Solar PV fed Two-Stage DC/DC Converter for Low-Voltage DC-Microgrid

In this paper, a novel distributed control algorithm for current sharing and voltage regulation in DC microgrids is proposed. The DC microgrid is composed of several distributed generation units, including buck converters and current loads. The considered model permits an arbitrary network topology and is affected by an unknown load demand and modeling uncertainties. The proposed control strategy exploits a communication network to achieve proportional current sharing using a consensus-like algorithm. Voltage regulation is achieved by constraining the system to a suitable manifold. Two robust control strategies of sliding mode type are developed to reach the desired manifold in a finite time. The proposed control scheme is formally analyzed, proving the achievement of proportional current sharing, while guaranteeing that the weighted average voltage of the microgrid is identical to the weighted average of the voltage references.

In dc microgrids, load power sharing and bus voltage regulation are two common control objectives. In this article, a consensus-based algorithm is presented to achieve proportional power sharing and regulation of weighted geometric mean of bus voltages in dc microgrids with ZIP (constant impedance, constant current, and constant power) loads simultaneously. By using the virtue of the Laplacian matrices of undirected connected graphs, a lemma is derived to assist the stability analysis of the dc microgrids. Thus, a sufficient condition that stabilizes the system with ZIP loads is established. In addition, with the help of a distributed voltage regulation error estimator, the tuning of the bus voltages can be realized without the requirement on initial voltage conditions. Through the Lyapunov synthesis, the large-signal stability of the closed-loop system is theoretically ensured for a wide range of load conditions. Finally, simulation studies are performed to validate the merits of the proposed consensus-based algorithm.

This paper addresses load current sharing and circulating current issues of parallel-connected dcdc converters in low-voltage dc microgrid. Droop control is the popular technique for load current sharing in dc microgrid. The main drawbacks of the conventional droop method are poor current sharing and drop in dc grid voltage due to the droop action. Circulating current issue will also arise due to mismatch in the converters output voltages. In this work, a figure of merit called droop index (DI) is introduced in order to improve the performance of dc microgrid, which is a function of normalized current sharing difference and losses in the output side of the converters. This proposed adaptive droop control method minimizes the circulating current and current sharing difference between the converters based on instantaneous virtual resistance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\bm{R}_{{\bf droop}}$</tex-math> </inline-formula> . Using <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\bm{R}_{{\bf droop}}$</tex-math> </inline-formula> shifting, the proposed method also eliminates the tradeoff between current sharing difference and voltage regulation. The detailed analysis and design procedure are explained for two dcdc boost converters connected in parallel. The effectiveness of the proposed method is verified by detailed simulation and experimental studies.

at microgrids, traditional approach to control the instabilities, is replaced by power electronic (PE) converters, which interfaced with non-conventional sources and battery storage. Due to intermittent nature of renewable sources, there is challenge to control the bus voltage regulation of DC microgrid. Therefore, inertia of DC microgrid is poorer than conventional grid. In this paper, different control strategy including PID controller, virtual capacitor, feedforward control of voltage control loop and current control loop are proposed and compared among of them to improve inertia of the DC microgrid with virtual inertia control circuit which enables the integration of storage and generation with improved dynamics and feed-forward control method are also used for suppressing the disturbances and its effect on dynamic response of DC-μG.

The integration of renewable energy sources in DC microgrids presents significant challenges in maintaining stable voltage regulation and efficient power management. Existing control approaches struggle to handle the inherent variability of renewable sources while maintaining optimal system performance. This paper presents an Adaptive Sinh Cosh Optimizer (ASCHO) integrated with cascade Fractional Order Proportional-Integral (FOPI) control to address these challenges. ASCHO enhances the original SCHO algorithm through dynamic parameter updating and improved exploration-exploitation mechanisms. The proposed control system demonstrates significant performance improvements, with ASCHO achieving a fitness value of superior to conventional approaches' values. In practical implementation, the system maintains voltage regulation within ±5V of the reference value and achieves a 70\% reduction in steady-state error (3.15V vs 10.39V) compared to SCHO. The method was extensively validated through 23 benchmark functions comprising unimodal, multimodal, and fixed-dimension multimodal functions, with ASCHO outperforming other algorithms in 17 functions. Comprehensive testing validates the system's robustness under varying solar irradiance, wind speed, and load demands. The results demonstrate faster rise times (0.00208s vs 0.00209s) and improved power management capabilities, while maintaining battery state of charge variations within 0.003%. The successful implementation establishes ASCHO as an effective solution to optimize DC microgrid control systems with high penetration of renewable energy.

This chapter is concerned with load sharing and voltage regulation in DC microgrids. In order to address this issue, a multi-objective optimization problem with tunable weighting coefficients is first formulated for DC microgrids. Second, a distributed control scheme, which only requires local communications among neighbors, is proposed to solve the optimization problem. It is shown that, the proposed distributed control scheme has remarkable advantages in improving reliability and scalability of microgrids.

The increasing penetration of dc distributed energy resources (DERs) and dc loads has motivated the utilization of dc microgrids (MGs). In this paper, a new two-level control scheme is proposed for accurate power sharing and appropriate voltage regulation in dc MGs during islanded operation mode. In the primary control level, a P-\dot V droop method is proposed to eliminate the dependency of power sharing among DERs on line resistances. Since the P V droop deviates the voltage derivative to a nonzero value, a voltage derivative restoration mechanism is adopted in the secondary control level to provide voltage stability. The secondary control level also compensates the voltage deviations by cascading an outer voltage control loop with the inner voltage derivative restoration loop. The secondary control signal is broadcasted to each DER via a unidirectional low-bandwidth communication link. Small signal stability of the proposed scheme is analyzed by studying the locus of the system eigenvalues. To verify the efficacy of the proposed method and compare it with the conventional scheme, real-time simulations in OPAL-RT lab setup are provided.

As the grid-connected AC-DC converter is the key equipment that links the low voltage direct current (LVDC) microgrids and the utility grid, the safe and stable operation of the converter is of great significance to improve the reliability of the LVDC microgirds. Since the converter's reliability is affected by the converter's topological structure and modulation strategy, a civil LVDC microgrid case study is analyzed, and the power losses, thermal stresses, lifetimes and reliability of components in the AC-DC converters with different topologies and modulation strategies are evaluated in this paper. The influence of these two factors on the reliability and efficiency of the converter is discussed, and the normalized weighted total harmonic distortion (NWTHD) with different modulation strategies is analytically determined and compared. The results show that when the converter operates with unity power factor in this application scenario, among the two-level converter, flying capacitor converter, neutral point clamped (NPC) converter and T-type converter, the T-type converter has the highest reliability. The effects of the converter's topology and modulation strategy on the capacitor reliability are relatively small. In view of reliability and power quality, third harmonic injection pulse width modulation (THIPWM) or space vector pulse width modulation (SVPWM) is more suitable for the converter in this microgrid.

In this article, we propose a novel four-stage distributed controller for a dc microgrid that achieves proportional power sharing and average voltage regulation for the voltages at actuated and unactuated buses. The controller is presented for a dc microgrid comprising multiple distributed generation units (DGUs) with time-varying actuation states, dynamic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$RLC$</tex-math> </inline-formula> lines, nonlinear constant impedance, current, and power (ZIP) loads, and a time-varying network topology. The controller comprising a nonlinear gain, proportional–integral (PI) controllers, and two dynamic distributed averaging stages is designed for asymptotic stability. This constitutes deriving passivity properties for the dc microgrid, along with each of the controller subsystems. Thereafter, design parameters are found through a passivity-based optimization using the worst-case subsystem properties. The resulting closed loop is robust against DGU actuation changes, network topology changes, and microgrid parameter changes. The stability and robustness of the proposed control are verified via simulations.

In the low-voltage (LV) ac microgrids (MGs), with a relatively high R/X ratio, virtual impedance is usually adopted to improve the performance of droop control applied to distributed generators (DGs). At the same time, LV dc MG using virtual impedance as droop control is emerging without adequate power flow studies. In this paper, power flow analyses for both ac and dc MGs are formulated and implemented. The mathematical models for both types of MGs considering the concept of virtual impedance are used to be in conformity with the practical control of the DGs. As a result, calculation accuracy is improved for both ac and dc MG power flow analyses, comparing with previous methods without considering virtual impedance. Case studies are conducted to verify the proposed power flow analyses in terms of convergence and accuracy. Investigation of the impact to the system of internal control parameters adopted by DGs is also conducted by using proposed method.

DC microgrid is a compact framework comprising interconnected nearby sources and loads. The renewable energy source used in DC microgrids being intermittent leads to the change in the power availability as well as the fault current levels. In such situations, detecting and clearing the faults is very important to protect the DC microgrid without compromising on fault clearing time and interruption of the load. This paper proposes a hybrid Cumulative Sum (CumSum) and Wavelet transform-based approach to detect the fault. The CumSum value raises the amplitude by averaging the fault current. Wavelet transforms obtain important fault current features by decomposing the current signal. The hybrid method of CumSum and Wavelet analysis proposed here enables the detection of the fault and differentiates the fault condition from sudden load variation. Additionally, it helps to recognize the location of the fault by the wavelet energy difference. The proposed scheme is tested with a developed ring-type low voltage DC (LVDC) microgrid hardware model under various fault conditions. The scheme is implemented using TMS320F28069 digital signal processors (DSP) of Texas Instruments. The hardware results are validated using MATLAB simulation. The proposed method performance is also compared with the existing methods used for DC microgrid protection. The outcome shows that the proposed method has a high accuracy of 98.72%, selectivity of 96.08%, and reliability of 99.01%. The execution time required by the proposed method is also less.

This study provides designing and techno-economic analysis of a low voltage (48 V) DC microgrid for domestic load application. In present DC microgrid, photovoltaic (PV) is considered as main power generator to deliver the load demand while fuel cell (FC) and battery are considered as backup power sources. To establish a 48 V DC bus, suitable DC-DC converter having wide input voltage range (22–65 V) is designed to stabilize the variable output voltage of PV and FC generators. The converter efficiency was found to be ≈ 87%. A techno-economic study is also carried out to assess the levelized cost of electricity in microgrid using different energy storage systems, i.e. battery and hydrogen. Lead-acid and lithium-ion are considered as battery storages while high-pressure cylinder and metal hydride storages are considered for hydrogen storage. It is found that PV-battery is most economical configuration; however, PV-FC-battery is better choice for high reliability of overall system. PV-FC-battery configuration has lower cost of electricity as compared to PV-FC alone system. In battery storage system, lithium-ion battery shows faster charging/discharging and efficient operation as compared to lead-acid battery application. In hydrogen storage system, if high-pressure hydrogen is considered, the levelized cost of electricity is reduced significantly as compared to metal hydride storage. However, metal hydride is much better choice for hydrogen storage as compared to the high pressure considering the safety concern.

Low-voltage dc (LVdc) microgrids emerge as a viable alternative to ac microgrids. A large research interest is noted toward fast and selective protection of dc grids, typically focusing on hybrid or full solid-state solutions. In this article, the use of fuses as short-circuit protection in low-voltage dc microgrids is evaluated. The main advantage of fuses is that they are simple, cheap, standardized, and have low steady-state losses. A theoretical basis is formed to model dc short-circuit currents in grids with a limited short-circuit availability. The outcomes are applied to evaluate the possibilities of fuse protection in LVdc grids. It was found that fuses are an effective means of protection, although the required amount of capacitance at the output of the voltage balancing converter can be high, which impacts the total system cost. A fuse-based protection strategy is presented that highlights the need for additional capacitance to clear faults compared to the necessary capacitance for system stability. An experimental setup was built to validate the claims.

This study presents an investigation of the stability of a direct current (DC) microgrid to control the voltage of the DC bus in a self-sufficient power production and consumption system. The DC microgrid is composed of a solar photovoltaic system, a wind turbine, and a device to store energy based on lithium batteries. The energy storage system remains consistently linked to the DC bus to alleviate fluctuations in DC voltage. In energy systems that incorporate several sources and experience voltage instability, it is necessary to implement an appropriate distributed control strategy. This technique analyses the voltage stabilisation by regulating the current of DC-DC converters. The aim of this study is to get insight into the operational efficiency of the closed-loop system. Therefore, the mathematical formulation of the DC-DC converter is obtained for both steady-state and dynamic operational scenarios. The derivation of the perturbed model of the system incorporates an analysis of both the steady-state and dynamic operational properties of the DC-DC converters. Additionally, an input-output linearisation approach is used to assess the stability of the developed system. The accuracy and efficacy of the stability of the system for a range of scenarios involving operation are confirmed by the simulation results performed in the MATLAB environment.

Microgrids (MG) are found to be the sound alter-natives for solving challenges posed by the conventional grid and conventional fuel based generation. As a paradigm shift Low Voltage DC Microgrids are emerging alternatives in rural electrification. As a part of this work, a standalone DC microgrid is considered consisting of a Solar PV, a Battery Energy Storage System (BESS) and loads interfaced together to a DC bus. The loads considered here are DC Homes of three categories designed based on the energy consumption levels and socio-economic factors of rural population. The PV power suffers the problem of intermittency due to the environmental factors such as insolation, temperature, etc. As a part of this work a practical non uniform insolation data of 24 hours collected at IIT Bhubaneswar campus is considered. Correspondingly a loading pattern is also designed for all the houses in view of real life energy consumption pattern in rural areas. In the context of variable nature in the source and load, a controller is designed to achieve the primary objective of maintaining constant DC bus voltage of 48V. Also, a proper energy management is designed to achieve the secondary objective of power balance in the system. Corresponding analysis of the system and results are obtained using simulations in MATLAB/SIMULINK platform.