Oscillation Performance and Wide‐area Coordination Control of Power System with Large‐scale Wind Farms

With the growing interconnection among distant power grids, low‐frequency oscillation problem across wide area arises in some modern power systems. This oscillation problem is commonly regarded as lack of damping in inter‐area oscillation modes in the sense of small signal stability. Solutions to this problem need to be implemented in the power systems. On the other hand, wind power especially largescale wind farms are increasingly integrated into modern power systems and bring new challenges to power system operation and control. The influence of wind power integration on system oscillation performance needs to be investigated. And according to some grid code requirements, power system oscillation damping improvement supplied by wind farms might become necessary. Under this circumstance, the present dissertation aims at studying oscillation damping control strategies using different components in the power system; investigating the possible influence of large‐scale wind power integration on system oscillation performance; developing oscillation mitigation strategies for wind farms; and coordinating various damping controllers in the power system. For the power system operation aspect, an optimal power flow (OPF) strategy is introduced, with system oscillation damping considered in the constraints of the optimization process. The method is validated by simulations in two test systems. For the power system control aspect, an optimal PMU placement method is used so as to provide full topological observability of the target power system and to provide wide‐area signals for damping controllers while minimizing the total PMU installation cost. Power system stabilizer (PSS) designs with both local input signal and wide‐area input signal are studied and compared. The PSS design is based on residue method and a residue identification technique is used. Furthermore, a particle swarm optimization (PSO) based coordinating strategy to select the locations, input signals and parameters of multiple PSSs is proposed. Simulation results show that this method is able to find a group of PSSs to improve the target mode damping to a certain level, while keeping the total magnitude of the PSSs low (so that the outputs from the PSSs could be kept low).
A supplementary damping controller is integrated in the control system of a static synchronous series compensator (SSSC). The design is also based on residue method and residue identification. Simulation results show the effectiveness of this damping controller under different operating conditions of the SSSC. Influence of a direct‐drive‐full‐convertor based wind farm ancillary frequency control and voltage control on power system oscillation performance is investigated by observing the oscillation damping change in relation with the change of the ancillary controller parameters. Furthermore, the forced oscillation in the power system activated by the wind power oscillation due to wind shear and tower shadow effects is analyzed. The forced oscillation amplitude is found to be dependent on the location of the wind farm, the amplitude and frequency of the wind power oscillation and the damping ratio of the system oscillation modes. To mitigate the forced oscillation, two types of controllers are designed to reduce the wind power oscillation amplitude and to increase system oscillation mode damping ratio, respectively. The former controller is implemented in individual wind turbines; the latter controller is implemented in the wind farm level as a supplementary damping controller.
Finally, the coordinating selection and parameter design strategy for PSS is extended for all types of damping controllers (including PSS in synchronous generator, SSSC damping controller and wind farm damping controller). Simulation results show the effectiveness of this strategy for coordinating various damping controllers.