METHOD FOR MONITORING CHANGE IN CAPACITANCE IN ELECTRIC SYSTEM AND ELECTRIC SYTEM

1 . A method for monitoring a change in a capacitance in an electric system, the 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, the method comprising:

a) providing 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;

b) providing 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 monitoring resulting AC voltage components in each of the two capacitances; and

c) determining on the basis of a difference between the monitored AC voltage components a change in at least one of the two capacitances.

2 . The method of claim 1 , wherein in step c) the change in the at least one of the two capacitances is determined by comparing the difference between the monitored AC voltage components to a predetermined threshold value or to a previously obtained difference value.

3 . The method of claim 1 , wherein the inverter is pulse width modulation controlled according to a voltage reference and wherein the AC current component is provided by including a predetermined zero sequence component to the voltage reference or modulation reference of the inverter.

4 . The method of claim 3 , wherein the inverter is a three-level three-phase inverter having one middle DC pole and the electric system comprises a first capacitance connected between the positive DC pole of the inverter and the middle DC pole of the inverter, and a second capacitance connected between the middle DC pole of the inverter and the negative DC pole of the inverter, and wherein the predetermined zero sequence component comprises a third harmonic, or a multiple thereof, having a predetermined amplitude and phase difference with respect to a fundamental AC voltage.

5 . The method of claim 1 , wherein the inverter has more than one middle DC pole and steps b) and c) are performed for each middle DC pole and the respective capacitances connected thereto.

6 . The method of claim 1 , wherein steps b) and c) are performed continuously during step a) or at predetermined intervals.

7 . The method of claim 1 , wherein the monitoring of the AC voltage components in the capacitances comprises filtering measured voltage signals.

8 . (canceled)

9 . 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.

10 . The electric system of claim 9 , configured to determine the change in the at least one of the two capacitances by comparing the difference between the monitored AC voltage components to a predetermined threshold value or to a previously obtained difference value.

11 . The electric system of claim 9 , wherein the inverter is pulse width modulation controlled according to a voltage reference and wherein the electric system is configured to provide the AC current component by including a predetermined zero sequence component to the voltage reference or modulation reference of the inverter.

12 . The electric system of claim 11 , wherein the inverter is a three-level three-phase inverter having one middle DC pole and the electric system comprises a first capacitance connected between the positive DC pole of the inverter and the middle DC pole of the inverter, and a second capacitance connected between the middle DC pole of the inverter and the negative DC pole of the inverter, and wherein the predetermined zero sequence component comprises a third harmonic, or a multiple thereof, having a predetermined amplitude and phase difference with respect to a fundamental AC voltage.

13 . The electric system of claim 9 , wherein the inverter has more than one middle DC pole and the electric system is configured to perform the providing of the AC current component and the determining of the change in the at least one of the two capacitances for each middle DC pole and the respective capacitances connected thereto.

14 . The electric system of claim 9 , configured to perform the providing of the AC current component and the determining of the change in the at least one of the two capacitances continuously during the providing of the AC voltage or at predetermined intervals.

15 . The electric system of claim 9 , configured to filter measured voltage signals during the monitoring of the AC voltage components in the capacitances.

16 . The electric system of claim 9 , wherein the inverter is a neutral-point-clamped inverter or an active neutral-point-clamped inverter.

17 . 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.

18 . 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.