Solar photovoltaic power conditioning units
We describe a photovoltaic power conditioning unit comprising: both dc and ac power inputs; a dc link; at least one dc-to-dc converter coupled between dc input and dc link; and a dc-to-ac converter coupled between dc link and ac output. The dc-to-dc converter comprises: a transformer having input and output windings; an input dc-to-ac converter coupled between dc input and input winding; and an ac-to-dc converter coupled between output winding the dc link. The output winding has a winding tap between the first and second portions. The ac-to-dc converter comprises: first and second rectifiers, each connected to a respective first and second portion of the output winding, to the dc link and winding tap; and a series inductor connected to the winding tap. Rectifiers are connected to the winding tap of the output winding via the series inductor wherein the series inductor is shared between the first and second rectifiers.
1. A power conditioning unit for delivering power from a photovoltaic dc power source to an ac mains power supply, the power conditioning unit comprising:
a dc power input to receive dc power from a photovoltaic dc power source;
an ac power output for delivering ac power to the ac mains power supply;
a dc link coupled between the dc power input and the ac power output;
first and second dc-to-dc converters coupled between the dc power input and the dc link, wherein the first and second dc-to-dc converters are coupled to the dc power input; and
a controller operative to control whether one or both of the first and second parallel coupled dc-to-dc converters are used based on a sensed level of power on the dc power input,
wherein each of the first and second dc-to-dc converters comprises a transformer having a primary winding, a secondary winding including a winding tap, an output inductor winding, and a plurality of rectifiers, and
wherein the output inductor winding is connected between the winding tap and the plurality of rectifiers.
2. The power conditioning unit of claim 1 , wherein the output inductor winding comprises a winding on a core of the transformer.
3. The power conditioning unit of claim 2 , wherein the winding tap is between first and second portions of the secondary winding, wherein the first and second portions of the secondary winding of the transformer are windings on respective first and second legs of the core the transformer, and wherein the output inductor wiring is on a third leg between the first and second legs of the core of the transformer.
4. The power conditioning unit of claim 3 , wherein the output inductor winding further comprises windings on the first and second legs of the core, each wound in an opposite sense to a sense of windings of the primary winding.
5. The power conditioning unit of claim 1 , wherein each of the first and second dc-to-dc converters comprises an input dc-to-ac converter that has a set of power switching devices to switch the dc power to the primary winding of the transformer, the power conditioning unit further comprising at least one switching controller to control the set of power switching devices such that switching is synchronized between the input dc-to-ac converters.
6. The power conditioning unit of claim 5 , wherein the switching is synchronized such that a phase angle between switching of the corresponding set of power switching devices in the input dc-to-ac converters is substantially 90 degrees or 180 degrees.
7. The power conditioning unit of claim 1 , wherein the controller is configured to sense an output power of the power conditioning unit and to control, responsive to the sensed output power, whether one or both of the dc-to-dc converters operate to feed dc power from the dc input to the dc link.
8. The power condition unit of claim 7 , wherein the controller is configured to switch off one of the dc-to-dc converters by gradually reducing the dc power fed to the dc link by the dc-to-dc converter.
9. The power conditioning unit of claim 7 , wherein each of the first and second dc-to-dc converters comprises an input dc-to-ac converter that has a set of power switching devices to switch the dc power to the input winding of the transformer, the power conditioning unit further comprising at least one switching controller to control the power switching devices such that the switching is synchronized between the input dc-to-ac converters, and configured to switch off one of the dc-to-dc converters by gradually reducing the dc power fed to the dc link by the dc-to-dc converter to be switched off.
10. The power conditioning unit of claim 9 , wherein the controller is configured to switch off one of the dc-to-dc converters by controlling a duty cycle of a pulse width modulated waveform controlling switching of at least one of the power switching device.
11. The power conditioning unit of claim 9 , wherein the-controller is configured to switch off one of the dc-to-dc converters by controlling a switching frequency of at least one of the power switching devices to gradually move the switching frequency away from a resonant frequency of the dc-to-dc converter.
12. The power conditioning unit of claim 9 , wherein the controller further comprises a circuit configured to sense a level of ripple on a capacitor of the input dc-to-ac converter of one of the first and second dc-to-dc converters to sense an available power from the photovoltaic dc power source.
13. A method of operating a power conditioning unit, the method comprising:
for each of a pair of dc-to-dc power converters, converting dc power from a dc power source to ac power by using a transformer of the dc-to-dc power converter, wherein the transformer comprises a primary winding, a secondary winding having a winding tap, and an output inductor winding;
converting the ac power back to dc power for a dc link using a plurality of rectifiers, wherein the output inductor winding is connected between the winding tap and the plurality of rectifiers;
converting the dc power at the dc link to ac output power for an ac mains; and
controlling the pair of dc-to-dc power converters based on a power level of the dc power source or the ac output power.
14. The method of claim 13 , wherein the output inductor winding that is coupled to the winding tap of the transformer is part of the transformer.
15. The method of claim 13 , wherein the plurality of rectifiers comprises a first rectifier that is connected to a first portion of the secondary winding and a second rectifier that is connected to a second portion of the secondary winding.
16. The method of claim 13 , wherein controlling the pair of dc-to-dc power converters comprises switching between each of the pair of dc-to-dc power converters for converting dc power from the dc power source.
17. The method of claim 16 , wherein each of the pair of dc-to-dc power converters are switched in order to synchronize a phase of the ac power converted by the dc-to-dc power converter.
18. The method of claim 13 , wherein controlling the pair of dc-to-dc power converters comprises, for one of the pair of dc-to-dc power converters, gradually reducing dc power to the dc link in order to switch off the dc-to-dc power converter.
19. A power conditioning unit comprising:
a dc power input to receive dc power from a photovoltaic dc power source;
an ac power output for delivering ac power to an ac mains power supply;
a dc link coupled between the dc power input and the ac power output;
first and second dc-to-dc converters coupled in parallel between the dc power input and the dc link and configured to receive the dc power; and
a controller operative to control whether one or both of the first and second dc-to-dc converters are used based on a sensed level of power on the dc power input,
wherein:
each of the first and second dc-to-dc converters comprises a transformer having a primary winding, a secondary winding comprising first and second portions, and a winding tap between the first and second portions, an output inductor winding, a plurality of rectifiers, and an ac-to-dc converter coupled between the output inductor winding of the transformer and the dc link,
the output inductor winding is connected between the winding tap and the plurality of rectifiers, and
the plurality of rectifiers comprise first and second rectifiers, each of the first and second rectifiers connected to one of the first and second portions of the secondary winding, the output inductor winding, and the dc link.
20. The power conditioning unit of claim 19 , wherein the first and second portions of the secondary winding of the transformer are windings on respective first and second legs of a core of the transformer, and the output inductor wiring is on a third leg between the first and second legs of the core of the transformer.
21. The power conditioning unit of claim 20 , wherein the output inductor wiring further comprises windings wound on the first and second legs of the core, each wound in an opposite sense to a sense of windings of the primary winding.
22. The power conditioning unit of claim 19 , wherein each of the first and second dc-to-dc converters comprises an input dc-to-ac converter that has a set of power switching devices to switch the dc power to the primary winding of the transformer, the power conditioning unit further comprising at least one switching controller to control the set of power switching devices such that switching is synchronized between the input dc-to-ac converters.
23. The power conditioning unit of claim 22 , wherein the switching is synchronized such that a phase angle between switching of the corresponding set of power switching devices in the input dc-to-ac converters is substantially 90 degrees or 180 degrees.
24. The power conditioning unit of claim 19 , wherein the controller is configured to sense an output power of the power conditioning unit and to control, in response to the sensed output power, whether one or both of the first and second dc-to-dc converters operate to feed dc power from the dc input to the dc link.
25. The power condition unit of claim 24 , wherein the controller is configured to switch off one of the first and second dc-to-dc converters by gradually reducing the dc power fed to the dc link by the one of the first and second dc-to-dc converters to be switched off.
26. The power conditioning unit of claim 24 , wherein each of the first and second dc-to-dc converters comprises an input dc-to-ac converter that has a set of power switching devices to switch the dc power to the input winding of the transformer, the power conditioning unit further comprising at least one switching controller to control the power switching devices such that the switching is synchronized between the input dc-to-ac converters, and configured to switch off one of the dc-to-dc converters by gradually reducing the dc power fed to the dc link by the one of the dc-to-dc converters to be switched off.
27. The power conditioning unit of claim 26 , wherein the controller is configured to switch off one of the dc-to-dc converters by controlling a duty cycle of a pulse width modulated waveform controlling switching of at least one of the power switching device.
28. The power conditioning unit of claim 26 , wherein the-controller is configured to switch off one of the dc-to-dc converters by controlling a switching frequency of at least one of the power switching devices to gradually move the switching frequency away from a resonant frequency of the dc-to-dc converter.