Enabling wind turbine generators to participate in power grid frequency regulation for enhanced stability
"This dissertation focuses on wind farm frequency regulation capability to maintain the grid frequency stability. Wind plant power controllers are designed and tested for participation in grid frequency restoration. The small signal stability on sub-optimal operating points is analyzed and a de-loaded operating point is proposed. In the first paper, a new frequency regulation scheme is developed for the wind turbine/generator/converter trio that will provide the capability to participate in restoring frequency in a way similar to the droop response of conventional generators. Output active power adjustment can be realized by both converter and pitch angle control in addition to inertial response of the wind turbine. This helps in maintaining instantaneous power balance as well as in longer term frequency regulation. In the second paper, Inertia controller, pitch angle controller and rotor speed controller are introduced for wind plant output active power adjustment. The control schemes are developed for wind generators to equip them with the capability to participate in restoring gird frequency. With the proposed controllers, the wind plant can operate as a synchronous generator and is able to output larger or smaller amounts of power as required so as to contribute to grid frequency restoration. The controllers are tested on a wind plant operating in a 4-bus test system for verification of the grid frequency performance under various system dynamic conditions. In the third paper, a reduced-3rd order doubly fed induction generator (DFIG) wind turbine dynamic model is described in details. This model along with an active power controller integrated with a power system is investigated for small signal dynamic stability for both sub- and super-synchronous operating conditions. Assuming constant wind speed and blade pitch angle the mechanical power of the machine is modeled as a function of the rotor speed. The system is initialized and linearized around the sub- and super-synchronous operating points. The eigenvalues and participation factors are calculated for a constant power reference as well as 90% P max reference. Both the open loop and closed loop controls on DFIG active power are applied for eigenvalue calculation. The results show that the DFIG operating at the sub-synchronous operating points is unstable"--Abstract, page iv.
Missouri University of Science and Technology
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