30.

T. Rösmann and S. Soter, "Regenerative Operation of DC-Series Machines in Pitchsystems for Multimegawatt Windturbines" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1--7.

Abstract:
Modern windmills and multimegawatt turbines are nearly exclusively equipped with electrical pitchsystems.The pitchsystem itself has to fulfill the following functions: In the case of strong wind conditions during normal operation, the system is responsible to limit the generator torque. On the other hand, the feathering position of the blades is needed to brake down the turbine in stop or emergency situations. In modern windturbines this aerodynamical brake is necessary, because the mechanical brake is designed as a holding brake and is not able to prevent overspeed conditions. The last function described is safety relevant. Because of this, pitch systems are equipped with an backup energy storage. So the blade can be put into feathering position in any case. DC-machines are preferred in pitch systems because of their capability to be supplied directly by the DC-storage unit without any control or powerelectronics. The motor must be designed especially for this battery driven operation because current and speed during emergency battery drive are depending on the mechatronical behaviour of the machine. Actually two different DC machine types are used in pitch systems: DC-series and DC-compound machines. DC- compound machines were introduced because of their natural 4Q operation area. But an actual development allows the regenerative operation also for DC-series machines and makes them very cost attractive for multimegawatt pitchsystems.

29.

S. Soter and R. Wegener, "Development of Induction Machines in Wind Power Technology" in 2007 IEEE International Electric Machines & Drives Conference, 2007, pp. 1490--1495.

Abstract:
This paper gives an overview of the development of the induction machine as a generator for wind turbines from the middle of the 20th century up to now. After a short history chapter the different energy conversion concepts are presented. At first a squirrel cage machine is direct coupled to the grid (Danish concept). To adjust the machine speed the second step is a wound rotor machine with collector rings to change rotor resistance. This concept allows the first adaption of the operating point of the wind turbine and the induction machine. Another concept is to connect the stator of a squirrel cage machine with a full inline voltage source converter to the grid to get the flexibility in rotational speed and to control the reactive power. The doubly fed induction machine is the latest development. This concept uses a bidirectional voltage source converter in the rotor circuit with a rated power of only 30{%} of the rated generator power. Now it is possible to change the rotational speed and the reactive power independently and in a wide range. A generalized control scheme of a modern doubly fed induction machine (DFIG) is shown in the last part to explain the variability and the range of application. Today over 70{%} of the wind turbines are build up with DFIG.

28.

R. Wegener, F. Senicar, C. Junge and S. Soter, "Low Cost Position Sensor for Permanent Magnet Linear Drive" in 2007 IEEE 2nd International Conference on Power Electronics and Drive Systems (PEDS), 2007, pp. 1367--1371.

Abstract:
This paper deals with a custom made low cost sensor for measuring the position of a permanent magnet linear motor. The principle how to measure position and movement direction with two analog hall sensor elements is described. The following simulated and detailed error and failure treatment is very important to know exactly the performance and the possibilities of this low cost sensor element. Afterwards this position sensor is build and some measurements with a linear machine is done. After filtering, the accuracy of the two signals is high enough to be an input of a converter control to determine the correct current which has to be injected. If there is another higher ranking closed-loop control, e.g. pressure, flow or force, in the control system this low cost sensor is sufficient and works very well. It is possible to implement the very small sensor in the housing of the linear drive. This sensor costs less than 15 dollar and can not be compared to a very precise working linear senor for some hundred dollar in order to position the linear drive very exact but the accuracy is high enough to build a lower ranking closed-loop control and to stabilize a complex control system of converter, linear drive and load.

27.

F. Bertling and S. Soter, "Real-time prediction of the steady state temperature of circuit components as a tool for power electronic circuit testing" in PCIM Europe, 2007.

26.

F. Bertling and S. Soter, "A novel converter integrable impedance measuring method for islanding detection in grids with widespread use of decentral generation" in International Symposium on Power Electronics, Electrical Drives, Automation and Motion, 2006. SPEEDAM 2006, 2006, pp. 503--507.

Abstract:
This paper proposes a method for measuring the impedance of the public grid for islanding detection by grid connected converters performing decentral power injection. Based on subharmonic (noncharacteristic to the grids fundamental frequency) test current injection and grid voltage frequency component detection to give a defined value for the frequency dependent impedance, a modulation of the subharmonic test currents amplitude with a pulse pattern assigns each converter an individually detectable {\textquotedbl}signature{\textquotedbl} for its test signal. This allows a larger number of converters to be operated within a grid segment without interference of their measurements causing unintended grid disconnections

25.

R. Wegener, S. Soter and T. Rösmann, "Facility Employing Standard Converters for Testing DFIG Wind Generators up to 30kW" in 2006 IEEE 15th International Symposium on Industrial Electronics (ISIE), 2006, pp. 1025--1029.

Abstract:
This paper presents a test facility for a double fed induction generator (DFIG) with a rated power of approximately 30 kW. The stator of the machine is directly connected to the grid and the rotor is fed with variable voltage and frequency. Actually such machines are used in wind generators with a power of more than 500 kW with special built converters. In the presented test facility the used converter consists of two standard low cost voltage source converter units with connected DC-link. The firmware of these converters are normally not suitable for feeding the rotor because there are not able to calculate the slip frequency and phase in real time. In the presented solution this is programmed in two application modules plugged into the converter. The equivalent network parameters of the DFIG are determined by the converter. These are necessary for the development of an analytical model of the system to set up a closed loop active and reactive power control. The test system provides the ability to control both power types decoupled and with a linear characteristic

24.

R. Wegener, S. Soter and T. Rösmann, "Operation of double fed induction generators with unmodified low cost standard converters" in 2006 37th IEEE Power Electronics Specialists Conference, 2006, pp. 1--5.

Abstract:
This paper presents how to operate a double fed induction generator (DFIG) with a rated power of approximately 30kW with unmodified low cost standard converters. The stator of the machine is directly connected to the grid and the rotor is fed with variable voltage and frequency from two standard low cost voltage source converters. Actually such machines are used in wind generators with a rated power of more than 500 kW with special build converters. In the presented solution the rotor of the DFIG is fed with two DC-Link connected standard voltage source converters. The original software is designed to calculate speed and phase of the rotor by receiving a speed signal from the rotor shaft. Therefore it is necessary to add two small application modules to modify the firmware externally to realize the calculation of slip frequency and phase in real time. The equivalent network parameters of the DFIG are determined by the converter itself by using an autotune function. With this parameters it is possible to develop an analytical model of the system to set up a closed loop active and reactive power control. The test system provides the ability to control both power types decoupled and with an approximately linear characteristic.

23.

R. Wegener, S. Soter and C. Deipenbrock, "Sensorless Determination of the Exact Rotor Position of a Blocked Permanent Magnet Machine" in PCIM Asia 2006, 2006.

22.

F. Bertling and S. Soter, "Improving grid voltage quality by decentral injection of current harmonics" in IECON 2005 - 31th Annual Conference of the IEEE Industrial Electronics Society, 2005, pp. 3 pp.

Abstract:
This research project deals with the development of an adjustable converter for injection of fuel cell power. A DC/DC converter stage increasing the voltage level is connected to a DC/AC inverter for grid current injection via a high capacitance DC link. This decouples the momentary power drawn from the energy source from the momentary power being injected into the grid, allowing arbitrary output current waveforms without an increase of input current ripple. Conventionally, pure fundamental frequency currents are considered the optimum waveform regarding the injected currents impact on the grid. This paper demonstrates how grid voltage harmonics caused by nonlinear loads can be minimized by adapting the converters injected current shape. In contrast to harmonics compensation in industrial applications where the compensation current is determined by measuring the local loads current harmonics, the intended decentral application in private houses suggests a different approach to improve overall grid voltage quality. The digital signal processor used for the converters control and monitoring functions calculates the frequency spectrum of the measured grid voltage run by digital Fourier transformation (DFT) and determines the current harmonics necessary to counteract the present voltage harmonics. The resulting current frequency spectrum is modulated into a time dependant current shape that is added to the fundamental frequency current run determined by the requested amounts of active and reactive power. This way, harmonic currents of the local loads are compensated without changes in the house installation for measuring the loads currents. Also current harmonics of neighbouring houses are compensated.

21.

F. Bertling and S. Soter, "Increasing efficiency of an DSP-controlled converter by on-the-fly changing of the gate drive method" in 2005 IEEE 6th International Conference on Power Electronics and Drive Systems (PEDS), 2005, pp. 448--451.

Abstract:
This paper demonstrates how bypassing the MOSFETs body diodes in a full bridge application by freewheeling of the transformer current through the MOSFETs of one input leg decreases the power losses, depending on the operating point. A gate drive control via a modern digital signal processor is described that allows an on-the-fly change of the gate drive method so that freewheeling is activated or deactivated as required to obtain lowest losses in a wide operating range. Unlike other approaches to bypass the MOSFETs body diodes, this technique requires no additional circuit parts (e.g. external freewheeling diodes). The effect of freewheeling on power losses is illustrated by measurements of the current and voltages run, MOSFET temperatures and the characteristic curve of power losses