Photovoltaic power conditioning units
We describe a photovoltaic (PV) panel system comprising a PV panel with multiple sub-strings of connected solar cells in combination with a power conditioning unit (microinverter). The power conditioning unit comprises a set of input power converters, one connected to each sub-string, and a common output power conversion stage, to provide power to an ac mains power supply output. Integration of the micro-inverter into the solar PV module in this way provides many advantages, including greater efficiency and reliability. Additionally, embodiments of the invention avoid the need for bypass diodes, a component with a high failure rate in PV panels, providing lower power loss and higher reliability.
1. A photovoltaic (PV) system comprising:
a PV module including a plurality of sub-strings of series connected solar cells arranged in loops; and
a plurality of series coupled conversion stages coupled to a DC link, each conversion stage coupled to a respective one of the plurality of sub-strings and including a DC output, one or more bypass diodes and a sub-string level Maximum Power Point Tracking (MPPT) circuit;
wherein each MPPT circuit includes a control circuit that adjusts a DC output voltage of one or more of the conversion stages to maintain a DC link voltage above a peak AC mains voltage.
2. The PV system of claim 1 , wherein each conversion stage of the plurality of conversion stages includes a voltage-increasing DC-to-DC power converter.
3. The PV system of claim 1 , wherein the one or more bypass diodes includes two diodes connected in parallel.
4. The PV system of claim 1 , wherein MPPT is performed by monitoring current and voltage for each sub-string of the plurality of sub-strings.
5. The PV system of claim 1 further comprising a sub-string sensing system and at least one input power converter controller coupled to the sub-string sensing system to control a level of power conversion of one or more of the plurality of conversion stages responsive to a voltage of one of the plurality of sub-strings.
6. A photovoltaic (PV) system for delivering power to an AC grid, the PV system comprising:
a PV panel comprising a plurality of sub-strings of series connected solar cells, each sub-string of the plurality of sub-strings having an electrical connection at either end;
a set of series connected input power converters, one coupled to each sub-string of the plurality of sub-strings, wherein each input power converter includes one or more bypass diodes, a Maximum Power Point Tracking (MPPT) circuit and an output coupled to a common DC link; and
a common output power conversion stage coupled to the common DC link and configured to provide power to the AC grid;
wherein each MPPT circuit includes a control circuit that adjusts a DC output voltage of one or more of the input power converters to maintain a voltage of the common DC link above a peak AC grid voltage.
7. The PV system of claim 6 , wherein each input power converter of the set of input power converters includes a voltage-increasing DC-to-DC converter.
8. The PV system of claim 6 , wherein the common output power conversion stage includes a DC-to-AC converter that converts DC voltage received from the common DC link to AC power delivered to the AC grid.
9. The PV system of claim 8 , wherein outputs of each of each input power converter of the set of input power converters are coupled to the common DC link.
10. The PV system of claim 6 , wherein the one or more bypass diodes includes two diodes connected in parallel.
11. The PV system of claim 6 , wherein MPPT is performed by monitoring current and voltage for each sub-string of the plurality of sub-strings.
12. The PV system of claim 6 further comprising a sub-string sensing system and at least one input power converter controller coupled to the sub-string sensing system to control a level of power conversion of one or more of the set of input power converters responsive to a voltage of one of the plurality of sub-strings.
13. A method of providing power to an AC grid from a photovoltaic (PV) panel, the PV panel comprising a plurality of sub-strings of series connected solar cells, the method comprising:
receiving, with a conversion stage, power from each sub-string of the plurality of sub-strings, wherein the conversion stage includes one or more bypass diodes and a sub-string level Maximum Power Point Tracking (MPPT) circuit;
combining power from a plurality of series connected conversion stages and conducting the combined power on a common DC bus;
adjusting a DC output voltage of one or more of the plurality of series connected conversion stages to maintain a voltage of the common DC bus above a peak AC grid voltage; and
converting, using a DC to AC converter, the combined power on the common DC bus to AC power for delivery to the AC grid.
14. The method of claim 13 , wherein the conversion stage includes a voltage-increasing DC-to-DC power converter.
15. The method of claim 13 , wherein DC outputs of each of the plurality of conversion stages are coupled to the common DC bus.
16. The method of claim 13 , wherein the one or more bypass diodes includes two diodes connected in parallel.
17. The method of claim 13 , wherein MPPT is performed by monitoring current and voltage for each sub-string of the plurality of sub-strings.
18. The method of claim 13 further comprising a sub-string sensing system and at least one input power converter controller coupled to the sub-string sensing system to control a level of power conversion of one or more of the plurality of conversion stages responsive to a voltage of one of the plurality of sub-strings.