Patent ID: 7248489

Claim:
A method for controlling the mains bridge of a four-quadrant PWM frequency converter provided with a direct-voltage intermediate circuit when power is flowing into the supply network, said mains bridge comprising a controlled mains bridge (INU) for rectification of the mains voltage to produce a DC intermediate-circuit direct voltage (U DC ) consisting of arms conducting during the positive and negative half cycles of the phase voltage of each phase of the supply network, each arm having a switch unit formed by a diode (D 1 –D 6 ) and a power semiconductor switch (V 1 –V 6 ), e.g. an IGBT, connected in inverse-parallel with it, wherein the mains bridge (INU) is controlled in such a way that, when power is flowing into the supply network, the switch (V 1 , V 2 , V 3 ) (conducting during the positive half cycle) in the upper arm of each phase is turned on substantially for as long a time as the supply network phase voltage (U L1 , U L2 , U L3 ) corresponding to it is the most positive of all the phase voltages and the switch (V 4 , V 5 , V 6 ) (conducting during the negative half cycle) in the lower arm is turned on substantially for as long a time as the corresponding phase voltage is the most negative of all the phase voltages, wherein in the method compromises the following steps: creating a virtual three-phase network having the same frequency with the actual supply network is, controlling the power semiconductor switches are controlled on the basis of the voltages of the virtual network, when power is flowing into the supply network, measuring the supply network phase currents, and making inferences are made as to whether the virtual network is correctly synchronized with the supply network and, if necessary, the phase shift of the virtual network is corrected so as to keep it synchronized with the supply network based on the curve form of the measured phase currents.