International Transactions on Electrical Energy Systems最新文献

In practical three-phase PWM rectifiers, external disturbances, such as changes in load and grid voltage, and internal disturbances, such as DC capacitor uncertainty, have caused major problems, including DC-link voltage fluctuations, harmonic distortion, and power factor degradation. They can also cause problems with power balance, stability, system dynamics, reduced efficiency, and increased stress. This paper proposes an enhanced linear state observer (ELSO) with a PLL-less voltage-oriented control (PLL-less VOC) method for a three-phase PWM rectifier in the synchronous coordinate frame (dq coordinate), aiming to simultaneously improve the DC-link voltage dynamics, disturbance rejection ability, and the grid current control loop’s transient response. The ELSO is suggested instead of the load current sensor not only to estimate the disturbance but also to estimate the overall switching losses of the PWM rectifier, resulting in good DC-link voltage disturbance rejection and transient fluctuation suppression, high robustness against DC capacitance change, low size and cost, and high reliability. Furthermore, contrary to the traditional VOC method, where the PLL and Park transforms are important parts for controlling the rectifier in the dq coordinate, the PLL-less VOC method can control the grid currents directly in the dq coordinate without needing PLL and Park transforms, which can offer a fast transient response and low computational burden. The frequency domain analysis of the ELSO is used to identify its gains and enhance its stability and disturbance rejection capability. Finally, hardware-in-the-loop (HIL) simulation using the OPAL-RT 4510 real-time simulator is also given to confirm the superiority and efficacy of the suggested control method.

This paper explores an integration of virtual synchronous generators into voltage source converter–based VSC-HVDC systems to enhance grid stability and performance. VSGs emulate the inertia and damping characteristics of traditional synchronous generators, thereby providing essential frequency support and improving transient stability during disturbances. The study focuses on design considerations, control strategies, and the impact of VSG incorporation on system dynamics. Through detailed simulations and analyses, it is demonstrated that VSGs can significantly reduce the rate of change of frequency during transient events, thus enhancing the overall robustness and reliability of VSC-HVDC systems. In addition, the practical challenges of implementing VSG-based control are addressed, and solutions are proposed to optimize system integration and performance.

Existing studies on decentralized control of islanded cascaded-type AC microgrids mainly focus on either dispatchable or nondispatchable DGs. However, islanded cascaded-type AC microgrids may contain both types of DGs. To address this issue, a decentralized control scheme is proposed that integrates both dispatchable and nondispatchable DGs for the islanded cascaded-type AC microgrids. By performing this method, frequency synchronization and power factor angle consistency for all DGs are obtained in a communication-free manner. One dispatchable DG is regulated to maintain constant voltage amplitudes at the point of common coupling (PCC). A high-quality power supply for load demands is achieved. Meanwhile, the nondispatchable DG is controlled to output active power based on its available capacity for using renewable energy efficiently. Next, a stability analysis of the proposed control for the cascaded-type AC microgrids is performed. Finally, simulations and experiments are executed to showcase the efficacy of the proposed scheme.

The renewable energy mobile offshore platform, which adopts the combined power supply of renewable energy and energy storage, is an important carrier for the development and utilization of marine resources. The randomness of renewable energy generation has a more prominent effect on the bus voltage stability and transient voltage deviation of the power system with small capacity and low inertia. Considering the operation and maintenance characteristics of the offshore platform, a virtual inertia control method for small- and medium-sized wind turbines is proposed. Firstly, by analyzing the characteristics of the renewable energy microgrid of the unattended offshore platform, considering the operating environment with high average wind speed at sea, the mechanical inertia in the wind turbine is selected as the energy source of virtual inertia. The structure of the wind power generation unit is analyzed, and small signal modeling is carried out. A virtual inertia control method based on power droop is proposed, and the rotational inertia and the damping coefficient are obtained from the characteristics of transient and steady-state analysis of the system. Finally, the DC microgrid experiment platform of the offshore platform is constructed, and it is verified that the proposed method makes full use of the characteristics of the offshore platform to enhance system inertia and improve the operational stability of the offshore platform DC microgrid system.

In order to improve the self-healing control ability of AC-DC hybrid distribution network and ensure the stability of distribution network operation, a coordinated oscillation control strategy for AC-DC hybrid distribution network based on mixed-integer linear programming (MILP) is proposed. By constructing a multiscale reactive power optimization model and considering the comprehensive cost minimization goal including network loss cost, regulation cost, and operation cost, the operation efficiency and reliability of distribution network are improved. In the simulation experiment, the AC-DC hybrid distribution network of a regional power supply company is taken as the experimental object by using MATLAB software, and the effectiveness of the proposed method is verified by setting different operation scenarios. The results show that in Scenario 1, the voltage of the proposed method is between 1.02 and 1.04 U, and the voltage fluctuation range is small. The proposed method can keep the voltage variation within the rated control range, which shows a more stable trend than other methods, whether in the scene where wind farms or photovoltaic fields work alone or in the scene where they work together with energy storage equipment. In addition, when simulating a single-phase fault, compared with the case without this method, this method can locate the fault area more accurately and significantly improve the reliability of fault detection. The significance of this study is to provide an effective mathematical model and control strategy to deal with the complex dynamic behavior of AC-DC hybrid distribution network. This not only enhances the adaptability of the power grid to various uncertain factors but also improves the economy and security of the power grid.

The increase in electric vehicle (EV) penetration has triggered a surge in demand for battery charging and swapping infrastructure. However, in diverse urban regions, an unbalanced allocation of charging stations (CSs) and battery swapping stations (BSSs) can lead to inefficient resource utilization and diminished revenue for these facilities. Therefore, it is urgent to set up a system of operational and economic indexes to evaluate the proportional planning of CSs and BSSs. The traditional weighting method of the index system cannot accurately describe the relationship between the indexes. As a result, an evaluation method of CS and BSS operational economy based on dynamic weighting indexes is proposed. Firstly, from the perspectives of EV owners, CSs, BSSs, power grid, traffic, and environment, a social and economic index system of CSs and BSSs is established, and quantifiable indexes related to the operational economy are extracted. Then, the copula entropy (CE) is used to improve the CRITIC objective weighting method, reflect the correlation between indexes, and align it with the objective law. The selected indexes are weighted by the subjective and objective combination weighting method. Finally, the simulation is conducted based on the practical data of the urban power grid in China. The rationality of the proposed operation economic indexes and the effectiveness of the improved weighting method are verified.

Unbalanced voltage and harmonics are major challenges in a microgrid. Single- and two-phase loads and short circuit can make unbalanced voltage. Besides, nonlinear loads create harmonic components in the grid. Thus, compensating both of the unbalanced voltage and harmonic distortion is necessary. Otherwise, they could cause low power quality, resonance, and stability issues. The present paper suggests a combination of distribution static synchronous compensator (DSTATCOM) and shunt active power filter (SAPF) to address the unbalanced voltage and harmonic pollution in hybrid grid-connected microgrid. A four-wire distributed static synchronous compensator has been adopted to deal with the negative- and zero-sequence components of the microgrid, extracting these components by using second-order generalized integrator (SOGI). To solve the harmonic issue, a SAPF is presented to dynamic compensation of grid current and voltage harmonic components in addition to DSTATCOM. The SAPF relies on estimating the fundamental frequency positive-sequence component (FFPSC) of the load current by applying the second-order sequence filter (SOSF). Since combining the reference signals of voltage balancer and harmonic eliminating at one controller is impossible, two independent devices are presented. Although both of the equipment have a same conception, their control and operation are different. So, simultaneous incorporation of distributed static synchronous compensator and SAPF for mitigating the unbalancing and distortion in the grid-connected microgrid based on hybrid solar and wind power is the work’s novelty. Some merits of this scheme include simple structure, fast real-time controller, low computational burden, and more effective operation. Robustness and effectiveness of such very simple scheme are evaluated by simulation in MATLAB/Simulink environment. Simulation report demonstrates that the total harmonic distortion (THD) of grid voltage and current decrease well under 1.44% and 2.33%, respectively. Meanwhile, the voltage unbalance factor (VUF) of negative and zero sequence of grid voltage reduces to 1.1% and 0%, respectively.

Although the development of electric vehicle (EV) technology offers opportunities for reducing CO₂ emissions through the electrification of transportation, the integration of EVs into distribution systems poses a significant challenge to the reliable operation of existing protection systems. As the penetration level of EVs continues to rise, the fault current characteristic of the distribution system changes, resulting in load de-energization, equipment damage, and reduced reliability. This paper develops a protection scheme for preserving coordination between main and backup overcurrent relays considering various penetration levels and locations of integrated EVs. By modifying the EV charger control system, the proposed scheme limits the fault current contribution of adopted EV charge stations into the fault point. The developed scheme does not alter the structure of the available protection system of the distribution network and is compatible with both old and nonprogrammable relays. Furthermore, it does not require communication links. The effectiveness of the proposed scheme is validated through several case studies on the Isfahan distribution network. The findings indicate that the operating time of the backup relay in the conventional protection system exceeds the thermal limit at 100% penetration level of EVs installed upstream of this relay as it reaches 1810 ms, while by using the proposed strategy, this time reduces to 776 ms, preserving protection coordination between the main and backup relays.

Since renewable energy is rapidly growing in the active distribution networks, the integrated energy system coupled with energy storage is a promising way to address the intermittent issues of renewable sources. This paper proposes an optimal planning model for the hydrogen-based integrated energy system (HIES) considering power to heat and hydrogen (P2HH) and seasonal hydrogen storage (SHS) to take full advantage of multienergy complementarity. To tackle the unstable factors introduced by renewable sources and varying loads, we apply robust optimization and stochastic programming theory to improve the robustness of the planning results. Meanwhile, we also consider the N-1 contingency constraints to make the technology selection, capacity allocation, and economic operation more reliable. The complex constraints resulting from producing two independent binary variables are converted into mixed integer linear constraints, which can be solved effectively using the nested column-and-constraint generation algorithm. Numerical simulation demonstrates the effectiveness of the P2HH and SHS in reducing the total cost of the HIES planning.

In isolated microgrids, the dynamic performance of the inverter output voltage is degraded due to the connection of unbalanced and nonlinear load, load switching, and significant perturbation of filter parameters. A compensation control structure based on the residual generator integrated with an optimization algorithm is proposed to improve the power quality of the inverter output voltage in this paper. First, the structure and mathematical model of the inverter are analyzed. The sensitivity results show that the residual generator–based compensating control structure improves the robustness of the inverter compared to the conventional V/f control. Second, a controller structure with lower order is derived with reduced-order theory for easier engineering implementation. Then, the compensation controller parameters are optimized online using the gradient descent optimization algorithm to improve the load disturbance’s dynamic compensation performance and reduce the power quality degradation resulting from parameter perturbation. Finally, the PEK-530 inverter platform developed by GWinstek is utilized for experimental verification. When the inverter is tested with unbalanced loads and nonlinear loads at load switching and with LC filter parameter perturbation, the three-phase unbalance, total harmonic distortion rate, d-axis voltage fluctuation, and total harmonic distortion rate of the inverter output voltage are 3.2%, 5.4%, ±2 V, and 0.5%, respectively, and are further reduced to 0.3%, 2.8%, ±0.2 V, and 0.22%, after the compensation controller is optimized by the gradient descent method. The experimental results show that the proposed improved control strategy can effectively improve the disturbance’s fast compensation performance, robustness to inverter parameter perturbation, and the power quality.