The Investigation Of Fault Characteristics And Protection Technology For A Grid-connected Variable-speed Constant-Frequency Wind Power Generation System

Accelerate the development of renewable clean energy and its efficient utilization has become one of the key national policies in China. Wind power generation, as a representative of the renewable energy utilization, has become one of the important developing strategies. High penetration of wind farm integration will have an impact on short circuit capacity, protection coordination, system transient stability, voltage control and power quality, which can significantly cause deterioration of relay performance, safety and stability of the whole utility (large power) grid. Many technical measures need to be adopted to mitigate the negative effeteness.In this background, the paper study on the external fault characteristic of doubly-fed induction generator (DFIG) and related protection technology. This is a basic work for the grid-connected wind power's stable operation in our country, which has certain theoretical significance and practical value.With the new energy utilization development and intelligent large-scale network research achievements, the first summarize the present status of wind industry; then combining the impact of typical wind farms connected into the gird, some questions that should be further studied are provided in the thesis. And in particular, the details and issues of protection technology for grid integrated with DFIG are reviewed.For variable-speed constant-frequency operation and low voltage ride through (LVRT), the operational mechanism of DFIG in steady-steady and transient-stated were studied, and a complete set of simulation system was established for fault analysis. Focus on its rotor side power decoupling control mechanism and the wind changes under the small disturbance regulation of rotor inverter process is analyzed and simulated.For LVRT operation, the over-current and over-voltage induced in the rotor were researched in the external fault. On this basis, the simulation model of active crowbar protection, with considered the release resistance and set values of crowbar circuit, were installed.This paper also discussed how the fault locations, action of crowbar protection and the proportional and integral coefficient of rotor side inverter affect the magnitude and decay time of doubly-fed induction generator (DFIG)'s short-circuit current. The simulation experiments verifies the "multi-waveform" fault characteristics of DFIG current fed into the grid, which is due to the multi rotor exciting models. Based on this, a grid connecting-point protection scheme for wind farm is proposed. The scheme was composed of advanced current protection and low voltage protections, which can not only reliably remove the fault, happened on the tie line, but also ensure the wind farm to operate under fault condition. The proposed protection scheme is of great significance to realizing the reliable operation and fast fault restoration of the grid with high penetration wind generators.To further increase the wind turbine's LVRT ability from the grid side, feeder protection and reclose action strategies were improved. For preventing wind farm caused mis-operation, this paper proposes that check the maximum wind short-circuit current level, or add complementary elements of the direction for feed current relay. From the Angle of feeder to prevent overvoltage, two kinds of technology solutions were puts forward, for help realizing the cooperate principle between reclosing and interconnection of wind power. At last the simulation results verify the effectiveness of the proposed measures, can make full use of wind power.With The high penetration of DG integration, traditional current protection operation condition could be more complicated and security and stability problem would be more austere in large-scale interconnected power source. The paper constructs a new protection system using the wide-area information to indentify the fault location. Introduced the protection system structure and function division, discussed the core of protection system-based on wide area information for fault location algorithm.a numerical example is given to illustrate the wide-area protection system based on the information can be more effectively determine the fault location, and this methods can guarantee the safe and stable operation of power grid under high permeability DG access.Finally the achieved research is concluded and further prospective on development of limited wide area intelligent protection is highlighted.