1 Introduction

In the past decade, the increase in energy demand emphasized the necessity for the improvement of power systems efficiency and sustainability through the connection of different energy distributed generators (DGs). There are several environmental and operational problems associated with traditional energy sources such as fossil fuels. Therefore, more renewable energy sources (RES) diversity is necessary. (RES) such as solar, wind, wave, and hydropower, offer clean energy sources that ensure sustainable power supply. This combination of RES is used for the transition to a more sustainable energy system [1]. The integration of the hybrid AC/DC MG offers various advantages that improve the overall performance and reliability of the electrical power system. These benefits are summarized as follows [2]: improved system stability through power sharing in both directions between the grid and the connected MG, and increased reliability due to the use of interlinking converters. It also results in enhancing the system efficiency due to integrating AC and DC components, and improved energy management flexibility with the existence of energy storage systems (ESS). Finally, the integration causes grid resilience with the ability to operate in islanded mode, and reduced power losses through power flow control. These advantages lead to a more resilient, efficient, and clean energy system that can successfully handle a wide range of energy sources and loads. Furthermore, the hybrid network structure helps reduce the AC/DC multistage conversion [2]. However, the development of a hybrid AC/DC MG offers significant challenges. These challenges include: the coordination complexities between both AC and DC components, achieving power quality standards, and establishing a reliable communication infrastructure. Overcoming these problems is of crucial significance to optimize the performance, efficiency, and reliability of hybrid AC/DC MG systems [3]. Implementing an effective optimization technique and ensuring smooth transitions between the grid’s two modes of operation islanding to (grid-connecting or vice versa) is also a critical requirement of the system. DC-based MGs have several advantages over AC-one such as no reactive power circulation, higher overall efficiency due to fewer interface converters, ease of voltage transformation using DC-DC converters, and the ability to integrate a variety of DC-based units, since recently DC-based loads became more popular such as electric vehicles. These advantages make DC-based MGs a better choice to be integrated since they meet the energy generation and distribution requirements [4]. In addition, DC MG has the advantage of not needing to address phase and frequency, whereas an AC MG benefits from utilizing the existing AC systems but requires additional considerations. Hybrid AC/DC MGs, on the other hand, combine the advantages of both AC and DC systems by directly connecting both AC and DC-based equipment (DGs and loads) with minimal interface parts, resulting in fewer conversion stages and energy losses. They simplify control strategies, allow for easy voltage transformation using transformers on the AC side and DC-DC converters on the DC side, and provide an efficient way to integrate RES and electric vehicles with minimal changes to the existing distribution grid [5].

MGs may be divided into three categories based on their network configuration: DC MGs, AC MGs, and hybrid AC/DC MGs, as seen in Fig. 1. As shown in the figure the use of neural network algorithms is essential for different operational aspects. The optimization strategies based on neural network techniques enhance the efficiency of the power system by utilizing a different method for controlling and optimizing the MG’s operation. Through the neural network, the central control of the MG can intelligently determine the mode of operation and manage the power consumption of each local controller based on real time data and system conditions [6]. Hybrid AC/DC MGs combine the advantages of easy connection with distributed energy sources and the utilization of the existing AC distribution network, with AC/DC converters allowing bidirectional power between the grid and the connected MGs and achieving voltage management between distribution networks [7].

Fig. 1
figure 1

Typical structure of different MG topology

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A hybrid AC/DC MG consists typically of an AC network with connected DGs and loads, a DC network with the same as AC, a utility grid coupled at a point of common coupling, and an interface stage which is represented by bidirectional power converters. Hybrid MGs can also be classified into three types based on how DGs and ESS are connected to the main bus and how the main bus is interconnected with the utility grid: AC coupled, DC coupled, and AC–DC coupled.

  1. 1.

    AC coupled hybrid MG: In this arrangement, the primary AC bus is connected to DGs (such as photovoltaic (PV), wind …), ESS, and the utility grid. Bidirectional interfacing converters link ESS with the main AC-bus. Most of the RES connected to the bus is connected through different types of power converters. AC generation systems are the most widely used and straightforward structure. Despite the potential inefficiencies in the connecting converters, the AC system is still the most used architecture.

  2. 2.

    DC coupled hybrid MG: In a DC coupled MG arrangement, DGs, energy storage elements, and loads are connected to the main DC bus through different converter topologies. Interfacing converter facilitating the connection of DC bus to the main grid.

  3. 3.

    AC–DC coupled hybrid MG: In this arrangement, loads, ESS, and DGs are connected to both AC and DC buses. Regarding interfaces between the AC and DC subgrids, interlinking converters limit higher interfacing stages to save costs and improve overall efficiency. This topology is a very attractive choice for upcoming smart grids.

In the integration and operation of hybrid AC/DC MG systems, each of these topologies has benefits and challenges, highlighting the need to choose the best structure corresponding to system requirements.

The operational complexities regarding the stability during different grid operating modes are one of the integration limitations. Furthermore, efficient power sharing techniques between AC and DC subsystems, regulating parallel operation of connected converters to minimize problems such as nonlinear load behavior and circulating currents, and ensuring coordinated protection schemes for system reliability are of significant importance. These problems emphasize the challenges of integrating hybrid MG systems and the importance of addressing these challenges through the implementation of an efficient design, control, protection, and optimization methodologies to produce a reliable hybrid AC/DC MG [3]. Figure 2 summarizes the different issues faces the hybrid AC/DC distribution network from different points of view [2, 3, 8, and 9]. Operational challenges in hybrid AC/DC MGs, as shown in Fig. 2, include ensuring stability during grid-connected and islanded modes. In addition, managing power sharing between AC and DC subsystems, and addressing issues related to the parallel operation of power converters.

Fig. 2
figure 2

Different challenges facing the hybrid microgrid

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The rest of the manuscript is divided into seven sections each section overviews a different operational aspect related to hybrid MG: Sect. 2 illustrates the typical structure of the hybrid distribution network with all of its components. Section 3 gives a state of the art review on system planning given the most common strategies. Section 4 the same is repeated for the energy management strategies for the hybrid network. Section 5 discusses the common challenges faces the development of a protection schemes and suggested solutions. Section 6 discusses some of the proposed control and optimization strategies is discussed in Sect. 7. In addition Sect. 8 is used to demonstrate the different practical project and future trends. Finally, Sect. 9 is the conclusion section.

2 Hybrid network structure

The main idea of the implementation of a hybrid AC/DC network is to connect the existing AC distribution network to a several DC and AC sub-grid through an interlinking converter (ILC) [10]. The ILC can function in both grid-connected and islanding modes. In grid-connected mode, the ILC regulates the voltage of the DC distribution network by drawing power from the AC grid. During grid outage or high energy demand, the ILC changes into the islanding mode to regulate the voltage of the AC distribution network [11]. During this condition power converters deliver the power generated by wind and solar (connected RES) systems from the AC/DC MGs to the grid. Furthermore, an (ESS) is critical in managing the voltage of the DC distribution network and supplying energy to support the intermittent nature of RES.

According to [12,13,14] a hybrid AC/DC distribution helps mitigate the need for multistage conversion. The network consists of interconnected MGs, one based on AC and the other based on DC, connected through a power electronic converter. The basic components of the hybrid network can be summarized as follows:

2.1 Traditional distribution network

It can be represented as the existing system infrastructure which includes power lines, transformers, and substations that transfer electricity from power generation sources to end consumers. This network serves as the backbone of the electrical grid. This network is in responsible for supplying power to residential, commercial, and industrial locations, maintaining the continuity of electricity supply to fulfill consumer demands [15]. The traditional distribution network has improved over time. The main function of a distribution network is to efficiently transfer power across a large area, while maintaining important services to communities. It consists of several components, which work together in a different manner to transmit electricity from power plants to individual customers. The traditional distribution network is fundamental in enabling the efficient distribution of electricity and maintaining a stable and reliable power supply for users.

2.2 Renewable energy sources (RES)

Refers to energy sources that replenish themselves naturally and do not decrease over time. These sources are considered sustainable and environmentally friendly and can be used as an alternative to conventional fossil fuels [16]. Examples of RES include solar energy, wind energy, hydroelectric power, biomass, wave, and geothermal energy.

Solar energy utilizes the power of the sun by transforming it into electricity using PV panels or solar thermal systems. Wind energy uses wind turbines to transform the wind’s kinetic energy into electrical energy. Hydroelectric power generates electricity by utilizing the energy of moving or falling water. Biomass energy is obtained from organic matter, such as plant materials or agricultural waste, which can be burned or transformed into biogas to provide energy. Geothermal energy uses heat from the Earth’s core to produce power or provide heating and cooling.

RES is becoming more and more essential to the power system to make the transition to a low-carbon and more sustainable energy system. Some of the advantages they provide are lower greenhouse gas emissions, less dependency on fossil fuels, and the possibility of achieving energy independence. Integrating RES, such as solar panels and wind turbines, into a hybrid distribution network enables the production of clean renewable energy with minimum environmental impact and facilitates the development of a more sustainable energy future [17].

2.3 Energy storage system

These systems are designed to store excess energy generated by renewable sources or during periods of low demand for later use when demand is high or during power outages. ESS plays a crucial role in balancing the supply and demand of electricity, improving grid stability, and optimizing the integration of RES into the power system [18, 19]. There are various types of ESS used in hybrid distribution networks, including battery energy storage, pumped hydro storage, compressed air energy storage, flywheel energy storage, and thermal energy storage.

  • Battery energy storage systems (BESS) are one of the most popular types of ESS. Rechargeable batteries are used to store electrical energy, which can be charged and discharged as needed. In a hybrid network, BESS is especially crucial because it can react quickly to meet unexpected demand fluctuations or out-of-balance fluctuations in the production of renewable energy. Additionally, they can give backup power during grid outages, enhancing the power system’s reliability and resiliency. [20].

  • Pumped hydro storage is another widely used form of energy storage. It involves using excess electricity to pump water from a lower reservoir to a higher reservoir and then releasing it to generate electricity during periods of high demand [21].

  • Compressed air energy storage (CAES) stores energy in the form of compressed air, which is then released to drive turbines and generate electricity when needed [22].

  • Flywheel energy storage systems store energy in the form of a rotating mass that can be accelerated or decelerated to release or absorb energy [23].

  • Thermal energy storage systems store energy in the form of heat or cold and can be used for heating or cooling purposes when required [24].

Energy storage systems provide multiple benefits in a hybrid distribution network. They help balance the intermittent nature of RES, improve grid stability, provide backup power during outages, and better utilization of RES. Additionally, they can contribute to reducing peak demand and optimizing the overall operation and efficiency of the network. This capability is especially critical in compensating for the drop in power generation caused by the absence of solar energy, often referred to as the “duck curve” [25].

2.4 Loads

The hybrid network accommodates both AC (most of the existing industrial and residential loads are AC) and DC loads, such as electric vehicles. These loads can be directly connected to each bus type or connected through converters, depending on the specific requirements of the loads.

2.5 Converters

Power electronic converters are essential components of hybrid AC/DC networks because they facilitate electrical power to be efficiently converted, controlled, and transferred between AC and DC systems. These converters are responsible for converting AC to DC and vice versa, enabling smooth integration of the two subgrids together and to the main grid and achieving optimal operation of the hybrid network [26]. Here are some types of power electronic converters commonly used in hybrid AC/DC networks:

AC/DC converters: These converters are utilized to transform AC power from the conventional distribution network or AC-based MG into DC power. They typically employ rectification methods, such as diode rectifiers or thyristor-based rectifiers, to convert the smooth AC voltage into a pulsating DC voltage. In addition, DC/AC converters are employed to link AC loads to the DC bus of the hybrid network or to connect a DC based DG such as PV to the AC-side. This allows the incorporation of AC-powered devices within the primarily DC network and vice versa [27].

Bidirectional converters: In (ESS) like (BESS), bidirectional converters are utilized to perform the dual functions of charging and discharging energy. These converters allow power to flow in both directions between the ESS and the AC/DC networks. They enable the ESS to be charged when there is low demand or an excess of renewable energy generation, and to discharge stored energy during periods of high demand or when renewable energy is low. Furthermore, these converters are commonly used to connect both types of MGs together with the utility.

DC/DC converters: DC/DC converters convert DC power from one voltage level to another voltage level. These converters are used in the hybrid network to match voltage levels between various components, such as RES, ESS, and DC loads [28]. These converters help to optimize power flow between network components and guarantee that available energy resources are used efficiently.

Power electronic converters in a hybrid AC/DC network can also provide voltage regulation, harmonic suppression, and maximum power point tracking (MPPT) for RES. These converters help maintain the quality, stability, and reliability of the power supply in the hybrid network. By combining these components, a hybrid AC/DC network provides flexible and efficient solutions that benefit from both AC and DC systems. The network’s converters are used to ensure smooth integration and conversion of power between AC and DC systems, which leads to stable and sustainable operation.

3 Hybrid AC/DC network: system planning and design

Network planning is the process of designing and optimizing a network’s infrastructure and configuration to have an efficient and reliable operation. Regarding AC–DC hybrid transmission and distributed energy resource (DER) systems, network planning involves defining the layout, topology, and interconnections of both AC and DC components in order to integrate RES, ESS, and other DERs (DGs and energy storage elements). This planning considers voltage classification, source-load-storage partitioning, network configuration, connection structures, and load transfer paths. The objective of network planning is to build a system that can efficiently control power flow and ensure system stability. In addition, maximizes the utilization of available resources while satisfying operational objectives and meeting its limitations. [29].

In [30], a planning model for hybrid AC/DC MGs is presented, which determines the optimal allocation of DC feeders. The objective of the model is to minimize overall investment and operation costs, including investments in (DER), converters, distribution lines, and MG operation, while ensuring the reliable operation of DC feeders. The problem is formulated as a mixed integer second-order cone programming (MISOCP) or mixed integer linear programming (MILP) problem, allowing for efficient optimization of the placement of DC feeders. This optimization aims to supply DC loads economically and securely within the hybrid AC/DC MG. According to [31] the proposed planning approach for the AC/DC hybrid distribution network involves converting certain AC lines into DC lines and establishing a DC network. This transformation enables the integration of DC components with AC ones in the network. A planning model is developed for the AC/DC hybrid distribution network with (DGs) using multi-scenario technology with timing characteristics. The goal of this approach is to optimize the network structure by considering several factors such as DG configuration, identification of lines to be converted to DC, selection of access locations, and the choice between DC and AC for newly constructed lines. The objective is to enhance network efficiency, reduce overall costs, improve power quality, and enhance the capacity to accommodate DG units within the network. Ref [32] a planning model is presented that proposes a flexible investment strategy to address long-term development uncertainties. This strategy involves dynamic investment decisions that can be adjusted over time to account for new information as it becomes available. By doing so, the model allows for planning decisions to adapt and respond to evolving uncertainties. Such as changes in load demand and (DG) capacity. This ensures that the system remains resilient and cost-effective. The proposed algorithms in the model utilize a multistage scenario tree as a main strategy to capture uncertainty. This scenario tree represents possible system states over time, considering stochastic parameters for load demand and DG capacity at each stage. By utilizing scenario generation and reduction methods, the constructed tree represents various outcomes as uncertainties gradually unfold. This enables the simulation of different scenario paths and the evaluation of planning decisions in response to evolving conditions. However, the proposed approach faces some limitations such as the complexities of managing and optimizing the flexible investment strategy over multiple stages under uncertainty. In addition, the dynamic nature of the planning model, allowing for adjustments based on new information, may introduce some challenges in terms of computational complexity and decision-making processes. Furthermore, reserving disposable funds for adjustments during the implementation stage could increase the overall cost of the expansion planning, particularly if the proportion of funds reserved is not accurate. The proposed two-stage iterative approach in [33] addresses the optimal sizing problem of hybrid MGs. In the first stage, a genetic algorithm is used to determine the optimal design and investment decisions. This stage focuses on finding the most suitable sizing solutions that can accommodate both normal operation and transient states. In the second stage, a nonlinear solver is employed to solve the operational problem based on the design decisions obtained from the genetic algorithm stage (first stage). This two-stage approach effectively addresses operational challenges in hybrid MG systems. In [34] the planning models and evaluation of a system with multiple MGs are investigated, considering the coupling effects of interconnection structure and operation mode. The authors establish system planning models and utilize Bender’s decomposition methods to face the complex combinatorial model. The proposed methods aim to enhance the controllability, flexibility, reliability, and energy management of the system by integrating multiple MGs using a mixed AC/DC interconnection approach.

4 Hybrid AC/DC network: energy management

Energy management strategies play a significant role in optimizing the distribution of power within a hybrid MG. This can be achieved by effectively coordinating the generation units, storage elements, and distribution of energy. These strategies involve various tasks such as forecasting load and generation profiles, implementing control mechanisms to balance supply and demand, and integrating (ESS) to enhance the stability of the system. By employing advanced algorithms and optimization techniques, energy management systems ensure the efficient utilization of resources, facilitate bidirectional power flow and enable smooth coordination between AC and DC subgrids. Moreover, these strategies help maximizing the utilization of (RES), minimizing operational costs, and enhancing the overall performance of the MG by providing backup power during outages and optimizing energy sharing among different components of the MG [34]. According to [7, 35,36,37] the energy management system (EMS) has two main classes centralized and decentralized structures as shown in Fig. 3.

Fig. 3
figure 3

Typical structure for both centralized and decentralized control strategy

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In [36] the proposed (EMS) for hybrid AC/DC MGs addresses energy management challenges in remote communities by optimizing operational costs through efficient energy generation and distribution. The system provides a reliable power supply by carefully balancing energy flow to prevent outages, which is very important, especially in areas with limited grid access. Furthermore, the MG integrates water desalination units to provide clean water, fulfilling a critical need in remote regions. The EMS considers the technical operational limits and supports the safe and efficient operation of the MG components, thereby enhancing overall system performance and sustainability. This can be achieved by implementing real time supervisory control on both the generation and demand sides. On the generation side, a combination of renewable and nonrenewable generation units is integrated, with renewable sources controlled using MPPT techniques and nonrenewable units controlled through droop parameters to meet the demand. On the demand side, optimal operational schedules are developed for residential appliances and water desalination units. These schedules are designed to satisfy customer requirements while minimizing operating costs. The EMS also controls the AC/DC interlinking converters, enabling bidirectional power flow between the AC and DC sections of the MG.

In [38] a robust optimal power management system (ROPMS) is proposed for a hybrid AC/DC MG. This system functions as a supervisory control, overseeing the power flow within the MG by solving an optimization problem. Its primary objective is to efficiently manage power by maximizing the utilization of renewable resources, minimizing the usage of fuel-based generators, and extending the lifetime of batteries. The ROPMS considers various factors, such as meeting the power demand requirements, and optimizing the utilization of renewable resources, while considering operational constraints. It also addresses uncertainties in resource output power and generation forecast errors to ensure stability and robustness. Several EMS utilizes hierarchical control as in [39, 40]. The proposed EMS in [7] utilizes a hierarchical control structure with a centralized control type, where the central controller is an intelligent load controller (ILC). Subcontrollers are used to manage the distributed power supply and the (ESS). An important aspect of this system is the implementation of artificial neural network (ANN) training, which enhances the decision-making processes in different operational modes. During normal operation, the ILC performs secondary control to maintain the voltage of the DC distribution network. However, in the event of a fault or disruption on the AC grid, the system can switch to a stand-alone operation mode. In this mode, the ILC independently controls the voltage of the AC distribution network, ensuring the stability and reliability of the MG. This approach enhances the system’s ability to manage power effectively and adapt to different scenarios, ensuring the stability and functionality of the MG.

5 Hybrid AC/DC network: protection challenges and strategies

Although the hybrid network offers many benefits compared to the conventional grid, it encounters various operational challenges in terms of control, management, and protection. One of the most challenging issues in implementing a hybrid network is the development of a reliable protection scheme. Integrating (DGs), particularly those based on inverters, introduces additional protection challenges due to the limited current contribution caused by interfacing converters. Additionally, the interconnection of (MGs) with different characteristics complicates the protection system since both MG sides behave differently during fault conditions [2, 9, 41, and 42]. Numerous technical studies have been conducted to investigate the protection of AC MGs [43] and more recently, for the protection of DC MGs [44, 45]. However, limited research has been carried out regarding the protection of hybrid networks. Most of the existing studies regarding the operation of hybrid networks concentrate on system simulation, design, planning, and control, but less attention on developing a protection system. Moreover, the few studies that do address protection tend to focus on either the AC or DC side, without considering the impact of the other side.

The proposed protection scheme in [46] proposes a protection scheme for a hybrid AC/DC grid with modular multilevel converters (MMCs), including main protection, local backup protection, and remote backup protection for DC lines. The main protection component includes several schemes such as current differential protection, overcurrent protection, and unbalanced current protection, considering both positive and negative-pole currents. The local backup protection operates in coordination with the main protection to prevent false tripping by simultaneously considering low-voltage and current criteria. It employs voltage and current direction criteria to discriminate fault types and ensure accurate fault identification. The remote backup protection extends the protection coverage to upstream lines of the converter and utilizes two-terminal current direction information to selectively determine fault location. The primary objectives of this scheme are to enhance fault identification, ensure selectivity, and increase the speed of DC line protection.

In [47] the proposed protection scheme focuses on enhancing the reliability of DC and hybrid AC–DC MGs. For DC MGs, the protection scheme utilizes both voltage and current measurements to derive protection decisions, improving accuracy compared to existing techniques. It involves dividing the system into segments/zones with circuit breakers at each end to isolate faulted parts while keeping the rest of the system operational during a fault. In the case of hybrid AC–DC MGs, a novel protection scheme is introduced to detect faults from the DC side. This method aims to provide backup protection to the AC grid in case the primary protection system fails to detect a fault. By utilizing oscillations in DC current caused by AC side faults, the scheme can effectively identify and isolate faulted sections without de-energizing the entire MG. Ref [48] proposes a protection scheme that aims to improve AC line distance protection in AC/DC hybrid systems. It establishes a fault supplementary network and a distance protection criterion based on fault component energy to accurately identify fault locations. The scheme includes a directional protection criterion to eliminate the impact of DC system faults and provides tolerance against fault resistance. Several other distance-based protection schemes which is used as a solution to the mal-operation of distance protection applied in a hybrid network [49,50,51,52]. The proposed protection scheme in [53] combines Discrete Wavelet Transform (DWT) and Linear Discriminant Analysis (LDA) for fault detection and classification in a hybrid MG system. It involves measuring both voltage and current waveforms, preprocessing the data using DWT, calculating several statistical features from the decomposed signal, and using LDA for fault detection, classification, and faulty line identification. The same in [54] the proposed protection scheme combines wavelet transform (WT) and decision tree (DT) techniques for the protection of distribution lines in a hybrid MG system during islanding mode. It involves preprocessing and feature extraction using WT to analyze signal values at the relaying point. The scheme is designed for fault detection, fault classification, and faulty line identification. Testing across various fault scenarios shows improved accuracy compared to other classifiers, making the scheme effective for ensuring distribution line protection in the MG system. However, these two schemes neglect the different disturbances that affect the protection scheme such as islanding, changing system irradiation, and changing grid mode. In addition, the scheme does not consider the protection coordination between the AC and DC sides. Furthermore, the scheme only concentrated on a small part of the network, mostly on the AC side. The scheme proposed in Ref. [7] aims to protect both the AC and DC sides and coordinate between the protective relays on the two sides. Each side employs a distinct protection scheme, with coordination achieved through a state diagram. However, the proposed scheme faces several issues explained in the following points. The protection scheme for the DC side is based on the current derivative which is sensitive to fault resistance and system configuration. As for the AC side protection, it is based on the current magnitude which is insufficient for such a system. As for the coordination algorithm, it only considers AC as a backup to the DC side and not the other way around. It also neglected all the disturbances that can affect the protection scheme such as islanding and changing system irradiation. The proposed protection scheme for hybrid distribution networks with AC/DC MGs in [41] utilizes wavelet analysis for fault detection, classification, and coordination. It is designed to be fast-acting for fault detection on both AC and DC sides, emphasizing the importance of fast fault clearing, especially on DC-side, during the capacitor discharge time to prevent component damage. The scheme aims to achieve reliable protection coordination between AC and DC MGs. The fault diagnosis technique presented in [55] employs an artificial neural network (ANN) to identify open-phase faults in the AC–DC converter of series hybrid electric vehicles (SHEVs). To train the ANN, a novel pattern derived from simulation and experimental data is utilized, including four distinct switch fault levels, and variations in capacitor size, load, and speed. The primary objective of this method is to ensure accurate and comprehensive fault detection, and the experimental results confirm the validity of the simulation outcomes, thus validating the approach’s effectiveness. Figure 4 summarizes the most common protection challenges [56, 57] and proposed strategies for both AC microgrid [58,59,48, 60,61,62,63,64] and DC MG [65,66,67,68,69].

Fig. 4
figure 4

Most common protection challenges and proposed strategies for both AC microgrid and DC microgrid

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6 Hybrid AC/DC network: control strategies of the system

The control strategies for managing a hybrid AC/DC MG effectively include nonlinear control techniques, interlinking converter control for power sharing with nonlinear loads, model predictive control, sliding mode control surfaces for efficient progress of state variables, and the application of passivity theory to improve system efficiency. These strategies are crucial for optimizing the performance and reliability of hybrid MG systems [3].

In [70] the proposed scheme combined central and local voltage control strategies in hybrid AC/DC MGs to regulate nodal voltages in real time by optimizing globally and controlling locally. The MG central controller (MGCC) performs optimal power flow based on predicted PV outputs to calculate voltage and power settings, considering PV output uncertainty. The MGCC generates optimal local voltage control curves using a piecewise linear curve fitting method. These curves are then implemented into local controllers (LC), which measure node voltage, calculate power values, and adjust power output for closed-loop voltage control. This strategy combines centralized optimization with fast local control to ensure effective voltage regulation without heavy communication requirements. By combining the centralized optimization capabilities of the MGCC with the fast response and control speed of the local controllers, this strategy ensures optimal voltage control effectiveness while maintaining real time regulation of nodal voltages in hybrid AC/DC MGs. Ref [71] proposes an adaptive neural network that tracks the (MPP) of renewable energy resources by continuously adjusting the control parameters based on the input data. Regarding the hybrid AC/DC MG, the adaptive neural network is used to optimize the power flow between the RES and the grid by dynamically adjusting the operating conditions to maximize energy production. The authors of [72] propose a reinforcement-learning-based online optimal control (RL-OPT) method that improves the performance of hybrid energy storage systems (HESS) in AC/DC MGs by optimizing the charging and discharging profile, enhancing direct controllability for transient performance regulation, and considering both grid-connected and islanded operation modes for optimal control in different scenarios. This method addresses challenges such as uncertainties, disturbances, and system dynamics to enhance system efficiency and power quality in MG applications. The hierarchical control strategy for AC/DC hybrid MGs in [73] involves a systematic approach where the lower converters and loads are managed through a central controller. This strategy considers different factors for islanding and grid-connected modes. In grid-connected operation, stability of AC bus voltage and frequency is ensured, while in islanding operation, maintaining stability of AC bus voltage and frequency becomes crucial. The control architecture adopted for this strategy is structured hierarchically for effective management and operation of AC/DC hybrid MG systems. Hierarchical control of parallel AC–DC converter interfaces for hybrid MGs is discussed in [74]. A three-level control system is implemented, including primary control using the droop method, secondary control to eliminate voltage deviation, and tertiary control for connection to external DC systems. Experimental results show successful implementation of the designed system. The study emphasizes practical application and control systems for hybrid MGs, focusing on both standalone and grid-connected operation modes. It presents detailed control strategies for efficient interaction between AC and DC components in hybrid MG topology. Figure 5 summarizes the Hierarchical control strategies.

Fig. 5
figure 5

Hierarchical control strategies

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7 Hybrid AC/DC network: optimization methods used in hybrid networks

Optimization methods are of great importance for enhancing the performance and efficiency of hybrid MGs. These methods include various approaches that aim to minimize costs and reduce losses through power flow optimization. The use of metaheuristic techniques and mathematical models in design optimization ensures the most effective sizing of components in terms of cost. In order to improve the grid performance, significant attention is given to battery storage optimization and minimizing operational costs. Ongoing research focuses on developing optimal sizing strategies, considering uncertainties and cost reduction in the design of hybrid MGs [3].

The research paper in [75] introduces a novel optimization algorithm that utilizes graph theory principles. It has been tested on various AC/DC test networks featuring different power converter operating modes and DG models. The algorithm successfully demonstrates the viability and effectiveness of this technique in optimizing hybrid AC/DC MGs. Its primary objective is to enhance the control capabilities of AC/DC hybrid (DER). The research paper in [76] introduces a robust optimization algorithm for effectively managing uncertainties in hybrid AC/DC distribution networks. The algorithm follows a two-stage approach, where a master-problem and a subproblem are solved iteratively using the column-and-constraint generation (C&CG) algorithm. In the master-problem, variables from both the first stage and secon -stage optimization are considered to find the optimal solution in the worst-case scenario. The sub-problem focuses on maximizing and minimizing objectives under uncertainties, with dual variables associated with constraints. The algorithm’s performance improves when the predicted fluctuation range closely aligns with the actual deviation, highlighting the significance of selecting an appropriate predicted fluctuation range. Ref [77] presents an optimization algorithm that utilizes an adaptive multiplier strategy to improve power flow calculations in AC/DC hybrid distribution networks. The algorithm focuses on minimizing the maximum power difference by iteratively adjusting the correction coefficient (μ). This adjustment is based on comparing power differences within the network. By incorporating adaptive strategies for (μ) and modifying step sizes, the algorithm aims to enhance convergence and optimize the solution for power flow distribution calculations in the network. The authors of [78] introduce an equilibrium optimizer algorithm (EOA) as an optimization method specifically designed for addressing the optimal power flow (OPF) problem in hybrid AC/DC power grids. EOA considers multiple objectives simultaneously, including minimizing total generation costs, environmental emissions, power losses, and bus voltage deviations. Notably, it incorporates adaptive dynamic control parameters that simulate dynamic and equilibrium states, thereby enhancing its effectiveness in achieving optimal fitness. In terms of performance, EOA outperforms other algorithms such as particle swarm optimization PSO in solving the OPF problem. The coordination control algorithm proposed for boost and main converters in a hybrid MG in [79] aims to ensure smooth operation under various supply and demand scenarios. This algorithm is derived from the fundamental control algorithms of grid-interactive inverters and focuses on optimizing power flow and maintaining stability within the hybrid MG system. By effectively coordinating the operation of the boost and main converters, the algorithm helps to maximize the power generated by renewable energy sources, balance power flow between AC and DC buses, and ensure stability under changing demand and supply conditions.

8 Comprehensive overview of current practices and future directions in hybrid AC/DC distribution networks

Hybrid AC/DC distribution networks are gaining traction as a way to overcome limitations of traditional AC systems and integrate renewable energy sources more effectively.

8.1 Comparative analysis of hybrid AC/DC networks

Table 1 represents a comprehensive analytical comparison for hybrid AC DC distribution network. The studies considered in the table are divided into several aspects such as system structure, protection and control [80].

Table 1 Comparative study of the hybrid AC/DC network in different aspect
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8.2 Practical projects of hybrid networks

Practical implementation of hybrid AC/DC power systems have been conducted in various countries, including:

  • The FPX AC/DC hybrid systems, a small-scale hybrid system developed by life safety power in the USA, are designed to provide hybrid AC/DC power solutions [81].

  • The Ultranet project in Germany is a 400-kV-hybrid AC/DC overhead line project that integrates AC and DC systems on multi-circuit support structures. This project aims to enhance power transmission capabilities by combining DC-transmission facilities with VSC-converters into an existing AC grid. The project involves operating a long DC line parallel to multiple AC circuits in close proximity, presenting challenges in ensuring system security [82].

  • The ANGLE-DC project by Scottish Power Energy Networks in North Wales involves converting existing 33 kV AC circuits into DC operation using specially designed cascaded power converters to establish a ± 27 kV DC link voltage. The project aims to address challenges like increased renewable power generation, growing electricity demand, and voltage stability concerns [83].

  • Solar Homaya hybrid home system: This system aims to provide cost-effective access to energy by reducing reliance on the traditional power grid. It can supply both AC and DC loads through a smart integration of solar and grid power, leading to substantial savings on electricity bills [81].

8.3 Future trends of hybrid AC/DC network

The future trends of hybrid AC/DC networks involve the utilization of flexible control capabilities for the development of power distribution systems for the energy internet. Additionally, the development of smart and intelligent systems with uninterrupted, secure, and safe power flow is a key principle for the futuristic approach in an AC/DC microgrid environment. This includes the enhancement of energy efficiency through advanced communication systems, such as optical wireless, and the implementation of self-healing capabilities. Furthermore, the integration of IoT technology with hybrid AC/DC renewable energy systems is anticipated to play a significant role in the future of power distribution systems, enabling seamless data flow between various elements and facilitating control operations and remote monitoring. [75]. The current trends and developments in local and global control strategies for DGs and power converters in hybrid microgrids are focused on addressing the complexities of a hybrid AC/DC microgrid. These strategies aim to ensure the stability and reliability of the system in the face of uncertain loading, grid outages, and intermittent renewable energy sources. The study in [84] discusses the evolution of control strategies, including droop control, constant voltage control, constant current control, mode-adaptive droop control, and modified DC droop control for DC microgrids. Additionally, it highlights the importance of global control strategies, which require significant research due to the increased complexity induced by communication channels, cyber security, and the dominance of low inertia inverter based DGs.

9 Conclusion

This paper aims to summarize the existing literature on hybrid AC/DC MGs, providing insights into their current status and addressing the challenges associated with the operation of the hybrid network. It presents various planning methods, stability control techniques, and energy management strategies implemented in different operating modes of hybrid AC/DC MGs, along with their advantages and disadvantages. The paper also discusses different optimization strategies, including hierarchical methods, and intelligent methods for the optimal operation of hybrid AC/DC MGs. Furthermore, a comprehensive literature survey is conducted on several protection strategies, and different protection challenges facing the hybrid network. As a result, it’s concluded that the integration of a hybrid AC/DC distribution system into the existing network is crucial due to the countless advantages provided by the hybrid network. This integration provides flexibility and adaptability in planning decisions, ensuring optimal solutions, while reducing total costs. Overall, this paper serves as a resource for researchers seeking to gain a deep understanding of the current state of hybrid AC/DC MGs and their performance.