Zhangjie Liu

Stability Analysis and Stabilization Methods of DC Microgrid With Multiple Parallel-Connected DC–DC Converters Loaded by CPLs

Constant power loads may yield instability due to the well-known negative impedance characteristic. This paper analyzes the factors that cause instability of a dc microgrid with multiple dc–dc converters. Two stabilization methods are presented for two operation modes: 1) constant voltage source mode; and 2) droop mode, and sufficient conditions for the stability of the dc microgrid are obtained by identifying the eigenvalues of the Jacobian matrix. The key is to transform the eigenvalue problem to a quadratic eigenvalue problem. When applying the methods in practical engineering, the salient feature is that the stability parameter domains can be estimated by the available constraints, such as the values of capacities, inductances, maximum load power, and distances of the cables. Compared with some classical methods, the proposed methods have wider stability region. The simulation results based on MATLAB/simulink platform verify the feasibility of the methods.

Stability analysis of DC microgrids with constant power load under distributed control methods

Abstract Constant power loads (CPLs) often cause instability due to its negative impedance characteristics. In this study, the stability of a DC microgrid with CPLs under a distributed control that aims at current sharing and voltage recovery is analyzed. The effect of the negative impedance on the behavior of distributed controller is investigated. The small-signal model is established to predict the system qualitative behavior around equilibrium. The stability conditions of the system with time delay are derived based on the equivalent linearized model. Additionally, eigenvalue analysis based on inertia theorem provides analytical sufficient conditions as a function of the system parameters, and thus it leads to a design guideline to build reliable microgrids. Simulations are performed to confirm the effectiveness and validity of the proposed method.

First-order Generalized Integrator Based Frequency Locked Loop and Synchronization for Three-Phase Grid-connected Converters under Adverse Grid Conditions

This paper presents an alternative frequency adaptive grid synchronization technique named HDN-FLL, which can accurately extract the fundamental positive- and negative-sequence components and interested harmonics in adverse three-phase grid voltage. The HDN-FLL is based on the harmonic decoupling network (HDN) consisting of multiple first order complex vector filters (FOCVF) with a frequency-locked loop (FLL), which makes the system frequency adaptive. The stability of the proposed FLL is strictly verified to be global asymptotically stable. In addition, the linearization and parameters tuning of the FLL is also discussed. The structure of the HDN has been widely used as a prefilter in grid synchronization techniques. However, the stability of the general HDN is seldom discussed. In this paper, the transfer function expression of the general HDN is deduced and its stability is verified by the root locus method. To show the advantages of the HDN-FLL, a simulation comparison with other gird synchronization methods is carried out. Experimental results verify the excellent performance of the proposed synchronization method.

Optimal decentralized economical-sharing criterion and scheme for AC microgrids

In order to address the economical dispatch problem in islanding microgrids, this paper proposes a criterion and a decentralized economical-sharing scheme. The criterion is to judge the optimal economical-sharing whether can be realized via the decentralized approach. Then an optimal decentralized economical-sharing scheme is proposed. This overcomes the limitations of traditional centralized control, where the central controller and external information exchange are needed. The frequency is used as a carrier driven the objective function convergence, meanwhile the final frequency can be controlled within the allowable ranges and the DGs can share loads with optimal decentralized economical operation. An islanded microgrid model has been developed through simulations and experiments to verify the performance of the proposed method.

A stabilization method of LC input filter in DC microgrids feeding constant power loads

In DC microgrids, constant power loads (CPLs) would lead to a destabilizing effect that may cause the main bus voltage oscillation. Hence, most researcher are exploring the stabilization methods for DC/DC buck converters feeding CPLs. In practical applications, buck converters are always cascaded with LC input filters to prevent the switching harmonics from returning into the source. With the addition of input filters, the system stability would be more severe and the complexity is increasing. In this paper, for buck converters with LC input filters, a linear feedback control is proposed to overcome the negative impedance characteristic and stabilize the system. Moreover, the accurate load demand sharing among DGs and high quality of load voltage are guaranteed. Simulation and experimental results verify the effectiveness of the proposed methods.