Interharmonic Analysis Model of Photovoltaic Grid-connected System with Extended Dynamic Phasors

The interactions between randomly fluctuating power outputs from photovoltaic(PV) at the DC side and background voltage distortions at the AC side could generate interharmonics in the PV grid-connected system(PVGS). There is no universal method that can reveal the transmission mechanism of interharmonics and realize accurate calculation in different scenarios where interharmonics exist in the PVGS. Therefore, extended dynamic phasors(EDPs) and EDP sequence components(EDPSCs) are employed in the interharmonic analysis of the PVGS. First, the dynamic phasors(DPs) and dynamic phasor sequence components(DPSCs) are extended into EDPs and EDPSCs by selecting a suitable fundamental frequency other than the power frequency. Second, an interharmonic analysis model of the PVGS is formulated as a set of state space equations. Third, with the decoupling characteristics of EDPSCs,generation principles and interactions among the interharmonics in the PVGS are presented by the sequence components,and its correctness is verified by simulation and experiment.The presented model can be used to accurately calculate the interharmonics generated in the PVGS both at the AC and DC sides. Because of the decoupling among the EDPSCs, the set of state space equations can effectively describe the principle.

Dynamic phasor-based hybrid simulation for multi-inverter grid-connected system

To realize the efficient transient simulation of a grid-connected power generation system based on multiple inverters, this paper proposes a hybrid simulation method integrating the models of electromagnetic transient and dynamic phasors. Based on a demonstration of the concepts and properties of dynamic phasors, the models of single-phase and three-phase inverters described by dynamic phasors are established first. Considering the numerical compatibility problem between dynamic phasors and instantaneous values, an interface scheme between dynamic phasors and instantaneous values is designed, and the efficiency and precision differences of various transformation methods are compared in detail.Finally, by utilizing MATLAB/Simulink, a hybrid simulation platform of a multi-inverter grid-connected system is built, and the efficiency and accuracy of the hybrid simulation are validated via comparison with the full electromagnetic transient simulation.

Dynamic Phasor Modeling and Low-frequency Oscillation Analysis of Single-phase Vehicle-grid System

In practical operations,low-frequency oscillation(LFO)occurs and leads to converter blocking when multiple electrical rail vehicles at the platform are powered by the traction network.This paper proposes a small-signal model in state-space form for multiple vehicle-grid systems based on a dynamic phasor.This model uses the phasor amplitude and phase as variables to accurately describe the dynamics of the converter phase-domain control.An eigenvalue based-method is introduced to investigate the LFO with advantages of acquiring all oscillatory modes and analyzing participation factors.Two low-frequency dominant modes are identified by eigenvalues.Mode shape reveals that one of the modes involves the oscillations between the grid-connected converters and the traction network,and the other one involves the oscillations among these converters.Then the sensitivities of these two low-frequency modes to different system parameters are analyzed.Participation factors of system state variables,when the number of connected vehicle increases,are compared.Finally,the theoretical analysis is verified by nonlinear time-domain simulations and the modal analysis based on the estimation of signal parameters via the rotational invariance techniques(ESPRIT)method.

Improved State-space Modelling for Microgrids Without Virtual Resistances

Power converters and their interfacing networks are often treated as modular state-space blocks for small-signal stability studies in microgrids;they are interconnected by matching the input and output states of the network and converters.Virtual resistors have been widely used in existing models to generate a voltage for state-space models of the network that require voltage inputs.This paper accurately quantifies the adverse impacts of adding the virtual resistance and proposes an alternative method for network modelling that eliminates the requirement of the virtual resistor when interfacing converters with microgrids.The proposed nonlinear method allows initialization,time-domain simulations of the nonlinear model,and linearization and eigenvalue generation.A numerically linearized small-signal model is used to generate eigenvalues and is compared with the eigenvalues generated using the existing modelling method with virtual resistances.Deficiencies of the existing method and improvements offered by the proposed modelling method are clearly quantified.Electromagnetic transient(EMT)simulations using detailed switching models are used for validation of the proposed modelling method.

Least square and Kalman based methods for dynamic phasor estimation:a review

The characterization of sinusoidal signals with time varying amplitude and phase is useful and applicable for many fields.Therefore several algorithms have been suggested to estimate main aspects of these signals.Within no standard approach to test the properties of these algorithms,it seems to be helpful to discuss a large class of algorithms according to their properties.In this paper,six methods of estimating dynamic phasor have been reviewed and discussed which three of them are based on least square and others are based on Kalman filter.Taylor expansion is used as a first step and continued with least square or Kalman filter in accordance with the proposal observer of each method.The theoretical processes of these methods are briefly clarified.The characterizations have been made by some tests in time and frequency domains.The tests include amplitude step,phase step,frequency step,frequency response,total vector error,transient monitor,noise,sample number,computation time,harmonic and DC offset which build a framework to compare the different methods.