Research On Emergency Control Of Power Grid Based On Power Controllability Improving Of Doubly-fed Wind Turbine Under Grid Fault

With the continuous construction of large-scale wind power bases,a sending-end grid with high wind penetration gradually is established.The frequency and voltage regulation capability and safety magin decrease because of the accessing of large-scale wind power.The active power imbalance cansed by a single grid fault may lead to power angle instability and power flow transfer.The existing emergency control methods,including electrical braking,fast valving,generator tripping,load shedding and so on,mainly rely on the control resources like synchronous generators(SGs).Due to problems like the large costs,less control resources,slow recovery and so on,the application scope and control effects of emergency control based on SGs are restricted in the sending-end grid with high wind penetration.Existing emergency control methods may not be able to effectively prevent the development of grid fault.Therefore,the instability,cascading failure and blackout of sending-end grid and even the receiving-end grid can hardly be avoided.With the development of converter control technologies,the control of converter on a doubly-fed induction generator-based wind turbine(DFWT)can be maintainted under a slight grid fault.It can also be recovered after a fault ride-through control is utilized.Thus it is feasible to utilize the power controllability of DFWT under fault to implement the emergency control of power grid.However,the power controllability of DFWT under fault has not be clearly modelled and fully developed in existing researches.It also has not be fully utilized in the emergency control of power grid.The emergency control methods based on the power controllability of DFWT have not be perfected.Given this situation,this paper aims at the effective participation of DFWT in the emergency control of power grid.The main innovative research results are as follows:(1)The power controllability of DFWT under grid fault has been accurately described.The operation mechasim and transient chararcteristrics is analyzed firstly.The influence factors of controllable power range of DFWT are analyzed in detail.The constraints of rotor current,rotor voltage,stator voltage and rotor speed on the controllable power range of DFWT are derived by the space vector model and the mathematical analytical method.Thus the power controllability of DFWT under grid fault is accurately modelled.(2)The the power controllability of DFWT under grid fault considering the coupling of DFWT-grid has been improved.Firstly,the coupling relationship between the DFWT and the power grid is analyzed.Then the expansion of controllable power range of DFWT under the effects of coupling characteristics is studied.Based on this,a control method for grid connecting duration improvement of DFWT is proposed.Thus,the power controllability of DFWT under grid fault is improved in terms of capability and available duration,which provides therical basis for the emergency control of power grid.The effectiveness of proposed method is verified by simulation results.(3)A power angle stability control method based on the improved power controllability of DFWT under grid fault is proposed.Firstly,the effect of DFWT on the power angle stability based on the power angle characteristic curve of SG is qualitatively analyzed.The evaluation index of power angle stability considering voltage amplitude-phase characteristics is proposed.The changing principle of power angle stability of power grid under the power regulation of DFWT is quantificationally studied by using the evaluation index.The controllable power range of DFWT for power angle stability improvement is presented.Then the power angle stability control method based on the improved power controllability of DFWT under grid fault is proposed.The simulation results indicate that the proposed method fully utilizes the power controllability of DFWT to improve power angle stability of power grid by considering the amplitude and phase angle variations of grid voltage.The power angle oscillations of power angle caused by the active power imbalance in the sending-end grid can be effectively suppressed by utilizing the proposed method.(4)An emergency overload control method based on power controllability expansion of DFWT is proposed.Firstly,the machasim of power controllability expansion of DFWT is analyzed based on the realization of motor operation and torque balance of DFWT.Then a method for power controllability expanding of DFWT is proposed.The power controllability of DFWT under motor operation considering safety constraints is studied.Finally,the emergency overload control method based on power controllability expansion of DFWT is proposed.The simulation results indicate that the proosed emergency overload control method has the advantages of rapid response,stable and adjustable power absorption and fast recovery.The power flow transfer caused by the active imbalance in the sending-end grid can effectively be suppressed by using the proposed method.This paper establishes an accurate model of power controllability of DFWT under grid fault.Then the power controllability of DFWT under grid fault is improved in terms of capability and available duration by considering the effects of coupling between the DFWT and the power grid.A power angle stability control method based on the improved power controllability of DFWT is proposed by considering the amplitude and phase angle variations of grid voltage.A emergency overload control method based on the power controllability expansion of DFWT is proposed to suppress the power flow transfer in the sending-end grid.The discussions on the power controllability of DFWT under grid fault in this paper benefits the control theories and methods of DFWT.The proposed emergency control methods based on the power controllability of DFWT enriches the utilization of DFWT which is an effective supplement for existing emergency control methods based on SGs in the sending-end grid.