Distributed substring architecture for maximum power point tracking of energy sources
View Patent ↗A photovoltaic array system includes multiple strings of series coupled photovoltaic modules to provide current at a voltage dependent on the number of modules in each string and their operating efficiency. Each string is coupled to a DC-to-DC converter to convert the current from each string for output to a DC bus. An inverter is coupled to DC bus to convert the current from the strings of series coupled photovoltaic modules to AC current at a grid-compatible voltage.
1. A photovoltaic array system comprising:
a first string of series-coupled photovoltaic modules, said first string being coupled to a first DC-to-DC converter, wherein the first DC-to-DC converter converts a DC output of the first string for output to a DC bus having DC rails;
a second string of series-coupled photovoltaic modules, said second string being coupled to a second DC-to-DC converter, wherein the second DC-to-DC converter converts a DC output of the second string for output to the DC bus in parallel to the output of the first converter; and
an inverter coupled to the DC bus, said inverter converting the DC outputs of the first and second DC-to-DC converters to an AC output, wherein the DC-to-DC converters are adapted to maintain bus stability by a concerted autonomous current response of the DC-to-DC converters and inverter by controlling a duty cycle of a pulse width modulated signal to a DC-to-DC converter switch responsive to sensed voltage of the DC rails, wherein the voltage of the DC rails is sensed by the DC-to-DC converter, and each DC-to-DC converter autonomously derates its output linearly once the sensed voltage passes a pre-determined threshold, with the aggregate effect of controlling the DC bus voltage in cooperation with other units coupled to the DC rails.
2. The photovoltaic array system of claim 1 wherein the DC-to-DC converters provide maximum powerpoint tracking.
3. The photovoltaic array of claim 2 wherein the maximum powerpoint tracking for each string is independent of other strings.
4. The photovoltaic array of claim 3 wherein each dc to dc converter provides a set collection voltage of approximately 380 V to the DC bus.
5. The photovoltaic array system of claim 1 wherein at least one string comprises between 4 and 10 modules.
6. The photovoltaic array system of claim 1 wherein the number of modules in a string is optimized to provide a voltage output suitable for efficient conversion to a regulated voltage.
7. The photovoltaic array system of claim 1 wherein at least one string includes a number of modules such that a voltage of the string provided to its corresponding dc to de converter is approximately 240V.
8. The photovoltaic array system of claim 1 wherein the action of each DC-to-DC converter is independent of the other DC-to-DC converters.
9. The photovoltaic array system of claim 1 wherein the DC bus comprises two conductors, the two conductors operating at substantially equal and opposite voltage with respect to neutral.
10. The array of claim 1 , wherein the DC bus includes power line carrier communication between the inverter and the DC-to-DC converters.
11. The system of claim 10 wherein the power line carrier communication includes Frequency Shift Keying encoding for communications.
12. The system of claim 10 wherein the communications are referenced to earth around.
13. The photovoltaic array of claim 12 wherein the DC-to-DC converters regulate the DC bus voltage such that inverter does not need an input stage.
14. The photovoltaic array of claim 1 and further comprising a switch in each DC-to-DC converter to controllably disconnect from the corresponding photovoltaic modules.
15. The photovoltaic array of claim 1 wherein each DC-to-DC converter is adapted to provide a regulated DC voltage to the DC bus.
16. The photovoltaic array of claim 15 wherein a DC-to-DC converter modulates its output to limit the voltage of the DC bus.
17. A method comprising:
receiving DC current on DC rails from multiple sets of series coupled photovoltaic modules, the current being at a voltage dependent on the number of modules in each set;
independently converting the current from each set to a selected DC set collection voltage;
providing the current from each set at the set collection voltage to a set collection line; and
providing the currents from each set via the set collection line to an inverter to convert the currents to an AC current, wherein the current from each set at the set collection voltage is autonomously modified by each set and the AC current from the inverter is autonomously modified to maintain bus stability, with the aggregate effect of controlling the DC bus voltage by controlling a duty cycle of a pulse width modulated signal to a DC-to-DC converter switch responsive to DC-to-DC converter sensed voltage of the DC rails, wherein each DC-to-DC converter autonomously derates its output linearly once the sensed voltage passes a pre-determined threshold in cooperation with other units coupled to the DC rails.
18. The method of claim 17 and further comprising:
receiving DC current from at least one other source of energy;
independently converting the current from the other source of energy to the selected DC set collection voltage; and
providing the converted current from the other source of energy to the set collection line.
19. The method of claim 17 and further comprising providing maximum powerpoint tracking while converting the current.
20. A photovoltaic array system comprising:
a first string of series-coupled photovoltaic modules, said first string being coupled to a first DC-to-DC converter, wherein the first DC-to-DC converter converts a DC output of the first string for output to a DC bus having DC rails;
a second string of series-coupled photovoltaic modules, said second string being coupled to a second DC-to-DC converter, wherein the second DC-to-DC converter converts a DC output of the second string for output to the DC bus in parallel to the output of the first converter; and
an inverter coupled to the DC bus, said inverter converting the DC outputs of the first and second DC-to-DC converters to an AC output, wherein the DC-to-DC converters are adapted to maintain bus stability by a concerted autonomous current response of the DC-to-DC converters and inverter, with the aggregate effect of controlling the DC bus voltage by controlling a duty cycle of a pulse width modulated signal to a DC-to-DC converter switch responsive to sensed voltage of the DC rails, wherein the voltage of the DC rails is sensed by the DC-to-DC converter, and each DC-to-DC converter autonomously Berates its output linearly once the sensed voltage passes a pre-determined threshold, in cooperation with other units coupled to the DC rails wherein each DC-to-DC converter observe the DC bus voltage and limits its output power to maintain selected limits on the DC bus voltage.