| Summary: | 碩士 === 國立清華大學 === 電機工程學系 === 97 === As the number of wind installations has grown worldwide at unprecedented rates in recent years, the average size of installations has increased due to the advent of larger capacity machine, variable speed technology, and an increasing number of off-shore sites. The raises the concern that widespread tripping of wind generators following disturbances could lead to propagation of transient instabilities and could potentially cause local or system wide blackouts. This has provoked many utilities to adopt fault ride-through (FRT) capability for wind turbines.
This thesis will focus on the fault ride-through capability enhancement of conventional doubly-fed induction generators (DFIG), which are widely used in wind power generation. Basic operation principles and control algorithms of DFIGs and the corresponding converter for stability studies will be presented first. Simulation verifications are performed by PSCAD. In this thesis, technical enhancements of the FRT capability are achieved through the following steps:
1.Dynamical mechanisms of the linearized DFIG will be conducted by eigenvalue analysis. It will be shown that the DFIG system under the conventional current control method will become unstable if the grid voltage is sufficiently low.
2.The FRT capability can be enhanced by advanced control algorithms in addition to the conventional current control methods. The most straight forward method is the so-called crowbar protection. The DFIG and its associated converter system have to be protected against severe grid faults with considering proper converters and crowbar switching. Alternatively, the FRT can be achieved by applying the advanced nonlinear control theory, including state feedback linearization and the input-output feedback linearization. In this thesis, the sliding mode control theory is applied for FRT enhancement. Both 5th order and the reduced 3rd order DFIG system will be utilized for sliding controller synthesis. Simulations of one-machine-infinite-bus system and a two-area-four-machine system are performed to verify dynamical characteristics of the proposed sliding control. Performance comparisons among existing FRT enhanced control laws will also be examined through numerical explorations
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