Connecting an inverter in a solar power plant with shifted potential center point

A process of connecting an AC output of a transformerless inverter of a solar power plant to an internal AC power grid at an input side of a galvanic isolation,

• while at least one photovoltaic generator is connected to a DC input of the inverter,
• while an external AC power grid is present at an output side of the galvanic isolation and
• while an offset voltage for shifting a potential center point of the at least one photovoltaic generator with regard to earth potential is applied, the process comprising:
• synchronizing the inverter with the internal AC power grid, while all current-carrying lines of the AC output of the inverter are still galvanically separated from the internal AC power grid;
• matching a potential center point of the current-carrying lines of the AC output of the inverter and a potential center point of the internal AC power grid, while all current-carrying lines of the AC output of the inverter are still galvanically separated from the internal AC power grid and while only one of the potential center points of the current-carrying lines and of the internal AC power grid is shifted by the offset voltage;
• and
• galvanically connecting the current-carrying lines of the AC output of the synchronized inverter to the internal AC power grid after synchronizing and matching.

An inverter system, comprising:

• a transformerless inverter configured to receive an intermediate DC voltage across current-carrying input lines at an input thereof, and generate an AC voltage across output lines defined by an output line and a center point line at an output thereof;
• a transformer having a primary winding and a secondary winding, wherein the center point line of the transformerless inverter is coupled to a center point of the primary winding of the transformer and an offset voltage potential;
• a relay configured to selectively couple the output line of the transformerless inverter to a phase input of the primary winding of the transformer;
• an offset connection circuit configured to selectively connect one of the current-carrying input lines to near an earth ground potential;
• and
• a control circuit configured to activate the offset connection circuit to connect one of the current-carrying input lines to near the earth ground potential prior to activating the relay to couple the transformerless inverter to the transformer.

An inverter system, comprising:

• a transformerless inverter configured to receive an intermediate DC voltage across current-carrying input lines at an input thereof, and generate an AC voltage across output lines defined by an output line and a center point line at an output thereof;
• a transformer having a primary winding and a secondary winding;
• a relay configured to selectively couple the output line of the transformerless inverter to a phase input of the primary winding of the transformer;
• an intermediate voltage circuit coupled between the current-carrying input lines of the transformerless inverter, wherein the intermediate voltage circuit comprises first and second series impedances coupled together at a midpoint node;
• an offset connection circuit configured to selectively connect the midpoint node of the intermediate voltage circuit to an offset voltage potential;
• and
• a control circuit configured to activate the offset connection circuit to connect the midpoint node to the offset voltage potential prior to activating the relay to couple the transformerless inverter to the transformer.