Manfred Winkelnkemper

Low output frequency operation of the Modular Multi-Level Converter

Modular Multilevel Converters (MMLC) based on series connected half-bridges achieve high phase voltages, need little or no filters and feature redundancy and modularity. In contrast to the similar series connected H-bridge converters an expensive and lossy transformer is not necessary. The capacitors buffer power fluctuations at fundamental and second harmonic frequency, therefore the capacitor voltage ripple magnitude increases with decreasing phase current frequencies and will become infinite at zero phase current frequency. This is a problem in variable speed drive applications where phase current frequencies from starting from zero are needed. An operating mode for low phase current frequencies which enables MMLCs to magnetize and start induction machines with quadratic torque loads is presented and the achievable torque-speed characteristic is derived. It is also shown that rotor flux optimization also reduces the capacitor voltage ripple at low torque. The method has successfully been tested in an experimental converter.

Active Neutral-Point-Clamped Multilevel Converters

This paper proposes a multilevel power conversion concept based on the combination of neutral-point-clamped (NPC) and floating capacitor converters. In the proposed scheme, the voltage balancing across the floating capacitors is achieved by using a proper selection of redundant switching states, and the neutral-point voltage is controlled by the classical dc offset injection. Experimental results are illustrated in the paper to demonstrate the system operation

Active-neutral-point-clamped (ANPC) multilevel converter technology

This paper proposes a multilevel power conversion concept based on the combination of neutral-point-clamped (NPC) and floating capacitor converters. In the proposed scheme, the voltage balancing across the floating capacitors is achieved by using a proper selection of redundant switching states, and the neutral-point voltage is controlled by the classical dc offset injection. Experimental results are illustrated in the paper to demonstrate the system operation

A modular direct converter for transformerless rail interties

Transformerless rail interties are of high interest for the interconnection of the European 15kV/16.7Hz rail supply with the 50Hz industrial grid due to the direct connection to the rail supply without a 16.7Hz transformer. The MMLC is an emerging topology which offers high blocking voltages, high modularity and very high power quality. Two transformerless interties are presented and compared which are the “indirect MMLC” and the “direct MMLC”. The design for a 15MVA intertie and the calculation of the currents and voltages are the presented in detail. The cell capacitances are determined from the internal branch currents and voltages. Semiconductors loss calculations have been carried out in detail. The comparison of the indirect and the direct MML shows clear advantages of the direct MMLC.

6 MVA five-level hybrid converter for windpower

For the further improvement of todays full converter systems for windpower applications a 6 MVA converter prototype has been developed. It is utilizing the ANPC5L topology as inverter on the grid-side and the simple 2-quadrant ANPC3L topology as rectifier for a permanent magnet synchronous generator (PMG). ANPC stands for Active Neutral Point Clamped, which is the base for the derivation of multiple topologies, i.e. 3-, 5- and higher levels. The ANPC5L converter is realized as a hybrid converter based on IGCT (Integrated Gate Commutated Thyristor) presspack devices and IGBT (Insulated Gate Bipolar Transistor) modules. The ANPC5L is modulated at 2 kHz. The IGCTs, used as inner switches, are operated at fundamental frequency, which allows a substantial reduction in silicon size. To meet the harmonic limits of the IEEE 519 no additional passive filter is required. To fulfill the more stringent VDEW requirements a simple LC filter is implemented. On the permanent generator side a simplified 2-quadrant 3 level converter (2Q-ANP3L), with 50% less turn-off switches than in a conventional NPC converter, is used. The operation of this simple topology with a discontinuous modulation allows the adjustment of the power factor of the permanent magnet synchronous generator for all operation conditions. The 6 MVA converter has been designed, built and tested in the power lab. The dimensions of the 6 MVA converter prototype, represents one of the most compact designs achieved for windpower applications today.

Transformerless multi-level converter based medium voltage drives

A new concept for transformerless medium voltage drives is presented, which avoids any common-mode inductors. By introducing a small capacitive filter on the grid side with a low-impedance ground connection at the star-point, a dominant common-mode voltage capacitive coupling to ground is achieved. No significant common-mode voltages or currents are reaching the grid in low- and high-impedance grounded systems. It is shown, that the additional common-mode stress on the medium voltage machine windings and related bearing current effects are well under control even for standard electrical machines and retrofit applications. The demonstrated concept can be applied to any kind of VSI based multi-level converter, as for example for ANPCML and MMLC converter solutions.

Control of a 6MVA hybrid converter for a permanent magnet synchronous generator for windpower

To further optimize the cost and performance of todays full converter systems for high power windpower applications a 6 MVA prototype has been designed, built and is under final test now. It contains the new 4-quadrant 5 level active neutral point clamped converter (ANPC) topology on the grid side and the simple 2-quadrant 3 level ANPC topology on the permanent magnet synchronous generator side. Both ANPCs require special modulator feature for proper converter control. In this paper the windpower system control and its special modulator features are explained in detail. Experimental results are proving full modulator functionality.