The inherent unlimited high switching frequency of the sliding mode controller (SMC) is limited by practical constraints of the hysteresis modulation (HM) technique. The inductor current and output voltage of a converter can be regulated using a combination of HM-SMC. However, HM-SMC results in a variable switching frequency operation, which is not preferred due to Electromagnetic Interference (EMI) issues. In this paper, an interval fuzzy controller is designed and developed as a solution to enable HM-SMC. In addition, a robust sliding surface is proposed, which provides an improved dynamic response. The two proposed controllers’ compatibility with one another has been tested via experiments such as a step change in input voltage, load resistance variation, and finally, a step change in output voltage reference value. The test results validate that while the interval type-2 fuzzy maintains a constant switching frequency with acceptable dynamic responses, it successfully regulates the state variables of the system. A comparison of the performance of the proposed control method with existing techniques in the literature is presented.

Ultracapacitors are energy storage devices that have shown outstanding capability in a vast spectrum of applications, mainly in energy storage systems required to deliver short bursts of electrical energy. Ultracapacitors possess high power density while batteries possess high energy density. In this paper, a hybrid energy storage device comprising a lithium-ion ultracapacitor module and a lead acid battery was modeled, built, and tested for vehicular start–stop application, which requires a much larger number of engine cranking events than conventional vehicles. The combination of a lead acid battery with Li-ion ultracapacitors was chosen due to the fact that the vast majority of vehicles utilize lead acid batteries to crank the internal combustion engine. This allows retrofitting this hybrid setup in conventional vehicles along with the start–stop feature without inflicting damage to the already installed lead acid battery. The start–stop feature puts high stress on the lead acid battery, contributing to its faster aging. This feature is commonly found in hybrid vehicles to save the unnecessarily burned fuel during idling. This paper discusses aging of the lead acid battery as a result of being used in hybrid vehicles equipped with start–stop when used alone versus when used in the hybrid setup. The paper shows cranking tests performed on a number of cars to obtain voltage, current, power, and energy requirements for combustion engine cranking. Mathematical derivation, analysis, and an energy storage age estimation method are also presented. A set of cranking events followed by capacity checks performed on two automobile energy storage systems, one being a lead acid battery alone and the other being the proposed hybrid module, show the advantage of integrating the ultracapacitor module with the lead acid battery to extend its life span almost fivefold in a hybrid automobile.

In this study, an extended Lyapunov-function-based control strategy that assures global asymptotic stability is proposed for single-phase UPS inverters. The Lyapunov function is formed from the energy stored in the inductor and capacitor due to the fact that the system states converge to the equilibrium point if the total energy is continuously dissipated. It is shown analytically that the classical Lyapunov-function-based control leads to a globally asymptotically stable system at the expense of steady-state errors in the output voltage, which exist due to the lack of outer voltage loop in the control input. Therefore, an extended Lyapunov-function-based control strategy is proposed, which eliminates the steady-state error without destroying the global stability of the closed-loop system. The steady state and dynamic performance of the proposed control strategy has been tested by simulations and experiments under resistive and diode bridge rectifier loads. The results obtained from a 1-kW inverter demonstrate that the developed control strategy not only offers global stability, but also leads to good quality sinusoidal voltage with a reasonably low THD, almost zero steady-state error in the output voltage, and fast dynamic response under linear and nonlinear loads.

A microgrid can be defined as a grid of interconnected distributed energy resources, loads and energy storage systems. In microgrid systems containing renewable energy resources, the coordinated operation of distributed generation units is important to ensure the stability of the microgrid. A microgrid needs a successful control scheme to achieve its design goals. Undesirable situations such as distorted voltage profile and frequency fluctuations are significantly reduced by installing the appropriate hardware such as energy storage systems, and control strategies. The multi-agent system is one of the approaches used to control microgrids. The application of multi-agent systems in electric power systems is becoming popular because of their inherent benefits such as autonomy, responsiveness, and social ability. This study provides an overview of the agent concept and multi-agent systems, as well as reviews of recent research studies on multi-agent systems’ application in microgrid control systems. In addition, a multi-agent-based controller and energy management system design is proposed for the DC microgrid in the study. The designed microgrid is composed of a photovoltaic system consisting of 30 series-connected PV modules, a wind turbine, a synchronous generator, a battery-based energy storage system, critical and non-critical DC loads, the grid and the control system. The microgrid is controlled by the designed multi-agent-based controller. The proposed multi-agent-based controller has a distributed generation agent, battery agent, load agent and grid agent. The roles of each agent and communication among the agents are designed properly and coordinated to achieve control goals, which basically are the DC bus voltage quality and system stability. The designed microgrid and proposed multi-agent-based controller are tested for two different scenarios, and the performance of the controller has been verified with MATLAB/Simulink simulations. The simulation results show that the proposed controller provides constant DC voltage for any operation condition. Additionally, the system stability is ensured with the proposed controller for variable renewable generation and variable load conditions.

This paper proposes a control scheme based on an optimal triple phase-shift (TPS) control for dual active bridge (DAB) DC–DC converters to achieve maximum efficiency. This is performed by analyzing, quantifying, and minimizing the total power losses, including the high-frequency transformer (HFT) and primary and secondary power modules of the DAB converter. To analyze the converter, three operating zones were defined according to low, medium, and rated power. To obtain the optimal TPS variables, two optimization techniques were utilized. In local optimization (LO), the offline particle swarm optimization (PSO) method was used, resulting in numerical optimums. This method was used for the low and medium-power regions. The Lagrange multiplier (LM) was used for global optimization (GO), resulting in closed-form expressions for rated power. Detailed analyses and experimental results are given to verify the effectiveness of the proposed method. Additionally, obtained results are compared with the traditional single phase-shift (SPS) method, the optimized dual phase-shift (DPS) method, and TPS method with RMS current minimization to better highlight the performance of the proposed approach.

Online adaptive protection scheme provides an appropriate protection to microgrids for various fault conditions irrespective of bidirectional power flow and changing network topology in a grid-connected or islanded mode of operation. The success of this scheme can, however, be undermined as a result of the operational uncertainties caused by the failure in various components in protection system: namely, communication links and circuit breakers. This paper proposes a framework for assessing the impact of such uncertainties on the reliability of centralized adaptive microgrid protection schemes. It first derives appropriate Markov chain models to describe the dynamic uncertainties associated with lines, communication links, and circuit breakers. The Markov models are then used in a Monte Carlo algorithm to evaluate the reliability indices for an adaptive protection scheme. The proposed framework is tested on a sample microgrid in Simulink-Matlab and Java Agent DEvelopment platform, assuming that the respective communication middleware is carried out by multi-agent and communication simulator. It has been shown that the selection of communication technology in an adaptive protection scheme requires special attention to its failure rate and data transmission bit error as well as data transfer speed. In fact, communication technologies with high failure rate and bit error show poor reliability indices that can cause unwanted delays in fault clearance, particularly when GOOSE time sequence latency is considered in the analysis. Besides, an increase in the fault duration can increase the electromagnetic and thermal stresses inflicted on circuit breakers, further undermining the reliability of the protection system.

The power and voltage levels of renewable energy resources is growing with the evolution of the power electronics and switching module technologies. For that, the need for the development of a compact and highly efficient solid-state transformer is becoming a critical task in-order to integrate the current AC grid with the new renewable energy systems. The objective of this paper is to present the design, implementation, and testing of a compact multi-port solid-state transformer for microgrid integration applications. The proposed system has a four-port transformer and four converters connected to the ports. The transformer has four windings integrated on a single common core. Thus, it can integrate different renewable energy resources and energy storage systems. Each port has a rated power of 25kW, and the switching frequency is pushed to 50kHz. The ports are chosen to represent a realistic industrial microgrid model consisting of grid, energy storage system, photovoltaic system, and load. The grid port is designed to operate at 4.16kVAC corresponding to 7.2kV DC bus voltage, while the other three ports operate at 500VDC. Moreover, the grid, energy storage and photovoltaic ports are active ports with dual active bridge topologies, while the load port is a passive port with full bridge rectifier one. The proposed design is first validated with simulation results, and then the proposed transformer is implemented and tested. Experimental results show that the designed system is suitable for 4.16kVAC medium voltage grid integration.

In this article, a new topology for a grid-connected solar photovoltaic inverter for the direct connection to the medium-voltage grid is proposed. This topology employs an LLC resonant converter with a high-frequency (HF) isolation transformer in the dc–dc stage. The output of the dc–dc stage is a rectified sine wave voltage and current at the line frequency. An unfolder inverter interfaces between this dc stage and the grid. A combined phase-shift and frequency control method is used to control the LLC resonant converter. The phase-shift angle and switching frequency values of the LLC resonant converter are regulated to track the reference current signal for the whole operation range. The Lagrange multiplier method is applied to find the optimal trajectory to calculate the optimal phase-shift angle and switching frequency pairs for any operation condition by considering power converter and HF transformer losses to achieve the highest efficiency at a varying current. The transformer leakage and magnetization inductances are properly designed to provide a zero-voltage switching (ZVS) for a wide operation area, and additional resonant inductor requirement is removed. The LLC converter operates in a ZVS region except in a narrow band around the zero-current crossings of the inverter output. Using an HF transformer in the LLC resonant converter, a bulky line frequency transformer requirement is eliminated, and thus, a more compact and efficient design is obtained. The proposed topology is validated by the simulation and experimental results.

The increasing viability of wide band gap power semiconductors, widespread use of distributed power generations, and rise in power levels of these applications have increased interest and need for medium voltage converters. Understanding the definitions of insulation coordination and their relationship to applications and methodologies used in the test environment allows system engineers to select the correct insulation materials for the design and to calculate the required distances between the conductive surfaces, accessible parts and ground accurately. Although, design guidelines are well established for low voltage systems, there are some deficiencies in understanding and meeting the insulation coordination requirements in medium voltage, medium frequency applications. In this study, an overview on standards for insulation coordination and safety requirements is presented to guide researchers in the development of medium voltage power electronic converters and systems. In addition, an insulation coordination study is performed as a case study for a medium frequency isolated DC/DC converter that provides conversion from a 13.8 kV AC system to a 4.16 kV AC system.

A decentralized adaptive scheme is proposed in this article for protection coordination for microgrids with topological and operational uncertainties. This protection coordination scheme is performed in two stages. In the first stage, a traditional fault protection is deployed to clear the fault. If the fault is isolated successfully, the protection system returns to the regular fault-free state. If the fault is not cleared due to operational uncertainties in the protection system, the second stage of protection coordination is initiated. In this stage, based on a federation structure, a set of agents around the fault location negotiate to reach the best protection coordination strategy upon the occurrence of a single or multiple fault(s). Considering the operational uncertainties of the respective circuit breakers and communication links, the best fault clearance strategy is determined by analyzing the probability of correct operation of all proposed strategies and considering the least number of probable load outages. Two major factors impacting the probability of correct operation are fault currents flowing through the respective circuit breakers and the latency of the communication links. The efficiency of the proposed scheme is demonstrated by comparing the simulation results with their experimental counterparts and those obtained using the conventional centralized and decentralized adaptive methods.

Bidirectional LLC resonant converters are becoming increasingly more attractive in grid-connected applications with integrated photovoltaic (PV) systems and energy storage systems (ESS). In this study, a multiport bidirectional LLC resonant converter for grid-tied systems is proposed. In addition, a region based control schema with a modified maximum power point tracking (MMPPT) control derived from the incremental conductance method is introduced to control both the forward (discharge mode) and backward (charge mode) operation of the proposed converter. The proposed topology consists of an integrated bidirectional converter for the battery system, a bidirectional LLC resonant converter, a voltage source inverter for grid-tied operation and the PV system. The proposed topology is modelled with MATLAB/Simulink and validated for different operation conditions.

In this study, analysis and control of a highly efficient, high-power full-bridge unidirectional resonant LLC solid-state transformer (SST) are discussed. A combination of pulse frequency modulation and phase-shift modulation is utilized to control this resonant converter for a wide load range. The converter is designed to maintain soft switching by using a resonant circuit to minimize the switching loss of the high-frequency converter. Zero-voltage-switching (ZVS) is achieved for the H-bridge converter. The ZVS boundary for the proposed combined control method is also analyzed in detail. The experimental setup for the suggested configuration was implemented, and the performance of the proposed control scheme and resonant LLC SST have been verified with test results. The proposed combined control scheme improves control performance. The obtained results show that, the proposed system can regulate output voltage and maintain soft switching in a wide range of load. Thus, the efficiency of the system is improved and an efficiency of 97.18% is achieved.

In this article, a four-port solid-state transformer and a control scheme to control the power flow and output voltage are studied, developed, and tested. The converter consists of three ports with H-bridge converters and one port with a diode bridge rectifier. The ports with H-bridge converters are capable of bidirectional power routing as well as reactive power contribution in a volt/Var control scenario. The diode bridge rectifier provides a dc voltage for load connection. Different arrangements for the four transformer windings are analyzed, simulated, and compared to achieve an optimal design. A new control strategy, which uses a combination of phase-shift and duty cycle control, is employed to control the flow of power between the converter branches and to regulate the output voltage. While phase-shift control ensures the balance of power on each port based on a reference value, the duty cycle control keeps the load voltage at a desired voltage level. A high-level control scheme is employed to determine the power references for all ports according to the load demand, generation capacity of the distributed generation system, and state of charge of the energy storage. The performance of the proposed system is validated with simulation and experimental analysis. A prototype is designed and built with 10-kW power rating at each port. The operating frequency of the system is designed at 100 kHz to obtain a very compact size for the whole converter.

“Water Withdrawal and Consumption Reduction for Electrical Energy Generation Systems“

Water is the greatest resource for life on earth. Various human activities affect the quality and quantity of this precious resource, and there are many initiatives to ensure water resources are protected from overuse, pollution, and industrial and agricultural waste. Since the energy sector is the second largest consumer of water after agriculture, water and energy systems are highly interlinked. Specifically, a significant amount of water is used in the energy generation process primarily for producing steam and for cooling processes, the water used for cooling processes will be returned back to the reservoir. Consequently, most fossil-based power plants in addition to consuming water, impact the water resources by raising the temperature of water withdrawn for cooling. Limited water resources can also affect the ability to generate electric power to meet the demand. Therefore, integrated planning for the interleaved energy and water sectors is essential for both water and energy savings. This paper describes a comprehensive study that analyzes and quantifies water withdrawals and consumption of various electricity generation sources such as coal, natural gas and renewable sources. The study has developed a general model to determine the water consumption and impact for various energy generation scenarios and to minimize the amount of water consumption while considering several limitations and restrictions. A case study performed for the state of California indicates that quantification of water consumption can be formulated and potential opportunities for water saving can be identified.

“Development of an Equivalent Circuit for Batteries Based on a Distributed Impedance Network“

This paper presents an original equivalent battery model based on a distributed RC circuit. The model virtually includes an infinite number of time constants to reflect the battery behavior, especially during the dynamic stress test. The model has much fewer parameters than the traditional battery model and so reduces the parameterization effort significantly. The self-discharge is inherently implemented in the model as well. The second order partial differential equation of the model is derived and both explicit and numerical solution approaches are introduced to incorporate the model in any simulation environment or analytical response analysis. Incorporating the effect of the state of charge on the parameters of this model is more easily accomplished by changing the trends of the parameters as opposed to each of them individually. The model matches to various battery type and it is validated on a 30 Ah Lithium-Ion battery from 1 C to 8 C discharge pulse current. Furthermore, the results are compared to the classic battery model.

“Chance-Constrained Optimization of Energy Storage Capacity for Microgrids“

The optimal storage capacity is a crucial parameter for stable and reliable operation of microgrids in an islanded mode. In this context, an analytical method is developed to robustly formulate and analyze energy storage capacity deploying chance constrained stochastic optimization. More specifically, the goal is to determine an appropriate size for an energy storage to reach a specific loss of load probability (LOLP) in a microgrid with large penetration of renewables considering generation and load forecast error. The total cost is minimized over optimal storage capacity as well as over generators power, while accounting for generation and storage power and energy constraints. It is postulated that the shortage/surplus power will be derived from/injected to the storage system. However, due to stochastic nature of load and renewables and an inevitable forecast error, the renewable generation output or the load power may not be accurately acquired. Thus, the total storage power and energy constraints are posed as chance constraints, for which conservative convex approximations are employed for tractability. In particular, to overcome the difficulty brought about by the large size of the optimization problem, a separable (distributed) structure is pursued, and the dual decomposition method is adopted to obtain optimal solutions. Numerical tests verify the effect of prior knowledge in modeling the uncertainty in optimal choice of storage capacity.

“Data-driven predictive model of reliability estimation using degradation models: a review“

The concept of the industrial internet of things (IIoT) provides the foundation to apply data-driven methodologies. The data-driven predictive models of reliability estimation can become a major tool in increasing the life of assets, lowering capital cost, and reducing operating and maintenance costs. Classical models of reliability assessment mainly rely on lifetime data. Failure data may not be easily obtainable for highly reliable assets. Furthermore, the collected historical lifetime data may not be able to accurately describe the behavior of the asset in a unique application or environment. Therefore, it is not an optimal approach anymore to estimate the reliability based on classical models. Fortunately, most of the industrial assets have performance characteristics whose degradation or decay over the operating time can be related to their reliability estimates. The application of the degradation methods has been recently increasing due to their ability to keep track of the dynamic conditions of the system over time. This study reviews general approaches for the most important degradation-based reliability estimation models proposed by several researchers during last few decades. The most commonly applied deterministic and stochastic degradation models are reviewed in this study. Furthermore, a roadmap for adopting the degradation-based reliability estimation models based on the concept of the IIoT is explained in detail.

“Analytical Study Based Optimal Placement of Energy Storage Devices in Power Systems to Support Voltage and Angle Stability“

Larger penetration of Distributed Generations (DG) in the power system brings new flexibility and opportunity as well as new challenges due to the generally intermittent nature of DG. When these DG are installed in the medium voltage distribution systems as components of the smart grid, further support is required to ensure a smooth and controllable operation. To complement the uncontrollable output power of these resources, energy storage devices need to be incorporated to absorb excessive power and provide power shortage in time of need. They also can provide reactive power to dynamically help the voltage profile. Energy Storage Systems (ESS) can be expensive and limited number of them can practically be installed in distribution systems. In addition to frequency regulation and energy time shifting, ESS can support voltage and angle stability in the power network. This paper applies a Jacobian matrix-based sensitivity analysis to determine the most appropriate node in a grid to collectively improve the voltage magnitude and angle of all the nodes by active/reactive power injection. IEEE 14, 24, and 123-bus distribution system are selected to demonstrate the performance of the proposed method. As opposed to most previous studies, this method does not require an iteration loop with a convergence problem nor a network-related complicated objective function. 

“A Dual Half-Bridge LLC Resonant Converter With Magnetic Control for Battery Charger Application“

In this paper, a dual half-bridge LLC resonant converter with magnetic control is proposed for the battery charger application. The primary switches are shared by two LLC resonant networks, and their outputs are connected in series. One of the LLC resonant converters is designed to operate at the series resonant frequency, which is also the highest efficiency operating point, and the constant output voltage characteristic is achieved at this operating point. The second LLC resonant converter adopts magnetic control to regulate the total output current and voltage during both constant current charge mode and constant voltage charge mode. Meanwhile, the function decoupling idea is adopted to further improve the system efficiency. The significant amount of the power is handled by the LLC resonant converter operating at the series resonant frequency, whereas the second LLC resonant converter fulfills the responsibility to achieve closed-loop control. By carefully designing the resonant networks, the zero-voltage switching for primary switches and zero-current switching for secondary diodes can be achieved for whole operation range. A 320-W experimental prototype is built to verify the theoretical analysis, and the maximum efficiency is measured about 95.5%.

“Decentralised resilient autonomous control architecture for dynamic microgrids“

Microgrids serve as an integral part of future power distribution systems. Typically, microgrids are managed by centralised controllers. There are two major concerns about using a single centralised controller. The controller can become a performance and reliability bottleneck for the entire system, where its failure can bring the entire system down. Excessive communication delays can also degrade the system performance. As a solution, a true decentralised control architecture for microgrids is proposed, designed, developed, and tested here. Distributing the controls to local agents decreases the possibility of network congestion to occur. Decentralisation will also enhance the reliability of the system since the single point of failure is replaced by a distributed architecture. The proof-of-concept of true decentralisation of microgrid control architecture is implemented using Hardware-in-the-Loop Platform. Device level and system level controller and interaction models are defined for a self-coordination. Also, microgrid energy management system (EMS) and control case scenarios are demonstrated. The experimental results show the robustness of the proposed architecture.

“Reliability Analysis of a Decentralized Microgrid Control Architecture“

Reliability enhancement of microgrids is challenged by environmental and operational failures. Centrally controlled microgrids are susceptible to failures at high probability due to a single-point-of-failure, e.g., the central controller. True decentralization of microgrid architecture entails elimination of the central controller, attaining a parallel configuration for the system. In this paper, decentralized microgrid control architecture is proposed as a solution for reliability degradation over time, and analyzes the reliability aspects of centralized and decentralized control architectures for microgrids. Degree of importance of a single controller in centralized and decentralized architectures is determined and validated by Markov chain models. Results confirm that higher reliability is achieved when true decentralization of control architecture has been adopted. Challenges of implementing a true decentralized control architecture are discussed. Hardware-In-the-Loop simulation results for microgrid controller failure scenarios for both architectures are presented and discussed.

“Foundational Support Systems of the Smart Grid: State of the Art and Future Trends“

The digitization of the grid is becoming a reality. This transformation has raised various challenges on hardware, controls, communication, and operation. The massive amount of data generated by smart-grid technology can create a practical problem and a reliable infrastructure is required to ensure an uninterrupted operation of smart grid subsystems. Grid modernization is facing a wave of effort to target and to advance every aspect of the smart grid. With the fast paced technology development, the need for intensive resources for smart grid research arises. This paper is intended as a compilation of smart grid research that attracts researchers. State of the art technologies and future trends in smart grid research are discussed within the scope of communications and computing. Challenges of the grid modernization process are tiered and discussed. Sample research efforts have been included for various research paths.

“Dynamic Modeling and Control of a Synchronous Generator in an AC Microgrid Environment“

A microgrid system model is created to assess transient and steady-state stability during periods of separation from the grid. A secondary control is constructed to dispatch energy sources according to user selected setpoints and participation factors. Two sources-an energy storage device and a synchronous generator-are controlled to share active and reactive load burdens. A system load models the difference between solar and wind-powered generation and load. First-order differential equations are written to describe the system, and implemented in Simulink. The model components carry a high level of detail to capture all relevant modes. Systems include an eight-state salient pole synchronous machine, an AC8B regulator, a prime mover, an equivalent π cable, an RL microgrid load, an ideal battery, a simple inverter model, and a detailed LCL filter at the inverter output. A detailed model of the inverter primary control is included, and a secondary level control, which distributes the power error according to user defined participation factors, is proposed.

“A Comprehensive Review on Constant Power Loads Compensation Techniques“

Microgrid, because of its advantages over conventional utility grids, is a prudent approach to implement renewable resource-based electricity generation. Despite its advantages, microgrid has to operate with a significant proportion of constant power loads that exhibit negative incremental impedance and thus cause serious instability in the system. In this paper, a comprehensive review is presented on accomplished research work on stabilization of dc and ac microgrid. After reviewing these, microgrid system stabilization techniques are classified with required discussions. As found out in this paper, the stabilization techniques can basically be classified as compensation done: 1) at feeder side; 2) by adding intermediate circuitry; and 3) at load side. Finally, after analyzing the merits and drawbacks of each generalized technique, several infographics are presented to highlight the key findings of this paper.

“Stability improvement of microgrids in the presence of constant power loads“

Renewable energy sources, the most reasonable fuel-shift taken over the naturally limited conventional fuels, necessarily deal with the self-sustainable microgrid system rather than the traditional grid distribution system. In practice, the microgrid system experiences the challenge of instability due to the Constant Power Load (CPL) from many electronic devices. In this paper, AC microgrid stability, besides the stability analysis and pole zero movement, is thoroughly investigated for each and every considerable parameter of the system in order to improve the stability scenario. After demonstrating all cases regarding the instability problem, the storage based virtual impedance power compensation method is introduced to retain the system stability and extend the loading limit of the microgrid system. Here, in this proposed method, both the active and reactive power compensation techniques are suggested for a potential solution. Besides this, a PID controller is implemented to maintain the constant terminal voltage of CPL via current injection method from storage. All the cases and results are rigorously scrutinized in the virtual platform such as MATLAB/Simulink with appreciable aftermaths. Hardware implementation of the proposed method is also conducted to find out its effectiveness.

“Development of Lyapunov redesign controller for microgrids with constant power loads“

In microgrid applications, stability issues have been raised into a matter of concern due to the continually increasing modern electronic loads and inverter-based power electronic loads. In this paper, adopting storage system based load side compensation technique, a Lyapunov redesign controller is proposed to considerably improve the microgrid stability in the presence of constant power loads. Besides the controller design, the robustness analysis of the proposed Lyapunov redesign controller against parameter uncertainties is depicted. After that, the proposed Lyapunov redesign controller robustness against parametric uncertainties, frequency variations, and additive white Gaussian noise (AWGN) is presented. Later, performance of the PID and Lyapunov redesign controller is compared in the case of nonlinearity, parameter uncertainties, and noise rejection to justify the selection of the Lyapunov redesign controller over the PID controller. Besides reckoning the necessary calculations, all of the results are verified in MATLAB/Simulink with the appreciable aftermath.

“A multi-predictor model to estimate solar and wind energy generations“

Recent technological developments in renewable energy systems and significant growth of solar and wind energy have made these 2 renewable sources potential viable alternatives for conventional energy sources. However, due to intermittent nature, their reliability and availability are not similar to traditional sources. Hence, it is crucial to estimate the solar and wind availability and contribution more accurately. There are various factors affecting the generation capacity of renewable sources. There has been a vast research on the impact of factors related to climate condition such as wind speed, air temperature, and humidity on renewable energy generation. However, there are several other factors with indirect impact on renewable capacity and generation mostly overshadowed by the climate factors. In this study, a multi-predictor regression model is developed and presented for solar and wind energy generation capacity across the USA. Our study of 50 states shows how the generation capacity can be affected by several indexes including human development index. Variables with the more significant impacts have been chosen using a regression analysis. A recommendation on the best transformation of the response variables and sensitivity analysis of the results has also been presented. The results provide a model to estimate the generation capacity using significant predictors. For instance, the impact of population growth on the wind turbine generation can be explored using these models.

“An overview of the environmental, economic, and material developments of the solar and wind sources coupled with the energy storage systems“

There is a constant growth in energy consumption and consequently energy generation around the world. During the recent decades, renewable energy sources took heed of scientists and policy makers as a remedy for substituting traditional sources. Wind and photovoltaic (PV) are the least reliable sources because of their dependence on wind speed and irradiance and therefore their intermittent nature. Energy storage systems are usually coupled with these sources to increase the reliability of the hybrid system. Environmental effects are one of the biggest concerns associated with the renewable energy sources. This study summarizes the last and most important environmental and economic analysis of a grid-connected hybrid network consisting of wind turbine, PV panels, and energy storage systems. Focusing on environmental aspects, this paper reviews land efficiency, shaded analysis of wind turbines and PV panels, greenhouse gas emission, wastes of wind turbine and PV panels’ components, fossil fuel consumption, wildlife, sensitive ecosystems, health benefits, and so on. A cost analysis of the energy generated by a hybrid system has been discussed. Furthermore, this study reviews the latest technologies for materials that have been used for solar PV manufacturing. This paper can help to make a right decision considering all aspects of installing a hybrid system. Copyright © 2017 John Wiley & Sons, Ltd.

Modular Multilevel Converter for Wind Energy Storage Applications

This paper presents a medium-voltage wind energy conversion system with integrated storage that implements power electronics converter based on modular multilevel topology. The proposed converter has a storage system integrated into its modular cell structure. This paper analyzes the proposed topology and presents detail sizing procedure for both converter and storage system in order to meet new grid connection standards and fault ride through (FRT) requirements. In order to verify the system operation, high detail model of the converter leg is used to simulate independent power control on ac and dc sides of the converter needed for the successful implementation of the FRT control.

An overview of microgrid protection methods and the factors involved

Microgrid is a distribution system composed of a set of micro-generators which are added to a network. One serious challenge facing a microgrid network is designing a proper protection scheme. This is because the fault current in a microgrid is constantly changing due to the presence of distributed generation (DG) resources at all levels of the distribution network and to the fact that they can operate in two islanding and normal modes. This makes the conventional methods inappropriate for microgrid protection. That is, new schemes should be developed for this purpose. This paper is a summary of studies recently carried out in the field of microgrid protection. Along this line, the structure and topology of microgrids are reviewed first. Afterwards, the protective challenges facing DG-equipped distribution networks are discussed. Then, the innovative methods proposed to solve these problems are analyzed. These methods can be divided up into six main categories: changing settings of protective devices at presence of DG units, disconnecting pertinent DG units when faults occur, creating a balance among different DG technologies, using fault current limiter, using smart transformers, and adaptive protection. In so doing, we consider the factors involved in selecting each method. The factors identified are microgrid type and topology, DG type, communication type and delay time, method of fault detection and analysis, relay type, fault type, method of grounding, and use of smart transformers in microgrid. It is hoped that this work will be useful to the researchers in the field of microgrid protection in finding relevant references and designing appropriate methods.

A Novel Method for Fault Detection in Future Renewable Electric Energy Delivery and Management Microgrids, Considering Uncertainties in Network Topology

The use of solid state transformers (SSTs) in microgrids has created a new kind of network called Future Renewable Electric Energy Delivery and Management (FREEDM) microgrid. The FREEDM microgrid provides an appropriate means for enhanced energy management, loss reduction, and network flexibility by reducing the number of converters used for a variety of AC-DC links. In this work, we propose a novel method for fault detection in FREEDM microgrids when considering uncertainties in network topology. The proposed method makes use of the Clarke and S-transforms to characterize the transients in three-phase current and voltage waveforms in the event of a fault. The extracted features of the waveforms will be used to form appropriate indices for detection, location, and characterization of the fault. The main feature of the proposed method is its capability to operate in a dynamic microgrid with varying topology. The performance of the proposed method is investigated by applying it to a sample FREEDM microgrid with ring and radial structures. It is shown that the proposed method is well capable of fault detection and diagnosis while being able to differentiate between short-circuit faults and switching transients due to variations in the network topology.

The Role of Energy Storage in a Microgrid Concept: Examining the opportunities and promise of microgrids

A Microgrid is a cluster of distributed generation (DG), renewable sources, and local loads connected to the utility grid. A microgrid provides a solution to manage local generations and loads as a single grid-level entity. It has the potential to maximize overall system efficiency, power quality, and energy surety for critical loads. The Microgrid Exchange Group, an ad hoc group of expert and implementers of microgrid technology, has defined a microgrid as a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island mode.

Transition Management of Microgrids With High Penetration of Renewable Energy

Microgrids are receiving attention due to the increasing need to integrate distributed generations and to ensure power quality and to provide energy surety to critical loads. Some of the main topics concerning microgrids are transients and stability concerns during transitions including intentional and unintentional islanding and reconnection. In this paper, the standard IEEE 34 bus distribution feeder is adapted and managed as a microgrid by adding distributed generations and load profiles. Supervisory power managements have been defined to manage the transitions and to minimize the transients on voltage and frequency. Detailed analyses for islanding, reconnection, and black start are presented for various conditions. The proposed control techniques accept inputs from local measurements and supervisory controls in order to manage the system voltage and frequency. An experimental system has been built which includes three 250 kW inverters emulating natural gas generator, energy storage, and renewable source. The simulation and experimental results are provided which verifies the analytical presentation of the hardware and control algorithms.

Dynamic Performance Improvement and Peak Power Limiting Using Ultracapacitor Storage System for Hydraulic Mining Shovels

This paper presents a concept to improve the dynamic response, efficiency, productivity, and fuel consumption of hydraulic mining shovels (HMSs) using a high power energy storage system to supplement the existing diesel-powered generators. A 2.25-MW ultracapacitor system is applied to improve the sluggish dynamic response of the diesel engine during sudden load changes and to limit the engine power requirements by maintaining an optimal engine speed. When the load rate of change increases above a specified rate limit, the ultracapacitor system supplements power to support loads in order to limit the power ramp rate for the generator. In addition to the engine dynamic improvement, the ultracapacitor system supports the engine during peak power demand, allowing for a smaller engine size for the machine, which results in higher fuel savings and less exhaust emissions. An interleaved bidirectional dc/dc converter is designed to interface between the ultracapacitor bank and main dc bus. Sophisticated multilayer control strategies are designed, implemented, and applied to the system to perform complete machine power management while keeping the dc bus voltage stable. Modeling and experimental results for an 8000-hp HMS with the novel electrification architecture have also been presented.

Comprehensive modeling of turbine systems from wind to electric grid

A detailed model of a full conversion wind turbine is presented in this paper. Most of the systems found in a modern wind turbine are included in the model and an optimal level of model complexity is achieved to allow the design, analysis and simulation of controls and subsystem interactions within a turbine under various conditions. Mechanical, electrical and control subsystems were modeled to achieve flexibility for future expansion and properties that can be analyzed. Beside describing the overall wind turbine (WT), simulation system integration, which couples a variety of freeware and commercial software, the paper introduces a variable speed control for wind turbine with stall power regulation. It also describes the analysis of the interactions between components which cannot be observed with a simple WT model.

Development of an Electrical Model for Lithium-Ion Ultracapacitors

Accurate knowledge of the electrical characteristics of an energy storage device is of high importance when utilized in any power system to achieve proper charging and discharging modes without inflicting any damage to its structure. In this paper, a Li-ion ultracapacitor, a hybrid type of energy storage, is thoroughly studied. This type of ultracapacitors has high energy density, high power density, high efficiency, long cycle life, and superior performance under high temperatures. Testing, analysis, and modeling of this energy storage device are presented in details. A battery-based model and a capacitor-based model are proposed. A series of dc and ac tests has been conducted at various temperatures to develop an electrical model spanning, the entire domain of temperatures under which the ultracapacitor can normally and safely operate. These models have been verified for model accuracy via simulation and comparison with the test results for both a single ultracapacitor cell and a 360 V ultracapacitor module built in the laboratory. Both coulombic and energy efficiency have also been investigated at different temperatures and current ratings.

Modeling and Management of Batteries and Ultracapacitors for Renewable Energy Support in Electric Power Systems–An Overview

Energy storage devices and systems are playing a major role in all electrical systems from small electronics devices and automotive systems to the utility grid. The main objective of this article is to review energy storage devices, management, control, interface, and demonstrations for electrical power systems. Various types of energy storage systems are discussed, but the main focus is on batteries and ultracapacitors. Different types of batteries and their electrical models are explained. Three major types of ultracapcitors are also discussed. The battery management system and its functions, controls, and hardware are discussed. Various power electronics-based interface systems for battery and ultracapcitor charging and discharging are presented. Applications of energy storage systems for utility applications, including renewable firming, power shifting, and ancillary services, are discussed.

Lithium-Ion Capacitor Energy Storage Integrated With Variable Speed Wind Turbines for Power Smoothing

Utilization of wind energy in modern power systems creates many technical and economical challenges that need to be addressed for successful large scale wind energy integration. Variations in wind velocity result in variations of output power produced by wind turbines. Variable power output becomes a challenge as the share of wind energy in power systems increases. Large power variations cause voltage and frequency deviations from nominal values that may lead to activation of protective relay equipment, which may result in disconnection of the wind turbines from the grid. Particularly community wind power systems, where only one or few wind turbines supply loads through a weak grid such as distribution network, are sensitive to supply disturbances. Energy storage integrated with wind turbines can address this challenge. In this paper, Li-ion capacitors are investigated as a potential solution for filtering power variations at the scale of tens of seconds. A novel topology and control technique has been introduced to integrate capacitors and power conversion circuitry. Modeling and scaled-down experimental results are provided to verify the theoretical analyses.

Multi-Level Wind Turbine Inverter to Provide Grid Ancillary Support

More utility companies now require wind power plants to participate in grid support functions including frequency, voltage and inertia support. However, typical wind turbine generator configurations cannot provide this support. In this paper, a multi-level inverter based wind turbine power conversion system is investigated. The developed inverter interfaces between the DC bus of a wind turbine power conversion system supported by energy storage elements and the grid. Two control techniques are proposed for capacitor-based and battery-based storage systems to provide grid active and reactive power support. Details of control implementation for grid interface, frequency and voltage droop support are presented. Simulation and experimental results are discussed to verify the viability of the proposed system and control techniques.

Multi-Inverter Controls and Management of Energy Storage for Microgrid Islanding

The electrical power distribution system in this country and worldwide is vulnerable to faults on the feeders or on the central power system due to natural and man-made events. The outages are not only expensive but also can result in security risks. The best solution to overcome such power outages is to form microgrids in the distribution system, with their own generation and load management strategies. However, managing microgrids with high reliability is difficult – there is no inertia in the microgrid, unlike the central power system, and control and stability of microgrids is a concern. The performance of microgrids is affected by the presence of the high penetration and intermittent renewable sources such as wind and solar. Performance of microgrids is enhanced with energy storage that adds “inertia” and provides stability to the microgrids. To improve reliability and energy management, multi-inverter energy storage offers several advantages. Energy storage has also been receiving increasing attention to address a variety of technical challenges in the management of electric power. This article addresses some of the issues of microgrids by using energy storage devices and in particular a multi-inverter energy storage system that allows for distributed storage.

Microgrid Generation Capacity Design With Renewables and Energy Storage Addressing Power Quality and Surety

Microgrids are receiving attention due to the increasing need to integrate distributed generations and to insure power quality and to provide energy surety to critical loads. Since renewables need to be in the mix for energy surety, a high renewable-energy penetrated microgrid is analyzed in this paper. The standard IEEE 34 bus distribution feeder is adapted and managed as a microgrid by adding distributed generation and load profiles. The 25 kV system parameters are scaled down to 12 kV and renewable sources including solar PV and wind turbines, an energy storage system, and a diesel generator for islanded mode have been added to the 34-bus system. The distribution generations (DG) and renewables are modeled in detail using PSCAD software and practical constraints of the components are considered. The monitoring of the microgrid for measuring power quality and control requirements for these DGs and storage are modeled to maintain the power quality of the system when loads are varied. Renewable sources are modeled with seasonal variation at different locations. The microgrid is monitored at number of buses and the power quality issues are measured and indexes are calculated. This paper proposes a generalized approach to design (determine the capacity requirements) and demonstrates the management of microgrids with metrics to meet the power quality indexes.

Active Torque Control for Gearbox Load Reduction in a Variable-Speed Wind Turbine

With the advent of power electronics, the size, weight, and cost of power converters have been drastically reduced while efficiency is improved. The use of variable-speed wind power generators has seen considerable growth. The use of gearboxes in the wind turbines allows for smaller size, lower weight, and higher speed generators. However, gearboxes have shown to be one of the least reliable components of the wind turbines. In this paper, we propose a method that can extend the life and reliability of wind turbine gearboxes by reducing the mechanical stress on gearbox components. Reduction of mechanical stress is achieved by the generator torque control that minimizes resonant torsional vibrations within a drivetrain caused by variations in wind velocity. A detailed model for the drivetrain of a 750-kW wind turbine, including a gearbox is presented. Experimental results are used to calculate the parameters of the gearbox. A controller is designed to adjust the generator torque at the end of the drivetrain to remove the unwanted and damaging torque variations from the drivetrain. Simulation results verify the effectiveness of the proposed method.

A Hybrid System of Li-Ion Capacitors and Flow Battery for Dynamic Wind Energy Support

Wind farm output power fluctuations create adverse effects on the voltage, frequency, and transient stability of the utility grid. Short-term wind farm power variations with high ramp rates can cause voltage instabilities, particularly if the farm is located in weak-grid areas. The integration of wind energy with energy storage devices to support the short-term shortcomings of wind energy is discussed in this paper. A turbine level hybrid configuration of an energy storage system is used to limit the power ramp rates and apply power smoothing. The proposed energy storage devices are the zinc bromide flow battery and lithium-ion capacitors. The actual models for the battery and capacitors used in this study are derived from laboratory tests. The wind farm power is also modeled using measured wind speed data. A new concept has been introduced to evaluate the effectiveness of energy storage system for wind energy support. The analysis shows that significant improvements can be made to shape the output power of the farm using energy storage systems.

Series Voltage Compensation for DFIG Wind Turbine Low-Voltage Ride-Through Solution

This paper introduces a new solution for doubly fed induction generators to stay connected to the grid during voltage sags. The main idea is to increase the stator voltage to a level that creates the required flux to keep the rotor side converter current below its transient rating. To accomplish this goal, a series compensator is added to inject voltage in series to the stator side line. The series converter monitors the grid voltage and provides compensation accordingly to accomplish this aim. Since the turbine and converter stay connected, the synchronization of operation remains established during and after the fault and normal operation can be resumed immediately after the fault is cleared. To keep the current at its minimum, a control strategy has been developed to keep the injected voltage and line voltage in phase during and after the fault.

A Linear Permanent Magnet Generator for Powering Implanted Electronic Devices

Permanent magnet (PM) machines provide high efficiency, compact size, robustness, lightweight, and low noise. These features qualify them as the best suitable machine for medical applications. The system presented in this paper is a self-contained, small size, and reliable device that can continuously provide power. The core of the system is a linear generator that consists of two layers of PMs and one layer of coils. It generates power from multidirectional body movements. The movement of the device causes the coil layer to move. The relative movement of the coils versus PMs, on two sides, creates a varying flux in the windings. This change in flux produces voltage in the winding and can be converted into electrical power if a load is connected. The best place to implement this device to produce continuous power is on a muscle inside the body that is linked to the respiratory system. Design, simulation, implementation, and testing of the generator are presented in this paper. The testing results reveal that the generator can produce up to 1 mW of power in the body.

Modeling of Zinc Bromide Energy Storage for Vehicular Applications

Energy storage devices such as lithium-ion and nickel-metal hydrate batteries and ultracapacitors have been considered for utilization in plug-in hybrid electric vehicles (HEVs) and HEVs to improve efficiency and performance and reduce gas mileage. In this paper, we analyze and model an advanced energy storage device, namely, zinc bromide, for vehicular applications. This system has high energy and power density, high efficiency, and long life. A series of tests has been conducted on the storage to create an electrical model of the system. The modeling results show that the open-circuit voltage of the battery is a direct function of the battery’s state of charge (SOC). In addition, the battery internal resistance is also a function of SOC at constant temperature. A Kalman filtering technique is also designed to adjust the estimated SOC according to battery current.

A dynamic LVRT solution for Doubly-Fed Induction Generator

Doubly-fed induction generators (DFIG) have become the most common type of wind turbine generators. However, this type of generator is susceptible to grid side low voltage and short circuits, due to existence of a power electronics converter on the rotor side. When a short circuit or voltage sag happens on the grid side, the rotor current of the generator tends to rise, which could cause damage to the rotor converter. Design and implementation of a series converter on the stator side is presented in this paper to limit the current rise in the rotor. This system includes an active DC/AC inverter, three series transformers and a DC bus capacitor. To lower the rating of the components and make the system viable for practical solutions, an exponential decaying sinusoidal voltage instead of a pure sinusoidal voltage is applied by the converter during short circuit.

Indoor Power Harvesting Using Photovoltaic Cells for Low-Power Applications

Utilization of low-power indoor devices such as remote sensors, supervisory and alarm systems, distributed controls, and data transfer system is on steady rise. Due to remote and distributed nature of these systems, it is attractive to avoid using electrical wiring to supply power to them. Primary batteries have been used for this application for many years, but they require regular maintenance at usually hard to access places. This paper provides a complete analysis of a photovoltaic (PV) harvesting system for indoor low-power applications. The characteristics of a target load, PV cell, and power conditioning circuit are discussed. Different choices of energy storage are also explained. Implementation and test results of the system are presented, which highlights the practical issues and limitations of the system.

Output Power Smoothing for Wind Turbine Permanent Magnet Synchronous Generators Using Rotor Inertia

Due to wind speed variations, the output power of wind turbines fluctuates. This power fluctuation can cause frequency deviations and power outage, particularly when wind power penetration is significant. Energy storage devices, such as batteries, ultracapacitors, super inductors, and flywheels can be utilized in a hybrid system to solve this problem. These methods are effective, but they impose a significant additional cost to the system. This article presents a novel control method to mitigate the power fluctuations using the rotor inertia as an energy storage component. Therefore, the additional energy storage system is not required. The proposed method is also modified to obtain better energy capturing efficiency. The efficiency of the algorithm is evaluated based on developed equations. The energy extracting capability using this method is comparable to other methods such as the maximum power extraction algorithm. Simulation studies for various cases are performed on a permanent magnet synchronous generator, which verify the theoretical analysis.

Wind power smoothing using rotor inertia aimed at reducing grid susceptibility

Due to wind speed variations, the output power of wind turbines fluctuates. These power fluctuations make the wind power undispatchable. Furthermore, they can cause frequency deviations and power outage particularly when wind power penetration is significant. This paper presents a novel control method to mitigate the power fluctuations using the rotor inertia as an energy storage component. Therefore, the additional energy storage system is not required. The proposed method is also modified to obtain better energy capturing efficiency. The method is analysed using mathematical and physical characteristics of the system. The efficiency of the algorithm is evaluated based on obtained equations. The energy extracting capability using this method is comparable with other methods such as maximum power extraction (MPE) algorithm. Simulation results for various cases are performed on a permanent magnet synchronous generator that verifies theoretical analysis.

A three-phase uninterruptible power supply with an adaptive reference waveform generator

In this paper, a new digital deadbeat controller along with a modified adaptive reference signal generator are designed, implemented and applied to a three-phase series-parallel line-interactive uninterruptible power supply (UPS). This kind of UPS system provides input power factor correction, output voltage conditioning with higher efficiency compared with other kinds of UPS. It consists of a series and a parallel converter. The series converter deals with voltage-based distortions while the parallel converter cancels the current-based disturbances. The objective of the controller is to achieve deadbeat dynamic response for the parallel and series converters. The adaptive filter provides the reference signal for the load voltage and source current without any phase shift. The proposed controller adjusts the current of the parallel converter and voltage of the series converter with two and four sampling periods, respectively. Simulation and experimental results are presented, which show the viability of the proposed controller and reference signal generator for this topology.

An On-Line UPS System With Power Factor Correction and Electric Isolation Using BIFRED Converter

This paper presents the design consideration and performance analysis of an on-line, low-cost, high performance, and single-phase uninterruptible power supply (UPS) system based on a boost integrated flyback rectifier/energy storage dc/dc (BIFRED) converter. The system consists of an isolated ac/dc BIFRED converter, a bidirectional dc/dc converter, and a dc/ac inverter. It provides input power factor correction, electric isolation of the input from the output, low battery voltage, and control simplicity. Unlike conventional UPS topologies, the electrical isolation is provided using a high frequency transformer that results in a smaller size and lower cost. Detailed circuit operation, analysis, as well as simulation and experiment results are presented. A novel digital control technique is also presented for UPS inverter control. This controller follows the reference current and voltage of the inverter with a delay of two and four sampling periods, respectively.

Digital Control of Three-Phase Series-Parallel Uninterruptible Power Supply Systems

In this paper, a new digital deadbeat controller is designed, implemented, and applied to a three-phase series-parallel line-interactive uninterruptible power supply (UPS). This kind of UPS system provides input power factor correction, output voltage conditioning, and high efficiency. The objective of the controller is to achieve deadbeat dynamic response for the parallel and series converters. The proposed controller adjusts the current of the parallel converter and voltage of the series converter with two and four sampling periods, respectively. A reduced-parts topology is also introduced that has less number of power electronics components as well as switching functions. The power flow of the system in the presence of current and voltage harmonics is discussed. Simulation and experimental results are presented, which show the viability of the proposed controller for this topology.

Full Digital Current Control of Permanent Magnet Synchronous Motors for Vehicular Applications

Due to high efficiency, compactness, and less generated noise of permanent magnet synchronous motors, they have received a lot of attention for traction applications in land, sea, and undersea vehicles. In this paper, a new digital deadbeat controller is designed, implemented, and applied to a permanent magnet synchronous machine. The objective of the controller is to achieve a deadbeat dynamic response for the speed of the machine. The analysis of the controller shows that it adjusts the current and voltage of the machine with two and four sampling periods, respectively. A robust simple sensorless method is used to estimate the position and velocity of the rotor. The experimental results of a laboratory prototype is presented, which show the viability of the proposed controller for this machine.

Effects of Power Quality Distortions on Electrical Drives and Transformer Life in Paper Industries: Simulations and Real Time Measurements

With the increasing of non-linear loads in electric power system, power quality distortion has become a serious issue in recent years. In paper and pulp industries, due to presence of concentrated high power non-linear loads such as electric drives, this problem is a greater concern. In this paper, the impacts of power quality distortions such as voltage sag and swell on the operation of electric devices in Mazandaran Wood and Paper Industries (MWPI) are discussed. Using the actual data, the effects of voltage sag on the main production line motor drive is investigated. In addition, the effects of harmonics on the distribution transformers such as increasing their losses and decreasing their life are analyzed

This paper presents an advanced modeling and simulation technique applied to DC/DC power electronic converters fed through renewable energy power sources. The distributed generation (DG) system at the Illinois Institute of Technology, which employs a phase-l system consisting of a photovoltaic-based power system and a phase-2 system consisting of a fuel cell based primary power source, is studied. The modeling and simulation of the DG system is done using the generalized state space averaging (GSSA) method. Furthermore, the paper compares the results achieved upon simulation of the specific GSSA models with those of popular computer-aided design software
simulations performed on the same system. Finally, the GSSA and CAD software simulation results are accompanied with test results achieved via experimentation on both the PV-based phase-1 system and the fuel cell-based phase-2 power system.

A hybrid transformer model for determination of partial discharge location in transformer winding

Partial discharges are well known as a source for insulation degradation in power transformers. A hybrid transformer model is introduced to simulate the transformer winding transient response. Transformer structural data is used to determine the hybrid model parameters. Calculations of the hybrid transient model parameters are based on the parameters of the lumped parameter equivalent transformer model and electromagnetic rules. Modern computation techniques and optimizations are employed beside this model for PD location using the multi conductor transmission line model and also to analyze its propagation aimed at achieving (i) more reliable simulation results (ii) less computational time (iii) accurate results for a wide range of frequency. The simulation results on a 66 kV, 25 MVA fully interleaved winding are presented. The measurement results on this winding are employed to validate this model

Determination of Partial Discharge Propagation and Location in Transformer Windings Using a Hybrid Transformer Model

Experimental experiences prove that partial discharge (PD) is a major source of inner insulation system failure in power transformers. If the deterioration of the insulation system caused by PD activity can be detected at an early stage, preventive maintenance measures may be taken. Due to the complex structure of the transformer, accurate determination of PD location is one of the major challenges in front of power utilities. This problem comes to be vital in open access systems. A hybrid transformer model and its application for locating partial discharge and studying its propagation along the transformer windings are proposed in this article. It is based on the structural data of the transformer for equivalent model parameters determination. The lumped parameter equivalent model for winding sections is constructed satisfying the quasi-static condition. Then an algorithm is developed to use the constructed matrices for PD propagation and location studies by application of the multi-conductor transmission line equivalent model. The results of the study are presented for a 66 kV/25 MVA transformer with fully interleaved winding.

Application of wavelet analysis to the determination of partial discharge location in multiple-α transformer windings

The inner insulation system is a critical component of a power transformer. Its degradation may cause the device to fail while in service. If deterioration of the insulation system caused by Partial Discharge (PD) activity can be detected at an early stage, preventive maintenance measures can be taken. Due to the complex structure of power transformers, accurate locating of PD is not an easy task and is one of the main challenges in front of power utilities. Locating PD is more difficult in transformers with multiple-α windings. This problem comes to be vital in open access systems. A method for locating partial discharge within multiple-α windings is proposed, which is based on structural data of a transformer. A 66 kV/25 MVA transformer with fully interleaved winding and connected tap winding is used as test object. Wavelet transform is employed to process the partial discharge signals. Wavelet transform analysis method is a powerful tool for processing transients and non-stationary or time varying signals. Since the wavelet transform provides multi-scale analysis and time–frequency domain localization, it is particularly suitable to process the partial discharge signals. In order to improve the accuracy of the partial discharge location, a new technique for extracting Partial Discharge signals is introduced. Applying wavelet transform to a signal produces a wavelet detail coefficient distribution throughout the time-scale, which depends on the mother wavelet chosen. This technique is based on the capability of the chosen mother wavelet for generating coefficients with maximum values. The wavelet based de-noising method proposed in this paper can be successfully employed to extract PD pulse from the measured signal. It can provide enhanced information and further infer the original site of the PD pulse through capacitive ratio method. The method is described in details and the applications to determine the partial discharge location in multiple-α windings are explored.