Studies On Active Power Control Of DFIG-based Wind Turbine Generators Under Grid Fault Condition

Currently,wind power is rapidly growing and replacing the traditional fossil energy.However,unlike synchronous generators used in traditional fossil energy sources,wind turbine generators(WTGs)are mostly connected to the grid via power electronic devices,which lacks the ability to spontaneously respond to grid demands and has weaker over-current capability.Consequently,large-scale wind power integration into the power system will weaken the grid support capacity,which can significantly affect system stability during grid faults.To ensure the stability of the power system under faults,it is important for wind turbines to possess the capability to support the grid.At present,China has introduced grid codes to specify the characteristics of WTGs under grid fault conditions.It is required that WTGs should be able to maintain grid-connected when the grid voltage is higher than 0.2 p.u.,and that they should be able to provide reactive power to support the grid voltage.However,the grid codes focus on reactive power,and less on active power.This leads to the lack of guidance for the active power control of the current WTGs during fault ride-through(FRT),which is prone to stability problems caused by unreasonable active power control.To address the above issues,this paper focus on doubly-fed induction generator(DFIG)-based WTGs,investigating the active power control during FRT.In terms of the impact of WTG active power control on the internal energy balance of the WTG itself,the mode-switching-based pitch angle control scheme in order to achieve quick power balance is researched.In terms of the phase-locked loops(PLL)synchronization stability problem caused by WTG active power control,the WTG output constraints of the existence of stable equilibrium points(SEPs)is researched.In terms of the WTG mechanical load and electrical stress,the WTG output constraints of mechanical and electrical safety is researched and methods to enhance the active power output capacity are proposed.Finally,within the above-mentioned output constraints,the relationship between active power imbalance of the power system and the voltage/frequency is studied,and an adaptive active power control scheme is proposed.The main work and contributions of this thesis are as follows.(1)To address the problem of wind turbine rotor speed fluctuation caused by active power imbalance,an improved pitch angle control scheme based on mode-switching is proposed by switching the pitch control target from the rotor speed zero steady-state error to the power balance during FRT.The proposed control scheme can reduce the hysteresis of the pitch angle control for speed fluctuation,and help to balance the wind power and electromagnetic power,which can improve the rotor speed stability and shorten the duration of speed fluctuation.(2)To address the PLL synchronization instability caused by the non-existence of the SEP,the influence of the Thevenin equivalence parameter of the grid is analyzed,and the mapping relationship between the range of the grid equivalence parameter during the fault and the grid equivalence parameter before the fault is drawn.Based on this,the active/reactive current reference constraints to ensure the existence of SEP are proposed.Firstly,the Id-Iq plane is divided into stable region,PLL deceleration region,and PLL acceleration region.The correspondence between the grid Thevenin equivalent parameters and the region boundaries is analyzed.Then,the relationship between the grid equivalent parameter boundaries and the pre-fault short-circuit ratio and impedance ratio is established.Finally,the active/reactive current reference constraints are proposed,so that the operating point of the WTG always falls in the stability region,which can ensure the existence of SEP and avoid the problem of PLL synchronization instability.The effectiveness of the proposed constraints has been verified based on converters of wind power manufacturers.(3)To address the mechanical load and electrical stress of DFIG-based WTGs during FRT,the electrical stress and mechanical load constraints are quantified and the corresponding active power control capability enhancement methods are proposed.First,to address the mechanical load constraint,the impact of electromagnetic torque dip and recovery on the transient torque of the drive shaft is quantified based on the two-mass drive shaft model,and the results show that the pre-fault torque and the degree of torque dip are the main factors that constrain the active recovery rate.Finally,based on the results of the electrical and mechanical constraints analysis,an active control capability enhancement method is proposed,which enhances the active output capability by optimizing the power distribution between the stator and the GSC as well as adaptive adjusting the active power recovery rate.(4)The propagation of fault voltage in the power system and its impact on the power system active power balance are analyzed for the active power balance problem during fault ride-through,and a WTG active power control scheme based on voltage and frequency to adapt to different fault conditions is proposed within the abovementioned PLL synchronization constraints and electrical/mechanical safety constraints.First,for the dynamic process of frequency shift caused by active power imbalance after grid fault,the active power imbalance caused by fault voltage,load/generator tripping,and other factors,as well as its reflection on the voltage and frequency at the WTGs are analyzed.Then,based on this,the WTG is made to adjust its active power output according to the voltage and frequency to cope with the active power imbalance under grid fault conditions.This can improve the frequency stability of the power system during FRT.