Fig. 5 shows the MMC developed by the Osaka Institute of Technology. It is comprised of a total of 24 modules, 8 modules for each phase. Two module circuits are mounted on each module circuit board. It has a capacity of 10 kVA, and the PWM carrier frequency is 20 kHz or less.
Fig. 6 shows the system configuration used during the experiment. 400 VDC is generated through a rectifier connected to a three-phase system. Then it is converted to three phase 200 V/400 Hz by a two-level inverter, after which it is isolated by a high frequency transformer, and then input to the MMC. The operation of the MMC is then evaluated, as it converts to 400 VDC. Furthermore, as the output of the MMC is input directly to the 2-level inverter, only the power loss of the converter is supplied from the three-phase system.
Fig. 7 shows the setup for the experiment. To analyze the MMC operation, we had to monitor a total of 12 channels simultaneously, including the capacitor voltage of the module, and the currents and voltages of the switching devices. A typical oscilloscope does not have enough channels, so we used a high speed GEN7tA multichannel recorder. Also, the GEN7tA has a sampling of 2 MS/s, and so we were able to measure superharmonics. Moreover, we were able to measure the power of the MMC simultaneously by using the eDrive dynamic power analyze function.
Fig. 8 shows an example of the measurement results of the capacitor voltages for each module. The result clearly shows that the capacitor voltages fluctuate due to MMC operation. The reference potentials of the capacitor voltages are different for each module. Therefore, if a typical oscilloscope is used, we would need a differential probe for each measurement point, whereas with the GEN7tA, the voltage input part is isolated (max. input voltage: ±1000 V), so that it is possible to measure the voltage at many points without the use of differential probes.