Abdullah Emhemed

Analysis of Transient Stability Enhancement of LV-Connected Induction Microgenerators by Using Resistive-Type Fault Current Limiters

In this paper an analytical method by which the transient stability of an induction machine is maintained regardless of the fault clearance times is introduced. The method can be applied in order to improve the transient stability of a large penetration of low-voltage (LV) connected microgeneration that can be directly interfaced by single-phase induction generators within domestic premises. The analysis investigates the effectiveness of using resistive-type superconducting fault current limiters (RSFCLs) as remedial measures to prevent the microgenerators from reaching their speed limits during remote faults, and hence improving their transient stability. This will prevent unnecessary disconnection of a large penetration of LV-connected microgeneration and thus avoiding the sudden appearance of hidden loads, and unbalanced voltage conditions. The minimum required value of a resistive element of RSFCL for mitigating the transient instability phenomena of LV-connected microgeneration based on the system and connected machine parameters is determined. The analytical method has been validated by conducting informative transient studies by using detailed models of a small microwind turbine with constant mechanical output interfaced directly within residential dwellings by a single-phase induction generator, a transient model of resistive superconducting fault current limiter (RSFCL), and a typical suburban distribution network with residential loads. All the models are developed in the time-domain PSCAD/EMTDC dynamic simulation.

An Advanced Protection Scheme for Enabling an LVDC Last Mile Distribution Network

Low voltage direct current (LVDC) distribution systems have the potential to support future realization of smart grids and enabling of increased penetration of distributed renewables, electric vehicles, and heat pumps. They do, however, present significant protection challenges that existing schemes based on dc fuses and conventional electro-mechanical circuit breakers cannot manage due to the nature of dc faults and slow device performance. Therefore, this paper presents an advanced protection scheme that addresses the outstanding challenges for protecting an LVDC last mile distribution network. The scheme takes advantage of advanced local measurements and communications that will be naturally integrated in smart grids, and the excellent level of controllability of solid state circuit breakers. It thus provides fast dc fault detection and interruption during dc transient periods, in addition to achieving fault limitation and fast reliable restoration. The introductory part of the paper quantifies the potential benefits of LVDC last mile distribution networks, and discusses the potential LVDC architectures that best utilize the existing plant. Based on the new LVDC architectures, a typical U.K. LV network is energized using dc and modeled, and is used as a case study for investigating the protection issues and evaluating the new protection scheme performance through simulation.

Transient performance analysis of low voltage connected microgeneration

The growing awareness of the environmental impacts of large-scale thermal generating units has stimulated interest in microgeneration that is installed within domestic or commercial premises. This paper investigates the transient response to be expected from a range of microgeneration units that could typically be connected. The paper examines the impact of fault locations, typical fault clearance times and generator/prime mover technologies on the ability of microgenerators to maintain stability when subject to disturbances during and after clearing of both local low and remote medium voltage faults. The paper also presents the study of the step voltage changes occurring due to the simultaneous reconnection of a large number of microgenerators within a small area of the network. Two types of technologies are considered in this paper: a small diesel engine driving a three-phase synchronous machine connected within commercial premises; and a small microwind turbine interfaced directly within a residential dwelling by a single-phase induction generator.

Simulation-based validation of smart grids - status quo and future research trends

Smart grid systems are characterized by high complexity due to interactions between a traditional passive network and active power electronic components, coupled using communication links. Additionally, automation and information technology plays an important role in order to operate and optimize such cyber-physical energy systems with a high(er) penetration of fluctuating renewable generation and controllable loads. As a result of these developments the validation on the system level becomes much more important during the whole engineering and deployment process, today. In earlier development stages and for larger system configurations laboratory-based testing is not always an option. Due to recent developments, simulation-based approaches are now an appropriate tool to support the development, implementation, and roll-out of smart grid solutions. This paper discusses the current state of simulation-based approaches and outlines the necessary future research and development directions in the domain of power and energy systems.

Validation of Fast and Selective Protection Scheme for an LVDC Distribution Network

Low-voltage direct-current (LVDC) distribution systems potentially enable more efficient power distribution and wider uptake of distributed renewables and energy storage. They do, however, present significant fault protection and safety challenges. To address these, the use of advanced protection techniques or significant system redesign is required. This paper reviews these protection key challenges, and presents experimental results of a prototype advanced protection scheme designed to help enable LVDC distribution networks for utility applications. The developed scheme is DC current direction-based and uses multiple intelligent electronic devices relays in combination with controllable solid-state circuit breakers to detect and locate DC faults. This scheme provides selective protection tripping within submillisecond timescales. A scaled laboratory demonstrator that emulates an LVDC distribution network is used as a test platform. It allows for the characterization of transient behavior for various fault conditions and locations. The developed protection algorithm is implemented in LabVIEW, and its performance against such fault conditions is tested within this environment.

Protection analysis for plant rating and power quality issues in LVDC distribution power systems

Low Voltage DC (LVDC) distribution systems have the potential to be considered as an efficient platform for facilitating the connection of more distributed energy resources. The applications of LVDC are still at an early stage due to the lack of mature experience and standards. Over and above, the protection challenges that are presented by integrating DC installations in existing AC systems are one of the key issues that are delaying the wide uptake of LVDC technologies. In response to these issues, this paper discusses the international installation progress of LVDC systems and their relevant standards in different sectors. This includes data centres, buildings, and utility last mile distribution systems. The paper also investigates the impact of using traditional LV protection methods on the performance of a faulted LVDC network, and on the associated post-fault power quality performance. A typical UK LV network is energised using DC and modelled in PSCAD, and used for the protection studies under different DC fault conditions.

Overview paper on: low voltage direct current (LVDC) distribution system standards

Low Voltage Direct Current (LVDC) systems have recently been recognised as one of the key enabling technologies that can facilitate the connection of more distributed renewables with improved efficiency and enhanced controllability. This is in addition to the potential provision of increased power flow capacity which is required to meet the anticipated growth in electric transport and heat demand. However, there is still a shortage of mature experience and practical technical solutions that can support the uptake of such systems and increase commercial interest. One of the barriers is the lack of standards necessary to increase industry confidence and for the development of cost effective technical solutions that will accelerate the commercialization of LVDC technologies. Most of the existing international standards focus on alternating current (ac) systems with limited areas covering direct current (dc). Recently, new standard activities at national and international levels have begun to cover specific LVDC applications. However, it is still not clear whether these activities, in addition to existing standards, are sufficient and comprehensive to provide the necessary tools for best practice system design. This paper therefore reviews and evaluates the available LVDC standards within the context of the establish ac system to determine the state of the art of dc standardization and the areas where future work is required.

Cyber-physical energy systems modeling, test specification, and co-simulation based testing

The gradual deployment of intelligent and coordinated devices in the electrical power system needs careful investigation of the interactions between the various domains involved. Especially due to the coupling between ICT and power systems a holistic approach for testing and validating is required. Taking existing (quasi-) standardised smart grid system and test specification methods as a starting point, we are developing a holistic testing and validation approach that allows a very flexible way of assessing the system level aspects by various types of experiments (including virtual, real, and mixed lab settings). This paper describes the formal holistic test case specification method and applies it to a particular co-simulation experimental setup. The various building blocks of such a simulation (i.e., FMI, mosaik, domain-specific simulation federates) are covered in more detail. The presented method addresses most modeling and specification challenges in cyber-physical energy systems and is extensible for future additions such as uncertainty quantification.

Fault analysis of an active LVDC distribution network for utility applications

Low Voltage DC (LVDC) distribution systems are new promising technologies which can potentially improve the efficiency and controllability of existing LV distribution networks. However, they do introduce new challenges under different fault conditions. Therefore, this paper investigates the performances of an active LVDC distribution network with local solar photovoltaics (PVs) and energy storages under different short-circuit faulted conditions. A typical UK LV distribution network energized by DC is used as a test network, and modeled using PSCAD/EMTDC. The LVDC is interfaced to the main AC grid using fully controlled two-level voltage source converter (VSC), and supplies DC and AC loads through DC/DC converter and DC/AC converter respectively. The response of an LVDC with such converters combination with different topologies and fault management capabilities are investigated through the simulation analysis.

Using real-time simulation to assess the impact of a high penetration of LV connected microgeneration on the wider system performance during severe low frequency

In addition to other measures such as energy saving, the adoption of a large amount of microgeneration driven by renewable and low carbon energy resources is expected to have the potential to reduce losses associated with producing and delivering electricity, combat climate change and fuel poverty, and improve the overall system performance. However, incorporating a substantial volume of microgeneration within a system that is not designed for such a paradigm could lead to conflicts in the operating strategies of the new and existing centralized generation technologies. This paper investigates the impact of tripping substantial volumes of LV connected microgeneration on the dynamic performance of a large system during significant low frequency events. An initial dynamic model of the UK system based on a number of coherent areas as identified in the UK Transmission Seven Year Statement (SYS) has been developed within a real time digital simulator (RTDS) and this paper presents the early study results.