Low Short-Circuit Ratio Connection of Wind Power Plants
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- Institutt for elkraftteknikk 
Primary factor for the site selection during the planning process of the large modern wind farms is the wind climate, which is usually favorable at remote and offshore locations where the public grid is not particularly strong. Among the consequences of this solution is the necessity to connect wind farms to weak points of the grid and the necessity to reach this point by the means of long connection lines. All the mentioned factors result in a low short circuit ratio connection of wind farms becoming a frequent condition to deal with. Wind turbine manufacturers and wind farm operators have already faced various engineering problems concerning the wind farms, operating in weak grids. One of them is inability to transfer the desired amount of the active power along the needed distance due to the lack of transmission capability. Besides that the system has to operate at the tip of its PV curve, which makes it vulnerable to voltage instability in case of sudden changes in a system, for instance a load connection or a short circuit. Furthermore, all the modern wind farms are using power electronic converter based drivetrain system, which has numerous advantages in terms of controllability but also demonstrates much lower short circuit current capabilities, compared to the previously used synchronous generator technology. That minimizes the modern wind turbines contribution to fault recovery, which in cases of severe faults might cause a wind power plant to violate the grid codes requirements, resulting in unfavorable consequences for the wind farm operator. Among the consequent instabilities, reported by the wind turbine manufacturers are the slow voltage recovery after system faults and oscillatory voltage instability in response to small disturbances. A peculiarity of the previously held studies is that they are constrained by a range of case-dependent parameters, due to the fact that vast majority of the offered solutions propose the refinement of the turbine voltage controllers gains. The proposed solutions have demonstrated limited positive effect, however they are not universal, due to the fact that the voltage controllers are tuned on a case to case basis. Therefore this thesis carries out a systematic analysis of the nature of the occurring phenomenas instead of a case study solving. The investigated system is modeled, using the per-unitized conventional power system elements, with an emphasis on their mutual relations and not bound by the magnitudes. Therefore system behavior is not conditioned by any specific case peculiarities. On the contrary, the integral dependences are being tracked and the solutions are supplemented by the mathematical derivations, based on the fundamental power system laws. Due to this approach the reason of the occurring instabilities has been detected, explained and resolved. Lack of transmission capabilities, shown by the simulations lays in the insufficient accuracy of the simplified power system modeling with the shunt capacitances neglected. The system, modeled with the shunt capacitances included does not possess the above-mentioned problems. Power system oscillations have been eliminated by means of controller tuning and insufficient voltage recovery has been overcome by means of partial reactive power compensation. The recommendations on modeling and control refinement are given, based on the derived dependences and tracked properties of the high impedance grid with high wind power penetration.