Investigation On Key Strategies Of DFIG Wind Power System Based On Virtual Synchronous Generator Control

In order to implement the goals of “2030 carbon emission peak” and “2060 carbon neutrality”,China proposed in the “14th Five-Years Plan” to build a clean,low-carbon,safe and efficient energy system,and to increase the scale of wind power.When the wind power generation is in an unprecedented development opportunity,a high proportion of new energy and a high proportion of the development trend of power electronics also bring new challenges to the safe operation of the power system at the same time.The doubly-fed wind power based(DFIG)under traditional power control strategy does not have the ability to build and support the grid,and the stability of power systems containing a high percentage of new energy sources will decrease due to low rotational inertia and capacity.In addition,power electronic converters are the control and grid-connected interface DFIG.It will change the impedance characteristics of the power system dominated by traditional synchronous generators(SG),and bringing potential stability risks.The novel power system with "double-high" characteristics in the future will inevitably require higher support ability of DFIG.Virtual synchronous generator(VSG)control is a control strategy that emulates the operating characteristics of synchronous generators to make DFIG with similar frequency support capability and impedance characteristics of SG,which has become a hot research topic.Due to the wind resources and power load inverse distribution,China mostly take centralized development,long-distance transmission of wind power operation mode.The grid-connected wind turbine in this mode is actually a grid with extremely complex operating conditions,bringing both transient and steady-state problems to the grid-connected operation of DFIG.In terms of transient problems,when symmetrical or asymmetrical grid faults occur,DFIG under VSG control will suffer from output power and electromagnetic torque oscillations,output current power quality degradation and other problems,which will also threaten the safety of DFIG operation in case of serious faults.In terms of steady-state problems,DFIG under VSG control exists in two centralized access forms,AC and UHVDC.When DFIG is transmitted via AC lines,series capacitor compensators are usually added to enhance the transmission capacity due to the long lines,however,a number of sub-synchronous oscillations caused by the coupling of DFIG and series compensators have occurred in recent years.When DFIG is transmitted via the UHVDC system,the DC delivery grid with high proportion of wind power and high proportion of power electronics also has problems such as low inertia and degradation of power quality.The current VSG for DFIG is designed under ideal grid conditions and lacks adaptive strategies for the above problems.Therefore,it is necessary to study the improved control strategy of DFIG under VSG control in fault voltage,AC series complementary grid,and UHVDC grid to enhance the grid adaptation.Against this background,this paper presents a systematic and in-depth study of the grid adaptation and active support functions of DFIG under VSG control.Then,some novel and important conclusions can be summarized as follows:1.For the problems of electromagnetic transient component oscillation and over rotor current in DFIG caused by symmetric fault voltage,an improved VSG control strategy is proposed for DFIG under fault voltage based on air-gap flux feedback to improve the low voltage ride-through capability of DFIG under fault grid based on VSG control.An over rotot current suppression strategy based on rotor current peak detection is also investigated to reduce the rotor overcurrent peak while accelerating the decay rate of the transient flux components during symmetrical voltage faults.For the problems of DFIG caused by asymmetric fault voltage,such as negative sequence component,power torque oscillation and power quality degradation,a resonator-based component control strategy is proposed on the basis of VSG control.The control objectives of stator current three-phase balance,rotor current three-phase balance,smooth stator power and electromagnetic torque,and smooth stator reactive power can be achieved respectively by setting different control feedback component.In addition,in order to improve the output current power quality of DFIG systems under asymmetric grids,a cooperative control of the total output current using a grid-side converter(GSC)is investigated to reduce the system output current harmonics.2.For the problems of grid-connected operation oscillation of DFIG under the AC series complementary grid,the impedance model of DFIG under VSG control is established.The results of time domain simulation measurements and FFT frequency analysis are compared to verify the correctness of the impedance model and the validity of the stability analysis.Based on the impedance model,the influence law of different parameters such as inertia coefficient,damping coefficient,flux feedback proportionality coefficient,integration coefficient and DFIG rotor speed on the stability of DFIG under VSG control in series complementary grid is analyzed.For the problems of sub-synchronous resonance(SSR)in high series complementarity grids,an SSR suppression technique based on virtual rotor leakage inductance is proposed to improve the stability of DFIG and accelerate the decay of SSR.Impedance design requirements based on impedance models is obtained to improve to control strategies.The SSR suppression method based on virtual rotor leakage inductance is investigated to improve the adaptability and SSR of DFIG under VSG control in high series complementarity grid by considering the sub-synchronization stability and power angle relationship.3.For the problem of stability in the delivery end of UHVDC transmission due to the reduction of SG capacity,a method to enhance the stability of the delivery end grid based on VSG control of DFIG wind farms is proposed.Based on the analysis of impedance characteristics of SG,DFIG and UHVDC converter stations,the influence laws of different SG capacities and short-circuit ratios on system stability are studied.The stability enhancement method of DFIG wind farm under VSG control is also studied.For the problem of power quality degradation in the power grid in the delivery end of UHVDC transmission due to operating points changing,the active power quality support technology for DFIG wind farms under VSG control is proposed.The design of flexible power quality support technology for each unit in a wind farm based on operating conditions and total harmonic reduction targets without real-time communication.The units in the wind farm can flexibly and autonomously adjust their power quality support capacity according to their operating status.