Publikationen

This paper shows the possibility of using standardized communication system components to shorten the development time of complex power electronics converter control. It can be seen that the combination of MATLAB/Simulink with EtherCAT fieldbus is an excellent alternative to classical microcontroller-based solutions. Investigations were carried out on a modular multilevel converter topology (MMC), which was described in [1]. The elementary demands on the communication and control system, as well as the development process from simulation to implementation, are briefly reported. Finally, the performance of the overall system will be examined and the impacts of discrete signals and propagation delay on the Total Harmonic Distortion (THD) of the output voltage are discussed.

The Modular Multilevel Converter (MMC) is a proven topology in HVDC, but it is also a suitable solution for medium voltage applications such as wind turbines. To increase the modularity and the scalability of this topology for medium voltage applications, a real-time communication bus between the submodules and the central control unit is implemented as an efficient way of control. This real-time communication is realized using EtherCAT communication. To verify the advantages of this control strategy a 3-kV 350-kVA demonstrator of a medium voltage MMC topology with EtherCAT communication is constructed and set up. The description and the analysis of this demonstrator with focus on the EtherCAT communication are presented in this paper. The communication path, the synchronization and the control including the modulation method are discussed, investigated and validated within this paper.

This paper investigates the possibility of applying medium-voltage modular multilevel converters in 5-MW onshore wind turbines. The focus is on the reduction of higher order harmonics in comparison to classical 3-level medium-voltage converters. The simulation of the 7-level 5-MW converter shows that the harmonic limits in accordance with the actual grid codes can be met without an additional grid side filter. The paper starts with a description of the detailed grid and converter model parameters, followed by the grid compatibility simulation according to the standard grid codes. Subsequently, a setup of a 3-kV 350-kVA MMC demonstrator with commonly used low-voltage components is presented which will afterwards serve to verify the simulation results.

In the last years pulse-controlled inverters using Insulated Gate Bipolar Tran sistors (IGBTs) have reached an outstanding position in the field of motor control and energy distribution. The power losses of the IGBTs can be lowered by the use of resonant techniques also called soft switching techniques. Hence it is possible to reduce cooling efforts, to raise switching frequency or to increase efficiency. In most cases the aim is an one-to-one replacement of the hard swit ched inverter. The Auxiliary Resonant Commutated Pole Inverter (ARCPI) has become a promising candidate to reach this aim, however, even this topology has got some drawbacks, e.g. balancing problems with the capacitive voltage divider in the DC link. This thesis starts with an excursus in the technology of modern power semi conductors in consideration of silicon carbide as a new base material. Subse quently a review of the most important resonant converter structures follows including a proposal for a structured method for their classification. Theoretical and experimental analysis of a total of four Resonant Commuta ted Pole commutation cells is done. The starting point is the classic voltage fed ARCPI. Results achieved in an optimised test set-up of a 1MVA converter for a motor-generator of a flywheel storage system are summarised. The three-phase current source ARCPI only needs two auxiliary switches. Therefore and due to the reason, that its capacitive voltage divider is easily stabilised by connecting it to the star point of the load, this solution is particularly attractive. Hence a three-phase inverter is built and measured. A new commutation cell, the so called Resonant Commutated Twin Pole (RCTP) cell, is derived using the laws of network theory. Its functionality is proved in a single-phase setup connected to a current source. The more promising voltage fed RCTP cell is examined in a three-phase inverter. All phases are realised using different technologies. Power losses in hard and soft switched mode with and without silicon carbide diodes are directly compared to each other. The theses is concluded by an appraisal of the new concept and by an outlook to possible realisations.

The Resonant Commutated Twin Pole Inverter (RCTPI), a novel resonant pole inverter topology using only main switches, is introduced. One phase of a voltage source RCTPI consists of four switches arranged in two symmetric half-bridges called the twin poles. Each pole carries half of the phase current. The switches are turned off as zero voltage switch (ZVS) and zero current switch (ZCS) respectively and turned on under ZCS both. Switching losses in the semiconductor devices can be reduced in all operating points by soft switching, while overall losses are reduced only at high load currents. The concept can be realised using IGBTs in standard packages. Single switch modules, half- and full-bridge modules and some six-packs are suitable – this is an advantage compared to many other resonant topologies.

To reduce the losses of power inverters, resonant topologies replace hard switching power inverters. This paper shows the first investigations and measurements carried out to design a three phase current link power inverter in ARCP (Auxiliary Resonant Commutated Pole) topology as well as the finally measurements and results achieved with the complete system.

SiC-JFETs are promising for many future applications but have a normally-on behaviour, which usually necessitates an auxiliary MOSFET to achieve self-blocking capability. However this characteristic is predestinated to current source inverter topologies as an attractive alternative for fast switching applications. The paper presents practical investigations o SiC-JFETs with a modified driver stage in a current source inverter and for comparison in a voltage source inverter topology. Measurement results of both topologies at a frequency of 250 kHz are included. Furthermore differences in the minimising of switching losses for each topology are shown.

Synchronous machines with air-gap permanent magnet excitation are advantageous in many modern applications. As a result of the construction principle the stator inductivities are often very low. Therefore, common voltage source inverters with sinusoidal output currents are not automatically the best supply solution for these machines. Either load reactors have to be used to smooth the output current or the high du/dt-rates of faster switches produce EMI problems and overvoltages at the machine windings. Investigations have shown that sine-wave machine currents are not the most effective way to feed these drives. This paper presents a novel way to overcome these problems by using a current source IGBT-inverter. The investigated system consists of a pulse-rectifier, a DC-current-link and an inverter with an output capacitor bank to decouple the stator currents from the DC-link. A simple, modular inverter construction is proposed. It makes no use of snubbers and is scalable over a wide power range. Practical experiments to prove the feasibility have been carried out with positive results.

Zero Voltage Switching (ZVS) as applied in the Auxiliary Resonant Commutated Pole Inverter (ARCPI) can reduce drastically the relatively high switching losses of High Voltage IGBT-Modules. Thus the switching frequency can be much higher than in conventional hard switched converters. This paper describes the construction of a scaleable ARCPI-structure. The structure was tested successfully at first in an 80 kVA inverter test-setup, while this paper presents first measurements and experiences with an 1 MVA test-setup with 3.3 kV-IGBTs. Switching frequencies up to 5 kHz were achieved with a rated load current of 800 ARMS.

This paper illustrates the development of an ARCPI with a simple, fully digital control which works on the basis of pre-calculated switching time values stored in a look-up table of a PLD. Although the load current is the only measured input value of the control system there is no need for special driver-stages with a zero-voltage detection possibility. The PLD-control can be easily adjusted to cope with different input parameters and therefore it can be used in full power range without hardware changes. The paper presents the calculation models for the values of the resonant components and the switching times and compares the theoretical results whith the measurements carried out on two different test-setups in the mid and high power range.