Research On Wind Farm Cluster Voltage Security And Its Assessment

Wind power generation is one type of renewable energy power generation, which is the most mature and has the greatest economic benefits except hydropower currently. The development of large-scale wind farm cluster and long-distance delivery is the main way of wind power development in China. At present, eight bases of ten million kilowatts level of wind power are intended to plan and set up in China.Judging from the present operating condition of the built wind power bases, due to the larger capacity of wind power bases and weaker structure of grid, serious problems are produced in power delivery and consumptive. The security and stability of gird is confronted with greater pressure especially in the case of high level of wind power output. Subjected to outside wind speed conditions, the wind power generation has randomness, fluctuation, intermittence characteristics and so on. When large-scale wind farm clusters are connected to the grid composed of controlled synchronous power, the original grid is changed into a complex stochastic-deterministic coupling power system. In such system, a large wind farm cluster is consisted of a large number of wind turbine generators and thus occupies a huge area in space. As a result, unique spatial-temporal distribution characteristic is often exhibited. The coupling and interaction between large-scale wind farm cluster and power system is complex because of the unique internal structure of large-scale wind farm cluster. And wind power output changes by the wind speed, showing strong randomness and volatility. These features are performed extremely obvious in large-scale cascading trip-off events of wind turbine generators. From large-scale cascading trip-off events, we can see that greater differences appeared in security and stability performance between such stochastic-deterministic coupling power system and conventional power system. In conventional power system, voltage stability of the system is often accompanied with angle stability simultaneously. So it’s even difficult to distinguish between these two kinds of stability frequently. As to wind turbine generators, variable-speed operation can be achieved. As a result,’angle stability’ in traditional meaning existed no longer, whereas voltage and frequency stability are performed further. Especially the voltage stability plays a leading role in large-scale cascading trip-off events of wind turbine generators. Therefore, it’s of both important theoretic and practical meaning to do the research on large-scale wind farm cluster and voltage stability of the system. Based on this, this paper is focused on voltage security of wind farm cluster, combined with several large-scale cascading trip-off events of wind turbine generators occurred in recent years, from two aspects of static voltage stability and dynamic voltage stability, and does analysis and assessment of voltage security of wind farm cluster, and puts corresponding improvement measures forward. Specific studies include:1) A static voltage stability model of a wind farm cluster is established, and stability assessment and voltage pre-evaluation is made. Aimed at static voltage stability of wind farm cluster, catastrophe features of static voltage variation are analyzed and its applicability for catastrophe theory is indicated. A static voltage stability model of a wind farm cluster is established by applying catastrophe theory, based on the measured trajectories, considering the mutual coupling effect inside the wind farm cluster. A method for modeling the external characteristics is provided essentially. So calculation is simple and practical. Based on the established model, stability assessment and voltage pre-evaluation are made. It’s proved the validity and applicability of the proposed model and method through simulation cases. A new way is provided for static voltage security analysis of wind farm cluster.2) Dynamic voltage equivalence model of wind farm cluster is built from the perspective of dynamic voltage. In traditional wind farm equivalence models, accurate simulation of total active power and reactive power output variation is more considered, lacking equivalence model of wind farm clusters from dynamic voltage angle. Based on this, firstly, dynamic voltage process of wind turbine generator is analyzed. Secondly, considering the randomness of wind speed and the fuzziness of equivalence, a clustering method of wind farms based on cloud models is proposed. And equivalence models of wind farm cluster and collection system are built. Finally, simulation cases are built and the simulation results prove that the equivalence model can reflect the dynamic process of cascading trip-off events of wind turbine generators. This provides modeling support for further analyzing dynamic stability of power systems including large-scale wind farms.3) Assessment for dynamic voltage security of wind farm clusters is made. Currently, assessment for wind farm voltage is more focused on the static voltage stability, lacking the voltage security assessment from the perspective of dynamic voltage. Based on this, aimed at dynamic voltage security of wind farm, combined with cascading trip-off evens of wind turbine generators, an index system for wind farm assessment is refined. At the same time, considering the fuzziness of assessments, the Interval Analytic Hierarchy Process (IAHP) method is applied to assess the voltage security of wind farm clusters. According to the assessment results, the indexes sensitivity is analyzed. Improvement measures for improving the dynamic voltage security of wind farm are provided. A reference for the planning and operation of wind farms is provided.4) Voltage and reactive power emergency control strategy of wind farm cluster is investigated using SVC and DFIG. The current voltage and reactive power control strategies of wind farms are aimed at the voltage fluctuation problems at the point of a single wind farm accessing to grid on the condition of wind speed fluctuation, ignoring emergency control strategies of wind farm clusters under faults. Based on this, mainly considering probable large disturbances occurred in power grid, especially large-scale cascading trip-off events of wind turbine generators, the control characteristics of various reactive power compensation equipments in wind farms are analyzed. Based on unique spatial-temporal distribution characteristics exhibited in cascading trip-off events of wind turbine generators, voltage and reactive power emergency control strategy of wind farm cluster is brought forward. Models are built and simulated in the simulation software. The simulation results show that the proposed control strategies can suppress the large-scale cascading trip-off evens of wind turbine generators. It provides technique support for voltage control of large-scale wind farm cluster.