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In this paper, we illustrate the effect of adding a hypothetical 100-MW doubly fed induction generator (DFIG) wind power plant to a weak transmission system. The effects of various wind plant load factors (100, 60 and 25 % of nameplate rating) are investigated. System performance is compared to a 100-MW conventional synchronous generator interconnected at the same location. The conventional generator is installed some distance away. The simulations demonstrated that DFIG generators provide a good damping performance under these conditions. These results support the conclusion that modern wind power plants, equipped with power electronics and low-voltage ride-through capability, can be interconnected to weak power grids without reducing stability. To conduct the studies, we selected an area of the Western Electricity Coordinating Council power system that is electrically far from major generation centers and is weakly connected to the bulk transmission system. The area contains large motor loads. We observed the dynamic response of large motors in the vicinity, especially their ability to ride through fault events. The studies were conducted using positive sequence phasor time-domain analysis. Index Terms--Low voltage ride through, power system, renewable energy, stability, variable-speed generation, weak grid, wind energy, wind farm, wind power plant, wind turbine.

Renewable-energy systems that are under development need a simulation-based analysis to ensure system stability, power quality, and reliability. Such an analysis may reveal design modifications that need to be made before the system is implemented in the field. The modular simulator RPM-Sim, available on the National Renewable Energy Laboratory Web site (http://wind.nrel.gov/designcodes/simulators/rpmsim/), facilitates a low-cost application-specific study of the dynamics of the wind-solar-diesel hybrid power systems. This paper discusses the principal modules of the simulator and provides several examples of a simulation-based analysis of the renewable-energy systems. These examples illustrate the importance of a simulation-based study of the grid-connected and stand-alone or autonomous systems. The RPM-Sim is open-ended and can be easily expanded. Index Terms--Modeling and simulation of power systems, peak-power tracker (PPT), renewable-energy systems, stiff grid, water pumping, weak grid, wind-farm aggregation.

We analyzed a power system network that consisted of two types of power generation: wind turbine generation and diesel generation. The power quality and the interaction of diesel generation, the wind turbine, and the local load were the subjects of investigation. From an energy-production standpoint, producing as much wind energy as possible is advantageous in terms of decreasing the fuel consumption of the diesel engines and reducing the level of pollution. From the customer point of view, good power quality at the receiving end is important. The purpose of this paper is to show the impact of the wind power plant on the entire system. In addition, we discuss how the startup of the wind turbine and the transient condition during load changes affect the system's voltage and frequency. Index Terms--Diesel generator, hybrid power system, power quality, renewable energy, wind turbine.

Permanent-magnet (PM) generators have been used for wind turbines for many years. Many small wind-turbine manufacturers use direct-drive PM generators. For wind-turbine generators, the design philosophy must cover the following characteristics: low cost, light weight, low speed, high torque, and variable-speed generation. The generator is easy to manufacture and the design can be scaled up for a larger size without major retooling. A modular PM generator with axial flux direction was chosen. The permanent magnet used is NdFeB or ferrite magnet with flux guide to focus flux density in the air gap. Each unit module of the generator may consist of one, two, or more phases. Each generator can be expanded to two or more unit modules. Each unit module is built from simple modular poles. The stator winding is formed like a torus. Thus, the assembly process is simplified and the winding insertion in the slot is less tedious. We built a prototype of one unit module and performed preliminary tests in our laboratory. Follow-up tests will be conducted in our laboratory to improve the design. Index Terms - Axial flux, modular generator, permanent magnet, wind energy.

To optimize the operation of a wind turbine, variable turbine speeds are desirable. To determine the advantages of a variable-speed turbine, the annualized energy production of the turbine needs to be considered. This is done by using a Rayleigh probability distribution to determine the number of hours of a particular wind speed which occur in a given year. This distribution is used with the aerodynamic power generated at a given average wind speed to determine the total energy generated during the year. The total energy for fixed- and variable-speed turbine control are compared. The variable-speed turbines show improved energy production over the constant-speed systems. Index Terms - Variable speed, wind energy, wind turbine.

The application of photovoltaics (PV's) has been increasingly popular, especially in remote areas, where power from a utility is not available or is too costly to install. PV-powered water pumping is frequently used for agriculture and in households. Among many available schemes, the system under study consists of a PV array, a variable-frequency inverter, an induction motor, and a water pump. The inverter feeds the induction motor, which drives the water pump. To seek the optimum power output of the PV array, the inverter is operated at variable frequency, to vary the output of the water pump. The inverter is operated to generate a six-step quasi-square wave, instead of a pulsewidth modulated (PWM) voltage output, to reduce the switching losses. The inverter acts as both a variable-frequency source and a peak-power tracker of the system, thus, having the number of switches minimized. The system is a low-cost design, with a simple control strategy. The dc bus is supported by a dc capacitor; thus, a balance-of-power flow must be maintained to avoid the collapse of the dc-bus voltage. Another advantage of the system is that the current is limited to an upper limit of the PV-array current. Thus, in case a short circuit is developed, the motor winding and the power semiconductor switches can be protected against excessive current flow. Index Terms - Peak-power tracker, photovoltaic, PV array, renewable energy, village electrification, water pumping.

This paper proposes a frequency‐control scheme for Doubly Fed Induction Generator (DFIG)‐based (Type‐3) wind turbines to improve the primary‐frequency‐control when the grid power balance is disturbed. The increasing penetration level of renewable energy sources, like wind power plants, reduces the total available inertia of modern grids, which deteriorate the frequency response in case of sudden power‐mismatches. The proposed closed‐loop participation of wind power plant interacts with the thermal units to reduce the frequency nadir and frequency settling‐time, during the inertial and primary stages. The designed disturbance observer decreases the uncertainties in the estimation of grid parameters, which results in robust PI performance in adjusting the ancillary power provided by wind turbines. Certain measures considered within the control loop to limit the rate‐of‐change‐of‐frequency within the permissive range to avoid the protective relays tripping. Comparative simulations studies on modified IEEE 9‐bus and 68‐bus test systems verify the effectiveness and advantages of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a new series compensated induction generator/battery supply topology which provides a constant voltage and frequency at the terminals, allowing minimum current harmonic distortion while at the same time providing a source and sink of real and reactive power. With appropriate control of the reactive power, the speed of the generator is allowed to vary within a relatively wide range. This technique can be further expanded by applying ac capacitors in parallel with the load to lessen the burden of the PWM inverter. Both simulation and experimental results are in agreement with the theory which leads to the conclusion that the system could be feasible for isolated power generation systems (wind, hydro, diesel, or hybrid generation).

The most common practice for starting a single-phase induction machine (SPIM) is to install a starting capacitor in series with the auxiliary winding. In some applications, two capacitors are used. One is used during the starting period to help create the starting torque. The other one is used during the running condition to improve efficiency. This paper discusses the possibility of using an electronic switch in parallel with the running capacitor, thereby providing the equivalent of a starting capacitor. The capacitor is shorted during each cycle to vary the effective size of the ac capacitor. By using this method, only one capacitor is used for both the starting and running condition, and a similar starting performance can be obtained when compared with the conventional method using two capacitors.

Integration of inverter-based resources (IBR) as a distributed generator (DG) in a microgrid has been a trend. Considering that DG integration transforms the short circuit behavior in terms of magnitude and direction in the system, the conventional protection scheme becomes no longer relevant. The major concern of the traditional protection scheme in a microgrid is the relay operating time to clear the fault in a coordinated manner. This paper presents a novel voltage-current model for directional over-current relay (DOCR) that gives the relay capability to reduce the overall relay time operation. The state-of-the-art of the proposed relay architecture relies on the adaptive pickup current threshold, which varies with the voltage magnitude, to anticipate the small fault current caused by the IBR. The improvement of relay sensitivity offers faster tripping time in a coordination scheme. The proposed method is assessed in the IEEE 13 bus distribution system with the integration of DG. The outcome of this study indicates that the total relay time operation using the proposed method is 2.363s. Compared to the conventional method based on the standard equation, the proposed technique significantly reduces the total relay operating time up to 47.67%. [ABSTRACT FROM AUTHOR]

This article proposes a flexible control scheme (FCS) of a photovoltaic (PV) power plant to support the voltage stability and provide fast recovery after the grid faults in a power system with high penetration renewable energy sources (RES). To achieve these goals, the proposed FCS includes two control loops: a reactive current (IQ) injection loop and a frequency-damping control (FDC) loop. The IQ injection loop injects IQ in proportion to the terminal voltage of the PV during a fault. After the fault clearance, the FDC loop provides active and/or reactive power modulation to stabilize system frequency; then, the FDC loop puts the system recovery forward by damping out the remaining synchronous machines' frequency oscillations. The FDC loop comprises active power damping function without PV power curtailment and reactive power damping function. We verified the proposed FDC's performance in a two-area system for various penetration levels of inverter-based resources using a PSCAD simulator. We investigated the control schemes' effectiveness by comparing the efficiency and the response in mitigating the post-fault oscillations and restoring the voltage regulation. [ABSTRACT FROM AUTHOR]

Hybrid Energy Storage System has been widely applied in aerospace, electric vehicle, and microgrid applications. The advantages are that they include complimentary technologies with both high power and energy capabilities. HESS have the potential to be useful to the bulk power systems, for example to increase the value of energy produced by variable generation resource through enabling participation in ancillary service markets. Actualizing the benefits of HESSs requires optimizing the combined ramping capability of aggregated HESS resources. To provide quality ancillary service, source of HESSs locating at multiple sites of variable generation resource must be optimally sized and cohesively controlled. Optimizing aggregated resources can be formulated as an optimization problem. Successfully solving this problem not only requires using effective solution methods but also depends on accurately and rapidly setting the necessary parameters in the problem formula. This article proposes a novel framework with double-layer structure to solve the optimization problem of aggregating ramping capability. Program developed on the upper layer focuses on solving the optimization of aggregating ramping capability among multiple HESSs and is compatible with most existing optimization algorithms. Program on lower layer of the framework targets at optimizing the local control of a single HESS based on a thorough analytics of HESS operation strategy presented in this article. Result of local optimization is also provided to upper layer program for updating the parameters in formula of optimization problem. Real time hardware-in-the-loop test is conducted to verify the performance of the optimization framework developed. The work presented is expected to provide guidance for implementing this framework in practical operation. [ABSTRACT FROM AUTHOR]

The characteristics of an overcurrent relay (OCR) in protection coordination are determined by adjusting the current and time delay function. This characteristic is adjusted once, fixed, and expected to overcome any disturbances in the system, which raises the risk of incorrect operation of the protection system, such as an overlapping trip or the incapability to limit the current exposure in the protected equipment. This article summarizes different efforts to overcome the modern challenges of overcurrent protection coordination (OCPC) in low or medium voltage industrial and commercial distribution systems. A brief background is introduced from a general perspective and then specifically describes in detail the problem of equipment coordination with a thorough link among mathematical concepts and fundamentals in practice. The operation challenges and ideas to avoid incorrect performance, under normal operation, are discussed. Finally, the article proposes a new optimization method based on the combination of the firefly algorithm and target remedy to overcome such challenges. The optimization considers the combination of inverse-time (ANSI 51) and definite-time (ANSI 50) function in OCR. The method is described and validated in the IEEE 242.2001 to meet the existing regulation of protection coordination. By assessing the critical cases, this article is expected to enhance the OCPC under different fault scenarios with complex coordination of time-current curves. [ABSTRACT FROM AUTHOR]

New challenges regarding system stability and efficiency arise when power systems operate with a high penetration level of inverter‐based renewable sources (IBRSs) and few synchronous generators. Since IBRSs have been on the rise, to secure the stable operation of future power systems, IBRSs will be required to support systems without having to rely on remaining synchronous generators. Also, to efficiently manage the uncertainty of renewable production, power‐to‐gas technology can provide the required flexibility. This study proposes modelling and a control coordination scheme (CCS) of a wind‐to‐hydrogen (W2H) set to optimise electricity production from a variable‐speed wind turbine generator (WTG) while helping balance between supply and demand in a system. To achieve this, a grid‐forming (GFM) inverter‐based WTG is modelled and a set of electrolyser and fuel cell is integrated at the DC circuit of a GFM‐WTG to be coordinated. Furthermore, the CCS offers an opportunity to reduce the investment cost for deploying a W2H set by utilising the control capabilities of a WTG and reducing the need for an additional device. The performance of the proposed W2H set with the CCS was verified considering the variations in system load and wind speed by using Power System Computer Aided Design (PSCAD)/ElectroMagnetic Transients including Direct Current (EMTDC). [ABSTRACT FROM AUTHOR]

Quaternary pumped storage hydropower (Q-PSH) technology, as one of the new advanced-PSH technology, has been developed by taking advantage of Conventional-PSH (C-PSH) and Adjustable Speed-PSH (AS-PSH). By combining adjustable-speed pump unit and conventional hydropower turbine unit in the quaternary configuration, Q-PSH has the more competitive capability of providing fast power support in the future high renewable penetrated power system. Acting as energy storage (ES), Q-PSH provides promising power supply to deal with the uncertainty and variability from renewable energy generation. This paper focuses on the dynamic modeling of Q-PSH technology employing full-converter machine and the impact of Q-PSH on the frequency response in a system. The detailed model of Q-PSH is developed and implemented in the IEEE 14-bus system based on GE Positive Sequence Load Flow (PSLF) platform, which captures the dynamic of multiple operation modes, especially hydraulic short-circuit (HSC) operation mode. Several cases are set up to reveal the advantages of Q-PSH technology when power electronic based renewable energy generation is deployed in the system. Sensitivity studies of the controller in pump governor show the impact of parameters in pump response performance. The comparison case illustrates the impact of frequency response provided by the Q-PSH in the system. [ABSTRACT FROM AUTHOR]

System strength and inertia inherently provided by synchronous generators (SGs) empower a power system to ride through voltage and frequency disturbances. The requirements of system strength and inertia were not enforced in the National Electricity Market (NEM) of Australia since SGs dominated the generation fleet in the past. However, the increasing wind and solar generation continuously displaces SGs and consequently reduces system strength and inertia in the NEM. This paper proposes a formulation of system strength and inertia constrained generator dispatch to reassure NEM operational security in light of emerging high renewable penetration. A fault current iterative solver is developed to evaluate system strength, in which the current limitation and voltage control logic of inverter-based generators, and the fault current contribution from VAR compensators are properly modelled in the phasor domain. The system strength contribution factor of an SG is defined to linearize system strength constraint for unit commitment (UC). System and sub-network inertia constraints are also formulated for the UC to limit the rate of change of frequency (RoCoF) in the event of generator/interconnector trip. The proposed generator dispatch formulation can fully meet system strength and inertia requirements in the NEM. [ABSTRACT FROM AUTHOR]

Synchronous generators are operating for less time than before or being decommissioned in the National Electricity Market (NEM) of Australia, due to the proliferation of asynchronous wind and solar generation. Sub-networks of the NEM will face inertia shortages in the near future. This paper develops a formulation of zonal inertia constrained generator dispatch for power systems with a diversified generator portfolio including synchronous generators, synchronous condensers, inverter-interfaced generators and energy storages. Zonal inertia constraints are formulated in unit commitment and optimal power flow to limit the rate of change of frequency (RoCoF) in the event of network separation. Load frequency relief is also considered to reduce the ramp rate requirement of primary reserve. The proposed formulation can reduce the average cost of primary reserve and maintain zonal inertia adequacy to constrain RoCoF in case of the trip of the interconnector(s). [ABSTRACT FROM AUTHOR]

As the deployment of wind and solar energy increases in the USA, energy storage (ES) will play an important role in future electric power grids to help manage the variability from high penetration levels of renewable generation, ES can provide promising power/energy demand to coordinate with renewables generation, as a virtual power plant (VPP). There is an industry need for the capability in power system studies to model ternary pumped storage hydropower (T-PSH), a pumped storage technology that offers increased system benefits. This study presents a comprehensive vendor-neutral dynamic model of T-PSH in GE's commercial software positive sequence load flow. A new governor model is developed with detailed gate valve modelling and a shared-penstock function. Specifically, this model is designed to simulate the seamless transition among three operation modes: generation mode, pumping mode, and hydraulic short-circuit mode. The developed T-PSH model has been tested and validated on a 10-bus test system. A comparison study of T-PSH to conventional pumped storage hydropower has also been conducted on the Western interconnection system. [ABSTRACT FROM AUTHOR]

As the deployment of wind and solar energy increases in the USA, energy storage (ES) will play an important role in future electric power grids to help manage the variability from high penetration levels of renewable generation, ES can provide promising power/energy demand to coordinate with renewables generation, as a virtual power plant (VPP). There is an industry need for the capability in power system studies to model ternary pumped storage hydropower (T‐PSH), a pumped storage technology that offers increased system benefits. This study presents a comprehensive vendor‐neutral dynamic model of T‐PSH in GE's commercial software positive sequence load flow. A new governor model is developed with detailed gate valve modelling and a shared‐penstock function. Specifically, this model is designed to simulate the seamless transition among three operation modes: generation mode, pumping mode, and hydraulic short‐circuit mode. The developed T‐PSH model has been tested and validated on a 10‐bus test system. A comparison study of T‐PSH to conventional pumped storage hydropower has also been conducted on the Western interconnection system. [ABSTRACT FROM AUTHOR]

This paper proposes a dynamic frequency support scheme of a supercapacitor energy storage system (SCESS) in coordination with run-of-the-river-based pumped storage hydropower (PSH) to enhance the short-term frequency stability in a power system that has a high penetration of renewable energy. To achieve this, the proposed coordinated frequency controller (CFC) enables the SCESS and PSH plant to provide the frequency response. The CFC scheme employs a dynamic droop characteristic in parallel with an integral controller and a distribution function. The dynamic droop characteristic determines the power production for frequency regulation employing a variable gain, which varies with the total capacity of the frequency control units and magnitude of the system frequency error with time; the gain increases with the frequency error, and thereby, arresting the frequency nadir at a higher level than in the conventional droop characteristic. In addition, the distribution function dispatches the power from the dynamic droop and integral controller to the control units in proportion to their headrooms; furthermore, the distribution function considers a sudden loss of generation from an SCESS by its operational constraint during the frequency support. Thus, the proposed dynamic frequency support scheme can enhance the short-term frequency stability for a frequency event. [ABSTRACT FROM AUTHOR]

Growing trends in the deployment of inverter‐based renewable energy will decrease the inertia and frequency control capability of electric power systems by replacing conventional power plants; thus, the frequency of future power systems might be dynamic. This study proposes a capability‐coordinated frequency control (CCFC) scheme of a virtual power plant (VPP) including adjustable‐speed pumped storage hydropower (AS‐PSH), a wind power plant (WPP), and an energy storage system to support the frequency nadir and reduce the steady‐state error of system frequency. The CCFC scheme is based on a hierarchical‐control structure in which a CCFC organises the output of local frequency control units. To support the frequency nadir, the CCFC dispatches weighted frequency errors that are proportional to the available headroom of the units; thus, the errors are forwarded separately with a system frequency error to the primary control loop of each unit and thereby arrest the frequency nadir at a higher value than a system without the CCFC. To reduce the steady‐state error of the system frequency, the CCFC determines a partial active power command by additionally feeding an integrator of the CCFC with a modified frequency error that depends on the unit with the largest control. [ABSTRACT FROM AUTHOR]

Power electronic-based wind turbine generators (WTGs) are capable of providing inertial response to the grid by releasing kinetic energy from the turbine blade; thus, as conventional power plants are retired, the reduction of online inertia can be compensated by designing frequency controls for the WTGs. Deployment of energy storage technology for renewable generations has been increased to have the renewables centralized with a system operator as an independent power supply by making up for their nature of generation. In addition, the cost of energy storage has dropped over time and global research activities on energy storage have been funded by private industries and governments. This paper investigates the opportunity of deploying an energy storage on a doubly fed induction generator (DFIG)-based WTG to respond to the system frequency, and then explores dynamic capabilities of the energy storage-embedded DFIG to boost its contribution while the frequency response is being provided by the power system's online inertia; thus, enabling an effective delivery of ancillary services to the system. To do this, the dynamic models of DFIG and energy storage are combined and simulated under the different operating conditions of a DFIG, such as subsynchronous, synchronous, and supersynchronous operations, using the PSCAD. [ABSTRACT FROM AUTHOR]

This research examines the structural and vibrational characteristics of double-sided flux-concentrating transverse flux machines (TFMs), designed for direct-drive application. Two prototypes of TFM with different stator cores, one with Quasi U-Core and the other with E-Core, have been used for this study. Three-dimensional finite element analysis has been carried out to determine the no-load and with-load performances of the TFMs along with their fluctuating axial electromagnetic force densities acting on the stator teeth. The deformation response of the stator cores was observed in the static structural analysis. Acceleration and noise measurements were experimentally obtained to characterize the vibrational performance of the prototypes. [ABSTRACT FROM AUTHOR]

This paper proposes a multi-timescale operational approach for a three-phase unbalanced distribution system containing hourly scheduling at substation level and minutes power flow operation at feeder level. In the substation scheduling model, the objective is to minimize the system cost with variable renewable generations. The given error distribution model of renewable generation is formulated as a chance constraint and derived into a deterministic form by Gaussian mixture model with genetic algorithm-based expectation-maximization. In the feeder scheduling model, the system cost is further reduced with the optimal power flow (OPF) at a timescale of minutes. Considering the nonconvexity of the three-phase unbalanced OPF problem in distribution systems, the semidefinite programming is used to relax the problem into a convex problem and a distributed computation approach is built based on alternating direction method of multiplier. The IEEE 123-bus distribution system, University of Denver campus distribution system, and the IEEE 8500-bus distribution system are used as the test bench for the proposed approach. The numerical results demonstrate the effectiveness and validity of the proposed method. [ABSTRACT FROM AUTHOR]

An introduction is presented which discusses various reports published within the issue including microgrid and virtual power plant for power supply, voltage reactive power control of a power system, and virtual power plant real-time energy management along with a brief profile of guest editors.

This paper presents the design considerations in cogging torque minimization in two types of transverse flux machines. The machines have a double stator-single rotor configuration with flux concentrating ferrite magnets. One of the machines has pole windings across each leg of an E-Core stator. Another machine has quasi-U-shaped stator cores and a ring winding. The flux in the stator back iron is transverse in both machines. Different methods of cogging torque minimization are investigated. Key methods of cogging torque minimization are identified and used as design variables for optimization using a design of experiments (DOEs) based on the Taguchi method. A multilevel DOE is proposed as an optimization method to reach an optimum solution with minimum simulations. The case study is analyzed in a two-level DOE optimization. Finite element analysis (FEA) is used to study the different effects. Two prototypes are fabricated for validating the FEA results. [ABSTRACT FROM AUTHOR]

This paper presents a novel simulation approach to evaluate new ancillary service controls in the context of large-scale wind power integration. We adopt and compare different types of turbine inertial control methods with the proposed modifications to cope with realistic wind conditions in the field. The simulation procedure is started with the software-based simulation, in which we employ the high-fidelity wind turbine simulator FAST that models a real three-bladed controls advanced research turbine (CART3). The advantages of using FAST are that it can provide convincing simulation results and address the interactions between turbine electrical and mechanical systems. The developed controller model is then rapidly prototyped for the real-time simulation with the hardware-in-the-loop scheme. CART3 will respond to a virtual frequency event triggered in the emulated electric grid modeled in a digital real-time simulator. The introduced simulation platform streamlines the procedure of designing turbine auxiliary controls, and these simulations results give insights on the turbine controls and their impacts on the interconnected power system, as well as the effects on turbine mechanical components. For example, the results indicate that the inertial controls tend to reduce the out-of-plane mechanical loadings in region 2, while such loadings are dominated by the pitch actions in region 3. [ABSTRACT FROM AUTHOR]

High penetration of wind power plants may have an adverse impact on power systems' stability by reducing the inertia, and problems like frequency stability could appear due to total inertia in the system being reduced, making the power system more vulnerable to disturbances. However, most recent grid codes include an emulation inertia requirement for wind power plants, because modern wind turbines are capable of providing virtual inertia through power electronic converter controls to improve frequency stability issues. Because of this, it is necessary that the inertia estimation analyze and quantify the impact of the inertia reduction in power systems. In this paper, an implementation of a methodology for the inertia estimation of wind power plants is presented. It is evaluated through synchrophasor measurements obtained from a Real-Time Digital Simulator (RTDS) implementation, using industrial Phasor Measurement Units (PMUs). This methodology is based on the swing equation. Furthermore, a comparison of the results obtained between two professional tools RSCAD and DIgSILENT PowerFactory is performed, in order to evaluate the accuracy and the robustness of the methodology. This methodology is applied for the inertia estimation of an equivalent of the southeast zone of the Mexican power system, where there is a large-scale penetration of wind power plants. The results demonstrate that this methodology can be applied in real power systems using PMUs. [ABSTRACT FROM AUTHOR]

An innovative power conversion system to convert energy from a variable-frequency wind-powered induction generator to a fixed frequency output. A standard six-switch dc link Current Regulated Pulse Width Modulated (CRPWM) inverter is simultaneously modulated with two current components. A three-phase balanced current component at the induction generator's optimum operating frequency transfers energy from the generator to the converter. A single phase zero sequence current component at a fixed 60 Hz frequency transfers energy from the converter through a zero sequence filter to the load. Unity power factor output is shown both in simulation and experiment, though any arbitrary power factor output may be readily commanded. Maximum power capture for a variable-speed wind turbine is achieved using proven control techniques. This method uses only half of the active power switching devices of conventional conversion methods. Simulation and experimental verification are shown. Index Terms--DC-AC power conversion, induction generators, inverters, power conversion, wind energy, wind power generation.

This paper covers the operation of variable-speed wind turbines with pitch control. The system we considered is controlled to generate maximum energy while minimizing loads. The maximization of energy was only carried out on a static basis and only drive train loads were considered as a constraint. In low to medium wind speeds, the generator and the power converter control the wind turbine to capture maximum energy from the wind. In the high-wind-speed regions, the wind turbine is controlled to maintain the aerodynamic power produced by the wind turbine. Two methods to adjust the aerodynamic power were investigated: pitch control and generator load control, both of which are employed to regulate the operation of the wind turbine. Our analysis and simulation show that the wind turbine can be operated at its optimum energy capture while minimizing the load on the wind turbine for a wide range of wind speeds. Index Terms--Pitch controlled, renewable energy, variable speed, wind turbine generator.

This paper presents the design considerations and analysis of a double-sided transverse flux machine (TFM) for direct-drive wind turbine applications. The proposed TFM has a modular structure with quasi U-core stators and toroidal ring windings. The rotor is designed with ferrite magnets in a flux-concentrating setup to achieve high air gap flux density. This paper presents the different aspects of the TFM, taking into consideration the critical geometric parameters affecting the back-EMF waveform, phase inductance, torque density, and power factor. Three-dimensional finite element analysis (3D-FEA) was used to analyze and optimize the critical TFM design parameters to meet the performance specification. A proof-of-concept prototype was developed based on the proposed design considerations. The prototype was experimentally tested at different operating points and the results showed good correlation with the 3D-FEA results. [ABSTRACT FROM AUTHOR]

This paper proposes an approach for distribution system load forecasting, which aims to provide highly accurate short-term load forecasting with high resolution utilizing a support vector regression (SVR) based forecaster and a two-step hybrid parameters optimization method. Specifically, because the load profiles in distribution systems contain abrupt deviations, a data normalization is designed as the pretreatment for the collected historical load data. Then an SVR model is trained by the load data to forecast the future load. For better performance of SVR, a two-step hybrid optimization algorithm is proposed to determine the best parameters. In the first step of the hybrid optimization algorithm, a designed grid traverse algorithm (GTA) is used to narrow the parameters searching area from a global to local space. In the second step, based on the result of the GTA, particle swarm optimization is used to determine the best parameters in the local parameter space. After the best parameters are determined, the SVR model is used to forecast the short-term load deviation in the distribution system. The performance of the proposed approach is compared to some classic methods in later sections of this paper. [ABSTRACT FROM AUTHOR]

In this paper, a novel E-Core transverse flux machine (TFM) is proposed. The machine has a double stator-single rotor configuration with flux concentrating ferrite magnets and pole windings across each leg of an E-Core stator. E-Core stators with the proposed flux-concentrating rotor arrangement result in better magnet utilization and higher torque density. The machine also has a modular structure facilitating simpler construction. This paper presents a single phase and a two-phase version of the E-Core machine. Case study for a 1 kW, 400 r/min machine for both the single phase and two-phase TFM is presented. [ABSTRACT FROM PUBLISHER]

This paper proposes a temporary frequency-support scheme of a doubly fed induction generator (DFIG) that can improve the frequency nadir while ensuring rapid frequency stabilization, particularly for high wind power penetration levels (WPPLs). Upon detecting a disturbance, the power reference is increased by the incremental power and maintained for a preset period. The proposed incremental power varies with the rotor speed and WPPL. Then, to force the rotor speed to converge to a stable operating range, the reference decreases with the rotor speed. During the deceleration period, the proposed scheme releases less kinetic energy, which helps in the rapid recovery of the rotor speed. During the acceleration period, to accelerate the rotor speed recovery, the reference smoothly decreases with time and rotor speed until it reaches the maximum power point tracking curve. The test results, which are based on the IEEE 14-bus system, demonstrate that even though less kinetic energy is released, the proposed scheme can improve the frequency nadir while rapidly recovering the rotor speed under various wind conditions and penetration levels, but it is particularly effective for higher penetration levels. The scheme helps providing a promising solution to the ancillary services of a DFIG in a power system with high WPPLs. [ABSTRACT FROM PUBLISHER]

To mitigate the degraded power system inertia and undesirable primary frequency response caused by large‐scale wind power integration, the frequency support capabilities of variable‐speed wind turbines is studied in this work. This is made possible by controlled inertial response, which is demonstrated on a research turbine – controls advanced research turbine, 3‐bladed (CART3). Two distinct inertial control (IC) methods are analysed in terms of their impacts on the grids and the response of the turbine itself. The released kinetic energy in the IC methods are determined by the frequency measurement or shaped active power reference in the turbine speed–power plane. The wind turbine model is based on the high‐fidelity turbine simulator fatigue, aerodynamic, structures and turbulence, which constitutes the aggregated wind power plant model with the simplified power converter model. The IC methods are implemented over the baseline CART3 controller, evaluated in the modified 9‐bus and 14‐bus testing power grids considering different wind speeds and different wind power penetration levels. The simulation results provide various insights on designing such kinds of ICs. The authors calculate the short‐term dynamic equivalent loads and give a discussion about the turbine structural loadings related to the inertial response. [ABSTRACT FROM AUTHOR]

This paper proposes a power-smoothing scheme for a variable-speed wind turbine generator (WTG) that can smooth out the WTG's fluctuating power caused by varying wind speeds, and thereby keep the system frequency within a narrow range. The proposed scheme employs an additional loop based on the system frequency deviation that operates in conjunction with the maximum power point tracking (MPPT) control loop. Unlike the conventional, fixed-gain scheme, its control gain is modified with the rotor speed. In the proposed scheme, the control gain is determined by considering the ratio of the output of the additional loop to that of the MPPT loop. To improve the contribution of the scheme toward maintaining the frequency while ensuring the stable operation of WTGs, in the low rotor speed region, the ratio is set to be proportional to the rotor speed; in the high rotor speed region, the ratio remains constant. The performance of the proposed scheme is investigated under varying wind conditions for the IEEE 14-bus system. The simulation results demonstrate that the scheme successfully operates regardless of the output power fluctuation of a WTG by adjusting the gain with the rotor speed, and thereby improves the frequency-regulating capability of a WTG. [ABSTRACT FROM PUBLISHER]