Implications of capacitor voltage imbalance on the operation of the semi-full-bridge submodule

Abstract

An investigation of the voltage imbalance of the two capacitors of the semi-full-bridge submodule is performed. Since the capacitances are not exactly the same, there may be a difference between the capacitor voltages. The resulting current-spike when they are connected in parallel has been analyzed in a full-scale laboratory experiment.

Low Loss Submodule Cluster for Modular Multilevel Converters Suitable for Implementation with SiC MOSFETs

“…For other states the capacitors may be charged differently, caused by having different shares of the arm current flowing through the capacitors, e.g., inserting +2V C (d), and by a mismatch of the capacitor values. The implications of parallel connecting capacitors with a voltage imbalance, and controlling this voltage imbalance to acceptable levels has been discussed in the scope of the SFB in [7]. The CDZC topology has the advantage, that the switches in series to the capacitors provide additional controllability.…”

“…If this voltage is higher than the difference between the ac terminal and the dc terminal, the fault can be controlled during a stiff dc-side fault as during the normal operation with full dc-side voltage. In the case of a pole-to-ground fault, the converter arm needs to provide an ac voltage equal to the peak of the ac lineto-neutral voltageV ac , which is determined by M , see (7). Combined with (5), this yields that M is limited to less than M max if such a fault should be controllable (8).…”

“…During that time interval most submodules are in double-zero bypass state, reducing conduction losses significantly. The SFB, investigated in [5]- [7], is a topology created by double-connecting two FB submodules and merging the switches between the connections to one (Fig. 1d).…”

Implications of Capacitor Voltage Imbalance on the Operation of the Semi-Full-Bridge Submodule

“…Note that part of this paper has been published in the Proceedings of the 2017 19 th European Conference on Power Electronics and Applications (EPE'17 ECCE Europe), Warsaw, 11-14 Sep., 2017 [29]. However, the parameter study of the maximum difference in the capacitor voltages has been considerably extended and twice as much measurement results are presented in this paper.…”

“…A reference case has been assumed, where the peak arm currentÎ arm = 900 A, ϕ = 0 • , C = 4 mF and k = ±5 %. A variation of the parameters ϕ, k,Î arm , and C is presented in the Reference arm voltage and currents[29].…”