Patent ID: 8400793
Filing Date: 2013-03-19
Classification: H02M

Abstract:
1. A method for providing a switching order signal to a cell of a cascaded two-level converter arranged to convert between alternating current and direct current, said cell comprising a capacitor parallel-connected with two series-connected semiconductor devices, which each in turn comprises at least one respective transistor with an anti-parallel connected diode, said cascaded two-level converter comprising at least two of said cells cascade connected and arranged in a phase between a first pole and a second pole of a direct voltage side, said phase being divided into two phase arms, a first phase arm being connected between said first pole and an ac phase output, and a second phase arm being arranged between said ac phase output and said second pole, comprising the steps of: measuring voltages of said capacitors of said at least two cells; calculating a compensated voltage reference based on a voltage reference and said measured voltages of said capacitors, said voltage reference representing the fundamental frequency output voltage on the ac phase output and corresponding to a desired ac current to be output on an ac-side; using said compensated voltage reference to calculate a switching order signal for said cell; and providing said switching order signal to said cell, wherein said step of calculating a compensated voltage reference comprises utilizing the equation: wherein r is said compensated voltage reference, u v ref is said voltage reference, u cn is the sum of the cell capacitor voltages of said first phase arm and u cn is the sum of the cell capacitor voltages of said second phase arm.