Control system and method

1. A method for controlling the operation of a drive unit comprising a plurality of switching modules arranged to receive a DC electricity supply and generate an AC electricity supply for driving a load from the received DC electricity supply, the AC electricity supply being generated by switching of the plurality of switching modules between a conducting state and a non-conducting state in accordance with a switching period, the method comprising:

receiving one or more characteristics associated with each of the switching modules;

generating, for each switching module of the plurality of switching modules, an error signal with a comparator that compares a characteristic of a respective switching module with an equivalent characteristic associated with one or more other switching modules of the plurality of switching modules; and

controlling a time period during which the respective switching module is in its conducting state in accordance with the generated error signal,

wherein controlling the time period includes applying a per switching period correction with a delay circuit to correct for problems occurring in each individual switching period and an offset factor to compensate for long-term strains placed on one or more of the switching modules, and wherein the offset factor is summed with the per switching period correction to produce a delay demand that is indicative of an amount by which a switch-on time of each switching module needs to be delayed.

2. The method according to claim 1 , wherein controlling the time period comprises shortening the time for which the respective-switching module is in its conducting state in accordance with the generated error signal.

3. The method according to claim 2 , wherein the time period during which the respective switching module is in its conducting state is shortened by delaying the switch-on time of the respective switching module with the delay circuit in accordance with the generated error signal.

4. The method according to claim 1 , wherein generating comprises, for each switching module of the plurality of switching modules, comparing the characteristic of the respective switching module with an average of the equivalent characteristic associated with each switching module of the plurality of switching modules.

5. The method according to claim 4 , wherein controlling the time period comprises shortening the time period during which the respective switching module is in its conducting state when the characteristic of the respective switching module is greater than the average of that characteristic for each switching module of the plurality of switching modules.

6. The method according to claim 1 , wherein one of the one or more characteristics is a temperature associated with the respective switching module.

7. The method according to claim 1 , wherein one of the one or more characteristics is an electric current associated with the respective switching module.

8. The method according to claim 1 , wherein one of the one or more characteristics is an accumulated electric current associated with the respective switching module.

9. The method according to claim 1 , wherein one of the one or more characteristics is indicative of a strain placed on the respective switching module.

10. The method according to claim 1 , wherein one of the one or more characteristics is determined by measuring the characteristic.

11. The method according to claim 1 , wherein one of the one or more characteristics is determined by modelling the characteristic.

12. The method according to claim 1 , wherein the time period during which the respective switching module is in its conducting state is controlled by a control signal for operating the switching of the respective switching module.

13. An apparatus for controlling the operation of a drive unit comprising a plurality of switching modules arranged to receive a DC electricity supply and generate an AC electricity supply for driving a load from the received DC electricity supply, the AC electricity supply being generated by switching of the plurality of switching modules between a conducting state and a non-conducting state in accordance with a switching period, the apparatus comprising:

a processor including at least one comparator and at least one delay circuit, the processor arranged to:

receive one or more characteristics associated with each of the switching modules;

generate, for each switching module of the plurality of switching modules, an error signal with the comparator that compares a characteristic of a respective switching module with an equivalent characteristic associated with one or more other switching modules of the plurality of switching modules; and

control a time period during which the respective switching module is in its conducting state in accordance with the generated error signal,

wherein the processor is arranged to control the time period by applying a per switching period correction with the delay circuit to correct for problems occurring in each individual switching period and an offset factor to compensate for long-term strains placed on one or more of the switching modules, and wherein the offset factor is summed with the per switching period correction to produce a delay demand that is indicative of an amount by which a switch-on time of each switching module needs to be delayed.

14. A drive unit for providing an AC electricity source for driving a load, the drive unit comprising:

an input arranged to receive a DC electricity source;

an output arranged to produce an AC electricity source for driving a load;

a plurality of switching modules arranged between the input and the output, the plurality of switching modules arranged to receive the DC electricity supply and generate an AC electricity supply for driving the load from the received DC electricity supply, the AC electricity supply being generated by switching of the plurality of switching modules between a conducting state and a non-conducting state; and

the apparatus according to claim 13 configured to control at least one of the plurality of switching modules.

15. The drive unit according to claim 14 , wherein a plurality of groups of the plurality of switching modules are provided, wherein each group of the plurality of switching modules has an associated apparatus according to claim 13 and each apparatus according to claim 13 is communicatively coupled.

16. A non-transitory computer readable medium comprising computer readable code operable, in use, to instruct a computer to perform the method of claim 1 .

17. The apparatus according to claim 13 , wherein the processor is arranged to control the time period by applying the per switching period correction prior to each change of state of the respective switching module.

18. The apparatus according to claim 13 , wherein the processor is arranged to control the time period by applying the per switching period correction twice per cycle.

19. The method according to claim 1 , wherein applying the per switching period correction includes applying the per switching period correction prior to each change of state of the respective switching module.

20. The method according to claim 1 , wherein applying the per switching period correction includes applying the per switching period correction twice per cycle.