Research On Low And High Voltage Ride-through Control Of Self-Synchronous Voltage Source Wind Turbines

With the increasing penetration of renewable energy generation in the power system,the problem of frequency and voltage stability of the power system becomes more and more prominent.Self-synchronous voltage source control technology represented by Virtual Synchronous Generator(VSG)enables wind generators to show inertia and damping characteristics to the grid and support the grid frequency and voltage,and its application in wind turbine is important for the safe and stable operation of the power system.On the other hand,line commutated converter(thyristor dominated)based high voltage direct current(LCCHVDC)is currently the mainstream of large-scale wind power outgoing mode,there will be a low and high voltage fault at the sending terminal alternating current(AC)grid when the failure of phase commutation occurs and a high voltage fault at the sending terminal AC grid when the dc blocking occurs,threatening the safety of the wind turbine power electronics operation,resulting in self-synchronous voltage source wind turbines face the risk of off-grid.Off-grid of wind turbines will cause reactive/active power shortages in the sending terminal AC grid and deteriorate the system voltage/frequency operation.Therefore,it is necessary to investigate low voltage ride-through(LVRT)and high voltage ride-through(HVRT)techniques for selfsynchronous voltage source wind turbines in order to support/suppress the AC grid voltage amplitude and keep the wind turbines on-grid as much as possible.Firstly,the basic model of doubly-fed induction generator(DFIG)and grid side converter(GSC)of permanent magnet synchronous generator(PMSG),the traditional vector control strategy and the basic principle of self-synchronous voltage source control strategy are introduced.Then,a self-synchronous voltage source control strategy for DFIG using excitation current control is presented.Then,a self-synchronous voltage source control strategy based on the virtual internal potential connected to the virtual impedance ideal voltage source model is proposed for the PMSG,and the virtual internal potential is regulated by an automatic voltage regulator(AVR)to realize the steady-state voltage response.Next,the LVRT and HVRT control of a self-synchronous voltage-sourced DFIG is investigated.A strategy to improve the response speed and excitation characteristics of the AVR during fault is proposed to achieve better support/suppression of the grid voltage.A transient current reference value is designed during fault,and a proportional-integral-resonant(PIR)controller is introduced to ensure that the current tracks the reference value without difference,and to achieve accelerated decay of the free component of the magnetic flux.The negative sequence feedforward compensation control is introduced to suppress the negative sequence components in asymmetric fault,and the compensation coefficient can be adjusted to suppress the negative sequence components to different degrees.The simulation results show that the proposed LVRT and HVRT control strategy can achieve fast active support/suppression of the grid voltage,effectively accelerate the decay of the magnetic flux during transient,and effectively suppress the negative sequence components during asymmetric fault.Then a LVRT and HVRT control strategy for a self-synchronous voltage source type PMSG is proposed.This paper proposes the method of blocking the AVR link and switching the virtual impedance during the fault period to realize the LVRT and HVRT control of PMSG.The principle and method for determining different virtual impedance values are also proposed based on the output constraint of the GSC.For the negative sequence component in the asymmetric fault,a negative sequence virtual impedance method is proposed to suppress it,and a PIR controller is introduced to realize the non-differential tracking of the current to the reference value.The simulation results show that the proposed control strategy can achieves excellent LVRT and HVRT performance,and the negative sequence component in asymmetric faults are effectively suppressed.Finally,the power loss and power superposition effect in the station of the selfsynchronous voltage source type DFIG are analyzed to obtain its whole-station power output characteristics and voltage distribution characteristics.A method is proposed to determine the excitation current-voltage sensitivity coefficient of the DFIG,and a station-level coordinated control strategy is proposed based on this coefficient.The simulation results show that the proposed control strategy effectively improves the support/suppression effect of the selfsynchronous voltage source type DFIG wind farm station on the grid voltage.