| With the development of Modular Multilevel Converter Based High Voltage Direct Current(MMC-HVDC)and Line Commutated Converter Based High Voltage Direct Current(LCC-HVDC),Hybrid multi-infeed Direct Current System(HMIDC)has attracted a lot of attentions in recent years.When multiple LCC-HVDC and MMC-HVDC converter stations are located in the same or more adjacent AC buses,there exists harmonic interaction between different converter stations through the AC subsystem.Against this background,this paper analyzes the harmonic impedance characteristics of main components on the AC side.Then,the impedance analysis model of harmonic interaction is established to analyze and evaluate the harmonic interaction in the HMIDC system.Finally,a hybrid three-infeed HVDC system is built by PSCAD/EMTDC for verification.Firstly,the harmonic impedance characteristics of the main components in the system are derived and analyzed.Based on the switching function model,the modulation and transfer law of harmonics between AC side and DC side is analyzed,and the analytical method and time domain simulation method to calculate the equivalent harmonic impedance of the ac side are further derived.The results shows that the harmonic impedance of LCC-HVDC converter can be calculated through the modulation law,and the harmonic impedance of MMC HVDC converter can be calculated by the simplified model only considering the current inner control loop.The calculation results and simulation results are basicly consistent in wide frequency band.Finally,the harmonic impedance characteristics of the AC filter,the end of the AC system,overhead transmission line and loads are analyzed in turn.All of them can be calculated through system and component parameters.According to the above harmonic impedance characteristics,the harmonic interaction analysis model of HMIDC system is established,the harmonic interaction mechanism between different inverter stations of AC sides is analyzed by using the harmonic impedance matrix.The concept of harmonic interaction factor is introduced to evaluate the harmonic interaction intensity of AC side.The influence of the harmonic impedance parameters in HMIDC system on the harmonic interaction is analyzed,and the above results are verified by the simple test system of hybrid three-infeed HVDC system and the IEEE39 node test system.Finally,combining with the modulation law of LCC-HVDC,harmonic mutual impedance and harmonic transfer impedance of DC side is defined to analyze harmonic transfer between one LCC-HVDC's DC side to another HVDC's AC or DC side.the above results are verified by the simple test system of hybrid three-infeed HVDC system and the IEEE39 node test system.The results shows that the harmonic of dc side can also be transfer between different inverter stations through the AC subsystem.The harmonic mutual impedance and harmonic transfer impedance of DC side can reflect the intensity of this transfer effects. |
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