Research On Stability Control Of Grid-connected Converter And System Under Grid Impedance Changes

The control technology of the grid-connected converter and system plays an important role in the safe and stable operation of the new energy power generation system.With the continuous improvement of new energy penetration,the power grid presents many complex states.In particular,long-distance power transmission,series compensation,and multi-machine parallel coupling lead to increasingly complex grid impedance characteristics.In addition,because of the strong randomness and volatility of new energy power generation,the grid-connected power and the converter number are constantly changing,which further causes the grid impedance to fluctuate significantly.However,most existing new energy power generation uses current-controlled mode(CCM)converters,usually designed based on specific grid states.Therefore,it is difficult to adapt to complex grid impedance changes,leading to a series of instability problems.To address this problem,this thesis firstly studies the plant-level and unit-level stability of grid-connected converters and systems with different control modes under typical power grids,and analyzes the adaptability of different control modes to different types of grid impedance.Then,for grid-connected system stability,a capacity optimization configuration method based on the hybrid-mode grid-connected system is proposed.Moreover,for grid-connected converter stability,a multi-model-based converter adaptive control strategy is proposed.Based on the proposed method,the stable operation of the grid-connected converter and system under high permeability and complex grid impedance conditions is realized.The main innovations of this thesis are as follows:1)Under complex grid impedance,the grid-connected system may have both plantlevel harmonic resonance and unit-level circulating current.Aiming at this problem,the two-level stability constraints and stability characteristics of both the CCM and voltagecontrolled mode(VCM)grid-connected systems under typical power grids are studied.Moreover,an equivalent simulation method for multi-converter grid-connected systems based on the double-machine equivalent model is proposed,which can simultaneously simulate the plant-level and unit-level stability of grid-connected converter systems.Analysis shows that this model is the most miniature scale model capable of simultaneously simulating these two levels of stability.2)The CCM grid-connected system is prone to instability at the plant-level under weak grids.To address this problem,this thesis proposes to connect VCM converters to the system forming a hybrid-mode grid-connected system(HMGCS),thereby enhancing the system stability under weak grids.However,the VCM converters need to reserve active power,and their connection will reduce the utilization rate of new energy.To address this problem,this thesis proposes an optimization capacity configuration method for the HMGCS.In the method,the capacity of the VCM converters will be adjusted in real-time.Thereby,the co-optimization of stability margin and power generation efficiency can be realized.3)The active power control bandwidth of the VCM converter is generally low,and it is challenging to meet the rapidity of the maximum power point tracking and automatic power generation control.To address this problem,a rapid active power control strategy based on the pre-filter is proposed,which can improve the active power control bandwidth of the VCM inverter without affecting its stability.4)Aiming at the problem that a single control mode is difficult to adapt and stabilize under the conditions of grid impedance type and parameter change,an adaptive stability control strategy for grid-connected converters based on the multi-model is proposed.The estimation method for different grid states is studied,and the grid-connected converter performs multi-model control switching accordingly,so as to adapt to complex grid impedance changes.Furthermore,the state tracking method and the disturbance-free switching strategy are proposed.Finally,an experimental platform of the multi-converter grid-connected system is built to verify the effectiveness of the proposed method.