Modular multilevel converter based medium voltage DC amplifier for ship board power system

Abstract

In this paper, the Modular Multilevel Cascaded Converter based on Double-Star Bridge Cells(MMCC-DSBC) for Medium Voltage DC (MVDC) amplifier is proposed. The medium voltage DC (MVDC) amplifier system is required to validate new technologies, new high power non-linear loads based on power electronics, in all electric ships as part of the proposed medium voltage (MVDC) ship power system. Thyristor based medium voltage DC amplifier (6 kV to 24 kV DC from 4.2 kV (L-L) AC three phase system) with Series Active Inverter (SAI) and DC side transformer in series with DC bus has been reported in the literature, where the series active Inverter compensates the slow dynamics of the Thyristor converter and facilitates fast dynamic response for step changing loads. However, this system has disadvantages as shown in Table III. This paper first shows a control method for AC to DC operation of MMCC-DSBC converter to control the wide DC output voltage in both buck and boost modes from fixed AC source. The simulation results show that the MMCC-DSBC converter provides a variable DC voltage from 6 kV to 24 kV DC and also the extended wide range of output DC voltage — 1 kV to 24 kV — from fixed 4.2 kV (L-L) AC three phase system. It is also shown that this topology will provide fast dynamic response compared to Thyristor based Line Commutated Converter (LCC) without the need of Series Active Inverter, DC side transformer in series with DC bus. This paper also shown the the control method to operate the MMCC-DSBC converter in STATCOM mode-to provide reactive power support to the AC side grid-during DC side pole to pole fault. Simulation results are presented on EMTDC/PSCAD platform to validate the controls.