Determination of the load capability of a DC voltage source which is connectable to an electric power grid via an inverter and a grid disconnect switch

• US 20100127576A1
• Filed: 11/20/2009
• Published: 05/27/2010
• Est. Priority Date: 11/25/2008
• Status: Active Grant

• Current Assignee: SMA Solar Technology AG
• Original Assignee: SMA Solar Technology AG
• Inventors: Ahlborn, Alexander, Reichenbaecher, Wolfgang, Groote, Wilfried
• Granted Patent: US 8,362,658 B2
• Time in Patent Office: Days
• US Class Current: 307/151
• CPC Class Codes: H02J 3/38 Arrangements for parallely feeding a single network by two or more generators, converters or transformers
• CPC Class Codes: H02M 7/72 using discharge tubes with control electrode or semiconductor devices with control electrode

Abstract

A method for determining a load capability of a direct current (“DC”) voltage source connectable to an alternating current (“AC”) power grid having a predetermined frequency via an inverter and a grid disconnect switch includes controlling, while the grid disconnect switch is in an open position, the inverter so as to convert a DC input voltage to a test frequency different than the predetermined frequency of the AC power grid so as to increase a test load acting on the DC voltage source above an internal consumption of the inverter at the predetermined frequency. The DC input voltage is measured while the grid disconnect switch is in the open position.

Claims

1. A method for determining a load capability of a DC voltage source connectable to an AC power grid having a predetermined frequency via an inverter and a grid disconnect switch, comprising the steps of:

controlling, while the grid disconnect switch is in an open position, the inverter so as to convert a DC input voltage to a test frequency different than the predetermined frequency of the AC power grid so as to increase a test load acting on the DC voltage source above an internal consumption of the inverter at the predetermined frequency; and

measuring the DC input voltage while the grid disconnect switch is in the open position.

2. The method as recited in claim 1, wherein the controlling is performed so as to successively convert the DC input voltage to different test frequencies.

3. The method as recited in claim 2, wherein the controlling is performed so as to vary the test load acting on the DC voltage source in a stepped manner.

4. The method as recited in claim 1, wherein the test frequency is at least 10 percent lower or higher than the predetermined frequency.

5. The method as recited in claim 1, wherein the test frequency is at least 20 percent lower or higher than the predetermined frequency.

6. The method as recited in claim 1, wherein the test frequency is at least 40 percent lower or higher than the predetermined frequency.

7. The method as recited in claim 1, wherein the test frequency is at least 60 percent lower or higher than the predetermined frequency.

8. The method as recited in claim 1, wherein the test frequency is at least 100 percent lower or higher than the predetermined frequency.

9. The method as recited in claim 4, wherein the predetermined frequency is in a range from 50 to 60 Hz and the test frequency is in a range from 500 Hz to 50 percent of a clock frequency at which fast-clocked switches of the inverter are clocked.

10. The method as recited in claim 1, further comprising the step of tuning the test frequency to a frequency near a respective resonant frequency of at least one an EMC (electromagnetic compatible) filter coupled between the inverter and the grid disconnect switch and an RLC component of the inverter, wherein the frequency is different than the respective resonant frequency.

11. The method as recited in claim 10, wherein the inverter generates an AC current is via at least one of a frequency-dependent impedance of the EMC filter and an RLC component of the inverter.

12. The method as recited in claim 11, wherein the inverter is configured as a load resistance for the AC current.

13. The method as recited in claim 10, further comprising the step of determining the test load by measuring at least one of the AC current and an AC voltage across the EMC filter or the RLC component of the inverter.

14. The method as recited in claim 1, wherein the controlling includes adapting the test frequency to the inverter and an environment of the inverter using a learning algorithm.

15. The method as recited in claim 14, wherein the learning algorithm is carried out in steps during switching of the DC voltage source into the AC power grid via the inverter.

16. The method as recited in claim 14, wherein the learning algorithm uses a database of reference data.

17. The method as recited in claim 1, further comprising steps of:

determining whether the load capability exceeds a predetermined load capability for continuous operation of the inverter on the grid; and

closing the grid disconnect switch so as to switch the DC voltage source into the AC power grid via the inverter after it is determined that the load capability exceeds the predetermined load capability.

18. The method as recited in claim 1, wherein the measuring the DC input voltage is performed while the inverter is enabled for four-quadrant operation.

19. A device for connecting a DC voltage source to an AC power grid having a predetermined frequency, comprising:

an inverter and a grid disconnect switch configured to connect the DC voltage source to the AC power grid;

a measuring device configured to measure a DC input voltage of the inverter; and

a controller configured to control the inverter while the grid disconnect switch is in an open position so as to convert the DC input voltage to a test frequency different than the predetermined frequency of the AC power grid so as to increase a test load acting on the DC voltage source above an internal consumption of the inverter at the predetermined frequency.

20. The device as recited in claim 19, further comprising an EMC filter interconnected between the inverter and the grid disconnect switch.

21. A method for determining a load capability of a DC voltage source connectable to an AC power grid having a grid frequency via an inverter and a grid disconnect switch, comprising:

opening the grid disconnect switch;

operating the inverter at a test frequency different from the grid frequency, wherein the test frequency is selected such that the internal power consumption of the inverter is increased compared to the internal power consumption of the inverter at the grid frequency; and

closing the grid disconnect switch should a voltage provided by the DC voltage source exceeds a predetermined voltage threshold.

22. The method as recited in claim 21, further comprising synchronizing the inverter with the AC power grid before closing the grid disconnect switch.