• Energy Research

Energy management is an enabling technique to guarantee the reliability and economy of hybrid electric systems. This paper proposes a novel machine learning-based energy management strategy for a hybrid electric bus (HEB), with an emphasized consciousness of both thermal safety and degradation of the onboard lithium-ion battery (LIB) system. Firstly, the deep deterministic policy gradient (DDPG) algorithm is combined with an expert-assistance system, for the first time, to enhance the “cold start” performance and optimize the power allocation of HEB. Secondly, in the framework of the proposed algorithm, the penalties to over-temperature and LIB degradation are embedded to improve the management quality in terms of the thermal safety enforcement and overall driving cost reduction. The proposed strategy is tested under different road missions to validate its superiority over state-of-the-art techniques in terms of training efficiency and optimization performance.

A novel unified control of the dc–ac interlinking converters (ICs) for autonomous operation of hybrid ac/dc microgrids (MGs) has been proposed in this paper. When the slack terminals in the ac and dc MGs are available, the ICs will operate in autonomous control of interlinking power between the ac and dc subgrids, with the total load demand proportionally shared among the existing ac and dc slack terminals. With a flexible control variable added in power control loop, design of the interlinking power control, and droop features of ac and dc MGs can be decoupled. Moreover, this control variable can be tuned flexibly according to different power control objectives, such as proportional power sharing in terms of capacity (which is considered in this paper), interlinking power dispatch, and other optimal power dispatch algorithms, ensuring a well-designed flexibility and compatibility. Furthermore, if the dc MG or the ac MG loses dc voltage control or ac voltage and frequency control capability due to failures of operation of its slack terminals, the ICs can automatically and seamlessly transfer to dc MG support or ac MG support control modes without operation mode detection, communication, control scheme switching, and control saturation. In order to enhance the stability of the proposed unified control in different modes with different control plants, a phase compensation transfer function has been added in the power control loop. After thorough theoretical analysis and discussions, detailed simulation verifications based on PSCAD/EMTDC and experimental results based on a hardware experimental MG platform have been presented.

Direct power generation near gas fields offers numerous benefits, including optimized economic efficiency and reduced environmental impact. Moreover, building on-site greenhouses emerges as a promising approach to further minimize carbon emissions and residual heat, greatly promoting resource utilization. However, such power plants generally have access to a weak grid due to their remote locations, and they also contain nonlinear local loads, such as the grow lights in the greenhouses. Consequently, the generation system is susceptible to power quality issues, manifested in overvoltage and harmonics. To address these issues, a smart back-to-back converter is employed to interconnect the gas turbine generator and the utility grid. This smart converter not only enhances power quality but also offers potential ancillary services that contribute to the dynamics of the gas generation system, such as damping low-frequency oscillation among parallel-connected generators. In this paper, three control configurations for the back-to-back converter are developed, enabling the coordinated regulation of exported active power, AC voltage, and DC-link voltage in either a grid-following or grid-forming manner. Furthermore, comparative studies are conducted to provide guidelines for selecting an appropriate control strategy that ensures stable operation under various short circuit ratios. A practical gas cogeneration system is chosen to evaluate the performance of the back-to-back converter, and real-time simulations based on RT-LAB are carried out to validate the effectiveness of the methodology.

Today, more sustainable and renewable energy sources are being integrated into power systems to address the increasing concerns of energy costs, energy security, and greenhouse gas emissions. For these sustainable and renewable energy-based power generation systems, the grid-interfacing power electronics are critical links that essentially integrate the energy source, energy storage, or even small networks (e.g., microgrids) to the main grid. As this sustainable energy integration (especially for dispersed and small capacity sources) is mostly at the distribution level, the wide adoption of grid-interfacing converters also brings unprecedented opportunities for more active and flexible distribution systems in the future with both ac and dc networks. To operate such a system with high penetration of sustainable and renewable energy sources, a number of power electronics technologies are essential: the topology and pulsewidth modulation (PWM) of grid-interfacing power electronics, voltage/current control strategies, power flow regulation, energy management, power quality control, and so on. If designed and controlled properly, the sustainable energy systems can also help to provide ancillary functions to the power systems. The main objective of this Special Edition is to collect the latest developments in sustainable energy integration technologies.

In this paper, a coordinated control method is proposed for three-phase ac/dc hybrid microgrid power electronic interlinking unit with dual converters sharing both dc and ac links. Unlike the conventional dual converters-based system where the same control strategy is applied to both converters, converter1 in this proposed system is controlled by a closed-loop current regulation algorithm while converter2 employs voltage controller with coupled adaptive virtual impedance. With the flexible control of the coupled virtual impedance, the system performance is enhanced in the following three aspects. First, in grid-tied operation mode, the output power difference between dual converters is used as an input to tune the coupled virtual impedance of converter2 for a rapid dynamic power response. Second, the proposed coordinated control with inherent voltage support is able to achieve a seamless transfer without additional efforts. Third, during system islanded operation, a generalized power sharing and an active PCC voltage harmonic and unbalanced components mitigation are achieved via the adjustment of the coupled virtual impedance. Finally, the scalability of the proposed method in a system with more converters is briefly discussed and verified.

Microgrids are being developed as a building block for future smart grid system. Key issues for the control and operation of microgrid include integration technologies and energy management schemes. This paper presents an overview of grid integration and energy management strategies of microgrids. It covers a review of power electronics interface topologies for different types of distributed generation (DG) units in a microgrid, a discussion of energy management strategies, as well as the DG interfacing converter control schemes. Considering the intermittent nature of many renewable energy based DG units, the ancillary services of DGs using the available interfacing converter rating are also discussed in the paper.

AbstractThe cost of energy should be minimised to increase market penetration of wind energy conversion systems. The existing wind farm control strategies are not helpful since they speed up the damage in upstream wind turbines. Therefore, a control strategy is proposed to maximise the energy yield while equalising lifetime of one of the most fragile sub‐system of the wind turbines. The DC‐link capacitor bank of the back‐to‐back converters is considered to this end. The proposed control strategy can be directly applied in systems based on type 4 wind turbines with full power converters. This exposition reveals the formulation of the control strategy by analysing the strengths and weaknesses of the existing strategies based on the maximum power output. The lifetime‐based control strategy is extended to formulate a multi‐stage reactive power dispatch method with the help of a total energy‐based strategy. The effectiveness of all control strategies is validated using the model‐based simulations.