Systems and methods for increasing output current quality, output power, and reliability of grid-interactive inverters

• US 10,651,724 B2
• Filed: 10/26/2018
• Issued: 05/12/2020
• Est. Priority Date: 07/11/2011
• Status: Active Grant

• Current Assignee: Generac Power Systems Incorporated (Generac Holdings Incorporated)
• Original Assignee: Chilicon Power, LLC (Generac Holdings Incorporated)
• Inventors: Jones, Christopher Richard, Kral, Alexandre Rudolph
• Primary Examiner(s): Pham, Emily P
• Application Number:
  US16/172,523
• Publication Number:
  US 20190207506A1
• Time in Patent Office: 564 Days
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 2300/26 involving maximum power point tracking control for photovoltaic sources
• CPC Class Codes: H02J 3/38 Arrangements for parallely feeding a single network by two or more generators, converters or transformers
• 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 1/12 Arrangements for reducing harmonics from ac input or output
• CPC Class Codes: H02M 1/44 Circuits or arrangements for compensating for electromagnetic interference in converters or inverters
• CPC Class Codes: H02M 7/537 using semiconductor devices only, e.g. single switched pulse inverters
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers
• CPC Class Codes: Y02E 40/30 Reactive power compensation

Abstract

Various enhancements to grid-interactive inverters in accordance with embodiments of the invention are disclosed. One embodiment includes input terminals configured to receive a direct current, output terminals configured to provide an alternating output current to the utility grid, a controller, an output current sensor, and a DC-AC inverter stage comprising a plurality of switches controlled by control signals generated by the controller. In addition, the controller is configured to: generate control signals that cause the switches in the DC-AC inverter stage to switch a direct current in a bidirectional manner; measure the alternating output current; perform frequency decomposition of the output current; and generate control signals that cause the switches in the DC-AC inverter stage to switch current in a way that the magnitude of a plurality of unwanted current components is subtracted from the resulting output current.

Claims

1. A grid-interactive inverter configured to receive direct current and to provide alternating current to a grid, comprising:

input terminals configured to receive a direct current;

output terminals configured to provide an alternating output current to the grid;

a controller;

at least one sensor capable of measuring alternating output current provided to the grid;

a DC-AC inverter comprising a plurality of switches controlled by control signals generated by the controller;

wherein the controller is capable of generating the control signals that cause the plurality of the switches in the DC-AC inverter to switch a direct current in a bidirectional manner across output terminals of the grid-interactive inverter to provide an alternating output current to the grid;

wherein the controller is capable of dynamically generating the control signals that cause the plurality of the switches in the DC-AC inverter to switch current in a bidirectional manner across the output terminals in response to the measured alternating output current to provide reactive current to the grid.

2. The grid-interactive inverter of claim 1, wherein the DC-AC inverter is an H-bridge inverter comprising four MOSFETs configured to receive PWM control signals from the controller, where the PWM control signals configure the four MOSFETs to switch current in a bidirectional manner through an output filter connected to the output terminals of the grid-interactive inverter.

3. The grid-interactive inverter of claim 1, wherein the controller is configured to perform frequency decomposition of the measured alternating output current by performing a Fast Fourier Transform of the measured output current.

4. The grid-interactive inverter of claim 1, further comprising:

a startup power check circuit connected to the input terminals of the grid-interactive converter and comprising at least one resistor connected in series with a switch between the input terminals; and

an input voltage sensor configured to measure the voltage across the input terminals;

wherein the controller is configured to determine a minimum input power that can be supplied by a photovoltaic module connected to the input terminals by measuring a voltage across the input terminals of the grid-interactive inverter when the switch is configured to permit current to flow through the at least one resistor connected in series between the input terminals.

5. The grid-interactive inverter of claim 1, wherein the controller is configured to sample an output current sensor and turn off the plurality of the switches in the DC-AC inverter in response to a predetermined number of the samples within a grid cycle having magnitudes exceeding an overcurrent threshold.

6. The grid interactive inverter of claim 1, wherein:

the at least one sensor comprises an output current sensor;

the controller is capable of measuring alternating output current provided to the grid using the output current sensor;

the controller comprises a current control loop capable of performing frequency decomposition of the alternating output current in order to generate feedback to the controller; and

the controller is capable of using the feedback to generate the control signals that cause the plurality of the switches in the DC-AC inverter to switch current in a manner that results in the alternating output current waveform meeting a desired leading or lagging power factor.

7. The grid interactive inverter of claim 1, wherein the controller is capable of dynamically generating the control signals that cause the plurality of the switches in the DC-AC inverter to achieve four-quadrant operation of the alternating output current and voltage waveforms.

8. The grid-interactive inverter of claim 1, further comprising:

a DC-DC converter comprising a plurality of switches connected to a primary coil of a transformer, and a full bridge rectifier connected to an output of a secondary coil of the transformer, where the plurality of switches in the DC-DC converter are controlled by the control signals generated by the controller; and

a DC link capacitor configured to link the DC-DC converter and the DC-AC inverter;

wherein the controller is configured to generate control signals that cause the plurality of the switches in the DC-DC converter to switch the direct current received via the input terminals in a bidirectional manner across the primary coil of the transformer;

wherein the transformer is wound so that the output voltage of the secondary coil is greater than the input voltage of the primary coil; and

wherein the DC link capacity or provides a direct current to the DC-AC inverter.

9. The grid-interactive inverter of claim 8, wherein the DC link capacitor is a polypropylene metal film capacitor.

10. The grid-interactive inverter of claim 8, further comprising:

an output voltage sensor;

wherein the controller is capable of making a power measurement of the alternating output current provided to the utility grid using the output current sensor and the output voltage sensor; and

wherein the controller is configured to select a new switching frequency and duty cycle for the plurality of the switches in the DC-DC converter based upon at least the measured power of the alternating current provided to the utility grid.

11. The grid interactive inverter of claim 10, further comprising:

an input current sensor; and

an input voltage sensor;

wherein the controller is capable of making a power measurement of the direct current received via the input terminals of the grid-interactive inverter; and

wherein the controller is further configured to select the new switching frequency and duty cycle for the plurality of the switches in the DC-DC converter based upon the ratio of the measured power of the direct current received via the input terminals to the measured power of the alternating output current provided to the utility grid.

12. The grid-interactive inverter of claim 10, wherein the controller is further configured to select the new switching frequency and duty cycle for the plurality of the switches of the DC-DC converter based upon the measured power of the alternating output current provided to the grid using a look up table.

13. The grid-interactive inverter of claim 11, wherein the controller is further configured to select the new switching frequency and duty cycle for the plurality of the switches of the DC-DC converter by perturbing at least one of the frequency and duty cycle of the plurality of the switches in the DC-DC converter and making a power measurement of a resulting alternating output current to determine whether an increase in the ratio of the measured power of the direct current received via the input terminals to the measured power of the alternating output current provided to the grid results.

14. The grid-interactive inverter of claim 8, wherein the controller is configured to perform a maximum power point tracker process.

15. The grid-interactive inverter of claim 14, wherein the controller is configured to disable the maximum power point tracker process and to control the voltage on the DC link capacitor to establish a predetermined voltage for the direct current input.

16. The grid-interactive inverter of claim 8, further comprising an autonomous grid inrush current limiter circuit comprising:

at least one resistor connected in series with the DC link capacitor;

a switch in parallel with the at least one resistor connected in series with the DC link capacitor; and

a capacitor charging circuit comprising a capacitor and at least one resistor connected in series with the capacitor, where the capacitor charging circuit is connected to the output terminals of the grid-interactive inverter and configured to charge the capacitor in the capacitor charging circuit using current drawn from the grid;

wherein the time constant of the capacitor and the at least one resistor in the charging circuit is longer than the time constant of the DC link capacitor and the at least one resistor in series with DC link capacitor; and

wherein the ON state of the switch connected in parallel with the at least one resistor connected in series with the DC link capacitor is controlled by the voltage level of the capacitor in the capacitor charging circuit such that the current flows through the switch when the voltage level of the capacitor exceeds a predetermined threshold.

17. The grid-interactive inverter of claim 16, wherein the switch in parallel with the at least one resistor connected in series with the DC link capacitor is a low ON state resistance MOSFET switch.

18. The grid interactive inverter of claim 1, further comprising:

a communication module that utilizes the grid cycle to perform symbol time alignment;

wherein the communication module is configured to transmit data using ON/OFF keying;

wherein the communication module is configured to encode transmitted data using an error correction code; and

wherein the communication module is configured to adapt the number of symbol transmissions per grid cycle.

19. The grid interactive inverter of claim 18, wherein the error correction code is a Golay code.

20. The grid interactive inverter of claim 1, wherein:

the controller is capable of generating the control signals that cause the plurality of the switches in the DC-AC inverter to switch current bidirectionally across the output terminals to achieve a desired power factor; and

the controller is capable of dynamically generating the control signals to offset local reactive load demands by changing the desired power factor in response to the measurement of the alternating output current that is provided to the grid.

21. The grid interactive inverter of claim 20, wherein the desired power factor (PF_desired) is determined by the ratio of the fundamental real (I_fund) and imaginary (Q_fund) amplitudes of the alternating output current such that:

PF_desired=cos(Angle(I_fund/Q_fund)).