| With the continuous development of power electronic technology,the integration scale of multiple sources and loads which are based on the power electronic converters is expanding in the power system.The problem of harmonic distortion also becomes ever more important.Multiple sources and loads have developed into the typical harmonic sources in the current power system,and the scale will be further enhanced with the development of the power system.The integration of large-scale power electronic equipment not only causes serious harmonic distortion but also may cause parallel or series resonance.Moreover,it may even lead to issues such as grid disconnection of renewable energy and wide-band oscillation,seriously threatening the safe and stable operation of the power system.The types of harmonic sources in the power system are diverse with multiple sources and loads integration.Due to differences in topology,control strategies,and application scenarios,the harmonic generation mechanism is more complex.Meanwhile,renewable energy resources including photovoltaic and wind power,as well as non-linear loads are influenced by the environment,user usage habits,and other factors.The harmonics generated by harmonic sources are characterized by randomness.In addition,the harmonic characteristics are not only related to their characteristics and operating conditions but also are affected by adjacent harmonic sources.The interactions among multiple harmonic sources result in increased difficulty in analyzing the harmonic characteristics of the power system.The paper focuses on the power electronic power systems with multiple sources and loads integration.The multi-frequency harmonic coupling models are constructed for various power electronic converters with clear structures and parameters,and the multi-frequency harmonic dynamic analysis model was established based on the measured data of multiple sources and loads.The degree of differential coupling of multi-frequency harmonics and their dynamic characteristics of multiple sources and loads are conducted.Then,the harmonic transfer analysis and power flow calculation methods for power systems are proposed.The harmonic transmission law and distribution characteristics of the power system with multiple sources and loads integration are clarified.Furthermore,considering the dynamic uncertainty of sources and loads,a probabilistic power flow method with source and load correlation is proposed.The dynamic harmonic state of the power system with multiple sources and loads is analyzed,laying a theoretical foundation for the high-quality power supply of the novel power system.The specific research contents are as follows:(1)Aiming at the widespread nonlinear power electronic devices,the harmonic characteristic analysis model for multiple types of power electronic converters is constructed based on mechanism analysis,to clarify the generated harmonic characteristics under non-ideal conditions.Firstly,the operational switching characteristics and harmonic coupling relationships are analyzed.Then,the harmonic coupling matrix models of semi-controlled and fully-controlled power electronic converters are established to reveal the coupling relationship between different voltage and current harmonic orders.Further,the harmonic coupling matrix model considering the unbalanced power supply is established to improve the generality of the harmonic coupling model,especially to analyze the impact of unbalanced voltage on non-characteristic harmonics.In this way,the harmonic coupling characteristics under an unbalanced power supply and the resulting non-characteristic harmonics are clarified,which can provide the model basis for analyzing the characteristics of power electronic harmonic sources in the power system.(2)Considering the variety of renewable energy resources and nonlinear loads in practical systems,it is difficult to obtain topology and control parameters.The data-driven dynamic harmonic coupling model for multiple sources and loads is established to clarify the dynamic harmonic coupling characteristics.Firstly,the harmonic characteristics and dynamic laws for typical renewable energy resources and nonlinear loads are established by mining the harmonic characteristics from measured data.The probability characteristics for each frequency are also clarified.Then,considering the harmonic coupling relationship,the data-driven dynamic harmonic coupling model is established.Meanwhile,the harmonic coupling model is simplified based on the harmonic generation characteristics of typical sources and loads.Finally,the dynamic harmonic coupling model considering the harmonic characteristic differences is established.The harmonic evaluation method is also proposed to analyze the harmonics of sources and loads under dynamic operating conditions,which can provide theoretical support for the harmonic evaluation of the power system.(3)The harmonic transfer analysis and power flow calculation method in the power system is proposed for harmonic problems caused by the integration of power electronic devices.The influence of source and load interaction on the power system harmonics is analyzed.Firstly,based on the harmonic coupling matrix model constructed from mechanism analysis and measured data,the source and load equivalent harmonic coupling model and the harmonic equivalent models for key components suitable for harmonic transfer analysis and power flow calculation are constructed.Then,based on the multi-level structure of the power system,the harmonic transmission laws and interactions in the power system are analyzed considering the impact of key components on harmonics.Finally,the harmonic power flow calculation method based on the equivalent harmonic model of source and load is proposed.On this basis,the system harmonics with large-scale nonlinear sources and loads integration can be evaluated,providing support for the formulation of harmonic contribution evaluation and related mitigation measures.(4)Considering the dynamic characteristics of the power system with multiple sources and loads,the time-varying probabilistic harmonic power flow method combined with the influence of source and load correlation is proposed,and dynamic features on harmonic characteristics of source and load are analyzed.Firstly,based on the daily time-varying state of source and load,a clustering method is used to determine the typical time-varying state,which can maintain the possible transition process,and also reduce the burden to calculate the probability of state transition.Then,the source and load uncertainty models are established according to the power supply and consumption.The random characteristics of renewable energy resources and electrical loads are also analyzed.Moreover,considering the possible correlation effects between sources and loads,an improved point estimation method is used to extract uncertainty features of sources and loads.Finally,based on the typical time-varying state and uncertainty characteristics,a time-varying probabilistic harmonic power flow analysis method for power systems is proposed combined with the established dynamic harmonic coupling matrix model,to clarify the dynamic variations of harmonics and analyze the harmonic probabilistic state accurately of power systems. |
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