Research Of Short Circuit Current Calculating Method Based On Annular Grid

With the development of modern ship toward large-scale, automated direction, thedemand for power performance and stability of the ship is increasing sharply. Because of itsstructural features, traditional tree-supply network has its own limitations in terms ofstability. Yet, in annular power supply network, generators can constitute a closed loopthrough connecting lines or buses, achieve multipath supply by forming multiple loopsbetween generator and the load so that both stability and reliability can be improved. Shortcircuit is the most common fault of shipboard power system, the most serious fault in whichis three-phase short circuit, and the three-phase short circuit current is not only foundationof switch setting value but also important technical indicators for reasonable choice ofprotective devices. Therefore, the research of short circuit current on ship annular powersupply network is of great significance.First of all, this paper starts from the transient process of synchronous motor to studyprofoundly the electromagnetic relationship of synchronous motor and establish theprimitive equations of synchronous generator. After that, it uses the method of coordinatetransformation to transform the primitive equation established by the abc system into dq0system, followed by the analysis of three-phase short circuit transient process of thesynchronous generator and achieved the short circuit current expression of synchronousgenerator. Combining with the GB (3321-82) and structural characteristics of the annularpower supply network, the paper ignores the transient time constant, considers supertransient time constant and draws a method to calculate the short circuit current of theannular power supply network of four generators. In the algorithm, the original generatorand its series impedance are equivalent to one equivalent generator, and equivalent withother parallel generators, then through a series of transformations, ultimately equivalent tothree-phase short circuit of a single generator in series with impedance, thereby calculatingthe short-circuit current.Secondly, Visual Basic6.0, a development platform, is used to design a softwarespecialized in calculating the short circuit current of the annular power supply network,which contains visual interfaces for parameters input of the generator and line as well asselection of the short-circuit point. Thus, efficiency is improve work by facilitating quicklycalculated short-circuit current. Finally, use Simpowersystems toolbox in Matlab/Simulink, mathematical model ofsynchronous generator and its excitation system, static load and line impedance are linkedin the form of element modules in simulink environment. After establishing the model ofthe entire ship annular power supply system, the join the fault module and the measuringmodule, and set the parameters of each element. By using variable step size ode23tb methodon three-phase short circuit annular network simulation, the short circuit current peak isdrawn and compared with the calculated short-circuit current peak in order to verify thefeasibility of the algorithm. Thereby, an accurate and simple method to calculate the shipannular power supply system short circuit current is obtained.