Impact Of Voltage Source Converter Controlled Power Devices On Power System Dynamic Interaction

With the continuous application of Flexible AC Transmission Systems(FACTS)and the large-scale centralized integration of wind turbine generators(WTGs),the penetration level of Voltage Source Converter(VSC),as the core components of above equipments,is constantly improving,result in increasingly significant development for the trend of power electronic in power systems.As classic issues focused on the stable operation of power systems,low frequency oscillation(LFO)and subsynchronous oscillation(SSO)present new features,and no consensus qualitative conclusions have been reached yet.Therefore,it is urgent to conduct a more in-depth mechanism research on the oscillation instability caused by FACTS and WTGs based on VSCs.The VSC-based power equipments mainly affect the power system stability from two aspects:one is that the power injection of VSC changes the power flow,affecting the oscillation modes;the other is that the dynamic interactions between VSC control links and power systems,affecting the oscillation modes.Different from the traditional VSC auxiliary control to improve the weakly damped oscillations,this paper focus on the strong dynamic interaction between VSC and the system,studying the instability mechanism of LFO and SSO induced by VSC-based devices,e.g.,Unified Power Flow Controller(UPFC),Static Synchronous Compensator(Static Synchronous Compensator(STATCOM)and Direct-Drive Permanent Magnet Synchronous Generator(PMSG).The main works and innovations are as follows:(1)Published literatures mostly focuses on the phase angle controlled UPFC,but there are few studies on vector controlled UPFC for research of auxiliary damping control and interactions between multiple controllers.From the two above aspects,the paper makes a comprehensive quantitative comparison analysis of two control strategies of UPFC for the first time,and clarifies the advantages and disadvantages of the control feature under each control strategy.Moreover,for the phase angle controlled UPFC,the optimal positioning choice of multiple auxiliary damping channels is studied,and coordination algorithm of multi-controller damping system is proposed for damping the multi-mode oscillations.For the vector controlled UPFC,the interactions between multi-controllers of UPFC under varied operating conditions is discussed.(2)The main functions of UPFC are power flow regulation,voltage support,and dynamic stability improvement.Different from the traditional works that auxiliary damping control of UPFC to improve the weak damping oscillations,the potential threat of the LFO of the system induced by control characteristics of the VSC of UPFC is studied.Through the damping torque contribution provided the grid-connected UPFC to the system,it is newly discovered that the strong interaction between the UPFC and the system will deteriorate the damping of the electromechanical oscillation mode,causing the system to be instability.On this basis,a multivariable(four-input and four-output)model in which the UPFC subsystem and the synchronous machine subsystem are interconnected is established.It is revealed in mechanism that the strong interaction is caused by the open-loop modal coupling of the two subsystems.(3)The FACTS auxiliary control can effectively suppress the subsynchronous oscillations of the synchronous machine's shafts and wind farm.Different from the traditional works that FACTS device to improve the stability of SSO,the paper study the potential risks of SSO of synchronous machine's shafts and wind farm induced by the FACTS itself.Firstly,by establishing a single-variable(single input single output)interconnection model based on the synchronous machine's shaft,dynamic interaction between UPFC and synchronous machine's shaft is studied.Secondly,for the wind farm of PMSG with a STATCOM,the asymmetric variable(two input and one output)interconnection model is established based on the power-voltage of the STATCOM access point.The subsynchronous control interaction between STATCOM and multiple PMSGs is investigaed and influencing factors of stability are analysis.(4)Positive resistance criterion and positive net damping criterion of impedance analysis,and open modal coupling are the three main analysis methods for SSO caused by grid-connected WTGs.They provide different explanations from the standpoints of negative resistance,negative net damping and modal resonance respectively about why the grid-connected WTGs may cause the SSOs.For a same case of the SSOs caused by the WTGs,different results may be obtained when three methods are applied separately.In order to explore the reasons for different results,and the relationship between negative resistance and oscillation instability,this paper compares three methods by calculating the control parameter stability region of PMSG,revealing the differences and connections between the three methods,which helps to gain better understanding on the limitations of the methods for studying the instability mechanism of SSO caused by grid-connected PMSG in a power system.