Converters In Microgrids Research Articles

Model predictive control—Based distributed control algorithm for bidirectional interlinking converter in hybrid microgrids

Model predictive control—Based distributed control algorithm for bidirectional interlinking converter in hybrid microgrids

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 Single-Objective Modulated Model Predictive Control for a Multilevel Flying Capacitor Converter in a DC Microgrid

This paper presents a single-objective modulated model predictive control for a bi-directional DC-DC flying capacitor (FC) converter in a microgrid. The presence of FC facilitates the converter to integrate a low-voltage battery to a high-voltage DC bus at reduced voltage stress on its power switches. The converter in such a configuration demands a multi-objective controller to accomplish DC bus and FC voltage regulations, and bi-directional power flow. The proposed controller realizes these multiple control objectives by determining optimum duty ratio for the power switches using a single-objective cost function based on battery current. In doing so, the converter realizes its multiple control objectives without weighting factors in the cost function and operates its power switches at a fixed switching frequency. The proposed controller also eliminates an additional control loop by utilizing an improved dynamic reference model to generate appropriate battery current reference for the DC bus voltage regulation and bi-directional power flow. Finally, the proposed system is validated experimentally under step-response of DC bus voltage, load, PV power, and system parameter variations, and compared with a finite control set model predictive control to prove its effectiveness.

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.

Analysis of the Droop Control Method in DC Microgrids with Regard to the Types of Load

The droop control approach divides load demand among voltage source converters in microgrids in a decentralized manner. Different factors, such as the microgrid line impedances, have an impact on the power sharing accuracy of the droop approach. This research evaluates the droop controller's power-sharing accuracy for the first time with respect to three different load types: constant impedance (CI), constant current (CC), and constant power (CP). Additionally, the impact of DC microgrid configuration on the accuracy of power sharing is investigated in this work. To support the conclusions, the simulation results in the Matlab/Simulink software environment are shown.

Seamless Switching Control Technology for the Grid-Connected Converter in Micro-Grids

In order to ensure the reliable power supply of the local load in the micro-grid (MG), a seamless switching control technology (SSCT) suitable for grid-connected converter (GCC) is proposed. This technology includes silicon-controlled rectifiers (SCR) forced shutdown control strategy (SCR-FSCS) and three-loop control strategy (TLCS). The SCR-SSCT adjusts the load voltage in real time to form a back voltage at the grid-connected inductor, which greatly reduces the SCR shutdown time and ensures the reliability of local load power supply. The TLCS can easily realize the switching between the current source mode and the voltage source mode of the GCC. An experimental platform is established to carry out the relevant experiments. The experimental results show the rationality and effectiveness of the theoretical analysis and the proposed control technology.

Equivalency between Adaptive Notch Filter PLL and Inverse Park PLL by Modeling and Parameter Adjustment

Synchronism algorithms play an important role in microgrid converters, and they are fundamental to the power flow control between the converters and the main grid. In island operation, synchronism algorithms provide important frequency and voltage data for converters to operate properly at steady state, and also connect and disconnect from the main grid. In this context, this work presents modeling and parameter design of two structures of single-phase PLLs: the inverse park PLL (Park-PLL) and the adaptive notch filter PLL (ANF-PLL). The derivation of the linearized model of Park-PLL, previously discussed in the literature, is detailed. In addition, as main contribution this paper, a linearized model of the ANF-PLL is presented and compared with the first one. As a result, it is possible to demonstrate equivalency between the two evaluated PLL structures when their parameters are adjusted by the proposed methodology. Practical and simulated results validate the parity of the PLL structures in terms of the dynamic response and steady-state error.

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.

Robust Linear Control of Boost and Buck-Boost DC-DC Converters in Micro-Grids with Constant Power Loads

Power distribution systems nowadays are highly penetrated by renewable energy sources, and this explains the dominant role of power electronic converters in their operation. However, the presence of multiple power electronic conversion units gives rise to the so-called phenomenon of Constant Power Loads (CPLs), which poses a serious stability challenge in the overall operation of a DC micro-grid. This article addresses the problem of enhancing the stability margin of boost and buck-boost DC-DC converters employed in DC micro-grids under uncertain mixed load conditions. This is done with a recently proposed methodology that relies on a two-degree-of-freedom (2-DOF) controller, comprised by a voltage-mode Proportional Integral Derivative (PID) (Type-III) primary controller and a reference governor (RG) secondary controller. This complementary scheme adjusts the imposed voltage reference dynamically and is designed in an optimal fashion via the Model Predictive Control (MPC) methodology based on a specialized composite (current and power) estimator. The outcome is a robust linear MPC controller in an explicit form that is shown to possess interesting robustness properties in a wide operating range and under various disturbances and mixed load conditions. The robustness and performance of the proposed controller/observer pair under steady-state, line, and mixed load variations is validated through extensive Matlab/Simulink simulations.

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.

Bidirectional DC-DC converter for microgrid in energy management system

ABSTRACT DC-DC converter plays a major role in microgrid and energy storage system using operational stability and synchronised power delivery. In this paper, an energy management control algorithm is proposed based on closed-loop proportional-integral (PI) to solve and analyse the issues of unstable DC link voltage. During grid-connected mode, a hybrid energy storage system (HESS), i.e. super-capacitor (SC) and battery units are charged from different hybrid energy resource, i.e. solar and wind. In the absence of energy resources (solar or wind), the energy storage units (supercapacitor and battery) need to supply the grid, which is the main aim of this work. Initially, the energy management control is used to control the generated power from wind and solar for the stability requirement. The MPPT technique has been considered here to make system flexibility and compatibility with different PV cell and wind energy. This proposed system successfully tracked the maximum power, which is analysed using the combined development of galactic swarm optimisation (GSO) based incremental conductance (IC) algorithm. Using GSO in MPPT should reduce the steady-state oscillation and get high convergence rate. The whole work is implemented in Matlab/Simulink platform and the performance of proposed is compared with existing algorithms. Abbreviation: HESS: Hybrid Energy Storage System; MPT: Maximum PowerPoint; PWM: Pulse Width Modulation; SOC: State of Charge; PMSG: Permanent Magnet Synchronous Generator; PI: Proportional Integral; AC: Alternating Current; DC: Direct Current; GSO: Galactic Swarm Optimization

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.

Advanced Control Methods for Power Converters in DG Systems and Microgrids

In recent years, different advanced control methods have been successfully proposed as alternatives to conventional cascaded linear controllers for power converters in distributed generation systems and microgrids. The prime movers of this research are strong capabilities of advanced controllers to improve the dynamic performance and robustness of power electronic converters in these applications. This article first introduces the key roles and functionalities of voltage source converters in distributed generation systems and microgrids. Then, it describes how these functionalities are traditionally achieved by using linear controllers, and addresses their fundamental dynamic performance limitations. Afterward, the most prominent advanced control methods are overviewed. In this context, the implementation principles, advantages, and disadvantages of prominent model- and data-based advanced control methods are critically discussed and experimentally compared. This article ends with a discussion about promising research directions in the area of advanced control for power electronic converters.

Modeling, Control, Estimation, and Optimization for Microgrids: A Fuzzy-Model-Based Method [Book News

This timely book explores the relevant challenges in microgrids such as modeling, control, protection, uncertainty, stability, centralized local and coordination control, power quality, and economic operation. It includes eleven chapters divided into four parts. The first part comprises three chapters and is devoted to fuzzy modeling and local control of microgrid converters. Part II covers centralized, decentralized, and distributed FLC architectures. The third part is devoted to the operational and optimization aspects of microgrid energy management. And finally, part fourt details several aspects related to cyberphysical systems’ control of microgrids and time delay switch cyberattacks. This book is very important for advanced students and scientists willing to further explore FLC theory. It is also a valuable tool for students, graduate-level teachers, and practicing professionals who need to understand and master microgrid models and controllers in their own way.

가상관성을 적용한 분산전원의 동 특성 분석

The distributed generations(DGs) such as wind turbine, photovoltaic(PV), fuel cells, and energy storage system(ESS) are normally connected through power conversion system interface to power system. The high penetration of DGs will bring significant challenges to the power system. The power conversion systems have no rotating inertia or damping mechanism, thus the inertia and damping can’t be provided to power system, which makes the power system have a low inertia, and weakness the frequency stability and regulation capability. In addition, there is a constant voltage constant frequency(CVCF) converter in microgrid. So, if a CVCF converter fails, the system can be shut down. In this paper, the virtual inertia was applied to the power conversion system of ESS and PV system for analyzing transient response characteristics when the disturbance occur in microgrids. Especially, when the microgrid operation mode was switched from the grid-connected mode to the islanded mode and CVCF converter fail, the transient response has been compared under with virtual inertia and without virtual inertia. The simulation results are presented using MATLAB/SIMULINK.

Design and implementation of high efficient dual control modular resonant converter for DC microgrid

A modular structured 12-kW resonant converter with effective zero voltage switching (ZVS) is proposed to integrate renewable energy lower voltage source to the microgrid. In this study, duty cycle control is adopted until ZVS fails and thereafter, sequential and parallel adjustment of duty cycle and the switching frequency is proposed for effective output voltage regulation. The proposed control logic ensures ZVS and output voltage regulation over wide load range even at very light loads compared to conventional control methods. Further, it maintains very high system efficiency with narrow range of frequency operation. Hence, it is easy to implement using a conventional digital controller. To minimise ripple current, each of the four converters is provided with 90 ° phase delayed gate pulses under the proposed control scheme. This study presents an optimal design of the tank circuit elements assimilating high permeable powder cores to reduce magnetic loss and operation with IGBT switches at an optimum point to reduce conduction loss for enhanced system efficiency. A 12-kW, 400–1600 V developed prototype is tested and experimental results are discussed.