Converters In DC Microgrids Research Articles

Large-Signal Stability Improvement of DC-DC Converters in DC Microgrid

In DC microgrids, constant power loads (CPLs) reduce the effective damping of the DC-DC converter and may induce destabilizing effects into the DC-DC converter. To overcome such problems regarding CPL and ensure large-signal stability of DC-DC converters in DC microgrids, some feedforward terms are added to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V$</tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$I$</tex-math></inline-formula> droop-based dual-loop controller for a DC-DC converter based on the large-signal model. It is proven that the feedforward terms can not only improve the transient response but also guarantee the exponential stability of the closed-loop system in the whole operating range in regards to a large-signal manner, which is verified by using a singular perturbation model. Moreover, a disturbance observer is designed to estimate the output current, thereby enabling the removal of the current measurement sensor. The proposed technique can be easily plugged into a pre-defined <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V$</tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$I$</tex-math></inline-formula> droop-based dual-loop controller without an additional sensor being required. Ultimately, both simulation and experimental tests verify the effectiveness of the proposed method.

Reduced-Order Aggregate Model for Large-Scale Converters With Inhomogeneous Initial Conditions in DC Microgrids

In practical microgrids, the inhomogeneous initial values are widely appeared due to soft-starting operation. If traditional model order reduction approaches are applied, the input-output maps error between the original system and reduced-order system is large. To address this problem, this paper proposes a reduced-order aggregate model based on balanced truncation approach to provide the preprocessing approach for the real-time simulation of large-scale converters with inhomogeneous initial conditions in DC microgrid. Firstly, the standard linear time-invariant model with inhomogeneous initial conditions is established through non-leader multiagents concept. To end this, it is convenient for scholars to build complex system modeling with switched topology. Furthermore, the full system is divided into two components, i.e., the unforced component with nontrivial initial conditions and forced component with null initial conditions. Moreover, this paper presents an aggregated approach that involves independent reducing component responses and combining reducing component responses. Based on this, the input-output maps error is reduced. Then, the approximated error estimate of the reduced-order aggregate model regarding large-scale converters in DC microgrid is first provided, which provides prior knowledge and theoretical basis for DC microgrid designers. Finally, the simulation results illustrate the accuracy of the proposed approach.

A New Control Strategy for Bi-directional DC/DC Converter in DC Microgrid

The bi-directional DC / DC converter in DC microgrid is a typical nonlinear system which has large voltage disturbance during lead accumulator charging and discharging. In order to solve the problem of voltage disturbance, the linearization of the converter is realized by exact feedback linearization, and the sliding mode controller is designed by using exponential approximation law. The simulation results show that the method has fast response speed, strong anti-interference ability and good steady-state characteristics.

Comparison between alternative droop control strategy, modified droop method and control algorithm technique for parallel-connected converters

Most of the active current sharing methods are based on a communication network. The communication link is also used with the improved droop control methods to achieve a precise load current sharing and regulate the voltage at the common DC bus. Conversely, the conventional droop method that is considered a decentralized method becomes more attractive for controlling parallel-connected converters in DC microgrids. The conventional droop methods' main drawbacks are associated with the unequal load current sharing and voltage deviation at the common DC bus. In this paper, the modified droop method as a conventional droop method is augmented with a virtual droop and adaptive voltage control gains to improve the load current sharing and the voltage regulation, respectively. In contrast with other improved droop approaches, the control approach proposed in the paper does not require a communication link to exchange information between parallel modules. Instead, it uses the converters' theoretical load regulation characteristics to estimate the voltage set point for each converter locally. The proposed virtual resistive gain manipulates the modified droop method to regulate each module's droop gain, which ensures equal current sharing. The proposed method also eliminates the tradeoff between current sharing difference and voltage regulation by implementing the adaptive voltage control, which compares the estimated voltage at the point of common coupling with the rated bus value and adjusts the droop gains based on the compared values to ensure a constant voltage at various load conditions. The load current sharing and voltage restoration improvements of the proposed method versus the modified droop method and the control algorithm technique are observed in this paper. The proposed method's effectiveness is demonstrated by MATLAB/Simulink simulation and validated by an experimental prototype.

Adaptive Control of Fuel Cell Converter Based on a New Hamiltonian Energy Function for Stabilizing the DC Bus in DC Microgrid Applications

DC microgrid applications include electric vehicle systems, shipboard power systems, and More Electric Aircraft (MEA), which produce power at a low voltage level. Rapid developments in hydrogen fuel cell (FC) energy have extended the applications of multi-phase parallel interleaved step-up converters in stabilizing DC bus voltage. The cascade architecture of power converters in DC microgrids may lead to large oscillation and even risks of instability given that the load converters considered as loads feature constant power load (CPL) characteristics. In this article, the output DC bus voltage stabilization and the current sharing of a multi-phase parallel interleaved FC boost converter is presented. The extended Port-Hamiltonian (pH) form has been proposed with the robust controller by adding an integrator action based on the Lyapunov−Energy function, named “Adaptive Hamiltonian PI controller”. The stability and robustness of the designed controller have been estimated by using Mathematica and Matlab/Simulink environments and successfully authenticated by performing experimental results in the laboratory. The results have been obtained using a 2.5 kW prototype FC converter (by two-phase parallel interleaved boost converters) with a dSPACE MicroLabBox platform. The FC main source system is based on a fuel reformer engine that transforms fuel methanol and water into hydrogen gas H2 to a polymer electrolyte membrane FC stack (50 V, 2.5 kW).

High Gain Switched-Inductor-Double-Leg Converter With Wide Duty Range for DC Microgrid

In a dc microgrid, efficient high gain converters are needed to raise the voltage level of low voltage power sources such as photovoltaic, fuel cells, etc. In this article, a high-gain switched-inductor-double-leg converter for dc microgrid is proposed. The proposed converter is capable of providing higher gain devoid of using any transformer, coupled inductor, and multiple voltage lifting techniques, e.g., triple lift, quadruple lift, super lift, etc. The operating modes of the converter are controlled using three switches in double duty mode. Compared to single duty converter, the double duty converter provides a flexibility in selection of duty cycle for switch to achieve desired output voltage and controlling inductor current ripple magnitude by selecting appropriate duty cycles. Moreover, two duty cycles make the converter capable of achieving high gain with wide duty range and an individual switch does not need to operate at very large duty cycle to achieve high voltage gain. The topological description, operating principles, steady-state voltage gain analysis during continuous conduction mode and discontinuous continuous mode, boundary condition, and voltage and current analysis, efficiency analysis, comparison and design of the proposed are presented. The proposed converter is tested in laboratory to validate its feasibility and performance.

Sliding Mode Control of Bi-directional DC/DC Converter in DC Microgrid Based on Exact Feedback Linearization

The nonlinear property of bi-directional DC/DC converter in DC Microgrid will cause large voltage disturbance. To solve the above problems, a exact feedback linearization method based on nonlinear differential geometry theory is proposed to realize the linearization of the converter. Moreover, considering the approaching speed of the linearized Bruno standard model, a sliding mode controller is designed by using the exponential approach law. The simulation results show that the method has fast response speed, strong antiinterference ability and good steady-state characteristics.

An Autonomous Control Scheme of Global Smooth Transitions for Bidirectional DC-DC Converter in DC Microgrid

In dc microgrid, energy storage system (ESS) plays a crucial role to provide short-or-long term and high-quality electric energy. The different control strategies for bidirectional dc-dc converter (BDC) of ESS in grid-tied and islanded modes pose challenges to the coordination control of the dc microgrid. This paper proposes an autonomous control scheme for the BDC in dc microgrid. The proposed control scheme is based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V^2-P$</tex-math></inline-formula> droop control and unifies bus voltage regulation and power regulation in a single control structure. Thus, global smooth transition between various operation modes can be achieved without any control strategy changes, which avoids various mode switch detection mechanisms and improves system stability. Furthermore, the proposed control scheme is fully decentralized which reduces the reliance on communication, and enhances the reliability of the microgrid. On the other hand, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V^2-P$</tex-math></inline-formula> droop approach eliminates the negative effect of widespread constant power loads (CPLs) in dc microgrid. The stability of the proposed control method is illustrated, and the design guideline of some critical control gains is addressed. Finally, the effectiveness of the proposed control scheme is validated by the real-time hardware-in-loop (HIL) platform.

Extending protection selectivity in low voltage DC microgrids using compensation gain and artificial line inductance

Vulnerability of power electronic converters in DC microgrids in case of fault occurrence in DC cables necessitates using a fast fault detection and isolation scheme. In this paper, a modified, fast and selective protection scheme has been presented, which provides the required tripping of low voltage DC (LVDC) microgrids. This protection scheme has been developed based on the natural characteristics of the fault current, in which the first and second derivatives of the fault current have been employed to define thresholds for discriminating between faulted and non-faulted situations. To enhance fault detection capability of the protection scheme, definition of thresholds have been improved by introducing compensation gains, which makes the thresholds adaptive to amplitude of line current. Furthermore, in order to extend time validity of the thresholds which are calculated by approximated equations, artificial line inductance (ALI) technique has been proposed. Since no communication link is involved and all measurements are local, any types of fault can be detected without time delay. As a result, fault detection is achieved within 250 μs. To show the superiority of the proposed protection scheme, different case studies are investigated and results are compared with the earlier study.

Review on Advanced Control Technologies for Bidirectional DC/DC Converters in DC Microgrids

DC microgrids encounter the challenges of constant power loads (CPLs) and pulsed power loads (PPLs), which impose the requirements of fast dynamics, large stability margin, high robustness that cannot be easily addressed by conventional linear control methods. This necessitates the implementation of advanced control technologies in order to significantly improve the robustness, dynamic performance, stability and flexibility of the system. This article presents an overview of advanced control technologies for bidirectional dc/dc converters in dc microgrids. First, the stability issue caused by CPLs and the power balance issue caused by PPLs are discussed, which motivate the utilization of advanced control technologies for addressing these issues. Next, typical advanced control technologies including model predictive control, backstepping control, sliding-mode control, passivity-based control, disturbance estimation techniques, intelligent control, and nonlinear modeling approaches are reviewed. Then the applications of advanced control technologies in bidirectional dc/dc converters are presented for the stabilization of CPLs and accommodation of PPLs. Finally, advanced control techniques are explored in other high-gain nonisolated (e.g., interleaved, multilevel, cascaded) and isolated converters (e.g., dual active bridge) for high-power applications.

A novel protection scheme for low voltage DC microgrid using inductance estimation

Vulnerability of power electronic converters in DC microgrids when pole to ground and pole to pole fault occur necessitates using an effective fault detection and isolation. Most of the proposed fault detection methods use a communication system to exchange data among protection units, which could be an Achilles heel for the entire protection scheme. In this paper, fault detection based on estimation of equivalent inductance using simplified fault current equation is proposed. In the presented method, only local measurement is required and communication system is avoided due to reliability issues. In order to improve equivalent inductance estimation, an artificial line inductance (ALI) is implemented at both ends of each line. Since estimated equivalent inductance only depends on protection zone inductances including line inductance and ALIs, any change in topology of the microgrid does not affect operation of the protection scheme. By employing the proposed protection scheme, fault detection and isolation with a sampling frequency of 8000 Hz is achieved in less than 0.2 ms. A protection scheme should be able to disconnect the faulted section even if one of the main protection units fails; hence, by defining different protection zones for each protection unit, backup protection is realized. Eventually, to show significance of the proposed protection scheme, several case studies are investigated and results are compared with earlier studies.

A novel cascaded control to improve stability and inertia of parallel buck-boost converters in DC microgrid

To deal with the issues caused by the high integration of renewable resources, the parallel operation of the converters is generally required to improve the system stability and reliability. To support the parallel operation of buck-boost DC/DC converters, they are generally regulated by a cascaded control strategy, which includes droop control loop, and nested proportional-integral (PI) based voltage and current control loops. However, it can provide barely inertia support and needs precise system model to reach the desired performance. In order to overcome these drawbacks, this paper proposes a novel cascaded control strategy for parallel operation of buck-boost DC/DC converters. The proposed cascaded control strategy involves a voltage synchronization loop, a virtual inertia control (VIC) loop, and two nested NTSMC based voltage and current control loops. The voltage synchronization controller can enable the plug and play requirements by synchronizing the output voltage of the converter with the voltage at the point of common coupling (PCC) before connecting a DC/DC converter to the DC microgrid. The VIC can improve the inertia of the converter-interfaced DGs. The nested-loop NTSMC based voltage and current controller can improve the robustness of converter system. Hardware experiments demonstrate the improved performance of the proposed control strategy.

Energy Management with Bi-Directional Converters in DC Microgrids

The application of Bi-Directional converter has been well notified in Electric Vehicles and the energy flow between the Grid-2-Vehicle and Vehicle-2-Grid are flexible, reliable, efficient and with reduced number of hardware components as compared with conventional DC-DC converters. Using these benefits, the Bi-Directional Converters are used in the DC Microgrids to charge the storage elements in the grid connection mode and to discharge the storage elements in the islanded mode of operation. In this paper, the balanced energy flow between the Storage Elements (Charging Mode) and DC loads (Discharging Mode) using Bi-Directional Converters with Fuzzy control technique is proposed. With this system, the Demand Response objectives like minimization of energy cost and awareness to the social welfare are achieved. The proposed Bi-Directional Converters system is developed using MATLAB-Simulink software and the energy flow between the battery and utility grid or DC load waveforms are shown.

Output Impedance Modeling and High-Frequency Impedance Shaping Method for Distributed Bidirectional DC–DC Converters in DC Microgrids

Output impedance characteristics of interfaced converters of energy sources are the important indicators to assess the anti-interference ability of the dc microgrids. However, the output impedance is often approximately modeled by neglecting the impacts of delay units. In order to accurately study the anti-interference ability, the output impedance of an energy storage system is modeled with the consideration of the coupling characteristics of the current control gains and the delays caused by digital control and switching. The constraint mechanism of the coupling factors to impact the effective bandwidth of current loop in a load current feedforward system is also revealed. Then, an impedance shaping method is proposed to improve the anti-interference ability by optimally shaping the high-frequency output impedance, and also to improve system performances in terms of increasing the bandwidth of inner loop and the stable margin of outer loop. The simulation and experimental results both clearly validate the correctness and feasibility of the proposed impedance shaping method.

Current-Limiting Droop Control Design and Stability Analysis for Paralleled Boost Converters in DC Microgrids

In this brief, a novel current-limiting droop controller for paralleled dc-dc boost converters loaded by constant impedance Z, constant current I, or constant power P loads in a dc microgrid is proposed to guarantee closed-loop stability and power sharing. Using an improved version of the recently proposed nonlinear current-limiting controller, an inherent current-limiting property is guaranteed for each converter independently of the load type or magnitude variations. Then, sufficient conditions to ensure closed-loop stability for the entire dc microgrid system with a constant Z, I, or P load are analytically obtained. Hence, compared with existing droop control methodologies, the proposed controller ensures accurate power sharing, tight voltage regulation, and closed-loop stability with a current limitation when connected to Z, I, or P load, for multiple paralleled boost converters, which introduce nonlinear dynamics. To verify the effectiveness of the proposed controller and the stability analysis, simulation results for the three parallel operated dc-dc boost converters with Z, I, and P loads and experimental results for two parallel operated dc-dc boost converters with a P load are performed under several changes of the load power demand.

Output Capacitance Minimization for Converters in DC Microgrids via Multi-Objective Tuning of Droop-Based Controllers

This paper proposes a controller tuning methodology for voltage-current droop-based DC-DC converters in DC microgrids to reduce the output capacitance. This minimization is cost saving and implies lower fault currents. However, it leads to higher DC voltage variability during load transients, which requires an output impedance shaping by control means to reduce over or undershoot. The proposed control structure and problem definition simultaneously takes into account that the solution must achieve the impedance shaping, performance and stand-alone stability objectives. This comprises a multi-objective problem which is effectively formulated here and, then, solved by a non-smooth $\mathcal {H}_\infty $ optimization technique that tunes all free parameters. For comparison purposes, this tuning methodology is applied to several droop proposals, and the proposed droop is able to reduce the output capacitance of bidirectional buck-type and boost-type half-bridge converters by 37.5% and 23.08%, respectively, with respect to previous proposals. The designs are validated in time and frequency domains by means of theoretical analysis and experimental results on DC microgrid prototypes with bidirectional buck-type or boost-type half-bridge converters.

Operating Cost Reduction of DC Microgrids Under Real-Time Pricing Using Adaptive Differential Evolution Algorithm

Virtual resistance-based droop control is widely adopted as secondary-layer control for grid-connected converters in DC microgrids. This paper presents an alternative usage of the virtual resistances to minimize the total operating cost of DC microgrids under real-time pricing. The total operating cost covers the running cost of utility grids, renewable energy sources (RES), energy storage systems (ESS), fuel cells, and power loss on the distribution lines. An adaptive Differential Evolution (ADE) algorithm is adopted in this paper to optimize the virtual resistances of the droop control for the grid-connected converters of dispatchable units, such that the power flow can be regulated. The performances of the proposed strategy are evaluated by the case studies of a 12-bus 380 V DC microgrid using Matlab and a 32-bus 380 V DC microgrid using a Real-Time Digital Simulator (RTDS). Both results validate that the ADE can significantly reduce the operating cost of DC microgrids and outperform the conventional Genetic Algorithm (GA) in terms of cost saving. Comparisons among the microgrids with different numbers of dispatchable units reveal that the cost saving is more prominent when the expansion of dispatchable units.

Study of A Bus Voltage Control Strategy for Two-Way AC/DC Current Converter in DC Microgrid

In an isolated DC microgrid consisting of photovoltaic cells, an energy storage unit and load, reasonable control of grid-connected two-way AC/DC converter can effectively realize coordinated power distribution and bus voltage stabilization of the microgrid. A mathematical model of grid-connected converter was established first in this paper, followed by a simulation analysis of the grid-connected converter under direct voltage control. In consideration of the fact that the ideal dynamic voltage response could not be realized by conventional control of the grid-connected converter, a seamless switching control strategy of inhibiting voltage fluctuation was proposed based on the voltage fluctuation analysis of the AC/DC converter, which improved the robust stability of the DC microgrid. The effectiveness and feasibility of the proposed control strategy were verified through the simulation results.

Research and Simulation of DC Microgrid Three-Phase AC-DC Converter Control Strategy Based on Double Loop

The new voltage and current double loop control strategy is proposed to solve the DC microgrid bus voltage fluctuation caused by loads fluctuation, parameters perturbation and unbalanced three-phase power supply. Firstly, the $dq$ axis mathematical model of three-phase AC-DC bidirectional converter in DC microgrid is analyzed and established, and then the controllers are designed according to the $dq$ axis mathematical model. The outer loop is a voltage loop based on variable gain linear extended state observer (VGLESO) and sliding mode theory. VGLESO can not only effectively overcome the problem of peak output of traditional high-gain LESO in the initial stage of operation, and ensure that the system has good startup characteristics, but also quickly track and compensate the total disturbance of the system without additional current sensors. The inner loop is a current loop based on adaptive PI, which can eliminate the influence of system parameters perturbation on bus voltage and improve the system’s adaptability. Under the action of the inner and outer loops, the system has good dynamic and static characteristics. Finally, the feasibility and correctness of the control strategy are verified by Matlab/Simulink.

A novel control strategy for mode seamless switching of PV converter in DC microgrid based on double integral sliding mode control

With the photovoltaic (PV) penetration increases in dc microgrid, the traditional PV converters with maximum power point tracking (MPPT) control, which are equivalent to current sources, can hardly meet the needs of coordinated operation. The PV converter should operate in both MPPT and constant voltage control (CVC) modes. When the operation mode changes, the PV converter needs to switch between current and voltage sources. Inevitably, the bus voltage instability would be caused. This paper presents a novel switching strategy for the smooth and seamless operation of the PV converter between MPPT and CVC modes. The PV converter under these two modes is always controlled as a voltage source, and its control variables are both the output voltage of PV arrays. When its operation mode changes from MPPT to CVC, a small offset signal based on DC bus voltage would be introduced to the reference voltage of inner loop of the PV converter, so as to shift the maximum power point (MPP) voltage. Meanwhile in the voltage inner loop, a double integral sliding mode controller (DISMC) is used to improve the performance and anti-interference ability of the PV converter. This method unifies MPPT and bus voltage adjustment strategy without any communication and changing any hardware structure and control parameters. When mode switching occurs, only the reference value of voltage inner loop would be slightly adjusted near the maximum photovoltaic power point, so the seamless transition for PV converter between the two modes can be ensured. Finally, the effectiveness of the mode seamless switching control strategy is demonstrated by the simulation and experimental tests.