Research On Small Signal Stability Of Connected Wind Power Generation Systems

With the fact of fossil energy exhaustion and global ecology environment deterioration, research and utilization of renewable resources became world concerned focus, now, countries all over the world invested enormous money and strongly researched wind power generation and related technology. Wind energy was a common renewable resource, which was clean, no pollution and inexhaustible, these features made wind power generation become one of the most commercial prospect new energy generation, it played important role in ecology environment improvement, energy structure optimization, sustainable and harmonious development of society and national economy. With rapid development of wind power generation, which greatly increased proportion in grid, large scale power generation connected with grid certainly had important influence on grid, special on grid frequency and stability. Which favourable measures were adopted for gird safe and stable operation when wind power generation system connected with grid, this problem was deeply researched and solved.Consideration for these facts, the effects of wind power generation system which connected with grid on power system, two types—constant speed constant frequency and variable speed constant frequency, three kinds of wind power generation systems—asynchronous induction machine, permanent magnet synchronous motor and doubly fed induction machine were researched, two questions were deeply studied: small signal stability of these wind power generation systems connected with grid and wind power generation system participated in grid primary frequency regulation after auxiliary frequency link was added; simulation model was established by MATLAB, three types wind power generation systems were analyzed by time domain simulation, correctness of small signal stability analysis was tested, some meaningful conclusions were received.For wind power generation system which included asynchronous induction generator, active power was delivered by wind power generation sets, some reactive power was absorbed from grid for field excitation at same time, reactive power was related to induction machine terminal voltage, which could not be determined in advance because closely associated with slip. Wind power generation system could not be simply regarded as PQ or PV node because induction machine itself characteristic was not considered. Wind power generation sets was equivalent to a impedance which variable to slip by simplified RX iteration method, stable state values were obtained after extended equations which included conventional power flow iteration equation and slip iteration equation were solved, iteration numbers were decreased and quadratic convergence of Newton algorithm was kept. On this base, the effects of parameters variation on system eigenvalues were analyzed, favourable measures are taken as much as possible for increasing system small signal stability:line inductance value is not suitable for large value, bus voltage value is properly raised; in order to keep terminal voltage stable, group switching capacitor compensation or reactive power compensation equipment is adopted for providing enough reactive compensation; induction motor stator、rotor inductance value and resistance value are as much as possible small, low loss and high efficiency are realized, stability is enhanced at same time; direct current capacitor value is suitable for quickening voltage loop dynamic response; pitch adjust system is applied to wind power generation system for filtering out redundant wind energy when wind speed too large, in order to ensure system stability; pitch adjust system time constant value is chosen suitably for ensuring adjustment rapidity.For direct driven permanent magnet synchronous machine and doubly fed induction machine wind power generation systems, their power control strategies were studied. Maximum power point tracking strategy based on tip speed ratio was applied, maximum wind energy utilization efficiency and maximum output power were reached under any wind speed when low rated wind speed; in order to protect wind power generation system safe and stable operation, variable pitch constant power strategy was used for avoiding output power exceed utmost limit when above rated wind speed. Theory analysis shows, system small signal stability is commendably kept in large wind speed variable section, power adjustment is realized under different wind speed. Load dynamic model was taken into consideration in small signal stability analysis process, which well reflects grid actual operation stateFor direct driven permanent magnet synchronous machine wind power generation system, grid side converter controller structure was designed based on droop characteristics by droop characteristics principle of micro power source controller in micro-grid, the effects of frequency and amplitude droop characteristics and parameters variation on system small signal stability were researched. Theory analysis shows, frequency and amplitude droop characteristics have greatly influence on system small signal stability, droop characteristics coefficient is chosen suitably in order to keep system safe and stable operation, theory analysis correctness was well tested by simulation results.Decoupling between Mechanical system and electromagnetic system of wind power generation sets was achieved because wind power generation sets connected with grid by power electronic interface technology, which did not participate actively in grid primary frequency regulation as convention power generation sets. With the increase of wind power penetration in gird, wind power output power random fluctuation property had greatly effect on grid frequency. Small signal stability of direct driven permanent magnet and doubly fed asynchronous wind power generation sets which added auxiliary frequency link were studied, wind power generation system proactively participated in grid primary frequency regulate, good effectiveness was reached. Simulation results show, wind power generation system timely and effectively participate in grid primary frequency regulation after auxiliary frequency link was introduced, grid dynamic frequency characteristic is improved in a certain extent, grid frequency is kept stable, system safe and stable operation is guaranteed...