Maximum power point tracking

1. A method of tracking a selected power point of a photovoltaic system, in which an operation point of the photovoltaic system is changed based on a change of operation point and a change of power generated by the photovoltaic system, the method comprising:

determining a current or voltage of the photovoltaic system;

determining a power of the photovoltaic system;

determining a change of power of the photovoltaic system with respect to a previous determined power;

changing the operation point of the photovoltaic system by changing a current or voltage reference of the system stepwise depending on the change of power and a direction of a previous change of the current or voltage reference;

storing determined values of current or voltage and the determined power (P(m)) of the photovoltaic system together with a time value (m);

reading from the stored values a previous value of determined power (P(m)) and its time value (m) having a current or voltage value equaling a present current or voltage value for determination of the change of power (ΔP) of the photovoltaic system; and

calculating the change of power from the current value of power (P(k)), previous value of power (P(k−1)), stored value of power (P(m)) having a current or voltage value corresponding to a present current or voltage, and the time value (m) of the stored value of power (P(m)).

2. The method of tracking a selected power point according to claim 1 , wherein the calculation of the change of power comprises:

calculating a difference between the current value of power (P(k)) and the previous value of power (P(k−1)) and subtracting a compensation term (P comp ) from the calculated difference, the compensation term being calculated by calculating a gradient (G) of change of power from a present operating point (k) and stored historic data having a same voltage or current value as the present voltage or current (m) as:

G

=

P

⁡

(

k

)

-

P

⁡

(

m

)

k

-

m

and multiplying the calculated gradient (G) by a time interval between the values of power for obtaining the compensation term as:

P comp =G *( k −( k− 1)).

3. The method of tracking a selected power point according to claim 1 , wherein if the stored values do not include the current or voltage value equaling the present current or voltage value, the method comprises:

changing the current or voltage reference in a direction opposite to a previous change of the current or voltage reference.

4. The method of tracking a selected power point according to claim 1 , comprising:

using the change of power (ΔP) for adapting a current or voltage step size.

5. The method of tracking a selected power point according to claim 1 , wherein the determination of the current or voltage of the photovoltaic system comprises:

measuring an output voltage or output current of a photovoltaic panel system of the photovoltaic system.

6. The method of tracking a selected power point according to claim 1 , wherein the determination of the current or voltage of a photovoltaic system comprises:

estimating an output voltage or output current of a photovoltaic panel system of the photovoltaic system.

7. The method of tracking a selected power point according to claim 1 , wherein the determination of the current or voltage of a photovoltaic system comprises:

using a reference value of the current or voltage.

8. The method of tracking a selected power point according to claim 1 , wherein the determination of the power of a photovoltaic system comprises:

determining power obtained from a panel system of the photovoltaic system.

9. The method of tracking a selected power point according to claim 1 , wherein the determination of the power of the photovoltaic system comprises:

determining power in a converter used in a photovoltaic system.

10. The method of tracking a selected power point according to claim 1 , wherein the determination of the power of the photovoltaic system comprises:

determining a power outputted by a photovoltaic system.

11. The method of tracking a selected power point according to claim 1 , comprising:

repeatedly determining a current or voltage of the photovoltaic system;

repeatedly determining a power of the photovoltaic system; and

repeatedly determining a change of power of the photovoltaic system with respect to a previous determined power.

12. An arrangement for tracking a selected power point of a photovoltaic system, in which arrangement an operation point of the photovoltaic system is changed based on a change of operation point and a change of power generated by a photovoltaic system, the arrangement comprising:

a processor configured to:

determine a current or voltage of a photovoltaic system;

determine a power of the photovoltaic system;

determine a change of power of the photovoltaic system with respect to a previous determined power;

change the operation point of the photovoltaic system by changing a current or voltage reference of the system stepwise depending on the change of power and a direction of a previous change of the current or voltage reference; and

a memory configured to:

store determined values of current or voltage, and determined power of the photovoltaic system together with a time value; and

wherein the processor is configured to determine the change of power (ΔP) of the photovoltaic system by:

reading from the stored values a previous value of determined power (P(m) and its time value (m) having a current or voltage value equaling a present current or voltage value; and

calculating the change of power from the current value of power (P(k)), previous value of power (P(k−1)), stored value of power (P(m)) having a current or voltage value corresponding to a present current or voltage, and the time value (m) of the stored value of power (P(m)).

13. The arrangement for tracking a selected power point according to claim 12 , wherein the calculation of the change of power comprises:

calculating a difference between the current value of power (P(k)) and the previous value of power (P(k−1)) and subtracting a compensation term (P comp ) from the calculated difference, the compensation term being calculated by calculating a gradient (G) of change of power from a present operating point (k) and stored historic data having a same voltage or current value as the present voltage or current (m) as:

G

=

P

⁡

(

k

)

-

P

⁡

(

m

)

k

-

m

and multiplying the calculated gradient (G) by a time interval between the values of power for obtaining the compensation term as

P comp =G *( k −( k− 1)).

14. The arrangement for tracking a selected power point according to claim 12 , wherein when the stored values do not include the current or voltage value equaling the present current or voltage value, the processor is configured for:

changing the current or voltage reference in a direction opposite to a previous change of the current or voltage reference.

15. The arrangement for tracking a selected power point according to claim 12 , wherein the processor is configured for:

using the change of power (ΔP) for adapting a current or voltage step size.

16. The arrangement for tracking a selected power point according to claim 12 , wherein the determination of the current or voltage of the photovoltaic system comprises:

measuring an output voltage or output current of a photovoltaic panel system of the photovoltaic system.

17. The arrangement for tracking a selected power point according to claim 12 , wherein the determination of the current or voltage of the photovoltaic system comprises:

estimating an output voltage or output current of a photovoltaic panel system of the photovoltaic system.

18. The arrangement for tracking a selected power point according to claim 12 , wherein the determination of the current or voltage of a photovoltaic system comprises:

using the reference value of the current or voltage.

19. The arrangement for tracking a selected power point according to claim 12 , wherein the determination of the power of a photovoltaic system comprises:

determining power obtained from a panel system of photovoltaic system.

20. The arrangement for tracking a selected power point according to claim 12 , wherein the determination of power of a photovoltaic system comprises:

determining power in a converter used in a photovoltaic system.