Device and method for detecting a fault current in a photovoltaic installation, and photovoltaic inverter comprising the device

A method for detecting a fault current (Ifault) on a PV installation, wherein a PV generator of the PV installation has at least a first PV string and a second PV string, which are connected via in each case two DC lines to a PV inverter of the PV installation, with a detection device that comprises at least one current transformer and an evaluation circuit connected to the at least one current transformer, wherein the current transformer comprises a current transformer jointly used by the first PV string and the second PV string, wherein a measurement signal of the jointly used current transformer represents a difference between a first summation current (Isum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (Isum,2) flowing from the second PV string toward the ground potential (PE), and wherein the fault current (Ifault), when it occurs, is a part of the first summation current (Isum,1) and/or a part of the second summation current (Isum,2), the method comprising:

• detecting a measurement signal of the at least one current transformer, wherein the measurement signal represents a difference between a first summation current (Isum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (Isum,2) flowing from the second PV string toward the ground potential (PE);
• and
• signaling a fault current (Ifault) when the detected measurement signal exceeds a first threshold value (STH,1).

A PV inverter comprising an AC output for connection to an AC voltage network and at least two DC inputs configured to connect to at least two PV strings of a PV generator, the PV inverter further comprising a detection device, comprising at least one current transformer and an evaluation circuit connected to the at least one current transformer, wherein the current transformer comprises a current transformer jointly used by a first PV string and a second PV string, wherein a measurement signal of the jointly used current transformer represents a difference between a first summation current (Isum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (Isum,2) flowing from the second PV string toward the ground potential (PE), and wherein a fault current (Ifault), when it occurs, is a part of the first summation current (Isum,1) and/or a part of the second summation current (Isum,2), wherein the detection device is configured to:

• detect a measurement signal of the at least one current transformer, wherein the measurement signal represents a difference between a first summation current (Isum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (Isum,2) flowing from the second PV string toward the ground potential (PE);
• and
• signal a fault current (Ifault) when the detected measurement signal exceeds a first threshold value (STH,1).