METHOD AND PHOTOVOLTAIC INVERTER FOR DETERMINING THE INSULATION RESISTANCE OF A PHOTOVOLTAIC SYSTEM TO GROUND

Abstract

The invention relates to a method and a photovoltaic inverter (2) for determining the insulation resistance (R.sub.iso) of a photovoltaic system (1) relative to ground (PE). According to the invention, the voltage required for the measurement can be provided by the intermediate circuit (6) in the form of the intermediate circuit voltage (U.sub.Zk), and the measuring device (13) is designed to actuate an input short-circuit switch (S.sub.Boost) for short-circuiting the DC input (3) with the AC disconnector (8) open, as a result of which the intermediate circuit voltage (U.sub.Zk) can be applied to the DC input (3) in the reverse direction, and the measuring device (13) is configured to record measured voltages (U.sub.M1, U.sub.M2) with the switch (S.sub.iso) of the voltage divider (14) open and closed, and to determine the insulation resistance (R.sub.iso) from the measured values of the two measured voltages (U.sub.M1, U.sub.M2) recorded with the switch (S.sub.iso) of the voltage divider (14) open and closed.

Claims

1: A method for determining the insulation resistance (R.sub.iso) of a photovoltaic system (1) relative to ground (PE), having a photovoltaic inverter (2) with at least one DC input (3) for connecting to at least one photovoltaic module (4) or a string (4′) of a plurality of photovoltaic modules (4), a DC/DC converter (5) with an input diode (D.sub.Boost), an intermediate circuit (6), a DC/AC converter (7), an AC disconnector (8), an AC output (9) for connection to a supply network (10) and/or consumer (11), a control device (12), and with a measuring device (13) with a voltage divider (14) containing at least two resistors (R.sub.1, R.sub.2), a switch (S.sub.iso) for connecting a resistor (R.sub.1) of the voltage divider (14) and a voltage measuring unit (15) for recording measured voltages (U.sub.M1) on at least one resistor (R.sub.2) of the voltage divider (14) with the switch (S.sub.iso) of the voltage divider (14) open and closed, and for determining the insulation resistance (R.sub.iso) from the temporal waveform of the recorded measured voltages (U.sub.M1), wherein the voltage required for the measurement is provided by the intermediate circuit (6) in the form of the intermediate circuit voltage (U.sub.Zk), and the measured voltages (U.sub.M1) are recorded with the AC disconnector (8) open while the DC input (3) is short-circuited with an input short-circuit switch (S.sub.Boost), which applies the intermediate circuit voltage (U.sub.Zk) to the DC input (3) in the reverse direction, wherein measured voltages (U.sub.M1, U.sub.M2) are each recorded with the switch (S.sub.iso) of the voltage divider (14) open and closed, and the insulation resistance (R.sub.iso) is determined from the measured values of the two recorded measured voltages (U.sub.M1, U.sub.M2) with the switch (S.sub.iso) of the voltage divider (14) open and closed.

2: The method according to claim 1, wherein the AC disconnector (8) is only closed if a defined minimum insulation resistance (R.sub.iso_min) is exceeded.

3: The method according to claim 1, wherein the insulation resistance (R.sub.iso) is measured by recording the measured voltages (U.sub.M1, U.sub.M2) during a specified time interval (Δt), in particular 1 s to 10 s.

4: The method according to claim 1, wherein the voltage (U.sub.DC) is measured at the DC input (3) of the photovoltaic inverter (2), and if the voltage (U.sub.DC) is below a preset limit value (U.sub.DC_limit), the DC input (3) is short-circuited with the input short-circuit switch (S.sub.Boost).

5: The method according to claim 1, wherein the system capacitance (C.sub.PV) is determined from the temporal waveform of the measured voltage (U.sub.M2) after the switch (S.sub.iso) of the voltage divider (14) is closed.

6: The method according to claim 1, wherein an energy storage device (18) is connected by closing a battery disconnector (17), and the measured voltages (U.sub.UM1, U.sub.M2) are recorded and used to determine the insulation resistance (R.sub.iso) and, if applicable, the system capacitance (C.sub.PV).

7: The method according to claim 6, wherein the exceeding of the defined minimum insulation resistance (R.sub.iso_min), the absolute insulation resistance (R.sub.iso), the falling below of the defined maximum system capacitance (C.sub.PV_max), and/or the absolute system capacitance (C.sub.PV) are each determined with and without the energy storage device (18) connected.

8: The method according to claim 1, wherein the exceeding of the defined minimum insulation resistance (R.sub.iso_min), the absolute insulation resistance (R.sub.iso), the falling below of the defined maximum system capacitance (C.sub.PV_max), and/or the absolute system capacitance (C.sub.PV) is displayed and/or stored.

9: The method according to claim 1, wherein the exceeding of the defined minimum insulation resistance (R.sub.iso_min), the absolute insulation resistance (R.sub.iso), the falling below of the defined maximum system capacitance (C.sub.PV_max), and or the absolute system capacitance (C.sub.PV) is determined within a measuring time (t.sub.m) of a maximum of 10 s.

10: A photovoltaic inverter (2) for determining the insulation resistance (R.sub.iso) of a photovoltaic system (1) relative to ground (PE), with at least one DC input (3) for connecting to at least one photovoltaic module (4) or a string (4′) of a plurality of photovoltaic modules (4), a DC/DC converter (5) with an input diode (D.sub.Boost), an intermediate circuit (6), a DC/AC converter (7), an AC disconnector (8), an AC output (9) for connection to a supply network (10) and/or consumer (11), a control device (12), and with a measuring device (13) with a voltage divider (14) containing at least two resistors (R.sub.1, R.sub.2), a switch (S.sub.iso) for connecting a resistor (R.sub.1) of the voltage divider (14), and a voltage measuring unit (15) for recording measured voltages (U.sub.M1) on at least one resistor (R.sub.2) of the voltage divider (14) with the switch (S.sub.iso) of the voltage divider (14) open and closed while the intermediate circuit voltage (U.sub.Zk) is applied to the DC input (3) and for determining the insulation resistance (R.sub.iso) from the recorded measured voltages (U.sub.M1), wherein the voltage required for the measurement can be provided by the intermediate circuit (6) in the form of the intermediate circuit voltage (U.sub.Zk), and the measuring device (13) is designed to actuate an input short-circuit switch (S.sub.Boost) for short-circuiting the DC input (3) with the AC disconnector (8) open, as a result of which the intermediate circuit voltage (U.sub.Zk) can be applied to the DC input (3) in the reverse direction, and the measuring device (13) is configured to record measured voltages (U.sub.M1, U.sub.M2) with the switch (S.sub.iso) of the voltage divider (14) open and closed, and to determine the insulation resistance (R.sub.iso) from the measured values of the two measured voltages (U.sub.M1, U.sub.M2) recorded with the switch (S.sub.iso) of the voltage divider (14) open and closed

11: The photovoltaic inverter (2) according to claim 10, wherein the measuring device (13) is connected to the AC disconnector (8) or the control device (12), so that the AC disconnector (8) can only be closed if a defined minimum insulation resistance (R.sub.iso_min) is exceeded.

12: The photovoltaic inverter (2) according to claim 10, wherein the measuring device (13) is designed to measure the insulation resistance (R.sub.iso) by the fact that the measured voltages (U.sub.M1, U.sub.M2) can be recorded with the switch (S.sub.iso) of the voltage divider (14) open and closed for a specified time interval (Δt), in particular 1 s to 10 s.

13: The photovoltaic inverter (2) according to claim 10, wherein at least one battery terminal (16) with at least one battery disconnector (17) and connected to the intermediate circuit (6) is provided for connection to at least one energy storage device (18), wherein the battery disconnector (17) is connected to the measuring device (13) or the control device (12) so that the battery disconnector (17) can be actuated during the recording of the measured voltages (U.sub.M1, U.sub.M2).

14: The photovoltaic inverter (2) according to claim 10, wherein the input short-circuit switch (S.sub.Boost) is formed by an existing boost switch (S.sub.Boost) of the DC/DC converter (5) implemented as a booster.

15: The photovoltaic inverter (2) according to claim 10, wherein the measuring device (13) is designed to measure the system capacitance (C.sub.PV) and to compare the system capacitance (C.sub.PV) with a defined maximum system capacitance (C.sub.PV_max), so that the AC disconnector (8) can only be closed if the defined maximum system capacitance (C.sub.PV_max) is not exceeded.

Description

[0035] The present invention will be explained in further detail by reference to the attached drawings. Shown are:

[0036] FIG. 1 a block diagram of a photovoltaic inverter designed according to the invention for determining the insulation resistance relative to ground;

[0037] FIG. 2 a simplified circuit diagram of a photovoltaic inverter designed according to the invention with photovoltaic modules connected thereto;

[0038] FIG. 3 a simplified circuit diagram of a photovoltaic inverter designed according to the invention with photovoltaic modules connected thereto having integrated electronics;

[0039] FIG. 4 a simplified circuit diagram of a photovoltaic inverter designed according to the invention with photovoltaic modules connected thereto having integrated electronics and a connected energy storage device;

[0040] FIG. 5 a flowchart illustrating the determination of the relative system capacitance and the relative insulation resistance;

[0041] FIG. 6 a flowchart illustrating the determination of the relative system capacitance and the relative insulation resistance of a hybrid inverter with connected energy storage device;

[0042] FIG. 7 a flowchart illustrating the determination of the absolute system capacitance and the absolute insulation resistance; and

[0043] FIG. 8 a flowchart illustrating the determination of the absolute system capacitance and the absolute insulation resistance of a hybrid inverter with connected energy storage device.

[0044] FIG. 1 shows a block diagram of a transformer-less photovoltaic inverter 2 of a photovoltaic system 1 designed according to the invention for determining the insulation resistance R.sub.iso relative to ground PE. The photovoltaic inverter 2 contains at least one DC input 3 for connection to at least one photovoltaic module 4 or a string 4′ of a plurality of photovoltaic modules 4. The photovoltaic modules 4 have a bypass diode D.sub.Bypass to enable a current flow if one of the photovoltaic modules 4 of a string 4′ is in shade. A DC/DC converter 5, which is often designed as a booster or an up-converter or a step-up converter, is arranged behind the DC input 3 of the photovoltaic inverter 2. An input diode (also a boost diode) D.sub.Boost is arranged in the DC/DC converter 5. This is followed by the intermediate circuit 6, a DC/AC converter 7, an AC disconnector 8 and an AC output 9 for connection to a power supply network 10 and/or consumers 11. The various components of the photovoltaic inverter 2 are controlled or regulated via a control unit 12. In order to be independent of the supply grid 10 even at night, when the photovoltaic modules 4 do not supply any voltage, suitable energy storage devices 18 are often connected to the photovoltaic inverter 2 via a battery connection 16. The energy storage devices 18 are connected to the photovoltaic inverter 2 via a battery disconnector 17, in order to be able to disconnect them from the photovoltaic inverter 2 also. The battery disconnector 17, which can also be integrated in the energy storage device 18, and the energy storage device 18 are usually connected to the control unit 12, which is represented by the dashed line. A power supply 21 supplies the components of the photovoltaic inverter 2 with electrical energy.

[0045] The photovoltaic system 1 has a certain system capacitance C.sub.PV relative to ground PE, which is composed of individual capacitances C.sub.PV,i relative to ground PE. In the equivalent circuit diagram, the total system capacitance C.sub.PV can be represented by a parallel connection of various system capacitances C.sub.PV,i. For example, certain capacitances C.sub.PV,i exist between the photovoltaic modules 4 and ground PE, as well as between any energy storage devices 18 and ground PE, which add up to the total system capacitance C.sub.PV. To prevent the residual current circuit breaker (not shown), which is intended to protect the photovoltaic system 1, from being triggered when the photovoltaic inverter 2 is connected to the supply grid 10 or consumers 11 with an inadmissibly high system capacitance value C.sub.PV present, it is important to regularly determine the system capacitance C.sub.PV of the entire photovoltaic system 1. For this purpose a measurement of the relative system capacitance C.sub.PV, i.e. determining whether the value is below the defined maximum system capacitance C.sub.PV,max, can be sufficient or else the absolute system capacitance C.sub.PV can be determined.

[0046] The photovoltaic system 1 also has a certain insulation resistance R.sub.iso relative to ground PE, which is also composed of individual partial insulation resistances R.sub.iso,i relative to ground PE. In the equivalent circuit diagram, the total insulation resistance R.sub.iso can be represented by a parallel connection of various partial insulation resistances R.sub.iso,i. For example, certain partial insulation resistances R.sub.iso,i exist between the photovoltaic modules 4 and ground PE, as well as between any energy storage devices 18 and ground PE, which add up to the total insulation resistance R.sub.iso. In order to prevent danger to persons or also the risk of destroying components of the photovoltaic system 1, regular determination of the actual insulation resistance R.sub.iso of the entire photovoltaic system 1 is important, often even mandatory. Either a measurement of the relative insulation resistance R.sub.iso, i.e. the exceeding of a defined minimum insulation resistance R.sub.iso_min, or the absolute insulation resistance R.sub.iso can be determined.

[0047] In most cases, both the system capacitance C.sub.PV and the insulation resistance R.sub.iso are determined simultaneously or in direct succession. For this purpose, a measuring device 13 is provided, which contains a voltage divider 14 comprising at least two resistors R.sub.1, R.sub.2 and a switch R.sub.iso for connecting a resistor R.sub.iso of the voltage divider 14. Using a voltage measuring unit 15, measured voltages U.sub.M1 are recorded on at least one resistor R.sub.2 of the voltage divider 14. A first measured value of the measured voltage U.sub.M1 is determined when the switch S.sub.iso is open and a second measured value of the measured voltage U.sub.M2 is determined when the switch S.sub.iso is closed. The system capacitance C.sub.PV can be determined from the temporal waveform of the measured voltage U.sub.M2 after the switch S.sub.iso is closed. This is achieved via the time constant of the temporal waveform of the measured voltage U.sub.M2 and knowledge of the resistance values R.sub.1, R.sub.2 of the voltage divider 14. The insulation resistance R.sub.iso is also determined from the two measured values U.sub.M1, U.sub.M2 and knowledge of the resistance values R.sub.1, R.sub.2 of the voltage divider 14. The voltage required for the measurement is provided in the form of the intermediate circuit voltage U.sub.Zk of the intermediate circuit 6. The necessary electrical energy is provided by a power supply 21, the DC input 3, or an energy storage device 18.

[0048] Previously, measurements of the system capacitance C.sub.PV and the insulation resistance R.sub.iso were provided at the start of the day when the photovoltaic modules 4 begin to generate a voltage. It is usually not necessary or not possible to make measurements during the night also. When the photovoltaic modules 4 are not generating any voltage, they are very highly resistive, which is why an exact measurement of the insulation resistance R.sub.iso would not be possible (see FIG. 2). Modern photovoltaic systems 1, in particular so-called hybrid systems with energy storage devices 18, make it necessary and practical to measure the system capacitance C.sub.PV and the insulation resistance R.sub.iso even during the night.

[0049] According to the invention, the measuring device 13 is designed to actuate an input short-circuit switch S.sub.Boost for short-circuiting the DC input 3 when the AC disconnector 8 is open, which allows the intermediate circuit voltage U.sub.Zk to be applied to the DC input 3 in the reverse direction. The circuit is therefore closed between the intermediate circuit 6 and the photovoltaic modules 4 in such a way that the current I (dashed arrow) flows in the forward direction of the bypass diodes D.sub.Bypass of the photovoltaic modules 4. This means that the system capacitance C.sub.PV and the insulation resistance R.sub.iso can also be reliably measured during the night when the photovoltaic modules 4 are particularly highly resistive. With the aid of the voltage measuring unit 15, a measured voltage U.sub.M1, U.sub.M2 is measured with the switch S.sub.iso open and closed respectively, and the system capacitance C.sub.PV and the insulation resistance R.sub.iso are determined from these measurements. The input short-circuit switch S.sub.Boost can ideally be formed by a boost switch S.sub.Boost of a DC/DC converter 5 implemented as a booster, which means that no dedicated hardware is required. The circuit and method are also suitable for photovoltaic modules 4 with integrated electronics 22 (see FIGS. 3 and 4), so-called MLPE (Module-Level Power Electronics), in which a reliable measurement of the system capacitance C.sub.PV or the insulation resistance R.sub.iso has not been possible up to now for certain circuits of the electronics 22. In addition, the system capacitance C.sub.PV or the insulation resistance R.sub.iso can be measured particularly accurately while taking into account any connected energy storage devices 18. It is only necessary to ensure that during the measurement of the measured voltages U.sub.M1, U.sub.M2 the energy storage device 18 is connected by closing the battery disconnector 17, so that a ground fault in the energy storage device 18 is also appropriately taken into account. Ideally, the system capacitance C.sub.PV and the insulation resistance R.sub.iso are measured both with the energy storage device 18 connected and with the energy storage device 18 disconnected. This allows the contribution of the energy storage device 18 to the system capacitance C.sub.PV and to the insulation resistance R.sub.iso to be measured separately and a fault in the photovoltaic system 1 to be better isolated or located more quickly.

[0050] If applicable, the measuring device 13 can be designed to measure the voltage U.sub.DC at the DC input 3 and if the measured voltage U.sub.DC is below a specified limit value U.sub.DC_limit the input short-circuit switch S.sub.Boost can be actuated or closed when determining the insulation resistance R.sub.iso and, if applicable, the system capacitance C.sub.PV. This ensures that during the night, when either no voltage or too low a voltage U.sub.DC is supplied by the photovoltaic modules 4, an exact determination of the measured values is possible.

[0051] For the sake of completeness, it should be noted that a photovoltaic inverter 2 can also have multiple DC inputs 3 for the connection of multiple strings 4′ of photovoltaic modules 4. The described method for determining the system capacitance C.sub.PV and the insulation resistance R.sub.iso can then be performed at each DC input 3. The connection to the supply grid 10 or to the consumers 11 is then only made for those strings 4′ of photovoltaic modules 4 for which the system capacitance C.sub.PV is below the defined maximum system capacitance C.sub.PV_max, or the AC disconnector 8 of the photovoltaic inverter 2 is only closed if the condition applies to all photovoltaic modules 4 and all components of the photovoltaic system 1. As a further condition for the closure of the AC disconnector 8, it is possible to check whether a defined minimum system capacitance C.sub.PV_min is exceeded, which indicates a closed DC disconnector at the DC input 3 of the photovoltaic inverter 2 (not shown).

[0052] FIG. 2 shows a simplified circuit diagram of a photovoltaic inverter 2 according to the invention with photovoltaic modules 4 connected thereto, with the system capacitance C.sub.PV and the insulation resistance R.sub.iso to ground PE in parallel with it shown symbolically. The photovoltaic modules 4 are shown in the equivalent circuit diagram as current sources with a parallel parasitic diode D.sub.P in the flow direction, parallel resistor R.sub.P and series resistor R.sub.S. If the cell of the photovoltaic module 4 is supplying current and a voltage U.sub.DC is present at the DC input 3 and if the voltage on the parasitic diode does not exceed the forward voltage D.sub.P, only the relatively small series resistance R.sub.S (usually in the mOhm range) is active. During the night, however, the cell of the photovoltaic module 4 does not supply any current, and so the relatively high parallel resistance R.sub.P (usually a few kOhm) comes into effect. In conventional measuring methods, this high parallel resistor R.sub.P would distort the determination of the system capacitance C.sub.PV and the insulation resistance R.sub.iso or render them impossible.

[0053] The photovoltaic modules 4 are each bridged by a bypass diode D.sub.Bypass, which is connected antiparallel to the flow direction of the solar current. The bypass diode D.sub.Bypass acts as a safety device in the photovoltaic module 4, through which the current is diverted via the bypass diode D.sub.Bypass in the event of shading or a defect in the photovoltaic module 4. The bypass diode D.sub.Bypass is usually located externally on the photovoltaic module 4. An equivalent circuit diagram of a single cell of the photovoltaic module 4 is shown. A string 4′ of multiple photovoltaic modules 4 is connected to the DC input 3 of the photovoltaic inverter 2. The following DC/DC converter 5 is designed as a booster and contains a boost switch S.sub.Boost arranged in parallel with the DC input 3, which is normally used to regulate the maximum input DC voltage U.sub.DC, and a boost diode D.sub.Boost in the direction of the desired current flow. The intermediate circuit 6 of the photovoltaic inverter 2 is represented by the intermediate circuit capacitor C.sub.Zk, to which the intermediate circuit voltage U.sub.Zk is applied. The measuring device 13 for determining the system capacitance C.sub.PV and the insulation resistance R.sub.iso includes the voltage divider 14, which has at least two resistors R.sub.1, R.sub.2, wherein the resistor R.sub.1 can be switched in or out via a switch S.sub.iso. A voltage measuring unit 15 for recording measured voltages U.sub.M1 across resistor R.sub.2 is arranged between the resistors R.sub.1, R.sub.2 at the centre of the voltage divider 14. According to the invention, the DC input 3 is short-circuited with an input short-circuit switch S.sub.Boost, which is formed here by the boost switch S.sub.Boost of the DC/DC converter 5. This creates a circuit through the system capacitance C.sub.PV and the insulation resistance R.sub.iso, according to which the intermediate circuit voltage U.sub.Zk is applied in the reverse direction to the photovoltaic modules 4 or the string 4′ of the photovoltaic modules 4 (shown by dashed lines). The current I thus flows according to the dashed lines in the forward direction of the bypass diodes D.sub.Bypass. This means that the system capacitance CPV and the insulation resistance R.sub.iso can also be reliably measured during the night when the photovoltaic modules 4 are particularly highly resistive. When the input short-circuit switch S.sub.Boost is closed, a measured voltage U.sub.M1 is recorded with the switch S.sub.iso of the voltage divider 14 open and a measured voltage U.sub.M2 with the switch S.sub.iso of the voltage divider 14 closed. The system capacitance C.sub.PV and the insulation resistance R.sub.iso can be determined from the measured values of the two measured voltages U.sub.M1, U.sub.M2 or the temporal waveform.

[0054] FIG. 3 shows a simplified circuit diagram of a photovoltaic inverter 2 designed according to the invention with photovoltaic modules 4 connected thereto, having integrated electronics 22, so-called MLPE (Module-Level Power Electronics). The equivalent circuit diagram of the electronics 22 contains a switch S.sub.E in addition to the diode D.sub.E and a high-resistance measuring resistor R.sub.M, via which the respective photovoltaic modules 4 can be deactivated. For safety reasons, for example, the photovoltaic modules 4 can be deactivated by opening the switch S.sub.E to ensure that no dangerous DC voltages are present on the supply lines to the photovoltaic inverter 2. In this case, the system capacitance C.sub.PV and, if applicable, the insulation resistance R.sub.iso could not be determined, or not reliably, with conventional methods because the current for measuring the system capacitance C.sub.PV and the insulation resistance R.sub.iso cannot flow through the string 4′ of photovoltaic modules 4 due to the open switch S.sub.E in the electronics 22. Due to the short-circuit of the DC input 3 according to the invention by means of the input short-circuit switch S.sub.Boost and the resulting reversal of the Intermediate circuit voltage U.sub.Zk at DC input 3, a current flow is possible here via the diodes D.sub.E integrated in the electronics 22. This means that the system capacitance C.sub.PV and the insulation resistance R.sub.iso can be reliably measured even when the photovoltaic modules 4 are deactivated, or during the night when they are very highly resistive.

[0055] FIG. 4 shows a simplified circuit diagram of a photovoltaic inverter 3 designed according to the invention with photovoltaic modules 4 connected thereto having integrated electronics 22 and a connected energy storage device 18 with integrated battery disconnector 17. For the remainder, the description of the photovoltaic inverter 2 according to FIGS. 1 and 3 applies. In this case, before recording the measured voltages UM it is additionally ensured that the battery disconnector 17 is closed, so that the energy storage device 18 is also taken into account in the measurement of the system capacitance C.sub.PV and the insulation resistance R.sub.iso. The portion of the system capacitance C.sub.PV,i and the insulation resistance R.sub.iso,i from the energy storage device 18 relative to ground PE is symbolically indicated. The insulation resistance R.sub.iso and, if applicable, the system capacitance C.sub.PV are each determined with and without the energy storage device 18 connected. In this way, the contribution of the energy storage device 18 to the system capacitance C.sub.PV and insulation resistance R.sub.iso can be determined separately and any insulation faults can be located more quickly.

[0056] FIG. 5 shows a more detailed flowchart of the determination of the relative system capacitance C.sub.PV and the relative insulation resistance R.sub.iso. After the method is started according to block 101, an initialization (block 102) takes place, during which phase the input short-circuit switch S.sub.boost is also closed. The block 200, which is enclosed in dashed lines, contains the method steps for determining the relative system capacitance C.sub.PV and the relative insulation resistance R.sub.iso, i.e. the test to determine whether the system capacitance C.sub.PV is below the defined maximum system capacitance C.sub.PV_max and whether the insulation resistance R.sub.iso is above the defined minimum insulation resistance R.sub.iso min. According to method step 202, the first measured voltage U.sub.M1 is measured with the switch S.sub.iso of the voltage divider 14 open. After this, the switch S.sub.iso is closed according to step 203. Then (step 205), the second measured voltage U.sub.M2 is measured with the switch S.sub.iso of the voltage divider 14 closed. After this, the insulation resistance R.sub.iso is determined according to step 206 and then according to step 207 it is ascertained whether the resistance is above the defined minimum insulation resistance R.sub.iso_min. From the temporal waveform of the measured voltage U.sub.M2 with the switch S.sub.iso closed, the system capacitance C.sub.PV is determined (step 208) and it is ascertained whether this is below the defined maximum system capacitance C.sub.PV_max (block 209). If both the insulation resistance R.sub.iso and the system capacitance C.sub.PV do not exceed or fall below the corresponding limit values, according to block 301 the switch S.sub.iso of the voltage divider 14 and the input short-circuit switch S.sub.boost are opened and then the photovoltaic inverter 2 is connected to the supply grid 10 and or the consumers 11 by closing the AC disconnector 8 (block 302).

[0057] If the insulation resistance R.sub.iso is below the defined minimum insulation resistance R.sub.iso_min, i.e. the query 207 returns a negative result, then according to method step 303 the procedure waits for a defined time and after a certain length of time (block 306) the measurement is restarted and processing jumps to method step 101. After a certain number of measurements or a certain time has been exceeded, an error message is issued according to block 304 and the switch S.sub.iso of the voltage divider and the input short-circuit switch S.sub.Boost are opened. Therefore, no sufficiently high insulation resistance R.sub.iso is measured. If the system capacitance C.sub.PV exceeds the defined maximum system capacitance C.sub.PV_Max, i.e. if the query 209 returns a negative result, an error message is output and the switch S.sub.iso of the voltage divider 14 is opened (block 305) and after a specified time period (block 306) processing returns to the start (block 101).

[0058] FIG. 6 shows the flowchart according to FIG. 5 for a hybrid inverter with a connected energy storage device 18. For the sake of simplicity the method steps within block 200 are not shown in as much detail here as they are in FIG. 5.

[0059] In addition to the method steps described in FIG. 5 with the energy storage device 18 connected, i.e. with the battery disconnector 17 closed, after determining the relative system capacitance C.sub.PV, i.e. whether the capacitance is below the defined maximum system capacitance C.sub.PV_max (block 207), and the relative insulation resistance R.sub.iso, i.e. whether the resistance exceeds the defined minimum insulation resistance R.sub.iso_min (block 209), the energy storage device 18 is disconnected by opening the battery disconnector 17 and the switch S.sub.iso of the voltage divider 14 is opened (block 401). If a fault occurs when the battery disconnector 17 is opened, an error message is issued according to block 402. Otherwise, the insulation resistance R.sub.iso (block 403) and the system capacitance C.sub.PV (block 406) are determined and compared with the defined minimum insulation resistance R.sub.iso_min (block 405) and the defined maximum system capacitance C.sub.PV_max (block 406). If the queries 405 and 406 return a positive result, according to block 407 the switch S.sub.iso of the voltage divider 14 and the input short-circuit switch S.sub.Boost are opened followed by the connection of the photovoltaic inverter 2 without a connected energy storage device 18 to the supply network 10 or to the consumers 11 by closing the AC disconnector 8 (block 408). If one of the queries 405 and 406 returns a negative result an error message is issued according to block 304 or 305 and after waiting for a specified time (block 306) the method for determining the relative system capacitance CPV and, if applicable, the insulation resistance R.sub.iso, is restarted.

[0060] FIG. 7 shows a more detailed flowchart of the determination of the absolute system capacitance C.sub.PV and the absolute insulation resistance R.sub.iso. The initialization according to block 102 is shown in more detail in this case. Accordingly, according to block 104, the voltage U.sub.DC at DC input 3 of the photovoltaic inverter 2 is measured and compared with a specified limit value U.sub.DC_limit (block 104). If the comparison returns a positive result, this is an indication that the photovoltaic module 4 is not supplying voltage (e.g. it is night-time or the switch S.sub.E of the electronics 22 of the photovoltaic module 4 is open) and the DC input 3 is short-circuited with the input short-circuit switch S.sub.Boost (block 105). Otherwise, the input short-circuit switch S.sub.Boost remains open (block 106) and the absolute system capacitance C.sub.PV and the absolute insulation resistance R.sub.iso are determined without a short-circuited DC input 3.

[0061] In addition to the method steps according to FIG. 5, the measurements of the first measured voltage U.sub.M1 with the switch S.sub.iso of the voltage divider 14 open and of the second measured voltage U.sub.M2 with the switch S.sub.iso of the voltage divider 14 closed are carried out here for a specified time interval Δt, in particular 1 s to 10 s, so that transient processes can decay and stable readings are obtained (blocks 201 and 204). Otherwise, the determination of the system capacitance C.sub.PV and the insulation resistance R.sub.iso proceeds as shown and described in FIG. 5.

[0062] Finally, FIG. 8 shows a flowchart according to FIG. 7 for a hybrid inverter with a connected energy storage device 18. During initialization (block 102), the energy storage device 18 is connected to the photovoltaic inverter 2 (block 103) by closing the battery disconnector 17. After that, the initialization corresponds to that shown in FIG. 7. For the sake of clarity the waiting times described in FIG. 7 (blocks 201 and 205) for the measurement of the absolute system capacitance C.sub.PV and the absolute insulation resistance R.sub.iso are not shown here, and block 200 is grouped together. If the measurement with the energy storage device 18 connected returns a positive result, the photovoltaic inverter 2 is connected to the supply network 10 or the consumers 11 with the energy storage device 18 connected (block 302), otherwise the measurement is repeated with the energy storage device 18 disconnected (blocks 401 to 406). If this measurement returns a positive result, the photovoltaic inverter 2 is connected to the supply network 10 or the consumers 11 with the energy storage device 18 disconnected (block 408). If this measurement also returns a negative result, the photovoltaic inverter 2 is not connected to the supply network 10 or the consumers 11 and the measurement is restarted at method step 101 after a specified time has elapsed (block 306).

[0063] The present invention enables a simple and reliable determination of the system capacitance C.sub.PV and, if applicable, the insulation resistance R.sub.iso of a photovoltaic system 1 relative to ground PE, in particular also during the night or when the photovoltaic modules 4 are deactivated, taking into account any energy storage devices 18 that are connected to the photovoltaic inverter 2.