Protection of Renewable Energy Systems

The recent progress in renewable energy (RE) technologies has led to the erection of RE power plants (REPPs) up to the order of several hundred megawatts. Unlike their predecessors, which generally appeared in the form of dispersed generation (DG) coupled mainly with distribution systems, such large REPPs are naturally part of high-voltage transmission networks and hold non-negligible proportions of the generation. On the other hand, RE-based DGs are becoming pervasive in modern distribution systems. As a result, the fault ride-through (FRT) requirement has become an essential part of modern grid codes. This dissertation investigates the challenges brought about by the FRT requirement now affecting protection of systems with which REPPs are integrated. On the transmission level, it explores the performance of distance relays that are installed at an REPP substation and protect the neighboring line. The analyses are founded upon time-domain simulation of detailed REPP models with FRT capability. The studies include squirrel cage induction generator and doubly-fed induction generator (DFIG)-based wind farms, as well as full-scale converter-interfaced REPPs. The exclusive fault behavior of REPPs is scrutinized to identify possible relay maloperations and their root causes. The relay malfunctions revealed by this dissertation are restricted to systems with REPPs, and are not among the known distance relay failures that can occur in conventional power systems. If a communication link with minimal bandwidth requirement is in place, distance relays provide non-delayed fast tripping over the entire length of the line. This feature is retained by devising modified relaying algorithms. On the distribution level, the dissertation examines the effects of RE-based DGs on directional relays and on fault type classification methods. DFIG-based wind turbines constitute an appreciable portion of today's DG power. Conventional directional elements are shown to be adversely affected when a distribution system incorporates DFIG-based wind DG. An effective method is proposed to identify the fault direction using the waveshape properties of fault signals. Microgrids are the building blocks of future smart distribution systems. Protective devices of smart and fault-resilient microgrids are not expected to trip the healthy phases during unbalanced short-circuits. Thus, some utilities as well as relay manufacturers have started contemplating single- and double-pole tripping for distribution systems. Selective phase tripping demands fault type classification. This dissertation reveals that existing industrial methods that exploit the phase difference between sequence currents and the magnitudes of phase and sequence currents misidentify the fault type in microgrids that include photovoltaic and/or Type IV wind DGs. Using phase and sequence voltages, two new classifiers are proposed to determine the fault type for not only microgrids with different DGs, but for any three-phase system.1 yea

Power Electronics Applications in Renewable Energy Systems

The renewable generation system is currently experiencing rapid growth in various power grids. The stability and dynamic response issues of power grids are receiving attention due to the increase in power electronics-based renewable energy. The main focus of this Special Issue is to provide solutions for power system planning and operation. Power electronics-based devices can offer new ancillary services to several industrial sectors. In order to fully include the capability of power conversion systems in the network integration of renewable generators, several studies should be carried out, including detailed studies of switching circuits, and comprehensive operating strategies for numerous devices, consisting of large-scale renewable generation clusters

Intelligent voltage dip mitigation in power networks with distributed generation

Includes bibliographical references.The need for ensuring good power quality (PQ) cannot be over-emphasized in electrical power system operation and management. PQ problem is associated with any electrical distribution and utilization system that experiences any voltage, current or frequency deviation from normal operation. In the current power and energy scenario, voltage-related PQ disturbances like voltage dips are a fact which cannot be eliminated from electrical power systems since electrical faults, and disturbances are stochastic in nature. Voltage dip tends to lead to malfunction or shut down of costly and mandatory equipment and appliances in consumers’ systems causing significant financial losses for domestic, commercial and industrial consumers. It accounts for the disruption of both the performance and operation of sensitive electrical and electronic equipment, which reduces the efficiency and the productivity of power utilities and consumers across the globe. Voltage dips are usually experienced as a result of short duration reduction in the r.m.s. (r.m.s.- root mean square) value of the declared or nominal voltage at the power frequency and is usually followed by recovery of the voltage dip after few seconds. The IEEE recommended practice for monitoring electric power quality (IEEE Std. 1159-2009, revised version of June 2009), provides definitions to label an r.m.s. voltage disturbance based upon its duration and voltage magnitude. These disturbances can be classified into transient events such as voltage dips, swells and spikes. Other long duration r.m.s. voltage variations are mains failures, interruption, harmonic voltage distortion and steady-state overvoltages and undervoltages. This PhD research work deals with voltage dip phenomena only. Initially, the present power network was not designed to accommodate renewable distributed generation (RDG) units. The advent and deployment of RDG over recent years and high penetration of RDG has made the power network more complex and vulnerable to PQ disturbances. It is a well-known fact that the degree of newly introduced RDG has increased rapidly and growing further because of several reasons, which include the need to reduce environmental pollution and global warming caused by emission of carbon particles and greenhouse gases, alleviating transmission congestion and loss reduction. RDG ancillary services support especially voltage and reactive power support in electricity networks are currently being recognized, researched and found to be quite useful in voltage dip mitigation

Increasing the capacity of distributed generation in electricity networks by intelligent generator control

The rise of environmental awareness as well as the unstable global fossil fuel market has brought about government initiatives to increase electricity generation from renewable energy sources. These resources tend to be geographically and electrically remote from load centres. Consequently many Distributed Generators (DGs) are expected to be connected to the existing Distribution Networks (DNs), which have high impedance and low X/R ratios. Intermittence and unpredictability of the various types of renewable energy sources can be of time scales of days (hydro) down to seconds (wind, wave). As the time scale becomes smaller, the output of the DG becomes more difficult to accommodate in the DN. With the DGs operating in constant power factor mode, intermittence of the output of the generator combined with the high impedance and low X/R ratios of the DN will cause voltage variations above the statutory limits for quality of supply. This is traditionally mitigated by accepting increased operation of automated network control or network reinforcement. However, due to the distributed nature of RES, automating or reinforcing the DN can be expensive and difficult solutions to implement. The Thesis proposed was that new methods of controlling DG voltage could enable the connection of increased capacities of plant to existing DNs without the need for network management or reinforcement. The work reported here discusses the implications of the increasing capacity of DG in rural distribution networks on steady-state voltage profiles. Two methods of voltage compensation are proposed. The first is a deterministic system that uses a set of rules to intelligently switch between voltage and power factor control modes. This new control algorithm is shown to be able to respond well to slow voltage variations due to load or generation changes. The second method is a fuzzy inference system that adjusts the setpoint of the power factor controller in response to the local voltage. This system can be set to respond to any steady-state voltage variations that will be experienced. Further, control of real power is developed as a supplementary means for voltage regulation in weak rural networks. The algorithms developed in the study are shown to operate with any synchronous or asynchronous generation wherein real and reactive power can be separately controlled. Extensive simulations of typical and real rural systems using synchronous generators (small hydro) and doubly-fed induction generators (wind turbines) have verified that the proposed approaches improve the voltage profile of the distribution network. This demonstrated that the original Thesis was true and that the techniques proposed allow wider operation of greater capacities of DG within the statutory voltage limits

Data-driven Protection of Transformers, Phase Angle Regulators, and Transmission Lines in Interconnected Power Systems

This dissertation highlights the growing interest in and adoption of machine learning approaches for fault detection in modern electric power grids. Once a fault has occurred, it must be identified quickly and a variety of preventative steps must be taken to remove or insulate it. As a result, detecting, locating, and classifying faults early and accurately can improve safety and dependability while reducing downtime and hardware damage. Machine learning-based solutions and tools to carry out effective data processing and analysis to aid power system operations and decision-making are becoming preeminent with better system condition awareness and data availability. Power transformers, Phase Shift Transformers or Phase Angle Regulators, and transmission lines are critical components in power systems, and ensuring their safety is a primary issue. Differential relays are commonly employed to protect transformers, whereas distance relays are utilized to protect transmission lines. Magnetizing inrush, overexcitation, and current transformer saturation make transformer protection a challenge. Furthermore, non-standard phase shift, series core saturation, low turn-to-turn, and turn-to-ground fault currents are non-traditional problems associated with Phase Angle Regulators. Faults during symmetrical power swings and unstable power swings may cause mal-operation of distance relays, and unintentional and uncontrolled islanding. The distance relays also mal-operate for transmission lines connected to type-3 wind farms. The conventional protection techniques would no longer be adequate to address the above-mentioned challenges due to their limitations in handling and analyzing the massive amount of data, limited generalizability of conventional models, incapability to model non-linear systems, etc. These limitations of conventional differential and distance protection methods bring forward the motivation of using machine learning techniques in addressing various protection challenges. The power transformers and Phase Angle Regulators are modeled to simulate and analyze the transients accurately. Appropriate time and frequency domain features are selected using different selection algorithms to train the machine learning algorithms. The boosting algorithms outperformed the other classifiers for detection of faults with balanced accuracies of above 99% and computational time of about one and a half cycles. The case studies on transmission lines show that the developed methods distinguish power swings and faults, and determine the correct fault zone. The proposed data-driven protection algorithms can work together with conventional differential and distance relays and offer supervisory control over their operation and thus improve the dependability and security of protection systems

Communication based loss-of-mains protection method by frequency correlation

Due to the increasing penetration of distributed generation (DGs) in the distribution network in high numbers and proportions, and its conspicuous impact on power system stability. This occurs during a wide system disturbance in the power system, the DGs will start to disconnect from the main source in large proportions. This will further affect the power system stability and causes damages to its components and DGs. This thesis investigates in the reliability, security, and efficiency of satellite and internet communications, specifically for loss of mains (LOM) protection and exploring the strengths, the weaknesses, the feasibility of each type of communications, and the requirements of communication system components. By using communications network to send Phasor Measurement Unit (PMU) data to DGs protection equipment that are connected at remote areas all over UK, the LOM protection can be improved, obtain synchronization, precision, and coordination among power protection components. Satellite communication is chosen as it makes a better communication method when it comes to the installation, construction, urban disruption, time saving, and the installation and annual cost on every participant. However, the high latency issue is approached and solved by making a few changes in the communication protocol format and the data requirements to reduce the effect of latency to a level that can be tolerated. This thesis presents the development of a novel LOM protection method based on communication and frequency correlation. The stability and sensitivity assessment will show that this method is highly secure and reliable. It can also withstand a communication delay of 120ms without causing any nuisance tripping, and have a relay response to LOM operation of a maximum of 1s. The thesis also presents a novel method in time delay estimation that has been developed for power system applications. This method is called the Linear Trajectory Path (LTP) and its performance fulfils the LOM synchronisation requirements by succeeding in determining the time delay between the two data streams within the tolerated estimation error of ±100ms.Due to the increasing penetration of distributed generation (DGs) in the distribution network in high numbers and proportions, and its conspicuous impact on power system stability. This occurs during a wide system disturbance in the power system, the DGs will start to disconnect from the main source in large proportions. This will further affect the power system stability and causes damages to its components and DGs. This thesis investigates in the reliability, security, and efficiency of satellite and internet communications, specifically for loss of mains (LOM) protection and exploring the strengths, the weaknesses, the feasibility of each type of communications, and the requirements of communication system components. By using communications network to send Phasor Measurement Unit (PMU) data to DGs protection equipment that are connected at remote areas all over UK, the LOM protection can be improved, obtain synchronization, precision, and coordination among power protection components. Satellite communication is chosen as it makes a better communication method when it comes to the installation, construction, urban disruption, time saving, and the installation and annual cost on every participant. However, the high latency issue is approached and solved by making a few changes in the communication protocol format and the data requirements to reduce the effect of latency to a level that can be tolerated. This thesis presents the development of a novel LOM protection method based on communication and frequency correlation. The stability and sensitivity assessment will show that this method is highly secure and reliable. It can also withstand a communication delay of 120ms without causing any nuisance tripping, and have a relay response to LOM operation of a maximum of 1s. The thesis also presents a novel method in time delay estimation that has been developed for power system applications. This method is called the Linear Trajectory Path (LTP) and its performance fulfils the LOM synchronisation requirements by succeeding in determining the time delay between the two data streams within the tolerated estimation error of ±100ms

Grid-Connected Renewable Energy Sources

The use of renewable energy sources (RESs) is a need of global society. This editorial, and its associated Special Issue “Grid-Connected Renewable Energy Sources”, offers a compilation of some of the recent advances in the analysis of current power systems that are composed after the high penetration of distributed generation (DG) with different RESs. The focus is on both new control configurations and on novel methodologies for the optimal placement and sizing of DG. The eleven accepted papers certainly provide a good contribution to control deployments and methodologies for the allocation and sizing of DG

Computational Intelligence Application in Electrical Engineering

The Special Issue "Computational Intelligence Application in Electrical Engineering" deals with the application of computational intelligence techniques in various areas of electrical engineering. The topics of computational intelligence applications in smart power grid optimization, power distribution system protection, and electrical machine design and control optimization are presented in the Special Issue. The co-simulation approach to metaheuristic optimization methods and simulation tools for a power system analysis are also presented. The main computational intelligence techniques, evolutionary optimization, fuzzy inference system, and an artificial neural network are used in the research presented in the Special Issue. The articles published in this issue present the recent trends in computational intelligence applications in the areas of electrical engineering