Pre-charging of MMC and power-up of a MMC-based multiterminal HVDC transmission system

High-voltage DC (HVDC) transmission lines offer highly efficient transport of electric energy. If applied to comparatively weak grids, self-commutated converters are the proper choice, because they allow stabilizing the grid in case of faults by injecting reactive power. At high voltages of e.g. 400 kV modular multilevel converters (MMC) fulfill all requirements. The first task to master is getting the converters and the grid into operation. For this a two-step pre-charging sequence is needed, because the control of the converter modules of the MMC is supplied from its own DC-link voltages. For testing all relevant aspects of a multiterminal HVDC transmission system a scaled-down test bench has been realized. Four modular multilevel-converters (MMCs) define the multiterminal DC system. With regard to practical demands, no explicit communication between the control systems is allowed for stable operation of the transmission system. This paper gives a short overview of the test bench and then concentrates on the pre-charging of the converters based on AC and DC voltage and demonstrates the powering up of the DC system.