Method and a control system for controlling parallel operating devices

1. A control system for controlling parallel operating devices configured to drive an operating quantity towards a target value, the control equipment comprising controllers for controlling the devices so that each controller is configured to control one of the devices at least partly based on a device-specific integral term maintained by the controller under consideration and relating to a time integral of a device-specific error signal available to the controller and indicative of a deviation of the operating quantity from the target value, wherein the control system further comprises a stabilizing system for computing, for each of the controllers, an arithmetic average of (i) the device-specific integral term maintained by the controller under consideration and (ii) one or more of the device-specific integral terms maintained by other ones of the controllers, and for correcting the device-specific integral term maintained by the controller under consideration towards the computed arithmetic average, wherein the stabilizing system is implemented with the controllers so that each of the controllers comprises:

a receiver for receiving the one or more of the device-specific integral terms maintained by the other ones of the controllers, and

an arithmetic section for computing the arithmetic average of the received device-specific integral terms and the device-specific integral term maintained by the controller under consideration, and for correcting the device-specific integral term maintained by the controller under consideration towards the computed arithmetic average.

2. The control system according to claim 1 , wherein at least one of the controllers comprises a transmitter for transmitting, to other ones of the controllers, the device-specific integral term maintained by the at least one of the controllers.

3. The control system according to claim 2 , wherein each of the controllers is configured to update and correct the device-specific integral terms according to the formula:

I _ n=I _ n ,prev+ e _ nΔt/Ti+K corr( I _ave−( I _ n ,prev+ e _ nΔt/Ti )),

where I_n is the device-specific integral term maintained by n th one of the controllers, I_n,prev is the previous value of the device-specific integral term, e_n is the device-specific error signal related to the nt h one of the devices, I_ave is the arithmetic average, Δt is a temporal length of one operating clock cycle, Ti is an integration time, and Kcorr is a correction factor.

4. The control system according to claim 2 , wherein each of the controllers is a proportional and integrative “PI” controller or a proportional-integrative-derivative “PID” controller.

5. The control system according to claim 1 , wherein each of the controllers is configured to update and correct the device-specific integral terms according to the formula:

I _ n=I _ n ,prev+ e _ nΔt/Ti+K corr( I _ave−( I _ n ,prev+ e _ nΔt/Ti )),

where I_n is the device-specific integral term maintained by n th one of the controllers, I_n,prev is the previous value of the device-specific integral term, e_n is the device-specific error signal related to the n th one of the devices, I_ave is the arithmetic average, Δt is a temporal length of one operating clock cycle, Ti is an integration time, and Kcorr is a correction factor.

6. The control system according to claim 1 , wherein each of the controllers is a proportional and integrative “PI” controller or a proportional-integrative-derivative “PID” controller.

7. A device-system comprising:

parallel operating devices for driving an operating quantity towards a target value, and

a control system according to claim 1 for controlling the parallel operating devices.

8. The device-system according to claim 7 , wherein the parallel operating devices are mechanically coupled electric machines whose rotational speeds have fixed relations with respect to each other, and the operating quantity has a fixed relation with the rotational speeds of the mechanically coupled electric machines.

9. The device-system according to claim 7 , wherein the parallel operating devices are electric power converters supplying a common direct voltage link, and the operating quantity is direct voltage of the common direct voltage link.

10. A plurality of controllers for controlling a plurality of devices, each device being one of parallel operating devices configured to drive an operating quantity towards a target value, each controller of the plurality of controllers comprising:

a control section for controlling the device at least partly based on a device-specific integral term maintained by the control section and relating to a time integral of a device-specific error signal available to the controller and indicative of a deviation of the operating quantity from the target value,

wherein each controller of the plurality of controllers further comprises:

a receiver for receiving one or more of device-specific integral terms related to other ones of the parallel operating devices, and

an arithmetic section for computing an arithmetic average of: (i) the received device-specific integral terms, and (ii) the device-specific integral term maintained by the control section, and for correcting the device-specific integral term maintained by the control section towards the computed arithmetic average.

11. The controller according to claim 10 , wherein the controller comprises a transmitter for transmitting, to a data transfer channel, the device-specific integral term maintained by the control section.

12. A method for controlling parallel operating devices configured to drive an operating quantity towards a target value, the method comprising:

controlling each of the devices at least partly based on a device-specific integral term relating to a time integral of a device-specific error signal indicative of a deviation of the operating quantity from the target value,

wherein the method comprises the following actions for each of the devices:

receiving one or more of the device-specific integral terms related to other ones of the devices,

computing an arithmetic average of (i) the device-specific integral term related to the device under consideration and (ii) the one or more of the device-specific integral terms related to other ones of the devices, and

correcting the device-specific integral term related to the device under consideration towards the computed arithmetic average.

13. The method according to claim 12 , wherein the device-specific integral terms are updated and corrected according to the formula:

I _ n=I _ n ,prev+ e _ nΔt/Ti+K corr( I _ave−( I _ n ,prev+ e _ nΔt/Ti )),

where I_n is the device-specific integral term related to n th one of the devices, I_n,prev is the previous value of the device-specific integral term, e_n is the device-specific error signal related to the n th one of the devices, I_ave is the arithmetic average, Δt is a temporal length of one operating clock cycle, Ti is an integration time, and Kcorr is a correction factor.

14. A non-volatile computer readable medium encoded with a computer program for stabilizing control of parallel operating devices configured to drive an operating quantity towards a target value, wherein the computer program comprises computer executable instructions for controlling a plurality of controllers of a programmable processing system to:

receive one or more of the device-specific integral terms related to other ones of the devices,

control each of the devices at least partly based on a device-specific integral term relating to a time integral of a device-specific error signal indicative of a deviation of the operating quantity from the target value,

compute, for each of the devices, an arithmetic average of (i) the device-specific integral term related to the device under consideration and (ii) the one or more of the device-specific integral terms related to other ones of the devices, and

correcting the device-specific integral term related to the device under consideration towards the computed arithmetic average.