Patent ID: 8385097

Claim:
A method for controlling a Voltage Source Converter configured to convert direct voltage into alternating voltage and conversely and having at least one phase leg connecting two opposite poles of a direct voltage side of the converter and comprising a series connection of switching elements, each of said switching elements having at least two semiconductor devices of turn-off type, at least two free-wheeling diodes connected in parallel therewith and at least one energy storing capacitor, a mid point of said series connection forming a phase output being configured to be connected to an alternating voltage side of the converter and to divide the phase leg into an upper valve branch and a lower valve branch, said at least two semiconductor devices of each of said switching elements being controlled to obtain, for each of said switching elements, one of a) a first switching state and b) a second switching state, in which for a) the first switching state, the voltage across said at least one energy storing capacitor or for b) the second switching state, a zero voltage, is applied across terminals of the each of switching elements, for obtaining a determined said alternating voltage on said phase output, wherein each of said switching elements is provided with an individual saw tooth voltage, the saw tooth voltages of the switching elements being identical but evenly distributed over the time with respect to each other with a phase shift of 2π/(pN) between adjacent saw tooth voltage, in which p is the pulse number defined as period of a reference alternating voltage/period of the saw tooth voltage and N is the number of switching elements in said phase leg, and is also provided with an individual said reference alternating voltage with the same frequency and phase as that of the other said switching elements, said switching elements are controlled according to a Pulse Width Modulation pattern so that each of said switching elements is switched to change from said first switching state to said second switching state and conversely each time the saw tooth voltage wave for the corresponding switching element crosses the reference alternating voltage belonging to the corresponding switching element, the voltage across said at least one energy storing capacitor of each of said switching elements is measured during the control of the converter and compared with a reference direct voltage, and a result of said comparison is used as a feedback control signal for setting the amplitude of said individual reference alternating voltage on the basis of the result of said comparison.