The existing research on hybrid energy storage system (HESS) stability mostly focuses on small-signal stability analysis methods. However, when large-signal disturbances occur, the intrinsic nonlinearity of the HESS becomes inevitable so that the small-signal studies are no longer valid, which is beyond the scope of small-signal stability analysis. To address this problem, a large-signal stability analysis method of the HESS is proposed based on the estimation of domain of attraction (DOA). This method can not only analyze the asymptotic stability of the operating point, but also answer the question of how large deviations from the operating point can be tolerated by the system. Specifically, the state space model and Takagi-Sugeno (T-S) fuzzy model of the whole system are first constructed. By using linear matrix inequality (LMI) and Lyapunov’s method, the DOA of the system at the operating point is estimated, and the large-signal stability of HESS is analyzed. On this basis, the influence of system parameters (e.g. virtual resistance, virtual inductance and PI regulators) and load power on the large-signal stability are studied, and the dominant parameters affecting the large-signal stability of the HESS is clarified, which provides a practical guiding basis for the parameter optimization design of the HESS to ensure the safe and stable operation of the system under extreme load switching conditions. The simulation results verify the feasibility and effectiveness of the proposed large-signal stability analysis method and the correctness of the analyzing results.

The hybrid power system has been widely adopted in the unmanned aerial vehicle (UAV) to meet various flight profiles. In this paper, an optimal sizing strategy for fuel cell (FC)-Li-ion battery-supercapacitor (SC) based hybrid power system in the UAV is proposed. The required energy from the FC is optimized based on the optimal efficiency range. In addition, using the frequency-based power split algorithm, a multi-objective function, minimizing the weight and economic cost, for the battery-SC subsystem is constructed. The PSO intelligent optimization algorithm is used to find the optimal sizing for the system. Finally, some simulations have been carried out to verify the correctness of the proposed algorithm.

With a large number of constant power loads (CPLs) connected to the DC microgrid (MG), the injected negative impedance drastically deteriorates the stability of the DCMG. To address this issue, a stabilization strategy for the DCMG with CPLs fed by the four-switch Buck-Boost (FSBB) converters is proposed in this paper. Due to the adopted DC machine technique, a virtual capacitor is paralleled to the output port of the FSBB converter to support the bus voltage in the presences of CPL power changes. The stability of the system is analyzed based on the Middlebrook impedance stability criterion, and the effectiveness of the proposed control strategy is verified by MATLAB/Simulink simulation.

For the fuel cell-ultracapacitor auxiliary power unit (FC-UC APU), the basic energy management objectives could be summarized as achieving dynamic load power sharing (i.e. the FC supplies the average load power while the UC buffers all the fluctuating load power), extending the service life, improving energy efficiency, and ensuring the system stability. Conventionally, the dynamic load power sharing objective is achieved by using centralized energy management system (EMS) which suffers from poor flexibility, scalability and reliability. What's more, the system stability can hardly be guaranteed because serious interactions between source converters and power-electronic interfaced loads, which behave as constant power loads (CPLs), always exist. In this paper, a completely decentralized EMS based on modified droop control and passivity-based control (PBC) is proposed to achieve the required objectives simultaneously in a decentralized way. First, the architecture and model of the FC-UC APU are described in detail. Then, analysis on the operational principle of the proposed EMS is deeply studied. Finally, simulation results verified the correctness of the theoretical analysis and effectiveness of the proposed decentralized EMS.

Droop-controlled inverter (DCI) is known for appealing characteristics in load sharing and stability enhancing. By simulating the external droop feature of synchronous generator, the DCI can also naturally offer necessary frequency/voltage regulation or provide active/passive power support when interfaced to the utility grid. However, with the growing penetration of distributed power generation system (DPGS), the large impedance in weak grid may arouse unexpected oscillation problem. In order to evaluate the stability performance of the DCI when it is interfaced to weak grid, the sequence- impedance model of DCI is established for the first time in this paper. The controller parameters of DCI are designed step by step under the theoretical guide. On this basis, the interactive oscillation risk between DCI and weak grid is analyzed in detail through Nyquist criteria. The results show that DCI exhibits excellent adaptability in weak grid integration. All these theoretical conclusions are tested and verified via simulations and experiments eventually.

In this article, a neural network (NN)-based robust adaptive fault-tolerant control (FTC) algorithm is proposed for a class of multi-input multi-output (MIMO) strict-feedback nonlinear systems with input quantization and actuation faults as well as asymmetric yet time-varying output constraints. By introducing a key nonlinear decomposition for quantized input, the developed control scheme does not require the detailed information of quantization parameters. By imposing a reasonable condition on the gain matrix under actuation faults, together with the inherent approximation capability of NN, the difficulty of FTC design caused by anomaly actuation can be handled gracefully, and the normally used yet rigorous assumption on control gain matrix in most existing results is significantly relaxed. Furthermore, a brand new barrier function is constructed to handle the asymmetric yet time-varying output constraints such that the analysis and design are extremely simplified compared with the traditional barrier Lyapunov function (BLF)-based methods. NNs are used to approximate the unknown nonlinear continuous functions. The stability of the closed-loop system is analyzed by using the Lyapunov method and is verified through a simulation example.

A simple mixed droop-v/f control strategy is proposed for the master inverter in a microgrid to achieve seamless mode transfer between grid-connected and autonomous islanding modes. The proposed control utilizes 1)droop control in grid-connected mode to achieve seamless transfer from grid-connected mode to islanding mode in case of intentional and unintentional islanding events, and 2) v/f control in islanding mode, which is realized by simply reducing the droop coefficient to zero gradually after islanding, to provide voltage and frequency references to the slave inverters. Pre-synchronization on the voltage and frequency between microgrid and utility grid is used for the reconnection of them in a seamless manner. The validity of the proposed method is verified by experiments performed by a 15kV A microgrid.

In this paper, a new hierarchical decentralized optimization architecture is proposed to solve the economic dispatch problem for a large-scale power system. Conventionally, such a problem is solved in a centralized way, which is usually inflexible and costly in computation. In contrast to centralized algorithms, in this paper we decompose the centralized problem into local problems. Each local generator only solves its own problem iteratively, based on its own cost function and generation constraint. An extra coordinator agent is employed to coordinate all the local generator agents. Besides, it also takes responsibility to handle the global demand supply constraint based on a newly proposed concept named virtual agent. In this way, different from existing distributed algorithms, the global demand supply constraint and local generation constraints are handled separately, which would greatly reduce the computational complexity. In addition, as only local individual estimate is exchanged between the local agent and the coordinator agent, the communication burden is reduced and the information privacy is also protected. It is theoretically shown that under proposed hierarchical decentralized optimization architecture, each local generator agent can obtain the optimal solution in a decentralized fashion. Several case studies implemented on the IEEE 30-bus and the IEEE 118-bus are discussed and tested to validate the proposed method.

This paper presents a distributed voltage unbalance compensation method for an islanded microgrid (MG) system. By using hierarchical control concept, we design a distributed compensation scheme in the secondary control layer, while the traditional inverter control methods including voltage and current PR control, droop control, virtual impedance are implemented in the primary layer. Different from most existing centralized compensation methods, no specified individual centralized secondary controller exists. We decompose the traditional centralized controller into several local secondary controllers. A novel distributed PI controller with negative sequence current feedback is designed in each secondary controller. By allowing them to exchange information with their neighboring controllers respectively, the unbalanced voltage in the sensitive load bus (SLB) can be compensated by all the distributed generators (DGs) cooperatively. In addition, the compensation effort of each DG is designed to be proportional to their power ratings respectively. An islanded microgrid system consisting of 4 DGs is built in MATLAB to validate the proposed method.

In this paper, a framework for stability analyses of a typical inverter-based islanded microgrid with a non-linear load is presented, namely ideal constant power load (CPL) which is supplied by tightly regulated power electronics converter. The whole dynamic model of the considered microgrid is firstly developed, based on which the small signal model of system is deduced at stable operating point. Afterwards, eigenvalue-based stability analysis and parametric sensitivity analysis are accomplished based on the obtained small signal model to predict the system's unstable regions and identify the effects of parameters on the stability boundaries. Simulation studies based on Matlab/Simulink are finally done to verify the theoretical analyses.

This paper presents a distributed cooperative control scheme for voltage unbalance compensation (VUC) in an islanded microgrid (MG). By letting each distributed generator (DG) share the compensation effort cooperatively, unbalanced voltage in sensitive load bus (SLB) can be compensated. The concept of contribution level (CL) for compensation is first proposed for each local DG to indicate its compensation ability. A two-layer secondary compensation architecture consisting of a communication layer and a compensation layer is designed for each local DG. A totally distributed strategy involving information sharing and exchange is proposed, which is based on finite-time average consensus and newly developed graph discovery algorithm. This strategy does not require the whole system structure as a prior and can detect the structure automatically. The proposed scheme not only achieves similar VUC performance to the centralized one, but also brings some advantages, such as communication fault tolerance and plug-and-play property. Case studies including communication failure, CL variation, and DG plug-and-play are discussed and tested to validate the proposed method.

Variable-speed fixed-pitch (VSFP) wind turbine is widely used in small-to-medium scale wind turbine markets due to its simple structure, low cost and high reliability features. However, power control, especially maximum power limitation control at high wind velocities, of such turbine concept is hard to realize and it has not been studied much in existing researches so far. To overcome this difficulty, we propose a simple power control strategy for VSFP wind turbines, covering the entire range of wind velocity. The proposed strategy contains a novel maximum power point tracking (MPPT) control method, a constant-speed (CS) soft-stalling control method and a constant-power (CP) soft-stalling control method. These three sub-control methods are appropriately combined in a way to realize seamless transitions among different operational modes (MPPT, CS and CP). The system structure and operational principle of the proposed strategy are thoroughly analyzed. Theoretical analysis is verified by simulation and experimental results performed by a 1.2kW fixed-pitch variable-speed wind turbine prototype.

A novel power control and energy management strategy is proposed to optimally control the power generated by the turbine and effectively manage the energy that supply to different loads for a stand-alone permanent magnet synchronous generator (PMSG)-based variable speed wind energy conversion system (WECS) consisting of a battery bank and a dump load. By simply regulating the dc bus voltage at different levels, the energy that will supply to the user's loads, charge to the battery and dissipate by the dump load can be optimally managed. Moreover, due to the energy management scheme proposed, the power control of the wind turbine can be decoupled, which means the power control system only needs to optimally control the generated power of the turbine follow the ideal power curve and the power will be supplied to different loads from the dc bus optimally. Thus, the control system is much simpler than the existing control strategies that used for the stand-alone WECS. To verify the effectiveness of the proposed power control and energy management strategy, a 1.2kW stand-alone WECS laboratory test-rig is established, the validity of the proposed strategy is verified through experiments.

Static and dynamic simulation methods are two commonly adopted techniques in constructing wind turbine simulator (WTS). To better understand the difference between static simulation method and actual wind turbine, their frequency-domain models which have never been discussed before are elaborated in this paper, and the reason why conventional static simulation method can not reproduce the dynamic behavior of a wind turbine and why dynamic simulation method should be adopted are carefully discussed. Then, the relationship between inertia of wind turbine and time constant of LFP are discussed based on the simplified model of dynamic WTS proposed in the paper, which reveals the ways to simulate different wind turbines with one hardware specification. An experimental platform which can accurately simulate both the static and dynamic characteristic of real wind turbines is implemented. The theoretical analysis and proposed strategy is verified by simulation and experimental results performed by a 7.5KW laboratory prototype.

The input current of the two-stage DC-AC inverter contains low frequency ripple whose frequency is twice that of the inverter's output voltage. The proportion of the low frequency current ripple's amplitude to the average current's can achieve as high as more than 30%. The stability of dc power system and life span of the input voltage source such as battery or fuel cell could be influenced seriously. The reason of low frequency input current ripple production is analyzed, and the new control strategy of the first stage DC/DC converter is proposed on this paper. Compared with the traditional control strategy, the resonant controller is introduced. The proposed new control strategy is effective in reducing the current ripple in the output filter inductor of the DC/DC converter. Moreover, the input current ripple of the two-stage DC-AC inverter can be decreased. The design of the resonant controller and the system transfer function are given in the paper. The design principles of system stability are also discussed. At last, the effectiveness of proposed new control strategy to reduce the input current ripple that exists in the two-stage DC-AC inverter is verified by simulation and experimental results.