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The energy transition is aimed to take advantage of the operational flexibility of hydropower to extend the integration of intermittent renewable sources. Consequently, the hydrogenerators will have to operate in regimes far away from their designed best-point operation. In order to accurately assess the impact, this paper presents a useful approach to determine the overall operating efficiency of synchronous generators under intermittent operation. An accumulated average efficiency (AAE) model is proposed and compared against an alternative approach; the weighted average efficiency (WAE) model. It is found that the WAE approach produces unrealistic low efficiencies when the generator operates in synchronous condenser mode (SCM) for long periods. In general, the AAE supersedes the WAE for all the different load distributions that were investigated. This was further illustrated by a worked example and by constructing more complex load distributions. A load distribution dominated by SCM yields a difference as high as $33.18 \,\%$ , while an even distribution deviates $1.43 \,\%$ in their respective efficiencies. Finally, a yearly on-site measurement of our studied $103 \,\mathrm{MVA}$ generator’s concentrated load distribution revealed a discrepancy of $0.67 \,\%$ , which is a significant deviation considering what the operating regime would mean in terms of economic implications. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate the sensorless voltage control schemes for the promising wound-rotor brushless doubly-fed induction generator (BDFIG) in both the stand-alone and grid-connected systems. For sensorless target, a new rotor position observer using the space vector flux relations of BDFIG is suggested. The proposed sensorless algorithm does not require any additional observers for obtaining the generator speed by directly detecting the rotor position, which may reduce the cost of the overall control system. For the voltage control purpose, a direct voltage control (DVC) scheme is applied for the stand-alone systems. However, this control strategy did not handle the control of the generated-voltage phase angle and also did not consider the phase sequence issue which are very essential provisions for the smooth connection target in grid-connected applications. Hence, DVC strategy cannot guarantee all the intended provisions for a soft and smooth connection process with the grid side and therefore, an effective grid-synchronization algorithm is proposed. A comprehensive analysis to confirm the functionality of the proposed control strategies is discussed through both simulation and experimental results. The presented study in this paper verifies the efficacy of the proposed schemes for sensorless voltage control target of BDFIG in both the stand-alone and grid-connected applications with a good transient behavior. [ABSTRACT FROM AUTHOR]

In this paper, a circulating-current-excited switched reluctance generator system with the diode rectifier is comparatively studied with a field-current-separated switched reluctance generator. By using a DC power source connected in series with the winding to provide the circulating current, a controllable excitation field is established in the proposed generator. To highlight the influence of this DC voltage source, the injected field current of the field-current-separated generator is kept the same as the circulating current of the circulating-current-excited generator. Then, both advantages and disadvantages of the generators are summarized. The circulating-current-excited switched reluctance generator system inherits both the advantages of good reliability of the separated-winding generator by using the diode rectifier and low copper loss of the integrated-winding machine by combining the AC and DC current in one set of windings. Furthermore, two more practically conventional field-current-separated generators are also compared with the proposed generator in terms of the external characteristics. These two conventional generators have different winding configurations. At last, the analysis is verified by experiments. [ABSTRACT FROM AUTHOR]

High temperature superconducting (HTS) technologies are expected to be a key enabler for lightweight and cost-effective direct-drive (DD) trains for large wind turbines. This paper reports the designing and basic experimental validation of the world's first full-scale DD HTS generator demonstrated on a commercial wind turbine. The HTS generator has its rotor with an HTS field winding working below 30 K, which is achieved by using off-the-shelf Gifford-McMahon cryocoolers. The stator of the generator is essentially conventional, except that the armature winding has four segments to limit fault torques in case of sudden short circuits due to converter failures. Compared to an existing DD permanent magnet generator on the turbine, the air gap shearing stress of the HTS generator is doubled, and the weight is reduced by 24%. The overall design requirements from the turbine integration perspective, as well as the topological considerations, are first described in this paper. The electromagnetic and cryogenic designs are then presented, followed by performance testing of HTS coils. The basic experimental validation shows that the cryogenic design is satisfactory and the measured no-load voltage matches the finite element calculation very well. [ABSTRACT FROM AUTHOR]

Interactions of identical inverters in weak grids have received wide attentions. However, in a practical wind farm, parameters of grid-side inverters and lengths of collecting lines are not completely the same. In this case, a multi-inverter system is not symmetric, and thus the interaction mechanism could be more complicated than that of identical inverters. To investigate the interaction mechanism of differently parameterized inverters, this paper proposes a clustering-based modeling method, in which inverters with identical parameters are classified as one group. The clustering-based model allows for efficient stability assessment using three low-order transfer function matrices, instead of a full-order dynamic model. The root locus analysis is applied in the clustering-based model for stability analysis. Results show that the interconnection of distinct groups of inverters deteriorates common current stability, but has no impact on interactive current stability. The simulation and rtlab-based experimental results validate the effectiveness of the proposed model and the interaction mechanism of differently parameterized inverters. [ABSTRACT FROM AUTHOR]

This paper presents an interfaced wind energy conversion system (WECS) using the switched reluctance generator (SRG). A bidirectional DC-DC converter is adopted as an interface stage to achieve optimal excitation voltage at the SRG side, while a voltage source converter regulates the DC-bus voltage. The SRG works with superior performance, which is characterized by a compromise between efficiency and torque ripple, using the optimal excitation parameters. In addition, the interface stage adds a filtering behavior, mitigating the voltage ripple produced by the SRG, thus improving the power quality reflected in the grid current. A comparison of optimized SRG systems, with direct connection and with interfaced connection, is conducted. Experimental results show that the interface converter improves the performance of the SRG in WECS. Although the extra converter efficiency is approximately 88%, the overall system efficiency is only reduced by up to 2.5% due to the SRG optimal parameters. Nonetheless, as advantages of the method, the torque oscillations are reduced by up to 9.5% in high speeds. Moreover, due to the converter filtering behavior, the grid current harmonic content is reduced by up to 6% in the comparison. Therefore, this research indicates that the proposed approach is a viable option for SRG in WECS, especially for scenarios where the DC-DC converter is already necessary, such as in a microgrid application. [ABSTRACT FROM AUTHOR]

Doubly fed induction generator (DFIG) based wind turbines are most employed for onshore applications because of their cost-effectiveness. The drivetrain improvement is barely studied due to the maturity of the DFIG based systems. This paper investigates two methods for improving the annual energy production (AEP) of the DFIG based wind turbines. They are referred to as short-circuited and $\bigtriangleup$ -Y-connected DFIGs. The origins of the AEP improvement are elaborated from the drivetrain perspective. The improvement is quantified by the aerodynamic model of the turbine and the steady-state model of the DFIG. The two methods are then compared when applied to six regions with different wind speed distributions. The AEP improvements at six regions are evaluated and compared to derive the feasibility of the methods for different locations. [ABSTRACT FROM AUTHOR]

Extension lengths of the stator and rotor end windings will obviously affect the electromagnetic field, fluid flow, and temperature of end components in the end zone of large water-hydrogen-hydrogen cooled turbine generator. In this paper, a 330 MW water-hydrogen-hydrogen cooled turbine generator is analyzed. Flow network of this turbine generator is established. The fluid velocity and the pressure (boundary conditions) are obtained at the different positions of this turbine generator after solving the equations of flow network. Three-dimensional transient electromagnetic field is calculated in the turbine generator end zone. The leakage magnetic field and the losses (heat source) of end components are determined in the turbine generator end zone under the different lengths of stator and rotor end winding extension. Based on these results, three-dimensional fluid-thermal coupled model is given in the end zone. The distribution of complex fluid velocity in the turbine generator end zone is determined. The temperature distribution of the stator end winding, clamp plate, electric screen, and finger plate is studied under the different lengths of stator and rotor end winding extension. The calculated temperature results agree well with the measured values. [ABSTRACT FROM AUTHOR]

This paper proposes a new slot-permanent-magnet (PM)-assisted variable flux reluctance generator (VFRG) for the hybrid electric vehicles. Compared with the existing dc-excited VFRG, auxiliary PMs are introduced into stator slots, which can fully utilize the redundant harmonics excited by single-layer concentrated armature winding and boost power density significantly. In this paper, the generator structure, operation principle as well as the winding restriction are discussed. Different slot pole combinations are fully compared in terms of both no-load and on-load characteristics. Some important design parameters are redefined for this slot-PM-assisted topology based on a sensitivity analysis. To release cost burden, both rare-earth-magnet and ferrite-magnet-assisted designs are considered into optimization. The feasibility of this new topology is verified by using finite element analysis and prototype testing. [ABSTRACT FROM AUTHOR]

A novel approach to synchronization of shore and ship power systems is presented in this paper, providing a blackout-free supply from the shore. An application of the shore-to-ship synchronization strategy, proposed in the paper, instead of the commonly used ship-to-shore synchronization, enables a fuller exploitation of the high dynamic offered by power electronic converters. Converters are increasingly being used in shore-to-ship supply arrangements in order to ensure the required levels of voltages and frequencies, specified in shore-supply related standards. Simulation results, supported by preliminary tests in small-scale experimental arrangements, have confirmed that shore-to-ship applications might significantly improve the dynamics of the synchronization as well as the parameters of the load transferring process, especially under distorted ship voltage conditions. [ABSTRACT FROM AUTHOR]

Design of a multiphase hybrid permanent magnet (HPM) generator for series hybrid electric vehicle (SHEV) application is presented in this paper. The proposed hybrid excitation topology together with an integral passive rectifier replaces the permanent magnet (PM) machine and active power electronics converter in hybrid/electric vehicles, facilitating the control over constant PM flux-linkage. The HPM topology includes two rotor elements: a PM and a wound field (WF) rotor with a 30% split ratio, coupled on the same shaft in one machine housing. Both rotors share a nine-phase stator that results in higher output voltage and power density when compared to three-phase design. The HPM generator design is based on a 3-kW benchmark PM machine to ensure the feasibility and validity of design tools and procedures. The WF rotor is designed to realize the same pole shape and number as in the PM section and to obtain the same flux-density in the air-gap while minimizing the WF input energy. Having designed and analyzed the machine using equivalent magnetic circuit and finite element analysis, a laboratory prototype HPM generator is built and tested with the measurements compared to predicted results confirming the designed characteristics and machine performance. The paper also presents comprehensive machine loss and mass audits. [ABSTRACT FROM AUTHOR]

This paper proposes a power mismatch elimination strategy for a medium-voltage modular multilevel converter (MMC) -based photovoltaic (PV) system. In the MMC-based PV system, each submodule of the MMC is energized by multiple PV generators, each interfaced with the dc port of the submodule by a corresponding isolated dual active bridge dc–dc converter. This configuration allows for a transformerless connection to the host grid and independent maximum power point tracking for the PV generators. The paper then proposes a power mismatch elimination strategy that ensures that the current delivered to the host grid is balanced in spite of unbalanced PV generator outputs. The proposed power mismatch elimination strategy employs a dc differential current to equalize the leg powers, and an ac differential current to stabilize the dc voltages of the submodules. The effectiveness of the proposed power mismatch elimination strategy is demonstrated by time-domain simulations conducted on a model of the PV system in PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

Due to the increasing penetration of distributed renewable energy generation systems, their participation in power system restoration is gaining a growing interest. For the black start of a system without synchronous generation, Wind Energy Conversion Systems (WECSs) must be able to control voltage and frequency. However, WECSs have been traditionally controlled as current sources. This paper proposes a decentralized control system for the black start of WECSs operating in parallel. In comparison with known methods for operation of distributed energy resources without synchronous generation, this paper specifically addresses the requirements of WECSs. Total wind power is regulated to match the demand during the black start process. This power is shared between WECSs according to the wind power available in each of wind turbine using a droop control characteristic between the voltage and active power. Using the so-called reactive power synchronization method, the WECSs can operate in synchronism even if wind power is significantly different in each wind turbine. Simulation results for a case study where two WECSs are connected in parallel are presented to validate the proposal. [ABSTRACT FROM AUTHOR]

In this paper, a fault-tolerant control of a symmetrical six-phase induction generator based on a variable structure strategy is experimentally tested and compared with a classical proportional-integral controller. For this, we use a linear feedback sliding mode controller with switched feedback gains in such a way to control the inner current loops of this machine. One main drawback of the classical three-phase induction generator is that it is no longer possible to generate electrical power if only one phase is lost on the stator side. To overcome this issue, we will use a squirrel cage six-phase induction machine that can still operate when at least three phases remain on the stator side. This article proposes to use a variable structure control in order to cope with unbalanced currents and also maintain the generation of power when a loss of the stator's phases occurs. The proposed controller shows its robustness and good regulation performance in healthy and faulty modes. The simulation and experimental tests were carried out with a 24 kW induction machine that represents 1/100 of the electrical power of a real wind turbine. [ABSTRACT FROM AUTHOR]

Transverse flux PM generator (TFPMG) is a capable option for direct-drive wind turbine (DDWT) applications due to its high-power characteristics at low speeds. NdFeB-based TFPMGs may suffer from a higher total cost and lower thermal capabilities compared to the ferrite-based TFPMGs. Not yet covered in the existing literature, in this paper a transverse flux ferrite PM generator (TFFPMG) is proposed, designed, and modeled, which can also resolve the unipolar flux generation and even-order harmonics in the flux linkage of the conventional TFPMGs that occurred in conventional TFPMGs through its innovative two-segment trapezoidal shape magnet structure. Due to the 3-D flux path nature in the proposed TFFPMG, either 3-D finite element analysis (FEA) or 3-D magnetic equivalent circuit (MEC) modeling should be used. As a computationally efficient modeling method, a nonlinear 3-D MEC is established to model the entire structure of the TFFPMG while the core saturation and nonlinear permeances are also fully considered to improve the modeling accuracy. The electromagnetic performance of the proposed TFFPMG modeled by 3-D MEC in various loading conditions is validated by both 3-D FEA and experimental results using a 2.5 kW TFFPMG. A close agreement between 3-D MEC modeling predictions, 3-D FEA, and test results confirms the reliability of the MEC modeling for the proposed TFFPMG. [ABSTRACT FROM AUTHOR]

In this paper, the control of a grid-connected wind energy conversion system (WECS), based on an induction machine fed by a matrix converter (MC), is presented. The MC is controlled using a space vector modulation algorithm with zero displacement factor at the input. Stability issues related to the operation of the WECS for the whole power and speed range are fully analyzed in this paper, using a simplified small-signal model. The implementation of a model reference adaptive system (MRAS) observer, for sensorless control of the proposed WECS, is also presented in this paper. The MRAS observer is implemented using the output voltage demand, avoiding the use of voltage transducers to measure the machine voltage. From the speed estimated by the MRAS observer, the electrical torque of the induction generator is regulated in order to drive the WECS to the operating point where the aerodynamic efficiency is maximized. Experimental results obtained with a 2.5-kW prototype are presented and fully discussed in this paper. [ABSTRACT FROM PUBLISHER]

This paper presents an intelligent wind speed sensor less maximum power point tracking (MPPT) method for a variable speed wind energy conversion system (VS-WECS) based on a Q-Learning algorithm. The Q-Learning algorithm consists of Q-values for each state action pair which is updated using reward and learning rate. Inputs to define these states are electrical power received by grid and rotational speed of the generator. In this paper, Q-Learning is equipped with peak detection technique, which drives the system towards peak power even if learning is incomplete which makes the real time tracking faster. To make the learning uniform, each state has its separate learning parameter instead of common learning parameter for all states as is the case in conventional Q-Learning. Therefore, if half learned system is running at peak point, it does not affect the learning of unvisited states. Also, wind speed change detection is combined with proposed algorithm which makes it eligible to work for varying wind speed conditions. In addition, the information of wind turbine characteristics and wind speed measurement is not needed. The algorithm is verified through simulations and experimentation and also compared with perturbation and observation (P&O) algorithm. [ABSTRACT FROM AUTHOR]

High temperature superconducting (HTS) generators could enable a lightweight and cost-effective direct drive (DD) wind turbines with large power ratings. The EU-funded EcoSwing project successfully demonstrated the world's first full-scale MW-class HTS generator on a commercial DD wind turbine. This paper focuses on the commissioning of the EcoSwing HTS generator on the wind turbine. The commissioning campaigns, including the rotor cool-down, excitation of the HTS field winding, and the power production of the generator, are presented in the paper. In the testing period, the generator was grid-connected for more than 650 hours and accumulatively produced more than 600 MWh to the grid. The target output power of the 3 MW class was reached. Throughout the real-life testing on the wind turbine, the generator performed well from the electromagnetic, thermal, and mechanical perspectives. Moreover, the generator even sustained three sudden short circuits in the converter system. The work reported has shown that HTS generators are technologically feasible for wind turbine applications, and the technology readiness level of HTS wind turbine generators has been improved to 6 $\sim$ 7 for the first time. [ABSTRACT FROM AUTHOR]

The proposed algorithm is based on the discrete Fourier transform (DFT) and a software-based synchronous sampling technique that is locked to the power system frequency using reconfigurable field-programmable gate arrays (FPGAs). The main purposes of this paper are to provide a new method of recording the high-resolution rotor angle of a synchronous machine and a practical method for deriving more accurate d-q-axis synchronous machine parameters. An FPGA is used in realizing the rotor-angle transducer and it simultaneously detects the frequency of the generator terminal voltage. The DFT algorithm is combined with an interpolation technique that downsamples the high-frequency sampled data using the measured power system frequency. This paper also describes a practical method for determining synchronous machine model parameters. The proposed algorithm for recording the generator dynamics was implemented in terms of Labview, Real-Time PCI eXtensions for Instrumentation (PXI) target, and the FPGA, and was applied to generator testing of a 1205-MVA thermal power unit. Its synchronous machine parameters were also derived using the recorded rotor angle. [ABSTRACT FROM AUTHOR]

The static exciter is dominating among large grid-connected generators due to the weak dynamic performance of conventional brushless exciters. In this contribution, a six-phase double-star outer pole permanent magnet rotating brushless exciter is evaluated with different active rectification topologies. Both thyristor-based and chopper-based topologies are considered. A high speed response brushless excitation system is obtained by replacing the conventional rotating diode bridge rectifier with the proposed active rectification topologies on the shaft. The given two-stage system generates its own excitation power directly from the shaft, contrary to static exciters. The selection of an appropriate rectification topology could minimize the rotor armature phase currents for a given generator field current. The objective is a high-power factor and a high utilization of the exciter machine. An optimal rectification topology makes higher ceiling currents was possible, improving the transient behavior of the synchronous generator. In this paper, we show that six-phase topologies add complexity, but improve exciter redundancy, increase the available ceiling voltage, and reduce the steady-state torque ripple. Experimental results are given for validating the models implemented for the analysis. [ABSTRACT FROM AUTHOR]

This paper examines a novel method of constructing large diameter generators using many layers of steel wire in place of laminations. The stator coreback is formed by winding thin steel wire around the outside of the armature coils and then encapsulating the structure in epoxy. This technique simplifies the manufacturing process by removing the requirement to build a large support structure to carry the laminations. The electromagnetic behavior of a wire coreback is very different from traditional laminations, however, and produces abrupt changes in flux density across its thickness. The material is difficult to model using conventional FEA techniques due to the large number of elements required to mesh the small diameter wire. This paper examines two alternative modeling approaches. Method 1 uses two-dimensional (2-D) FEA to model the steel wire as a lamination oriented in the “wrong” direction. Method 2 uses a quasi-analytic approach based on detailed 3-D FE analysis of a small section of the generator to capture the flux density profile in the airgap. The two models are benchmarked against a prototype generator tested in the laboratory, and it is shown that the quasi-analytical technique gives the most accurate prediction of performance. [ABSTRACT FROM PUBLISHER]

This paper analyzes the influence of the doubly fed induction generator (DFIG) based wind farm (WF) participating in the frequency response (FR) on the oscillation modes of the power system and proposes a hybrid modulated active damping scheme to mitigate the potential power oscillations. The FR control would excite additional lightly damped electro-mechanical oscillation mode and deteriorate the damping for the primary interarea oscillation mode inside the power system. Moreover, the internal damping effect on the turbine torsional oscillation would be weakened resulting shaft damage risk if FR is required by the WF. The frequencies of those modes are usually close to each other. By integrated hybrid multiple active damping loops into the active and reactive power controller of the WF respectively, the damping control on all the oscillation modes can be improved at the same time. The analysis and effectiveness of the proposed scheme in this paper are verified by the time-domain simulations. [ABSTRACT FROM PUBLISHER]

This paper presents the variable speed applications of a recently developed generator named Brushless Induction Excited Synchronous Generator (BINSYG) for offshore wind power applications. The machine is basically a Synchronous generator (SG) with its excitation fed from a three-phase Induction Machine (IM). Both the machines are embedded in the same machine frame and share the same magnetic core. The SG feeds power to the dc grid via a five-level Unidirectional Dual Flying Capacitor Active Neutral Point Converter. The converter is controlled such that the Total Harmonic Distortion (THD) of the current flowing through the stator of SG is below 5% and at unity power factor at SG terminal. The excitation developed by IM-type exciter is adjusted using a new control strategy that is proposed for Voltage Source Inverter, such that BINSYG can operate at wide speed range necessary for Wind Energy Systems (WES). The complete system is controlled without using ac voltage sensors and speed encoders. The proposed WES with its control algorithm is first modelled in MATLAB/Simulink. Further, rapid control prototyping verification in the form of real-time results using OPAL-RT is presented for a small scale (5.5 kVA) BINSYG feeding power to a 750 V dc grid. [ABSTRACT FROM AUTHOR]

Wind energy systems are expected to supply the majority of future generation from renewables due to the maturity of technology and the availability of energy resources. In this paper, in order to improve the ability of tracking a desired power reference at high wind speed region, a pitch control strategy based on error-based active disturbance rejection control (ADRC) approach is proposed for variable speed wind turbine system, providing a novel practically appealing control structure. The stability analysis of the proposed pitch controller based on singular perturbation theory is provided. Next, an improved coyote optimization algorithm (COA) fused with L $\rm {\acute{e}}$ vy flight and Sinusoidal map is proposed to address the controller parameters tuning issue. Then, the effectiveness of the proposed solution is verified on a 1.5 MW FAST simulator taking into account both multi-step and IEC turbulence wind conditions. Finally, an analysis in frequency domain of the pitch control system with the proposed control strategy is presented. [ABSTRACT FROM AUTHOR]

Modern wind turbines vary greatly in their drive train configurations. With the variety of options available, it can be difficult to determine which type is most suitable for on and offshore applications. A large percentage of modern drive trains consist of either doubly fed induction generators with partially rated converters or permanent magnet generators with fully rated converters. These configurations are the focus of this empirical reliability comparison. The turbine population for this analysis contains over 1800 doubly fed induction generators, partially rated converter wind turbines, and 400 permanent magnet generator fully rated converter wind turbines. The turbines analyzed are identical except for their drive train configurations and are modern MW scale turbines making this population the largest and most modern encountered in the literature review. Results of the analysis include overall failure rates, failure rates per operational year, failure rates per failure mode, and failure rates per failure cost category for the two drive train configurations. These results contribute toward deciding on the most suitable turbine type for a particular site, as well as toward cost of energy comparisons for different drive train types. A comparison between failure rates from this analysis and failure rates from similar analyses is also shown in this paper. [ABSTRACT FROM AUTHOR]

As the development of tidal turbines attracts more and more attention in recent years, reliable design and efficient control of tidal turbines are becoming increasingly important. However, the majority of existing tidal turbines still utilize traditional fixed ratio geared transmissions and the associated control designs focus on simple feedback controllers that use measurements or possibly estimates of the turbine itself or current local tidal profile. Therefore, the measurement and control are inevitably affected by the inherent delay with respect to the current tidal speeds. This paper proposes a novel tidal turbine with continuously variable speed hydrostatic transmissions and a nonlinear predictive controller that uses short-term predictions of the approaching tidal speed field to enhance the maximum tidal power generations when the tidal speed is below the rated value. The controller is designed based on an offline finite-horizon continuous time minimization of a cost function, and an integral action is incorporated into the control loop to increase the robustness against parameter variations and uncertainties. A smooth second order sliding mode observer is also designed for parameter estimations in the control loop. A 150 kW tidal turbine with hydrostatic transmission is designed and implemented. The results demonstrate that the averaged generator power increases by 6.76% with this preview based nonlinear predictive controller compared with a classical non-predictive controller. [ABSTRACT FROM AUTHOR]

For the comparative investigation on direct-drive permanent magnet (PM) wind power generators, this paper presents the analytical approach based on transfer relations. Considering various generator types, this paper provides analytical solutions for those generators’ characteristic prediction, and the analyzed results are verified. In addition, since our analysis models are designed to be connected to a grid, the power loss characteristics according to grid output power variation are presented considering harmonics in generator current caused by the grid-connected power converter. [ABSTRACT FROM PUBLISHER]

This paper presents a new hybrid wind turbine system that is formed by a continuously variable transmission connection of the turbine drive shaft with an air expander/compressor. A mechanical power split device is designed to synthesize the power delivered by the wind turbine and the air expander/compressor. A small-scale hybrid wind turbine system is mathematically modeled, analyzed, and validated using a laboratory-scale experimental test rig. By utilizing compressed air energy storage, it is shown that the hybrid wind turbine system can provide smooth power output under fluctuating wind speed conditions. Such a direct connection structure reduces the overall system cost by using one generator instead of two compared with the conventional CAES system structure. The study demonstrates the benefit of improved efficiency and flexibility brought to the turbine operation by the hybridization of wind energy and stored energy. [ABSTRACT FROM AUTHOR]

For the fixed-gain inertial control of wind power plants (WPPs), a large gain setting provides a large contribution to supporting system frequency control, but it may cause over-deceleration for a wind turbine generator that has a small amount of kinetic energy (KE). Further, if the wind speed decreases during inertial control, even a small gain may cause over-deceleration. This paper proposes a stable inertial control scheme using adaptive gains for a doubly fed induction generator (DFIG)-based WPP. The scheme aims to improve the frequency nadir (FN) while ensuring stable operation of all DFIGs, particularly when the wind speed decreases during inertial control. In this scheme, adaptive gains are set to be proportional to the KE stored in DFIGs, which is spatially and temporally dependent. To improve the FN, upon detecting an event, large gains are set to be proportional to the KE of DFIGs; to ensure stable operation, the gains decrease with the declining KE. The simulation results demonstrate that the scheme improves the FN while ensuring stable operation of all DFIGs in various wind and system conditions. Further, it prevents over-deceleration even when the wind speed decreases during inertial control. [ABSTRACT FROM PUBLISHER]

In this paper, considering the oceanic multiple degree of freedom motion, a double stator linear-rotary permanent magnet (DSLRPM) generator with Halbach PM array is proposed for direct drive ocean energy conversion. It focuses on the power maximization generation resulting in a highly optimized DSLRPM generator to improve the generated power of the ocean energy conversion system in complex ocean condition. Based on the power-oriented design concept, the whole design process is divided into two steps. In the initial design procedure, the operating domain design is conducted, and the design point is selected. In the optimization design procedure, the non-dominated sorting genetic algorithm II (NSGA-II) coupled with response surface methodology (RSM) and sensitivity analysis is proposed to be as an optimization design technique. Finally, a prototype based on the optimized results is manufactured. The experimental results agree with the simulated result, and they both reveal that the proposed method can offer an effective design and optimization for the DSLRPM generator, where the multiple oceanic motions and complex operating conditions are considered. [ABSTRACT FROM AUTHOR]

For the islanded dc microgrids, the photovoltaic (PV) generators and storages are the main components to provide power and maintain the system stability. To avoid the dc bus voltage rise, the overcharge and overdischarge of storages, the coordination between generations and storages is critically important. In this paper, a decentralized control method is proposed to realize the generation-storage coordination based on the dc bus voltage signaling. First, a novel state-of-charge (SOC) based droop control strategy is designed for storages to provide dc bus voltage support and balance their SOCs, which can prevent the overuse of a certain storage. Second, an adaptive power control strategy is applied to PV generators, which can seamlessly regulate the output power of PV generators off maximum power point tracking mode according to the dc bus voltage. Combining these two strategies through the dc bus voltage signaling, the generation-storage coordination can be realized. Furthermore, the stability of the proposed control method is also analyzed. All the conclusions are verified by the real-time hardware-in-loop tests. [ABSTRACT FROM AUTHOR]

This paper proposes a novel integration of doubly fed induction generator (DFIG)-based wind farms within microgrids. The main and probably the best feature of this method is that the voltage and frequency of the microgrid are controlled by the wind generators through droop characteristics. Therefore, this control structure is quite suitable for cases where the wind generators supply significant part of the load. This paper will adjust the standard droop method in order to force the DFIG to share the load according to their available wind powers, not only their ratings. This paper focuses on microgrid application; however, the method is quite applicable for ac grid and HVdc link connections as well. The proposed control methods are validated using PSCAD/4.2.1 Professional/EMTDC simulations. [ABSTRACT FROM PUBLISHER]

In this paper, the interactions of the torsional dynamics of the wind turbines and power modulation for oscillations in DFIG-based wind generators are investigated. One major concern of active power modulation is its interaction with wind turbine’s torsional dynamics. A study case with wind turbine torsional dynamics modeled is derived from our earlier work. Both active power modulation and reactive power modulation controls are developed in this paper. The paper successfully demonstrates the presence of interaction between active power modulation and torsional dynamics and the absence of such interaction in the case of reactive power modulation. Linear system analysis and root loci approach are employed to demonstrate such interaction. Simulation results in MATLAB/Simulink further verifies the analysis. The major contribution of this paper is: 1) the demonstration of the potential disadvantage of wind generation's active power modulation; and 2) the identification of reactive power modulation in wind generation as an alternative for interarea oscillation damping. [ABSTRACT FROM AUTHOR]

With the development of more electric aircraft, higher demand for electrical energy is put forward in generation systems. The constant speed drive is eliminated in variable frequency ac (VFAC) generation systems, which makes integrated starter-generator (SG) can be realized. Considering the inherent defects of wound rotor synchronous SG systems, such as the complex starting excitation methods and rotating rectifiers, this paper proposes a VFAC SG system based on hybrid excitation synchronous machines (HESMs) for aircraft applications. The key technology of HESM-based VFAC SG system is to regulate the field current in such a way that three major modes of VFAC SG system operation for aircraft applications, namely, engine starting operation, transition mode, and generating operation, can be realized effectively. The simulation model of HESM-based VFAC SG system is built and analyzed. A prototype HESM-based VFAC SG system has been designed and developed for experimental verification. The starting operation, transition mode, and generating operation of the prototype HESM-based VFAC SG system are implemented. The feasibility of HESMs to VFAC SG system for safety-critical aircraft applications has been validated. [ABSTRACT FROM AUTHOR]

A rotating diode failure is the most common electrical fault in brushless exciters. It brings a safety hazard to the generator excitation system so that the whole unit is forced to be shut down. It is necessary to detect and deal with this fault accurately. According to the electrical structure characteristics of brushless exciters, this paper analyzes the influence of an open circuit rotating diode on the armature current and armature magnetic field. It is proposed here that we install the magnetic field detection coil on the stator yoke and use the rotational frequency component in the induced voltage to diagnose the open circuit fault in the rotating diode online. In this paper, a two-dimensional model of a 22-pole 39-phase brushless exciter is established, and the new detection method is verified using a finite element simulation. This provides a new solution for rotating diode fault diagnosis in brushless exciters. The detection method exhibits good performance in terms of practicability, real-time usage, and versatility. [ABSTRACT FROM AUTHOR]

The belt-driven starter/generator (BSG), as a cost-effective solution, has been widely employed in hybrid electric vehicles (HEVs) to improve the stability and reduce the fuel consumption of the vehicles. It can provide more than 10% reduction in CO2. Electrical machine is the heart of the BSG system, which is functioned both as motor and generator. In order to optimize both aspects of motor and generator simultaneously, this paper presents a new multimode optimization method for the switched reluctance machines. First, the general multimode concept and optimization method are presented. The switched reluctance motor and the switched reluctance generator are the two operation modes. The optimization models are established based on motor and generator functions. Sensitivity analysis, surrogate models and genetic algorithms are employed to improve the efficiency of the multimode optimization. Then, a design example of a segmented-rotor switched reluctance machine (SSRM) is investigated. Seven design variables and four driving modes are considered in the multiobjective optimization model. The Kriging model is employed to approximate the finite element model (FEM) in the optimization. Finally, the optimization results are depicted, and an optimal solution is selected. The comparison between the initial and optimal designs shows that the proposed method can improve the foremost performance of the SSRM under all driving modes. [ABSTRACT FROM AUTHOR]

This paper presents a stand-alone wind-based dual-stator-winding induction generation (DWIG) system under unbalanced load conditions. The DWIG consists of a cage-rotor and a stator with two sets of three-phase windings with the same number of pole-pairs. The first winding is fed by a three-phase STATCOM and is used to excite the machine. The STATCOM main frequency is chosen to be equal to generator load frequency. Since two stator windings share the same working frequency, the output frequency is independent of the generator load power demand, and its prime mover speed. The second stator winding is connected to the load. Assuming a three-phase unbalanced load, the main aim of this paper is to use conventional proportional-integral (PI) controllers for three different control targets. In the first target, the PI controllers are used to regulate the load positive-sequence-voltage while simultaneously removing the load negative-sequence-voltage. In the second and third control targets, the positive-sequence-voltage is regulated while simultaneously removing either the load negative-sequence-current or the load active power double-frequency-component. A separate PI regulator is used to regulate the STATCOM dc-link voltage by obtaining the rotor reference speed. Some simulation and experimental results are presented to prove the validity and effectiveness of the proposed system. [ABSTRACT FROM PUBLISHER]

This paper studies the electromagnetic torque (EMT) of a nonsalient-pole synchronous generator with the interturn short circuit of field windings (ISCFW). First, the virtual displacement method serves as a basis for analyzing the postfault steady-state harmonic characteristic of EMT. The interactions among various space magnetic fields with different orders, speeds, and rotation directions generated by the stator and rotor are considered. A general conclusion on the EMT characteristics under the fault is then derived. Second, based on the connection conditions of the rotor and stator windings under the fault, the voltage and flux relationships among each circuit are analyzed and the calculation model of the EMT under the fault is built. Through comparisons among the simulations, experiments, and theoretical analyses, the correctness of the calculation model is verified. In addition, a three-pole-pair nonsalient-pole synchronous generator after transformation is used for the case study. The calculation and analysis results indicate that ISCFW generates ac pulsation components with their harmonics closely linked to the stator winding configuration. This paper deepens the fault mechanism of ISCFW and lays a foundation for fault monitoring based on mechanical characteristics. [ABSTRACT FROM AUTHOR]

Rotor winding interturn short-circuit (RWISC) occurring in turbine generators has been plaguing power producers with respect to production safety and economy. RWISC is characterized by impaired fundamental component of the exciting magnetomotive force and resultantly reduced no-load electromotive force E0. In this paper, an inverse reasoning method is proposed to analyze RWISC. First, it is assumed that the generator's rotor winding is running in good condition and the generator's electromagnetic power (virtual power, VP) is inversely calculated from the obtained operational data on generators. Second, the difference between the virtual power and the actual electromagnetic power is calculated and then used to determine whether an RWISC occurs on the turbine generator. Three real cases of severe rotor winding shorts have demonstrated that this method proposed in this paper not only enables to diagnose RWISC, but also allows determining the RWISC level and its trending accurately. The method is appropriate for RWISC diagnosis, whether offline or online, and can be applied to make the generator sets in the power plant run safer, more economical, and more stable. [ABSTRACT FROM AUTHOR]

This paper presents a robust nonlinear distributed controller design for islanded operation of microgrids in order to maintain active and reactive power balance. In this paper, microgrids are considered as inverter-dominated networks integrated with renewable energy sources (RESs) and battery energy storage systems (BESSs), where solar photovoltaic generators act as RESs and plug-in hybrid electric vehicles as BESSs to supply power into the grid. The proposed controller is designed by using partial feedback linearization and the robustness of this control scheme is ensured by considering structured uncertainties within the RESs and BESSs. An approach for modeling the uncertainties through the satisfaction of matching conditions is also provided in this paper. The proposed distributed control scheme requires information from local and neighboring generators to communicate with each other and the communication among RESs, BESSs, and control centers is developed by using the concept of the graph theory. Finally, the performance of the proposed robust controller is demonstrated on a test microgrid and simulation results indicate the superiority of the proposed scheme under different operating conditions as compared to a linear-quadratic-regulator-based controller. [ABSTRACT FROM AUTHOR]

Torque-speed characteristics of an oscillating wave energy converter (WEC) is necessary to determine its optimal power point and to extract the corresponding design parameters for the generator and power converters used to generate power from the WEC. In this paper, analytical expressions for the mechanical torque on the WEC due to the oscillating surge of seawater are derived, and the dynamic model of the WEC is developed. A method for hardware emulation of the WEC is proposed by controlling the torque of an electric motor. The reference torque of the motor is derived by solving the mechanical dynamics of the WEC system in a real-time computing device considering a typical sea wave input. The emulated WEC dynamics is validated through variable loading on a generator coupled with the emulator. Experimental results support the existence of an optimal power point of the derived WEC model. [ABSTRACT FROM AUTHOR]

This paper proposes an enhanced control scheme for a direct drive variable speed wind turbine with an interior permanent magnet (IPM) synchronous generator. The proposed control scheme incorporates maximum torque per ampere (MTPA) trajectory and maximum power extraction, which ensures the generation of required torque for maximum power extraction with minimum stator current. This in turn minimizes stator copper loss and excessive heat generated in the IPM synchronous generator. The performance of the proposed control scheme has been validated through rigours simulation and experimental studies under varying wind speed conditions. The simulation and experimental results demonstrate the efficacy of the proposed control method. [ABSTRACT FROM AUTHOR]

The 12-phase synchronous generator–rectifier system is widely used in mobile platforms that require high system security. In the system, the stator internal short circuit is a kind of common fault that may compromise the security of the generator. This paper gives detailed analysis on the internal phase-to-phase short-circuit faults, including the faults within the same group of Y-connected phases and those between different groups of Y-connected phases in the 12-phase system. A multiloop mathematical model is built to calculate all the voltages and currents when the faults occur in the system. In the model, the additional short-circuit loop between different phases, as well as the fault effects on both the air-gap magnetic field and the circuit topology of the system, are taken into account. Then a series of fault experiments are carried out on a 12-phase model machine. In view of the good agreement of experiment and simulation results, the effectiveness of the mathematical model and the simulation method is confirmed. Finally, the electric characteristics of the internal phase-to-phase short-circuit faults are summarized, and the fault mechanism is revealed. [ABSTRACT FROM AUTHOR]

The low cost of diesel generators have led to their increased presence in hybrid microgrids. The presence of low-inertia devices in such microgrids makes the stability of hybrid microgrids a pressing concern. This paper analyzes the impact of equal and unequal active power sharing conditions on the dynamic stability of a hybrid microgrid. The sensitivity of low-frequency eigenvalues to changes in active power droop gains is studied and a “domain of stability” region is proposed to assess the stability of hybrid microgrids under different power sharing conditions. The different operating regions within the domain of stability are defined. Based on the sensitivity analysis and the domain of stability, the optimal droop gain for the diesel generator to improve the stability of the microgrid is determined. Time-domain simulations are carried out in the MATLAB/Simulink software environment and are used to validate the small-signal stability analysis results. [ABSTRACT FROM AUTHOR]

Fast synchronization of generators and microgrids will be a critical technology in future power systems with high penetration of nondispatchable power resources. Existing synchronization methods rely on generator controls and their performance is limited by the generator characteristics. Speed of microgrid synchronization is further limited by the communication link among generation units. These factors lead to a slow and sometimes faulty synchronization, predominantly because of the phase mismatch during interconnection. This paper frames the generator synchronization problem as a phase-locked loop (PLL) design problem and introduces a voltage-source converter (VSC) based synchronizer for implementing the PLL-based active synchronization method. The VSC-based synchronizer is connected at the point of common coupling and it relies only on local measurements for control. It ensures zero phase error during interconnection by taking the advantage of the fact that the phase is not regulated in the generator controls. Relation between the synchronizer rating, control design, and synchronization speed is developed using describing function analysis of frequency and phase control loop gains. The operation and performance of the VSC-based synchronizer is demonstrated using simulations of a 555 MVA, 24 kV synchronous generator. [ABSTRACT FROM AUTHOR]

This paper presents modal analysis of a large offshore wind farm using permanent magnet synchronous generator (PMSG)-type wind turbines connected to a voltage source converter HVDC (VSC-HVDC). Multiple resonant frequencies are observed in the ac grid of offshore wind farms. Their control is crucial for the uninterrupted operation of the wind farm system. The characteristics of oscillatory modes are presented using modal analysis and participation factor analysis. Sensitivity of critical modes to wind turbine design parameters and their impact on closed loop stability of the system are discussed. A comparison between a full wind farm model and an aggregated model is presented to show differences in the characteristics of critical modes observed in the models, and implication of using the models for stability studies It is concluded that robust control design is important for reliable operation of the system. [ABSTRACT FROM PUBLISHER]

This paper proposes one control paradigm that can operate in both grid-connected and islanded modes, hence does not need any sort of an islanding detection method. The proposed method automatically and seamlessly rides through a fault on the grid side, and controls the microgrid's voltage and frequency during islanded operation. During an islanded operation, it utilizes the combination of distributed generation-energy storage similar to the prime mover of a synchronous generator to control the frequency. A comprehensive active and reactive power control is proposed that minimizes the usage of a local fossil-fuelled auxiliary generator. The method is based on expanding the so-called non-detection zone to all operational scenarios including islanded mode, hence having small “undetectable” voltage and frequency deviation. As soon as the grid is reconnected, the distributed generator is automatically and seamlessly synchronized with the grid. This is achieved through keeping phase-locked loop as a part of the operation in islanded mode without altering its phase angle. The proposed method is validated using PSCAD/ EMTDC simulation. [ABSTRACT FROM PUBLISHER]

This paper presents a new overall control strategy for small-scale wind energy conversion systems (WECS). The purpose of the proposed strategy is to utilize a maximum power point tracking control (MPPT) to maximize the captured energy when the wind speeds are below the rated speed. For high wind speed region; two stall controllers are developed: The constant speed stall controller which will limit the rotational speed of the generator to its rated value, and the constant power stall controller, which will regulate the captured power to be within the system rating. For the MPPT control, a modified perturb and observe (P&O) algorithm is utilized, where the dc side current is used as a perturbation variable and the dc-link voltage slope information is used to enhance the tracking speed and stability of the algorithm. For the speed and power regulation operation during high wind speeds, the system is controlled in the stall region to limit the rotational speed and the power of the generator. A stabilizing control loop is proposed to compensate for the stall region instability. A new mode transfer control strategy is developed to effectively control the transition between different modes of operation while ensuring the system stability without using any preknowledge of the system parameters. A 1 kW hardware prototype is developed and tested to validate the proposed new control strategy. [ABSTRACT FROM PUBLISHER]

Partial shading of photovoltaic (PV) power generators causes typically multiple maximum power points (MPPs) on the electrical characteristics of the generators. In this paper, the effect of partial shading on the number of MPPs has been studied by using MATLAB Simulink simulation models of a real PV generator composed of 18 series-connected PV modules. In most cases, partial shading conditions lead to the occurrence of multiple MPPs, but also only one MPP can be present despite of the partial shading. Conditions, which lead to one or multiple MPPs in electrical characteristics of PV generators, have been studied systematically. It was found out that reasons for having one or more MPPs originated directly from physical properties of PV modules: the ratio of MPP current to short-circuit current, power losses in bypass diodes, and the value of the parasitic shunt resistance of PV cells. These phenomena are thoroughly explained in this paper. [ABSTRACT FROM PUBLISHER]

The objective of this paper is to optimize direct drive permanent magnet synchronous generators for offshore direct drive wind turbines in order to reduce the cost of energy. A 6 MW wind turbine design is assumed and parametric electromagnetic and structural generator models are introduced for a surface-mounted magnet generator topology (using magnets with high \rmBH{\rm{max}}) and a flux-concentrating variant (using magnets with lower \rmBH{\rm{max}}). These are optimized using a hybrid genetic algorithm and pattern search process and the results show that the surface-mounted permanent magnet generator produces the lower cost of energy. The choice of objective function is addressed and it is found that a simplified metric incorporating generator cost and losses proxy produces similar designs to a full cost of energy calculation. Further steps to improve the quality of the model include the effect of generator mass on the design and cost of the turbine tower and foundation, which can add €0.4 m to the turbine cost. Further optimizations are carried out to show the impacts of magnetic material costs (doubling this leads to a €1.1/MWh increase in cost of energy) and generator diameter limits (increasing the upper limit from 6 to 8 m leads to a 0.9% drop in cost of energy) have on the choice of optimum independent variables. [ABSTRACT FROM AUTHOR]