Research On Improvement Of New Energy Accessing Capacity In Power Electronized Grid-connected System With Oscillation Stability Limits

Under the target of new power system,new energy generations are widely injecting into the grid.The grid strength is gradually weakened.The oscillation problem,which is caused by the grid-source interaction,has become one of the key factors on system stable operation and the accessing capacity of new energy.For the power electronized system with various scenes and complex source-grid impedance,the frequency control characteristics of the synchronous generators is gradually deteriorated with grid weakening.The applicability of the existing impedance modeling method under fixed frequency point for the extreme weak grid condition with low frequency regulation capacity is unknown.The reactive power compensation capacitor and network interconnection lead the grid-source impedance to be more complex.Oscillation suppression method designed in fixed weak grid and fixed injecting node are needed to further revise.The internal relationship between the accessing capacity of each cluster and the maximum accessing capacity of the whole distribution area is ambiguous.For facing the improvement limitation of accessing capacity of distributed new energy caused by frequent oscillations,on the basis of impedance analysis method,this paper is researched in the following three aspects,which are impedance modeling considering multi-scenario grid weakening,accessing capacity improvement for single cluster and coordinated capacity allocation among multi-infeed clusters.The main works are as follows:(1)Considering the characteristics of grid impedance and frequency controllability under different penetration levels,impedance models in multi-scenes are built.For the traditional weak grid which is equivalent to the infinite power grid series with grid impedance,three-dimensional impedance of single inverter is built by considering the interconnection of the inverter’s AC and DC ports.And the effect of inverter’s control parameters and operation point on the oscillation occurrence are analyzed.Based on the multi-paralleled impedance model,along with increasing new energy,the source-grid impedance characteristics and oscillation constraint are analyzed.For the extreme weak grid with high penetration level and low frequency regulation capacity,considering the deterioration of synchronous generator’s frequency controllability,the impedance modeling method of synchronous generator-inverter with voltage,current and frequency disturbance is proposed,which covers impedance characteristics and frequency disturbance characteristics.The weakening reasons for grid strength and frequency control characteristics,and the mechanism of low-frequency oscillation are analyzed.Considering the weak grid with reactive power compensation capacitor,the impedance characteristics with shunting capacitor and the oscillation constraint on the new energy accessing capacity are analyzed.(2)For the single infeed cluster,location optimization of new energy clusters,optimized parameter design method and optimized control loop are proposed.Considering the difference of source-grid impedance at different injecting nodes,location optimization of new energy clusters is proposed for maximizing the new energy accessing capacity.Further,based on impedance matching of AC and DC ports,the optimization method of DC bus capacitance is studied.Combining the equation covering the coupling effect of PLL and the grid impedance and the impedance characteristics in DC port,the optimization scheme of inverter’s control parameters with dual constraints of AC and DC stabilities are proposed.Under inductive weak grid condition,distributed damping reconstruction scheme is carried out,which can realize oscillation suppression and self-stability.Considering the shunting capacitor in weak grid,the composite adaption method,which is composed of high frequency capacitive feature cancellation loop and damping reconstruction loop,is proposed.The oscillation suppression ability of the clusters in complex grid-connected condition is improved.(3)For the requirement of maximizing the injecting level of new energy,phase margin sensitivity coordination based optimal capacity allocation of new energy clusters is proposed.The impedance of complex network is established by applying the equivalent conversion of polygon-star.Combining with the inverter model,the aggregated impedance model of multi-infeed system and its stability margin indices are obtained.The accessing capacity under equal proportional allocation and non-proportional allocation among clusters and the sensitivity coordination requirement are analyzed.The phase margin assessment function with each cluster capacity is built.By applying the Lagrange multiplier method for solving the extreme value conditions,it is presented that the phase margin sensitivity of each cluster should be set consistent to obtain optimal capacity allocation with the largest phase margin under fixed accessing capacity.In order to obtain the maximal injecting capacity in regional grid,based on this efficient principle,capacity allocation optimization scheme of multi-infeed new energy clusters based on phase margin sensitivity coordination,which is only realized by simple numeric iterations,is carried out.The applications of the proposed scheme in the pre-planning scenario required highly efficient solution and the grid fault scenario required rapid evaluation are studied.Finally,the experiment platform with multi-parallel inverters and real-time digital simulation platform are built,and the validity of the above researches are verified.It demonstrates that the proposed multi-scenes impedance,cluster location optimization,parameter optimization,advanced control loop and sensitivity coordination based allocation method have positive significance for improving the accessing capacity of new energy and promoting the safe operation of the system.