Stefan Soter

Development of Induction Machines in Wind Power Technology

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.

Low Cost Position Sensor for Permanent Magnet Linear Drive

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.

Zero sequence current elimination for dual-inverter fed machines with open-end windings

A dual-inverter allows to increase easily the maximum available power of a given inverter. When connecting two equal inverters to a machine with open end windings, the available power will raise by factor 3√. However, the dual inverter also introduces a new degree of freedom for the current, which is the zero component current. The dual inverter is known for developing zero component currents, which have three times the frequency of the fundamental current, and can be of a rather large amplitude. This paper analyses the origin of these zero component currents and shows an example of how to eliminate them. Moreover it shows the possibility to suppress zero component currents by using only software algorithms modifying the current control loop. There is no need for extra components. Moreover there are no drawbacks in the quality of control as well as regarding the maximum available voltage.

Reduction of detent force caused by the end effect of a high thrust tubular PMLSM using a genetic algorithm and FEM

In order to meet industrial safety standards in eccentric presses, the detent force of the acting high force tubular permanent magnet linear synchronous machines (PMLSM) should be reduced. The detent force is caused by two components: the slot effect and the end effect. Both effects are based on the reluctance change between permanent magnet (PM) and stator teeth. The slot effect is already optimized by the closing slot technique which is proven by measurements presented in this paper. The main topic is the reduction of detent force by using auxiliary poles at the end of the machine. Therefore a special genetic algorithm (GA) is developed which rates the simulation results of the FEM and produces new auxiliary poles. The combined simulation tries to find an optimal size and position for auxiliary poles to reduce the whole detent force. The numerical calculations propose a minimized detent force caused by the located poles, which is independent from the length of the machines (2, 3 or 5 modules).

Pressure control of a nonlinear system with a linear-PMSM and a standard inverter

This paper deals with a high dynamic hydraulic pressure control. The pressure is generated by an inverter driven linear drive which applies a force on a hydraulic cylinder. The linear drive has to generate a pressure profile into the hydraulic system and has to keep it at a constant value after reaching the demanded pressure. The controlled system, containing hydraulic oil, dissolved air and overall three spring packs, is showing a highly non-linear characteristic. The control cascade of the inverter is based on a standard position control loop, which is enhanced by a PI based pressure controller, a dynamically adapted position feed-forward and an active anti-windup of the controller in order to provide optimal control behavior of the nonlinear system.

Development and Test of a High Force Tubular Linear Drive Concept with Discrete Wound Coils for Industrial Applications

This paper deals with the development of a tubular permanent magnet linear drive with radial magnetized armature and discrete wound coils mounted on a star-shaped stator part. The rated force of the developed machine is 500 N per segment. This presented particular design results in a very economic product because all primary parts, except of the permanent magnets and coils, are made of standard non-laminated steel and are optimized for easy production and assembly. The control of this machine with a specially built low cost linear sensor based on the Hall-effect is also presented. The suitability of the design is proven by the demonstration of a prototype with measurements of thrust and cogging force.

SOGI based grid fault detection for feeding asymmetrical reactive currents to fulfill LVRT requirements

Due to the increasing wind power penetration, grid codes of system operators require low voltage ride through (LVRT) capability for wind turbines (WT). Additionally the WT has to support the power system stability in LVRT cases by supporting the grid with reactive power. The amount of reactive power feed-in depends on the type of grid fault and the depth of the voltage dip. Therefore this paper shows a reliable grid voltage monitoring consisting on a second order generalized integrator (SOGI) structure. The resulting phase locked loop (PLL) is tolerant against grid faults and the amplitudes and phase angles of the individual phase voltages are detected.

System analysis and optimization of a pressure control for a hydraulic-Clutch-Brake-Combination actuated with a linear drive

This paper deals with the analysis of an electro-hydraulic system and conception of a pressure control. The controlled system consists of a permanetmagnet linear actuator, a hydraulic pressure line and a Clutch-Break-Combination (CBC). The linear actuator generates a force which causes the pressure in the hydraulic line. Depending on the system-pressure, the Clutch-Brake-Combination brakes or clutches with a different torque. Because of the integrated prestressed springs of the CBC, the controlled system is very nonlinear. In addition there are different disturbances, which makes it hardly controllable. The linear actuator is driven with a standard inverter.

Inverter based voltage sag generator with PR-controller

Due to the increasing wind power penetration, grid codes of system operators require low voltage ride through (LVRT) capability for wind turbines (WT). Additionally the WT has to support the power system stability in LVRT cases. To evaluate the LVRT capability of grid connected converter, a voltage sag generator (VSG) is required to emulate grid failures. This paper introduces a three phase programmable inverter based VSG, which is equipped with a cascaded control structure consisting of proportional resonant (PR) current controller and PR voltage controller. The described VSG is able to emulate all required voltage sags, propagated through a delta star connected transformer, very precisely. The control structure has been simulated and tested successfully on a 2MW full power testbench.

Optimization of a high force tubular linear drive concept with discrete wound coils to fulfill safety standards in industrial applications

This paper deals with the next step of development of a tubular permanent magnet linear drive concept for industrial applications up to 3000N thrust force where low cogging forces are required to fulfill safety standards. The presented linear drive concept is designed for easy production and assembly of a few hundred units per year. It can be manufactured on standard production machines because all ferromagnetic parts are made of standard not-laminated steel. This results in a very economic product. Further more the drive concept includes an internal low cost position sensor based on the hall-effect. The different steps of development are proven by measurements of thrust and cogging forces of different prototypes.