Research On Power System Online Transient Stability Numerical Simulation

With the overall interconnection development among regional power grids, the grid structure, control means and operating mode become more and more complex, and the issues of stability become much more striking. Under the circumstances, the operators need to identify the system stability level in time, which requires the system transient stability assessment for the large power system to be accomplished online as fast, precisely and accurately as possible. As a reliable tool for the transient stability analysis, the transient stability numerical simulation is expected to be applied online early. The main work of thesis is to enhance the calculating speed and precision of numerical algorithm that can fulfill the requirements of the online system transient stability assessment.At the beginning of this thesis, various mathematical models and numerical algorithms for the transient stability analysis were introduced; the existing common algorithms for the hybrid system's mathematic model, i.e. the differential-algebraic equations, all have their respective shortcomings. Enlightened by the bi-directional iteration technique and the physical relationship among the power system elements, the conception of machine-network interface and the equations-solution algorithm were proposed. Such solution scheme includes two operations called"the interfacing equation's forming for forward substitution"and"backward evaluation for the generator subsystem's variables", which is virtually equivalent to the simultaneous solution. This solution algorithm inherits the both advantages of the existing methods on the calculating precision and the element flexibility, and can be used conveniently in dealing with the nonlinear components. Besides, to improve calculation efficiency, two kinds of sparse storage format and solver were used respectively on the large scale network matrix and small scale generator subsystem matrix.In the research process, it was found that the numerical oscillation arose when the improper treatment was employed in dealing with the nonlinear components. By investigating on the fundamental problem of numerical oscillation in numerical simulation, the generation mechanism of numerical oscillation was pointed out: the implicit trapezoid method introduces the variable's pre-mutated value into differential equation (DE), so the following cyclical compensation during the DE solving process which tries to balance the difference before and after the mutation acts as the numerical oscillation. High frequency dominates the characteristics of this numerical oscillation. From the view of digital filter, a new oscillation eliminating method called mean-value oscillation-elimination method was presented by the author. The new method's validity and convenience were proved by contrast with traditional methods on the oscillation-elimination effect. Equipped with the method for oscillation elimination, an effective treatment scheme was summarized for handling the nonlinear components and applied successfully in the calculation examples.According to the basic ideas and algorithms mentioned above, a power system transient stability online analysis program was coded by C++ language. Through the simulation results'comparison with the commercial simulation software BPA on both the north china power grid actual system and the IEEE 4-generators 11-buses test system, it was proved that the algorithm and the model realization could meet the requirements of the online system transient stability analysis on calculating speed and precision.