Method and device for recognising faults in a photovoltaic (PV) generator

Abstract

A method for detecting a potential-induced degradation (PID) of PV modules of a PV installation includes operating a PV generator at a maximum power point (MPP), at a first generator voltage (U.sub.1) and first generator current (I.sub.1), and at a second generator voltage (U.sub.2) and second generator current (I.sub.2), where a first power (P.sub.1) at the first generator voltage (U.sub.1) is in a predefined first ratio V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1, with the power (P.sub.MPP) at the maximum power point (MPP) of the PV generator, and where a second power (P.sub.2) at the second generator voltage (U.sub.2) is in a predefined second ratio V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1) of the PV generator, and where a quantity Y that characterizes a progress of the potential-induced degradation (PID) is determined from the values of the voltages (U.sub.1, U.sub.2) and/or the currents (I.sub.1, I.sub.2).

Claims

1. A method for detecting a potential-induced degradation (PID) of PV modules of a PV installation, having a measurement pass, comprising: operating a PV generator of the PV installation at a maximum power point (MPP) with values of generator voltage (U.sub.MPP) and generator current (I.sub.MPP) associated with the maximum power point (MPP), operating the PV generator of the PV installation at a first generator voltage (U.sub.1) and detection of a first generator current (I.sub.1) associated with the first generator voltage (U.sub.1), operating the PV generator of the PV installation additionally at a second generator voltage (U.sub.2) and detection of a second generator current (I.sub.2) associated with the second generator voltage (U.sub.2), operating the PV generator at a third generator voltage (U.sub.3), and detecting a third generator current (I.sub.3) associated with the third generator voltage (U.sub.3), wherein the first generator voltage (U.sub.1) dictates that a first power (P.sub.1), with P.sub.1=U.sub.1*I.sub.1, of the PV generator at the first generator voltage (U.sub.1) is in a predefined first ratio (V.sub.1), with V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1, with the power (P.sub.MPP) at the maximum power point (MPP) of the PV generator, and wherein the second generator voltage (U.sub.2) dictates that a second power (P.sub.2), with P.sub.2=U.sub.2*I.sub.2, of the PV generator at the second generator voltage (U.sub.2) is in a predefined second ratio (V.sub.2), with V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1) of the PV generator, and wherein the third generator voltage (U.sub.3) dictates that a third power (P.sub.3), with P.sub.3=U.sub.3*I.sub.3, of the PV generator at the third generator voltage (U.sub.3) is in a predefined third ratio (V.sub.3), with V.sub.3=P.sub.3/P.sub.1 and V.sub.3<1, with the first power (P.sub.1) of the PV generator, and a relationship in the form (U.sub.3<U.sub.1<U.sub.2) or a relationship in the form (U.sub.2<U.sub.1<U.sub.3) applies to the generator voltages (U.sub.1, U.sub.2, U.sub.3) in accordance with their values, wherein a characteristic quantity Y that characterizes a progress of the potential-induced degradation is determined from the values of the first, the second and the third generator voltage (U.sub.1, U.sub.2, U.sub.3) and/or the first, the second and the third generator current (I.sub.1, I.sub.2, I.sub.3), wherein during a measurement pass a first approach and, with staggered timing from the first approach, a further approach are effected for one of the generator voltages (U.sub.1, U.sub.2, U.sub.3), wherein the further approach to the respective generator voltage (U.sub.1, U.sub.2, U.sub.3) also results in a further generator current (I.sub.1,2, I.sub.2,2, I.sub.3,2) being detected, and wherein values of the further generator current (I.sub.1,2, I.sub.2,2, I.sub.3,2) are compared with applicable values of the generator current (I.sub.1, I.sub.2, I.sub.3) of the first approach to the respective generator voltage (U.sub.1, U.sub.2, U.sub.3), and wherein an applicable measurement pass is used to determine the characteristic quantity Y only when an absolute value of a difference between the generator current (I.sub.1, I.sub.2, I.sub.3) and the further generator current (I.sub.1,2, I.sub.2,2, I.sub.3,2) is below a predefined threshold value ΔI.

2. The method as claimed in claim 1, wherein the third power (P.sub.3) of the PV generator at the third generator voltage (U.sub.3) is equal to the second power (P.sub.2) of the PV generator at the second generator voltage (U.sub.2).

3. The method as claimed in claim 1, wherein the first approach to one of the generator voltages (U.sub.1, U.sub.2, U.sub.3) is effected at a beginning of the measurement pass, and the further approach to the respective generator voltage (U.sub.1, U.sub.2, U.sub.3) is effected at an end of the measurement pass.

4. The method of claim 1, wherein the first generator voltage (U.sub.1) denotes a maximum power point of the PV generator (MPP).

5. The method of claim 1, wherein the characteristic quantity Y is determined from a first parameter (W.sub.1) that takes into consideration a relative voltage width ΔU=U.sub.2−U.sub.3 around an operating point at the power P.sub.1 within a PU graph.

6. The method of claim 1, wherein the characteristic quantity Y is determined from a second parameter (W.sub.2) that takes into consideration a discrete-point fill factor (FF) within an IU graph.

7. The method as claimed in claim 1, wherein the characteristic quantity Y is determined from a third parameter (W.sub.3) that takes into consideration a difference between the first generator current (I.sub.1) and a point on an imaginary straight connecting line between the points (U.sub.2, I.sub.2) and (U.sub.3, I.sub.3) within an IU graph at the location of the first generator voltage (U.sub.1).

8. The method as claimed in claim 1, wherein the characteristic quantity Y is determined from a fourth parameter (W.sub.4) that takes into consideration a first current difference (I.sub.1−I.sub.2), a second current difference (I.sub.3−I.sub.1) or a ratio of the first and second current differences between the generator currents (I.sub.1, I.sub.2, I.sub.3) as per (I.sub.1−I.sub.2)/(I.sub.3−I.sub.1) at the respective generator voltages (U.sub.1, U.sub.2, U.sub.3) within an IU graph.

9. The method as claimed in claim 1, wherein the characteristic quantity Y is determined from a fifth parameter (W.sub.5) that takes into consideration a first voltage difference (U.sub.2−U.sub.1),a second voltage difference (U.sub.1−U.sub.3) or a ratio of the first and second voltage differences between the generator voltages (U.sub.1, U.sub.2, U.sub.3) as per (U.sub.1−U.sub.3)/(U.sub.2−U.sub.1) within an IU graph.

10. The method as claimed in claim 1, wherein the characteristic quantity Y is determined from a combination of at least two parameters, selected from a group comprising a first (W.sub.1), a second (W.sub.2), a third (W.sub.3), a fourth (W.sub.4), a fifth (W.sub.5) and a sixth (W.sub.6) parameter; wherein the first parameter (W.sub.1) takes into consideration a relative voltage width ΔU=U.sub.2−U.sub.3 round the operating point at the power P.sub.1 within a PU graph; and wherein the second parameter (W.sub.2) takes into consideration a discrete-point fill factor (FF) within an IU graph; and wherein the third parameter (W.sub.3) takes into consideration a difference between the first generator current (I.sub.1) and a point on an imaginary straight connecting line between the points (U.sub.2, I.sub.2) and (U.sub.3, I.sub.3) within the IU graph at the location of the first generator voltage (U.sub.1); and wherein the fourth parameter (W.sub.4) takes into consideration a first current difference (I.sub.1−I.sub.2), a second current difference (I.sub.3−I.sub.1) or a ratio of the first and second current differences between the generator currents (I.sub.1, I.sub.2, I.sub.3) as per (I.sub.1−I.sub.2)/(I.sub.3−I.sub.1) at the respective generator voltages (U.sub.1, U.sub.2, U.sub.3) within the IU graph; and wherein the fifth parameter (W.sub.5) takes into consideration a first voltage difference (U.sub.2−U.sub.1), a second voltage difference (U.sub.1−U.sub.3) or a ratio of the first and second voltage differences between the generator voltages (U.sub.1, U.sub.2, U.sub.3) as per (U.sub.1−U.sub.3)/(U.sub.2−U.sub.1) within the IU graph; and wherein the sixth parameter (W.sub.6) takes into consideration a second gradient m.sub.2=(I.sub.1−I.sub.3)/(U.sub.1−U.sub.3), a first gradient m.sub.1=(I.sub.2−I.sub.1)/(U.sub.2−U.sub.1) a ratio of the first and second gradients m.sub.1/m.sub.2 within the IU graph.

11. The method as claimed in claim 10, wherein the determination of the characteristic quantity Y involves the at least two parameters being weighted differently.

12. The method as claimed in claim 1, wherein in addition to the determination of the characteristic quantity Y a check is performed to determine whether a change in a value of the characteristic quantity Y relative to values of the characteristic quantity Y from preceding measurements is attributable to a change in a parallel resistance R.sub.par that characterizes the potential-induced degradation (PID) of the PV modules, wherein the check is performed taking into consideration at least one parameter from the first generator voltage (U.sub.1), the second generator voltage (U.sub.2), the third generator voltage (U.sub.3) and a no-load voltage U.sub.0 of the PV generator.

13. The method as claimed in claim 1, wherein a start of the measurement pass is effected under time control and/or under event control.

14. The method as claimed in claim 1, further comprising generating a warning signal when the characteristic quantity Y is outside a predefined tolerance range.

15. A method for detecting a potential-induced degradation (PID) of PV modules of a PV installation, having a measurement pass, comprising: operating a PV generator of the PV installation at a maximum power point (MPP) with values of generator voltage (U.sub.MPP) and generator current (I.sub.MPP) associated with the maximum power point (MPP), operating the PV generator of the PV installation at a first generator voltage (U.sub.1) and detection of a first generator current (I.sub.1) associated with the first generator voltage (U.sub.1), operating the PV generator of the PV installation additionally at a second generator voltage (U.sub.2) and detection of a second generator current (I.sub.2) associated with the second generator voltage (U.sub.2), operating the PV generator of the PV installation additionally at a third generator voltage (U.sub.3), and detecting a third generator current (I.sub.3) associated with the third generator voltage (U.sub.3), wherein the first generator voltage (U.sub.1) dictates that a first power (P.sub.1), with P.sub.1=U.sub.1*I.sub.1, of the PV generator at the first generator voltage (U.sub.1) is in a predefined first ratio (V.sub.1), with V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1 , with the power (PMPP) at the maximum power point (MPP) of the PV generator, and wherein the second generator voltage (U.sub.2) dictates that a second power (P.sub.2), with P.sub.2=U.sub.2*I.sub.2, of the PV generator at the second generator voltage (U.sub.2) is in a predefined second ratio (V.sub.2), with V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1) of the PV generator, wherein the third generator voltage (U.sub.3) dictates that a third power (P.sub.3), with P.sub.3=U.sub.3*I.sub.3, of the PV generator at the third generator voltage (U.sub.3) is in a predefined third ratio (V.sub.3), with V.sub.3=P.sub.3/P.sub.1 and V.sub.3<1, with the first power (P.sub.1) of the PV generator, and a relationship in the form (U.sub.3<U.sub.1<U.sub.2) or a relationship in the form (U.sub.2<U.sub.1<U.sub.3) applies to the generator voltages (U.sub.1, U.sub.2, U.sub.3) in accordance with their values, and wherein a characteristic quantity Y that characterizes a progress of the potential-induced degradation (PID) is determined from a sixth parameter (W.sub.6) that takes into consideration a second gradient m.sub.2=(I.sub.1−I.sub.3)/(U.sub.1−U.sub.3), a first gradient m.sub.1=(I.sub.2−I.sub.1)/(U.sub.2−U.sub.1), a ratio of the first and second gradients m.sub.1/m.sub.2 within an IU graph.

16. A method for detecting a potential-induced degradation (PID) of PV modules of a PV installation, having a measurement pass, comprising: operating a PV generator of the PV installation at a maximum power point (MPP) with values of generator voltage (U.sub.MPP) and generator current (I.sub.MPP) associated with the maximum power point (MPP), operating the PV generator of the PV installation at a first generator voltage (U.sub.1) and detection of a first generator current (I.sub.1) associated with the first generator voltage (U.sub.1), operating the PV generator of the PV installation additionally at a second generator voltage (U.sub.2) and detection of a second generator current (I.sub.2) associated with the second generator voltage (U.sub.2), wherein the first generator voltage (U.sub.1) dictates that a first power (P.sub.1), with P.sub.1=U.sub.1*I.sub.1, of the PV generator at the first generator voltage (U.sub.1) is in a predefined first ratio (V.sub.1), with V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1, with the power (P.sub.MPP) at the maximum power point (MPP) of the PV generator, wherein the second generator voltage (U.sub.2) dictates that a second power (P.sub.2), with P.sub.2=U.sub.2*I.sub.2, of the PV generator at the second generator voltage (U.sub.2) is in a predefined second ratio (V.sub.2), with V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1) of the PV generator, wherein a characteristic quantity Y that characterizes a progress of the potential-induced degradation (PID) is determined from the values of the first and second generator voltages (U.sub.1, U.sub.2) and/or the first and second generator currents (I.sub.1, I.sub.2), and shifting a generator potential (DC+, DC−) of the PV installation relative to a ground potential (PE) when the characteristic quantity Y is outside a predefined tolerance range.

17. A photovoltaic (PV) inverter, suitable for detecting a potential-induced degradation (PID) of PV modules of a PV installation, comprising: a DC input terminal configured to connect to a PV generator, an AC output terminal configured to connect the PV inverter to a power supply system, a DC/AC converter circuit configured to convert an input-side DC voltage into an AC voltage, a control circuit, connected to the DC/AC converter circuit, configured to deliver a predefined flow of power via the DC input terminal of the PV inverter, a current sensor configured to detect a generator current (I.sub.PV) at the DC input, a voltage sensor configured to detect a generator voltage (U.sub.PV) at the DC input, an evaluation circuit connected to the current sensor and the voltage sensor, wherein the PV inverter is configured to: operate a PV generator of the PV installation at a maximum power point (MPP) with values of generator voltage (U.sub.MPP) and generator current (I.sub.MPP) associated with the maximum power point (MPP), operate the PV generator of the PV installation at a first generator voltage (U.sub.1) and detection of a first generator current (I.sub.1) associated with the first generator voltage (U.sub.1), operate the PV generator of the PV installation additionally at a second generator voltage (U.sub.2) and detection of a second generator current (I.sub.2) associated with the second generator voltage (U.sub.2), wherein the first generator voltage (U.sub.1) dictates that a first power (P.sub.1), with P.sub.1=U.sub.1*I.sub.1, of the PV generator at the first generator voltage (U.sub.1) is in a predefined first ratio (V.sub.1), with V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1, with the power (PMPP) at the maximum power point (MPP) of the PV generator, wherein the second generator voltage (U.sub.2) dictates that a second power (P.sub.2), with P.sub.2=U.sub.2*I.sub.2, of the PV generator at the second generator voltage (U.sub.2) is in a predefined second ratio (V.sub.2), with V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1 ) of the PV generator, wherein a characteristic quantity Y that characterizes a progress of the potential-induced degradation (PID) is determined from the values of the first and second generator voltages (U.sub.1, U.sub.2) and/or the first and second generator currents (I.sub.1, I.sub.2), and wherein the PV inverter has a biasing unit configured to shift a generator potential (DC+, DC−) relative to a ground potential (PE) when the characteristic quantity Y is outside a predefined tolerance range.

18. The PV inverter as claimed in claim 17, wherein the PV inverter is configured to generate a warning signal when the characteristic quantity Y is outside a predefined tolerance range.

19. A photovoltaic (PV) installation, comprising: a PV generator, a biasing unit configured to shift a generator potential (DC+, DC−) relative to a ground potential (PE), and a PV inverter, comprising a DC input terminal configured to connect to the PV generator, an AC output terminal configured to connect the PV inverter to a power supply system, a DC/AC converter circuit configured to convert an input-side DC voltage into an AC voltage, a control circuit, connected to the DC/AC converter circuit, configured to deliver a predefined flow of power via the DC input of the PV inverter, a current sensor configured to detect a generator current (I.sub.PV) at the DC input, a voltage sensor configured to detect a generator voltage (U.sub.PV) at the DC input, an evaluation circuit connected to the current sensor and the voltage sensor, wherein the PV inverter is configured to: operate the PV generator of the PV installation at a maximum power point (MPP) with values of generator voltage (U.sub.MPP) and generator current (I.sub.MPP) associated with the maximum power point (MPP), operate the PV generator of the PV installation at a first generator voltage (U.sub.1) and detection of a first generator current (I.sub.1) associated with the first generator voltage (U.sub.1), operate the PV generator of the PV installation additionally at a second generator voltage (U.sub.2) and detection of a second generator current (I.sub.2) associated with the second generator voltage (U.sub.2), wherein the first generator voltage (U.sub.1) dictates that a first power (P.sub.1), with P.sub.1=U.sub.1*I.sub.1, of the PV generator at the first generator voltage (U.sub.1) is in a predefined first ratio (V.sub.1), with V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1, with the power (P.sub.MPP) at the maximum power point (MPP) of the PV generator, wherein the second generator voltage (U.sub.2) dictates that a second power (P.sub.2), with P.sub.2=U.sub.2*I.sub.2, of the PV generator at the second generator voltage (U.sub.2) is in a predefined second ratio (V.sub.2), with V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1) of the PV generator, wherein a characteristic quantity Y that characterizes a progress of potential-induced degradation (PID) is determined from the values of the first and second generator voltages (U.sub.1, U.sub.2) and/or the first and second generator currents (I.sub.1, I.sub.2), and wherein the generator potential (DC+, DC−) is shifted relative to the ground potential (PE) when the characteristic quantity Y is outside a predefined tolerance range.

20. A photovoltaic (PV) inverter, suitable for detecting a potential-induced degradation (PID) of PV modules of a PV installation, comprising: a DC input terminal configured to connect to a PV generator, an AC output terminal configured to connect the PV inverter to a power supply system, a DC/AC converter circuit configured to convert an input-side DC voltage into an AC voltage, a control circuit, connected to the DC/AC converter circuit, configured to deliver a predefined flow of power via the DC input of the PV inverter, a current sensor configured to detect a generator current (I.sub.PV) at the DC input, a voltage sensor configured to detect a generator voltage (U.sub.PV) at the DC input, an evaluation circuit connected to the current sensor and the voltage sensor, wherein the PV inverter is configured to: operate a PV generator of the PV installation at a maximum power point (MPP) with values of generator voltage (U.sub.MPP) and generator current (I.sub.MPP) associated with the maximum power point (MPP), beginning a measurement pass, operate the PV generator of the PV installation at a first generator voltage (U.sub.1) and detection of a first generator current (I.sub.1) associated with the first generator voltage (U.sub.1), operate the PV generator of the PV installation additionally at a second generator voltage (U.sub.2) and detection of a second generator current (I.sub.2) associated with the second generator voltage (U.sub.2), operating the PV generator at a third generator voltage (U.sub.3), and detecting a third generator current (I.sub.3) associated with the third generator voltage (U.sub.3), wherein the first generator voltage (U.sub.1) dictates that a first power (P.sub.1), with P.sub.1=U.sub.1*I.sub.1, of the PV generator at the first generator voltage (U.sub.1) is in a predefined first ratio (V.sub.1), with V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1, with the power (P.sub.MPP) at the maximum power point (MPP) of the PV generator, and wherein the second generator voltage (U.sub.2) dictates that a second power (P.sub.2), with P.sub.2=U.sub.2*I.sub.2, of the PV generator at the second generator voltage (U.sub.2) is in a predefined second ratio (V.sub.2), with V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1) of the PV generator, wherein the third generator voltage (U.sub.3) dictates that a third power (P.sub.3), with P.sub.3=U.sub.3*I.sub.3, of the PV generator at the third generator voltage (U.sub.3) is in a predefined third ratio (V.sub.3), with V.sub.3=P.sub.3/P.sub.1 and V.sub.3<1, with the first power (P.sub.1) of the PV generator, and a relationship in the form (U.sub.3<U.sub.1<U.sub.2) or a relationship in the form (U.sub.2<U.sub.1<U.sub.3) applies to the generator voltages (U.sub.1, U.sub.2, U.sub.3) in accordance with their values, wherein a characteristic quantity Y that characterizes a progress of the potential-induced degradation is determined from the values of the first, the second and the third generator voltage (U1, U2, U3) and/or the first, the second and the third generator current (I.sub.1, I.sub.2, I.sub.3), wherein during the measurement pass a first approach and, with staggered timing from the first approach, a further approach are effected for one of the generator voltages (U.sub.1, U.sub.2, U.sub.3), wherein the further approach to the respective generator voltage (U.sub.1, U.sub.2, U.sub.3) also results in a further generator current (I.sub.1,2, I.sub.2,2, I.sub.3,2) being detected, and wherein values of the further generator current (I.sub.1,2, I.sub.2,2, I.sub.3,2) are compared with the applicable values of the generator current (I.sub.1, I.sub.2, I.sub.3) of the first approach to the respective generator voltage (U.sub.1, U.sub.2, U.sub.3), and wherein the applicable measurement pass is used to determine the characteristic quantity Y only when an absolute value of a difference between the generator current (I.sub.1, I.sub.2, I.sub.3) and the further generator current (I.sub.1,2, I.sub.2,2, I.sub.3,2) is below a predefined threshold value ΔI.

21. A photovoltaic (PV) inverter, suitable for detecting a potential-induced degradation (PID) of PV modules of a PV installation, comprising: a DC input terminal configured to connect to a PV generator, an AC output terminal configured to connect the PV inverter to a power supply system, a DC/AC converter circuit configured to convert an input-side DC voltage into an AC voltage, a control circuit, connected to the DC/AC converter circuit, configured to deliver a predefined flow of power via the DC input of the PV inverter, a current sensor configured to detect a generator current (I.sub.PV) at the DC input, a voltage sensor configured to detect a generator voltage (U.sub.PV) at the DC input, an evaluation circuit connected to the current sensor and the voltage sensor, wherein the PV inverter is configured to: operate a PV generator of the PV installation at a maximum power point (MPP) with values of generator voltage (U.sub.MPP) and generator current (I.sub.MPP) associated with the maximum power point (MPP), operate the PV generator of the PV installation at a first generator voltage (U.sub.1) and detection of a first generator current (I.sub.1) associated with the first generator voltage (U.sub.1), operate the PV generator of the PV installation additionally at a second generator voltage (U.sub.2) and detection of a second generator current (I.sub.2) associated with the second generator voltage (U.sub.2), operate the PV generator of the PV installation additionally at a third generator voltage (U.sub.3), and detection of a third generator current (I.sub.3) associated with the third generator voltage (U.sub.3), wherein the first generator voltage (U.sub.1) dictates that a first power (P.sub.1), with P.sub.1=U.sub.1*I.sub.1, of the PV generator at the first generator voltage (U.sub.1) is in a predefined first ratio (V.sub.1), with V.sub.1=P.sub.1/P.sub.MPP and V.sub.1≤1, with the power (P.sub.MPP) at the maximum power point (MPP) of the PV generator, and wherein the second generator voltage (U.sub.2) dictates that a second power (P.sub.2), with P.sub.2=U.sub.2*I.sub.2, of the PV generator at the second generator voltage (U.sub.2) is in a predefined second ratio (V.sub.2), with V.sub.2=P.sub.2/P.sub.1 and V.sub.2<1, with the first power (P.sub.1) of the PV generator, wherein the third generator voltage (U.sub.3) dictates that a third power (P.sub.3), with P.sub.3=U.sub.3*I.sub.3, of the PV generator at the third generator voltage (U.sub.3) is in a predefined third ratio (V.sub.3), with V.sub.3=P.sub.3/P.sub.1 and V.sub.3<1, with the first power (P.sub.1) of the PV generator, and a relationship in the form (U.sub.3<U.sub.1<U.sub.2) or a relationship in the form (U.sub.2<U.sub.1<U.sub.3) applies to the generator voltages (U.sub.1, U.sub.2, U.sub.3) in accordance with their values, and wherein a characteristic quantity Y that characterizes a progress of the potential-induced degradation (PID) is determined from a sixth parameter (W.sub.6) that takes into consideration a second gradient m.sub.2=(I.sub.1−I.sub.3)/(U.sub.1−U.sub.3), a first gradient m.sub.1=(I.sub.2−I.sub.1)/(U.sub.2−U.sub.1), or a ratio of the first and second gradients m.sub.1/m.sub.2 within the IU graph.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The disclosure is explained and described in further detail below on the basis of exemplary embodiments depicted in the figures.

(2) FIG. 1 shows an equivalent circuit diagram of a solar cell arrangement using which the effect of the potential-induced degradation of PV modules is described.

(3) FIG. 2a shows a family of curves for current/voltage characteristics within an IU graph given progressive potential-induced degradation computed taking into consideration a decrease in parallel resistance R.sub.Par of the equivalent circuit diagram from FIG. 1.

(4) FIG. 2b shows a family of curves for power/voltage characteristics within a PU graph given progressive potential-induced degradation computed taking into consideration a decrease in parallel resistance R.sub.Par of the equivalent circuit diagram from FIG. 1.

(5) FIG. 2c shows a family of curves for current/voltage characteristics within an IU graph given increasing series resistance R.sub.Ser, computed taking into consideration the equivalent circuit diagram from FIG. 1.

(6) FIG. 2d shows a family of curves for power/voltage characteristics within a PU graph given increasing series resistance R.sub.Ser, computed taking into consideration the equivalent circuit diagram from FIG. 1.

(7) FIG. 3a shows the influence of a progressive potential-induced degradation on the parameter W.sub.1, which takes into consideration a relative voltage width within a PU graph.

(8) FIG. 3b shows the influence of a progressive potential-induced degradation on the parameter W.sub.2, which takes into consideration a discrete-point fill factor FF within an IU graph.

(9) FIG. 3c shows the influence of a progressive potential-induced degradation on the parameter W.sub.3, which takes into consideration a difference between the first generator current (I.sub.1) and a point on an imaginary straight connecting line within the IU graph.

(10) FIG. 3d shows the influence of a progressive potential-induced degradation on the parameter W.sub.4, which takes into consideration an asymmetry of current differences within the IU graph.

(11) FIG. 3e shows the influence of a progressive potential-induced degradation on the parameter W.sub.5, which takes into consideration an asymmetry of voltage differences within the IU graph.

(12) FIG. 3f shows the influence of a progressive potential-induced degradation on the parameter W.sub.6, which takes into consideration a gradient of the IU characteristic within the IU graph.

(13) FIG. 4 shows a flowchart for an embodiment of the method according to the disclosure for detecting a potential-induced degradation of PV modules of a PV installation.

(14) FIG. 5 shows a block diagram of a PV installation according to the disclosure with a PV inverter according to the disclosure that is configured to perform the method according to the disclosure for detecting a potential-induced degradation of PV modules.

DETAILED DESCRIPTION

(15) The disclosure relates to a method for detecting a potential-induced degradation (PID) of photovoltaic (PV) modules of a PV installation. The method is effected in the course of operation of the PV installation and is implementable inexpensively without additional sensor outlay. The method has a reduced infeed loss in comparison with known methods for detecting the potential-induced degradation. The disclosure relates moreover to a photovoltaic (PV) inverter for performing the method and to a photovoltaic (PV) installation having such a photovoltaic (PV) inverter.

(16) FIG. 1 shows an equivalent circuit diagram 100 of a solar cell arrangement, for example a PV module. The equivalent circuit diagram 100 has, in a known manner, a constant current source 101 that is connected up in parallel with a diode 102 and a parallel resistance R.sub.Par. The parallel circuit comprising constant current source 101, diode 102 and parallel resistance R.sub.Par is connected to the output 103 of the solar cell arrangement via a series resistance R.sub.Ser. The output 103 has a load resistance R.sub.load connected to it. The current I.sub.Ph produced by the constant current source 101 is dependent on the incident radiation on the solar cell arrangement, as symbolized by curly arrows in FIG. 1, and rises as the incident radiation increases. The current I.sub.Ph produced by the solar cell arrangement is split by the parallel circuit into

(17) a current I.sub.Di through the diode 102 connected in parallel with the constant current source 101,

(18) a current I.sub.Par through the resistance R.sub.Par connected in parallel with the constant current source 101, a useful current I that flows via the series resistance R.sub.Ser to the output 103 of the solar cell arrangement.

(19) The output 103 of the solar cell arrangement has a load resistance R.sub.load—for example a photovoltaic (PV) inverter—connected to it. The load resistance R.sub.load is therefore supplied with an electrical power P as per P=U*I by the solar cell arrangement. The power P drawn from the solar cell arrangement is therefore governed by the product of the voltage U present at the output 103 of the solar cell arrangement and the useful current I associated with this voltage. A high load resistance R.sub.load results in a state denoted by a negligibly small useful current I (I≈0) and a no-load voltage U.sub.0 associated with this zero-useful-current state. By contrast, an extremely small load resistance R.sub.load results in a state in which almost all of the current I.sub.Ph produced by the constant current source 101 drains in the direction of the load resistance R.sub.load. In this case, the voltage U at the output 103 of the solar cell arrangement collapses to negligibly small values (U≈0). Variation of the load resistance R.sub.load from relatively high to very low values delivers the known current/voltage characteristic of the solar cell arrangement within an IU graph.

(20) A progressive potential-induced degradation of the solar cell arrangement can be described by means of a change in the parallel resistance R.sub.Par connected in parallel with the constant current source 101 and the diode 102 over time. Specifically, the parallel resistance R.sub.Par decreases as the degree of the potential-induced degradation increases, resulting in an increasing proportion of the current I.sub.Ph produced by the constant current source 101 draining via the parallel resistance R.sub.Par. This proportion is therefore lost from the useful current I.

(21) FIG. 2a depicts a family of curves 200 for current/voltage characteristics of a solar cell arrangement in an IU graph. The individual characteristics of the family of curves 200 denote current/voltage characteristics of solar cell arrangements—for example of PV modules or strings of PV modules—with a different degree of the potential-induced degradation. Within the family of curves 200 for the current/voltage characteristics, the value of the parallel resistance R.sub.Par has been changed, given otherwise identical parameters, in order to depict the influence of a progressive potential-induced degradation on the current/voltage characteristic. Specifically, the value of the parallel resistance of the individual current/voltage characteristics within the family of curves decreases in the direction of the arrow 214. For each current/voltage characteristic, three respective operating points 211, 212, 213 are marked by applicable crosses on the characteristic curve. For the sake of clarity, the three operating points 211, 212, 213 are provided with reference signs only on one current/voltage characteristic by way of example. The first operating point 211 has an associated first generator voltage U.sub.1, first generator current I.sub.1 and first power as per P.sub.1=U.sub.1*I.sub.1. In this situation, the first power P.sub.1 is in a fixed first ratio V.sub.1 with a power P.sub.MPP at the maximum power point of the solar cell arrangement. By way of example, the first ratio V.sub.1=1.00 is chosen in this case. This is intended to be understood purely by way of example and in nonlimiting fashion, however. It is thus also possible for the power P.sub.1 to be lower than the maximum possible power P.sub.MPP of the solar cell arrangement and to be in a first ratio V.sub.1<1.00 therewith, for example V.sub.1=0.99. A second operating point 212 is characterized by a second generator voltage U.sub.2 and a second generator current I.sub.2 and denotes an operating point at which the PV installation generates a second power P.sub.2 as per P.sub.2=U.sub.2*I.sub.2. In this situation, the second power P.sub.2 is in a fixed second ratio V.sub.2=P.sub.2/P.sub.1 relative to the first power P.sub.1 of the first operating point 211 (in this case by way of example: V.sub.2=0.90). A third operating point 213 is denoted by a third generator voltage U.sub.3 and a third generator current I.sub.3. At the third operating point 213, the solar cell arrangement generates a third electrical power P.sub.3 as per P.sub.3=U.sub.3*I.sub.3 that is likewise in a fixed third ratio V.sub.3=P.sub.3/P.sub.1 with the first power P.sub.1 of the first operating point 211 (in this case by way of example: V.sub.3=0.90).

(22) FIG. 2b illustrates a family of curves 220 for power/voltage characteristics of a solar cell arrangement in a PU graph that have been computed using the equivalent circuit diagram from FIG. 1 and with identical parameters as in FIG. 2a. In this case too, a first 211, a second 212 and a third 213 operating point are again marked for each of the power/voltage characteristics by crosses. Each of the three operating points 211, 212, 213 is denoted by respective applicable values of generator voltage U.sub.1, U.sub.2, U.sub.3, generator current I.sub.1,I.sub.2 I.sub.3 (not depicted in FIG. 2b) and power P.sub.1, P.sub.2, P.sub.3, which are each identical to the values from FIG. 2a. In this respect, the operating points 211, 212, 213 of FIG. 2b are also identical, in principle, to the operating points 211, 212, 213 from FIG. 2a. For the sake of clarity, the operating points 211, 212, 213 are again provided with reference signs only on one of the power/voltage characteristics by way of example. Analogously to FIG. 2a, only the value of the parallel resistance R.sub.Par from FIG. 1 has also been varied in the family of curves 220 for the power/voltage characteristics in FIG. 2b given otherwise identical parameters, in order to illustrate the influence of the progressive potential-induced degradation on the power/voltage characteristic. In the family of curves 220, the parallel resistance R.sub.Par for the individual power/voltage characteristics decreases in the direction of the arrow 224.

(23) FIG. 2c illustrates a family of curves 240 for current/voltage characteristics of a solar cell arrangement in an IU graph that have been computed using the equivalent circuit diagram from FIG. 1. Whereas, for the computation of the family of curves in FIG. 2a, the parallel resistance R.sub.Par has been varied while the series resistance R.sub.Ser is kept constant, FIG. 2c now shows a family of curves for which the series resistance R.sub.Ser has been varied while the parallel resistance R.sub.Par has been kept constant. On each curve of the family of curves, the first operating point 211—in this case the MPP operating point—the second operating point 212 and a third operating point 213 are again marked by crosses. For the sake of clarity, the applicable values of the first generator current I.sub.1 and of the first generator voltage U.sub.1 are depicted merely at the first operating point 211. FIG. 2d shows a family of curves 260 for power/voltage characteristics in a PU graph, where the series resistance R.sub.Ser has been altered while the parallel resistance R.sub.Par has been kept constant in the same way as in FIG. 2c. Rising series resistances R.sub.Ser within the families of curves 240, 260 are symbolized by means of arrows 244, 264 in each of FIG. 2c and FIG. 2d.

(24) From the graph in FIG. 2c, it becomes clear that, when the series resistance increases, the first generator voltage U.sub.1—in this case the MPP voltage—initially decreases sharply, while no further change in the first generator voltage U.sub.1 can be observed at an advanced time. Since the no-load voltage U.sub.0 of the curves within the family of curves does not change, this response also applies, analogously, to the ratio of first generator voltage U.sub.1 and no-load voltage U.sub.0 as per U.sub.1/U.sub.0. This response differs in a fundamental manner from the family of curves from FIG. 2a, where the parallel resistance R.sub.Par has been varied while the series resistance R.sub.Ser has been kept constant. In that case, initially a merely negligible change in the first generator voltage U.sub.1—in this case likewise the MPP voltage—can be observed as the parallel resistance R.sub.Par decreases. Only at an advanced time is it possible to observe a sharp decrease in the first generator voltage U.sub.1. The no-load voltage U.sub.0 in FIG. 2a also shows a similar response. It initially hardly changes, and decreases sharply within the family of curves only at an advanced time. This response is also reflected in a comparison of the power/voltage characteristics shown in FIGS. 2d and 2c. A corresponding response to that of the first generator voltage U.sub.1 can also be recorded for the profile of the second generator voltage U.sub.2 and of the third generator voltage U.sub.3 within the families of curves in FIGS. 2a-2d.

(25) By now evaluating at least one of the generator voltages U.sub.1, U.sub.2, U.sub.3 and possibly also the no-load voltage U.sub.0, in particular the trends thereof, in successive measurements, it is possible to clearly distinguish a change in the parallel resistance R.sub.Par from a change in the series resistance R.sub.Ser. Thus, initially only the decrease in the parallel resistance R.sub.Par without a significant change in the first generator voltage U.sub.1 occurs, but not the increase in the series resistance R.sub.Ser. Since only the change in the parallel resistance R.sub.Par, but not the change in the series resistance R.sub.Ser, indicates a potential-induced degradation of the PV modules, it is possible to plausibilize whether or to what extent the change in the characteristic quantity Y is attributable to a potential-induced degradation of the PV modules of the solar cell arrangement.

(26) In the description of the figures that follows—in particular within FIG. 3a to FIG. 3f—the index a on the reference signs denotes a respective state that still has no significant potential-induced degradation. By contrast, the index b denotes a state that has a potential-induced degradation that is already further advanced.

(27) FIG. 3a illustrates the influence of a potential-induced degradation on a first parameter W.sub.1, which takes into consideration a relative voltage width ΔU=U.sub.2−U.sub.3 within a PU graph. A power/voltage characteristic 301.sub.a and a further power/voltage characteristic 301.sub.b are depicted by way of example. The power/voltage characteristic 301.sub.a therefore characterizes a solar cell arrangement, e.g. a PV module or a series connection of PV modules in the form of a string, in a state without significant potential-induced degradation. By contrast, the power/voltage characteristic 301.sub.b denotes the same solar cell arrangement as that of the characteristic 301.sub.a, but now with a potential-induced degradation that is already further advanced. In accordance with FIG. 2b, the first 311.sub.a, 311.sub.b the second 312.sub.a, 312.sub.b and the third 313.sub.a, 313.sub.b operating points are each symbolized by crosses on each of the power/voltage characteristics 301.sub.a, 301.sub.b in this case too. Each of the first 311.sub.a, 311.sub.b, second 312.sub.a, 312.sub.b and third 313.sub.a, 313.sub.b operating points have associated applicable values of generator voltage U.sub.1a,b, U.sub.2a,b, U.sub.3a,b, generator current I.sub.1a,b, I.sub.2a,b, I.sub.3a,b (not depicted in FIG. 3a) and power P.sub.1a,b as per P.sub.1a,b=U.sub.1a,b*I.sub.1a,b, P.sub.2a,b=U.sub.2a,b*I.sub.2a,b, P.sub.3a,b=U.sub.3a,b*I.sub.3a,b. By way of example, the second power P.sub.2a,b is equal to the third power P.sub.3a,b each time in this case. The two are in a fixed ratio V.sub.2=P.sub.2a,b/P.sub.1a,b=V.sub.3=P.sub.3a,b/P.sub.1a,b=0.90 with the first power P.sub.1a,b. Specifically, the values of the second V.sub.2 and third V.sub.3 ratios may be different than 0.90 and in particular also different than one another. There is moreover a depiction of a relative voltage width ΔU.sub.a=U.sub.2a−U.sub.3a around the first operating point 311.sub.a of the power/voltage characteristic 301.sub.a and of a relative voltage width ΔU.sub.b=U.sub.2b−U.sub.3b around the first operating point 311.sub.b of the further power/voltage characteristic 311.sub.b. The first parameter W.sub.1a,b takes into consideration this relative voltage width ΔU.sub.a,b. In this situation, the first parameter W.sub.1a,b may be equal to the relative voltage width ΔU.sub.a,b, but it can likewise also include still further terms besides the term of the relative voltage width ΔU.sub.a,b.

(28) It becomes clear that the relative voltage width ΔU.sub.a,b and hence also the first parameter W.sub.1a,b increases as the potential-induced degradation progress ΔU.sub.b>ΔU.sub.a. The effect of the increase is boosted if the first parameter W.sub.1a,b is equal to the dimensionless variable ΔU.sub.a,b/U.sub.1a,b or is computed as per W.sub.1a,b=ΔU.sub.a,b/P.sub.1,b. In one embodiment of the disclosure, the first parameter W.sub.1a,b is used for determining the characteristic quantity Y characterizing the progress of the potential-induced degradation. This means that the characteristic quantity Y is a function of at least the first parameter W.sub.1a,b, but possibly also of still further parameters (Y=Y(W.sub.1a,b, . . . )).

(29) FIG. 3b illustrates the influence of a potential-induced degradation on the second parameter W.sub.2, which takes into consideration a discrete-point fill factor FF within an IU graph. Similarly to FIG. 3a, there are again two characteristics depicted, but now a current/voltage characteristic 302.sub.a and a further current/voltage characteristic 302.sub.b. As in FIG. 3a, the current/voltage characteristic 302.sub.a corresponds to a state without significant potential-induced degradation, while the further current/voltage characteristic 302.sub.b denotes a state with a potential-induced degradation that is already further advanced. On each of the current/voltage characteristics 302.sub.a, 302.sub.b, the first 311.sub.a, 311.sub.b, the second 312.sub.a, 312.sub.b and the third 313.sub.a, 313.sub.b operating points are each again marked by crosses. Again, each of the first 311.sub.a, 311.sub.b, second 312.sub.a, 312.sub.b and third 313.sub.a, 313.sub.b operating points have associated applicable values of generator voltage U.sub.1a,b, U.sub.2a,b, U.sub.3a,b, generator current I.sub.1a,b, I.sub.2a,b, I.sub.3a,b and power P.sub.1a,b as per P.sub.1a,b=U.sub.1a,b*I.sub.1a,b, P.sub.2a,b=U.sub.2a,b*I.sub.2a,b, P.sub.3a,b=U.sub.3a,b*I.sub.3a,b (not depicted in FIG. 3b).

(30) The discrete-point fill factor FF is obtained from the operating points 311.sub.a,b, 312.sub.a,b and 313.sub.a,b characterizing the discrete points. The significance of the discrete-point fill factor corresponds to an area ratio of a first (smaller) rectangle 315.sub.a,b relative to a second (larger) rectangle 316.sub.a,b within the IU graph. In this situation, the area of the first rectangle 315.sub.a,b is obtained as per (U.sub.1a,b−U.sub.3a,b)*(I.sub.1a,b−I.sub.2a,b). Analogously, the area of the second rectangle 316.sub.a,b is obtained as per (U.sub.2a,b−U.sub.3a,b)*(I.sub.3a,b−I.sub.2a,b). This results in a discrete-point form factor FF as per FF=(U.sub.1a,b−U.sub.3a,b)*(I.sub.1a,b−I.sub.2a,b)/((U.sub.2a,b−U.sub.3a,b)*(I.sub.3a,b−I.sub.2a,b)).

(31) The discrete-point form factor FF defined in this way decreases as the potential-induced degradation progresses. The second parameter W.sub.2a,b takes into consideration this discrete-point fill factor FF. In this situation, the second parameter W.sub.2a,b may firstly be equal to the discrete-point fill factor defined above. However, the discrete-point fill factor FF may—besides the terms shown—also include still further terms. In one embodiment of the disclosure, the parameter W.sub.2a,b is used to determine the characteristic quantity Y characterizing the progress of the potential-induced degradation. This means that the characteristic quantity Y is a function of at least the second parameter W.sub.2a,b, but possibly also of still further parameters (Y=Y(W.sub.2a,b, . . . ))

(32) FIG. 3c depicts the influence of a progressive potential-induced degradation on the third parameter W.sub.3 within the IU graph. In this situation, the IU graph and the current/voltage characteristics 302.sub.a, 302.sub.b depicted therein are equal to that/those of FIG. 3b. Therefore, reference is made to the description of FIG. 3b for the general description of the graph structure and of the depicted current/voltage characteristics 302.sub.a, 302.sub.b.

(33) The third parameter W.sub.3 takes into consideration a difference between the first generator current I.sub.1 and a point on an imaginary straight connecting line 320.sub.a,b within the IU graph. The imaginary straight connecting line 320.sub.a,b connects the second operating point 312.sub.a,b having the coordinates (U.sub.2a,b, I.sub.2a,b) to the third operating point 313.sub.a,b having the coordinates (U.sub.3a,b, I.sub.3a,b) within the IU graph. The difference between the first generator current I.sub.1a,b and the point on the imaginary straight connecting line 320.sub.a,b is computed at the location of the voltage U.sub.1a,b each time and is symbolized by a distance arrow 321.sub.a,b in FIG. 3c. The parameter W.sub.3a,b takes into consideration this difference and therefore more or less the length of the distance arrow 321.sub.a,b. As the potential-induced degradation progresses, the length of the distance arrow 321.sub.a,b and therefore also the value of the third parameter W.sub.3a,b decreases.

(34) In one embodiment of the disclosure, the third parameter W.sub.3a,b is used to determine the characteristic quantity Y. Therefore, the characteristic quantity Y is a function of at least the third parameter W.sub.3a,b, but possibly also of still further parameters (Y=Y(W.sub.3a,b, . . . )).

(35) FIG. 3d depicts the influence of a progressive potential-induced degradation on a fourth parameter W.sub.4 within the IU graph. Again, the IU graph with the current/voltage characteristics 302.sub.a, 302.sub.b depicted therein is equal to that/those of FIG. 3b, which is why reference is made to the description of FIG. 3b for the general description of the graph structure and of the depicted current/voltage characteristics 302.sub.a, 302.sub.b.

(36) The fourth parameter W.sub.4 takes into consideration an asymmetry of current differences within the IU graph. This is an asymmetry between a first ΔI.sub.1 and a second ΔI.sub.2 current difference. The first current difference is computed as per ΔI.sub.1=|I.sub.1a,b−I.sub.2a,b| and is symbolized by a distance arrow 331.sub.a,b in FIG. 3d. The second current difference is computed as per ΔI.sub.2=|I.sub.3a,b−I.sub.1a,b| and is symbolized by a distance arrow 330.sub.a,b. A measure used for the asymmetry may be the ratio of the first to the second current difference as per ΔI.sub.1/ΔI.sub.2=|I.sub.1a,b−I.sub.2a,b|/|I.sub.3a,b−I.sub.1a,b|. If the asymmetry is high, the ratio assumes high values, whereas it assumes the value ΔI.sub.1/ΔI.sub.2=1.00 in the symmetrical case. As potential induced degradation progresses, the asymmetry changes and hence so does the value of the fourth parameter W.sub.4a,b taking into consideration this asymmetry. For the case depicted in FIG. 3d that the second generator voltage U.sub.2 is higher than the third generator voltage U.sub.3, the ratio ΔI.sub.1/ΔI.sub.2 decreases as the potential-induced degradation progresses, whereas in the other case (U.sub.3>U.sub.2), not depicted, the reciprocal of the ratio as per ΔI.sub.2/ΔI.sub.1 decreases as potential-induced degradation progresses. By means of a suitable choice of numerator and denominator, the fourth parameter W.sub.4 can thus be defined such that a desired change—i.e. decrease or increase—in the fourth parameter W.sub.4 as potential-induced degradation progresses can be set.

(37) With knowledge of a basic profile of the current/voltage characteristic 302.sub.a, 302.sub.b, however, it may also be sufficient to determine the asymmetry of the current differences by ascertaining either just the second ΔI.sub.2 or just the first ΔI.sub.1 current difference. To exclusively determine the second current difference ΔI.sub.2, and to exclusively determine the first current difference ΔI.sub.1, just two operating points, e.g. only the first operating point 311 and the second operating point 312, are sufficient. Evaluating the ratio of the first ΔI.sub.1 and the second current difference as per ΔI.sub.1/ΔI.sub.2, on the other hand, requires at least three operating points, in particular the first 311, the second 312 and the third 313 operating point.

(38) In one embodiment of the disclosure, the fourth parameter W.sub.4a,b is used to determine the characteristic quantity Y. Therefore, the characteristic quantity Y is a function of at least the fourth parameter W.sub.4a,b, but possibly also of still further parameters (Y=Y(W.sub.4a,b, . . . )).

(39) FIG. 3e depicts the influence of a progressive potential-induced degradation on a fifth parameter W.sub.5 within the IU graph. For the general description of the graph structure and of the depicted current/voltage characteristics 302.sub.a, 302.sub.b, reference is again made to the description of FIG. 3b.

(40) The fifth parameter W.sub.5 takes into consideration an asymmetry of voltage differences. This is in particular the asymmetry of a first ΔU.sub.1 and a second ΔU.sub.2 voltage difference. The first voltage difference is computed as per A U.sub.1=|U.sub.2a,b−U.sub.1a,b| and is symbolized by a distance arrow 341.sub.a,b in FIG. 3e. The second voltage difference is computed as per ΔU.sub.2=|U.sub.1a,b−U.sub.3a,b| and is symbolized by a distance arrow 340.sub.a,b in FIG. 3e.

(41) The measure used for the asymmetry may be the ratio of second to the first voltage difference as per ΔU.sub.2/ΔU.sub.1=|U.sub.1a,b−U.sub.3a,b|/|U.sub.2a,b−U.sub.1a,b|. With knowledge of the basic response of the current/voltage characteristic 302.sub.a,b during progressive potential-induced degradation, however, the knowledge of just either the first ΔU.sub.1 or the second ΔU.sub.2 voltage difference is sufficient to obtain a piece of information about the asymmetry of the voltage differences. To exclusively determine the second voltage difference ΔU.sub.2, and to exclusively determine the first voltage difference ΔU.sub.1, just two operating points, e.g. only the first operating point 311 and the second operating point 312, are sufficient. Evaluation of the ratio of the second ΔU.sub.2 and the first voltage difference as per ΔU.sub.2/ΔU.sub.1, on the other hand, requires at least three operating points, in particular the first 311, the second 312 and the third 313 operating point.

(42) In one embodiment of the disclosure, the fifth parameter W.sub.5a,b is used to determine the characteristic quantity Y. Therefore, the characteristic quantity Y is a function of at least the fifth parameter W.sub.5a,b, but possibly also of still further parameters (Y=Y(W.sub.5a,b, . . . )). In this case too, the fifth parameter W.sub.5 can be defined by means of a suitable choice of the ratio ΔU.sub.2/ΔU.sub.1 or of the reciprocal for the ratio as per ΔU.sub.1/ΔU.sub.2 such that a progressive potential-induced degradation produces a desired change−i.e. decrease or increase—in the fifth parameter W.sub.5.

(43) FIG. 3f depicts the influence of the progressive potential-induced degradation on a sixth parameter W.sub.6 within the IU graph. Owing to the identity of FIG. 3f to FIG. 3b in respect of the graph structure and the depicted current/voltage characteristics 302.sub.a, 302.sub.b, reference is again made in this regard to the corresponding description of FIG. 3b.

(44) The sixth parameter W.sub.6 takes into consideration a first gradient as per m.sub.1=(I.sub.2a,b−I.sub.1a,b)/(U.sub.2a,b−U.sub.1a,b), a second gradient as per m.sub.2=(I.sub.1a,b−I.sub.3a,b)/(U.sub.1a,b−U.sub.3a,b), or a ratio of the first and second gradients as per m.sub.1/m.sub.2 within the IU graph. In this situation, the first gradient m.sub.1 corresponds to the gradient of a straight connecting line 351.sub.a,b between the first 311.sub.a,b and the second 312.sub.a,b operating points, while the second gradient m.sub.2 characterizes the gradient of a straight connecting line 350.sub.a,b between the 313.sub.a,b and the first 311.sub.a,b operating point.

(45) As potential-induced degradation progresses, the second gradient m.sub.2 becomes more negative and has a larger absolute value, while the first gradient m.sub.1 becomes more positive and has a smaller absolute value. Accordingly, the ratio of the first to the second gradient as per m.sub.1/m.sub.2 decreases as the degree of the potential-induced degradation rises, to finally strive toward the value m.sub.right/m.sub.left=1.00.

(46) A sensitivity of the sixth parameter W.sub.6 to the progressive potential-induced degradation can still be increased by virtue of the first operating point 311.sub.a,b already having a reduced power P.sub.1 in comparison with the power P.sub.MPP at the maximum power point of the PV installation and the first ratio being chosen as per V.sub.1=P.sub.1/P.sub.MPP<1.00, for example V.sub.1=0.90. In the case of the second gradient m.sub.2, this results in a more positive and, in terms of absolute value, smaller gradient than for V.sub.1=1.00. In the case of the first gradient m.sub.1, it results in a more negative and, in terms of absolute value, larger gradient in comparison with V.sub.1=1.00.

(47) To exclusively determine the second gradient, and to exclusively determine the first gradient, just two operating points, e.g. the first operating point 311 and the second operating point 312, are sufficient. Evaluating the ratio of the first m.sub.1 and the second m.sub.2 gradient, on the other hand, requires at least three operating points, for example the first 311, the second 312 and the third 313 operating point. In order to increase the sensitivity of the sixth parameter W.sub.6 further even taking into consideration the ratio of the first to the second gradient as per m.sub.1/m.sub.2, it is advantageous if additionally a fourth operating point 314 is also approached and the values of a fourth generator voltage U.sub.4 and of a fourth generator current I.sub.4 that are associated with the fourth operating point are ascertained. In this situation, the fourth operating point 314 is chosen such that a fourth power P.sub.4=U.sub.4*I.sub.4 at the fourth operating point 314 is equal to the first power P.sub.1 at the first operating point, but the fourth generator voltage U.sub.4 is closer to the second generator voltage U.sub.2 than is the case for the first generator voltage U.sub.1. In this case, the first operating point 311 is thus closer within the IU graph to the third operating point 313, while the fourth operating point 314 is closer to the second operating point 313. Therefore, a second gradient as per m.sub.2=(I.sub.1a,b−I.sub.3a,b)/(U.sub.1a,b−U.sub.3a,b) and a first gradient as per m.sub.1=(I.sub.2a,b−I.sub.4a,b)/(U.sub.2a,b−U.sub.4a,b) can be ascertained. Accordingly, this results in a ratio of the first and second gradients as per m.sub.1/m.sub.2.

(48) FIG. 3f depicts the case in which the second generator voltage U.sub.2 is higher than the third generator voltage U.sub.3. Here, the ratio of the first to the second gradient m.sub.1/m.sub.2 decreases as potential-induced degradation progresses. In the other case (U.sub.3>U.sub.2), not depicted, on the other hand, the reciprocal of the ratio as per m.sub.2/m.sub.1 decreases. Here too, a suitable choice of numerator and denominator allows the sixth parameter W.sub.6 to be defined such that a desired change—i.e. decrease or increase—in the sixth parameter W.sub.6 as potential-induced degradation progresses can be set.

(49) In one embodiment of the disclosure, the sixth parameter W.sub.6a,b is used to determine the characteristic quantity Y. Therefore, the characteristic quantity Y is a function of at least the sixth parameter W.sub.6a,b, but possibly also of still further parameters (Y=Y(W.sub.6a,b, . . . )).

(50) FIG. 4 illustrates a flowchart for an embodiment of the method according to the disclosure for detecting a potential-induced degradation of PV modules of a PV installation. The explanation that follows is provided by way of example using an embodiment of the method with a ratio of magnitudes for the first U.sub.1, second U.sub.2, and third U.sub.3 generator voltages as per U.sub.3<U.sub.1<U.sub.2. However, it is transferable mutatis mutandis to a ratio of magnitudes for the generator voltages as per U.sub.2<U.sub.1<U.sub.3.

(51) After a start at S401a, the method initially enters an MPP mode of the PV generator of the PV installation, which is denoted by S401b. In the MPP mode, the PV generator is operated at an MPP maximum power point P.sub.MPP by means of an MPP tracking. The MPP tracking results in each of the present values of the MPP generator voltage U.sub.MPP, the MPP generator current I.sub.MPP and the MPP generator power P.sub.MPP=U.sub.MPP*I.sub.MPP being ascertained and being stored as first generator voltage U.sub.1, first generator current I.sub.1 and first power P.sub.1. In this respect, the first operating point 311 corresponds to the MPP operating point. On the basis of the present values of the MPP operating point I.sub.1, U.sub.1, P.sub.1, act S401c is used to check whether the present MPP operating point is within a predefined range that is standard for the normal MPP mode. By way of example, it is thus possible to check whether the PV generator is operated in a limited mode, which can occur as a result of the specification by a network operator, for example. In this case, a controller superordinate to the MPP tracking can prevent the PV generator or the PV installation from actually being operated at the MPP operating point. At the same time, it is thus checked whether the currently present MPP operating point is actually a global rather than just a local MPP operating point. If the MPP operating point is not in the range that is standard for the PV generator, then a measurement pass of the method for detecting a potential-induced degradation would result in incorrect values, and the method branches back to the MPP mode at S401b. If the MPP operating point is within the predefined operating range, the method transfers to S401d. Within act S401d, a check is performed to determine whether an entry condition for activating a measurement pass of the method for detecting the potential-induced degradation is satisfied. This entry condition can be provided under time control as a result of the expiration of a timer and/or event control—e.g. if the present MPP power P.sub.1 is within a predefined range. If the entry condition is not satisfied, the method branches back to the MPP mode at S401b. If the entry condition is satisfied, on the other hand, the measurement pass starts and the method branches to S402a.

(52) A left-hand third operating point 313 is now initially approached. To this end, the present generator voltage U.sub.PV is lowered at S402a. Act S402b is used to cross-check whether a minimum generator voltage U.sub.PV,min required for operation of the PV installation has been reached. If this is the case, then the measurement pass terminates, since the method would otherwise ascertain incorrect values. If the minimum generator voltage U.sub.PV,min required for operation of the PV installation has not yet been reached, however, act S402c is used to check whether a present power of the PV generator P.sub.PV is in a third ratio V.sub.3 with the power P.sub.1 of the PV generator. If this is not yet the case, the present generator voltage U.sub.PV needs to be lowered further and the method branches back to S402a. If the present power of the PV installation P.sub.PV is in the predefined third ratio V.sub.3 with the power P.sub.1 of the first operating point 311, on the other hand, then the third operating point 313 of the PV generator has been reached. At S402d that now follows, the values of a third generator voltage U.sub.3 and of the third generator current I.sub.3, possibly also of the third power P.sub.3=U.sub.3*I.sub.3, that are associated with the third operating point 313 are stored.

(53) Subsequently, a second operating point 312 is approached, but with the first operating point 311 being crossed a second time. For this purpose, the present generator voltage U.sub.PV is initially increased again at S403a. For the increase, act S403b is used to check the extent to which a maximum generator voltage U.sub.PV,max for operation of the PV generator has been reached. If this is the case, the method terminates. If the maximum generator voltage U.sub.PV,max has not yet been reached, however, act S403C is used to check whether present generator voltage U.sub.PV already corresponds to the first generator voltage U.sub.1. If this is not yet the case, the method branches back to S403a and the present generator voltage U.sub.PV is increased further. If the present generator voltage U.sub.PV is equal to the first generator voltage U.sub.1, the present power of the PV generator is ascertained and compared with the previously stored first power P.sub.1 of the first operating point 311. Only if the present power P.sub.PV of the PV generator corresponds to the first power P.sub.1 within predefined tolerance limits can it be assumed that the first operating point 311 has not changed during the measurement pass hitherto and the measurement pass can be continued. If the present power P.sub.PV of the PV generator is outside the predefined tolerance limits, however, the measurement pass of the method terminates.

(54) When the measurement pass is continued, act S405a is now used to increase the present generator voltage U.sub.PV further. This further increase in the generator voltage U.sub.PV occurs at S405a, S405b and S405c, the structure of which is similar to that of acts S403a, S403b and S403c. At S405c, however, a check is now performed to determine the extent to which a present power of the PV installation P.sub.PV is in a second predefined ratio V.sub.2 with the first power P.sub.1 of the first operating point 311. If this is not yet the case, the method branches back to S405a and the present generator voltage U.sub.PV is increased further. If the present power of the PV installation P.sub.PV is in the second predefined ratio V.sub.2 with the first power P.sub.1 of the first operating point 311, on the other hand, then the second operating point 312 has been reached. The method branches to S405d, in which the right-hand values of the second generator voltage U.sub.2 and of the second generator current I.sub.2, possibly also of the second power P.sub.2, are stored.

(55) After the values associated with the second operating point 312 are stored, the present generator voltage U.sub.PV is lowered further. The present generator voltage U.sub.PV is lowered again to the value of the first generator voltage U.sub.1 associated with the first operating point 311. This lowering occurs at S406a, S406b and S406c similarly to acts S402a, S402b and S402c already dealt with, which is why reference is made to the description given with said method acts for the details. Only the check on the branch back within act S406c is different than that of act S402c. Specifically, at S406c for branching back, a check is performed to determine whether present generator voltage U.sub.PV corresponds to the first generator voltage U.sub.1 of the first operating point 311.

(56) If the present generator voltage U.sub.PV does correspond to the first generator voltage U.sub.1, the present power P.sub.PV of the PV installation is ascertained once again and compared with the previously stored first power P.sub.1. Only when the present power P.sub.PV of the PV generator corresponds to the first power P.sub.1 within predefined tolerance limits can it be assumed that the first operating point 311 has not changed during the entire measurement pass. In this case, an evaluation of the characteristic quantities Y is now effected at S408 on the basis of the stored values of the first U.sub.1, second U.sub.2 and third U.sub.3, generator voltages, the first I.sub.1, second I.sub.2 and third I.sub.3 generator currents, and possibly the first P.sub.1, second P.sub.2 and third P.sub.3 powers. If the present power P.sub.PV of the PV generator does not correspond to the power P.sub.1 at S407 taking into consideration predefined tolerance limits, however, the measurement pass terminates, since a change in the external conditions (for example: incident radiation and/or temperature) can be assumed and therefore a determination of the characteristic quantity Y would result in incorrect values.

(57) When the characteristic quantity Y has been determined at S408, a measurement pass of the method according to the disclosure is at an end. A further measurement pass follows the already ended measurement pass under time and/or event control and the flowchart is executed again. Specifically, it is thus possible for successive measurement passes to be performed at a time interval of e.g. a day, a week or a month.

(58) FIG. 5 depicts a block diagram of a PV installation 501 according to the disclosure with a PV inverter 502 according to the disclosure. A DC input 512 of the PV inverter 502 has a PV generator 503 connected to it. An AC output 511 of the PV inverter 502 is connected to a power supply system 515. The PV inverter 502 has a DC/DC converter unit 506, a DC link circuit 505 and a DC/AC converter unit 504. The output side of the DC/AC converter unit 504 is connected to the AC output 511 of the PV inverter 502 via a system isolating relay 509. In this arrangement, the DC/AC converter unit is configured to convert an input-side DC voltage, for example the DC voltage of the DC link circuit 505, into an output-side AC voltage.

(59) Further, the control unit 508 of the PV inverter 502 is connected to the DC/AC converter unit 504. The control unit 508 is suitable, if need be in conjunction with the DC/DC converter unit 506 and/or the DC/AC converter unit 504, for controlling the operation of the PV inverter 502 such that a predefined flow of power via the DC input 512 of the PV inverter 502 is observed. Moreover, the control unit 508 is suitable, if need be in conjunction with the DC/AC converter unit 504 and/or the upstream DC/DC converter unit 506, for setting a particular generator voltage U.sub.PV at the DC input 512 of the PV inverter 502.

(60) Moreover, the PV inverter 502 has a current sensor 514 for detecting a generator current I.sub.PV, and also a voltage sensor 513 for detecting a generator voltage U.sub.PV. An evaluation unit 507 is connected to the current sensor 514 and the voltage sensor 513 and, at least for the purpose of a bidirectional data interchange—to the control unit 508. The PV inverter 502 is also configured to perform the method according to the disclosure for detecting a potential-induced degradation of PV modules.

(61) The PV inverter 502 further has a biasing unit 510 connected to the control unit 508. The biasing unit 510 has an associated DC voltage source and switching unit. If the characteristic quantity Y ascertained during the method according to the disclosure is outside a predefined tolerance range and hence an already further advanced potential-induced degradation of PV modules of the PV generator is indicated, the control unit 508 activates the biasing unit 510 under time and/or event control. In this situation, the biasing unit 510 is used to shift a generator potential of a DC input 12—in this case: for example the negative DC input “DC−”—relative to a ground potential PE. In this way, a potential-induced degradation of the PV modules that has already set in can be fixed again at least to a certain degree, and the life of the PV generator and of the PV installation 501 can therefore be extended. Alternatively or cumulatively, the control unit 508 generates a warning signal 516 that is transmitted to the operator of the PV installation 501 via a communication unit, not depicted in FIG. 5. This informs the operator about a critical state arising in the PV installation 501.

(62) In the embodiment depicted in FIG. 5, the DC/AC converter unit 504 has an upstream DC/DC converter unit 506. In this situation, the upstream DC/DC converter unit 506 is only optional and not absolutely necessary, however. It is also possible for the PV inverter 502 to have no DC/DC converter unit 506. In this case, the DC/AC converter unit 504 is configured, if need be in conjunction with the control unit 508, so as to set a prescribed generator voltage U.sub.PV at the DC input 512 of the PV inverter 502. Equally, it is possible for the PV inverter 502 to be a multistring inverter having multiple DC inputs 512 for connecting different PV generators 503 or PV generator elements. In this situation, the individual PV generator elements are each connected to the common DC link circuit 505 via a DC/DC converter 506. Every single PV generator element can use the control unit 508 to execute an independent MPP tracking. Equally, the PV inverter 502 is equipped, for each of the different DC inputs 512, with in each case a current sensor 514 for detecting a generator current I.sub.PV and a voltage sensor 513 for detecting a generator voltage U.sub.PV at the applicable DC input 512. It is therefore capable, in conjunction with the control unit 508 and the evaluation unit 507, of performing the method according to the disclosure for each of the PV generator elements connected to the PV inverter. In this situation, a measurement pass of the method can be effected for the individual PV generator elements independently of one another in succession or at the same time.