Control Strategy Of Unified Power Flow Controller With Synchronous Generator Characteristics

Unified Power Flow Controller(UPFC)can solve the problem of insufficient long-distance transmission capacity of large-scale renewable energy which is caused by the reverse distribution characteristics of China’s renewable energy supply and power demand.However,there are still problems in the “double-high” power system which is composed of high-penetration renewable energy and a high proportion of power electronic devices,such as reduced moment of inertia and weakened system damping.Virtual Synchronous Generation(VSG)can make power electronics equipments imitate the behavior of traditional synchronous generators,and it is an important technology to realize the grid-friendly and stand-alone independent operation for renewable energy generation.Aiming at the application scenario of large-scale renewable energy stations(RES)connected to grid,this paper focuses on the control technology of UPFC with VSG function.There are three topics analyzed and discussed: basic control strategy,system stability and control optimization.The main research work of this thesis is as follows:(1)To solve the problem of “low inertia” and “weak damping” in renewable energy fields,the control strategy of UPFC with VSG functionality can realize the decoupling control between the grid and the renewable energy station.The function of UPFC is expended,and the output voltage fluctuations from RES are eliminated as well.The feasibility of VSG applied to UPFC is discussed.The operation scheme of UPFC with an energy storage system is expounded,and the power control mechanism of UPFC is analyzed which provides the basic theory for designing the control strategy.The UPFC control strategy with VSG(UPFC-VSG)and the control command distribution scheme are designed.The constant voltage amplitude and the phase angle output by VSG are used as the control command in the shunt converter(SHC),while the voltage amplitude and phase angle output by VSG are used as the control command in the series converter(SEC).The independent control of both two ac systems can be achieved and the behavior of traditional synchronous generator can be imitated.The characteristics of UPFC-VSG is compared with the Static Synchronous Compensator(STATCOM)in voltage support capability and main circuit parameters.The simulation model and experimental platform are built.The verification analysis is carried out under different conditions.The results show the effectiveness of the proposed control strategy and theoretical analysis.(2)To analyze the influence of UPFC-VSG on the stability of the renewable energy grid-connected system,the impedance modeling method is proposed.The coupling influence of the internal and external UPFC-VSG is considered as well as the inconsistency reference coordinate system.The impedance model of UPFC-VSG can be regarded as the basic theory for the subsequent system stability analysis.The coupling relationship is analyzed between the UPFC and RES.The output impedance model of RES can be used as an additional impedance in the impedance model of SHC,which realizes the decouple modeling between RES and UPFC-VSG.The impedance modeling method of UPFC-VSG considering the influence of internal coupling is proposed so that the interaction problems between SEC and SHC in AC systems can be solved.The VSG control coordination system is considered as the only reference coordination system.The impedance model of SEC/SHC is also built in the same reference coordination system,so that the accurate impedance in dq frame can be derived.A real-time simulation platform is built to analyze and verify the effectiveness of the impedance model.(3)To solve the stability analysis problem in the complex system composed of UPFC-VSG,the grid and RES,the system stability judgment method is proposed.The oscillation and instability mechanism of RES composed of full-power grid-connected inverters is analyzed in detail.The stability judgment method is present,which can be extended to a complex system composed of multiple subsystems and grid-connected points.So that the stability of the renewable energy grid-connected system and UPFC-VSG can be analyzed.The influence of controller parameters on output impedance and system stability is analyzed.The influence of UPFC-VSG on the stability of RES grid-connected system under weak grid conditions is analyzed by combing the real-time simulation cases and impedance model.(4)To solve the problem of capacity and power losses increasing due to the inappropriate phase angle in UPFC-VSG,the capacity design scheme and optimal control strategy of UPFC-VSG based on dynamic phase angle control are proposed.The mathematical model among phase angle,apparent power and design capacity of SEC/SHC is established.The influence of phase angle on power of SEC/SHC and design capacity of UPFC-VSG is analyzed in detail.The capacity design scheme of UPFC-VSG based on phase angle optimization is proposed,which can reduce the system design capacity of UPFC-VSG.The dynamic phase angle control strategy considering the constraints of capacity design parameters is proposed,which can reduce the power of SEC and SHC in UPFC-VSG.The simulation platform is built to verify the effectiveness of the proposed control strategy.