Low Voltage Ride Through is an important feature for wind turbine systems to fulfill grid code requirements. In case of wind turbine technologies using doubly fed induction generators the reaction to grid voltage disturbances is sensitive. Hardware or software protection must be implemented to protect the converter from tripping during severe grid voltage faults. In this paper two methods for low voltage ride through of symmetrical grid voltage dips are investigated. As a basis, an analysis of the rotor voltages during grid fault is given. First, the conventional hardware method using a crowbar is introduced. Then the stator current reference feedback solution is presented. Both methods are investigated and compared by simulation results using 2 MW wind turbine system parameters. Measurement results on a 22 kW laboratory DFIG test bench show the effectiveness of the proposed control technique.

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The growing amount of electric energy generated from distributed energy resources (DER), mainly of renewables, requires their appropriate integration into the electrical grid. These distributed renewable power sources are mainly feeding-in the power via converters and can be controlled well. Investigation of their grid integration means investigating their control and hardware concept in the dedicated grid environment. Measurements at downscaled systems in a laboratory environment are a time-and cost-efficient way to investigate this. The properties of the electrical grid, voltage amplitude, frequency and phase as well as the grid impedance can vary substantially by time, the grid impedance moreover is frequency dependent. These properties have to be emulated for the measurements of the grid integration. A dedicated research laboratory test-bench has been developed. The laboratory comprises special developed systems for grid voltage emulation, sag generation, grid impedance emulation, grid voltage correction and grid impedance analysis. Here, the design and realization of these systems in the laboratory test-bench for investigations on grid integration of distributed renewable energy resources are presented. Representative measurements for each system prove their functionality.

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