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Synchronous Reference Frame Hysteresis Current Control for Grid Converter Applications

This paper investigates the operation of space-vector-based hysteresis current control in the synchronous reference frame (SRF) for grid-connected voltage source converters. The space-vector-based approach allows for the systematic application of zero-voltage vectors and prevents high switching frequencies caused by phase interaction. The average switching frequency can be minimized by always selecting the zero-voltage vector that is closest to the previous switching state. Implementation in the SRF ensures a simple structure based on two three-level hysteresis comparators and a switching table that fits well into the traditional structure of vector-oriented current control. For operation in a grid-connected converter with an LCL filter, the current references to the hysteresis controllers are given by a dc-link voltage controller and an active damping mechanism for suppressing filter oscillations. Results from simulations and laboratory experiments are presented to illustrate the features of the investigated approach compared to traditional phase-current hysteresis controllers.

Control and Fault Handling in a Modular Series-Connected Converter for a Transformerless 100 kV Low-Weight Offshore Wind Turbine

A transformerless wind power generator concept with the potential to achieve 100 kV dc output is proposed in this paper. In this paper, the modular ac/dc converter system suitable for such a high-voltage generator is analyzed for normal operation and fault-tolerant modes. The control synthesis is addressed, based on an assumption that the control can be designed modularly. Additionally, the investigated control system can facilitate fault-tolerant operation without changing the main structure. The robustness of the module control system is analyzed, and the limits for fault-tolerant operation are investigated. The theoretical studies are supported by transient analysis through simulations of the turbine in EMTDC/PSCAD. Finally, the generator/converter solution is verified experimentally. The laboratory results were obtained using a 45 kW prototype with three generator segments and three converter units in series.

Synchronous reference frame hysteresis current control for grid converter applications

This paper presents the application of space-vector based hysteresis current control in the synchronous reference frame for grid connected voltage source converters. The space-vector based approach allows for systematic application of zero-voltage vectors and prevents high switching frequencies caused by phase interaction. The average switching frequency can be minimized by always selecting the zero-voltage vector that is closest to the previous switching state. For operation in a grid connected converter with LCL-filter, the current references to the hysteresis controllers are given by a DC-link voltage controller and an Active Damping strategy for preventing filter oscillations. Results from simulations and laboratory experiments are presented to discuss the features of the investigated approach compared to traditional phase-current hysteresis controllers.

Fault Ride-Through Testing of Wind Turbines

Grid codes concerning low-voltage fault ride-through capabilities of wind turbines are not (yet) harmonized, varying from country to country. Full-scale field testing of wind turbines with respect to all such codes may therefore not be practical. IEC 61400-21 is currently under revision, and the last committee draft (CD 2006) presents a standardized physical test for characterizing a wind turbines response to a voltage-dip. The results of such physical tests may be used to validate numerical models of the turbine, which in turn can be used for modelling grid code compliance assessment of particular country codes. Success with such modelled validation will provide confidence in numerical simulations. The hypothesis of this paper is that the use of numerical models, that have been validated against the standardized tests, can reduce the number of physical full-scale tests needed. This paper assesses to what degree a validated simulation model is capable of predicting the fault ride-through capabilities of a fixed speed induction generator. This is done by comparing measurements on a wind-turbine emulator in a laboratory with predictions from a validated simulation model of the laboratory emulator. Simulations and laboratory measurements show excellent agreement. The validated simulation model accurately predicts the fault ride-through capability of the direct grid connected induction generator. It is concluded that the proposed methodology is promising and thus has the potential to increase the efficiency of grid code assessments. Full-scale field tests will still be required to validate the modelling completely, but using numerical models reduces the number of field tests needed.