Instability of current control loops in grid-connected converters can cause problems in wind power systems. This phenomenon has gained increasing attention in recent literature. Often, the current control loop is designed with conventional methods neglecting the presence of grid resonances. If this approach would bear the risk of instability when grid resonances occur, it could mean a large risk for wind turbine operation. This paper investigates under which circumstances such an control loop can become instable. It evaluates the influence of a grid voltage feedforward (GVFF) on stability of the current control by transfer function analysis and simulation. The results show that with GVFF, such a simple design approach yields stable current control loops, while without GVFF, instability can exist. The simulation and analysis are validated by experimental results.
Commonly used speed-control concepts permit speed ranges up to two-three times the base speed via voltage to frequency (V /f ), that is, field-weakening control. This paper teaches the extension of the speed-control range up to nine times the base speed through online reconfiguration of the motor windings via electronic switches. The switchover of the windings, either from a winding with p 1 poles to p 2 poles, or from series to parallel connection of the number of turns per phase-so-called (V /N ·f ) control-lasts less than a 60-Hz cycle. Such fast switchover causes small transients only, and therefore, this concept is applicable to most variable-speed drives.Index Terms-Change of number of poles, change of number of turns N , electronic switching of windings, increase of speed range, induction motor, voltage to N frequency (V /N ·f ) control.
This paper describes a joint research project in the field of grid integration of distributed renewable energy sources. Several research groups cooperate in an interdisciplinary approach. Main focus of this research project is the analysis of both influence and possibilities of multi-agent systems (MAS) which control consumers and renewable energy resources (RES) in future energy systems. Analysis and solutions are provided in the fields of control of power electronics that connects the RES to the grid and the interaction between multiple renewable energy resources in the grid. A detailed modelling of the connecting distribution and overlaying transmission networks is used for the analysis of distributed generation systems and their influence on the different voltage levels of the power grid. A combined simulation environment for different scenarios of single distribution grids controlled by MAS and the overlaid transmission grid in an overall system model has been developed, so both grid areas can be analysed in adequate accurateness. Furthermore the integration of grid and power market in one simulation environment is extended for analyses of frequency stability.
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