| Due to the application of large-capacity DC power transmission and the large-scale access of renewable energy,a large number of power electronic equipment is put into the power system,which brings new challenges to the electromagnetic transient simulation of the power system.In order to simulate the AC/DC hybrid power grid quickly and accurately,it is necessary to continuously research and innovate the simulation theory.The lumped components are modeled as lossless transmission lines in the Transmission Line Modeling Method(TLM).The advantage is that decoupled simulation is achivevd to improve the simulation speed as the transmission delay.This thesis focuses on the TLM theory,and application in real-time simulation of DC grid.Firstly,the principle of TLM for solving nonlinear resistance is briefly introduced.A damping TLM model is proposed to eliminate the numerical oscillation caused by sudden change of non-state variables.The damped TLM model and the undamped TLM model are analyzed in the aspects of z-domain,frequency domain,and numerical analysis,revealing the zero-pole distribution,impedance(admittance)characteristic,local truncation error,computational stability,and convergence.The principle of damped TLM eliminating numerical oscillation is explained and illustrated by an example.Secondly,a DC breaker modeling method for small time-step simulation is proposed taking the advantages of TLM.After modeling the arrester which is regarded as a nonlinear resistor by TLM model,and using the small time-step switch model,the admittance matrix of the DC breaker keeps unchanged.A simplified hybrid DC circuit breaker model is used for explanation,and a simple test circuit is implemented on two field programmable gate arrays(FPGAs).Parallel computing is achieved by decoupling by the inductance.The results illustrate that the proposed DC circuit breaker modeling method can realize real-time in a small time-step,and the simulation is accurate and efficient with parallel computing.Finally,the theory of transmission line decoupling network is further studied.The concept of Node Network and Load Network are proposed,and the general principle of dividing network is given.By selecting some networks as Node Networks and Load Networks appropriately,the accuracy can be improved and the computing resources can be optimized.For the special case where the Node Network is a ordinary port and multi-ports are connected in parallel at this port through parallel capacitors or series inductors,a specific decoupling method is proposed.The correctness of the theory is verified by an example.When the parameters are selected appropriately,results are of high accuracy. |
