Method Of Time-varying Amplitude/Frequency Symmetrical Components For Network Asymmetrical Fault Analysis Excited By Power Electronic Devices

With the rapid development of power electronic technologies,the penetrations of power electronic devices in the power systems source-network-load keep increasing,which has been widely applied to the renewable energy generations,the ultra high voltage DC transmissions,the flexible AC transmissions systems and the AC frequency conversion transmissions.The output characteristics of the power electronic devices are obviously different from those of the traditional synchronous generators and other electromagnetic transformation equipments,which will bring new challenges to the security and stable operation of the power electronized power systems and their relay protection.For the reasonable design and selection of electrical equipments,the setting and installation of relay protection devices,it is necessary to analyze the network fault,especially the network asymmetrical fault,in order to obtain the amplitude/frequency and their characteristics of network voltage/current in the process of fault.However,the traditional network fault analysis is based on the phasor method,which considers the network fault analysis is an open-loop dynamic process analysis of network passive components excitated by the voltage with constant amplitude/frequency.It not only fails to recognize the time-varying characteristics of voltage/current’s amplitude/frequency of each node in the fault process,but also fails to realize that the network fault analysis should base on the dynamic process of closed-loop system composed of device excitation-response and network excitation-response.Especially,due to the electromagnetic transformation devices such as synchronous generators in power systems has been replaced by power electronic devices such as wind turbines and photovoltaic generators on a large scale,the amplitude/frequency of the devices will fluctuate in a large range during the transient process.It is not easy to think that network fault analysis is an open-loop analysis excitated by the voltage with constant amplitude/frequency.In addition,in order to analyze the network asymmetrical fault,the method of symmetrical components based on phasor has been proposed in the traditional power systems,which transforms the complex three-phase asymmetrical system analysis into a simple three-phase symmetrical system analysis.However,the voltage/current of each node in the network presents the time-varying amplitude/frequency characteristics under the excitation of the power electronic devices,so it is not easy to describe the voltage/current by phasor.Therefore,the traditional method of symmetrical components can not be applied to the network asymmetrical fault analysis excitated by power electronic devices.Thus,based on the basic structure and function of devices and network,firstly,the essence of the time-varying amplitude/frequency of power electronic devices’ internal voltage during fault process is expounded.Then the network asymmetrical fault analysis excitated by power electronic devices is the solution of the non-homogeneous linear differential equation system,which is excited by the voltage with time-varying amplitude/frequency and constrained by the fault boundary conditions is pointed out.And by the form of non-homogeneous linear differential equations’ solution and time-domain simulation,the time-varying amplitude/frequency essence of network voltage/current which are excited by the voltage with time-varying amplitude/frequency is illustrated.Finally,the method of time-varying amplitude/frequency symmetrical components for the network asymmetrical fault analysis under the excitation of power electronic devices is proposed,and the simulations is used to verify its accuracy.The method of time-varying amplitude/frequency symmetrical components proposed in this paper can simplify the network asymmetrical fault analysis under the voltage excitation with time-varying amplitude/frequency,and can provide important theoretical basis for the design and selection of electrical equipments,the configuration and parameter setting of relay protection devices and accident analysis in power systems.