Research On Impact And Stability Margin Of Phase Sequence Exchange Technology On Power System

With the expansion of power network scale,the stability of regional power grid is becoming increasingly prominent.In order to solve the problem of system oscillation and power angle stability,a new exploratory solution-phase sequence exchange technology is proposed.The phase sequence exchange needs to complete a series of operations,such as opening,exchanging phase and reclosing,and it belongs to discontinuous control,which may cause great impact on the system in the process of closing;on the other hand,the phase sequence exchange can restore the stability of the unstable system and improve the stability margin of the system.Around the two aspects of phase sequence exchange technology,this paper makes the following preliminary research:1.Based on the theoretical calculation,the impulse current and impulse electromagnetic torque produced in the process of phase sequence exchange are obtained.On this basis,the necessary conditions for the system to withstand the impact of phase sequence exchange is determined,and the type of system generator are considered in two different scenarios:steam turbine unit and water turbine unit.The conclusion shows that for steam turbine unit,when the ratio of system tie line reactance to generator sub transient reactance(Xs/X"d)is greater than 1.1591,the impact of phase sequence exchange on the system is bearable;for water turbine unit,the ratio of system tie line reactance to generator sub transient reactance(Xs/X"d)should be greater than 1.9629.Finally,the corresponding verification is given through the simulation test,which lays a theoretical foundation for the feasibility of the application of the phase sequence exchange technology.2.The optimal control mode combining closing phase independence control and optimal power angle control is proposed to minimize the impact of phase sequence exchange technology.Firstly,the transient process of closing is analyzed,and the expression of instantaneous value of impact current of phase sequence exchange closing is deduced in theory.By analyzing the characteristics of periodic component and aperiodic component of current,the conclusion is drawn:closing phase independence can reduce the aperiodic component of impact current,and the optimal power angle control can be used to carry out phase sequence exchange which can reduce the periodic component of impulse current.Finally,the effectiveness of the proposed optimal control method is verified by simulation test.3.The phase sequence exchange technology improves the stability of the system and the stability margin of the system.Based on the energy function method,the system energy reduced by phase sequence exchange technology is calculated theoretically,and then the stability region of phase sequence exchange is solved.By defining the gain coefficient of stability margin,the effect of phase sequence exchange on system stability margin is analyzed quantitatively.Through the example and simulation verification,it is concluded that the phase sequence exchange at the power angle 150 degrees has the largest stability region and the most significant improvement of system stability margin.