Flexible power point tracking (FPPT) is a common and significate photovoltaic control problem under partial shading conditions. Based on CPP and MPP distribution analysis, both constant power point ...tracking and maximum power point tracking could be mathematically described by a single objective optimization. As one of the recent meta‐heuristic algorithms, which avoid the limitations of conventional scanning methods, the carnivorous plant algorithm (CPA) solves non‐convex optimization problems with fast speed, high accuracy, and simple calculation. However, it exhibits several drawbacks such as inappropriate parameters, over‐searching, and feasible solution ignored issues. Therefore, a global FPPT strategy based on modified CPA (mCPA) is proposed, which includes a search objective integration, efficient non‐convex search space skipping strategy, and adaptive tuning parameters according to sensitivity analysis. By statistical experiments compared with eight popular meta‐heuristic algorithms, the mCPA can correctly converge to the global MPP under various scenarios with the highest accuracy of 99.0%. Meanwhile, the observations of dynamic experiments demonstrate the effectiveness of the proposed strategy in terms of fast time response (0.5–3 s) for global FPPT under various uniform and non‐uniform insolation conditions.
Based on statistical results (more than 2430 MATLAB simulations and hard‐in‐loop experiments) with a comprehensive comparison index, the CPA is selected and modified to serve as the core optimizer for the GFPPT optimization problem. Based on the features of MH and GFPP, a novel modification strategy is proposed to reduce searching time, including an integrated search objective, non‐convex region skipping strategy based on equivalent or‐der‐preserving mapping, and adaptive tuning parameters. A novel and efficient mCPA‐based GFPPT control algorithm is designed to automatically track the GMPP or CPP of a photovoltaic array under any PSC, which is also applicable to other MH‐based methods.
The negative-sequence reactive power-conductance (Q-G) droop control strategy is a conventional method to compensate for the output unbalanced voltages in an islanded MG. Nevertheless, the conflict ...between unbalanced voltage compen-sation and negative-sequence reactive power sharing, caused by the impedance mismatching of distribution lines and distributed generators, has not been solved only by Q-G droop control. In this paper, a distributed cooperative secondary unbalanced voltage control strategy is proposed to decrease the output voltage unbalance factor (VUF) of each droop-controlled DG, as well as to further enhance the negative-sequence reactive power sharing effectiveness among DGs by properly shifting up and down the Q-G droop characteristics of each DG. An algorithm for adaptive VUF weight coefficient is proposed to better suppress VUF under severe imbalance conditions. Furthermore, a negative-sequence small-signal model of an MG under an unbalanced condition, considering the communication delay time of the proposed SUVC, is established to analyze the system's stability and transient performance under the influence of some critical parameters. Finally, the effectiveness of the proposed strategy is validated by the simulation results from a real-time emulator of StarSim HIL.
In this paper, an optimal secondary voltage control (SVC) strategy has been proposed for an islanded multibus microgrid (MG). The proposed strategy consists of a multiobjective function, power flow ...equality constraints, and four sets of inequality constraints. The multiobjective function aims to achieve a compromise between multibus voltage regulation and reactive power distribution among voltage modulation units by taking full advantage of both droop-controlled and PQ-controlled distributed energy resource (DER) units in the MG. Power flow equations for an islanded MG with or without secondary frequency control were considered as equality constraints. The limits of DER capacities, bus voltages, power flows on distribution lines, and system frequency must be accommodated during the process of solving the proposed SVC control strategy iteratively using the primal-dual interior-point method. Finally, simulation results have been presented to validate the feasibility of the proposed optimal SVC strategy.
•A rate of the change of DC voltage and AC frequency (RoCoX) controller for an interlinking converter (ILC) is proposed to equalize the dynamic deviation of an HMG and reduce the power ...oscillation.•The relationship between the RoCoX droop coefficient and the dynamic performance of the HMG is revealed by the eigenvalue analysis of a small-signal model of an HMG.•Considering both the dynamics and equalization of RoCoX, a guideline for adaptively adjusting the droop coefficient is presented.
With the increasing application of virtual inertia in microgrid design, AC and DC subgrids in hybrid microgrids (HMG) present diverse inertia characteristics. However, an HMG cannot achieve an optimal dynamic response by interconnecting multiple AC and DC virtual inertia scenarios. Therefore, a droop controller with a normalized rate of the change of DC voltage and AC frequency (RoCoX) is proposed to minimize the HMG's steady and dynamic deviations and reduce the power oscillation of the interlinking converter (ILC). The effect of the proposed controller on dynamic performance and equalization of RoCoX is analyzed using a small-signal model of an HMG with various inertias applied at both subgrids. The coefficient of the RoCoX controller is adaptively adjusted with the constraint of equalization of RoCoX and dynamic performance. The proposed method is verified using a Starsim hardware-in-the-loop experimental platform under various working conditions.
Most current proton exchange membrane fuel cell (PEMFC) fault diagnosis models are not updated after offline training is completed, so they cannot learn new data features. For this problem, a PEMFC ...fault diagnosis model based on an equivalent circuit and an online sequential extreme learning machine (OS-ELM) framework is proposed. Under different operating conditions, the range of equivalent circuit element values is different, which is conducive to fault diagnosis. Element values constitute a dataset, from which OS-ELM is used to learn data features, and PEMFC fault diagnosis and isolation can be accomplished. OS-ELM has online learning capability. During the operation of PEMFC, equivalent circuit fitting is performed on multiple sets of sample data obtained using electrical impedance spectroscopy (EIS). Optimized fuzzy C-means (FCM) is used to cluster and partition sample data for online updating of OS-ELM models. The experimental results show that the proposed PEMFC fault diagnosis model not only has the speed and accuracy of fault diagnosis but also has the ability to learn fault characteristics online.
The growing demand for high-efficiency power con-verters with silicon carbide (SiC) devices at high switching frequencies has fueled an interest in non-auxiliary circuit-based soft switching ...techniques, particularly the variable switching frequency control for zero-voltage switching (VF-ZVS). Besides increasing the Total Harmonic Distortion (THD), applying the full ZVS range to three-phase SiC/Si Hybrid Active Neutral Point Clamped (HANPC) converters leads to a rise in conduction loss. This issue is exacerbated by the use of Si IGBT, as it generates significantly more conduction loss than SiC MOSFET in medium and small power HANPC. Consequently, a power loss optimized VF-ZVS (OVF-ZVS), based on a precise loss model, is proposed to enhance efficiency by considering the effect of the current ripple on device loss. This OVF-ZVS optimizes the switching frequency to minimize the total loss of three-phase switching devices across all load ranges. Meanwhile, the variable switching frequency range and LCL parameters are optimized to strike a balance between superior output current quality, power density, etc. Simulation and exper-imentation using a 10kW HANPC converter prototype illustrate the effectiveness of the proposed comprehensive optimization, further highlighting the reduction in THD and device loss.
All-switches zero voltage switching (ZVS) and lower current stress are the most important challenges for improving the efficiency of dual-active-bridge (DAB) converters operating in the bidirectional ...full-load range and wide voltage range applications. Triple-phase-shift (TPS) has three control degrees of freedom and it is a more flexible way to extend the ZVS range in all kinds of operating conditions. In order to improve the efficiency of DAB converters, a new optimal current stress in TPS (OCS-TPS) control is proposed to minimize the current stress as well as achieve all-switches ZVS within the bidirectional full-load range and wide voltage range applications. Firstly, seven specific switching modes satisfying the ZVS conditions under step-up and step-down are selected from 12 TPS switching modes. Secondly, the issue of current stress minimization is turned into an extreme optimization problem with equality constraints of desired transmission power and inequality constraints of ZVS conditions in selected switching modes. The Karush-Kuhn-Tucker (KKT) algorithm is employed to solve this optimization issue. Then two switching modes are chosen to form the high-load range ZVS and low-load range partial-switches ZCS/ZVS in the case of step-down and step-up operations respectively. Therefore, the phase-shift ratios of selected switching modes in the low-load range are further optimized by adding an offset value to achieve all-switches ZVS. Finally, the effectiveness of the proposed strategy is verified by the experimental results.
•A coordinated control strategy of the converter based on the constant cut-off area;•Commutation area theory and the coupling mechanism of the system;•Improvement of transient characteristics of ...UHVDC system.
The Ultra High Voltage Direct Current (UHVDC) systems under hierarchical connection, while improving the voltage support and current assimilative capacity of the receiving-end system, also bring new challenges to the security and stability of the power grid. An AC short-circuit fault at the receiving-end system can easily induce commutation failures (CFs) of the high-end and low-end converter at the same time, which seriously affects the stability of the system. Therefore, it is urgent to study the coordinated control strategy for the commutation failures of UHVDC systems under hierarchical connection. This paper analyzes the coupling mechanism and CFs characteristics of the high-end and low-end converters of a UHVDC system under hierarchical connection. Based on the commutation area theory of the three-phase full-wave bridge circuit, a coordinated control strategy is proposed to suppress CFs of the non-faulty layer converter of the UHVDC system under hierarchical connection. It can reserve enough extinction area for the non-faulty layer converter after an AC system failure occurs, thus preventing the occurrence of commutation failure. Finally, based on PSCAD/EMTDC, Zhundong-Wannan ±1100 kV UHVDC was built and the effectiveness of the proposed control strategy was simulated and verified. The simulation results show that the proposed control strategy can effectively suppress the simultaneous commutation failure of inverter station converters when symmetrical faults and asymmetrical faults of different levels occur in the receiving-end grid. This has important engineering significance for improving the stability of the DC transmission system.
The zero-voltage-switching variable frequency (ZVS-VF) control can achieve ZVS without auxiliary circuit. Nevertheless, it will increase switching losses under light load and conduction losses under ...heavy load, thus affecting efficiency. This is particularly evident in scenarios with varying dc side voltage and load power (e.g., photovoltaic systems). To address this issue, our proposed continuous space vector modulation (CSVM)-improved discontinuous space vector modulation (IDSVM) hybrid strategy can improve the efficiency at both light and heavy loads under various dc side voltages when employing ZVS-VF to a 3L inverter. This is achieved by leveraging the losses performance of diverse modulations, and CSVM is chosen to reduce switching losses under light load, while IDSVM is selected to limit conduction losses under heavy load. Additionally, with the proposed hybrid modulation, the full ZVS range can be realized in the 3L inverter. The modulation transition point is determined by the power losses analysis, ZVS range analysis, and smooth transition constraint. It can be adaptively adjusted to achieve smooth transition of different modulation methods under varying operating conditions. The experimental results from a 10-kW 3L active neutral-point-clamped (ANPC) inverter demonstrate that the proposed strategy improves efficiency by 3.7% for light load and 1.94% for full load compared to the conventional constant frequency control.
•A novel modeling method for microgrid system based on measurement data is proposed.•The proposed model is applicable to adjust the PI parameters of the secondary voltage and frequency ...controller.•The performance of the proposed method is examined by two experiments.
A novel modelling method is proposed in this paper for modelling the point of common coupling (PCC) voltage and frequency response (VFR) of the microgrid (MG) via system identification techniques. The proposed modelling method uses the voltage and frequency data sampled at PCC to identify the VFR model without knowing the features of MG, such as the MG’s topology, the models of the distributed generators (DGs) and loads connected to it. Compared with the commonly used mechanism modelling method, the proposed method greatly simplifies the process of modelling MG. The obtained identified VFR model can mimic the characteristics of the voltage and frequency response at the PCC with simple structure and low order, which can significantly reduce the computational cost. Based on the simplified model, a new guide for parameters tuning of secondary voltage and frequency controllers in the MG system is presented. The proposed modelling method has been validated on a MG experimental platform, and the obtained VFR model has been applied to the design of the secondary voltage and frequency controllers. The designed results also have been validated on the experimental platform, and the experimental results show the optimized secondary controllers have favorable stability and dynamic performance.
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