Stability Analysis For PMSG-based Wind Farm Integrated Through Modular Multilevel Converter

With the rapid development of flexible high voltage direct current(HVDC)transmission technology,the grid integration of large-scale wind farms by modular multilevel converter based HVDC has become an important means to achieve the consumption and transmission of renewable energy.However,both the wind farm and the MMC station contain a large number of power electronic devices and a variety of control loops,the coupling mechanism of the two systems is complex,which increases the difficulty of control parameter design.At the same time,the randomness and uncontrollability of wind power fluctuation and the complexity of internal harmonic dynamics of MMC also bring potential stability problems to the system.It is reported that in recent years,a number of projects of wind farm integrated through MMC-HVDC have experienced oscillatory instability phenomenon,which has aroused widespread concern in the academic and industrial circles.This paper focuses on the stability of the permanent magnet synchronization generator(PMSG)based wind farm interconnected with MMC from three aspects:small signal modeling and oscillation mode of interconnected system,oscillationmechanism analysis of MMC internal modal,dynamic interaction mechanism analysis of control loops between PMSG-based wind farm and MMC station.1)Operation principle and control parameters design of PMSG-based wind farm integrated through MMC In order to ensure the stability of PMSG-based wind farm and MMC before interconnection.Firstly,the operation principle and the detailed control structure of PMSG-based wind farm and MMC station are introduced.And then the control parameters of each control loop of the grid side converter(GSC)of PMSG-based wind farm and MMC station are designed.Finally,the frequency domain and time domain simulation results show that the designed control parameters ensure that the PMSG-based wind farm and the MMC station have sufficient stability margin and good dynamic response characteristics before the interconnection.The analytical design method of the control parameters is also suitable for other power electronic converter application scenarios and has good versatility.2)Operation principle and control parameters design of PMSG-based wind farm integrated through MMCEigenvalue analysis based on small signal model is one of the important methods for system stability analysis and oscillation mode recognition.Firstly,the PMSG-based wind farm and the MMC station are decomposed into several basic unit modules,then the small signal model of the interconnected system is established by combining the basic units according to the input-output relationship,and the accuracy of the small signal model is verified by PSCAD electromagnetic transient time domain simulation.Based on the small signal model of the interconnected system,the eigenvalue method is used to analyze the influence of the output power of the wind farm on the stability of the interconnected system.It is found that as the power increases to the rated power,there are two kinds of oscillation instability modes in the interconnected system.In the first type of oscillation instability mode,the participating variables are mainly the internal state variables of MMC,and the participation degree of wind farm state variables is very small.Based on the example in this paper,The oscillation frequency of this kind of oscillation instability mode is about the sub-/supersynchronous oscillation frequency of 30.2Hz and 69.8Hz based on the parameters in this paper,and the internal oscillation instability of MMC can be effectively suppressed by Circulating current suppress controller(CCSC).The participating variables in the second type of oscillation instability mode mainly come from phase-locked loop(PLL)of wind farm and alternating current(AC)bus voltage-frequency(V/F)control of MMC,while the participation degree of internal state variables of MMC is very small.Based on the parameters in this paper,the oscillation frequency of this kind of oscillation instability mode is about 3.8Hz.This kind of oscillation instability mode can be effectively suppressed by reducing the PLL control bandwidth of PLL or increasing the proportional coefficient of V/F control.3)Internal modal oscillation mechanism of MMC in interconnected systemSince the relevant variables of wind farm basically do not participate in the internal oscillation mode of MMC,the interconnected system model can be reduced before analyzing the internal modal oscillation mechanism of MMC,the dynamic process of wind farm is ignored,and the wind farm is equivalent to current source.Eigenvalue analysis and time domain simulation show that the reduced-order model can accurately reflect the internal harmonic dynamics of MMC,and verify the rationality of the reduced-order model.The eigenvalue information of MMC is analyzed in detail based on the reduced-order model,and the internal relationship between oscillation damping,frequency and the parameters of MMC main circuit is revealed,which leads to the construction of the internal equivalent circuit of MMC.Based on the internal equivalent complex frequency domain circuit model of MMC,the impedance ratio criterion for judging the internal stability of MMC is derived,and the mechanism of internal oscillation instability of MMC is revealed.According to the results of mechanism analysis,it is further proposed that virtual resistors are added to the DC equivalent circuit of MMC arm to improve the damping of the circuit,so as to restrain the internal oscillation of MMC and improve the stability of the system.The oscillation suppression strategy of additional virtual resistance is realized by using DC current state feedback control of MMC.The effectiveness of the proposed suppression strategy and the correctness of the theoretical analysis results are verified in the PSCAD electromagnetic transient simulation environment.4)Mechanism of interaction between the control of wind fame and the control of MMC in interconnected systemIn order to reveal the control interaction mechanism of the interconnected system,a stability analysis method based on complex frequency domain small signal circuit model is proposed,and a complex frequency domain small signal circuit model of the interconnected system is developed.In this model,different control links are equivalent to circuit impedance,and the mechanism of control interaction can be explained by classical circuit theory.Furthermore,the impedance ratio criterion which can judge the stability of the system is derived by using the circuit model,and the interaction mechanism between the PLL control of wind farm and the V/F control of MMC station is studied based on the model and stability criterion,it is shown that the PLL control of wind farm is equivalent to parallel equivalent impedance Zpll(s)at the exit of wind farm,and the V/F control of MMC is equivalent to series t impedance ZHVDC(s)at the ac bus of PCC,after the wind farm is interconnected with MMC,the impedance Zpll(s)and ZHVDC(s)are in the same loop,so the dynamic process of PLL control and V/F control is coupled.Within the control bandwidth of PLL,the ZPll(s)is capacitive impedance with negative resistance,while the ZHVDC(s)is inductive impedance with positive resistance.The phase difference between the two impedances is close to or more than-180 degrees,and there is a risk of oscillation instability,and the oscillation frequency increases proportionally with the increase of the control bandwidth,but is always within the range of the control bandwidth of PLL.The impedance ZHVDC(s)is reshaped by adjusting the V/F control parameters of MMC,which suppresses the oscillation instability caused by the interaction between PLL control and V/F control,and ensures a sufficient stability margin for the interconnected system.