Search

Wind farms featuring a series-connected dc collection system have been shown to offer advantages in terms of total conversion efficiency and amount of offshore-deployed equipment. To ensure proper exploitation of these benefits, it is necessary to determine the ratings of wind turbine converters. These ratings must be sufficient to cover the expected operating conditions over the life of the wind farm, without unnecessarily oversizing the equipment. As shown in this article, the variable string current results in an interdependence of operating points among wind turbines that has to be considered for sizing these converters. This article proposes a generic sizing framework for such single-string, series-connected dc wind farms. This framework is applied to three wind farm configurations featuring differential power processing, voltage-source converters, and diode-bridge rectifiers and buck converters. Finally, a case study for a 450 MW offshore wind farm demonstrates the implementation of this sizing methodology and quantifies the design tradeoffs between converter ratings and energy production enabled from those converters. In two of the studied wind farm configurations, significant rating reductions are achievable while still avoiding energy curtailment for >99.7% of the energy production of an equivalent ac wind farm. [ABSTRACT FROM AUTHOR]

Secondary control strategies in hybrid ac/dc microgrids have been conventionally designed independently for each side of the microgrid, neglecting the interaction between the ac and dc sides due to the power transferred through the interlinking converter. This has a negative effect on the power sharing and dynamic response of the system. In this article, a novel distributed secondary control strategy for hybrid ac/dc-microgrids is presented which coordinates the control actions of the ac and dc sides. The proposed control scheme simultaneously regulates ac-voltage magnitude and frequency, as well as the dc-voltage magnitude, by including the interlinking converters in the control strategy via a distributed consensus approach. This improves the power-sharing accuracy and secondary control restoration of the variables in both ac and dc sides of the microgrid. Moreover, the proposed methodology avoids the circulating currents which are typically produced in systems with multiple interlinking converters. Several simulation and experimental tests, such as load impacts and plug-and-play capability, are presented to validate the performance of the proposed control strategy using a 24-kW hybrid ac/dc-microgrid laboratory prototype. [ABSTRACT FROM AUTHOR]

The analysis of wind turbine output power measurements from the offshore wind farm Horns Rev 1 demonstrates a significant likelihood of wind turbine output powers to be very similar at a given time within offshore wind farms. This paper exploits this observation by proposing a new offshore wind farm configuration with DC collection system and series-connected wind turbines based on partial power processing converters (PPPCs) and diode-bridge rectifiers. In the proposed wind farm configuration, PPPCs are only required to process output power differences among wind turbines in a wind farm to achieve maximum power point (MPP) operation, yielding a potential for efficiency and sizing improvements. This paper addresses major design considerations at wind farm, wind turbine, and PPPC levels. System operation of the wind turbine design is derived, alongside with a matching control system and HVDC-link current scheduling algorithm. The proposed wind farm is successfully tested for low voltage ride through, power curtailment, inertia response, and communication system outage scenarios. Time-transient simulations of a 30-turbine series string using measured and artificial wind speed profiles demonstrate that wind turbines can achieve MPP operation while only a fraction of power needs to be processed by the PPPCs. [ABSTRACT FROM AUTHOR]

This paper presents a novel distributed control strategy for frequency control, congestion management, and optimal dispatch (OD) in isolated microgrids. The proposed strategy drives the distributed generators (DGs) within the microgrid to a dispatch that complies with the Karush–Kuhn–Tucker (KKT) conditions of a linear optimal power flow (OPF) formulation. The controller relies on local power and frequency measurements, information from neighboring DGs, and line-flow measurements transmitted through a communications network. Extensive simulations show a good performance of the controller against sudden changes in the load, congested lines and availability of DGs in the microgrid, and the ability to successfully drive the system to an optimal economic operation. [ABSTRACT FROM AUTHOR]

Battery energy storage systems (BESSs) can be very beneficial to power systems and microgrids for various applications. With increasing sales of electric vehicles (EV), the availability of used electric vehicle batteries (EVBs) is on the rise, which has received significant attention in recent years. The retired EVBs, after repurposing, can serve as an alternative option to new batteries in a BESS. In addition, when a microgrid operator desires to install a BESS, the optimal decisions such as installation year, energy and power size, replacement year, and the number of cycles to failure, that are normally overlooked, have to be determined. Therefore, this paper proposes a comprehensive and novel framework for planning and operation of the BESS based on repurposed EVBs. A novel linearized BESS sizing model is proposed that obtains the BESS optimal decisions regarding design and operation. Various new, modified, and linearized relationships for the BESS have been included in the planning model to ensure that the replacement year of the BESS is optimally determined. Several classes of EVs with multiple drive cycles are clustered and integrated within the proposed framework. [ABSTRACT FROM AUTHOR]

This paper proposes a generalized and efficient power-flow algorithm for islanded hybrid ac/dc microgrids. The algorithm considers the microgrid operational aspects, i.e., absence of a slack bus, unbalanced ac subgrid, droop-controlled ac and dc voltages and ac frequency, and coupling between the ac frequency and dc voltage through interlinking converters. To attain high computational efficiency, the algorithm adopts three features. First, it models the ac subgrid elements in sequence components, thereby dividing the subgrid's set of equations into three smaller sets for faster parallel solution. This approach also accurately represents the different types of ac distributed generators. Second, the algorithm sequentially solves for the power-flow variables of the ac and dc subgrids, thus reducing the number of equations to be solved simultaneously, once again for further computational cost alleviation. Third, the algorithm implements the quadratically convergent Newton–Raphson technique to solve the decoupled sets of equations. The proposed algorithm is validated through comparisons with time-domain simulations, in MATLAB/Simulink, for test hybrid ac/dc microgrids of different configurations. Moreover, three case studies are introduced to examine the proposed algorithm's effectiveness in solving large-scale microgrids, to investigate its limits-enforcement capabilities, and to evaluate its performance as compared to conventional methods. [ABSTRACT FROM AUTHOR]

Given the expected impact of electric vehicle (EV) charging on power grids, this paper presents a novel two-step approach for the smart operation of EVs with four-quadrant chargers in a primary distribution feeder, accounting for the uncertainties associated with EVs, and considering the perspectives of both the utility and the EV owners. In the first step of the proposed approach, the mean daily feeder peak demand and corresponding hourly feeder control schedules, such as taps and switched capacitor setpoints, considering the bidirectional active and reactive power transactions between EVs and the grid, are determined. A nonparametric bootstrap technique is used, in conjunction with a genetic algorithm-based optimization model, to account for EV uncertainties and discrete variables. In the second step, the maximum possible power that can be given to connected EVs at each node, while providing active and/or reactive power to maintain the peak demand value and corresponding feeder dispatch schedules defined in the first step, is computed every few minutes in a way which is fair to the EVs. The proposed approach is validated using the distribution feeder model of a real primary feeder in Ontario, Canada, considering significant EV penetration levels. The results show that the proposed approach could be implemented in practice to properly operate EVs, satisfying feeder, and peak demand constraints, which would be better than the business-as-usual practice or a popular heuristic method in terms of number of tap operations, system peak demand, and voltage regulation. [ABSTRACT FROM AUTHOR]

This paper proposes a generic steady-state modeling and power-flow analysis approach for droop- and isochronously controlled microgrids. The proposed framework adopts symmetrical sequence component models, rather than phase-coordinate models, of microgrid elements. Such approach immensely reduces the power-flow execution time, as it breaks down the system model into independent equation sets with considerably reduced sizes. To render the proposed approach practical and generic, it integrates different types and control schemes of distributed generation (DG), including synchronous generator-based DG (SGDG) and electronically interfaced DG units. Furthermore, it incorporates unbalanced loads and feeders, transformer connections, different load characteristics, and configurations, as well as microgrid droop features. A novel power-flow algorithm based on a modified Newton–Raphson method is proposed to solve for the microgrid steady-state voltage magnitudes, angles, and frequency. The accuracy of the models and algorithm is verified through comparison with detailed time-domain simulations in MATALB/Simulink. Additionally, the proposed approach is shown to outperform the reported Newton-Trust Region approach in generality, accuracy, and performance. Two case studies, incorporating IEEE 123-node test microgrid, are further performed to examine the effectiveness of the proposed approach in solving complicated droop-controlled microgrids, and to examine the behavior of droop-controlled DGs in isochronous microgrids. [ABSTRACT FROM AUTHOR]

With the increased penetration of electric vehicle (EV) chargers in distribution systems, there is a need to understand and minimize their impact on medium-voltage (MV) and low-voltage (LV) networks. Thus, this paper proposes a three-stage algorithm to coordinate the operation of four-quadrant EV chargers with other volt/var control devices in MV and LV distribution feeders. The first stage operates on a day-ahead basis and defines the load tap changer and capacitor schedules while minimizing the peak load associated with EVs in the distribution system. The second and third stages update their operation every five minutes, to fairly allocate the aggregated and individual EV loads in the MV an LV feeders, respectively, while minimizing active power losses and voltage deviations. The proposed technique is applied to CIGRE’s North-American MV and LV benchmark systems to demonstrate its ability to properly allocate EV loads, and improve distribution system performance in terms of losses and voltage deviations. [ABSTRACT FROM AUTHOR]

This paper proposes a model of a single-phase bidirectional electric vehicle (EV) charger with capability of operating in all four quadrants of the P – Q plane. The steady-state and step responses of the proposed model are used to validate it based on the actual responses of a bidirectional charger prototype for different P – Q requests. The model can be used efficiently in time-domain simulations that require models of a number of EV chargers, such as EV integration studies in low-voltage (LV) distribution networks. A practical case study is presented to demonstrate and test the proposed smart charger and model, investigating the provision of vehicle-to-grid (V2G) for active and reactive power in an LV residential distribution network. These results demonstrate the advantages of the presented charger model for developing V2G strategies in distribution networks. [ABSTRACT FROM PUBLISHER]

This paper presents the mathematical formulation and control architecture of a stochastic-predictive energy management system for isolated microgrids. The proposed strategy addresses uncertainty using a two-stage decision process combined with a receding horizon approach. The first stage decision variables (unit commitment) are determined using a stochastic mixed-integer linear programming formulation, whereas the second stage variables (optimal power flow) are refined using a nonlinear programming formulation. This novel approach was tested on a modified CIGRE test system under different configurations comparing the results with respect to a deterministic approach. The results show the appropriateness of the method to account for uncertainty in the power forecast. [ABSTRACT FROM PUBLISHER]

This paper provides a detailed comparative analysis of optimal sizing of battery-only, ultracapacitor-only, and battery–ultracapacitor hybrid energy storage systems (ESSs) for a plug-in electric city bus (PECB). It is shown how the configuration affects the optimal size of the ESS. The problem of optimal sizing of the storage system for the PECB is formulated and solved for a specific set of vehicle parameters, battery, and ultracapacitor cell data, as well as a specific daily drive cycle. A modified particle swarm optimization algorithm is used to solve the optimal sizing problem. The optimization platform developed by the authors and used for this study is highly flexible and accepts different component data, vehicle parameters, and drive cycles as the input for determining the optimal sizes of the storage system. [ABSTRACT FROM PUBLISHER]

Batteries are the most commonly used electrical energy storage devices. The cost and performance of an electric vehicle (EV) are strongly affected by the proper selection of technology, number, and arrangement of the battery cells used. This paper formulates the problem of optimal sizing of the battery unit (BU) for a plug-in EV (PEV), given the specifications of the vehicle, battery cell data, and drive cycle. This paper employs a metaheuristic optimization algorithm to find the optimal size of the BU with the objective of minimizing the overall cost. Feasibility of the optimization results in terms of respecting the constraints is verified via simulation. The developed optimization platform is highly flexible and allows use of different vehicle designs, component data, and drive cycles. [ABSTRACT FROM PUBLISHER]

Access to energy in many of the world?s remote communities is still restricted; these locations only have access to simple and inexpensive local energy sources, such as biomass for cooking and kerosene lamps or candles for lighting. The World Bank and the International Energy Agency (IEA) perceive this energy deficit as a major obstacle to achieving community economic development as well as to obtaining adequate access to health services and clean water. Electricity is a flexible, modern source of energy that is considered to be one of the principal driving forces that stimulate community development and access to basic services in remote locations. Governments, private institutions, and nongovernmental organizations have gradually recognized these energy needs and have established electrification programs at the national and regional levels that aim at the gradual electrification of remote locations. [ABSTRACT FROM PUBLISHER]

The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature. [ABSTRACT FROM PUBLISHER]

This paper presents the mathematical formulation of the microgrid's energy management problem and its implementation in a centralized Energy Management System (EMS) for isolated microgrids. Using the model predictive control technique, the optimal operation of the microgrid is determined using an extended horizon of evaluation and recourse, which allows a proper dispatch of the energy storage units. The energy management problem is decomposed into Unit Commitment (UC) and Optimal Power Flow (OPF) problems in order to avoid a mixed-integer non-linear formulation. The microgrid is modeled as a three-phase unbalanced system with presence of both dispatchable and non-dispatchable distributed generation. The proposed EMS is tested in an isolated microgrid based on a CIGRE medium-voltage benchmark system. Results justify the need for detailed three-phase models of the microgrid in order to properly account for voltage limits and procure reactive power support. [ABSTRACT FROM PUBLISHER]

The paper investigates renewable energy alternatives to reduce diesel fuel dependency on electricity generation in Ontario's remote northern communities; currently, these communities use diesel fuel as the sole energy source to produce electricity. The current operation is complex, involving several stakeholders, high operating costs, and a considerable CO2 footprint. Several of these communities have electric load restrictions that limit further building construction and economic growth. This preliminary work discusses the barriers for renewable energy (RE) projects in northern Ontario communities by analyzing the current economic structure, the high capital costs, the available natural resources, and the installation and operation complexity. Also, a detailed analysis of six scenarios is presented; three scenarios consider a solar and/or wind-diesel system with a low RE penetration of 7% without any excess energy, whereas other three scenarios increase the RE penetration to 18%, requiring a dump load, an additional small diesel engine, or a battery storage system. The proposed systems reduce fuel consumption, operating costs and CO2 emissions, considering the investment, operation and maintenance costs and constraints in remote regions. [ABSTRACT FROM PUBLISHER]

Common-mode voltages (CMVs) can lead to premature failure of the motor insulation system in medium-voltage current-source-fed drives. By analyzing the CMV values at all switching states under different operating conditions of a current-source-inverter (CSI)-based motor drive, this paper first indicates that the CMV peaks are produced by the zero states in most of the cases. The nonzero-state (NZS) modulation techniques employed in voltage-source converters are adapted for use in a space-vector-modulated current-source converter (CSC) to reduce the CMV magnitude. For NZS modulation in CSCs, the nearest three-state (NTS) modulation sequences are designed with good low-order harmonic performances in their linear modulation region of m _a ≥ 0.67 and with no increase in the device switching frequency. A combined active-zero-state (AZS) modulation technique is also proposed as compensation, for a lower modulation index in the range of 0.4–0.67, when a compromise is made between the dc-link current minimization and high input power factor control. The simulation and experimental results are provided to validate the CMV reduction effects and harmonic performances of the NTS and combined AZS modulation methods in CSI-fed drives. [ABSTRACT FROM AUTHOR]

Partial shading of a photovoltaic array is the condition under which different modules in the array experience different irradiance levels due to shading. This difference causes mismatch between the modules, leading to undesirable effects such as reduction in generated power and hot spots. The severity of these effects can be considerably reduced by photovoltaic array reconfiguration. This paper proposes a novel mathematical formulation for the optimal reconfiguration of photovoltaic arrays to minimize partial shading losses. The paper formulates the reconfiguration problem as a mixed integer quadratic programming problem and finds the optimal solution using a branch and bound algorithm. The proposed formulation can be used for an equal or nonequal number of modules per row. Moreover, it can be used for fully reconfigurable or partially reconfigurable arrays. The improvement resulting from the reconfiguration with respect to the existing photovoltaic interconnections is demonstrated by extensive simulation results. [ABSTRACT FROM PUBLISHER]

A mismatch in a photovoltaic array implies differences in the I\–V characteristics of the modules forming the array. This can lead to significant energy losses known as mismatch losses. The mismatch and the resulting losses can be reduced by altering the interconnection of the array. This paper proposes an optimal total cross tied interconnection, based on a thorough mathematical formulation that can significantly reduce mismatch losses. The improvement over existing photovoltaic array interconnections has been demonstrated by extensive simulation results. [ABSTRACT FROM PUBLISHER]

Voltage-source inverter (VSI) topology is widely used for grid interfacing of distributed generation (DG) systems. However, when employed as the power conditioning unit in photovoltaic (PV) systems, VSI normally requires another power electronic converter stage to step up the voltage, thus adding to the cost and complexity of the system. To make the proliferation of grid-connected PV systems a successful business option, the cost, performance, and life expectancy of the power electronic interface need to be improved. The current-source inverter (CSI) offers advantages over VSI in terms of inherent boosting and short-circuit protection capabilities, direct output current controllability, and ac-side simpler filter structure. Research on CSI-based DG is still in its infancy. This paper focuses on modeling, control, and steady-state and transient performances of a PV system based on CSI. It also performs a comparative performance evaluation of VSI-based and CSI-based PV systems under transient and fault conditions. Analytical expectations are verified using simulations in the Power System Computer Aided Design/Electromagnetic Transient Including DC (PSCAD/EMTDC) environment, based on a detailed system model. [ABSTRACT FROM PUBLISHER]

This paper presents modeling guidelines and a benchmark system for power system simulation studies of grid-connected, three-phase, single-stage Photovoltaic (PV) systems that employ a voltage-sourced converter (VSC) as the power processor. The objective of this work is to introduce the main components, operation/protection modes, and control layers/schemes of medium- and high-power PV systems, to assist power engineers in developing circuit-based simulation models for impact assessment studies, analysis, and identification of potential issues with respect to the grid integration of PV systems. Parameter selection, control tuning, and design guidelines are also briefly discussed. The usefulness of the benchmark system is demonstrated through a fairly comprehensive set of test cases, conducted in the PSCAD/EMTDC software environment. However, the models and techniques presented in this paper are independent of any specific circuit simulation software package. Also, they may not fully conform to the methods exercised by all manufacturers, due to the proprietary nature of the industry. [ABSTRACT FROM AUTHOR]

Advanced transportation technologies, such as hybrid-electric, battery-electric, and hybrid-fuel-cell vehicles, rely on high-power dc/dc converters to interconnect and manage their power sources. Boost converters are particularly useful in stepping up the voltage from a battery or fuel cell to match the traction motor operating voltage range. High-frequency high-efficiency operation is crucial so that the sizes of the passive components can be reduced and fuel economy can be improved. This paper proposes a new regenerative-snubber circuit that greatly reduces the switching losses of the insulated-gate bipolar transistor in the boost converter, so that high-switching-frequency operation can be achieved. The circuit uses no additional magnetic components, enjoys a simple control strategy, and results in very high efficiency while significantly reducing the mass of the converter. Two 5-kW boost converter prototypes, i.e., one with and the other without the regenerative snubber, have been built and tested to evaluate and demonstrate the impact of the proposed snubber circuit. [ABSTRACT FROM AUTHOR]