Search

The rapid development of new energy power generation technology and the transformation of power electronics in the core equipment of source-grid-load drives the power system towards the "double-high" development pattern of "high proportion of renewable energy" and "high proportion of power electronic equipment". To enhance the transient performance of AC/DC hybrid microgrid (HMG) in the context of "double-high," a π type virtual synchronous generator ( π -VSG) control strategy is applied to bidirectional interface converter (BIC) to address the issues of lacking inertia and poor disturbance immunity caused by the high penetration rate of power electronic equipment and new energy. Firstly, the virtual synchronous generator mechanical motion equations and virtual capacitance equations are used to introduce the virtual inertia control equations that consider the transient performance of HMG; based on the equations, the π -type equivalent control model of the BIC is established. Next, the inertia power is actively transferred through the BIC according to the load fluctuation to compensate for the system's inertia deficit. Secondly, the π -VSG control utilizes small-signal analysis to investigate how the fundamental parameters affect the overall stability of the HMG and incorporates power step response curves to reveal the relationship between the control's virtual parameters and transient performance. Finally, the PSCAD/EMTDC simulation results show that the π -VSG control effectively improves the immunity of AC frequency and DC voltage in the HMG system under the load fluctuation condition, increases the stability of the HMG system and satisfies the power-sharing control objective between the AC and DC subgrids. [ABSTRACT FROM AUTHOR]

This paper presents the design and validation of a novel adaptive islanding detection method (AIDM) for a hybrid AC/DC microgrid network using a combination of Artificial Intelligence (AI) and Signal Processing (SP) approaches. The proposed AIDM is aimed to detect and discriminate between the different fault/disturbance conditions that result in islanding and/or non-islanding conditions in a hybrid microgrid. For the islanding and non-islanding conditions detection by the AIDM, firstly, fault/disturbance signals are obtained from a test microgrid. Secondly, these signals are decomposed using Dual-Tree Complex Wavelet Transform. Thirdly, a Synthetic Minority Oversampling Technique (SMOTE) is applied for data preprocessing to increase the accuracy of the classifier. Finally, an artificial neural network (ANN) is used as the classifier for training and testing the proposed AIDM for different microgrid configurations and event scenarios. The proposed method is tested with different data categories from three different microgrid test systems with different scenarios. All modeling and simulations are executed in MATLAB Simulink Version 2023a. Results indicate that the proposed scheme could detect and discriminate between islanding and non-islanding conditions accurately in terms of dependability, precision, and accuracy. An average accuracy of 99–100% could be achieved when tested and validated with microgrid networks adapted from IEEE 13-bus systems. [ABSTRACT FROM AUTHOR]

In response to the complexity of the Jacobian matrix inversion process in the power flow algorithm for AC/DC microgrids, leading to large memory requirements and susceptibility to convergence issues, a novel power flow algorithm based on an improved unified iteration method for AC/DC microgrids is proposed. Firstly, the fundamental equations of the unified iteration method and the characteristics of DC systems are analyzed. The reactive power correction terms and voltage phase correction differences are removed from the modified equations of the unified iteration method, and result in a reduction in the order of the Jacobian matrix in the power flow algorithm. Subsequently, the improved IEEE 11-node system is subjected to simulation verification to attain precise power flow solutions for hybrid AC/DC microgrids. The theoretical analysis identifies the main influencing parameters of active and reactive power errors and assesses their impact factors. Finally, experimental validation of the improved power flow algorithm is carried out on a physical platform, clarifying the applicability range of the proposed method. The research results indicate that within allowable error margins, the proposed approach reduces the difficulty of Jacobian matrix inversion, resulting in an 80% increase in computational speed compared to the unified iteration method. It is suitable for microgrid systems with short electrical distances and small magnitudes of node voltage amplitudes and phase differences. [ABSTRACT FROM AUTHOR]

Islanded hybrid ac/dc microgrids (IHMGs), in which ac and dc subgrids are interconnected via interlinking converters (ICs), have emerged as a promising solution for efficient and zero-carbon power grids. In an IHMG, primary objectives of the secondary control for distributed generators (DGs) are to 1) restore the frequency of the ac subgrid ( $f$ ) and common dc bus voltage of the dc subgrid ( $V^{dc}$ ) to their nominal values; and 2) achieve global power sharing among all DGs throughout the entire IHMG. For conventional secondary control methods of DGs, communication lines of DGs and inter-domain communication lines between DGs and ICs are utilized to achieve the objectives, raising reliability and cost issues. In this study, a novel decentralized secondary control method for DGs in an IHMG is proposed, eliminating the need for communications of DGs and inter-domain communications. The effectiveness of the proposed method was validated through dynamic simulations using Simulink/MATLAB. The obtained results were compared with those of a conventional method.

Bidirectional interlinking converter (BIC) is the core equipment in a hybrid AC/DC microgrid connected between AC and DC sub-grids. However, the variety of control modes and flexible bidirectional power flow complicate the influence of AC faults on BIC itself and on DC sub-grid, which potentially threaten both converter safety and system reliability. This study first investigates AC fault influence on the BIC and DC bus voltage under different BIC control modes and different pre-fault operation states, by developing a mathematical model and equivalent sequence network. Second, based on the analysis results, a general accommodative current limiting strategy is proposed for BIC without limitations to specific mode or operation condition. Current amplitude is predicted and constrained according to the critical requirements to protect the BIC and relieving the AC fault influence on the DC bus voltage. Compared with conventional methods, potential current limit failure and distortions under asymmetric faults can also be avoided. Finally, experiments verify feasibility of the proposed method.

Growing concerns about the energy and environmental crisis are accelerating the transition to a sustainable energy generation landscape through the integration of distributed generators (DGs) into the electric power systems. Microgrids (MGs) have developed as autonomous, localized energy solutions for integrating DGs, improving grid functionality and reliability by supporting both grid-connected and islanded operational modes. Furthermore, the dc nature of numerous DGs and domestic appliances, along with advances in power electronics, have driven research away from traditional ac MGs towards dc and even hybrid ac/dc MGs. This transition introduces greater complexity and a wider range of control scenarios. To address the challenges of these increasingly complex systems, this paper reviews the application of artificial intelligence (AI) in the hierarchical control structures of ac, dc, and hybrid ac/dc MGs. AI techniques show significant promise in enhancing control and operation due to their ability to learn and adapt from the system operational data. However, integrating these AI algorithms into real-world applications remains challenging, with substantial progress still required to make these methods viable in practical scenarios. This review highlights the latest research efforts, identifies the key contributions of AI to MG control, and outlines the limitations and future research opportunities in the field.

In response to the complexity of the Jacobian matrix inversion process in the power flow algorithm for AC/DC microgrids, leading to large memory requirements and susceptibility to convergence issues, a novel power flow algorithm based on an improved unified iteration method for AC/DC microgrids is proposed. Firstly, the fundamental equations of the unified iteration method and the characteristics of DC systems are analyzed. The reactive power correction terms and voltage phase correction differences are removed from the modified equations of the unified iteration method, and result in a reduction in the order of the Jacobian matrix in the power flow algorithm. Subsequently, the improved IEEE 11-node system is subjected to simulation verification to attain precise power flow solutions for hybrid AC/DC microgrids. The theoretical analysis identifies the main influencing parameters of active and reactive power errors and assesses their impact factors. Finally, experimental validation of the improved power flow algorithm is carried out on a physical platform, clarifying the applicability range of the proposed method. The research results indicate that within allowable error margins, the proposed approach reduces the difficulty of Jacobian matrix inversion, resulting in an 80% increase in computational speed compared to the unified iteration method. It is suitable for microgrid systems with short electrical distances and small magnitudes of node voltage amplitudes and phase differences.

The hybrid AC/DC microgrid is an independent and controllable energy system that connects various types of distributed power sources, energy storage, and loads. It offers advantages such as a high power quality, flexibility, and cost effectiveness. The operation states of the microgrid primarily include grid-connected and islanded modes. The smooth switching between these two states is a key technology for ensuring the flexible and efficient operation of the microgrid. In this paper, the typical structure of an AC–DC hybrid microgrid and its coordination control strategy are introduced, and an improved microgrid model is proposed. Secondly, an adaptive current–voltage–frequency integrated control method based on signal compensation is proposed to solve the impulse current and voltage generated during the switching between a grid-connected state and an off-grid state. Finally, in response to unplanned grid-connected scenarios, an improved pre-synchronization control strategy based on BP neural networks is introduced to rapidly restore stable operation. The proposed control strategies enhanced the steady-state and transient stability of the hybrid wind–solar–energy storage AC/DC microgrid, achieving seamless grid-connected and islanded transitions without disturbances. The simulation and experimental results validated the correctness and effectiveness of the proposed theories. [ABSTRACT FROM AUTHOR]

Large-scale electric vehicle integration into the grid is expected in the imminent future. Congestion caused by these integrations can be addressed through a hybrid AC/DC microgrid. The DC sub-grid is exclusively dedicated to incorporating electric vehicles. Solar-powered charging station supported with a supercapacitor energy storage system in the DC sub-grid reduces the stress on the AC sub-grid. This paper presents a hybrid AC/DC microgrid voltage/frequency stability improvement strategy based on adaptive DC-link voltage regulation assisted by energy storage. The proposed volatility-based approach is an efficient strategy to enhance the microgrid stability during generation intermittency and load fluctuations. The combined control scheme of the interlinking converter and supercapacitor energy storage in the independent and hybrid operation modes achieve energy balance with the variations in solar irradiation and the addition/disconnection of electric vehicles. The proposed energy management strategy is tested in real-time under various volatile conditions to validate the performance. The hybrid microgrid with proposed energy management strategy provided a frequency improvement of 0.55% and voltage improvement of 5.4%. [ABSTRACT FROM AUTHOR]

In hybrid AC/DC microgrid, interlinking converter (ILC) can balance power flow between AC and DC microgrid, thus stabilizing AC frequency and DC voltage in islanding mode. As a topology of ILC, modular multilevel interlinking converter (MMILC) has advantages of good harmonic characteristics, high quality waveforms and low switching loss. A two-stage droop control strategy for MMILC is proposed. Firstly, based on the energy fluctuation characteristics of the sub-module capacitance, a new unified droop characteristic of AC and DC side for MMILC is established. Then, a mathematical relation between DC voltage and equivalent sub-module capacitor voltage is given to adjust power reference according to deviation of dc voltage. Also the dead zone and an allowable minimum operation range of the droop characteristic are set. Finally, the control performance of MMILC in different load profiles and overload conditions are simulated. The simulation results show that the proposed two-stage droop control strategy of MMILC can automatically compensate the power to both AC/DC sides in different load profiles. Under overload conditions, MMILC can instantaneous block the active power transmission, which validates the correctness and availability of the control strategy.

The idea of a microgrid is created by utilizing more diverse ac or dc distributed generation (DG) sources along with an energy storage system (ESS) and loads. The most efficient and reliable selection of ac and dc microgrids is a hybrid ac/dc microgrid. The hybrid microgrid largely overcomes the shortcomings of standalone ac or dc microgrids. A bidirectional interlinking converter (BIC) is utilized in the interface for controlling power flow between subgrids. In order to improve voltage and frequency regulation with effective power sharing, the BIC based on the proposed control scheme is implemented for power flow between ac and dc sub-grid in Islanding mode. The control scheme is modified based on conventional droop control with voltage and frequency variation in order to improve bus voltage and frequency regulation with effective power sharing for intermittent sources. The operation of the islanded hybrid ac/dc microgrid is performed with solar, wind, and energy storage system under variable generation and load conditions. In order to make robustness of the system, there are considered different cases for generation and load scenarios. In the transient state, the overshoot and settling time of frequency and voltage are improved, as well as the frequency and voltage regulations are found within the permissible limit in the steady state. Furthermore, the corresponding variations are shown in tabular form in the simulation result. The actual data of solar irradiance and wind speed have been taken from the National Renewable Energy Laboratory. The performance of the system is verified in MATLAB/Simulink environment.

Bidirectional Interface Converter (BIC) connecting the AC and DC buses is the hub of the hybrid microgrid in the energy interaction between the AC and DC subnets. Therefore, the research on the control strategy of BIC is of great significance for the stable operation of the microgrid. Rural distributed power supply and power load are complex and decentralized. AC/DC hybrid microgrid is very suitable for the rural distributed energy consumption and power supply. However, the access of a high proportion of new energy and power electronic equipment has made the power supply and load of rural AC/DC hybrid microgrid appear the multi-power electronics, leading to the low inertia and damping problems of BIC. In this study, Generalized Direct Current Electromotive Force (G-DCEMF) control strategy was proposed to effectively solve the problem of low inertia and damping in hybrid microgrids. Power sharing was also realized in the AC-DC side of the hybrid microgrid for better inertia and dynamic performance. Firstly, a connection was established between the AC bus frequency and the DC motor angular velocity, where the terminal voltage was equivalent to the DC bus voltage, while the BIC transmission power was expressed by the DC side voltage and current. Then, the electromotive force balance equation of the DC motor was incorporated into the BIC control to derive the G-DCEMF control equation. Secondly, the relationship was obtained between the virtual resistance value and the rated capacity of BIC in the generalized DC electromotive force control strategy. The virtual resistance was used to determine the active power deficit or surplus state between DC and AC subnets. The judgment results show that the BIC was used to control the power distribution, in order to verify the autonomous bidirectional power regulation of generalized DC electromotive force control. The power distribution was also achieved in the multi-BIC parallel operation. Thirdly, the stability of the system was evaluated using a small signal model. The improved control strategy was simple in structure and easy to model. Only the external control loop of BIC was modified without switching between control modes, thereby reducing power loss and instability caused by mode switching. Then, the dynamic response of BIC was analyzed under the control of generalized DC electromotive force. The value of the virtual inductance was obtained during this time. The virtual inductance was adjusted to improve the inertia of the system. The power distribution and dynamic response between subnets were adjusted to reduce the influence of the frequency of the AC bus or the voltage fluctuation of the AC/DC subnets. The anti-interference characteristics and dynamic performance of the AC/DC hybrid microgrid were improved effectively for better stability of the hybrid microgrid operation. Finally, compared with the traditional bidirectional droop control, the generalized DC electromotive force control can be expected to better adjust the power transmission, according to the load fluctuation of the AC/DC subnet. The finding can provide the inertia for the entire microgrid system, and then weaken the impact of unilateral microgrid power disturbance on other microgrids in the system. The AC/DC hybrid microgrid was established in the PSCAD/EMTDC simulation software. The effectiveness of the G-DCEMF control strategy was also verified. The G-DCEMF control can be expected to realize the bidirectional power flow of BIC for better system inertia, dynamic response and anti-disturbance performance in the stability of the microgrid. [ABSTRACT FROM AUTHOR]

Recently, the establishment of technical standards for grid connection has gained interest in academia and industry. These standards have focused on the reactive power control function of the grid-connected inverter and maintenance of grid operation, and include detailed information about the grid support function. However, remote control communication and control devices for grid support functions, and other distributed sources, such as wind power and energy storage systems, other than inverters have not been addressed. In this paper, the control of the interlinking converter (ILC) in a hybrid microgrid considering low voltage ride-through (LVRT) among grid support functions is investigated. The proposed method consists of an energy management system considering LVRT and a cooperative control scheme. In the energy management system, an algorithm capable of mode selection was constructed by applying the LVRT curve. Then, considering the LVRT situation, the allowable reactive power range of the ILC was mathematically analyzed through the cooperative control of the energy storage device and the ILC. The proposed method enables us to perform active and reactive power control of the ILC in a hybrid distribution network, considering the power factor under various conditions. This functionality, such as supplying reactive power, significantly contributes to the enhanced grid resilience with distributed power sources, including renewable energy. The proposed strategies were verified through experiments after configuring an experimental set of distributed power sources. [ABSTRACT FROM AUTHOR]

In grid-connected operations, a microgrid can solve the problem of surplus power through regeneration; however, in the case of standalone operations, the only method to solve the surplus power problem is charging the energy storage system (ESS). However, because there is a limit to the capacity that can be charged in an ESS, a separate energy management strategy (EMS) is required for stable microgrid operation. This paper proposes an EMS for a hybrid AC/DC microgrid based on an artificial neural network (ANN). The ANN is composed of a two-step process that operates the microgrid by outputting the operation mode and charging and discharging the ESS. The microgrid consists of an interlinking converter to link with the AC distributed system, a photovoltaic converter, a wind turbine converter, and an ESS. The control method of each converter was determined according to the mode selection of the ANN. The proposed ANN-based EMS was verified using a laboratory-scale hybrid AC/DC microgrid. The experimental results reveal that the microgrid operation performed stably through control of individual converters via mode selection and reference to ESS power, which is the result of ANN integration. [ABSTRACT FROM AUTHOR]

Bipolar hybrid ac/dc microgrid (MG) controls a DC and AC bus at the same time, supplies local loads with local resources, and offers multiple levels of dc voltages to resources and services. The applications are getting more popular in power systems. Due to its vital tasks in numerous applications, the interlinking converter (ILC) is an utmost crucial portion of the MG system. From this study, a 10‐switch converter is provided as an ILC for a hybrid MG, which gains from both two‐level (2L) and three‐level (3L) converters at the same time, and it can be used to a wider range of power levels. This work provides a space vector modulation (SVM) for a 10‐switch converter to maximize dc‐link voltage consumption and minimize total harmonic distortion (THD) compared with other modulation techniques. The behaviour of the suggested SVM is compared with that of sinusoidal PWM. The suggested SVM was evaluated using OPAL‐RT to show that it used DC bus voltage more reliably and produced significantly less THD than existing techniques. Furthermore, grid‐tied ILC's architecture is improved to reduce low‐order harmonics and used for dc‐link voltage balancing. A real‐time (RT) digital simulator (OP‐5700) was used to test the inverter control technique. In MG, this hardware‐in‐the‐loop simulation provides an excellent environment for designing and verifying system‐level control algorithms. Simulations were run in a MATLAB/Simulink environment and confirmed using an RT simulator to validate the feasibility of the suggested topology. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Engineering Technology is the property of Editorial Department of Electric Power Engineering Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper presents an adaptive power management strategy (PMS) that enhances the performance of a hybrid AC/DC microgrid (HMG) with an interlinking converter (IC) integrated with a hybrid energy storage system (HESS). The HESS is made up of a supercapacitor (SC), a battery, and a fuel cell (FC) with complementary characteristics. The modeling of the test system, which consists of an IC control structure with virtual synchronous machine-transient (VSM-T) droop and HESS control, is outlined in this article. The modified IC topology improves the power-sharing between sub-grids and enhances the steady-state and transient responses. The novelty of the proposed strategy is that the power ratio index-based power management approach is executed through a single controller using local measurements. Furthermore, the adaptive nature of the reference generation improves the active and reactive power-sharing of the IC and the simultaneous voltage regulation at AC and DC sub-grids. The performance of a HMG under varying loading and irradiation, distributed generation (DG) outage, and electric vehicle charging station (EVCS) charging are analyzed on a modified IEEE 13-bus HMG test system and validated using OPAL-RT real-time hardware-in-the-loop (HIL) experiments. Furthermore, the adaptive nature of reference generation in the proposed approach improves power-sharing during both volatile and normal loading. The proposed PMS provides a voltage improvement of 7.92% at the AC sub-grid, and 0.42% at the DC sub-grid of the HMG. [ABSTRACT FROM AUTHOR]

A fundamental strategy for utilizing green energy from renewable sources to tackle global warming is the microgrid (MG). Due to the predominance of AC microgrids in the existing power system and the substantial increase in DC power generation and DC load demand, the development of AC/DC hybrid microgrids (HMG) is inevitable. Despite increased theoretical efficiency and minimized AC/DC/AC conversion losses, uncertain loading, grid outages, and intermittent complexion of renewables have increased the complexity, which poses a significant threat toward system stability in an HMG. As a result, the amount of research on the stability, management, and control of HMG is growing exponentially, which makes it imperative to recognize existing problems and emerging trends. In this survey, several strategies from the most recent literature developed to address the challenges of HMG are reviewed. Power flow analysis, power sharing (energy management), local and global control of DGs, and a brief examination of the complexity of HMG's protection plans make up the four elements of the review technique in this article. During critical analysis, the test system employed for validation is also taken into consideration. A comprehensive review of the literature demonstrates that MILP is a frequently employed technique for the supervisory control of HMG, whereas tweaking bidirectional converter control is the most common approach in the literature to achieve efficient power sharing. Finally, this review identified the limitations, undiscovered challenges, and major hurdles that need to be addressed in order to develop a sustainable control and management scheme for stable multimode HMG operation. [ABSTRACT FROM AUTHOR]

The idea of a microgrid is created by utilizing more diverse ac or dc distributed generation (DG) sources along with an energy storage system (ESS) and loads. The most efficient and reliable selection of ac and dc microgrids is a hybrid ac/dc microgrid. The hybrid microgrid largely overcomes the shortcomings of standalone ac or dc microgrids. A bidirectional interlinking converter (BIC) is utilized in the interface for controlling power flow between sub-grids. In order to improve voltage and frequency regulation with effective power sharing, the BIC based on the proposed control scheme is implemented for power flow between ac and dc sub-grid in Islanding mode. The control scheme is modified based on conventional droop control with voltage and frequency variation in order to improve bus voltage and frequency regulation with effective power sharing for intermittent sources. The operation of the islanded hybrid ac/dc microgrid is performed with solar, wind, and energy storage system under variable generation and load conditions. In order to make robustness of the system, there are considered different cases for generation and load scenarios. In the transient state, the overshoot and settling time of frequency and voltage are improved, as well as the frequency and voltage regulations are found within the permissible limit in the steady state. Furthermore, the corresponding variations are shown in tabular form in the simulation result. The actual data of solar irradiance and wind speed have been taken from the National Renewable Energy Laboratory. The performance of the system is verified in MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

A Hybrid AC/DC microgrid (MG) can integrate distributed generation sources and distributed loads on the AC and DC side of the MG by eliminating many unnecessary power conversion devices, which is more flexible and efficient. However, to achieve reliable and economic operation of a hybrid AC/DC MG is challenging due to its complex structure. In this paper, a novel distributed event-triggered hierarchical control strategy is proposed to improve the economic operation of a hybrid AC/DC MG. For the primary control, distributed local controls of AC DGs, DC DGs, and interlinking converters (ICs) are realized by adopting the droop control method. For the secondary control, the distributed economic dispatch, distributed average bus voltage discovery, and distributed proportional power-sharing algorithms are first proposed; then, control objectives of voltage and frequency restoration and economic operation of the hybrid AC/DC MG are realized based upon the developed algorithms. Furthermore, the distributed secondary control is built upon an event-triggered mechanism developed in this paper, which can reduce the communication burden. The simulation results demonstrate the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

With the increasing integration of distributed generators (DGs), renewable energy sources (RESs), and energy storage systems (ESSs) in the hybrid AC/DC microgrid (HMG), the paralleled bidirectional interlinking converters (BILCs) play a significant role in the economic power interaction (EPI) between AC and DC subgrids. This paper proposes a distributed dynamic event-triggered control (ETC) strategy for the HMG to realize the global economic operation. In each subgrid, economic power sharing (EPS) among DGs can be achieved by economic droop controllers with no global information. The proposed distributed dynamic ETC includes a continuous-time part and a discrete-time part, where the former part realizes the EPI between two subgrids, while the latter part ensures the proportional power sharing between BILCs. Thus, a particular over-stressed BILC can be avoided, and system reliability and scalability are significantly improved. Additionally, only data from neighboring BILCs at the event-triggered times are involved, which greatly minimizes the communication burden and costamong BILCs. Furthermore, the asymptotic stability of the closed-loop system dynamics and the communication time delay stability are proven. Finally, the hardware-in-loop experimental results are conducted to demonstrate the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a novel hybrid AC/DC microgrid architecture incorporating a central energy storage system (ESS) for both the AC and the DC sub-grids. To ensure effective operation of the proposed hybrid AC/DC microgrid architecture, a coordinated control strategy between the central ESS and the inter-linking converter (ILC) is proposed in the paper. The proposed central ESS provides voltage and frequency control to the AC sub-grid by measuring voltage and frequency at the point of common coupling (PCC) of the AC sub-grid, while providing DC voltage support to the DC sub-grid by measuring DC sub-grid’s PCC voltage. Furthermore, per-unit frequency and the DC sub-grid PCC voltage are utilized to transfer power between the two sub-grids via the ILC. Therefore, two control strategies are coordinating with each other and maintain stable operation. The central ESS eliminates the use of multiple ILCs and complex control schemes for charge and discharge control to maintain same state of charge levels at each ESS. Effectiveness of the proposed coordinated controller is verified by applying various load disturbances, faults, and under different modes of operation. [ABSTRACT FROM AUTHOR]

Hybrid AC/DC microgrids are of special interest for energization due to their flexibility, low infrastructure investments, reduced conversion losses, and reliability against failures on the utility grid. In these systems, global economic dispatch needs communication between the AC and DC subgrids to achieve a near-optimal solution since the incremental costs of all generators need to be equalized. Hence, this paper proposes a distributed coordination between generators by means of a finite-time controller for the microgrid’s interlinking converter, which ensures an economic operation while taking care of the microgrid power utilization. The latter implies that a multi-objective control is performed by the interlinking converter, which uses shared variables of incremental costs and average powers from distributed generators in AC and DC sides. Also, an adaptive weighting method is proposed to adjust the control effort regarding the average power utilization of a side microgrid. The controller’s performance is verified through simulations and experimental setups. Results show that the proposed strategy is able to perform a trade-off between the two control objectives while achieving a finite-time convergence even though communication delays exist.

This paper proposes an energy management strategy (EMS) of a reconfigurable grid-tied hybrid ac/dc microgrid (HMG) architecture for commercial building (CB) applications. This HMG architecture consists of a multi-mode configuration (MMC) with renewable-based distributed generators (DGs), energy storages (ESs), and genset. The main advantage of this architecture is its capability to reconfigure its structure based on the predicted building load power (BLP) and renewable-based DGs power. However, minimizing the building electricity cost (BEC) and maximizing the reliability index (RI) of a reconfigurable grid-tied HMG architecture is a challenging task due to the stochastic behaviors of BLP and renewable-based DGs power. In this context, the BLP and renewable-based DGs behaviors are modeled using an artificial neural network approach to predict future time slot values. Then, system level models are developed for HMG energy sources and grid-tied converters/switches. Furthermore, BEC and RI models are developed based on the dynamic pricing of grid/HMG electricity cost and supplied energy. Then, an EMS is proposed for the developed reconfigurable grid-tied HMG architecture, which consists of a two-stage control strategy, i.e., stage I and stage II. Stage I control strategy minimizes BEC and maximizes RI using multi-objective particle swarm optimization (MOPSO) and MMC. Stage II control strategy generates control signals for HMG energy sources, converters, and grid-tied converters/switches based on stage I reference signals. Historical data is used to demonstrate the effectiveness of the proposed EMS for a reconfigurable grid-tied HMG architecture. Through numerical simulation studies, it is shown that the proposed EMS is capable of reducing BEC and increasing RI by concurrently enabling MMC of a reconfigurable grid-tied HMG architecture. [ABSTRACT FROM AUTHOR]

The fluctuation of production of renewable energy resources (RESs) is a big problem for its installation and integration in isolated residential buildings. A hybrid AC/DC microgrid facilitates the good operation of RESs with a storage system in standalone mode and the possibilities of smart energy management. In this paper optimization research of the hybrid ac/dc microgrid in isolated mode of operation is presented. The power system is supplied by various Renewable Energy Resources (RESs), Photovoltaic arrays (PVA), a Wind Turbine Generator (WTG), Diesel Generator (DG) and supported by Batteries Storage System (BSS) for short term storage. The main objective of this study is to optimize power flow within a hybrid ac/dc microgrid with regards to reliability in islanded mode. First a mathematical model optimized by mixed integer linear programming and solved by CPLEX solver with JAVA language is developed for an islanded RES system and then, based on the developed model, the power system control is simulated for different cases of off-grid mode. Simulation results have shown that the management strategy can maintain power balancing while performing optimized control and give a controllable loads and batteries charging/discharging powers, even with unpredictability of RESs powers outputs and arbitrary energy tariffs. Finally, the proposed algorithm respects the optimization in real-time operation under various constraints.%.

A hybrid ac/dc microgrid (MG) has the advantages of both ac and dc subgrids, and the operator should perform an accurate power flow analysis to ensure the MG's operational economy and reliability. This article proposes a Newton—Raphson (NR)-based sequential power flow algorithm for the hybrid ac/dc MG. First, the MG's islanded operation mode and the three-phase unbalance feature of ac subgrids are considered, and the droop control for distributed generations participating in the voltage and frequency regulation is modeled. Then, a sequential algorithmic framework is proposed to handle the coupling between ac and dc subgrids. Within this framework, the power flow of ac and dc subgrids is solved separately and iteratively with modified NR subalgorithms. The accuracy of the proposed sequential algorithm is validated in comparison with the results from MATLAB/Simulink, and the advantage of employing this algorithm is confirmed compared with an equivalence approach. The case studies demonstrated that the proposed algorithm can significantly improve the result accuracy with acceptable computational time. Finally, the convergence performance of the proposed algorithm is verified in the cases of varying ac and dc subgrid loading. [ABSTRACT FROM AUTHOR]

This paper proposed a distributed uniform controlstrategy for parallel bidirectional interlinking converters(BICs) to realize resilient operation of hybrid ac/dc microgrid (HMG). The proposed control strategy can coordinate the control objectives of BICs in different HMG operation modes with a uniform control structure, thus, to eliminate any mode transition detections. In normal condition, the power coordination between ac and dc subgrids can be achieved, while ac or dc voltage regulation can be maintained when main voltage source(MVS) fails in ac or dc subgrid. The coordination between parallel BICs relies on low-bandwidth neighbourhood communication. With the communication planned, accurate power sharing can be achieved. Communication fault ride-through capability is enabled with less power sharing accuracy. The system reliability and resilience of HMG under various fault conditions can be improved by the proposed control strategy. To verify the effectiveness of the proposed control strategy, the controller hardware-in-the-loop experiment results have been provide in this paper. [ABSTRACT FROM AUTHOR]

Historically, the development and design of distribution systems (DSs) and microgrids (MGs) were based primarily on alternating current (AC) as a traditional approach due to many advantages such as eliminating the need for synchronization, as well as the ease of integrating distributed energy resources (DERs). Recently, given the democratization of DER through local installations of renewable energy systems and appliances using power electronics, direct current microgrids (DC‐MGs) are gaining more and more momentum. In order to enable durable and economically viable use by integrating DC and AC DERs into microgrids, hybrid AC/DC microgrids (HMGs‐AC/DC) present one of the most promising approaches in eliminating the need for AC‐DC or DC‐AC conversions. The improvement of energy efficiency, protection, management, and control of this kind of systems are relevant research topics. This article provides an overview of theoretical works and industrial applications of hybrid AC/DC microgrids/distribution systems. In addition, an efficiency/energy‐losses study of different literature‐based works is discussed. Accordingly, a critical analysis is provided, and research perspectives related to this subject are outlined. This review article can be considered as a guide for future research on the efficiency and energy losses of hybrid AC/DC distribution systems/microgrids. [ABSTRACT FROM AUTHOR]

Poor power management of bidirectional interlinking converter (BIC) and sub-grids in a hybrid AC/DC microgrid can lead to a heavy or light load state of some distributed energy resources (DERs), and large deviations of AC bus voltage/frequency and DC bus voltage. This study proposes a distributed power management strategy based on the concept of coordination factors for hybrid microgrids to achieve accurate active power sharing among all AC/DC DERs (instead of the same within one sub-grid), reactive power sharing among AC DERs, and AC voltage/frequency and DC voltage restorations. Power management within a sub-grid is achieved by following the leader DER with the maximum coordination factor. Meanwhile, the BIC can establish a dynamic connection among all DERs by tracking the leaders of each sub-grid to realize power management between sub-grids. Compared to conventional methods, the proposed method can achieve accurate power sharing and voltage/frequency restorations simultaneously. Additionally, communication data and bandwidth can be significantly reduced. Finally, hardware-in-loop experiments are also performed to prove the feasibility of the proposed strategy, even during communication latency or failure. [ABSTRACT FROM AUTHOR]

In microgrids, energy storage is generally located based on power management criteria and economic objectives that lean on the system's steady‐state operation. However, the grid configuration, dynamics of primary energy sources, and storage type (AC or DC) impact its transient operation. This paper conducts a comparative study of the dynamic performance of Hybrid AC/DC Microgrids (HMGs) under the influence of Battery Energy Storage System (BESS) location, which has been neglected in the literature. The HMG performance is compared for one single BESS unit located either in AC or DC sub‐grid during various grid operational cases, considering interlinking converter (ILC) control topology that alters depending on the type of BESS. Two types of microgrid case studies with different structures have been considered, that is, a high inertia HMG and a converter‐interfaced low inertia HMG. Simulation results reveal that the HMGs with both BESS locations perform robustly in handling grid disruptions. However, the BESS located in the DC sub‐grid of a high inertia microgrid case presents an average of 40% higher quality of performance in key performance indicators compared to that of the AC sub‐grid BESS. Conversely, the performance of the low inertia HMG with the BESS connected to the AC sub‐grid is superior to that of the DC sub‐grid BESS, by an average of 65% and 54% in key performance indicators. [ABSTRACT FROM AUTHOR]

Summary: The intermittent nature of renewable energy generation and the variable AC and DC loads are major factors that offer great challenges in power management for AC/DC hybrid microgrids. In this context, this paper presents a coordinated AC frequency vs DC voltage control (CFVC) scheme for managing contemporary renewable energy‐based hybrid AC/DC microgrids. The proposed control strategy enables appropriate power interactions between the AC and DC subgrids while sharing power fluctuations in a coordinated way. Both the AC and DC subgrids support each other in accordance with their normalized relative changes in AC frequency and DC voltage, respectively. The proposed CFVC scheme is designed using fractional‐order‐proportional‐integral‐derivative (FOPID) controllers, and bacterial‐foraging optimization (BFO) method is employed for calculating the design parameters, viz. the controller gains and set‐point orders. A typical photovoltaic (PV) wind‐battery‐based hybrid AC/DC microgrid is modelled and investigated, and the usefulness of the proposed scheme is validated under the renewable energy variations and load perturbations. [ABSTRACT FROM AUTHOR]

This article proposed a selective power droop control for a hybrid interlinking converter (HIC) in connecting ac grid with low-voltage DC (LVDC) grid. The direct connection to the LVDC means relatively lower converter capacity, lower switching loss, higher efficiency, and lower cost in HIC. However, the small capacity converter and large capacitive coupling impedance of HIC can lead to a relatively lower reactive power compensation range. Therefore, the practical overcapacity situation may be more likely to occur in HIC, which deteriorates the system performance. To address this issue, the unique P-δ and Q-P-V droops are proposed for HIC with automatic selective factors adjustment control. With the proposed control method, the HIC can effectively regulate active/reactive powers and improve power quality. To demonstrate the validity of the proposed structure and its control method, a 5-kW HIC is built, and results are presented for poor power quality situations. The results convincingly indicate that the proposed HIC is efficient and capable of low capacity advantage, multifunction control strategy, and overcapacity operation. [ABSTRACT FROM AUTHOR]

In this paper, a novel distributed active power control method for interlinking converters (ICs) in an islanded hybrid ac/dc microgrid (IHMG) is proposed. The goal is to implement power sharing among all distributed generators (DGs) in an IHMG and among the ICs in a distributed manner, without the need for additional proportional-integral controllers. A mathematical formula for the active power references of the ICs is derived to achieve these objectives. Then, an estimator providing the data required for the reference calculation is proposed based on the modified dynamic consensus algorithm. In addition, a unified state-space model is derived for the entire IHMG, including the cyber-physical interactions, and a design method is presented for the parameters of the estimator. Finally, the effectiveness of the proposed method is verified by performing small-signal analysis and controller-hardware-in-the-loop verification. Compared to a previous distributed control method, the proposed method significantly improves the robustness in terms of communication delay and variation in the status of DGs while achieving the objectives. [ABSTRACT FROM AUTHOR]

High penetration of dynamic loads, such as induction motors (IMs) could give rise to sustained voltage/frequency and power oscillations in hybrid AC/DC microgrids during disturbances. Majority of the published literature has investigated these stability issues with aggregated models of IMs in hybrid AC/DC microgrids, which do not properly reflect the actual dynamics of parallel operating IMs; hence, power oscillation damping (POD) controllers must be designed explicitly considering various oscillations induced by parallel operating IMs. This paper proposes an adaptive neuro-fuzzy inference system (ANFIS) based POD controller to damp low-frequency oscillations (LFOs) induced by IMs in hybrid AC/DC microgrids. The proposed supplementary POD controller was embedded to the energy storage system (ESS) controller, which provides additional damping power proportional to the frequency deviation. The following two features namely: 1) ability to adjust the gain based on the frequency deviation, and 2) ability to handle more non-linearity in the system dynamics, make the proposed adaptive ANFIS based POD controller more unique compared to conventional POD controllers. The effectiveness of the proposed ANFIS-POD controller is verified using non-linear dynamic simulations considering a range of disturbances in a hybrid AC/DC microgrid and different combinations of parallel operating IMs. Results indicate improved oscillatory stability performance in the hybrid AC/DC microgrid with the proposed ANFIS-POD controller.

Self-governing small regions of power systems, known as “microgrids”, are enabling the integration of small-scale renewable energy sources (RESs) while improving the reliability and energy efficiency of the electricity network. Microgrids can be primarily classified into three types based on their voltage characteristics and system architecture; 1) AC microgrids, 2) DC microgrids, and 3) Hybrid AC/DC microgrids. This paper presents a comprehensive review of stability, control, power management and fault ride-through (FRT) strategies for the AC, DC, and hybrid AC/DC microgrids. This paper also classifies microgrids in terms of their intended application and summarises the operation requirements stipulated in standards (e.g., IEEE Std. 1547-2018). The control strategies for each microgrid architecture are reviewed in terms of their operating principle and performance. In terms of the hybrid AC/DC microgrids, specific control aspects, such as mode transition and coordinated control between multiple interlinking converters (ILCs) and energy storage system (ESS) are analysed. A case study is also presented on the dynamic performance of a hybrid AC/DC microgrid under different control strategies and dynamic loads. Hybrid AC/DC microgrids shown to have more advantages in terms of economy and efficiency compared with the other microgrid architectures. This review shows that hierarchical control schemes, such as primary, secondary, and tertiary control are very popular among all three microgrid types. It is shown that the hybrid AC/DC microgrids require more complex control strategies for power management and control compared to AC or DC microgrids due to their dependency on the ILC controls and the operation mode of the hybrid AC/DC microgrid. Case study illustrated the significant effects of microgrid feeder characteristics on the dynamic performance of the hybrid AC/DC microgrid. It is also revealed that any transient conditions either in the AC or DC microgrids could propagate through the ILC affecting the entire microgrid dynamic performance. Additionally, the critical control issues and the future research challenges of microgrids are also discussed in this paper.

In this article, a decentralized controller, based on the current mode virtual oscillator, is proposed to control the distributed power filters in dc systems. This communication-free solution only requires local dc bus voltage measurements as its control input. With the proposed control algorithm, the power filters can automatically share the target ripples based on their storage capacities, realizing modular and fault tolerant design. One typical application is that they can be placed on the dc subgrid of a hybrid ac/dc microgrid to attenuate the second-order harmonics from ac side. A frequency adaption mechanism is further embedded within the controller; hence, it can process any ripples at an unknown frequency for more general applications. To eliminate the circulating current caused by the power line dynamics, virtual impedance is employed to modify the feedback voltage measurement of the virtual oscillator. The proposed control algorithm has been verified by both simulation and down-scaled experiments. [ABSTRACT FROM AUTHOR]

Summary: The increasing penetration of AC and DC loads and renewable energy resources as distributed generations (DGs) in electrical power grids, hybrid AC/DC microgrid concept has been taken into consideration as one of the future grid structures. In this article, an islanded hybrid AC/DC microgrid including AC and DC subgrids, which are connected to each other through bidirectional converters, is considered. Bidirectional interlinking converters in hybrid AC/DC microgrid have an important role in power sharing among AC and DC subgrids. Hence, coordinated power control schemes based on AC frequency, DC voltage and common bus voltage are used in the control system of these converters. On the other hand, the primary droop control method is implemented in AC and DC subgrids to share the power among DGs, which causes frequency and voltage deviations in AC subgrid and voltage drop in DC subgrid. In order to tackle these problems, secondary control methods based on distributed cooperative control are proposed in both AC and DC subgrids. By these secondary control methods, which use a communication graph among DGs, a complicated communication network and central controller are not required and the system reliability is increased. Therefore, the accurate voltage and frequency regulation and active and reactive power sharing in AC subgrid, as well as voltage regulation and proper current sharing in DC subgrid are obtained. The effectiveness of proposed secondary control methods for the hybrid AC/DC microgrid is validated in comparison with an available consensus control method by simulation results conducted in MATLAB/SIMULINK software. [ABSTRACT FROM AUTHOR]

Secondary control strategies in hybrid ac/dc microgrids have been conventionally designed independently for each side of the microgrid, neglecting the interaction between the ac and dc sides due to the power transferred through the interlinking converter. This has a negative effect on the power sharing and dynamic response of the system. In this article, a novel distributed secondary control strategy for hybrid ac/dc-microgrids is presented which coordinates the control actions of the ac and dc sides. The proposed control scheme simultaneously regulates ac-voltage magnitude and frequency, as well as the dc-voltage magnitude, by including the interlinking converters in the control strategy via a distributed consensus approach. This improves the power-sharing accuracy and secondary control restoration of the variables in both ac and dc sides of the microgrid. Moreover, the proposed methodology avoids the circulating currents which are typically produced in systems with multiple interlinking converters. Several simulation and experimental tests, such as load impacts and plug-and-play capability, are presented to validate the performance of the proposed control strategy using a 24-kW hybrid ac/dc-microgrid laboratory prototype. [ABSTRACT FROM AUTHOR]

Today, conventional power systems are evolving to smart grids, which encompass clusters of AC/DC microgrids, interfaced through power electronics converters. In such systems, increasing penetration of the power electronics-based distributed generations, energy storages, and modern loads provide a great opportunity for power quality control. In this paper, an overview of the power quality control of smart hybrid AC/DC microgrids is presented. Different types of power quality issues are studied first, with consideration of real-world hybrid microgrid examples, including data centers, electric railway systems, and electric vehicles charging stations. It shows that compared to traditional centralized power quality compensations, smart interfacing power converters from distributed generations, energy storages, and loads, and the AC and DC subgrids interfacing converters are promising candidates for power quality control. To realize the smart interfacing converters' power quality control, both primary converters control and secondary system coordination are required. In this paper, a thorough review of the primary control of interfacing converters to integrate the power quality compensation are presented, with a focus on the hybrid AC/DC microgrid harmonics compensation and unbalance compensation. For multiple interfacing converters, the secondary control with system-level coordination and optimization for harmonics and unbalance compensation (considering both unbalance and harmonics in single-phase and three-phase systems) are also presented. Challenges like low switching frequency of interfacing converters, parallel interfacing converters operation, and interfacing converters communications are discussed, and typical solutions for primary and secondary controls to deal with them are presented. The paper also includes rich case study results.

Taking the consumption rate of renewable energy and the operation cost of hybrid AC/DC microgrid as the optimization objectives, the adjustment of load demand curves is carried out considering the demand side response (DSR) on the load side. The complementary utilization of renewable energy between AC area and DC area is achieved to meet the load demand on the source side. In the network side, the hybrid AC/DC microgrids purchase electricity from the power grid at the time-of-use (TOU) price and sell the surplus power of renewable energy to the power grid for profits. The improved memetic algorithm (IMA) is introduced and applied to solve the established mathematical model. The promotion effect of the proposed source-network-load coordination strategies on the optimal operation of hybrid AC/DC microgrid is verified.

In hybrid ac/dc microgrids, a bus converter system involving the use of a high-frequency dc transformer (DCT) plays a vital role to transmit power between dc and ac subgrids. However, the multiple control tasks and power transfer mode switching of bidirectional dc/ac converter create big challenges for the systematic controller design of DCT. To address this concern, a comprehensive coordinated frequency control (CCFC) is put forward in this article for the symmetrical CLLC-DCT to adapt the complicated hybrid ac/dc microgrid operation. By establishing the mathematical model on the voltage conversion gain (VCG) and active power transmission ratio (APTR), four operation modes are proposed based on power transfer conditions. The corresponding operators are derived based on the circuit feature to avoid the redundant manipulations. Frequency threshold is determined to avoid both overvoltage and low-voltage issues based on the known allowable range of VCG. The proposed CCFC scheme is detailedly realized by combining both the open-loop and closed-loop schemes to facilitate the design procedure. The prototype DCT demonstrates the proposed CCFC scheme can achieve a peak efficiency of 98.2% and satisfied performances on both VCG and APTR. [ABSTRACT FROM AUTHOR]

This article studies a hybrid AC/DC microgrid with bidirectional Γ-Z-source inverter as an interlinking converter (IC). The Γ-Z-source inverter is capable of providing high-voltage gainand it does not have the drawbacks of the conventional inverters. In this hybrid microgrid, DC-type energy sources and loads are connected in DC sub-grid and AC-type energy sources and loads are connected in AC sub-grid to reduce the extra power conversion stages in both AC and DC sub-grids. Therefore system volume and cost decreases and system reliability and efficiency increases. In DC sub-grid, the advantages of multi-port power electronic interface (MPEI) are utilised. The individual DC–DC power converter, a MPEI, is used for interconnecting DC distributed generators. Photovoltaic system and battery unit are used as DC distributed generators. The article presents the control of three operation modes including grid-connected mode, islanded mode and islanded mode with power flow management and the IC and distributed generators. For power sharing management, droop strategy is used. The performance of the proposed IC in all three operation modes is evaluated by time domain simulations in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Hybrid AC/DC microgrids (MGs) provide efficient integration of renewable sources into grids and the interconnection of multiple MGs can improve system reliability, efficiency and economy by energy sharing. In this paper, a distributed and robust energy management system is proposed for networked hybrid AC/DC MGs. For each individual MG, an adjustable robust optimization model is proposed to optimize its individual operational cost considering the uncertainty of the renewable generation and load demand. For the networked-MGs system, the energy sharing information of each MG is coordinated by the DC network to minimize the power transmission loss with network constraints. The overall optimization model is formulated, exactly convexified and solved in a distributed manner by the alternating direction method of multipliers (ADMM), where only limited information is required from each MG entity (i.e., the power injection to the network) and thus information privacy is guaranteed. Simulations of the networked hybrid AC/DC MGs are conducted to demonstrate the effectiveness of the proposed energy management system. [ABSTRACT FROM AUTHOR]