Method in connection with converter bridges

1. A method in connection with converter bridges connected in parallel, whereby the converter bridges are directly connected in parallel and the converter bridges are controlled independently on the basis of control variables, the method comprising the steps of

controlling the converter bridges to produce an output switching vector on the basis of the control variables,

determining the magnitude of a common-mode current component of the converter bridges, and

selecting a zero vector to be used during control as a switching vector on the basis of the sign of the common-mode current component to minimize common-mode current of the converter bridges.

2. A method as claimed in claim 1 , wherein the converter bridges are controlled by means of a method based on direct torque control, whereby

an electric model of the load of the converter bridges is formed,

a flux linkage vector is estimated on the basis of the electric model,

torque is estimated on the basis of the electric model, and

the switching vectors of the converter bridges are determined on the basis of the set value for the torque, the estimated value for the torque, the set value for the absolute value of the flux linkage vector and the estimated flux linkage vector.

3. A method as claimed in claim 1 , wherein the determination of the magnitude of the common-mode current component comprises a step of determining the magnitude of currents of all phases of the converter bridges and

summing the current magnitudes together to achieve the magnitude of the common-mode current.

4. A method as claimed in claim 2 , wherein the determination of the magnitude of the common-mode current component comprises a step of determining the magnitude of currents of all phases of the converter bridges and

summing the current magnitudes together to achieve the magnitude of the common-mode current.