Studies On Critical Protection Schemes And Anti-off-grid Strategies For Large-scale Long-distance-transmission Wind Farms

As an important part of renewable energy, wind energy has unparalleled advantage of being windely distributed, aboundant and inexhaustible, and it has relatively mature technology foundation and pollution-free. Therefore, relative techniques regarding to the generation, transmission and distribution of wind power have become a worldwide research focus in the field of electrical engineering. Since the major load centres are mostly far away from abundant areas of wind energy in China, large-scale concentrated wind farms with long-distance transmission lines have become an inevitable choice as a result. In resent years, relative scientific researches and engineering projects has already ranked at the world forefront with the rapid development of this kind of wind farm.However, enormous challenges still exist while the wind power is achieving striding development. Especially in recent years, several cascading off-grid incidents occurred and brough some inner theoretical problems regarding to the stability and fault tolerance of wind farm to the front. This arouses widespread concerns on how to improve the wind farm ride-through capability during fault existence and post-fault in relevant government agencies, industries and academia. However, existing methods concern mainly about the wind generator itself. As a matter of fact, there still exists a scope of research for enhancement in the aspects of protective relaying and the universal behavior relevant to the wind farm.In fact, if the wind farm universal behavior can be taken into consideration while desing the low voltage ride through strategy of a certain wind generator, and reasonable methods can be proposed for better performance of wind farm protective relaying, the wind farm stability and reliability will be greatly enhanced. To this aim, this thesis focuses on the large-scale concentrated wind farm with long-distance transmission lines, and the main emphasis is placed on the advanced protection and control strategies in different areas based on the voltage levels. The fulfilled research work can be concluded as follows:The RBFNN-based adaptive crowbar protection switching scheme for the DFIG:the wind farm LVRT standard and the dilemma that the conventional crowbar protection encounters are introduced in the first place. Secondly, the variation of the DFIG rotor faulted current is analyzed, with part of its duration being considered as a black box. Therefore, the radial basis function neural network is utilized for the black box modeling. Te adaptive crowbar protection scheme is finally put forward and assessed. DIgSILENT-based simulation results indicate that with the novel scheme, it becomes possible for the crowbar to be adaptively switched out at a certain reasonable moment, so as to generate reactive power within the faulted duration.The mal-operation risk assessment and selectivity enhancement for the feeder overcurrent protection:investigation is undertaken and the characteristics of the DFIG feeding current is analyzed under various fault conditions in the first place. Secondly, the feeder overcurrent protection is set according to currently-applied principles, based on which the corresponding mal-operation zone and the mal-operation risk are calculated under different system operation conditions. To prevent this, the superimposed positive sequence current is added into the overcurrent protection strategy as a new criterion, and the selectivity of the overcurrent protection is then fully guaranteed.The partition method and on-line fault location algorithms for the feeder single-phase earth fault based on the faulty phase current feature recognition:To improve the reliability of power supply, neutral un-effectively grounded system (NUGS) is adopted in the wind farm medium voltage area. A novel partition method is firstly introduced for the area of NUGS, and an on-line single-phase earth faulty location method based on the faulty phase current feature recognition is introduced and assessed according to the universal distribution characteristics of the high-frequency transient components. In comparison with existing location methods, the proposed method is independent of zero-sequence current measurement and overhead-line-cable hybrid system, and has the natural faulty feeder selection capability. The necessity of the proposed method is verified by PSCAD/EMTDC simulation and comparison studies.The identification of internal high-resistance single-phase earth fault on the wind farm transmission line:a novel relaying methodology combining the measured impedance of two ends is introduced, and a so-called "summation impedance relay" making use of these critical information is put forward and assessed. This relay is finally adopted into the impedance pilot protection as a unit protection without blind area. Theoretical and simulation studies indicate that the proposed method has satisfied resistive tolerance, is applicable under the condition of series capacitor accessing and can trip phase separately.