Research On Additional Control Of Virtual Synchronous Generator And Its Suppression Of Ultra-low Frequency Oscillation

With the increasing proportion of renewable energy,the integration of large numbers of power electronic converters makes the power system show a trend of low inertia and weak damping,resulting in reduced power system frequency stability.In addition,recent years there are cases of ultra-low frequency oscillations(ULFO)due to lack of damping of generators in power grids with hydropower units accounting for large proportion.The grid connected inverters can mimic the operation mechanism of traditional synchronous generators through virtual synchronous generator(VSG)technology,which can increase the system inertia and damping.The VSG technology has a wide application prospect.This paper studies additional control technology to improve the security and stability of the VSG,and the supporting effect of the VSG on the system frequency stability is validated with the ULFO problem.This paper firstly introduces the research status of VSG control technology and the ULFO problem,and the mathematical model and control method of the synchronverter based VSG are discussed in detail.Considering the regulating ability limit of the battery,additional control methods for the VSG under the restriction of battery charging/discharging performance are proposed in this paper.When the disturbance exceeds the regulation ability of the battery,the DC voltage offset is suppressed and eliminated,and the continuous charging/discharging power over-limit is avoided,which makes the VSG and the battery operate safely and stably.In order to study the frequency supporting effect of the VSG,the hydropower system with VSG integrated is chosen as the study case.Based on the damping torque analysis and the oscillation energy calculation,it is analyzed that the fundamental cause of the ULFO is the negative damping from the hydropower units,and the damping of the VSG to the power system is proven to be positive,which can effectively suppress the ULFO.Based on the analysis above,additional bandpass damping control loop is introduced to the VSG to improve its damping and suppression effect of the ULFO.The additional control methods proposed in this paper are all verified by simulations.