A method and an apparatus for charging a filtering capacitor in the direct-voltage intermediate circuit of a voltage-controlled PWM frequency converter comprising a rectifier (20) connected to an alternating-current source (U.sub.R, U.sub.S, U.sub.T), a direct-voltage intermediate circuit and an inverter unit (11)/inverter units for feeding a multi-phase alternating voltage (U.sub.U, U.sub.V, U.sub.W) of variable frequency into a load/loads (12), said rectifier bridge having controlled semiconductor switches (V1 V3), especially thyristors, in one arm, preferably in the upper arm, and diodes (D1 D6) in the other arm, preferably the lower arm, and said direct-voltage intermediate circuit being provided with a capacitor unit (C.sub.DC) of a relatively high capacitance for filtering the voltage. The filtering capacitor is charged by means of the semiconductor switches of the rectifier bridge by adjusting their firing angle according to the measured voltage of the capacitor unit, and the firing angle is adjusted by forming the sum of the measured voltage (U.sub.DC) of the capacitor unit and a predetermined limit voltage (U.sub.LIM) and comparing the said sum to the measured main voltages of the supply network.

The present invention provides a power converter system in which forward conversion capacities and reverse conversion capacities of semiconductor power converter apparatuses connected to a D.C. capacitor can be different from one another. The more suitable semiconductor power converter apparatuses can be selected in accordance with various loads. A power converter system includes semiconductor power converter apparatuses including a D.C. capacitor and power devices cooled by a cooling elements having an output terminal connected to a load. The semiconductor power converter apparatuses are connected in parallel with one another through the D.C. capacitor. Each of the power devices includes self-arc-suppressing semiconductor devices and diodes connected in anti-parallel with the self-arc-suppressing semiconductor devices. All of the semiconductor power converter apparatuses have the same circuit configuration. The power device of a first of the semiconductor power converter apparatuses has characteristics different from those of the power devices of the other semiconductor power converter apparatuses.

A converter circuit arrangement is disclosed for matching a variable DC voltage to a drive circuit for producing drive signals and having a three-point DC voltage intermediate circuit (2), which three-point DC voltage intermediate circuit (2) is formed by a first capacitor (3) and a second capacitor (4) connected in series with it, with one connection of the first capacitor (3) forming an upper connection (5) of the three-point DC voltage intermediate circuit (2), and the first capacitor (3) forming a center point connection (6) at the junction point with the second capacitor (4) and one connection of the second capacitor (4) forming a lower connection (7) of the three-point DC voltage intermediate circuit (2). A first partial converter system (8) and a second partial converter system (9) are provided, with the input side of the first partial converter system (8) being connected to a first pole (10) of a DC voltage source (11) which produces the DC voltage, and the input side of the second partial converter system (9) being connected to a second pole (12) of the DC voltage source (11), a first output (13) of the first partial converter system (8) being connected to the upper connection (5), and a first output (14) of the second partial converter system (9) being connected to the lower connection (7). Furthermore, a second output (15) of the first partial converter system (8) is connected in series with a second output (16) of the second partial converter system (9) via the center point connection (6). A method for matching a variable DC voltage is also specified.