Showing papers in "IEEE Transactions on Energy Conversion in 2013"

Analysis of Battery Lifetime Extension in a Small-Scale Wind-Energy System Using Supercapacitors

Abstract: Due to the variable characteristics of renewable generation, batteries used in renewable-power systems can undergo many irregular, partial charge/discharge cycles. In turn, this can also have a detrimental effect on battery lifetime and can increase project costs. This study presents a method of improving battery lifetime in a small-scale remote-area wind-power system by the use of a battery/supercapacitor hybrid energy storage system. The supervisory control algorithm and hardware implementation are described and projected long-term benefits of the proposed system are assessed by simulation. A representative dynamic model of the overall system, incorporating realistic wind-speed and load-power variations has been developed. An analysis is presented of the potential improvement in battery lifetime that is achievable by diverting short-term charge/discharge cycles to a supercapacitor energy-storage system. This study introduces a method by which supercapacitor energy storage systems and control algorithms can be evaluated and implemented in the application area considered. The composition of a prototype test system is described and experimental results are presented to demonstrate system feasibility.

Implemental Control Strategy for Grid Stabilization of Grid-Connected PV System Based on German Grid Code in Symmetrical Low-to-Medium Voltage Network

Abstract: In the last couple of years, the increasing penetration of renewable energy resulted in the development of grid-connected large-scale power plants. However, a high penetration harbors the risk of grid instability if the generating power plants are not able to support the grid. Therefore, grid stabilization, which depends on the system-type or grid of each country, plays an important role and has been strengthened by different grid codes. With this background, VDE-AR-N 4105 for photovoltaic (PV) systems connected to the low-voltage grid and the German Association of Energy and Water Industries (BDEW) introduced the medium-voltage grid code for connecting power plants to the grid and they are the most stringent certifications. In this paper, the control strategy of generating system is enhanced with VDE-AR-N 4105 and BDEW grid code, where both active/reactive powers are controlled. Simulation and experimental results of 100-kW PV inverter are shown to verify the effectiveness of the proposed implemental control strategy.

Design and Analysis of an MPPT Technique for Small-Scale Wind Energy Conversion Systems

Abstract: This paper proposes a maximum power point tracking (MPPT) algorithm for small-scale wind energy conversion systems. The proposed algorithm uses the dc current as the perturbing variable. The algorithm detects sudden wind speed changes indirectly through the dc-link voltage slope. The voltage slope is also used to enhance the tracking speed of the algorithm and to prevent the generator from stalling under rapid wind speed slowdown conditions. The proposed method uses two modes of operation: A perturb and observe (P&O) mode with adaptive step size under slow wind speed fluctuation conditions, and a prediction mode employed under fast wind speed change conditions. The dc-link capacitor voltage slope reflects the acceleration information of the generator, which is then used to predict the next step size and direction of the current command. The proposed algorithm shows enhanced stability and fast tracking capability under both high and low rate of change wind speed conditions and is verified using a 1.5 kW prototype hardware setup.

Evaluation of the Performance of a DC-Link Brake Chopper as a DFIG Low-Voltage Fault-Ride-Through Device

Abstract: The performance of the doubly fed induction generator (DFIG) during grid faults is attracting much interest due to the proliferation of wind turbines that employ this technology. International grid codes specify that the generator must exhibit a fault-ride-through (FRT) capability by remaining connected and contributing to network stability during a fault. Many DFIG systems employ a rotor circuit crowbar to protect the rotor converter during a fault. Although this works well to protect the generator, it does not provide favorable grid support behavior. This paper describes an experimental investigation of an alternative FRT approach using a brake chopper circuit across the converter dc link to ensure that the dc-link voltage remains under control during a fault. Two different approaches to chopper control are examined and the resulting FRT performance is compared with that of a conventional crowbar approach. The new chopper-based control methods are experimentally evaluated using a 7.5-kW DFIG test rig facility.

Quadrature PLL-Based High-Order Sliding-Mode Observer for IPMSM Sensorless Control With Online MTPA Control Strategy

Abstract: To improve performance of the field-oriented controlled position sensorless interior permanent magnet synchronous motors (IPMSM) drive, a model-based high-order sliding-mode observer with a software quadrature phase-locked loop (PLL) is proposed. A simplified method of designing the feedback gain matrix of the sliding-mode observer is introduced for easy application. The PLL coefficients are analyzed by considering the expected position estimation error and the operating condition. Compared with the conventional arc-tangent calculation method, the proposed quadrature PLL estimation method can be immune to the influence of the noise and distortion. To achieve the robust high-efficiency operation of the sensorless IPMSM, an enhanced online maximum torque per ampere (MTPA) control strategy without motor parameters dependent is adopted for the vector control drive. The optimal MTPA operation is achieved by searching the optimal current angle online to make the current amplitude be the minimum. The effectiveness of the proposed method is verified with a 2.2-kW IPMSM sensorless drive.

Two-Time-Scale Coordination Control for a Battery Energy Storage System to Mitigate Wind Power Fluctuations

Abstract: In this paper, a two-time-scale coordination control method to mitigate wind power fluctuations using a battery energy storage system (BESS) is proposed. Two-time-scale maximal power fluctuation restrictions (MPFRs) are set for the combined output of the wind farm and the BESS: the maximal fluctuation of the combined power in any 1- and 30-min time window must be kept within γ1min% and γ30min%, respectively, of the rated power of the wind farm. A flexible first-order low-pass filter (FLF) with an optimization of time constant Tf is developed to limit the power fluctuation under restriction with smaller BESS capacity. Then, a coordination control method is developed on base of the FLF, which contains: 1) PSO-based time constant real-time optimizing algorithm; 2) remaining energy level feedback control; 3) two-time-scale coordination control strategy. Finally, an estimation of BESS capacity and power rating for a wind farm to achieve the MPFRs is presented, and the whole method is tested in a time-domain simulation system.

Advancements in OCV Measurement and Analysis for Lithium-Ion Batteries

Abstract: Incremental open-circuit voltage (OCV) curves and low-current charge/discharge voltage profiles of a lithium-ion (Li-ion) battery are compared and evaluated for optimizing measurement time and resolution. Since these curves are often used for further analysis, minimizing kinetic contributions is crucial for approximating battery OCV behavior. In this context, an incremental OCV measurement is characterized by state of charge (SOC) intervals and relaxation times. Various constant low C-rates, SOC intervals, and relaxation times are tested for approximating OCV behavior. Differential capacity and voltage analysis is used to check whether the main electrode features can be resolved satisfactorily. An interpolation method yields additional data points for the differential analysis of incremental OCV curves. It is shown that incremental OCV measurements are suitable for an approximation of battery OCV behavior, rather than low current-voltage profiles. Furthermore, extrapolation of voltage relaxation enables the estimation of fully relaxed OCV.

Control of a Supercapacitor Energy Storage System for Microgrid Applications

Abstract: The proper operation of a microgrid requires storage devices that increase the inertia and avoid instability of the system. This paper presents the control of an energy storage system (ESS) based on supercapacitor in the context of grid-connected microgrids. The ESS is composed of AC/DC and DC/DC converters tied by a dc link. A single sliding mode strategy is proposed to control a bidirectional dc/dc converter, capable of working properly under all operating conditions. The switching devices are commanded by a single sliding function, dynamically shaped by references sent from the microgrid central controller. This feature facilitates the implementation and design of the control law and simplifies the stability analysis over the entire operating range. The effectiveness of the proposed control strategy is illustrated by experimental results.

Multicriteria Optimal Sizing of Photovoltaic-Wind Turbine Grid Connected Systems

Abstract: Power generation systems (PGSs) based on hybrid renewable energy are one of the promising solutions for future distributed generation systems. Among different configurations, hybrid photovoltaic-wind turbine (PV-WT) grid connected PGSs are the most adopted for their good performance. However, due to the complexity of the system, the optimal balance between these two energy sources requires particular attention to achieve a good engineering solution. This paper deals with the optimal sizing of PV-WT by adopting different multicriteria decision analysis (MCDA) optimization approaches. Sensitivity of MCDA algorithms has been analyzed, by considering different weighting criteria techniques with different fluctuation scenarios of wind speed and solar radiation profiles, thus highlighting advantages and drawbacks of the proposed optimal sizing approaches. The following study could be assumed as a powerful roadmap for decision makers, analysts, and policy makers.

Improvement of the Hilbert Method via ESPRIT for Detecting Rotor Fault in Induction Motors at Low Slip

Abstract: The traditional Hilbert method to detect broken rotor bar fault in induction motors is reviewed and its major drawbacks are clearly revealed, namely, deteriorating or even completely failing when a motor operating at low slip due to the fixed constraints of fast Fourier transform (FFT) is used in this method. To overcome this, the estimation of signal parameters via rotational invariance technique (ESPRIT) is then introduced to replace FFT, and an improved Hilbert method is thus presented by conjugating the Hilbert transform and ESPRIT together. Experimental results of a small motor in a laboratory and a large motor operating on an industrial site are reported to demonstrate the effectiveness of the improved Hilbert method.

Simple Neutral-Point Voltage Control for Three-Level Inverters Using a Discontinuous Pulse Width Modulation

Abstract: This paper proposes a neutral-point voltage balancing method for a three-level inverter that uses discontinuous pulse width modulation (DPWM). The performance of a three-level inverter system is affected by the neutral-point voltage unbalancing. Therefore, the voltages of series connected dc-link capacitors should be equal. Generally, the offset voltage is added to reference voltages in the conventional voltage balancing method. However, when using DPWM, the performance of conventional balancing method is limitary because turn-on and turn-off switching occurs at regular intervals. The proposed method is implemented by adjusting the discontinuous pulsewidth of positive and negative cycle. The proposed method maintains low switching losses and achieves the effective voltage balancing, without additional hardware and complex calculations. Simulation and experimental results confirm the feasibility and effectiveness of the proposed method.

Integrated System Identification and State-of-Charge Estimation of Battery Systems

Abstract: Accurate estimation of the state of charge in battery systems is of essential importance for battery system management. Due to nonlinearity, high sensitivity of the inverse mapping from external measurements, and measurement errors, SOC estimation has remained a challenging task. This is further compounded by the fact that battery characteristic model parameters change with time and operating conditions. This paper introduces an adaptive nonlinear observer design that compensates nonlinearity and achieves better estimation accuracy. A two-time-scale signal processing method is employed to attenuate the effects of measurement noises on SOC estimates. The results are further expanded to derive an integrated algorithm to identify model parameters and initial SOC jointly. Simulations were performed to illustrate the capability and utility of the algorithms. Experimental verifications are conducted on Li-ion battery packs of different capacities under different load profiles.

A Fast PV Power Tracking Control Algorithm With Reduced Power Mode

Abstract: This paper presents a fast maximum power point tracking (MPPT) control algorithm for the photovoltaic (PV) in a hybrid wind-PV system, in which the PV generatormay also need to work in a reduced power mode (RPM) to avoid dynamic overloading The two control modes, MPPT and RPM, are inherently compatible and can be readily implemented, without the need of a dumping load for the RPM Following the establishment of a dynamic system model, the study develops the guidelines to determine the variables of a direct hill-climbing method for MPPT: the perturbation time intervals and the magnitudes of the applied perturbations These results are then used to optimally set up a variable-step size incremental conductance (VSIC) algorithm along with adaptive RPM control The power tracking performance and power limiting capability are verified by simulation and experiment

Single-Phase DC Crowbar Topologies for Low Voltage Ride Through Fulfillment of High-Power Doubly Fed Induction Generator-Based Wind Turbines

Abstract: This paper presents two new single-phase crowbar protection topologies, for doubly fed induction generator (DFIG) based wind turbines. These crowbar topologies, can be suitable for fulfilling the low voltage ride through (LVRT) requirements demanded by the grid codes, in next generation of high-power DFIG-based wind turbines, allowing the possibility to use simpler and more cost effective crowbar designs, than the classic three-phase dc crowbar, commonly used among manufacturers. Therefore, this paper compares the differences of the classic three-phase dc crowbar and the new proposed ones, together with the study of the behavior of a prototype of 6.25 MW DFIG, when is affected with severe voltage symmetric and asymmetric voltage dips. Control actions and design ideas are discussed, trying to accomplish the LVRT requirements, concluding by means of simulation results that the proposed new single-phase crowbar topologies can be an attractive alternative crowbar solution, for high-power wind turbines.

Multi-Objective Model-Predictive Control for High-Power Converters

Abstract: This paper presents a multi-objective model-predictive control (MOMPC) strategy for controlling converters in high-power applications. The controller uses the system model to predict the system behavior in each sampling interval for each voltage vector, and the most appropriate vector is then chosen according to an optimization criterion. By changing the cost function properly, multiobjectives can be achieved. To eliminate the influences of one step delay in digital implementation, a model-based prediction scheme is introduced. For high-power applications, the converter switching frequency is normally kept low in order to reduce the switching losses; this is done by adding a nonlinear constraint in the cost function. However, to avoid system stability deterioration caused by the low switching frequency, an N-step horizontal prediction is proposed. Finally, the control algorithm is simplified using a graphical algorithm to reduce the computational burden. The proposed MOMPC strategy was verified numerically by using MATLAB/Simulink, and validated experimentally using a laboratory ac/dc converter.

Large-Scale PV Plant With a Robust Controller Considering Power Oscillation Damping

Abstract: Transmission voltage-level photovoltaic (PV) plants are becoming reality in many developed and developing countries around the world. Studies suggest that large-scale PV plants could have either positive or negative influence on low-frequency oscillation (LFO) depending on their locations and sizes. Given the fact that these plants cannot be placed in ideal locations to minimize their impact on LFO, it is important to consider designing a damping controller for flawless integration. In this paper, a minimax linear quadratic Gaussian-based power oscillation damper (POD) for a large-scale PV plant is proposed for interarea oscillation damping. A benchmark power system prone to power system oscillations is used to demonstrate the damping performance of the designed controller considering feedback signal transmission delay. The performance of the designed controller is evaluated under different operating conditions as compared to the classical POD at PV plant. Simulation results demonstrate that the proposed controller for a PV plant provides sufficient damping to the interarea mode for a wide range of operating conditions.

Thermal Modeling of Directly Cooled Electric Machines Using Lumped Parameter and Limited CFD Analysis

Abstract: This paper presents a practical approach to model thermal effects in directly cooled electric machines. The main focus is put on modeling the heat transfer in the stator winding and to the cooling system, which are the two critical parts of the studied machines from a thermal point of view. A multisegment structure is proposed that divides the stator, winding, and cooling system into a number of angular segments. Thereby, the circumferential temperature variation due to the nonuniform distribution of the coolant in the cooling channels can be predicted. Additionally, partial computational fluid dynamics (CFD) simulations are carried out to model the coolant flow in the cooling channels and also on the outer surface of the end winding bodies. The CFD simulation results are used as input to the analytical models describing the convective heat transfer to the coolant. The modeling approach is attractive due to its simplicity since CFD simulations of the complete machine are avoided. The proposed thermal model is evaluated experimentally on two directly cooled induction machines where the stator winding is impregnated using varnish and epoxy, respectively. A good correspondence between the predicted and measured temperatures under different cooling conditions and loss levels is obtained.

Adaptive Hybrid Maximum Power Point Tracking Method for a Photovoltaic System

Abstract: Recently, the importance of exploring the plausibility of renewable energy has been progressively increased, not only because of concerns over the shortage of current fossil fuels but also the consideration of sustainable development and the negative environmental impact caused by large scale use of fossil fuels. Among renewable sources, solar energy seems to be one of the promising energy sources for widespread application. Due to its inherent intermittency and fluctuation, one of the important research interests is to harness the maximum power possible from the solar energy falling on a panel. To this end, an efficient maximum power point tracker to harvest as much energy as possible is a key to improving the system's efficiency and performance. This paper presents a novel hybrid maximum power point tracking mechanism with adaptive perturbation size. The proposed method is implemented, analyzed, and evaluated in MATLAB/Simulink. Both numerical and experimental evaluation results prove that by using the proposed method, better tracking performance can be achieved and the power delivered at steady state can be increased by a factor of 7.31% compared with conventional methods.

A New Model for PMSG-Based Wind Turbine With Yaw Control

Abstract: In order to fully study small wind turbines (WTs), a comprehensive model that considers all mechanical and electrical aspects is necessary. The permanent magnet synchronous generator (PMSG)-based WT is one of the most common types of WTs that uses a full scale converter with a variable speed WT. In this paper, a new model is developed for a PMSG-based WT with yaw control scheme. The WT generator is connected to a grid by two back-to-back voltage source IGBT converters and a dc capacitor set between them. A precise mechanical model is necessary to simulate yaw control. The yaw control is used as a mechanical mechanism to adjust yaw error and protect small horizontal axis wind turbines (HAWT) against over speed and excess power. TurbSim and FAST are used to model a wind profile and the mechanical parts of the WT. Also, the WT generator and electrical controllers are modeled by Simulink. Field oriented control (FOC) method is developed on the voltage source converters (VSCs). Simulation results show the performance of the mechanical and electrical controllers in different conditions.

Improving Fault Ride-Through Capability of Fixed-Speed Wind Turbine by Using Bridge-Type Fault Current Limiter

Abstract: The interaction between wind turbines and grid results in increasing short-circuit level and fault ride-through (FRT) capability problem during fault condition. In this paper, the bridge-type fault current limiter (FCL) with discharging resistor is used for solving these problems. For this FCL, a control scheme is proposed, which uses the dc reactor current as control variable, to adjust the terminal voltage of induction generator (IG) without measuring any parameters of system. In this paper, the wind energy conversion system (WECS) is a fixed-speed system equipped with a squirrel-cage IG. The drivetrain is represented by a two-mass model. The analytical and simulation studies of the bridge-type FCL and proposed control scheme for limiting the fault current and improving FRT capability are presented and compared with the impact of the application of the series dynamic braking resistor (SDBR).

Axial Flux Segmented SRM With a Higher Number of Rotor Segments for Electric Vehicles

Abstract: Motors for in-wheel electric vehicle application should have high specific torque. In addition, these motors should be rugged, insensitive to vibration, and temperature variation. Therefore, segmented rotor switched reluctance motor (SSRM) could be an attractive alternative to permanent magnet-based motors, which are being used for this application. A limitation of SSRM is that it requires full pitch winding for its operation. Since, in-wheel motors have high diameter to axial length (D/L) ratio, SSRM of these dimensions would be bulky and has high copper loss due to full pitch winding. Hence, in this paper, a novel SSRM with nonoverlapping winding is proposed. This motor is an axial flux SSRM (AFSSRM) with toroidal winding arrangement. It has single-stator, dual-rotor configuration with 12 stator poles and 16 rotor segments on each rotor disc. Design procedure for AFSSRM is developed and a flowchart describing the design algorithm is presented. A finite element method-based simulation is carried out to verify the performance of the proposed AFSSRM. In order to validate the performance of the motor, a prototype is fabricated and experimental results are presented.

A Novel Approach for Broken Bar Fault Diagnosis in Induction Motors Through Torque Monitoring

Abstract: The cracked or broken bar fault constitutes about 5-10% of total induction motor failures and leads to malfunction as well as reduction of the motor's life cycle. This is the reason why there is continuous research on techniques for prompt detection. In this study, a study of the influences of the broken bar fault to the electromagnetic characteristics of the induction motor is presented, using an asynchronous cage motor and finite element method analysis. To this direction, two models have been created and studied: a healthy and one with a broken bar. Additionally, a new approach on the detection of the broken rotor bar fault through the electromagnetic torque monitoring is suggested and validated through experimental results.

Application of the Teager–Kaiser Energy Operator to the Fault Diagnosis of Induction Motors

Abstract: The diagnosis of induction motors through the spectral analysis of the stator current allows for the online identification of different types of faults. One of the major difficulties of this method is the strong influence of the mains component of the current, whose leakage can hide fault harmonics, especially when the machine is working at very low slip. In this paper, a new method for demodulating the stator current prior to its spectral analysis is proposed, using the Teager-Kaiser energy operator. This method is able to remove the mains component of the current with an extremely low usage of computer resources, because it operates just on three consecutive samples of the current. Besides, this operator is also capable of increasing the signal-to-noise ratio of the spectrum, sharpening the spectral peaks that reveal the presence of the faults. The proposed method has been deployed to a PC-based offline diagnosis system and tested on commercial induction motors with broken bars, mixed eccentricity, and single-point bearing faults. The diagnostic results are compared with those obtained through the conventional motor current signature analysis method.

Adaptive Power Capture Control of Variable-Speed Wind Energy Conversion Systems With Guaranteed Transient and Steady-State Performance

Abstract: This paper deals with the power capture control of variable-speed wind energy conversion systems. The control objective is to optimize the capture of wind energy by tracking the desired power output. Arbitrary steady-state performance is achieved in the sense that the tracking error is guaranteed to converge to any predefined small set. In addition, to maximize the wind energy capture, transient performance is enhanced such that the convergence rate can be larger than an arbitrary value, which further limits the maximum overshoot. First, an adaptive controller is designed for the case where known aerodynamic torque is assumed. Then, by utilizing an online approximator to estimate the uncertain aerodynamics, the need for the exact knowledge of the aerodynamic torque is waived to imitate the practical experience. With the aid of a novel output error transformation technique, both of the proposed controllers are capable of shaping the system performance arbitrarily on transient and steady-state stages. Meanwhile, it is also proved that all the signals in the closed-loop system are bounded via Lyapunov synthesis. Finally, the feasibility of the proposed controllers is demonstrated on an 1.5-MW three-blade wind turbine using the FAST (Fatigue, Aerodynamics, Structures, and Turbulence) code developed by the National Renewable Energy Laboratory.

Integrating Hybrid Power Source Into an Islanded MV Microgrid Using CHB Multilevel Inverter Under Unbalanced and Nonlinear Load Conditions

Abstract: This paper presents a control strategy for an islanded medium voltage microgrid to coordinate hybrid power source (HPS) units and to control interfaced multilevel inverters under unbalanced and nonlinear load conditions. The proposed HPS systems are connected to the loads through a cascaded H-bridge (CHB) multilevel inverter. The CHB multilevel inverters increase the output voltage level and enhance power quality. The HPS employs fuel cell (FC) and photovoltaic sources as the main and supercapacitors as the complementary power sources. Fast transient response, high performance, high power density, and low FC fuel consumption are the main advantages of the proposed HPS system. The proposed control strategy consists of a power management unit for the HPS system and a voltage controller for the CHB multilevel inverter. Each distributed generation unit employs a multiproportional resonant controller to regulate the buses voltages even when the loads are unbalanced and/or nonlinear. Digital time-domain simulation studies are carried out in the PSCAD/EMTDC environment to verify the performance of the overall proposed control system.

Current Frequency Spectral Subtraction and Its Contribution to Induction Machines’ Bearings Condition Monitoring

Abstract: Induction machines are widely used in industrial applications. Safety, reliability, efficiency, and performance are major concerns that direct the research activities in the field of electrical machines. Even though the induction machine is very reliable, many failures can occur such as bearing faults, air-gap eccentricity, and broken rotor bars. The challenge is, therefore, to detect them at an early stage in order to prevent breakdowns. In particular, stator current-based condition monitoring is an extensively investigated field for cost and maintenance savings. In this context, this paper deals with the assessment of a new stator current-based fault detection approach. Indeed, it is proposed to monitor induction machine bearings by means of stator current spectral subtraction, which is performed using short-time Fourier transform or discrete wavelet transform. In addition, diagnosis index based on the subtraction residue energy is proposed. The proposed bearing faults condition monitoring approach is assessed using simulations, issued from a coupled electromagnetic circuits approach-based simulation tool, and experiments on a 0.75-kW induction machine test bed.

Damping of Torsional Vibrations in a Variable-Speed Wind Turbine

Abstract: Torsional dampers are employed in wind turbines to damp vibrations in the drive-train. The conventional design is based on band-pass filters (BPF); however, its effectiveness can be compromised due to parametric uncertainty. To restore the performance of the damper, it is a common practice to re-tune it during the commissioning of wind turbines. To overcome this shortcoming, a model-based torsional damper was designed and its performance compared to the conventional approach when subjected to model uncertainty. A stability analysis was conducted and simulations were performed in Simulink. A real-time hardware-in-the-loop experiment was carried out, with experimental and simulation results showing good agreement. The proposed model-based torsional vibration damper showed a superior performance over the conventional BPF-based approach. Results also showed that the model-based damper can eliminate the need for retuning procedures associated with BPF-based designs.

Small Hydropower Plant With Integrated Turbine-Generators Working at Variable Speed

Abstract: Present-day small hydropower plants (SHPs) have a large development potential because of the increasing interest in renewable resources and distributed energy generation; however, the variable hydrological conditions that are found in run-of-the-river projects require operations over a wide range of water flow and head variations. Special control methods and system topologies are needed to maintain the high efficiency of energy conversion systems.This paper presents a complete SHP solution based on an innovative generation unit (hydro-set): a propeller turbine is integrated with a permanent magnet synchronous generator working at a variable speed in a grid-connected system that utilises an ac/dc/ac converter. The main elements of this energy conversion system are described. A dedicated control strategy is proposed and verified. All of the results presented here originate from an actual 150-kW SHP that contains two innovative hydro-sets working in parallel on the same river.

Integration of Offshore Wind Farm Using a Hybrid HVDC Transmission Composed by the PWM Current-Source Converter and Line-Commutated Converter

Abstract: This paper investigates the feasibility of the application of a hybrid HVDC transmission system for the grid integration of offshore wind farms. The proposed hybrid HVDC consists of a pulse width modulated current source converter (PWM-CSC) and a line-commutated converter (LCC). The PWM-CSC is connected to the offshore wind farm and the LCC connects the onshore grid. The hybrid topology takes advantages from self-commutated converters as well as LCCs. On the one hand, LCC-based HVdc is the most mature technology with the lowest power losses and lowest cost. On the other hand, PWM-CSC has the same features that a voltage source converter for offshore applications, i.e., the ability to operate without an external commutation voltage, reactive power control capability, and a relative small footprint. Moreover, both the PWM-CSC and the LCC are current source converters and hence the coupling can be effortlessly done. The control design for the entire system is presented and verified using numerical simulations. Simulations are performed using PSCAD/EMTDC under different conditions including changes in the wind speed and ac and dc faults.

Comparison of Cogging Torque Reduction in Permanent Magnet Brushless Machines by Conventional and Herringbone Skewing Techniques

Abstract: Cogging torque, as one of the main parasitic demerits of permanent magnet brushless machines, is of particular importance and primary concern during the machine design stage in many high-performance applications. Hence, numerous design techniques have been proposed and employed to effectively alleviate the cogging torque in permanent magnet brushless machines. The effects of rotor step skewing techniques including both conventional and herringbone styles on the cogging torque of permanent magnet brushless machine are comprehensively investigated and compared by synthesized 2-D and 3-D finite-element analysis in this paper. The results have revealed that both the conventional and herringbone rotor step skewing techniques can reduce the cogging torque significantly, but the latter is less effective than the former especially with small skewing step numbers. Moreover, the machine with herringbone rotor step skewing technique has rather peculiar and asymmetric cogging torque profiles, while the machine with conventional rotor step skewing technique exhibits normal and symmetric ones. The validity of the obtained results and findings is underpinned by the experiments on the prototype machine.