Low Voltage AC Microgrid Research Articles

Standards Commonly Used for Microgrids—A Research Project to Develop an Industry Microgrid Standard in Taiwan

This article presents standards commonly used for microgrid technology and practical tests of these standards before developing an industry microgrid standard. Standards of communication networks, Internet security, data and communications security, installation and protection of distributed generators and energy storage devices, electronic power interfaces of distributed energy resources, and interconnection of microgrids to electric power system are analyzed. The practical tests for the above standards are planned to perform at a low-voltage (380-V) AC microgrid test bed built by the Institute of Nuclear Energy Research, Taiwan. Microgrid configurations, installation of distributed generators and energy storage systems, operation conditions of the microgrid, microgrid energy management and control systems, and microgrid protection are mentioned in the proposed standard tests. The main contributions of this article are (i) to review and analyze common standards used for microgrids, (ii) to present research works from Taiwan to develop a microgrid standard for industry applications, and (iii) to propose practical tests of critical standards used for microgrids and how to perform these tests at a real-time low-voltage AC microgrid in Taiwan.

Hierarchical Control Scheme for Droop Controlled Parallel Three Phase Voltage Source Inverters in Low Voltage AC MicroGrid

The hierarchical control scheme for three phase parallel connected voltage source inverters (VSI), forming a low voltage AC MicroGrid (MG)is presented and analyzed in this paper. The proposed control scheme consists of two controllers, a local controller and a centralized controller. The local controller consists of inner control loops that are composed by a voltage and current proportional resonant controllers with selective harmonics compensation, and the power sharing controller that includes the droop control and the virtual output impedance loops in order to share properly the active and reactive power between the connected VSIs. In this system, the centralized controller is designed to restore the magnitude and the frequency deviations of the AC bus voltage produced by the power sharing controller. Simulation results for three parallel connected VSIs forming a low voltage AC MG that operates in islanded mode are provided to show the effectiveness of the proposed control scheme. Active and reactive power are properly shared as well as good harmonics compensation is achieved when both linear and non-linear loads are connected to the load bus.

A Generalised Fault Protection Structure Proposed for Uni-grounded Low-Voltage AC Microgrids

Abstract This paper presents three main configurations of uni-grounded low-voltage AC microgrids. Transient situations of a uni-grounded low-voltage (LV) AC microgrid (MG) are simulated through various fault tests and operation transition tests between grid-connected and islanded modes. Based on transient simulation results, available fault protection methods are proposed for main and back-up protection of a uni-grounded AC microgrid. In addition, concept of a generalised fault protection structure of uni-grounded LVAC MGs is mentioned in the paper. As a result, main contributions of the paper are: (i) definition of different uni-grounded LVAC MG configurations; (ii) analysing transient responses of a uni-grounded LVAC microgrid through line-to-line faults, line-to-ground faults, three-phase faults and a microgrid operation transition test, (iii) proposing available fault protection methods for uni-grounded microgrids, such as: non-directional or directional overcurrent protection, under/over voltage protection, differential current protection, voltage-restrained overcurrent protection, and other fault protection principles not based on phase currents and voltages (e.g. total harmonic distortion detection of currents and voltages, using sequence components of current and voltage, 3I0 or 3V0 components), and (iv) developing a generalised fault protection structure with six individual protection zones to be suitable for different uni-grounded AC MG configurations.

Available Fault Protection Methods of Ungrounded AC Microgrids Evaluated by Transient Simulation Results

This paper evaluates fault protection methods of ungrounded low-voltage (LV) AC microgrids (MGs) based on transient simulation results of a typical ungrounded LVAC microgrid. By considering operation characteristics of ungrounded MGs and a literature review on existing MG fault protection solutions in recent years, possible fault protection methods are proposed for an ungrounded AC MG. Transient simulation results of an ungrounded AC MG are obtained by line-to-line (LL) and line-to-ground (LG) faults, and operation transition tests of the microgrid between autonomous and grid-connected operation modes. Based on the simulation results, advantages and disadvantages of each ungrounded microgrid protection solution are highlighted. In order to get the optimal fault protection, combinations among some or all of possible fault protection solutions of an ungrounded LVAC microgrid are found out. As a result, main contributions of the paper contain: (i) proposing and analysing available fault protection solutions of ungrounded LVAC MGs, (ii) doing the transient simulations of a typical ungrounded microgrid under different disturbance cases, and (iii) suggesting the necessary combinations among proposed fault protection solutions of ungrounded MGs.

A novel fault protection system using communication-assisted digital relays for AC microgrids having a multiple grounding system

Abstract This paper develops a novel fault protection system for AC microgrids having a multiple grounding system. Communication-supported digital relays which have different protection modules are used for this novel microgrid protection system. The protection modules in a digital relay have various functionalities to protect AC microgrids from various fault types, such as: low/high-impedance ground faults and short-circuits. To effectively use the developed novel microgrid protection system, the first step is to reconfigure different microgrid structures into a standard microgrid configuration having a multiple-grounding system through using delta/wye-grounded or wye-grounded/wye-grounded transformers. Then, a fast–dependable–adaptable (FDA) fault protection algorithm is developed to protect AC microgrids from faults occurring at trunk lines, common buses or branch lines. At an islanded operation mode of AC microgrids, by using digital relays, a new protection module is designed to detect, locate, and classify the faults occurring at trunk lines and common AC buses, while other protection modules can use the existing protection methodologies (e.g. directional/non-directional overcurrent protection principles, etc.) to detect and clear the faults at source branches or load branches of the microgrids. At a grid-connected operation mode, standard overcurrent protection modules are applied for the FDA fault protection system. Simulation and experiment results obtained from various fault cases at a real low-voltage AC microgrid have validated the effective operation of the FDA fault protection system.

Control of inverters in a low-voltage microgrid with distributed battery energy storage. Part II: Secondary control

The first part of this work introduced a control architecture for a low-voltage AC microgrid with distributed battery energy storage working in isolated mode. A primary control layer, based on applying decoupled droop control methods to the inverters, was proposed and analyzed, demonstrating its capability to share power and keep frequency and voltage within prescribed ranges under load changes. This second part of the work extends the analysis of the primary control actuation and proposes a secondary control layer, supported over a communications network, to add improvements such as local reactive power dispatching and nominal frequency and voltage restoration.

Control of inverters in a low voltage microgrid with distributed battery energy storage. Part I: Primary control

This article proposes a control architecture for a low-voltage AC microgrid with distributed battery energy storage. The droop controlled inverters interact with the microgrid through the RL combination of their virtual resistive output impedance with the series impedance of a coupling transformer. To obtain independent association of frequency with active power, and voltage with reactive power, decoupled droop control techniques are proposed and analyzed. The resulting control can be adjusted for appropriate dynamic response without modifying the droop coefficients. The application in the microgrid in diverse operating conditions is verified by a detailed simulation. The resulting primary control layer offers the capability of being easily actuated from a secondary control layer for procedures such as flexible power dispatching and frequency restoration.