Control of VSC During Unsymmetrical AC Faults in Offshore Wind Farms

Abstract

This master thesis has analyzed the eects of unbalanced faults in the ACgrid of an oshore wind farm. The wind farm is connected to shore with aVoltage Source Converter (VSC) HVDC link, and the wind turbine genera-tors employs VSC technology in full scale converters. The oshore wind farmhave been modelled in Matlab SIMULINK, and there have been performeddierent short circuit simulations at various places in the oshore AC grid. Acontrol method designed to control positive and negative sequence currentsin parallel have been tested and compared to a standard control system dur-ing these short circuit simulations. The theory behind the modelled negativesequence current controller is that it can be used to control active poweroscillations, thus limiting DC voltage oscillations caused by this pheonmena.The simulation model built in Matlab SIMULINK could be further op-timized, and many challenges were faced to obtain a functioning and stablecontrol system. Due to this, the negative sequence current controller is onlyenabled in the wind turbine converter, and not in the HVDC link oshoreconverter. Still it should be able to dampen out the oscillations in the HVDClink. The results obtained from the simulations show that in general thenegative sequence current controller does not improve the oscillations on theHVDC link DC voltage signicantly. When it is enabled, it performs worsethan the standard controller during a three phase fault, which in theoryshould give equal results with the two control systems. This means that theproblem is probably not the theory of the negative sequnce current controller,but rather the implementation of the controller in the modelled system. Itis therefore recommended further work to obtain a more credible simulationmodel with better tuned controller parameters.This thesis has resulted in a Matlab SIMULINK model based on theNOWITECH reference turbine, and there are many possibilities for furtherresearch on the reference turbine, as well as general research on control ofoshore wind farms during fault ride throughs and other transient conditions.A brief overview over challenges and pitfalls when building a large simulationmodel in Matlab SIMULINK is available in Appendix A, this is very muchrecommended for students undertaking their rst large project in MatlabSIMULINK.