Application Of Phase Shifting Measures In Emergency Control Of Power System

In our country,large scale interconnected power grids,which are supported by UHV grids,are gradually being built.The scale of power system is expanding and the electric network composition is becoming more and more complex.In order to improve the transient stability of power system while faults occur,phase shifting transformer and AC excited generator were proposed to change the power angle at the two ends of the line by phase shifting in this paper.In this way,the power flow of the transmission line can be controlled and the power oscillation as well as the overload phenomenon on the transmission line can be suppressed,so that the system can complete the transient process and restore to stable operation quickly and safely.First of all,the working principle,basic theory and classification of phase-shifting transformer were discussed in this paper.Through discussing the quantity and location problem of phase-shifting transformers,the decision-making methods under different circumstances were summarized and a large scale power system whose total load is about 22000MW was taken as an example.The type and concrete model of phase-shifting transformer were decided and the relevant parameters of the phase-shifting transformer were derived and calculated.The power flow of each tie-line at different phase-shifting angles was simulated using PSASP.The results indicated that phase-shifting transformers can realize power flow transfer between tie-lines and make full use of the transmission capability of the tie-lines on the transmission section,which can reduce the tie-line overload caused by bipolar block faults of UHVDC transmission system.And then,the working principle of AC excited generator was analyzed and the mathematical model of the generator was established in this paper.Coordinate transformation,power angle characteristic analysis and operation state analysis were researched as well.Comparing with the traditional synchronous generator,the transient stability characteristics of AC excited generator was discussed.The state equation of a single-machine infinite-bus system was established in MATLAB and the short-circuit fault simulation was carried out to compare the transient stability of the generator under different excitation control strategy.The simulation results verified the good transient stability characteristics of the AC excited generator,and that the generator can significantly reduce the impact of the transient process on the power grid under reasonable excitation control strategy.Finally,the research contents and results were summarized and the future work was prospected in this paper.