Analysis And Enhancement Of The Maximum Accomodable Capacities Of Renewables In Power Systems With The Constraints Of Oscillation Stability

With the rapid development of renewables and loads,which are connected to grids by power electronical devices(PEDs)as interfaces,the modern power systems are featured by short-circuit-ratio weak-synchronized grids(called as weak grids).This causes oscillation issues,bringing great challenges to the secure operation of modern power systems.The current research shows that the oscillation issues are mainly related with the strong interaction between PEDs and network,among PEDs.However,the dynamics of PEDs are complicated and the number of PEDs are very large;besides,the control structure and parameters of PEDs are diverse,which makes the stability analysis of the multi-infeed PEDs power systems(MIPPS)very complicated.This also causes that the analysis and enhancement of the grid-accomodable capacities(GAC)of the renewables in modern power systems are very difficult.Besides,unlike synchronous generators,the dynamics of PEDs are faster and more complicated.Due to this,the PEDs may still lose stability and be tripped from grids during short-circuit faults,even that the voltage fault only maintains in dozens of milliseconds level,which limits the GAC of renewables in modern power systems.The previous researches mainly focus on the post-fault stability of the conventional power systems,dominated by synchronous generators.However,the instability mechanism of the PEDs during faults is lack of awareness,which also limits the GAC of renewables.To this end,this dissertation is centered around the GAC issues of renewables with the constraints of oscillation stability,including the key-factor analysis of the GAC issues of renewables,the evaluation of GAC issues of renewables,and the modified method of the GAC of renewables.The main contributions of this dissertation are summarized as follows.1.For the single-renewable infinite-bus system,we derive the relation between PED's capacity and the small signal stability during normal operation,propose a control-parametermodifying method of the SVG to enhance the small signal stability,analyze the key factors of oscillation stability which limit the GAC of the wind turbines(WTs)during short-circuit faults,and propose modified low-voltage ride-through(LVRT)method.1)In terms of key factors analysis,we derive the explicit relation between the PED's capacity and the small signal stability,and explain the key factors and their behavior rules causing oscillation instability during short-circuit faults,which limit the GAC of the renewables.Firstly,we use the impedance-modelling method to construct the impedance model and closedloop characteristic equation of the single-PED infinite-bus system in the synchronous reference.On this basis,we show the explicit relation between PED's capacity and the system's small signal stability.Then,we focus on the oscillation stability issue of type-4 WTs and double-fed induction generator(DFIGs)during short-circuit faults which limit the GAC of WTs.We construct the quasi-steady state model and linearized state-space model of the system during short-circuit faults.By the two constructed models,we analyze the instability mechanism of the type-3 WT and DFIG system during short-circuit faults from the viewpoint of equilibrium existence and small signal stability,respectively.We explain the key factors and their behavior rules for the oscillation instability issues.Besides,we compare the difference between the instability behaviors of the type-3 WTs and the DFIG WTs.2)In terms of the modified method of the GAC of renewables,we proposed a controlparameter-modifying method of the SVG to enhance the small signal stability during normal operation,and propose modified low-voltage ride-through(LVRT)methods for type-3 WTs and DFIG WTs,respectively.Firstly,we construct the linearized state-space model of the singlePED single-static-var-generator(SVG)infinite-bus system.On this basis,we use the parametersensitivity method to find the key control parameters that impact the system's small signal stability.Furtherly,we proposed a control-parameter-modifying method to enhance the system's small signal stability.Besides,we combine the online-parameter-identification method and trajectory planning method to propose the modified LVRT method for type-3 WTs and DFIG WTs,respectively.2.For the diverse renewable multiple-infeed system(RMIS),we analyze the key factors for the small signal stability issues which limiting the GAC of renewables,propose the renewables' GAC evaluation method with the constraints of small signal stability,and propose a SVG-based method to enhance the renewables' GAC.1)In terms of key factors analysis,we construct the explicit relation between PEDs capacities and the small signal stability of the RMIS,and analyze how the SVG impacts the small signal stability of the RMIS.Firstly,we review the generalized-short-circuit-ratio(g SCR)-based stability-analyzing method.Based on the g SCR-based stability-analyzing method,we find the explicit relation between PEDs capacities and the system's small signal stability.Then,we construct the frequency-domain impedance model and closed-loop characteristic equation of the RMIS with SVGs.By the modal-perturbation theory,we construct an equivalent homogeneous multiple-infeed system(EHMIS),of which the dominant eigenvalues are approximately the same as that of the RMIS with SVGs.Beisdes,we prove that the g SCRbased stability-analyzing method is suitable for the small signal stability issues of the RMIS with SVGs.Finally,we propose the calculating algorithm of the g SCR and critical g SCR(Cg SCR)of the RMIS with SVGs,and propose the evaluation method of grid strength and stability margin of the RMIS with SVGs.2)In terms of the renewables' GAC evaluation method and related modified method,we propose the GAC evaluation method with the constraint of small signal stability and propose a SVG-based method to enhance the GAC from the viewpoint of small signal stability.Firstly,we note that the system's small signal stability has a positive relation with the g SCR.On this basis,we convert the evaluation issues of renewables' GAC in RMIS with the constraints of small signal stability into the evaluation issues of the GAC with the constraints of g SCR,which highly simplifies the concerned optimal issues since it avoid constructing the system's detailed linearized state-space model.Finally,we propose the SVG's optimal placement method and parameter-modifying method to enhance the stability.stability of the RMIS.