| AC/DC hybrid power system has significantly enhanced the largecapacity,long-distance transmission,and flexible,fast power regulation in domestic power grid.However,wide utilization of the LCC-HVDC transmission also make the dynamic characteristics of the system very complicated.When analyzing these systems through time-domain simulation,the switching nonlinearity in LCC-HVDC system makes it difficult for the model to interchange efficiency with accuracy.In order to construct an accurate and efficient equivalent model of LCC-HVDC transmission system for the analysis of fault transient characteristics of large-scale AC / DC hybrid power system,this paper studies the dynamic parametric average-value modeling(PAVM)method of HVDC transmission system,including PAVM of HVDC transmission system in case of external asymmetrical fault in the AC network,and internal fault inside the HVDC converter,the network interface between PAVM and AC network considering the fault characteristics of HVDC system,and the efficient simulation modeling method of AC/DC hybrid system based on the proposed PAVM,the research work are as follow:(1)A dynamic average-value model of LCC-HVDC transmission system considering the asymmetrical external AC network is proposed.In one hand,to consider the influence of the asymmetric condition on the AVM.The research analyzes the composition of converter bus voltage when there is imbalance in the AC network.Based on the analysis,the amplitude and phase angle of the positive and negative sequence components are extracted to construct additional indices for characterizing the parametric functions.In the other hand,the AC and DC side currents and voltages of HVDC converter are transformed into multiple synchronous reference frame with difference frequency,using symmetrical component method and Fourier decomposition.After deriving the dq values of each frequency component,the parameter functions for each component can be obtained separately.By integrating the parametric functions with controlled current and voltage sources,the proposed PAVM of LCC-HVDC transmission system is finalized and able to predict the system transient under asymmetrical operation.Finally,the accuracy of the model and the advantages in simulation efficiency are verified by comparison between the detailed model and the proposed PAVM.(2)A dynamic average-value model of LCC-HVDC transmission system considering abnormal operation of the converter is proposed.In one hand,to consider the faulty switch inside the converter,the internal fault state of the converter is used as a separate key index for characterizing the parametric functions of LCC.In the other hand,all factors related to the critical voltage sag which would trigger commutation failure(CF)are adapted and reconfigured as key indices for characterizing the dynamic CF criterion.In addition,the sag instant would be input to a logic to identify the faulty switch.By integrating the parametric functions of LCC and critical voltages sag with controlled current and voltage sources,the proposed PAVM of LCC-HVDC transmission system is finalized able to predict the transient not only when switch inside LCC becomes opencircuit or short-circuit,but also when there is commutation failure inside the converter.Finally,the accuracy of the model and its advantages of simulation efficiency under multiple-converter scenario are verified by comparison between the detailed model and the proposed PAVM.(3)An enhanced interface for interfacing the PAVM with external network and its corresponding parametric functions soft-switching method are proposed.The dynamic average-value model using different interface formulation are numerically linearized at wild range of operating points,the numerical stability of these nonlinear systems is analyzed through Lyapunov’s first method.Based on the analysis,the interface formulation which can ensure the numerical stability both in rectifier and inverter is selected.An enhanced interface using a compensated current source is proposed to reduce the excessive numerical stiffness of the current source formulation.A softswitching technique is proposed to eliminate the overshoot in PAVM’s AC terminal voltages caused by the sudden change of parametric functions’ value.The soft switching method improve the accuracy of transient waveform during fault simulation.The improvement in numerical stability and accuracy brought by the proposed interface are verified by the comparison between the PAVM with and without adopting the proposed interface and soft-switching method.(4)The electromagnetic transient simulation model of AC-DC hybrid power system for fast simulation is constructed.Based on the PAVM of LCC-HVDC transmission system constructed above,the equivalent circuit of the AC system is constructed in the dq synchronous reference frame.And the dq equivalent circuit of positive and negative sequence coupling is deduced for the three-phase asymmetric circuit.Using the enhanced interface to connect the PAVM and the Combined with the controlled source combination in Chapter 4 and the external AC/DC network interface,the equivalent model of the AC/DC hybrid power system with multiple DC lines is constructed.The example test shows that the constructed model can accurately predict the AC-DC cascading failure of the system and effectively improve the solution efficiency.It is expected to be used as an online analysis tool for large-scale AC-DC hybrid power systems.In conclusion,this paper proposes dynamic average-value modeling method for LCCHVDC transmission system considering the external asymmetrical operation and internal abnormal operation,the dynamic CF criterion,the enhanced interface,and the parametric functions switching method designed for fault simulation.The dynamic CF criterion makes the model capable of simulating common fault conditions in HVDC systems.By integrating the model with the equivalent circuit of the AC system in synchronous reference frame,it forms an AC-DC hybrid system to realize accurate and efficient simulation of cascading fault between AC and DC subsystem,which can be used to verify the system protection strategy or enhance the dispatching system’ s online analysis capabilities. |
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