Research On The Small Signal Stability Of VSC-HVDC Connected To The Weak AC System

Compared with Line Commutated Converter based High Voltage Direct Current(LCC-HVDC),Voltage Source Converter based High Voltage Direct Current(VSC-HVDC)technology is more suitable for the power transmission with weak ac system as the full controlled power electronic devices are applied.The Commutation failure will not occur in the VSCs.Meanwhile,the active power and reactive power can be regulated independently.An AC system can be defined as ‘weak’ from two aspects: 1,small Short Circuit Ratio(SCR)looking into the Point of Common Coupling(PCC);2,AC system mechanical inertia is inadequate relative to the dc power infeed.When AC system is weak,the classical vector current controller of VSC is highly affected by AC system and will lose small signal stability.This situation is worse with the decreasing of AC system strength.The small signal stability problem of VSC connected to weak AC system is recognized as one of the most significant issues and it is closely related to the design of vector current controller and control parameters.To certificate the damping character and high responding speed of the VSC system,the small signal stability model of VSC connected to the weak AC system is established.This article gives the explanation of system oscillation mechanism and proposes the control strategy to improve the damping character of the system.There is a great significance of theory and valuable application of engineering about the research.This thesis proposes a small signal model which contains VSC-HVDC system and AC network which is represented by an equivalent synchronous generator series with equivalent impedance of the transmission line.The frequency character of AC network has been fully considered.Two converter topologies are contained in the model and they are two-level VSC and Modular Multilevel Converter(MMC).Based on the small signal model,the impact of AC network strength and control system parameters on system stability is analyzed.The novel control strategy is proposed.Meanwhile,the interaction mechanism of Multi-Infeed VSC-HVDC system is researched.In this paper,a series of theoretical and engineering valued conclusions are obtained.(1)The mathematical model of VSC-HVDC system and control strategyThe mathematical model of VSC-HVDC connected to the weak AC system is the basic of small signal stability research.This article establishes the non-linear mathematical equations of the VSC system.The AC network is replaced by an equivalent synchronous generator series with impedance.The vector current control is adopted as the main control system.The interfaces between different sub-systems are developed to fulfill the entire system.The small signal model is verified in the PSCAD/EMTDC time-domain simulation work.(2)Research of the damping characters of VSC-HVDC connected to the weak AC systemBased on the small signal model of VSC-HVDC connected to the weak AC system,the impact of AC network SCR and mechanical inertia has been researched.With the decreasing of SCR and mechanical inertia,the small signal stability is reduced.However,compared with SCR,the impact degree of inertia is less.Only when the SCR of AC network is very low,the system is going to be unstable with the enhancement of mechanical inertia.The impact of control system parameters on the small signal stable region is also been analyzed,such as the parameters of Phase-Locked-Loop,outer-loop controller,and the excitation system of generator.To analyze the impact of AC network frequency character on the small signal stability of VSC system,the comparison of stable region between synchronous generator AC network and Thevenin equivalent voltage source AC network has been proposed.It is concluded that the frequency character of AC network has great impact and if the equivalent synchronous generator is replaced by voltage source,the evaluation of power transfer limitation is overly optimized.With the comparison of stability region of MMC and two-level VSC,the system stability can be affected by the interaction between MMC inner dynamics and PLL controller.There will be a high frequency oscillation which can lead the system to be unstable.(3)The novel control system design for VSC connected to the weak AC systemA virtual resistor control based power synchronization control(PSC)is proposed to enhance the small signal stability of VSC connected to the weak AC system.The resistor control is added in the voltage modulation waveform.With the supplemented resistor control,the equivalent resistance of the converter is enhanced to damp the high frequency oscillation caused by PSC controller.The method is convenient to be realized and it needs no additional loss.The power transfer efficiency is not reduced with the additional control strategy.However,the responding speed of controller is also greatly reduced.A novel PLL controller is proposed to suppress the oscillation caused by the control system instability.The damping loop is added to the PLL to reduce the responding speed of PLL and enhance the small signal stability of VSC-HVDC system.All the control method mentioned above is the improvement of PSC control or vector current control.The mechanism is reducing the responding speed of controller to enhance the system stability.With the consideration of AC network strength is not a fixed value,the AC system strength adaption virtual impedance control method is proposed.This control method can compromise the responding speed and stability of control system.The control method can pretendedly change the equivalent impedance of ac system by changing the measurement point.If the AC system is weak,this control strategy can change part of network impedance to the valve side of PCC and enhance the SCR equivalently.If the AC system strength is enhanced to a certain value,the control strategy can reduce the value of virtual impedance and enhance the responding speed of controller.With the time-domain simulation in PSCAD/EMTDC,the effectiveness of the AC system strength adaption virtual impedance control method is validated.(4)Operating character of parallel connected dual-infeed VSC-HVDC integrated into the weak AC systemThe small signal stability of parallel connected dual-infeed VSC-HVDC system is investigated in this paper.The impact of electric proximity between parallel connected VSCs on system stability is analyzed.If the electric proximity is short,the control system will excite the high frequency oscillation between the inductance of electric proximity and reactive power compensation capacitance.The high frequency oscillation can be suppressed by additional virtual resistor control strategy.If the electric proximity between VSC exceeds a certain value,with the consideration of converter current limits,the control strategy is changed from AC voltage control to reactive power control.The small signal stability region of the system is narrowed.The virtual impedance control strategy is utilized to enhance the small signal stability of the parallel connected VSC system.