Research On Electromnetic Transient Simulation Of Power System Based On Branch Current Cutting Method

The process of electromagnetic transient simulation is to study the transient overvoltage and the transient overcurrent of power system after the fault or operation. Some electrical equipment in power system can be reasonably developed and designed, as well as be analyzed whether its operation is safe and reliable through the study of the overvoltage and the overcurrent. At present, the scale of power system is constantly expanding. So it has important significance to establish the electromagnetic transient simulation model and to study the network solution algorithm for ensuring the safety and reliability continues power supply of power system.The state equation and output equation of the generator, the transformer, the transmission line, the load and other network elements are discretized using the trapezoidal numerical integration method. The discrete dynamic equivalent model of the generator unit and the network element is established. The discrete dynamic equivalent model has a unified form of circuit, which is the Norton equivalent circuit of the parallel form of the pure resistance and the current source or the Thevenin equivalent circuit of series form of pure resistance and volt age source. Each element which is the composition of power system is used to establish the discrete dynamic equation model respectively. When parameters of the element in system change, just change the coefficient matrix of state equation of these elements, which does not affect the discrete dynamic equivalence structure of the element and is convenient to modify and maintain the program.Based on the characteristic of discrete dynamic equivalent model of transmission line, an electromagnetic transient simulation splitting algorithm is proposed, which decomposes the power system through cutting both ends of the transmission line. In a period of time, the original power system will be broken down into a series of sub networks with a small amount of terminal n odes, when the transmission line is separated from the power system by cutting both ends of it. In this period of time, the steady state system model can be established through replacing internal components of the sub network by their discrete dynamic equivalent model. The discrete dynamic equivalent model of each subsystem which is separated from the whole system is established through the linear network equivalence method. The model is equivalent to the atomic system in the split-suture time interval. The equivalent model of each sub network is stitched up by using the discrete time domain equivalent model of the transmission line, and the interconnected system which equivalent to the original system is obtained. The mutual admittance of the variables at the two ends of the transmission line is zero in the discrete dynamic equivalence model of transmission line. Therefore, the terminal variable of each sub network can be used as a set to write the electromagnetic simulation equation of the interconnected system independently, which reduces the difficulty of writing the simulation equation of system as well as the computation of the simulation.On this basis, the split algorithm of AC / DC power transmission system is proposed. The rectifier and inverter of the AC / DC system are regard as sub systems, and makes up the nonlinear interconnected system which is equivalent to the original system, together with equivalent linear system. In this way, the size of the nonlinear equation and the amount of computation are reduced. The proposed algorithm is used to simulation with examples named “3 generators and 9 nodes” and “10 generators and 39 nodes”, and is compared with the simulation results of PSCAD, in order to verify the correctness and effectiveness of the pro posed algorithm.