Arrangement for exchanging power

An arrangement for exchanging power, in shunt connection, with a three-phase electric power network, said arrangement comprising:

• a Voltage Source Converter having three phase legs with each a series connection of switching cells in the form of so-called H-bridges comprising two switching elements connected in parallel and each having at least two semiconductor assemblies connected in series and having each a semiconductor device of turn-off type and a rectifying element connected in anti-parallel therewith, each said switching cell further comprising at least one energy storing capacitor connected in parallel with said switching elements, mid points between semiconductor assemblies of each switching element forming terminals of the switching cell for connection to corresponding, terminals of adjacent switching cells for forming said series connection of switching cells, in which the three phase legs of the Voltage Source Converter are interconnected in a neutral point by forming a wye-connection,
• device configured to detect electrical conditions of said three-phase electric power network, and
• a control unit configured to control said semiconductor devices of said semiconductor assemblies of each switching cell and by that each switching cell dependent upon information received from said detecting means to deliver a voltage across the terminals of each said switching cell being zero, +U or −U, in which U is the voltage across said capacitor, for together with other switching cells of the phase leg deliver a voltage pulse being the sum of the voltages so delivered by each switching cell,
• characterized in that the arrangement further comprises a device connected to said neutral point of the wye-connected Voltage Source Converter and configured to provide a current path for a zero-sequence current, and that said control unit is configured, upon receipt of information from said detecting device causing a need to generate a negative-sequence current, to calculate a value for amplitude and phase position for a zero-sequence current for which, when added to said three phase legs upon generation of said negative-sequence current, the resulting energy stored in said energy storing capacitors in each said phase leg will be constant and to control the semiconductor devices of said switching cells of the phase legs to add such a zero-sequence current to the currents of each phase leg of the converter.