Research On Electromagnetic Transients Simulation And Equivalence Of Large Power Grids Using ATP-EMTP

With the continuous expansion of modern power systems,the successive commissioning of UHV AC/DC transmission projects and the wide access of distributed generation systems,the dynamic characteristics of modern power grids will become increasingly sophisticated.Power system simulation is a research methodology that simulates an actual circuit by establishing an appropriate mathematical model.The electromechanical transient simulation program based on positive-sequence fundamental phasors can simulate large-scale power grid,but it can not accurately simulate the fast transient characteristics of power electronic devices and the waveform distortion caused by non-linear components,and usually does not consider the high frequency characteristics of system parameters.While the Electromagnetic transient simulation program based on instantaneous value calculation can simulate the fast transient behavior of the system precisely and can simultaneously take into account the frequency characteristics of the system parameters.However,the scale of the network applied for electromagnetic transient simulation is generally small and the network needs to be reduced to implement simulation.Unfortunately,equivalence will produce simulation errors.The current data of power grids in China are mainly recorded in the database of electromechanical transient simulation software,and the data of the power grid suitable for electromagnetic transient simulation are scarce.Therefore,it is essential to convert the electromechanical transient data into electromagnetic transient data to perform electromagnetic transient simulation of large-scale power grid.Based on the distinguishing feature of storage format of the data in electromagnetic transient analysis software ATP-EMTP,the electrical data of the large grid recorded in electromechanical transient simulation software PSASP is converted into the input data file of ATP-EMTP.The component models converted include generators,transformers,transmission lines,Loads and reactive power compensation devices,etc.Based on the converted data,power flow calculation and short circuit current calculation of WSCC9 system and Shaanxi AC power grid are carried out.The comparison with the calculated results of PSASP verified the feasibility of using ATP-EMTP for large-scale power grid transient simulation.Transient phenomena in power systems involve a wide range of frequencies,ranging from 0 Hz to about 50 MHz.The frequency characteristics of network elements in the transient process must be carefully considered because these parameters have a significant influence on transient phenomena.In the electromagnetic transient simulation of the power system,the simulation of the characteristics of a component not only requires an accurate equivalent circuit,but also needs to simulate its frequency characteristics according to the frequency range considered.For large-scale power grids,the influence of the frequency characteristics of power system component parameters on the transient characteristics of the network must not be ignored.This paper mainly takes into account the frequency dependent characteristics of transmission line parameters,uses a rational method for frequency dependent network equivalence and calculates the port admittance rational function based on the autoregressive moving average algorithm.To figure out the unknown coefficients,an unconstrained system identification method,of which computing error criterion can be self-defined was applied.Then a single-port and multi-port frequency-dependent equivalent circuit for a three-phase balanced system is proposed.The effectiveness of the equivalent model is verified with several specific examples.Finally,by comparing with the equivalence method considering only the power frequency characteristics of the system,it is shown that the FDNE which consider the high frequency characteristics of the system not only can save the simulation time,but also can improve the accuracy of the equivalence,therefore,it is necessary to take FDNE as the tool to assess the electromagnetic transient characteristics of the large power grid,which has practical significance in engineering.