Distributed current balancing control

1. A system for controlling one or more motors, the system comprising a plurality of power converters, each power converter for providing power to the one or more motors, wherein the power converters are configured in a parallel configuration with each other and each power converters comprises:

an inverter for receiving an input voltage and converting this to an output voltage having an associated output current;

a module configured to modulate the output voltage using a modulation scheme;

a first feedback circuit configured to configured to generate a first signal, wherein the first signal corresponds to a summation of a first reference signal and a first measurement/calculation of the output current scaled by a first gain, wherein the first gain is set so as to reduce circulating currents between the power converter and one or more additional power converters in the parallel configuration due to imbalance of positive and/or negative sequence currents; and

a second feedback circuit configured to compare a second reference signal and a second measurement/calculation of the output current to produce an error value, wherein the error value is scaled by a second gain to produce a second signal, wherein the second gain is set so as to reduce circulating currents between the power converter and one or more additional power converters in the parallel configuration due to the imbalance of zero sequence currents,

wherein the modulation module is configured to receive the first signal and the second signal and adjust the modulation based on the first signal and the second signal so as to reduce circulating currents as aforesaid,

wherein the plurality of power converters are configured to control the one or more motors, wherein the first and second feedback circuits of each power converter are configured as aforesaid so as to reduce circulating currents between the power converters.

2. The system as claimed in claim 1 , wherein the input voltage is a DC input voltage and the output voltage is a three-phase output voltage.

3. The system as claimed in claim 1 , wherein the modulation scheme includes pulse width modulation, and the first reference signal used in the first feedback circuit is a setpoint for use in the pulse width modulation.

4. The system as claimed in claim 1 , wherein the first gain is set such that, for a range of values of the output current, a value of the first gain is chosen that reduces circulating currents between the power converter and one or more additional power converters in the parallel configuration due to imbalance of positive and/or negative sequence currents.

5. The system as claimed in claim 4 , wherein the first gain is preset or otherwise predetermined as part of a calibration or pre-processing routine.

6. The system as claimed in claim 5 , wherein the gain is preset or predetermined using frequency response analysis.

7. The system as claimed in claim 1 , wherein the error value corresponds to a difference between the second reference signal and the second measurement/calculation of the output current.

8. The system as claimed in claim 1 , wherein the modulation scheme uses space vector modulation, and the modulation module comprises a processor or circuitry configured to modify one or more vectors used in the space vector modulation based on the second signal.

9. The system as claimed in claim 8 , wherein the processor or circuitry of the modulation module is configured to adjust a length or duration of one or more vectors based on the second signal.

10. The system as claimed in claim 9 , wherein the processor or circuitry of the modulation module is configured to drive a modulation signal based on a zero state vector, which modulation signal is used to adjust the length or duration of the zero state vector by the processor or circuitry as aforesaid.

11. The system as claimed in claim 1 , wherein the modulation module is configured to adjust a modulation duty cycle based on the first signal and/or the second signal so as to reduce circulating currents as aforesaid.

12. The system as claimed in claim 1 , wherein the modulation module is configured to adjust a length or duration of one or more vectors of a space vector modulation scheme based on the first signal and/or the second signal to so as to reduce circulating currents as aforesaid.

13. The system as claimed in claim 1 , further comprising a structure configured to receive and hold multiple power converters, wherein the structure is configured to interconnect the power converters held and received within the structure such that a variable number of power converters may be interconnected using the structure as desired.

14. The system as claimed in claim 13 , wherein the structure is configured such that a single power converter can be connected thereto for providing power to the one or more motors in a standalone configuration, as well as up to N power converters connected thereto for providing power to one or more motors in a parallel configuration, wherein N is an integer greater than 1.