Keng-Yu Lien

Review on protection coordination strategies and development of an effective protection coordination system for DC microgrid

In this paper, common DC-fault detection methods have been reviewed. Effects of two line-to-line and line-to-ground fault types from various fault locations to operation of the DC system are presented. In addition, operation principle, strong and weak points of four main types of protection devices (including fuses, no-fuse circuit breakers, power-electronic protection devices and protective relays) are mentioned. Each different type of protection devices has the ability to protect and isolate different components of the DC microgrid (e.g. power converter, PV system, battery system, capacitor and others) under fault occurrences. The paper analyses possible protection coordination strategies of protection devices to ensure safety of any components in the DC microgrid. These summarized coordination strategies can be suitable for any DC microgrid configurations. In the next content, an effective protection coordination system of a real community-sized DC microgrid is developed, which use fast-acting fuses to replace no-fuse circuit breakers already installed at some certain locations in the DC microgrid. Aims of this improved protection coordination system are to shorten critical fault clearing time and get the cost effectiveness while still ensuring high selectivity, dependability and safety of the DC microgrid. As a result, the no-fuse circuit breakers placed at locations such as: output of PV arrays, output of the battery, output of the fuel-cell system, and terminals of power converters are efficiently replaced by the fast-acting fuses. Additionally, protection devices located at load feeders are suggested to use the relays to optimise the coordination time between main and back-up protection in case of faults occurring at the load feeders.

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

Abstract 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.

A fast computing algorithm for microgrid fault protection system using communication-assisted digital relays and initially experimental results

This paper presents a fast computing algorithm (FCA) for microgrid (μgrid) fault protection system that can online work with high adaptability and dependability functions and can get a critical fault clearing time shorter than two cycles beginning at fault inception points. Concretely, the FCA uses an overcurrent protection principle to protect branch lines that contain either loads or distributed energy sources in a μgrid system. In addition, in order to protect main lines in the μgrid from fault occurrences, a new fault protection method is developed through using communication-assisted digital relays. The main lines are understood as lines used to interconnect two or more distributed energy sources and not including any load branches along these lines. Two fundamental ways that are used to inspect operation of the H-grid protection system with the FCA are: (i) theoretical calculations from actual results of staged fault tests, motor-starting tests, and tests of μgrids operation transition between grid-connected and islanded modes which are conducted at a real low-voltage AC μgrid in Taiwan and (ii) doing different fault tests along with FPGA boards to evaluate the FCA from a lab environment. As a result, the FCA can quickly detect and identify different types of grounding fault and optimally isolate faulted sections. Fault clearing time of the FCA for occurrences at the main lines is about 0.5-2 cycles depending on the algorithms computation time and the communication time among the digital relays, while that at the branch lines is completely dependent on their fault current values.

A simplified and automated fault current estimation approach for grid-connected low-voltage AC microgrids

For a grid-connected AC microgrid (MG), when a fault occurs at the microgrid, adaptive overcurrent (OC) or directional overcurrent (DOC) relays must be activated as fast as possible to protect all distributed generation (DG) units and loads. Fault currents inside the grid-connected AC microgrid can be significantly varied because fault current contributions from the main grid and DG units are different depending on various fault locations, fault types, and high penetration of inverter-based distributed generators (IBDGs) and rotating-based distributed generators (RBDGs) in the microgrid. A traditional fault analysis method cannot be applicable for AC microgrids with the presence of both rotating-based distributed generators and inverter-based distributed generators. Therefore, this paper proposes a simplified and automated fault current estimation approach for grid-connected AC microgrids, which is effective to quickly and accurately calculate fault current contributions from IBDGs and RBDGs and the grid fault current contribution to any faulted sections in the microgrid. This simplified and automated fault current calculation approach is mainly focused on grid-connected and small-sized low-voltage (LV) AC microgrids with the support of communication system. Under the grid-connected operation mode of the MG, fault tripping current thresholds of adaptive OC/DOC relays can be properly adjusted by the proposed fault analysis method. In particular, relying on fault current distribution coefficients of IBDGs, RBDGs, and the utility grid, pick-up currents of the adaptive OC/DOC relays in the LV AC microgrid are instantly and accurately self-adjusted according to different MG configurations as well as the operation status of DG units during the grid-connected mode.