Method and apparatus for minimising a circulating current or a common-mode voltage of an inverter

1. A method for minimizing a circulating current or a common-mode voltage of an inverter comprising at least one of an inverter bridge and an active rectifier bridge, wherein modulation of the at least one of an inverter bridge and an active rectifier bridge comprises controlling a duty ratio during a switching cycle, wherein the switching cycle has a set length, the method comprising:

measuring a common-mode voltage of the inverter; and

one of:

controlling the length of the switching cycle of the inverter bridge based on the measured common-mode voltage; or

controlling the length of the switching cycle of the active rectifier bridge based on the measured common-mode voltage.

2. The method as claimed in claim 1 and configured for minimizing a common-mode voltage, wherein the inverter includes an active rectifier bridge, and the method comprises:

controlling the length of the switching cycle of the active rectifier bridge or the inverter bridge to synchronize the switching cycles of the active rectifier bridge and the inverter bridge.

3. The method as claimed in claim 1 for minimizing a circulating current of parallel-connected inverters, the method comprising:

for at least one parallel-connected inverter:

controlling the length of the switching cycle of the inverter's inverter bridge to synchronize the switching cycles of the inverter bridges of the parallel-connected inverters.

4. The method as claimed in claim 1 for minimizing a circulating current of parallel-connected inverters having active rectifier bridges, the method comprising:

for at least one parallel-connected inverter:

controlling the cycle length of the switching cycle of the inverter's active rectifier bridge to synchronize the switching cycles of the active rectifier bridges of the parallel-connected inverters.

5. The method as claimed in claim 3 , wherein the common-mode voltage is measured on the DC-side.

6. The method as claimed in claim 3 , wherein the common-mode voltage is measured on the AC-side.

7. The method as claimed in claim 5 , wherein controlling the length of the switching cycle includes minimizing an RMS value of the common-mode voltage.

8. The method as claimed in claim 6 , wherein controlling the length of the switching cycle includes maximizing an RMS value of the common-mode voltage.

9. The method as claimed in claim 1 , wherein controlling the length of the switching cycle includes:

changing the length of the switching cycle by a value of a first parameter;

determining a change in the common-mode voltage induced by the change in the length of the switching cycle; and

determining a new value for the first parameter based on the determined change in the common-mode voltage.

10. The method as claimed in claim 1 , wherein controlling the length of the switching cycle includes:

determining an instantaneous common-mode output voltage;

comparing the determined common-mode output voltage with the measured common-mode voltage; and

controlling the length of the switching cycle based on the comparison.

11. The method as claimed in claim 10 , wherein the common-mode voltage is measured on the DC side, and wherein controlling the length of the switching cycle includes:

controlling the length of the switching cycle to minimize an overlap between the determined instantaneous common-mode output voltage and the measured DC side common-mode voltage.

12. The method as claimed in claim 10 , wherein the common-mode voltage is measured on the AC side, and wherein controlling the length of the switching cycle includes:

controlling the length of the switching cycle to maximize an overlap between the determined instantaneous common-mode output voltage and the measured AC side common-mode voltage.

13. The method as claimed in claim 10 , wherein the common-mode voltage is determined from the switching states of the switches in the inverter bridge.

14. An inverter configured for minimizing a circulating current or a common-mode voltage of the inverter, the inverter comprising:

at least one of an inverter bridge and an active rectifier bridge, configured such that modulation of the at least one of an inverter bridge and an active rectifier bridge comprises controlling a duty ratio during a switching cycle, wherein the switching cycle has a set length; and

means configured to:

measure a common-mode voltage of the inverter; and

one of:

control the length of the switching cycle of the inverter bridge based on the measured common-mode voltage; or

control the length of the switching cycle of the active rectifier bridge based on the measured common-mode voltage.

15. The inverter as claimed in claim 14 , wherein the inverter is configured for minimizing a circulating current of an arrangement comprising parallel-connected inverters, wherein the inverter can include means configured to control the length of the switching cycle of the inverter's inverter bridge to synchronize the switching cycles of the inverter bridges of the parallel-connected inverters.