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With Monte Carlo simulation,this paper investigated Freedericksz transitions of biaxial nematics induced by an external magnetic field.In the simulation process,the pair potential in London's dispersion forces approximation was used.With three different orientations,nine different Freedericksz transitions were discussed by changing the molecular magnetic susceptibility and the directions of magnetic field.The transition of two short axes u and v,the transition of long axis w and short axis u,and the transition of long axis w and short axis v were all simulated.With the result of simulations,relations of the nine elastic constants can be discussed and they are fit with one elastic constant approximation.The result is practical in the sense that it works as a theoretical basis for developing biaxial display device.

With real-time data from WAMS and EMS, the paper proposes a kind of comprehensive dynamic stability prevention and control method including low-frequency vibration monitoring, oscillation source search and dynamic stability control strategy. This method on one hand observes multiple oscillation modes and damping characteristics that may be existing in the power grid on line from EMS and through real-time small signal stability analysis, combining the sensitivity analysis, dynamic stability control aided decision for every kind of mode is generated. On the other hand, data from WAMS and online Prony calculating method are used to fast detect the occurrence of low frequency oscillation. Once it occurs, oscillation propagation way will be tracked by the algorithm based on transient energy function and dynamic hybrid simulation to identify possible initial oscillation point. Oscillation pattern matching calculations start to choose the feasible real-time auxiliary control strategy from the matching small disturbance mode. In ideal condition, initial point position and oscillation inhibit quell measures will be showed which provides online automatic monitoring method for preventing dynamic instability of power grid. The method gets practical application in Henan interconnected power grid.

Along with the development of projects of power transmission from west to east in China, more power and load centers are connected by high-voltage AC&DC transmission lines, which pose challenges to the operation stability of power system. When a malignant fault takes place in the power system, especially DC faults with heavy power, it is likely to lead to swings and even collapse of the power system. Advanced SSC (Safety and Stability Control Device) equipment are designed at the AC/DC converter station or the focus location of power flow, the devices are linked by the high performance optical fiber commutation network. In the paper, the structure of SSC in China's Tian-Guang AC and DC parallel transmission system is analyzed, the trigger criterion of SSC is based on the measurement of power, the partial control strategies and some instances are given. It is important and necessary for hybrid power systems to use SSC equipment. Finally one conclusion is drawn about the operation problems and future development of SSC.