Quasi-Two-Level Flying-Capacitor-Converter for Medium Voltage Grid Applications

Abstract

Medium voltage converters are becoming more relevant in future grid applications. Especially more wind and PV generation systems will be have to added and integrated into the grid in order to achieve the required reduction in CO2 emissions. High power generation units will be integrated into the grid at medium voltage level. Furthermore meshed MV grids will require interties based on MV converter technology. Today's standard medium voltage converters are either based on the MMC technology or us 3L/5L approaches operated at low switching frequencies and containing bulky filters. Both concepts still lead to relatively high costs and low efficiencies which are major reasons for the slow spread of power converters in medium voltage grid applications.. Future grid applications demand less distortion, higher reliability and lower costs for converter systems at each voltage level. The Quasi-Two-Level operation of the flying capacitor multilevel converter with silicone-carbide (SiC) based semiconductors is a method for achieving these requirements for medium voltage converters. This paper presents a new concept for minimizing capacitance and balancing the capacitors with fast switching semiconductors. Moreover, a novel approach for limiting the overvoltage stress caused by the Quasi-Two-Level modulation of a flying capacitor converter is presented.

Load-Independent Voltage Balancing of Multi-Level Flying Capacitor Converters in Quasi-2-Level Operation

“…Considering all cells, the mean voltage deviation with CL balancing is approximately half that observed with OL balancing, i.e., 10.8% instead of 21.8%, respectively. The proposed CL balancing concept compared to methods proposed in the literature, e.g., [31,32], avoids the operation with opposite voltage ripple on FCs, therefore, the switch voltages are not unbalanced by a maximum peak-to-peak voltage ripple of the FCs, which significantly improves the symmetry of the switches' blocking voltages as shown above. This highlights the superiority of the proposed CL balancing concept.…”

“…The most straightforward approach is to implement a controller which uses measurements of the FC voltages and of the output current to achieve close tracking of the FC voltage references. However, as indicated in [32] due to the opposite voltage ripple on FCs, the switches can be operated with a strong asymmetry of the blocking voltages. To address this issue, alternatively the controller can be built to equalize the cell voltages, derived as differences between the voltages of capacitors adjacent to the switches, and hence ensuring equal voltage sharing among the series-connected switches.…”

“…With Q2L modulation, the intermediate voltage levels are only used during the switching transitions. The bridgeleg's output voltage thus transitions between the two DC voltage levels (positive and negative) in a staggered fashion [22][23][24][28][29][30][31][32][33][34]. Note that these staggered transitions of the the Q2L-MMC and Q2L-FCC topologies feature lower average dv/dt compared to the (MV) 2-level converters, which is beneficial for the design of EMI filters and magnetics such as medium-frequency transformers, and lowers the stress of the electric insulation [29,[35][36][37][38].…”

“…In this context, switching sequence refers to the order in which the individual FC cells are commutated during a single Q2L transition. The authors of [32] present a balancing algorithm incorporating all switching sequences based on FC voltage errors by prioritizing the FC with the largest voltage error. As shown in [32] due to the opposite voltage ripple on FCs, the switch voltages can by unbalanced by a maximum peak-to-peak voltage ripple of the FCs, which leads to a strong asymmetry of the switches' blocking voltages.…”

“…The authors of [32] present a balancing algorithm incorporating all switching sequences based on FC voltage errors by prioritizing the FC with the largest voltage error. As shown in [32] due to the opposite voltage ripple on FCs, the switch voltages can by unbalanced by a maximum peak-to-peak voltage ripple of the FCs, which leads to a strong asymmetry of the switches' blocking voltages. To mitigate this asymmetry, further investigations of active balancing methods are required, and experimental verification of such methods, which, to the knowledge of the authors, is so far missing in literature, is needed.…”