Stability Criterion Adaptation Analysis And Vector Stability Classification For Small-Signal Stability Of Renewable Power System

The construction of a new power system with high penetration of reneable energy devices is not only an important strategic direction of China’s energy revolution,but also the main technical path to realize the goal of "double carbon".The dynamic characteristics of reneable energy devices and synchronous machines are quite different.Therefore,the mathematical models,analytical methods and classification standards used in the stability analysis of traditional power system are difficult to be applied to the renewable power system.There are various stability analysis methods and stability criteria for renewable energy devices due to their complex control characteristics and diverse application scenarios.Although different stability criteria can be transformed into each other,they focus on different dominant variables and dynamic loops,reflecting different physical characteristics.Consequently,for the typical stability scenarios dominated by renewable energy devices,how to accurately select appropriate stability criteria and quantify the stability margin;how to scientifically explain the system’s instability mechanism and construct a stability classification method compatible with both traditional and new power system are difficult for stability analysis.To this end,this paper focuses on the small-signal stability issue of renewable power system and answers the two key questions of the adaptation of stability criterion and the scientific explanation of the stability mechanism.Moreover,the assessment principle and indexe of stability criterion’s adaptation are proposed;the identification method for stability mechanism of renewable energy devices and quantitative method for stability margins are offered;and a vector stability classification system is constructed.The above research enhances the stability perception of renewable power system.The main innovations of this paper are as follows:1.The construction mechanisms of classical stability criteria for synchronous machines and reneable energy devices are clarified,and the essential differences and connections of different stability criteria are explored.Moreover,based on the time-domain and frequency-domain models,the generalized construction idea of the stability criteria and its mathematical nature are clarified,and a unified description method for stability criteria is proposed.Based on the unified description method,it is shown that although different stability criteria can usually ensure the correct stability assessment results of whether the system is stable,different stability criteria select different forward transfer functions and input-output variables,which focus on different key dynamics loops and dominant variables.Therefore,different stability criteria correspond to differences in physical interpretation and ability to portray the degree of stability,and therefore apply to different stability analysis scenarios and issues.2.From the perspectives of stability,nominal performance and robustness,the possible limitations of the frequency-domain stability criterion for renewable power system are illustrated,and the qualitative assessment principle of stability criterion’s adaptation is proposed.The reasons for analysis errors or failures of certain stability criteria in specific scenarios are also explained.On this basis,a quantitative indicator,namely loop gain sensitivity,is proposed to assess the adaptation of various stability criteria.Here,the angle criterion of loop gain sensitivity can determine whether the stability margin obtained by a stability criterion is valid,and the magnitude criterion of loop gain sensitivity can analyze whether a stability criterion meet the robustness requirements.The physical nature and adaptation of several impedance-based stability criteria in renewable power system are also clarified.3.The matching law between the stability mechanism and the stability criterion is revealed,and a loop-gain-sensitivity-based dominant variable and stability mechanism identification method is proposed.The appropriate stability criteria,derived stability mechanisms,dominant variables and recommended stability control strategies for typical devices,such as synchronous machines,converters and doubly-fed machines under different grid conditions are analyzed.In view of the problem that renewable energy devices’ stability cannot be integrated into stability classification of traditional power system,the mathematical model for the stability analysis of the traditional and new power systems is unified by using vector modeling,and the instability mechanism of multiple devices is unified by using the dominant-output-variable-based method,so as to construct a vector stability classification system for renewable power system.The system classifies equipment stability from the vector perspective into three categories: phasedominated synchronous stability,amplitude-dominated voltage stability,and electrical resonance formed by their special combination.Issues such as the concept of wide-band oscillation,the distinction between equipment stability and system stability,and the extensibility of equipment vector stability are further discussed.4.In view of the multiple modes’ oscillation problem of grid-connected renewable energy device,a stability mode identification method based on the sensitivity of dominant eigenvalue trajectories is proposed.The method can effectively locate the key dynamic loops related to dominant oscillation modes and determine their stability classification by simply utilizing the terminal characteristics and information of renewable energy devices and power grid.On this basis,the necessity of stability margin function selection in multiple modes oscillation problems is illustrated,and a stability-mode-identification based stability margin assessment method is proposed.The research paradigm of stability mode identification-stability margin assessmentcritical factor analysis for the multiple modes’ oscillation problem of grid-connected renewable energy device is finally formed,which can effectively avoid the stability analysis errors caused by the improper selection of stability margin function.