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Electrical and electronic engineering
Double-fed induction generators (DFIG’s) became the predominant generator installed for wind generation applications in the mid 1990’s. Issues pertaining to the operation and control of DFIG’s subsequently became apparent, particularly in weak areas of the grid network. Ironically weak areas of the grid tend to be where the average wind speed is high and the usual location of wind farms. One of the issues that emerged was the quality of the voltage in the network at the point of common coupling (PCC) with the DFIG’s. An important issue is the question of voltage unbalance at the PCC. As part of this work, research was undertaken into the issue of voltage unbalance in a distribution network. Investigative studies were undertaken on a small wind farm connected to the Irish distribution network. The results obtained were then analysed and conclusions drawn, with recording of daily, weekly and seasonal variation of voltage unbalance. The behaviour of DFIG’s to varying levels of network voltage unbalance at the wind farm was analysed, and it was observed that the DFIG’s had difficulty remaining connected to the distribution network when voltage unbalance exceeded certain threshold levels. The behaviour of DFIG’s to the effects of grid network voltage unbalance is further investigated in this work. A literature review was undertaken of the effects that utility network voltage unbalance has on DFIG’s. Emerging from this research, the suitability of appropriate control schemes to alleviate the problems caused by grid voltage unbalance were investigated. Control techniques to improve performance of a DFIG during conditions of asymmetrical grid voltage including measures to control the rotorside and grid-side converters in a DFIG, were designed and then implemented in Matlab/Simulink and results showed improved behaviour. A synchronous generator system was similarly investigated and improvements shown. This research also includes development of a laboratory based DFIG test system. A DSP based digital microcontroller and interfacing hardware has been developed for a 5kVA DFIG laboratory based system. The system comprises of a machine set; a dc machine with common shaft coupling to a three-phase wound rotor induction machine. The dc machine emulates a wind turbine, and drives the induction machine in response to required speed. A converter has been constructed to control the rotor power of the induction machine. Interfacing schemes for the required feedback signals including voltage and current transducers and speed measurement were designed to enable control of both the rotor-side and grid-side converters of the DFIG. Grid/stator voltage oriented control is implemented to control both the rotor side and grid side converters respectively. An additional feature is the implementation of a single DSP controller, configured to control both the rotor side and grid side converters simultaneously. Initially the DFIG test rig was tested as a standalone system, with a load bank connected to the stator terminals of the induction machine. Testing of the DFIG was also conducted with the test rig connected directly to the grid, and the system operated in subsynchronous and super-synchronous modes of operation. Hardware and software solutions were implemented to reasonable success. The laboratory based test rig has been designed for operation as a rotor converter for a DFIG; however the converter can also be configured to operate as a system for a synchronous generator, or for operation as a machine drive. Further research may allow the rig to be used as a DFIG/UPQC (unified power quality controller) test bed.
Kearney, J. (2013) Grid Voltage Unbalance and The Integration of DFIG’s. Doctoral Thesis, Technological University Dublin. doi:10.21427/D7QK6V