Standalone DC Microgrid Research Articles

SoC-Based Output Voltage Control for BESS with a Lithium-Ion Battery in a Stand-Alone DC Microgrid

This paper proposes a new DC output voltage control for a battery energy storage system (BESS) with a lithium-ion battery based on the state of charge (SoC). The proposed control scheme was verified through computer simulations for a typical stand-alone DC microgrid, which consists of a BESS, photovoltaic (PV) panel, engine generator (EG), and DC load. A scaled hardware prototype for a stand-alone DC microgrid was set up in the lab, in which the proposed control scheme was loaded in a DSP controller. The experimental results were compared with the simulation results for performance verification. The proposed control scheme provides relatively lower variation of the DC grid voltage than the conventional droop control.

ENGLISH

ENGLISH

독립형 DC 마이크로그리드에 적용 가능한 BESS의 SoC를 기반으로 한 새로운 출력전압 제어기법

This paper proposes a new output voltage control scheme based on the SoC variation of the battery energy storage system (BESS) applicable for the stand-alone DC microgrid. The proposed control scheme provides relatively lower variation of the DC grid voltage than the conventional droop method. The performance of proposed control scheme was verified through computer simulations for a typical stand-alone DC microgrid which consists of BESS, photo-voltaic (PV) panel, engine generator (EG), and DC load. A scaled hardware prototype for the stand-alone DC microgrid with DSP controller was set up in the lab, and the proposed control algorithm was installed in the DSP controller. The test results were compared with the simulation results for performance verification and actual system implementation.

태양광 기반의 가변속 하이브리드 시스템을 위한 직류 전압 제어

Recently, the interest in DC systems to achieve more efficient connection with renewable energy sources, energy storage systems, and DC loads has been growing extensively. DC systems are more advantageous than AC systems because of their low conversion losses. However, the DC-link voltage is variable during operation because of different random effects. This study focuses on DC voltage stabilization applied in stand-alone DC microgrids by means of voltage ranges, power management, and coordination scheme. The quality and stability of the entire system are improved by keeping the voltage within acceptable limits. In terms of optimized control, the maximum power should be tracked from renewable resources during different operating modes of the system. The ESS and VSDG cover the power shortage after all available renewable energy is consumed. Keeping the state of charge of the ESS within the allowed bands is the key role of the control system. Load shedding or power generation curtailment should automatically occur if the maximum tolerable voltage variation is exceeded. PSIM-based simulation results are presented to evaluate the performance of the proposed control measures.

Modeling and Control of the Distributed Power Converters in a Standalone DC Microgrid

A standalone DC microgrid integrated with distributed renewable energy sources, energy storage devices and loads is analyzed. To mitigate the interaction among distributed power modules, this paper describes a modeling and control design procedure for the distributed converters. The system configuration and steady-state analysis of the standalone DC microgrid under study are discussed first. The dynamic models of the distributed converters are then developed from two aspects corresponding to their two operating modes, device-regulating mode and bus-regulating mode. Average current mode control and linear compensators are designed accordingly for each operating mode. The stability of the designed system is analyzed at last. The operation and control design of the system are verified by simulation results.

A Multivariable Optimal Energy Management Strategy for Standalone DC Microgrids

Due to substantial generation and demand fluctuations in standalone green microgrids, energy management strategies are becoming essential for the power sharing and voltage regulation purposes. The classical energy management strategies employ the maximum power point tracking (MPPT) algorithms and rely on batteries in case of possible excess or deficit of energy. However, in order to realize constant current-constant voltage (IU) charging regime and increase the life span of batteries, energy management strategies require being more flexible with the power curtailment feature. In this paper, a coordinated and multivariable energy management strategy is proposed that employs a wind turbine and a photovoltaic array of a standalone DC microgrid as controllable generators by adjusting the pitch angle and the switching duty cycles. The proposed strategy is developed as an online nonlinear model predictive control (NMPC) algorithm. Applying to a sample standalone dc microgrid, the developed controller realizes the IU regime for charging the battery bank. The variable load demands are also shared accurately between generators in proportion to their ratings. Moreover, the DC bus voltage is regulated within a predefined range, as a design parameter.

Standalone DC microgrids as complementarity dynamical systems: Modeling and applications

It is well known that, due to bimodal operation as well as existent discontinuous differential states of batteries, standalone microgrids belong to the class of hybrid dynamical systems of non-Filippov type. In this work, however, standalone microgrids are presented as complementarity systems (CSs) of the Filippov type which is then used to develop a multivariable nonlinear model predictive control (NMPC)-based load tracking strategy as well as Modelica models for long-term simulation purposes. The developed load tracker strategy is a multi-source maximum power point tracker (MPPT) that also regulates the DC bus voltage at its nominal value with the maximum of ±2.0% error despite substantial demand and supply variations.

Adaptive Droop Control Strategy for Load Sharing and Circulating Current Minimization in Low-Voltage Standalone DC Microgrid

This paper addresses load current sharing and cir- culating current issues of parallel-connected dc-dc 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 dcgrid voltage due tothe 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 con- trol method minimizes the circulating current and current sharing difference between the converters based on instantaneous virtual resistance Rdroop .U singRdroop 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 dc-dc boost converters connected in paral- lel. The effectiveness of the proposed method is verified by detailed simulation and experimental studies.

Coordinated Droop Control for Stand-alone DC Micro-grid

This paper introduces a coordinated droop control for the stand-alone DC micro-grid, which is composed of photo-voltaic generator, wind power generator, engine generator, and battery storage with SOC (state of charge) management system. The operation of stand-alone DC micro-grid with the coordinated droop control was analyzed with computer simulation. Based on simulation results, a hardware simulator was built and tested to analyze the performance of proposed system. The developed simulation model and hardware simulator can be utilized to design the actual stand-alone DC micro-grid and to analyze its performance. The coordinated droop control can improve the reliability and efficiency of the stand-alone DC micro-grid.

A Downsizing Strategy for Combinatorial PMSG Based Wind Turbine and Micro-SMES System Applied in Standalone DC Microgrid

This paper presents a combinatorial standalone permanent magnet synchronous generator (PMSG) based variable speed wind turbine (VSWT) and small-size superconducting magnetic energy storage (SMES) system into the DC microgrid system. The principal purpose of SMES system is to preserve power balance by absorbing power during peak wind generation and to release it during low power generation. This work accomplished by describing the optimized design of the SMES solenoid coil, ensuring the desired energy storage capacity based on the simulated annealing (SA) algorithm. More importantly, the new control technique is developed for bi-directional DC-DC converter to level output power of the wind turbine depending on the demand thereby reducing the capacity of the DC-DC converter system. Detailed simulation studies implemented in PSCAD/EMTDC corroborate the superior robustness and balancing performance of the proposed micro-SMES controller with an optimal coil size under various situations including variable wind speed. This combination will result in “scaling-factors” knowledge through downsizing strategy which will lead to the most efficient system from cost cutting, energy savings, and downsizing viewpoints.

독립형 DC 마이크로그리드의 최적화 설계와 협조적 제어

This paper describes the coordinated droop control method for stand-alone type DC micro-grid to improve reliability and utilization of distributed generations and energy storage. The stand-alone type DC micro-grid consists of several distributed generations such as a wind power generation, solar power and micro-turbine, and energy storage. The proposed method which is based on autonomous control method shows high reliability and stability through coordinated droop control of distributed generations and energy storage and also capability of battery management. The operation of stand-alone type DC micro-grid was analyzed using detail simulation model with PSCAD/EMTDC software. Based on simulation results, a hardware simulator was built and tested with commercially available components and performance of system was verified.