| With the wide integration of renewable energy in power system and the formation of large-scale AC-DC hybrid transmission system,the traditional electromechanical transient simulation based on single phase sinusoidal assumption cannot satisfy the demand for the accuracy of power system simulation.On the other hand,for the ABC three-phase modeling based electromagnetic transient simulation,the required simulation resources are too large,and it is difficult to meet the demand of the large-scale power system simulation as the time step is too small.Hybrid transient simulation technology combines the technical characteristics of both electromechanical transient simulation and electromagnetic transient simulation,and harmonizes the simulation scale,accuracy,efficiency and other problems.It has become one of the most effective means to study the modern power system.The main work of this paper is to study the error mechanism,accuracy improvement techniques,multi-port modeling of electromagnetic and electromechanical hybrid transient simulation and its application to actual large-scale AC-DC hybrid transmission system.The key issues include error mechanism of equivalent models,error mechanism of interaction process,advanced interfacing techniques,multi-port decoupling issues and modeling method as well as accuracy evaluation.A series of conclusions are derived which are quite applicable to real engineering.Firstly,electromagnetic and electromechanical transient hybrid simulation interface information analysis and equivalent model error mechanism analysis are undertaken and improved methods are given.In order to solve the non-unique solution problem caused by the power-voltage iteration,the voltage amplitude and phase of the electromagnetic-side interface node are introduced into the electromechanical side calculation,and an improved power source equivalent method is proposed.The method makes full use of the interface information and retains the characteristic of the power source equivalent which is not sensitive to the phase of the interface electrical information.The comparison of the typical components of power system in electromagnetic and electromechanical transient simulation modeling shows that the electromechanical transient modeling in which algebraic equations are used for a large number of circuits,transformers and other components in the grid makes the external system equivalent parameters obtained from the electromechanical transient data cannot accurately reflect the nonlinear characteristics of the external system,which is one of the reasons that influence the accuracy of hybrid simulation.Then,the method of obtaining the equivalent impedance of the external system based on time constant extration is proposed and verified with the IEEE39 bus system.The calculation method of complex asymmetrical fault occuring both in the electromechanical and electromagnetic side isproposed for hybrid simulation,thus the application range of hybrid simulation is enlarged.Then,the interaction error mechanism of electromagnetic and electromechanical transient hybrid simulation interface is studied,and the influence of phasor extraction algorithm,interaction step length,information interaction time delay and electromechanical side iteration process on simulation accuracy are analyzed.The error mechanism of dq-120 algorithm for phasor extraction caused by the decaying DC component is analyzed.An improved dq-120 algorithm based on Prony algorithm is proposed,which can effectively filter the DC component with information of 5 sampling points of the waveform after fault.As the interaction error is increased with the increase of the information transmission time delay,a integrated accuracy improvement techniques including variable time step interaction,Thevenin equivalent voltage prediction and Thevenin equivalent voltage 1st order holding are proposed.Thirdly,aiming at the situation of AC-DC hybrid grid with multi-infeed HVDC in China,the error mechanism of the multi-port circuit and the coupling relationship between ports before and after HVDC is introduced are analyzed.In order to reduce the error introduced by multi-port circuit and to ensure the simulation accuracy of electromagnetic side as far as possible within the RTDS(Real time digital simulator)modeling constraints,the GDM(Global Difference Measure)index of FSV(Feature Selected Validation)method is used to characterize the simulation results after port decoupling.Qualitatively analysis of the relationship between the strength of coupling and the simulation accuracy is undertaken,and the practical decoupling index is obtained with FSV qualitative evaluation as "good" and above.The process and method of establishing port modeling scheme by using port clustering method are proposed.In this paper,a mathematical model of multi-port euqivalent circuit for electromagnetic side simulation is established.Aiming at the problem that the equivalent impedance of positive sequence and negative sequence is not equal caused by the rotating equipment in the electromechanical side system,a method of correcting the equivalent parameter of the negative sequence circuit by compensating the negative sequence Thevenin equivalent voltage is proposed.The RTDS / CBuilder self-defined modeling method is introduced to establish the interface circuit considering the phase-to-phase coupling.At last,the integrated application of engineering research for China Southern Power Grid operation mode of the year 2014 is undertaken,the interface scheme for the application platform of China Southern Power Grid's "eight HVAC lines-eight HVDC lines" system based on the real-time simulation platform of RTDS is designed.The scheme integrates the accuracy improvement techniques as well as the multi-port modeling techniques.Through the appropriate decoupling of the rectifier side and theinverter side converter buses in the electromagnetic side system resonably,China Southern Power Grid 8 HVDC projects and 13 ports modeling demands are meet within the constraints of the maximum number of ports that can be modeled for a single interface by RTDS.Finally,electromagnetic and electromechanical transient transient hybrid simulation accuracy evaluation are carried out. |
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