Synchronization and Control of Modular AC- and DC-Sided Parallel-Connected Three-Level NPC Inverters

Inhalt:
This paper describes the synchronization and control of modular three-level NPC inverters, which are connected AC- and DC-sided in parallel. By this parallel arrangement without galvanic isolation, zero-sequence circulating currents are occurring due to several reasons. These circulating currents can cause the inverters to shut down, e.g. because of the saturation of the filter inductances. To avoid circulating currents, a control strategy in combination with a high-precision PLL-based synchronization method is introduced. The difference to existing methods is that this synchronization method does not require a communication between the inverters. It will use the information of the grid voltages, which are connected to every inverter in the system, to synchronize the PWM signals of each inverter. The performances of the synchronization method and the control strategies are proven by experimental tests. Measurements validate the theoretical considerations and results.