Dynamic 11kA Power Converter for Prototype Testing of Superconducting FAIR Magnets

In 2006 the set up of one test bench for superconducting magnets of FAIR was finished at GSI. A second one is under construction. After commissioning of both test stands magnetic measurements of up to 8 superconducting magnets per month will be possible. The dedicated power converter must supply DC currents and ramped currents up to 11000A and provide output voltages up to +/-100V. This gives an effective apparent power of 1.3 MVA. The maximum value for the rate of current change is 18kA/s for positive and negative current ramps. To avoid tracking errors on the current ramp and to reduce current ripple the power converter is equipped with a dynamic active filter. A similar power converter topology with active filter is in use in GSI since 1990 [1]. It consists of a 12 pulse silicon controlled rectifier (SCR), which carries the bulk of the load current, and a parallel active filter (PE) witch carries only a small current, but controls the accuracy and stability of the load current. The power converter which is installed at the test bench is based on a newer design of a power converter with active filter. As described in [2] the active filter is set up as a switch mode power converter. The main advantage is the reduction of conduction losses in the semiconductors of the active filter and leads to a more compact design of the system. Figure 1 shows the principle circuit diagram of the power converter. Because of the large power the SCRs are connected to the 20kV voltage level via 2 identically build transformers which are phase shifted by +/15° el by phase rotation of the primaries. Two smoothing reactors decouple the SCRs and the active filter PE, because both units are voltage sources by principle. The PE is connected to the 400V AC system and is designed for a nominal current of 100A. At the output of the PE a small passive LC-filter suppresses the switching frequency of 100 kHz. Whereas the configuration described in [2] needs a passive filter for the SCR there is no need for SCR filtering in the GSI approach because of the new developed control strategy.