Full State Regulation of the Modular Multilevel DC Converter (M2DC) Achieving Minimization of Circulating Currents

Abstract

The M2DC exploits the interleaving between the three legs of an MMC to realize a promising uninsulated DC/DC converter to interconnect HVDC grids. This paper details a current and energies decoupled model of the M2DC. The major idea proposed in this paper is focused on the full energy control generating optimal current references to minimize the internal currents magnitude. The energy sum and difference models are fully detailed. Both current and energy control loops are based on the model inversion principle in order to control all the state variables. The proposed control is based a dynamic control developed with the model inversion principle associated with an optimization of the current magnitude deduced from a quasi-static analysis. All dynamics of the system are then explicitly controlled, which guarantee a good dynamic behavior during the transient. Therefore, current and energy controls are presented in detail. Simulation results show the dynamic behavior of the converter for various operating points.

The Modular Multilevel DC Converter With Inherent Minimization of Arm Current Stresses

“…The ac current stresses go up as the dc input voltage is further stepped down, eventually rendering the topology impractical for certain applications, e.g., HVdc to MVdc interconnects or HVdc power tapping. Some works have focused on control methods to reduce the ac currents needed for a given P dc [27], [28], however, the resulting ac currents will still be inherently large when V dci and V dco are significantly different.…”

Impact of DC fault blocking capability on the sizing of the DC-DC Modular Multilevel Converter

“…The DC-DC Modular Multilevel converter (here denominated DC-MMC, but also known in literature as M2DC [12]) is a non-isolated DC-DC converter formed by the interconnection of several MMC converter legs in parallel, where the AC output ports have been connected together on the low voltage (LV) DC port through an AC filter. The AC filter can be of different types [14] and its principal function is to mitigate the propagation of AC currents into the DC grid and the magnitude of the AC circulating currents.…”

“…1 (b) shows the current circulation inside the circuit, highlighting the AC currents and the DC current of each DC side. A more detailed explanation of the working principle of the topology can be found in [12] where an energy based approach [15] for the topology control is proposed. The required AC power depends of the DC voltage transformation ratio :…”

“…(4) In order to reduce the converter losses [12], [13], the voltage magnitudes must be equal, i.e. | | = | | = .…”

“…The recent research on this circuit has increased given its advantages compared to other converters. Some of the works address the topology working principle, its modelling [10], [11], its control [12], the circuit sizing [13], and comparative studies [6]. However on all of these works the fault blocking capability is only mentioned by the inclusion of FB-SMs ignoring the impact on the costs and losses of including such feature.…”