Research On Active Frequency Regulation And Fault Ride Through Technology Of Grid-Friendly Direct-Driven PMSG Wind Turbines

The new power system with new energy as the main body presents the "double high characteristics" of a high proportion of new energy access and high power electronization,which leads to insufficient frequency stability of the system and frequent off-grid accidents of large-scale new energy units,which seriously endangers the safe operation of the power grid.Therefore,this paper focuses on the permanent magnet synchronous generator-wind turbine to improve its power grid frequency active supportability and high/low voltage fault ride-through ability;the following four aspects of research work are carried out:(1)Based on the traditional MPPT control,adding inertia support and primary frequency regulation can make grid-connected wind turbines show friendliness characteristics.It is essential to design the energy supply mode of regulation function reasonably,and the coordination control between different functions is a technical difficulty.The paper focuses on the single grid-connected direct-driven wind turbine,proposing a "coordinated control strategy of inertia support and primary frequency regulation" over full-range of wind speed.By regulating the active power reference of the controller,the proposed strategy adopts the rotor kinetic energy to realize inertial support.It uses the additional energy storage device combined with variable power tracking control to provide the energy of primary frequency regulation.This proposed strategy can reduce the need for energy storage configuration and solve the problem of frequency secondary drop.Besides,it avoids the issues of energy loss and insufficient regulating margin,which are caused by conventional spare capacity for load reduction.Finally,the feasibility and effectiveness of the theory are verified by simulation.(2)The frequent occurrence of blackouts due to wind farms off the grid has exposed the shortcomings of wind turbines in fault ride through and active participation in frequency regulation.However,there are few works of literature that comprehensively analyze these two requirements simultaneously.Based on the analysis of the principle and demand of fault ride-through and frequency regulation,this paper proposes a joint control of wind power-storage based on rotating reserve and DC side energy storage,which uses the mechanical kinetic energy of wind turbine to provide virtual inertia support,supercapacitor to realize primary frequency regulation and fault ride through.According to the industry standard,the power regulation formula and parameter design during fault and disturbance are analyzed quantitatively,and the logic control principle realizes multi-operation mode switching.Based on only increasing the energy storage,this strategy has brought many performance improvements to the unit:It ensures the active power balance during the HVRT/LVRT and satisfies the priority of providing dynamic reactive power to the grid to support the voltage recovery;At the same time,it effectively solves the problems such as the second drop of frequency,the waste of reserve capacity,and the lack of regulation margin in the traditional strategy,to improve the overall operation stability and economy of the PMSG.Finally,the strategy is verified by simulation.(3)In the off-grid accident of wind turbines,that caused by transient overvoltage accounts for a significant proportion.It is essential to enhance wind turbines’ high voltage ride-through(HVRT)capacity.Based on the analysis of the phasor control principle of the converter,it is theoretically concluded that the transient overvoltage of the power grid is easy to causes the converter on the grid side to reach the upper limit of its AC voltage regulation,resulting in AC/DC power coupling oscillation,causing instability or overvoltage trip protection.In this paper,based on the requirements of relevant standards,the HVRT transient optimal control was established.The filter inductor was used to mitigate the overvoltage impact by increasing the reactive power support.It met the requirements of voltage phasor control of the converter and increased the reactive current injected into the grid,which was conducive to voltage recovery.The improved control adopted subsection optimization design according to different voltages.Energy storage was added to the DC side to improve the reactive power support and the upper limit of the ride-through voltage.The theoretical analysis based on typical parameters and the simulation experiment based on MATLAB/Simulink proved the correctness of the fault characteristic analysis and transient optimal control.(4)In the "double-high" power system,the fault voltage is increasingly characterized by high and low continuous oscillation,which increases the probability of trip-off.Based on the virtual synchronous generator(VSG),the paper designs a high/low voltage continuous ride-through strategy:active power control improves VSG by designing power compensation terms to increase frequency support externally and reduce bus voltage fluctuation internally;Reactive power control is based on standards and supports voltage recovery by injecting reactive current.The active power flow is quickly and accurately balanced during low voltage through variable power tracking methods such as Overspeed power limitation and emergency pitch control.During high voltage,the controllability of GSC is increased by dynamically adjusting the DC bus voltage.Compared with the dual closed-loop control with current source characteristics,the application of VSG with voltage source characteristics is conducive to improving the support of the unit during fault.The designed strategy has stronger sustainability,can withstand long-time,multi-frequency,and large-scale continuous fault voltage impact,and improves the unit’s survivability under lousy working conditions.In conclusion,based on the whole study of the causes of power grid frequency fluctuation and high/low voltage faults,this topic analyzes the harm of insufficient frequency stability and wind unit off-grid to the safe operation of the system.After fully summarizing the shortcomings of the existing control schemes,this paper puts forward the corresponding improvement and optimization strategy to enhance the unit’s ability to participate in power grid frequency regulation and fault ride-through and make grid-connected wind turbines show friendliness characteristics.