Oscillation Stability Affected By Phase-locked Loop In Power Systems With Wind Power Integration

With increasing expansion scale and penetration level of renewable energy sources,such as wind power,the safe and stable operation of power system is facing new challenges.Wind turbines are connected to the power system via voltage source converters(VSCs)based on vector decoupling control.Moreover,phase-locked loop(PLL)is the critical component to implement the synchronous connection between the wind turbines and the power system.Wind turbines generally exhibit the characteristics of weak inertia to the power system,resulting in the relative reduction of overall system inertia.Virtual inertia control enables wind turbines to take part in frequency regulation of the power system.At the same time,virtual inertia control will make it possible for the wind turbines to interact with synchronous generators(SGs)in the power system.Thus power system low-frequency oscillations will be affected by the virtual inertia control of wind turbines inevitably.It is worthy of note that PLL provides a frequency measurement signal for the virtual inertia control.Furthermore,the dynamic interactions between wind turbines and the power system may trigger power system oscillation stability issues under certain conditions.The interactions caused by PLL mainly include the couplings among PLLs,VSCs for wind power integration,and SG shafts.Instability oscillations may be induced to the wind turbine itself or the power system if the interaction levels are stronger enough.Therefore,this dissertation mainly investigates the oscillation stability affected by PLL in power systems with wind power integration.The main research work of this dissertation is as follows:(1)The impact of virtual inertia control of wind turbines on power system low-frequency oscillation characteristics is investigated by taking PLL dynamics into account.A closed-loop interconnected system model is established and divided into two open-loop subsystems,which are the dynamic part subsystem corresponding to the PLL and the virtual inertia control,and the subsystem related to the rest of the power system.Based on the established model,the impact of virtual inertia control of the wind turbine on power system low-frequency oscillations is investigated under different dynamic interaction levels between the wind turbine and the power system.When the distributed distance between the open-loop PLL mode and an electromechanical oscillation mode(EOM)is far away on the complex plane,the dynamic interaction between the wind turbine and the power system is weak.The influence characteristics of virtual inertia control on the system stability depend on the integration locations of wind turbines.An estimation method based on the relationship between open-loop and closed-loop poles is proposed to quantitative study the effect of virtual inertia control on the power system stability,so as to provide reference basis for the installation location of virtual inertia control.When the open-loop PLL mode is close to the EOM on the complex plane,the near strong modal resonance may occur which leads to the considerable dynamic interaction between the wind turbine and the power system.Moreover,the virtual inertia control of the wind turbine will further strengthen the interaction,which will affect the small-signal stability of the power system significantly.Under the condition of modal resonance,the models of subsystems can be simplified by reducing their orders.Thus,with the dynamics of the PLL being taken into account,the impact of the virtual inertia control of the wind turbine on small-signal stability of power systems can be analyzed by the root-locus method on the basis of the following influencing factors:the position relationship between the open-loop PLL mode and the EOM,gain value of the virtual inertia control and the departure angles of open-loop poles.(2)The impact of dynamic interactions between VSC control loops of wind turbine,wind turbines in a wind farm as affected by PLL on power system small-signal stability is investigated systemically.A single-input and single-output closed-loop interconnected system model is established,wherein,the PLL part is treated as a feedback subsystem.Based on the established model,the impact mechanism of dynamic interactions between VSC control loops is revealed from the perspective of open-loop modal resonance.Under the condition of open-loop modal resonance,i.e.,an open-loop PLL mode is close to an oscillation mode of open-loop object subsystem on the complex plane,strong dynamic interactions between VSC control loops are caused which will make the coupled modes repulse each other to move towards opposite directions.It is very likely that open-loop modal resonance may lead one of the resonant modes to move along the imaginary axis,and degrade the system damping.An estimation method is proposed to predict the distribution positions of closed-loop resonant modes on the complex plane under the condition of open-loop modal resonance,and thus estimate the system damping degradation.Meanwhile,the impact on open-loop modal resonance from many factors is also explored,including system operation conditions(such as the connection strength with the grid,injected power level),and reactive power control structures and parameters.Results show that with the weakening of the connection strength with the grid,or the increasing injected power level,the interactions between subsystems are further strengthened,which may even lead to instability oscillations.In addition,constant voltage control mode is more conducive to the stable operation of the power system than constant reactive power control mode.(3)The impact of dynamic interactions between wind turbines and SGs caused by PLL on subsynchronous oscillations(SSOs)is investigated.A two-input and one-output closed-loop interconnected system model is established,wherein,the PLL and the rest of power system are modelled as open-loop feedback and object subsystems respectively.The mechanism of the SSOs caused by the grid connection of the wind turbines is revealed from the perspective of open-loop modal resonance.The open-loop modal resonance between PLL and SG shafts,and the cascading open-loop modal resonance between wind farm and SG considering the interaction between PLLs are studied respectively.Moreover,the impacts of system operating points and the grid connection impedance on open-loop modal resonance are analyzed.