Coherent Equivalence Method Of Grid-connected Inverters And Its Application In Dyanmic Analyses

With the global energy crisis becoming increasingly severe,the development and utilization of new energy sources has become the main way to solve this problem.The flexible control capability of the inverter makes it an important interface between renewable power generation(RPG)devices and the power grid.Compared with synchronous generators,the capacity of a single RPG device is small.As renewable energy is gradually replacing conventional energy,whether in distributed or centralized new energy generation applications,it is necessary to connect multiple renewable energy generators in parallel to meet the needs of "source-charge" balance.Therefore,a large number of grid-connected inverters are connected in parallel in RPG-integrated power systems.The inertia of inverters is low.The dynamic processes exhibit multiple time-scale characteristics,and there is interaction between parallel inverters.The above characteristics bring challenges to the control and stable operation of multiple grid-connected inverters and RPG-integrated power systems.In order to predict the dynamic behavior of RPG-integrated power systems and guide the control and protection strategies of the systems,an accurate model of paralleled inverters needs to be established.However,in systems connected with inverters in a large scale,if the detailed dynamic process of each inverter and its connection network is all included in the dynamic model,the order of the model will be very high and the calculation will be complicated.Coherent equivalence method is a reduced-order modeling method for multi-machine parallel systems.This equivalent model has clear physical meaning and can accurately characterize the dynamics of a system.To meet different dynamic analysis needs,this paper proposes a set of coherent equivalence methods for grid-connected inverters.The research focuses on the following three aspects: the coherency identification method of inverters,the aggregation method of coherent inverters,and two coherent equivalence methods for the interaction analysis of inverters and the dynamic analysis of the part of system outside inverters.The main work of this article is as follows:(1)In order to improve the accuracy of the coherent equivalence model of a multi-inverter parallel system,this paper uses the feature of Hamilton's action that it can represent the whole dynamic characterististics of a system,and then derives a coherency criterion that can comprehensively characterize the similarity of the dynamic characteristics of inverters from the perspective of energy conversion.By comparing mechanical systems and electrical systems,the application of the Hamilton principle to electrical systems is illustrated.In addition,the generalized Hamilton's action is defined to comprehensively characterize the energy exchange among energy storage elements of an inverter and the energy exchange of energy storage elements with the external system.According to the generalized Hamiltonian action expression of grid-connected inverters,the condition that the state variables of the coherent inverters are proportional is equivalent to the condition that the generalized Hamiltonian action is proportional.Then,a Hamilton's-action-based coherent criterion is proposed for inverters.(2)In order to make the coherent equivalent model applicable for different dynamic analyses senarios,this paper systematically proposes a set of aggregation modeling method for coherent inverters.On the one hand,when an inverter group is the external system and its integrated power grid is the research system,it is proposed to aggregate coherent inverters into a single-machine model.On the other hand,the aggregation method which represents multiple coherent inverters as one equivalent inverter has limitations in the interaction analysis of multiple inverters.This paper uses the controllability and observability to analyze the characteristics and the physical meaning of the interactive modes.Based on the interaction mechanism of coherent inverters,this paper proposes a novel aggregation modeling for the study of the stability of inverter groups,that is,representing coherent inverters by two equivalent inverters.(3)For the stability analysis of multi-inverter-paralleled systems,this paper explores the interaction characteristics among inverters of the same structure but with different parameters,and then proposes an equivalence model based on the two-inverter aggregation modeling of coherent inverters.Firstly,an open-loop subsystems interconnection model is proposed for the analysis of the interaction among inverters.By establishing the open-loop subsystems interconnection model of a two-inverter system with different parameters and a multi-inverter system with two coherent groups,the effects of parameter differences on the interaction and dynamic characteristics of the closed-loop system are analyzed.Besides,the characteristics of the interaction among coherent inverters and the interaction among multiple coherency groups are summarized.Based on the interaction mechanism of differently parameterized inverters,an equivalent model based on the two-inverter representation of coherent inverters is proposed to preserve the influence of interaction on the stability of paralleled inverters.(4)For the dynamic analysis of the part of a system outside inverters,this paper proposes a coherent equivalence method based on single-machine representation,and this equivalence method is applied to the low-frequency oscillation analysis of complex power systems.This paper takes an IEEE 30-node system connected to 12 direct-drive permanent magnetic synchronous generators as an example,and analyzes the dynamics interaction between paralleled inverters with different parameters and the synchronous generators through the participation factor method.At the same time,through the two steps,i.e.,the coherency identification of grid-connected inverters and the representation of coherent inverters as a single inverter,an aggregated model of the parallel inverters is established.The dynamic interaction analysis of the coherent equivalent model and the detailed model is verified.