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

Measurements, Models, Systems and Design

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Fault detection and location in DC systems from initial di/dt measurement

The use of DC for primary power distribution has the potential to bring significant design, cost and efficiency benefits to a range of power transmission and distribution applications. The use of active converter technologies within these networks is a key enabler for these benefits to be realised, however their integration can lead to exceptionally demanding electrical fault protection requirements, both in terms of speed and fault discrimination. This paper describes a novel fault detection method which exceeds the capability of many current protection methods in order to meet these requirements. The method utilises fundamental characteristics of the converter filter capacitance’s response to electrical system faults to estimate fault location through a measurement of fault path inductance. Crucially, the method has the capability to detect and discriminate fault location within microseconds of the fault occurring, facilitating its rapid removal from the network

European White Book on Real-Time Power Hardware in the Loop Testing : DERlab Report No. R- 005.0

The European White Book on Real-Time-Powerhardware-in-the-Loop testing is intended to serve as a reference document on the future of testing of electrical power equipment, with specifi c focus on the emerging hardware-in-the-loop activities and application thereof within testing facilities and procedures. It will provide an outlook of how this powerful tool can be utilised to support the development, testing and validation of specifi cally DER equipment. It aims to report on international experience gained thus far and provides case studies on developments and specifi c technical issues, such as the hardware/software interface. This white book compliments the already existing series of DERlab European white books, covering topics such as grid-inverters and grid-connected storag

Reverse Engineering of Short Circuit Analyses

The electrical distribution system has evolved with embedded computer systems that can better manage the electrical fault that occurred around the feeders. Such random events can affect the reliability indices of overall systems. Computerized management system for distribution operation has been improving with the advanced sensing technologies. The general research question is here to articulate is the responsiveness for utility crew to pinpoint the exact location of a fault based on the SCADA fault indicators from pole-mounted feeder remote terminal units (FRTUs). This has been a tricky question because it relies on the information received from the sensors that can conclude fault with logic\u27s of over currents. The merit of this work can benefit at large the grid reliability because of time-saving in searching the exact location of a fault. The main contribution of this thesis is to utilize the 3-phase unbalanced power flow method to incrementally search for narrowing the localization of electrical short circuits. This is known as the reversal of the typical short circuit approach where a location of the fault is presumed. The 3 topological configurations of simulation studied in this thesis exhibit the typical radial configuration of a distribution feeder have been researched based on unidirectional and bidirectional power flow simulation. The exact fault location is carried in two steps. Firstly, a bisection search algorithm has been employed. Secondly, an incremental adjustment to match the simulated currents of fault with the measurements is conducted. Finally, the sensitivity analysis of a search can be improved with the proposed algorithm that leads to matching of telemetered and calculated values. The analysis of exact fault location is carried in unidirectional and bidirectional flow of power. Distributed energy resources (DER) such as residential PV at a household level as well the wind energy changes affect the protective relaying within a feeder as well as the reconfigurability of the switching sequences. Furthermost, the bidirectionality of power flow in an unbalanced manner would also be a challenging issue to deal with the power quality in automation. Finally, the simulation results based on unidirectional and bidirectional power flow are extensively discussed along with the future scope

Impedance spectroscopy techniques for condition monitoring of polymer electrolyte membrane fuel cells

Energy continues to remain the spine of all human development. As we continue to make advances in various levels, the need for energy in quantity, and even more recently, quality, continues to increase. The fuel cell presents itself as a promising prospect to solve one of mankind’s current challenge - clean energy. The fuel cell is essentially an electrochemical conversion system which takes in fuel supply to produce electricity. Some key features make the fuel cell attractive as a power source. Firstly, its efficiency in practical applications is approximately 50% compared to the typical efficiency of 40% for a typical internal combustion engine [1]. Secondly, unlike the systems such as the internal combustion engine that typically releases carbon-monoxide which is a major greenhouse gas, the typical fuel cell system, produces just water and heat, alongside the useful electrical energy. These characteristics make it attractive as a clean energy supply capable of replacing the fossil-based supplies that are currently the mainstay. Unfortunately, the fuel cell is far cry from an ideal system. Despite significant advantages of the fuel cell as a power supply, various challenges still exist which have hindered its widespread acceptance and deployment. The fuel cell at its core is a highly multi-physics system and its operational intricacies makes it highly prone to a series of fault conditions. This begs the question of durability - an important requirement of a viable power source. Another challenge is the fact that humanity currently struggles with an efficient method of producing hydrogen which is the fuel of choice for the fuel cell. Given the promises of the fuel cell however, research efforts continue to increase to further improve its viability as an energy source competitive enough to meet mankind’s need of clean energy. This work presents results bordering on efficient diagnostic approaches for the fuel cell, aimed at improving the durability of the fuel cell. Particularly, two techniques targeted at improving the popular Electrochemical Impedance Spectroscopy (EIS) are presented. Conventional EIS takes significant amount of time, rendering it unsuitable for real-time diagnostics. Multi-frequency perturbation signals have been proposed to address this challenge. These however introduces concerns surrounding the accuracy of the resulting impedance measurement. Part of this work addresses some of the challenges with the fuel cell multi-sine impedance spectroscopy, such as measurement accuracy, by defining an optimized signal synthesis formulation. The proposed approach is validated in simulation and compared to the popular exponential frequency distribution approach using the appropriately defined error metric. Secondly, the chirp – as a frequency rich signal, is investigated as an alternative perturbation signal. Consequently, the use of the wavelet transform as an analysis tool of choice is presented. The characteristic nature of the chirp signal makes a broadband frequency sweep over time possible, hence enabling a faster impedance estimation. The resulting decomposition is harnessed for impedance calculation. The approach is tested in simulation and results for equivalent circuits are presented. It is shown that the resulting impedance spectrum well approximates the theoretical values. To further validate both techniques in practice, a low-cost active load is designed and built. The active load enables the injection of an arbitrary signal using the load modulation technique. The device is tested and benchmarked against commercial frequency response analyzer (FRA) using the conventional single sine EIS technique. Both approaches developed – the improved multi-sine scheme and the chirp signal perturbation are demonstrated with the aid of the active load on a single cell fuel cell station. Outcomes of the experiment show significant accuracy from the two techniques in comparison with results obtained from the FRA equipment which implements the single sine technique. In addition, the two schemes enabled impedance results to be taken in a few seconds, compared to conventional single sine EIS which takes several minutes. Impedance measurements are also carried out in the presence of two prominent faulty conditions – flooding and drying, using the developed techniques. This demonstrates the capability of the proposed system to perform real-time diagnostics of the PEMFC using impedance information

Feasibility of simultaneous intracranial EEG-fMRI in humans: a safety study

In epilepsy patients who have electrodes implanted in their brains as part of their pre-surgical assessment, simultaneous intracranial EEG and fMRI (icEEG-fMRI) may provide important localising information and improve understanding of the underlying neuropathology. However, patient safety during icEEG-fMRI has not been addressed. Here the potential health hazards associated with icEEG-fMRI were evaluated theoretically and the main risks identified as: mechanical forces on electrodes from transient magnetic effects, tissue heating due to interaction with the pulsed RF fields and tissue stimulation due to interactions with the switched magnetic gradient fields. These potential hazards were examined experimentally in vitro on a Siemens 3 T Trio, 1.5 T Avanto and a GE 3 T Signa Excite scanner using a Brain Products MR compatible EEG system. No electrode flexion was observed. Temperature measurements demonstrated that heating well above guideline limits can occur. However heating could be kept within safe limits (< 1.0 °C) by using a head transmit RF coil, ensuring EEG cable placement to exit the RF coil along its central z-axis, using specific EEG cable lengths and limiting MRI sequence specific absorption rates (SARs). We found that the risk of tissue damage due to RF-induced heating is low provided implant and scanner specific SAR limits are observed with a safety margin used to account for uncertainties (e.g. in scanner-reported SAR). The observed scanner gradient switching induced current (0.08 mA) and charge density (0.2 μC/cm2) were well within safety limits (0.5 mA and 30 μC/cm2, respectively). Site-specific testing and a conservative approach to safety are required to avoid the risk of adverse events

How to protect a wind turbine from lightning

Techniques for reducing the chances of lightning damage to wind turbines are discussed. The methods of providing a ground for a lightning strike are discussed. Then details are given on ways to protect electronic systems, generating and power equipment, blades, and mechanical components from direct and nearby lightning strikes

Analysis and synthesis of concepts for hybrid power electronic earth fault compensators for medium voltage grids

In den vergangenen Jahren ist das Energieübertragungssystem mit der Zunahme der Produktion angewachsen. Jedes Jahr werden daher neue Übertragungsnetze in Betrieb genommen und die bereits bestehenden Übertragungsnetzwerke werden erweitert. In der Vergangenheit wurde der Strom in großen Kraftwerken zentral erzeugt und vom Hochspannungsnetz übertragen. Jetzt werden jedoch zunehmend auch große Mengen der elektrischen Energie vom NS- und MS-Netz übertragen. Die Anlagen zur Nutzung der erneuerbaren Energien sind grundsätzlich auf der Mittelspannungseben angeschlossen. Die modernen Netze müssen somit nicht nur mit einer schwankenden Stromerzeugung sondern auch mit verschiedenen Fehlern umgehen können. Mit wachsender Netzausdehnung steigt auch die Wahrscheinlichkeit für einen Fehlereintritt. Folglich müssen neue Verfahren entwickelt werden, um die Zuverlässigkeit und Stabilität der Netze auch im Fehlermodus zu verbessern. Derzeit werden oft kompensierte MS-Netze zum Schutz vor einphasigen Erdfehlern verwendet, wobei der Neutralleiter entweder über eine Drossel oder einen Widerstand mit der Erde verbunden ist. Damit kann der Fehlerstrom begrenzt und die Netze im Fehlerfall weiter betrieben werden. Gleichwohl haben auch die modernen passiven Kompensationsanlagen Probleme mit der Abstimmgenauigkeit, den Abmessungen sowie aufgrund der Komplexität des Antriebssystems. Moderne leistungselektronische Kompensationsanlagen werden zunehmend in MS-Netzen eingesetzt, um die Blindleistung zu kompensieren und nichtlineare Lastströme zu filtern. Sie können außerdem verwendet werden, um den Fehlerstrom zu kompensieren und eine optimale Ausnutzung der Übertragungskapazitäten der Leitungen zu ermöglichen. Da diese innovativen leistungselektronischen Kompensationsanlagen bei relativ hohen Frequenzen arbeiten, können außerdem wertvolle Materialien wie Kupfer und Stahl, die für die 50-Hz-Drosseln notwendig sind, eingespart werden. Diese Arbeit widmet sich der Entwicklung eines Hochleistungs-MS-Wechselrichters sowie dessen zur Kompensation notwendigen Steuerungssystems. Der Kompensator dient dabei zur Eliminierung des einpoligen Erdfehlerstromes (Grund- und Oberschwingungskomponenten) und kann daher im Übertragungssystem als Äquivalent der Petersonspule oder des Widerstands betrachtet werden. Der auf der Hilbert-Transformation basierende Steueralgorithmus wird ebenfalls erörtert.In the last years, the power generation systems have increased constantly with the increase in production. Every year new distribution networks are put into operation. The already existing networks are expanded. Moreover, in the past the power had been generated centrally in large power plants and transmitted by the high-voltage transmission grid, now vast amounts of the electric energy are handled by the low- and medium-voltage grid. The renewable energy sources are basically united in medium voltage grids. The modern grids has to be able to handle the fluctuating power generation and various sort of faults. With the growing grids the fault chance increases. Consequently, the new methods have to be developed to improve the reliability and stability of the grids in fault modes. Currently, to protect from one-phase ground faults the compensated networks are used with the neutral connected with the ground through the reactor or resistor. It allows to limit the fault current and the networks be able to be operated. Nevertheless, the modern compensation devices have the problems with the tuning accuracy, dimensions and the complexity due to the drive system. The modern power electronic devices are used in MV grids to compensate the reactive power (STATCOMs) and to filter the non-linear loads currents. They could be used to compensate the fault current and to allow the optimal utilization of lines as well. Moreover, since these converters operate at relatively high frequencies, valuable materials like copper and steal, used for 50 Hz reactors, can be saved. This work is dedicated to the development of a high-power medium-voltage power converter and its control system. This converter is used to compensate the one-phase ground fault current (main and high frequency components) and therefore is considered as the equivalent of the reactor or resistor in the classical system. The control algorithm based on the Hilbert transformation is proposed as well