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The low-frequency operation of modular multilevel converters, required by variable-speed drives, is generally challenging within the established operation modes. Especially when rated torque beginning from zero speed is required and the rated machine speed is low, the installed module capacitance has to be very high. A possible solution to this problem is quasi-two-level PWM operation, which reduces the capacitance by more than an order of magnitude. Although the multilevel property of the modular multilevel converter has to be sacrificed, all other advantages, i.e. small voltage steps limiting the problems with long machine cables and bearing currents, modularity, scalability to higher voltage levels, and a straight-forward option to add redundancy, are retained from the conventionally operated modular multilevel converters. This thesis presents a derivation of the quasi-two-level PWM operation mode and proposes two suitable control methods. These methods are validated using a downscaled prototype of a modular multilevel converter. Simulations are conducted to evaluate the sensitivity of the proposed control to uncertainities in the knowledge of the converter parameters. Furthermore, the specific effects and properties of the operation mode, such as output voltage errors, reduced overvoltages with long machine cables, and the operation with flat-top modulation, are analyzed. In addition to the presented control methods, the design process is studied. First, the specific design trade-offs between the converter parameters are identified. The evaluation of trade-offs shows that quasi-two-level PWM operation is especially well-suited for modular multilevel converters with a high number of modules per branch. In the next step, the quasi-two-level PWM-operated modular multilevel converters are designed for several study cases of low-frequency medium-voltage drives. The design process utilizes the validated simulation model and derived analytic equations. Additionally, the validated model is also used to simulate a dynamic machine acceleration to confirm the plausibility of the designs. The resulting design indicators of the conducted designs are compared to those of the conventional operation mode of the modular multilevel converter, to the two-level voltage-source inverter based on series-connected IGBTs, and to the modular multilevel matrix converter, which were designed for the same study cases. The comparison to standard operation modes of the modular multilevel converter and to the modular multilevel matrix converter shows design advantages for the quasi-two-level PWM operation of modular multilevel converters. As expected, the output current distortion is significantly increased with the quasi-two-level PWM operation. Nevertheless, the current distortion is likely to be acceptable for medium voltage drives without additional filter requirements. In conclusion, quasi-two-level PWM-operated modular multilevel converters are a favorable alternative for applications in low-speed medium-voltage drives.

The modular multilevel converter offers benefits for medium and high voltage power conversion due to indirect series connection of devices. The drawback is a large amount of capacitance that has to be installed in each module, often prohibiting its use in cost sensitive applications. This letter proposes a new operation mode of modular multilevel topologies that allows reducing the capacitance by at least an order of magnitude. To achieve this, the multilevel operation is sacrificed, but many advantages of indirect series connection, such as proper voltage sharing, small voltage steps, limited voltage slopes, modularity, and scalability remain.

The present invention relates to a method for operating a resonant power converter (1) having a primary stage (2) and a secondary stage (3) at least one of which is actively operated and includes a half-bridge or full-bridge inverter (21) having a pull-up switching device (S1) coupled in parallel with a pull-up antiparallel diode (D1) and a pull-down switching device (S2) coupled in parallel with a pull-down antiparallel diode (D2), comprising the steps of: - alternately turning-off and turning on the pull-up (S1) and pull-down switching devices (S2) according to a periodic switching scheme, wherein after turning-off one of the switching devices (S1, S2) the complementary switching device (S2, S1) is respectively turned on after a delay time; - initiating an error handling, if after turning-off of one of the switching devices (S1, S2) a non-conductance state of the antiparallel diode (D1, D2) of the complementary switching device (S2, S1) has been determined.

This paper presents an IGCT unit that has been tailored for emerging soft-switching high-power applications. Redefining the state-of-the-art design and driving concepts, the size and consumption of the proposed unit can be reduced significantly. The paper investigates the operation of the IGCT with this gate unit at a ground-breaking 5 kHz continuous switching frequency. The experiments reveal the operation specifics and conclude a reliable operation.

The tutorial provides an overview of relevant technologies and concepts for high power DC-DC conversion at MW level, as well as challenges and opportunities in that field. Topics are covered with sufficient level of details to allow for understanding of design trade-offs associated with these conversion systems, expected performances and impact of various technologies on overall solution.

The invention relates to a gate unit (22) for controlling a gate commutated thyristor (21), comprising: - a voltage selector (26) for selectively applying a high supply potential (Vpos), a middle supply potential (Vmid), and a low supply potential (Vneg); - a nonlinear inductor (27) serially coupled between the output of the voltage selector (26); - a gate control unit (23) configured to control the voltage selector (26) to control switching of the gate commutated thyristor (21) in its turn-on state comprising a turn-on pulse generation, a positive-gate-voltage backporch operation, a negative-gate-voltage backporch operation and a retrigger pulse generation; wherein the nonlinear inductor (27) has a nonlinearity to have a high inductance during any of the backporch operations and to have a low inductance during the turn-on pulse generation and retrigger pulse generation.

The dc transformers based on an LLC converter operated in open loop near resonant frequency have several crucial advantages compared to other topologies. These advantages are the stiff voltage ratio with a natural power flow, the dc grid isolation via medium-frequency transformer, and the high efficiency due to the resonant converter nature. Despite these advantages, the open-loop operation does not provide any options to limit the transferred current during overload conditions in the dc grid. This is critical, since it prevents the grid from recognizing that either some local primary regulation or load shedding is required. This paper proposes a closed-loop control method that utilizes variable duty cycles to limit the current once the overload situation is detected. An applicable control scheme is derived and implemented. Since the duty-cycle modulation increases the losses of the converter, an online thermal model is implemented to protect the dc transformer during the overload conditions. The proposed current-limiting mode with the thermal model are validated using an exemplary low-voltage dc transformer prototype, demonstrating the general feasibility and overall good performance.

The present invention relates to a DC/DC converter (1) for transferring an active power between two DC subsystems (5, 6) comprising: - a first converter stage (3) and a second converter stage (4) both configured to be selectively operated as an active inverter or as a passive rectifier; - an AC coupling circuit (2) for coupling the first and the second converter stage (3, 4 ) by means of an AC power transfer; and - a control unit (10) configured to switch the operation mode of the first and second converter stages (3, 4 ) depending on a measurement of an amount of power transfer through the AC coupling circuit (2).