Power conditioning unit

1. A photovoltaic power conditioning unit for delivering power from a dc photovoltaic power source to an ac mains power supply, the photovoltaic power conditioning unit comprising:

an input for receiving power from said dc power source;

an output for delivering ac power;

an non-electrolytic energy storage capacitor;

a dc-to-dc converter having an input connection coupled to said input and an output connection coupled to the energy storage capacitor;

a dc-to-ac converter having an input connection coupled to said energy storage capacitor and an output connection coupled to said output; and

a power injection control block configured to control an amount of power transferred from said dc-to-ac converter to said ac mains power supply based at least in part on an amplitude of a fluctuating sinusoidal component of a dc voltage on said energy storage capacitor.

2. A photovoltaic power conditioning unit according to claim 1 , wherein said dc-to-dc converter comprises:

at least one power device for transferring power from the input connection of the dc-to-dc converter to the output connection of the dc-to-dc converter;

a sensing circuit for sensing a voltage on said input connection; and

a driver circuit for driving said at least one power device responsive to said sensing to control said power transfer.

3. A photovoltaic power conditioning unit according to claim 2 , wherein the dc-to-dc converter further comprises a reference voltage generator, and wherein the driver circuit is configured to substantially minimise a difference between the reference voltage and the sensed voltage.

4. A photovoltaic power conditioning unit according to claim 1 , wherein said dc-to-ac converter comprises:

at least one power device for transferring power from the input connection of the dc-to-ac converter to the output connection of the dc-to-ac converter;

a low-pass filter coupled to said input connection;

a sensing circuit for sensing an output from the low-pass filter and comparing with a reference; and

a driver circuit for driving said at least one power device responsive to said sensing to control said power transfer.

5. A photovoltaic power conditioning unit according to claim 4 , wherein the driver circuit is configured to substantially minimise a difference between the output from the low-pass filter and the reference.

6. A photovoltaic power conditioning unit according to claim 4 , wherein the sensing circuit is configured to sense an output voltage from said low-pass filter and compare with a reference voltage.

7. A photovoltaic power conditioning unit according to claim 4 , wherein the sensing circuit is configured to periodically compare said output from the low-pass filter with said reference.

8. A photovoltaic power conditioning unit according to claim 7 , wherein the sensing circuit is configured to periodically compare said output from the low-pass filter with said reference in synchronisation with the ac output.

9. A photovoltaic power conditioning unit according to claim 1 , wherein said energy storage capacitor has a capacitance of less than 15 microfarads.

10. A photovoltaic power conditioning unit according to claim 1 , wherein said energy storage capacitor comprises a film-type capacitor.

11. A photovoltaic power conditioning unit comprising:

a non-electrolytic energy storage capacitor;

a dc-to-ac converter having an input connection coupled to said energy storage capacitor and an output connection coupled for delivering ac power to an ac mains power supply; and

a power injection control block to control said dc-to-ac converter, wherein said power injection control block is configured to control an amplitude of an ac current provided to said ac mains power supply such that an amount of power transferred to said ac mains power supply is dependent on a peak amplitude of a fluctuating sinusoidal component of a dc voltage on said energy storage capacitor.

12. A control method for controlling a photovoltaic power conditioning unit for delivering power from a dc photovoltaic power source into an ac electricity supply, the method comprising:

inputting dc power from a dc power source;

voltage conditioning said dc power from said dc power source using a dc-to-dc power conversion stage;

injecting power into the said ac electricity supply from a dc-to-ac power conversion stage;

coupling power from said dc-to-dc power conversion stage to said dc-to-ac power conversion stage via a dc link;

buffering energy from said dc power source to said electricity supply using a non-electrolytic dc link energy storage capacitor connected in parallel between an output of said dc-to-dc converter and an input of said dc-to-ac converter; and

controlling said dc-to-ac converter power conversion stage to control an amount of power transferred to said ac electricity supply based, at least in part, on a fluctuating sinusoidal component of the non-electrolytic dc link energy storage capacitor.

13. A control method as claimed in claim 12 comprising controlling said first and second power conversion stages using two control blocks, using a first control block controlling the first dc-to-dc power conversion stage for voltage conditioning from said dc power source to the energy storage capacitor for energy buffering, and using a second control block for controlling said second, dc-to-ac power conversion stage for controlling power injection into the ac electricity supply.

14. A control method as claimed in claim 13 , wherein said voltage conditioning comprises controlling said first control block for said first power conversion stage to maintain a voltage at input terminals of the dc power source constant, hence maintaining a constant flow of power from said dc power source into said non-electrolytic dc link energy storage capacitor.

15. A control method as claimed in claim 12 , further comprising maintaining an average voltage on said non-electrolytic dc link energy storage capacitor constant by adjusting the amplitude of current injection into said ac electricity power supply by controlling said second control block controlling said dc-to-ac power conversion stage.

16. A control method as claimed in claim 15 , further comprising controlling a sinusoidal shape of said current injection into said ac electricity supply by controlling said second control block controlling said dc-to-ac power conversion stage.

17. A control method as claimed in claim 15 , further comprising sensing a voltage across said non-electrolytic dc link energy storage capacitor; conditioning the resulting signal using a Low Pass Filter (LPF); and controlling said second control block controlling said dc-to-ac power conversion stage using said resulting signal.

18. A control method as claimed in claim 17 , further comprising comparing a result of said signal conditioning to a reference to generate an error signal, sampling said error signal at periodic intervals, and using said sampled error signal for controlling said second control block controlling said dc-to-ac power conversion stage.

19. A control method as claimed in claim 12 , further comprising controlling the transfer of power from the dc power source into the electricity supply while maintaining the voltage and power of the dc power source constant by controlling a sinusoidal current injection into the electricity supply, whilst allowing a sinusoidal fluctuation on the average voltage across said non-electrolytic dc link energy storage capacitor.

20. A method as claimed in claim 12 , the method further comprising controlling said first, dc-to-dc power conversion stage to regulate a voltage of said dc power source to control power extracted from said dc power source to said energy storage capacitor.