A comprehensive overview of DC‐DC converters control methods and topologies in DC microgrids

Microgrids with large‐scale photovoltaic systems constitute a large part of distributed renewable generation in many grids around the world. Managing the performance of such microgrids and especially their interaction with the main power grid is a challenging task, because it requires the control of renewable resources. This paper presents a comprehensive overview of DC‐DC converter structures used in microgrids and presents a new classification for converters. This paper also provides an overview of the control techniques of DC‐DC converters in DC microgrids and the advantages and disadvantages of the control methods are discussed. In connection with the increasing penetration of distributed generation sources (DGS) and renewable sources in power systems and their power management has been raised as a major concern and methods for power management have been investigated in this paper. Finally, a DC microgrid system, which includes a solar system, wind turbine, and battery, is simulated in MATLAB/Simulink software and its performance is analyzed.


| INTRODUCTION
Recent technological advances and increasing concerns about global warming have prompted engineers to seek clean energy sources. 1 The microgrid can tackle the current energy crisis by reducing transmission losses. 2,3Microgrids often include technologies such as photovoltaic PV systems or microturbines that require power electronic converters to connect to the electrical grid. 4A DC microgrid based on renewable energy has the following components 5 : 1.A microgrid DC bus. 2. Photovoltaic (PV) panel.3. Wind turbine.4. Power electronic converters. 5. Hybrid energy storage system (ESS) is applied to provide the required energy in case of lack of energy.6. DC load.
Figure 1 shows the different classifications of microgrids.
DC microgrid can provide multiple voltage levels and high efficiency. 6,7A DC microgrid is shown in Figure 2.
A network of distributed generation units forms an AC microgrid system, as shown in Figure 3. 8 A hybrid microgrid is shown in Figure 4.The power grid is controlled by converters and connected through a static transfer switch (STS). 9,10arge PV-based microgrids can produce part of their energy needs locally. 11Advanced control methods are required to improve energy transfer, enable cost-effective operation, and ensure power supply.PV systems are one of the effective sources of distributed generation. 12,13ombining the DC distribution with the microgrid concept becomes attractive because: 1. Renewable energy sources are naturally DC, and the efficiency is increased due to fewer power conversion steps.2. A DC system is much simpler to control and manage than an AC system.[16] The first challenge in regulated DC microgrids is constant power loads. 17The second challenge stems from the pulsed power load problem that commonly occurs in indoor microgrids.][20] Various control strategies are available for DC microgrids, such as instantaneous power control, 21,22 F I G U R E 1 Different microgrid classifications.
profile-based control, 18 adaptive voltage and current control, 23,24 consensus-based control, 25 decentralized control, 26 and power filter algorithm-based control. 27In Xu et al. 28 the optimal control strategy for an autonomous microgrid to overcome frequency fluctuations was investigated.In Chen et al. 29 and Tani et al. 30 a frequency-based method to reduce DC bus voltage fluctuations is considered.Control strategies for microgrid-based converters have been carefully reviewed in the references. 31Research on the topology of electronic power converters and control methods in DC microgrids is increasing.Figure 5 shows the percentage of the number of articles published in this regard in the last decade.
Most of the reviewed literature refers to different topologies of power electronic converters in microgrids or problems related to control and power management systems, and all cases are not stated at once for comparison.This paper has presented a new classification for topology and control methods by comprehensively examining the topology and control methods of DC-DC converters in the DC microgrid.Also, the performance, application, and control system of DC microgrid are investigated and reviewed.The structure of the continuation of this article is as follows; in Section 2, the description of the DC microgrid is discussed, then in Section 3, the types of converter structures available in the microgrid are described.In Section 4, the control methods of DC-DC converters in the DC microgrid are reviewed, and in Section 5, the power management methods in the DC microgrid are introduced.In Section 6, hardware development in the field of DC-DC converters applied in microgrid is presented.In Section 7, the simulation and analysis of a typical DC microgrid is presented, and in Section 8, the conclusion is presented.

| DESCRIPTION OF DC MICROGRID
Due to the development and progress of power electronics, DC microgrids have been considered. 32Advantages of DC microgrids include higher reliability and efficiency. 33For this reason, DC microgrids are preferred in residential applications, electric vehicle charging stations, data centers, and so forth. 34Furthermore, the increasing demand for DC electrical loads has made research on generation using DC sources very attractive. 35 DC microgrid has the capability to operate in either grid-connected or stand-alone (island) mode.In the gridconnected mode, the microgrid is linked to the DC bus, and compensates for the lack of power.When operation is in the island mode, the microgrid operates without synchronizing with the main power grid. 36In both cases, various renewable energy sources, and energy storage systems, including batteries and supercapacitors, are connected to the microgrid. 37he battery has a high energy density and its controller is designed to generate or absorb the steady state power.On the other hand, the supercapacitor has a high power density and its controller is designed to generate or absorb the transient state power. 38istribution grids and ESSs are connected to each other using DC link by power electronic converters. 39,40C microgrid protection problems and how to solve the problems are presented in. 41,42A review on local control is briefly discussed in Dragicevic et al. 43 and Papadimitriou et al. 44 In Elsayad et al. 45 the general architecture of a DC microgrid with the existence of energy storage units (ESUs), is presented.

| INVESTIGATING THE TOPOLOGIES OF DC-DC CONVERTERS IN THE DC MICROGRID
Clean sources of energy are connected to the microgrid through the use of power electronic converters.In one type of division, DC-DC converters are classified into nonisolated and isolated. 46,479][50] The topology of these three converters is shown in Figure 6. 33 manage the power flow in the microgrid, DC-DC converters are required to match the voltage levels between the feeders. 51Bidirectional isolated DC-DC converters are commonly used in DC systems. 52Using the Dual Active Bridges (DAB) DC-DC converter is a suitable option as it allows for bidirectional power flow and high power density. 53igure 7 shows the schematic of the DAB converter. 54urrently, the dual-active-bridge converter (DAB) with symmetrical and isolated features is a suitable converter for DC power systems. 55,56The best PV power generation performance can be achieved by using power electronics. 57In the meantime, the topologies of the series resonant converter (SRC) have attracted the attention of many researchers. 58n DC microgrids, the use of converters can result in higher costs and system losses.Fortunately, these issues have been addressed through the development of multiport DC converters. 59A multiport converter is commonly utilized in microgrids to connect various DC networks. 60eanwhile, in Wang et al. 61 an isolated two-stage threeport converter topology has been suggested.
Several topologies of multiport DC converters have been presented in scientific literature. 62,63The topology presented in Saafan et al. 64 is a five-port converter that has the flexibility to connect different DC loads and sources and control the DC link. Figure 8 presents the multiport converter proposed in Saafan et al. 64 Multiport converters are suitable for integrating various sources (including energy storage sources) and have a higher voltage ratio than buck-boost converters. 65,66One of the applications of DC-DC converters in DC microgrids, which includes energy storage systems, is to adjust the voltage of the supercapacitor and the power between the battery and the supercapacitors. 67Also, bidirectional DC-DC converters are used to charge the batteries in the microgrid.Cornea et al. 68 a bidirectional converter, in Zhang et al. 69 a threelevel converter, in Wang et al. 70 a multiport bidirectional converter, and in Prabhakaran et al. 71 a four-port converter are proposed for the integration of the hybrid storage system in the DC microgrid.The converter proposed in Ahmadi et al. 72 is a voltage-balancing function for a DC microgrid.In 33 F I G U R E 7 Schematic of the DAB converter. 54athore et al. 73 a resonance converter is proposed to increase the voltage without a transformer, and in Xue et al. 74 a converter is proposed to reduce the voltage level in the microgrid.In Hou et al. 75 a converter with ultra-fast dynamic characteristics is presented to integrate several ESUs to balance the power flow between renewable energies.The converters used in the DC microgrid are generally divided into isolated and nonisolated categories.The classification of converters is presented in Figure 9.

| CONTROL METHODS OF DC-DC CONVERTERS IN DC MICROGRID
Control of DC microgrids is one of the main concerns of researchers. 76,77Centralized control is appropriate for small and local microgrids with limited data collection. 33he centralized control scheme is shown in Figure 10.Distributed control, unlike centralized control, does not require a central controller.The distributed control scheme is shown in Figure 11.
Nonlinear control technologies, including model predictive control (MPC), 78 sliding mode control (SMC), 79 adaptive control, 80 and intelligent control. 81,82In recent years, many studies have focused on the performance of MPC for BESS bidirectional converter control and power balancing in the microgrid. 83In MPC, the optimal switching mode of the converter is determined by a cost function and adopted to achieve better performance. 84The MPC control scheme is shown in Figure 12.In SMC control, the generated control input is fed directly to the power electronic converter switches that provide a fast response. 79The SMC control scheme is shown in Figure 13.
Control of DC-DC converters are important subject because their load and input source are variable, and it is suitable for increasing the robustness of the adaptive control method.The adaptive control scheme is shown in Figure 14. 86n Zolfaghari et al. 87 a new control method for power management of microgrids based on a PV system is proposed.In this approach to control the power of each inverter, Fuzzy Logic Controllers (FLCs) have been implemented.In Figure 15, the control methods of converters used in the DC microgrid are categorized.In Table 1, the control methods of DC-DC converters are compared.

| POWER MANAGEMENT STRATEGY IN DC MICROGRID
DC microgrids are a suitable choice for energy supply to remote areas.This has led to increased attention toward energy management methods. 112,113I G U R E Block diagram of centralized control. 33I G U R E Block diagram of distributed control. 33I G U R E Block diagram of MPC controller. 85hirugnanam et al. 114 presents a battery energy management system (BEMS) for microgrids, where PVs and diesel generators are the main power sources.The proposed BEMS can achieve the following: 1. Reduction of working hours of diesel generators.2. Reduction of PV power fluctuations.

Simultaneous management of several batteries with different features and increasing battery life.
Power management in microgrids can be challenging due to various factors.For instance, the output power of photovoltaic systems fluctuates with radiation changes.Therefore, to ensure a reliable and highquality energy supply, these factors must be considered into account while designing the power management system. 115n grid-independent microgrids, one of the challenges is the power balance in the presence of the photovoltaic system, and the operation of the PV system must be coordinated with the BESS with other units. 116he power management system of the DC microgrid is displayed in Figure 16, which illustrates its various operation modes.The flowchart demonstrates

Hierarchical control Correction of inconsistencies in the system by adding secondary layers
The problem in the central controller leads to the uncoordinated performance of the microgrid [88-90]   Secondary control A communication line with the central control is required A slower dynamic response to changes [32, 91]   The  [110, 111]   Highly flexible Flowchart of the power management strategy in Alidrissi et al. 117

SARVI and ZOHDI
| 2025 that the PV system has two modes, namely, the limited power mode (LPM) and maximum power point tracking (MPPT), which are determined by the amount of battery State of Charge (SoC). 117n Ríos et al. 54 a power management algorithm is proposed for balancing the power of PV and BESS systems, while considering the limitations of the BESS system's SoC.When the battery is discharging, the bidirectional converter regulates the voltage of the dc bus.Usually, the photovoltaic system can generate the maximum power from the solar panels.However, in some situations, the power electronic converter must help the system to work in MPPT mode. Figure 17 shows the operation mode of the system with the power management algorithm for the battery and PV sections. 54n Venayagamoorthy et al. 118 an intelligent dynamic energy management system for the microgrid has been presented.The power management method of a hybrid PV/battery system is proposed in

DC-DC converter topology Hardware implementation References
High-gain-high-power (HGHP) DC-DC converter SCALE driver board 2AP043512 in interface the control and power circuit [122]   Interleaved boost with voltage multiplier Converter control and drive by TMS320F28379D [123]   Bidirectional three-level DC/DC Converter Converter control by dSPACE-1202 [69]   Novel boost-SEPIC type interleaved DC-DC convert Converter control by TMS320f28335 DSP controller [6]   Voltage source converter Implement ability of the control strategy in DC microgrids PCU modeled in OPAL-RT [113]   Bidirectional buck/boost DC-DC converter Converter control and drive by dSPACE-1103 [124]   F I G U R E 19 Block diagram of the studied DC microgrid.
Mahmood et al. 119 In Neto et al. 120 a power management strategy (PMS) has been provided for controlling power flow in DC microgrids.

| HARDWARE DEVELOPMENT IN THE FIELD OF DC-DC CONVERTERS APPLIED IN MICROGRID
Connecting a physical system to the simulation environment is a new topic.Different control methods and different topologies have been presented for microgrids.
To validate the simulation results, we require hardware for comparison.In Schultze et al. 121 a microgrid was implemented by connecting it to a fuel cell through a DC-DC converter using a hardware-in-the-loop (HIL) simulation.The communication between the simulation environment and the physical fuel cell system is bidirectional.As shown in Figure 18, the HIL simulation consists of a DC-DC converter and a microgrid. 121n the Table 2, several pieces of equipment used to implement the hardware part of DC-DC converters are collected from scientific references.

| SIMULATION RESULTS
A DC microgrid system is simulated in MATLAB software and its outputs are analyzed.The studied DC microgrid consists of a PV system, wind with PMSG generator, battery, DC-DC bidirectional converter to  regulate voltage, and MPPT system for wind turbines and solar panels.The structure of the studied system is shown in Figure 19.The DC microgrid photovoltaic system consists of 22 solar panels in series and the maximum power point voltage and current of each PV panel is 30.3V and 7.10 A.
A resistive load is used for the DC microgrid output.The specifications of the DC microgrid and its components are shown in Table 3.
MATLAB software is used to simulate the system.Figure 20 shows the overall DC microgrid schematic in the MATLAB/Simulink environment.The curves of the PV, battery, and wind turbine outputs are shown in Figure 21.The output power of the wind turbine at different turbine speeds (in terms of perunits) is shown in Figure 22.
Battery voltage (in the nominal and discharge area) is shown in Figure 23.The voltage and current of the output load of the system are shown in Figure 24.
The wind turbine system is simulated with a constant speed of 12 m/s.At rated wind speed, the wind system generates 8 kW.The nominal power of the photovoltaic system is 4.6 kW.Considering that the bidirectional converter is used in the battery section, the converter can be charged and discharged.

| CONCLUSION
This paper discusses the topologies and control methods of DC-DC converters in DC microgrids, along with several power management system strategies.Also, the hardware used in DC-DC converters in the microgrid has been investigated.A comprehensive analysis and comparison of control methods are provided.The complexity of microgrids has made them require digital automation and intelligent management to be a suitable and reliable alternative to traditional power grid.Advances in technology make it possible for automated energy management to handle multiple components and variable conditions, optimizing reliability and costs.Effective use of energy storage systems such as batteries in microgrids ensures an uninterrupted supply of required energy.Using renewable energy to power a region can be beneficial for the environment and economically for the entire world.Advanced and intelligent control methods are robust to impedance instabilities, and in DC-DC converters in DC microgrids, the intelligent controller has a fast and accurate performance compared to other control algorithms.In a standalone DC microgrid, DC-DC converters increase or decrease the voltage from different levels.Nonisolated converters have fewer losses than isolated converters and are more suitable.Various strategies are available to control the converters in the microgrid.Linear control techniques cannot ensure steady system functioning.Advanced methods such as MPC, SMC, and fuzzy control are used instead.

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I G U R E 13 Block diagram of SMC Controller. 79F I G U R E 14 Block diagram of adaptive control. 86SARVI and ZOHDI | 2023 F I G U R E 15 Control methods of converters used in DC microgrid.T A B L E 1 Comparison of DC-DC converter control methods used in DC microgrid.

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Power management algorithm for the (A) battery and (B) PV part of the microgrid presented in Ríos et al.54

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I G U R E 18 Simulation of a DC/DC converter and a microgrid with a hardware simulation presented in Schultze et al. 121 T A B L E 2 The equipment used to implement the hardware part of DC-DC converters.

T A B L E 3
Photovoltaic system current 7.10 (A) MPPT algorithm P&O Wind turbine Wind turbine speed 12 m/s Nominal mechanical output power 8.5 KW Generator type PMSG Battery Voltage 300 (V) Current 6.5 (Ah) Resistive load 400 Ω

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I G U R E 22 Simulation results, turbine output power (pu) at different turbine speeds (pu).

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I G U R E 23 Simulation results, battery voltage in the discharge mode.F I G U R E 24 Simulation results, (A) voltage of DC microgrid output loads (B) current of DC microgrid output loads.