Analysis And Suppression Strategy Of Synchronous Frequency Resonance For Grid-forming Converter

As new energy sources continue to develop,China’s power system will gradually transform from the traditional power system dominated by synchronous generators to a new power system dominated by new energy generation.In the new power system,the proportion of non-synchronous machine power sources based on power electronic devices will continue to increase,leading to a continuous reduction in the short-circuit ratio,and the grid will become weaker and weaker.A voltage source converter(VSC)is commonly used as the interface device between new energy generation and the grid.Among them,the grid-forming VSC is undergoing rapid development,for it can provide voltage and frequency support and improve the stability of VSC when connected to weak grids.However,studies have pointed out that when considering the dynamic of the grid inductance,gridforming VSCs taking common power synchronous control(PSC)and virtual synchronous generator(VSG)control will inevitably suffer from synchronous frequency resonance(SFR).In order to solve the problem mentioned above,this paper investigates the mechanism of SFR in PSC-VSC and VSG-VSC and proposes corresponding SFR suppression strategies to provide a theoretical basis for the safe and stable operation of grid-forming VSCs.The main contents are as follows.Firstly,we take the coupling relationship between the active power control loop and reactive power control loop into account and establish the small-signal models of PSC-VSC and VSG-VSC in the s-domain.The open-loop gain and closed-loop transfer functions are also derived.Based on the closed-loop transfer function,it is verified that the root locus of the system will approach the imaginary axis and enter the right half-plane when the equivalent resistance of the inductive network is small,triggering SFR.Combining the classical Nyquist stability criterion,it is found that the non-minimum phase link is also an important cause of SFR.Finally,the system parameter ranges are classified according to the causes of SFR initiation.Secondly,an SFR suppression strategy using a notch filter is proposed.It is found that when using VSG control instead of PSC control,the same VSC is less likely to have SFR.In fact,the VSG control is essentially equivalent to PSC control with a first-order low pass filter(LPF).When the cutoff frequency of the LPF is relatively low,it can suppress the resonance peak near the synchronous frequency to some certain extent,but the system’s equivalent inertia coefficient will be relatively large.Additionally,if the LPF’s cutoff frequency is set too low,oscillations may occur at even lower frequencies.To suppress SFR under various parameters,this paper proposes an SFR suppression strategy using a notch filter.This strategy can not only eliminate the resonance peak but also make the system has a higher stability margin and stronger robustness to control parameters.The effectiveness of this strategy is verified by simulations and physical experiments.Finally,an SFR suppression strategy based on conjugate poles elimination is proposed.According to the characteristics of the grid-forming VSC’s small-signal model and the two reasons for SFR,it is proposed to add a group of auxiliary branches to eliminate the conjugate poles near the virtual axis.This strategy can also eliminate the non-minimum phase element in the open-loop gain.Compared with the traditional virtual resistance method,this strategy can suppress gridforming VSC’s SFR in a wider range of parameters.It also effectively increases the bandwidth of the power control loop and improves the response speed of the system.The effectiveness of this strategy is verified by simulations and controller hardware-in-the-loop experiments.