A multilevel modular DC–DC converter topology

Abstract

A multilevel modular DC-DC power conversion topology based on cascaded H-Bridge converters in a double П configuration is presented. The topology is intended to interconnect large power DC networks. A two level control hierarchy is used to regulate the DC voltage of each H-bridge module. At the top level, DC and circulating AC currents are used to control the total energy converter in all branches (both parallels and series) of each П arrange. At bottom level, the voltage balance of a converter branch, which comprises N H-bridge modules, is carried out by balancing (N-1) capacitor voltage deviations, with respect to the average capacitor voltage. The entire topology and control strategies are simulated in a PSIM environment. Simulation results with three H-bridge converters per branch are shown and preliminary experimental results with a low power prototype are also included.

Sizing and Short-Circuit Capability of a Transformerless HVDC DC–DC Converter

“…The previous results highlight the importance of a proper selection of the auxiliary ac voltage and midpoint dc voltage to avoid extremely high power ratings that lead to a low utilization of the installed power. Substituting (11) in (10b), the optimal power rating is: Fig. 7 shows the minimum power rating in terms of SMs as a function of k r (with V dco I dco = 1 pu) when the optimal values of V u and V dcm given by (11) are used.…”

“…Substituting (11) in (10b), the optimal power rating is: Fig. 7 shows the minimum power rating in terms of SMs as a function of k r (with V dco I dco = 1 pu) when the optimal values of V u and V dcm given by (11) are used. High voltages ratios require large circulating currents, thus, most of the current capability of the IGBTs is used to handle the ac currents, which limits the dc power transfer.…”

“…8 (black trace) shows the nominal dc power of each T-section (i.e. the transmitted dc power) for V dci = 300 kV and I max = 1 kA when the optimal values of V u and V dcm given by (11) are used. As the voltage ratio k r increases, the ac circulating currents also increase and the nominal dc power decreases.…”

“…However, inner ac voltages and circulating currents are needed to compensate for the energy drift on the MMC branches [7]. The double-Π topology creates a constant pole-to-pole voltage, however, the pole-to-ground voltages present a noticeable ripple [11]. This ripple is eliminated by means of coupled inductors and capacitors in [12], [13].…”

Integrated control of offshore wind farms and HVDC links with MML converters

“…Moreover, two parallel-connected Π converters arranged in a double-Π topology are used in order to cancel out the ac voltages of each Π-converter. In this way, a dc-dc conversion can be achieved with the double-Π converter topology [159].…”