Stability Improvement Of UHV AC-DC Power System Based On Unified Power Flow Controller

With the advent of national network interconnection,Ultra High Voltage(UHV)long-distance power transmission and large-scale wind power integration taking into effect,accompanying with multitudes of new equipment and components put into operation,the problem of power system stability is further highlighted.Especially,the dynamic stability problem has become one of the most important factors to restrict the improvement of transmission power limit of tie-line and the operation of the interconnected power system safely and steadily.Therefore,the dynamic stability and damping characteristics of UHV AC-DC power grid in new patterns have begun to attract extensive attention.It is a potential method to improve the stability of AC-DC hybrid system by using new Flexible AC Transmission(FACTS)devices to provide damping so that suppresses power system oscillation.At present,Unified Power Flow Controller(UPFC),as one of the most advanced FACTS devices,has many functions at the same time,including line power flow control,voltage and reactive power regulation,and damping system oscillation,which can be installed at the tie-lines so as to control the power flow flexiblely and suppress the voltage fluctuation effectively of system tie-line.In order to improve the stability of UHV AC-DC hybrid power system with UPFC,this thesis first briefly introduces the oversea and domestic research status of UHVDC transmission technology and FACTS technology.A new AC-DC hybrid system with UHVDC transmission is established for this paper's research background.DSATools,as the most advanced power system simulation software,is applied to carry out the theoretical research on the establishment of UHVDC system model and UPFC device model.Both UPFC_UDM and UHVDC_UDM model are proposed to be built based on the User Defined Modeling(UDM)method.Research contents in this thesis mainly include the following four aspects:(1)The overview of DSA software is briefly introduced,followed by the stability analysis mechanism of conventional AC power system based on DSA.Meanwhile,the core content and modeling mechanism of UDM technology in DSA is excavated furtherly with summarizing the detailed steps how to build custom models based on DSA.(2)With reference to the basic concept of conventional HVDC transmission system,the basic structure and principle of UHVDC transmission system is discussed in detail.The mathematical model of UHVDC system used in this paper is derived and the related control strategy and characteristics are also fututher analyzed.The creation of UHVDC_UDM system model is established based on the quasi-steady-state model of UHVDC system as the research object.(3)The structure principle,working mode and control characteristics of UPFC are described in detail firstly.Based on these theoretical knowledge,the steady-state mathematical models of UPFC are derived based on the equivalent voltage source model and the equivalent power injection model respectively.Meanwhile,the dynamic mathematical model of UPFC is deduced fully considering the dynamic regulation process of two side converters and DC capacitor.The core is the establishment of UPFC_UDM device model considering the UDM model of UPFC's bilateral converters and the design of the corresponding auxiliary control system model.(4)The feasibility verification of the established UPFC_UDM model and UHVDC_UDM model is carried out in the typical New England system and a simple two-machine system respectively.Then,the introduction of UHVDC transmission constructs a new two-area and four-machine AC-DC hybrid system.Putting this system as an example,and the study of the proposed UPFC_UDM model is carried out for improving the stability of UHV AC-DC hybrid system.