Method for monitoring change in capacitance in electric system and electric sytem

An electric system comprising:

• a multilevel inverter having a DC input comprising a positive DC pole, a negative DC pole, and at least one middle DC pole at an electric potential between the positive DC pole and the negative DC pole, and an AC output;
• and
• at least two capacitances connected in series between the negative DC pole of the inverter and the positive DC pole of the inverter, wherein the connection point between the at least two capacitances is connected to one of the at least one middle DC pole of the inverter,
• wherein the electric system is configured to:
• provide by the inverter an AC voltage to the AC output of the inverter from a DC voltage supplied to the DC input of the inverter;
• provide by the inverter, while providing the AC voltage to the AC output of the inverter, an AC current component to one of the at least one middle DC pole of the inverter, which AC current component is distributed between the two capacitances connected to the middle DC pole in question, and monitor resulting AC voltage components in each of the two capacitances;
• and
• determine on the basis of a difference between the monitored AC voltage components a change in at least one of the two capacitances.

An electric system comprising:

• a multilevel inverter having a DC input comprising a positive DC pole, a negative DC pole, and at least one middle DC pole at an electric potential between the positive DC pole and the negative DC pole, and an AC output;
• at least two capacitances connected in series between the negative DC pole of the inverter and the positive DC pole of the inverter, wherein the connection point between the at least two capacitances is connected to one of the at least one middle DC pole of the inverter;
• and
• a control arrangement comprising a processor and a memory storing instructions that, when executed by the processor, cause the control arrangement to:
• control the inverter to provide an AC voltage to the AC output of the inverter from a DC voltage supplied to the DC input of the inverter;
• control the inverter to provide, while providing the AC voltage to the AC output of the inverter, an AC current component to one of the at least one middle DC pole of the inverter, which AC current component is distributed between the two capacitances connected to the middle DC pole in question;
• and monitor resulting AC voltage components in each of the two capacitances;
• and
• determine on the basis of a difference between the monitored AC voltage components a change in at least one of the two capacitances.

An inverter controller structured to operate an inverter having a DC input comprising a positive DC pole, a negative DC pole, and at least one middle DC pole at an electric potential between the positive DC pole and the negative DC pole;

• an AC output;
• and at least two capacitances connected in series between the negative DC pole of the inverter and the positive DC pole of the inverter, wherein a connection point between the at least two capacitances is connected to one of the at least one middle DC pole of the inverter, the inverter controller comprising:
• a processing device;
• and
• a non-transitory computer readable medium structured to store a set of instructions which, when executed by the processing device, operate the inverter so as to:
• provide an AC voltage to the AC output of the inverter from a DC voltage supplied to the DC input of the inverter,
• provide, while providing the AC voltage to the AC output of the inverter, an AC current component to one of the at least one middle DC pole of the inverter, which AC current component is distributed between the two capacitances connected to the middle DC pole in question, and monitoring resulting AC voltage components in each of the two capacitances, and
• determine on the basis of a difference between the monitored AC voltage components a change in at least one of the two capacitances.