d) a DC power combiner connected to said DC-DC boost converters for combining the DC output from all DC-DC boost converters and allowing the said DC-DC boost converters to connect in parallel so that all DC currents are added together;

e) a DC-AC inverter connected to said DC power combiner and arranged to invert the DC power to AC power;

f) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

g) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

h) a digital microcontroller connected to said DC-DC boost converters, DC-AC inverter, load interface circuit, line sensing circuit, and load detector, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converter, perform maximum power point tracking (MPPT), perform DC-AC inversion and AC power synchronization, monitor AC current and voltage for generated power amount and status, perform powerline communications, and perform logic controls such as AC powerline switching and isolation;

i) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline;

j) a line sensing circuit connected to said internal AC powerline and said microcontroller for detecting the phase and zero-crossing point of the incoming AC power from the external AC powerline;

k) a solid state switch connected to said internal AC powerline and external AC powerline, and arranged to disconnect said internal AC powerline from the external AC powerline during the non-generation time; and

l) a power supply connected to said DC power combiner and arranged to supply DC power to the electronic components of said power inverter.

12. A scalable DC to AC power inversion system for providing AC power to an AC load from a plurality of individual DC power sources each having a DC power output port, comprising:

a) a plurality of power inverters, each of said power inverters having at least one DC power input port, an AC power input port, and an AC power output port;

b) said AC power output port of each inverter being connected in a daisy chain to the AC power input port of the next inverter, except for the AC power input port of the first inverter being left open, and the AC power output port of the last inverter being connected to the AC load; and

c) whereby said system is incrementally scalable by adding or subtracting DC power sources and daisy-chained inverters.

13. The system of claim 12, in which the output of each of said power inverters is single-phase AC or three-phase AC.

14. The system of claim 12, wherein each of the said power inverters comprises:

a) at least two DC power input ports;

b) one AC power output port arranged to supply AC power to an AC load;

c) for each DC power input port, a DC-DC boost converter arranged to convert the voltage of a DC power source to a higher DC voltage suitable for inversion;

d) a DC power combiner connected to said DC-DC boost converters for combining the DC output from all DC-DC boost converters and allowing the said DC-DC boost converters to connect in parallel so that all DC currents are added together;

e) a DC-AC inverter connected to said DC power combiner and arranged to invert the DC power to AC power;

f) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

g) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

h) a digital microcontroller connected to said DC-DC boost converters, DC-AC inverter, and load interface circuit, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converter, perform maximum power point tracking (MPPT), perform DC-AC inversion, monitor AC current and voltage for generated power amount and status, perform powerline communications, and perform logic controls such as AC powerline switching and isolation;

i) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline; and

j) a power supply connected to said DC power combiner and arranged to supply DC power to the electronic components of said power inverter.

15. The system of claim 14, wherein one of the said power inverters further comprises:

a) a load detector connected to said internal AC powerline and external AC powerline, and arranged to detect the impedance of the connected AC load;

b) said digital microcontroller further connected to the load detector and arranged to check the impedance of the AC load to determine if it is within predetermined specifications, initially energize the internal and external AC powerline, continually deliver AC power to the internal and external AC powerline to allow the other power inverters also connected on the same external AC powerline to synchronize the AC power being produced, continually check and determine whether the AC load is too large or too small for the power generation system to handle, and turn the power off and trigger an error signal if the load is too large or too small; and

c) a line sensing circuit connected to said internal AC powerline and said microcontroller, and arranged to detect if there is AC power on the internal AC powerline prior to the startup of the inverter, and to monitor the internal AC powerline for over voltage, under voltage, over current, or under current conditions.

16. A method of making a DC to AC power conversion system incrementally scalable, comprising:

a) providing a plurality of DC power sources and a plurality of DC to AC power inverters, said inverters each having an AC input port, an AC output port, and at least one DC input port;

b) connecting at least one of said DC power sources, respectively, to at least one of said DC input ports; and

c) providing AC power to an AC load.

17. The method of claim 16, further comprising:

a) daisy-chaining at least two of said inverters, said AC power output port of each inverter being connected in a daisy chain to the AC power input port of the next inverter, except for the AC power input port of the first inverter being left open, and the AC power output port of the last inverter being connected to the AC load; and

b) producing a total AC power that is the summation of the AC power supplied by each said inverter.

18. The method of claim 16, in which the output of each of said power inverters is single-phase AC or three-phase AC.

19. The method of claim 16, wherein each of the said power inverters comprises:

a) at least two DC power input ports;

b) one AC power output port arranged to supply AC power to an AC load;

c) for each DC power input port, a DC-DC boost converter arranged to convert the voltage of a DC power source to a higher DC voltage suitable for inversion;

d) a DC power combiner connected to said DC-DC boost converters for combining the DC output from all DC-DC boost converters and allowing the said DC-DC boost converters to connect in parallel so that all DC currents are added together;

e) a DC-AC inverter connected to said DC power combiner and arranged to invert the DC power to AC power;

f) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

g) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

h) a digital microcontroller connected to said DC-DC boost converters, DC-AC inverter, and load interface circuit, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converter, perform maximum power point tracking (MPPT), perform DC-AC inversion, monitor AC current and voltage for generated power amount and status, perform powerline communications, and perform logic controls such as AC powerline switching and isolation;

i) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline; and

j) a power supply connected to said DC power combiner and arranged to supply DC power to the electronic components of said power inverter.

20. The method of claim 19, wherein one of the said power inverters further comprises:

a) a load detector connected to said internal AC powerline and external AC powerline, and arranged to detect the impedance of the connected AC load;

b) said digital microcontroller further connected to the load detector and arranged to check the impedance of the AC load to determine if it is within predetermined specifications, initially energize the internal and external AC powerline, continually deliver AC power to the internal and external AC powerline to allow the other power inverters also connected on the same external AC powerline to synchronize the AC power being produced, continually check and determine whether the AC load is too large or too small for the power generation system to handle, and turn the power off and trigger an error signal if the load is too large or too small; and

c) a line sensing circuit connected to said internal AC powerline and said microcontroller, and arranged to detect if there is AC power on the internal AC powerline prior to the startup of the inverter, and to monitor the internal AC powerline for over voltage, under voltage, over current, or under current conditions.

21. A system for providing AC power to an AC load from a plurality of individual DC power sources each having a DC power output port, comprising:

a) a plurality of power inverters, each of said power inverters being connected to m DC power sources, where m is an integer greater than or equal to two, and having an AC power input port and an AC power output port;

b) said AC power output port of each inverter being connected in a daisy chain to the AC power input port of the next inverter, except for the AC power input port of the first inverter being left open, and the AC power output port of the last inverter being connected to an AC load;

c) each of the power inverters including;

i) m main DC-DC boost converters, each arranged to convert the voltage of a corresponding DC power source to a higher DC voltage suitable for inversion;

ii) m backup DC-DC boost converters, each arranged to convert the voltage of said corresponding DC power source to a higher DC voltage suitable for inversion;

iii) m DC input channel selectors, each constructed and arranged to connect its corresponding main DC-DC boost converter to said corresponding DC power source when the corresponding main DC-DC boost converter is working and connect the corresponding backup DC-DC boost converter to the DC power source when the corresponding main DC-DC boost converter is not working;

iv) a DC power combiner connected to said main DC-DC boost converters and said backup DC-DC boost converters;

v) a DC-AC inverter connected to said DC power combiner and arranged to invert the DC power to AC power;

vi) an internal AC powerline that allows the generated AC power to be sent to the AC load through an external AC powerline;

vii) a load interface circuit connected to said DC-AC inverter and to said internal AC powerline, said load interface circuit being arranged to filter high-frequency components out of the said DC-AC inverter'"'"'s AC output;

viii) a digital microcontroller connected to said main DC-DC boost converters, backup DC-DC boost converters, DC input channel selectors, DC-AC inverter, and load interface circuit, said microcontroller arranged to monitor the DC boost voltage, control the DC-DC boost converters, perform maximum power point tracking (MPPT), perform DC-AC inversion, monitor AC current and voltage for generated power amount and status, perform powerline communications, perform logic controls such as AC powerline switching and isolation, perform redundancy functions;

ix) a powerline modem connected to said microcontroller and said internal AC powerline through an interface circuitry arranged to establish a 2-way digital signal communication between the digital microcontroller and the outside world through the external AC powerline; and

x) a power supply connected to said DC power combiner and arranged to supply DC power to the electronic components of said power inverter;

d) one of the power inverters further including;

i) a load detector connected to said internal AC powerline and external AC powerline, and arranged to detect the impedance of the connected AC load;

ii) said digital microcontroller further connected to the load detector and arranged to check the impedance of the AC load to determine if it is within predetermined specifications, initially energize the internal and external AC powerline, continually deliver AC power to the internal and external AC powerline to allow the other power inverters also connected on the same external AC powerline to synchronize the AC power being produced, continually check and determine whether the AC load is too large or too small for the power generation system to handle, and turn the power off and trigger an error signal if the load is too large or too small; and

iii) a line sensing circuit connected to said internal AC powerline and said microcontroller, and arranged to detect if there is AC power on the internal AC powerline prior to the startup of the inverter, and to monitor the internal AC powerline for over voltage, under voltage, over current, or under current conditions;

e) each of the other power inverters further including;

i) said digital microcontroller arranged to perform AC power synchronization;

ii) a line sensing circuit connected to said internal AC powerline and said microcontroller for detecting the phase and zero-crossing point of the incoming AC power from the external AC powerline; and

iii) a solid state switch connected to said internal AC powerline and external AC powerline, and arranged to disconnect said internal AC powerline from the external AC powerline during the non-generation time.

22. The system of claim 21, in which the output of each of said power inverters is single-phase AC or three-phase AC.