Optimizing the roles of unit and non-unit protection methods within DC microgrids

The characteristic behavior of physically compact, multiterminal dc networks under electrical fault conditions can produce demanding protection requirements. This represents a significant barrier to more widespread adoption of dc power distribution for microgrid applications. Protection schemes have been proposed within literature for such networks based around the use of non-unit protection methods. This paper shows however that there are severe limitations to the effectiveness of such schemes when employed for more complex microgrid network architectures. Even current differential schemes, which offer a more effective, though costly, protection solution, must be carefully designed to meet the design requirements resulting from the unique fault characteristics of dc microgrids. This paper presents a detailed analysis of dc microgrid behavior under fault conditions, illustrating the challenging protection requirements and demonstrating the shortcomings of non-unit approaches for these applications. Whilst the performance requirements for the effective operation of differential schemes in dc microgrids are shown to be stringent, the authors show how these may be met using COTS technologies. The culmination of this work is the proposal of a flexible protection scheme design framework for dc microgrid applications which enables the required levels of fault discrimination to be achieved whilst minimizing the associated installation costs

A Review on Challenges and Solutions in Microgrid Protection

Protection of microgrid has become challenging due to the hosting of various actors such as distributed generation, energy storage systems, information and communication technologies, etc. The main protection challenges in the microgrid are the bi-directional power flow, protection blinding, sympathetic tripping, change in short-circuit level due to different modes of operation, and limited fault current contribution by converter-interfaced sources. This paper presents a comprehensive review of the available microgrid protection schemes which are based on traditional protection principles and emerging techniques such as machine learning, data-mining, wavelet transform, etc. A categorical assessment of the reviewed protection schemes is also presented. The key findings of the paper suggest that the time-domain and communication-assisted protection schemes could be suitable solutions to address the identified protection challenges in the microgrid

Protection of microgrid with high amounts of renewables : challenges and solutions

Microgrid is a small-scale network including generators, loads and storage system, which provides a friendly way for the penetration of renewables and releases the burden of transmission system arising from the increased energy demand. Moreover, since microgrid can operate in islanded mode, it can provide backup power to local consumers when the main grid is disconnected. However, the utilization of microgrid causes serious problems in the area of power system protection. The main issues comprise varied fault levels in different operating modes and fault detection in islanded microgrid particularly when the microgrid is dominated by inverter based DGs (IIDGs). In addition, to avoid non-necessary power losses raised from multi-stage power conversion of DC loads and generators, DC microgrid becomes another attractive choice, which further increases the difficult on designing protection system for the futuristic microgrid. In this paper, a comprehensive review of the existing issues and protection methods for AC and DC microgrids is presented. Furthermore, to facilitate better understanding to readers, the benefits and limitations of each method are discussed in depth. Potential protection tools for future microgrid are suggested at the end of this paper

Non-Pilot Protection of the Inverter-Dominated AC Microgrid

The main objective of this research is to develop reliable non-pilot protection and control strategies for the inverter-dominated microgrid. First, an improved Proportional-Derivative (PD) droop control strategy is proposed for enhanced disturbance response of the inverter-dominated AC microgrid. The proposed strategy significantly improves microgrid dynamic response and stability without requiring communication between distributed energy resources. Moreover, the impacts of large startup currents of induction motors on the stability and power quality of the inverter-dominated microgrid are investigates and recommendations for minimizing the associated adverse effects are made. Subsequently, a fast, selective, and reliable protection strategy for the inverter-dominated microgrid is introduced. The proposed protection strategy utilizes phase- and sequence-domain protective elements for reliable detection of symmetrical and asymmetrical faults without the need for communication signals or adaptive relays settings. The protection strategy is robust against the grid-connection mode of the microgrid and enables fuse protection of laterals. It can also be implemented on the existing commercially available relays. The acceptable performance of the proposed protection and control strategies is verified through numerous fault studies conducted on a realistic study system in the PSCAD/EMTDC software environment. Additionally, the proposed protection strategy is implemented in a SEL-351 relay and evaluated using the SEL-AMS industrial relay testing platform

Microgrid Renewable Energy Integration

The Microgrid is a small-scale electrical system that is designed to give Cal Poly students hands-on experience on power generation, system protection, distribution, and automation that would otherwise be very difficult to experiment in a large-scale model. To closely replicate the modern electrical grid, a renewable energy source shall be added to the Microgrid in conjunction with the existing synchronous generators. Electrical engineering student, Virginia Yan initiated this effort, namely Grid-Tied Solar System project [1], by designing and constructing a set of solar panels and microinverter for future connection to the Microgrid. The scope of Virginia’s project was, however, limited to designing and constructing the panels and microinverter. This Microgrid Renewable Energy Integration project aims to integrate the designed solar panels and microinverter to the Microgrid by testing the microinverter when running on islanded mode that replicates the Microgrid and eventually running with the Microgrid. The project develops test methods and solutions to enhance integration capability from the test results. In addition, this project implements basic power protection elements such as over-current and under-voltage. Protection schemes and monitoring are configured using Schweitzer Engineering Laboratories (SEL) relays, such as SEL-751 Feeder Protection Relay and SEL-735 Power Quality Meter. The success of the Microgrid Renewable Energy Integration project guarantees a smooth synchronization and secured operation of the microinverter to the Microgrid

Experimental Test bed to De-Risk the Navy Advanced Development Model

This paper presents a reduced scale demonstration test-bed at the University of Texas’ Center for Electromechanics (UT-CEM) which is well equipped to support the development and assessment of the anticipated Navy Advanced Development Model (ADM). The subscale ADM test bed builds on collaborative power management experiments conducted as part of the Swampworks Program under the US/UK Project Arrangement as well as non-military applications. The system includes the required variety of sources, loads, and controllers as well as an Opal-RT digital simulator. The test bed architecture is described and the range of investigations that can be carried out on it is highlighted; results of preliminary system simulations and some initial tests are also provided. Subscale ADM experiments conducted on the UT-CEM microgrid can be an important step in the realization of a full-voltage, full-power ADM three-zone demonstrator, providing a test-bed for components, subsystems, controls, and the overall performance of the Medium Voltage Direct Current (MVDC) ship architecture.Center for Electromechanic

Hardware-In-Loop Evaluation of Microgrid Protection Schemes

Distributed energy resources are becoming more common in distribution systems. Higher energy prices and increased interest in alternative energy sources are two of the driving forces behind this trend. Local utilities, however, anticipate very serious distribution system protection problems resulting from high penetration of these resources. The microgrid concept has been proposed as a possible solution to integrating distributed energy resources without adversely impacting the distribution system. Protection schemes have been proposed to work within this microgrid structure, but very little testing with real hardware is available. Without a practical solution for microgrid protection, backed by extensive studies, microgrids are unlikely to receive wide acceptance. This thesis outlines modeling of microgrids for protection testing using a real time digital simulator. In addition, the construction of a low voltage, low power, hardware-in-loop test bed using relays and an automation controller is detailed. The results of testing possible microgrid protection schemes using this apparatus are presented along with conclusions and suggestions for future work

System configuration, fault detection, location, isolation and restoration: a review on LVDC Microgrid protections

Low voltage direct current (LVDC) distribution has gained the significant interest of research due to the advancements in power conversion technologies. However, the use of converters has given rise to several technical issues regarding their protections and controls of such devices under faulty conditions. Post-fault behaviour of converter-fed LVDC system involves both active converter control and passive circuit transient of similar time scale, which makes the protection for LVDC distribution significantly different and more challenging than low voltage AC. These protection and operational issues have handicapped the practical applications of DC distribution. This paper presents state-of-the-art protection schemes developed for DC Microgrids. With a close look at practical limitations such as the dependency on modelling accuracy, requirement on communications and so forth, a comprehensive evaluation is carried out on those system approaches in terms of system configurations， fault detection, location, isolation and restoration

Increasing security of supply by the use of a local power controller during large system disturbances

This paper describes intelligent ways in which distributed generation and local loads can be controlled during large system disturbances, using Local Power Controllers. When distributed generation is available, and a system disturbance is detected early enough, the generation can be dispatched, and its output power can be matched as closely as possible to local microgrid demand levels. Priority-based load shedding can be implemented to aid this process. In this state, the local microgrid supports the wider network by relieving the wider network of the micro-grid load. Should grid performance degrade further, the local microgrid can separate itself from the network and maintain power to the most important local loads, re-synchronising to the grid only after more normal performance is regained. Such an intelligent system would be a suitable for hospitals, data centres, or any other industrial facility where there are critical loads. The paper demonstrates the actions of such Local Power Controllers using laboratory experiments at the 10kVA scale

An open platform for rapid-prototyping protection and control schemes with IEC 61850

Communications is becoming increasingly important to the operation of protection and control schemes. Although offering many benefits, using standards-based communications, particularly IEC 61850, in the course of the research and development of novel schemes can be complex. This paper describes an open-source platform which enables the rapid prototyping of communications-enhanced schemes. The platform automatically generates the data model and communications code required for an intelligent electronic device to implement a publisher-subscriber generic object-oriented substation event and sampled-value messaging. The generated code is tailored to a particular system configuration description (SCD) file, and is therefore extremely efficient at runtime. It is shown here how a model-centric tool, such as the open-source Eclipse Modeling Framework, can be used to manage the complexity of the IEC 61850 standard, by providing a framework for validating SCD files and by automating parts of the code generation process. The flexibility and convenience of the platform are demonstrated through a prototype of a real-time, fast-acting load-shedding scheme for a low-voltage microgrid network. The platform is the first open-source implementation of IEC 61850 which is suitable for real-time applications, such as protection, and is therefore readily available for research and education