| Wind energy has merits of being widely distributed, abundant, inexhaustible and pollution-free. Among the developments of various new energies, wind power generation is worthy of large-scale exploitation and commercial utilization since it is low cost and has relatively mature technology foundation. Therefore, efforts on the utilization of wind power have become a worldwide research focus in the field of electrical engineering. Centralized grid-connected wind farm, as well as active distribution network integrated with wind power generation, has experienced rapid development in China, and the domestic research of relevant subjects has already ranked at the world forefront. However, enormous challenges still exist while the wind power is achieving striding development. Especially in recent years, due to the mismatch of settings in wind farm protection/reclosure, and the insufficient LVRT capacity of wind turbine, accidents of cascading off-grid of large-scaled wind turbine occur frequently, which threatens the safety of power grid operation. 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. As a different utilization mode of new energy, micro-grid has gained widespread attentions for allowing flexible access of DG, high reliability on power supply and high energy utilization rate. However, the bottlenecks of development of this new type of power grid still exist due to the lagging study on its protection and control.In fact, a large wind farm can be regarded as a typical power-source type medium voltage distribution network, and a micro-grid with extensive access of small-capacity DG is a typical active medium-voltage (MV) or low-voltage (LV) distribution network. In recent years, there has been a great development trend in the micro-grid operating on user voltage (LV) level. However, the existing research on the protection and control of wind farm/micro-grid is still relatively lagging, and may pose a potential threat to their safe and reliable operation. In light of this, this thesis focuses on the optimal strategies on protection and emergency control of wind farm/micro-grid. The fulfilled research work can be concluded as follows:Based on the analysis of shortcomings in terms of selectivity and speed of existing wind farm protection, inverse time over-current protection criteria and their corresponding implementation schemes are proposed, which guarantee protection selectivity between the faulty collective feeder, the fuse, and sound collective feeders. With respect to three different scenarios, that is, conventional over-current protection that acquires phase currents only within its bay, substation area based protection that obtains all the currents of all collection feeders and in-feed line on the same bus, and installation of TV on the LV side bus of the power collecting substation, the configuration principles and setting schemes of the proposed inverse time over-current protection which simultaneously guarantee selectivity and operation speed are put forward and verified.By studying the single-phase ground fault characteristics of collection feeder of wind farm, a comprehensive faulty feeder selection scheme based on D-S evidence theory is proposed, which chooses three representative faulty feeder selection criteria for a specific grid, dynamically revises the weight values assigned to each criterion based on their success rates in the specific system to highlight the criterion of high success rate. By this means, a higher success rate in identifying faulty collection feeder can be achieved. A resonant model of wind turbine group-distributed parameter lines due to removal of collection feeder with single-phase fault from the system side is put forward, revealing the inherent mechanism of abnormal severe over voltage on the wind turbine terminal. Two strategies of single-phase fault isolation for wind farm are proposed respectively, one of which requires installation of circuit breaker on the turbine side of the feeder, and the other one is based on the smooth off-grid of the wind turbine. By this means, the overvoltage problem at the wind turbine terminal due to inappropriate fault isolation can be avoided effectively.In view of the LV micro-grid topology structure and operation characteristics, an adaptive regional protection scheme of limited selectivity for micro-grid is proposed in this thesis. This proposed scheme adopts the condition of low voltage as pick-up criterion, and adopts positive sequence superimposed component to form the directional protection criterion. By this means, this scheme is capable of being adapt to a variety of fault types and some special operating conditions, and fulfils protection requirements on sensitivity and reliability.Relevant reclosure characteristics of tie line connecting wind farm with power grid are analyzed from the perspective of guaranteeing system transient stability, and are used as the design basis of the emergency control strategy adaptive to the reclosing requirements for the tie line. An algorithm calculating the minimum amount of wind turbine needed to be shed in islanded wind farm is proposed. Furthermore, an Initiative wind-turbine-shedding emergency control strategy for unplanned islanded wind farm is proposed based on the study of frequency transient survivability of survived wind turbines within the wind farm. This proposed strategy can adapt to the frequency characteristics of unplanned islanding operation of the wind farm, and on top of fulfilling the requirements of reclosure time of the tie line, it also maximize the wind turbines capable of being re-connected to the main grid. |
PDF Full Text Request