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While much attention was given in the literature to improve the fault ride through (FRT) capability of a doubly-fed induction generator (DFIG)-based wind energy conversion system (WECS) during various faults at the grid side, not much attention was given to investigate and improve the performance of the DFIG-WECS under ferroresonance conditions. This paper investigates the impact of ferroresonance on the overall performance of a DFIG-WECS. The paper also presents a new technique to improve the high voltage ride through capability of the investigated system under such condition. The proposed technique relies on incorporating a superconducting coil within the dc-link of the DFIG grid-side and rotor-side converters. Energy exchange between the coil and the system is controlled through regulating the duty cycle of the dc-chopper interfacing the coil with the dc-link. In this regard, a simple but yet effective proportional-integral (PI) controller is employed to regulate the dc-chopper duty cycle. Parameters of the PI controller are optimized using elephant herding optimization algorithm. Simulation results verify the effectiveness of the proposed controller in improving the overall performance of a DFIG-WECS during ferroresonance events. [ABSTRACT FROM AUTHOR]

Light-emitting diodes (LEDs) are widely used in street lighting, landscape lighting, liquid crystal display (LCD) backlighting, healthcare, etc. As an important part of LED lighting products, LED driver plays a vital role in maintenance costs reduction, energy saving and lifetime prolongation. In this paper, a single-stage LED driver is proposed, which is integrated by a totem-pole bridgeless power factor correction (PFC) unit and several modular LCL-T resonant rectifiers. The totem-pole bridgeless PFC unit and the LCL-T resonant rectifiers are integrated by sharing switches, which can simplify the circuit and reduce the system cost. The output current can be regulated by applying the variable inductor control. Constant duty cycle and fixed switching frequency operation can be achieved as well. The operating principle and the performance of the proposed converter are analyzed in this paper. In addition, a precise parameter design method is proposed by considering the high-order harmonics. At last, an experimental prototype is established to validate the proposed parameter design and variable inductor control method. [ABSTRACT FROM AUTHOR]

The dc-dc converters with high step-up DC-voltage gain play a vital role in integrating low voltage DC sources. Though several converter topologies are reported in the recent past, attempts have been made to reduce the components, especially the switching devices, passive elements, converter losses, etc., of the converter. A novel DC-DC converter topology, viz., single switched impedance network (SSIN)-based converter with n-stage is proposed in this paper. The operation of the SSIN based converter in continuous and discontinuous conduction modes are discussed. The effect of parasitic elements on DC-voltage gain and efficiency of the SSIN is carried out. The small-signal model of the SSIN converter is derived. The performance of the SSIN converter is compared with the similar converter topologies. The 500 W prototypes of the SSIN with n = 1 and n = 2 are fabricated, and the experimental results are presented in this paper. To improve the converter performance, SiC-based semiconductor devices are used. The SSIN converter with two-stage has low switch loss at 500 W load, improved efficiency during high power and high output voltage operation, the switch stress of Vo/2, etc. [ABSTRACT FROM AUTHOR]

This paper compares transient responses of virtual oscillator control (VOC) to the conventional droop control method, under various inverter terminal voltage amplitude and frequency regulations. The system of interest is an islanded microgrid of three three-phase inverters sharing a resistive load. By letting the two controllers have similar steady-state droop characteristics, the virtual oscillator and droop controlled systems are compared in a small-signal framework with the aid of eigenvalue analyses. The analytical results are verified by simulating the two systems in MATLAB/Simulink. In the simulations, the third inverter in each system is connected to share the load when the other two inverters are already synchronized. Transients are compared by examining synchronization time after the third inverter is connected. The key finding is VOC outperforms droop control in terms of time responses when the terminal voltage frequency regulation range is allowed to be wide; otherwise, the droop control responds faster than the VOC technique. The same conclusions are found for two other load types: constant power load and nonlinear load. In addition, transient energy losses are investigated, which shows positive correlation with the time response. [ABSTRACT FROM AUTHOR]

An improved two-stage model predictive current control (MPCC) for three-phase three-level converters in the paper, which can simultaneously achieve common-mode voltage (CMV) reduction, effective dc-link capacitor voltage control, and decreased computational burden, is proposed. Compared to the other current control strategies, it has the capability of handling multiple control objectives and possesses a simple structure. Furthermore, it greatly reduces the CMV since the peak CMVs of all the candidate voltage vectors are constrained to be less than one sixth of the dc-link voltage. In addition, an optimal two-stage voltage vector selection mechanism is employed in the proposed method. It helps to significantly reduce the computational complexity. Simulation and experimental results are provided to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In multi-MW three-level neutral point clamped back-to-back (3L-NPC) power converters, power semiconductor devices are considered as most vulnerable components due to its thermo-mechanical fatigue stress. Particularly, rotor side converter (RSC) in doubly fed induction machine (DFIM) experiences high thermal stress during its operation around synchronous speed. It degrades the performance of power converter and reduces the lifetime availability of the drive. In practice, stabilization of temperature fluctuation across each power semiconductor device improves the efficiency and reliability of power converter. With respect to this prime aim, this paper proposes a carrier-based active thermal control method to regulate temperature fluctuation across power semiconductor devices in a multi-MW (5 x 5 MW) 3L-NPC power converters serving to a 250 MW asynchronous hydro-generating unit. To test the proposed active thermal control method, different operating conditions of a 250 MW DFIM hydro-generating unit (to be commissioned in Tehri pumped storage plant, India) are examined in MATLAB/PLECS environment. The designed active thermal control method effectively regulates mean junction temperature and temperature fluctuation of power semiconductor devices within a safe operating limit. The practical feasibility and adoptability of proposed active thermal control is examined through a 2.2-kW DFIM laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper presents a dual-hysteresis band control method for multi-drive systems based on a nine-switch inverter. The drive system contains two independent three-phase induction motors fed by a nine-switch inverter. Independent control of torque, speed, and stator current of two load stages is achieved. The proposed strategy offers a reliable control operation of both induction machines under individual load profiles. Instead of using independent sets of three-phase voltage source converters, the nine-switch inverter is used to drive separate machines. Thus, the volume and size of the power stage are noticeably reduced due to the use of fewer semiconductors. The theoretical concept and design steps of the proposed method are presented. The performance of the closed-loop independent motor control scheme is experimentally validated using a 3.2 kW lab-scaled nine-switch inverter prototype and Altera Cyclone IV Field-programmable gate array. [ABSTRACT FROM AUTHOR]

This paper investigates an offshore wind farm based on modular dual-active-bridge (DAB) isolated DC-DC converter. The DAB converter is adopted due to its compact design and the galvanic isolation between its input and output terminals. The synchronous generator in each wind turbine is operated individually with the generator-side rectifier to maximize the power production. The output terminals of multiple DAB converters are connected in series so that the high voltage for DC transmission is achieved and the expensive offshore step-up substation is eliminated. The DC transmission voltage is excluded from the primary side of the DAB transformer by its galvanic isolation. To balance the power sharing among the DAB converters, their input terminals are connected in parallel. In addition, the droop voltage balancing control is presented for the series offshore wind farm. As a result, the input and output voltages of each DAB converter are governed by the same droop characteristic, and the DAB converters’ output voltages are kept balanced. The time-domain simulation results show that the DC voltages of each DAB converter and the high voltage direct current (HVDC) transmission system remain stable during the temporal and spatial variations in the wind power generation. [ABSTRACT FROM AUTHOR]

This paper shows that the capability of voltage-source converters (VSCs) to control the real power output of wind turbine generators (WTGs) can be applied to smooth power fluctuations due to wind turbulence. The paper calls attention to instability that can arise from: 1) the wind turbine itself and 2) the doubly fed induction generator (DFIG). The paper presents GP/V(s) to characterize the dynamic behavior of WTGs. [ABSTRACT FROM AUTHOR]

Line-commutated rectifiers are often utilized in machine-converter systems and many energy conversion applications. Simulation of such power systems using detailed switching models of rectifiers is computationally expensive, and as an alternative for system-level studies, the so-called average-value modeling (AVM) techniques have become indispensable. The parametric AVM (PAVM) uses a computerized approach for establishing the key relationships between the averaged ac and dc variables. In this paper, a generalized PAVM (GPAVM) is proposed, which extends several previously proposed models. The new GPAVM includes the ac harmonics in thyristor-controlled rectifier models considering their nonlinear dependency on the line frequency. The new model is verified using detailed simulations and experimental results and is demonstrated to have better accuracy in a wider range of operating conditions and speeds/frequencies. [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]

This paper presents a direct frequency control for offshore wind farms (OWFs) connected to the diode-rectifier-based (DR-based) high voltage direct current (HVDC) link. The frequency control is guaranteed through reactive power balance at the DR station without a capacitor bank placed at the DR station; while AC-voltage magnitude is clamped by the DR, provided that the HVDC link voltage is imposed by the onshore inverter. The control system is implemented by a voltage source converter (VSC) connected to the DR station. The VSC is also used to compensate harmonic currents, leading to passive AC-filters being also removed from the DR station. The proposed control system provides comparative advantages in terms of DR station volume and OWF installation cost with other centralized controls where besides a VSC, a high-voltage large capacitor bank is needed to be placed at the DR station. A new average-value model (AVM) of the DR system is derived to test the proposed frequency control, while the stability of the system is evaluated through a small-signal analysis. The proposed control system performance and the accuracy of the proposed AVM are verified through a detailed switching model of the system using operation of a cascaded H-bridge VSC. [ABSTRACT FROM AUTHOR]

This paper presents a modification of the Vienna rectifier (VR) to obtain more than three voltage levels at the DC line connection, while preserving the unity power factor at the AC grid side. The proposed unidirectional multilevel there-phase rectifier, called the “Negev rectifier”, has a reduced device count and utilizes a unique novel Time Division Multiplexed based control scheme, which makes the challenging N-level VR expansion possible. The Negev rectifier can serve as a grid interface for supplying dc voltage to multilevel inverters with 4 up to 7 voltage levels, eliminating the burden of inverter-side voltage balancing control. The Negev hardware can be constructed by two complementary topologies, making it suitable for either high current or high voltage applications. [ABSTRACT FROM AUTHOR]

Due to the repeated switching, the detailed switch-level models of electrical machines coupled with power-electronic converters are computationally expensive and hard to linearize for small-signal frequency-domain analysis. Average-value modeling (AVM) has become an effective tool for small-signal analysis of power electronic systems and time-domain transient studies where the details of switching are not important and can be neglected. Recently, a parametric AVM (PAVM) approach has been developed for machine/diode rectifier systems. This paper extends the parametric approach to the machine/thyristor-controlled-rectifier systems, where the thyristor firing may be referenced to either the line voltages or the rotor position. An analytical average model for this system is also developed based on the recently proposed constant-parameter voltage-behind-reactance synchronous machine model. The new PAVM is compared against the original switching system, as well as the analytical AVM. It is shown that the PAVM can accurately predict both small-signal characteristics and large-signal transients of the original switching system in light and heavy modes, which represents an advantage over the analytical models which are typically implicit. [ABSTRACT FROM AUTHOR]

This paper investigates the operating modes and converter options of dual winding permanent magnet synchronous machines (DWPMSMs) that are used to drive accessories in hybrid electric vehicles. In this application, DWPMSM operate in three different modes: 1) motor-only; 2) generator-only; and 3) simultaneous motor–generator modes. Motor-only and generator-only operations can be treated independently as long as two winding sets are magnetically decoupled. Simultaneous motor–generator mode links two winding sets through a mechanical shaft. As part of this study, the impact of dual winding set usage on the machine's output and performance is also investigated. Experimental results are given for each mode. The converter options of generator winding were investigated as well. These options include uncontrolled rectifier with dc–dc converter and controlled rectifier. Two converters are compared for their impacts on efficiency and generator current harmonics. The relationship between speed and converter control variable is given for various back electromotive force constant values of the DWPMSM. [ABSTRACT FROM AUTHOR]

This paper deals with the sensorless vector control of a doubly fed induction generator (DFIG) whose stator is connected by a rectifier to a dc link, which also feeds the rotor-side inverter. A noticeable reduction of the costs can be achieved by this topology, because only a single inverter and a diode rectifier are needed instead of the traditional back-to-back converter. A sensorless stator-flux-oriented control is implemented by using an observer, which directly computes the necessary slip angle based on of the air-gap power and without the need of the stator flux estimation. The direct-axis component of the rotor current is adjusted to regulate the stator frequency near the rated value. In order to track the actual stator frequency, the phase-locked-loop approach is used. The control method is validated by simulations and experimental results obtained using a small-rating laboratory setup. [ABSTRACT FROM AUTHOR]

This paper addresses comparative investigation on the permanent magnet (PM) wind power generators’ performance considering PM flux directions while generated electrical energy is being stored through a mechanical energy storage system (MESS). Radial- and axial-flux PM generators are dealt with, and since their equivalent circuit parameters are almost identical, their comparison can be reasonable. During the energy conversion through the MESS, since a power converter is required, the phase current of each generator contains harmonic components due to diode rectifier with high influence on the generators’ performance. In this study, the electromagnetic characteristics of each generator are analyzed based on the measured current to consider the influence of those harmonics, and their performance evaluation is also performed by experiment. [ABSTRACT FROM AUTHOR]

Line-commutated rectifiers (LCRs) are extensively utilized in exciters of large electrical machines, ac–dc power systems of ships, vehicles, aircraft, and many other industrial applications. Efficient and accurate computer simulations are necessary to analyze various aspects of such systems under both normal and unbalanced/faulty operating conditions. The so-called parametric average-value modeling (PAVM) technique has been recently developed and shown to provide accurate and computationally efficient models of power-electronic converters for system-level simulations. In this paper, the PAVM methodology is extended to LCRs with internal faults. The new formulation considers the asymmetrical operation of rectifiers by including the ac-side characteristic (i.e., 5th, 7th, etc.) and non-characteristic (i.e., 2nd, 3rd, 4th, etc.) harmonics in both positive and negative sequences as well as dc components that may be present on ac variables. The proposed PAVM is verified by experimental measurements and detailed model and is demonstrated to have excellent accuracy under various operating modes and fault configurations. [ABSTRACT FROM AUTHOR]

In this paper, tools for the design of air-core tubular linear electric drives are established. The tools include numerical techniques to efficiently calculate coil inductance and magnet/coil coupling for arbitrary turn placement. The structure of the state model of the system is tailored to facilitate rapid computation enabling comprehensive design optimization. In addition, a null zone is included to efficiently capture the off-state in the diode rectifier model. The tools are highlighted in the design of an energy harvest system that consists of a machine, a diode rectifier, and a capacitive energy storage device. A comparison with hardware measurements is used for validation. [ABSTRACT FROM PUBLISHER]

Brushless excitation systems are widely used for synchronous machines. As a critical part of the system, rotating rectifiers have a significant impact on the system behavior. This paper presents a numerical average-value model (AVM) for rotating rectifiers in brushless excitation systems, where the essential numerical functions are extracted from the detailed simulations and vary depending on the loading conditions. Open-circuit voltages of the brushless exciter armature are used to calculate the dynamic impedance that represents the loading condition. The model is validated by comparison with an experimentally validated detailed model of the brushless excitation system in three distinct cases. It has been demonstrated that the proposed AVM can provide accurate simulations in both transient and steady states with fewer time steps and less runtime compared with detailed models of such systems and that the proposed AVM can be combined with AVM models of other rectifiers in the system to reduce the overall computational cost. [ABSTRACT FROM AUTHOR]

This paper presents a voltage and frequency control (VFC) and an average-value model (AVM) of a line-commutated converter for a rectifier station in an offshore wind farm (OWF) connected by a high-voltage direct current link. A capacitor bank is placed at the AC terminals of the rectifier station to perform VFC within the OWF. The proposed model uses the active and reactive power generated by the OWF as inputs, while the state variables are the voltage magnitude and phase angle at the capacitor bank bus. The proposed VFC is based on the orientation of the voltage vector at the capacitor bank bus toward a synchronous reference axis. It is then demonstrated that frequency control is achieved by regulating the reactive power balance at the capacitor bank bus, while voltage control is carried out by regulating the active power balance. Moreover, it is demonstrated that in a diode rectifier, although voltage cannot be controlled as in a thyristor rectifier, it is bounded within acceptable limits. In addition, small-signal study is performed to facilitate controller design and system stability analysis. VFC and the accuracy of the proposed AVM are validated by simulation, using both the proposed AVM and a detailed switching model. [ABSTRACT FROM AUTHOR]

This paper analyzes a topology consisting of a squirrel-cage induction machine and a Vienna rectifier for a wind energy conversion system. Simulation results reveal that this configuration is advantageous with respect to energy efficiency compared to a traditional six-switch two-level converter. In addition, it can have higher reliability, due to reduced blocking voltage stress across the semiconductors. The theoretical analysis and feasibility of the proposed configuration are validated by experimental results obtained from a prototype system. [ABSTRACT FROM PUBLISHER]

This paper presents a constant power soft-stall control scheme for a vertical-axis wind turbine (VAWT) fitted with a permanent magnet generator. Small-scale VAWTs are attractive due to their ability to capture wind from different directions without using yaw; this simplifies the design and gives the turbine tolerance to turbulence in urban areas. One difficulty with VAWTs is to prevent overspeeding and component overloading at excessive wind velocities. This paper suggests that necessary control can be achieved by controlling the electric power in order to operate the turbine in the stall region. This control strategy attenuates the stress in the system, while smoothing the power generated. It is shown that the controller can stabilize the nonlinear system using an adaptive current feedback loop. The controller does not need wind velocity or turbine speed, but is based on the current and voltage measured on the dc side of the rectifier connected to the permanent magnet synchronous generator. Maximum power point tracking (MPPT) control is provided in normal operation. Simulation and experimental results are presented. [ABSTRACT FROM AUTHOR]

This paper presents a modulation and controller design method for paralleled Z-source inverter systems applicable for alternative energy sources like solar cells, fuel cells, or variable-speed wind turbines with front-end diode rectifiers. A modulation scheme is designed based on simple shoot-through principle with interleaved carriers to give enhanced ripple reduction in the system. Subsequently, a control method is proposed to equalize the amount of power injected by the inverters in the grid-connected mode and also to provide reliable supply to sensitive loads on- site in the islanding mode. The modulation and controlling methods are proposed to have modular independence so that redundancy, maintainability, and improved reliability of supply can be achieved. The performance of the proposed paralleled Z-source inverter configuration is validated with simulations carried out using Matlab/Simulink/Powersim. Moreover, a prototype is built in the laboratory to obtain the experimental verifications. [ABSTRACT FROM AUTHOR]

This paper presents an adaptive dead-time compensation strategy to obtain fundamental phase voltage for inverter-fed vector-controlled permanent magnet synchronous motor drives. A phase dead-time compensation voltage (DTCV) to compensate the disturbance voltage due to undesirable characteristics of inverter, such as dead time, turn on/off time of switching devices, and on-voltages of switching devices and diode, is transformed into q-axis DTCV in the rotor reference frame. The relationship between q-axis DTCV and a dead-time compensation time (DTCT) is investigated. DTCT is identified online by using q-axis disturbance voltage, which is estimated by a disturbance observer. The amplitude of phase DTCV is adaptively determined according to the identified DTCT. The accuracy of identified DTCT is experimentally confirmed by calculating the mean absolute percentage error (MAPE) between calculated active power and measured one. MAPE for adaptive DTCT is within 5 % at any operating point and is less than that for the fixed DTCT. [ABSTRACT FROM AUTHOR]

The analytical derivation of the average-value models (AVMs) for synchronous-machine-fed rectifier systems depends on many factors and is challenging in general. Various models presented in the literature rely on a single operating mode and a dc filter configuration. A recently proposed parametric approach utilizes a detailed simulation as a basis to establish an AVM that uses a proper full-order qd generator model. This paper presents a procedure to extract the appropriate nonlinear algebraic functions to represent the rectifier averaged behavior. In contrast to existing methods, the proposed procedure does not require significant analytical derivations and/or computational resources and results in a highly accurate dynamic AVM. The proposed methodology is validated with the results of detailed simulation and hardware experiment, and is shown to be very accurate in predicting the large-signal time-domain transients as well as the small-signal frequency-domain characteristics. [ABSTRACT FROM AUTHOR]

Test results of active current balance (ACB) at Georgia Power's McDonough Plant show the effect of ACB applied to parallel thyristor bridges in generator excitation applications. This paper presents test results of ACB, a new technique that adjusts the average current in parallel leg thyristors to facilitate current balance between the parallel operating bridges. The unbalance between parallel thyristors is typically caused by differences in inductance feeding the thyristors and by the conducting characteristics of the thyristors. Balance is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are conducting carry more current during this interval and their average current is increased. Test results show that the current balance between the parallel operating thyristors in the bridges is significantly improved when ACB is enabled. The results of rapid changes in load while ACB is enabled are also shown. [ABSTRACT FROM AUTHOR]

Line-commutated converters (LCCs) are widely used in various high-power applications such as generator-rectifier systems, exciters, front-end rectifier loads, and classic high voltage dc systems. Among various techniques used for modeling LCC systems, the dynamic average value modeling (AVM) wherein the effect of switching is neglected or averaged over a prototypical switching interval has become indispensible since it results in continuous, linearizable, and computationally efficient models. The conventional AVMs can only predict the fundamental component of the ac voltages and currents, and neglect the harmonics injected at the ac side by the switching converter. In this paper, a recently proposed parametric average-value modeling (PAVM) approach is extended using multiple reference frame theory to include the significant harmonics of interest (e.g., fifth and seventh) for diode rectifiers. The new PAVM is verified against the detailed simulation in steady-state and transient studies, and is effective in predicting the transient waveforms, while achieving significant computational advantage (speed up) in time-domain simulation over conventional models. [ABSTRACT FROM PUBLISHER]

High-pulse-count rectifier systems (with more than six pulses) are often used in high-power applications. Dynamic average value modeling (AVM) has proven to be indispensable for simulation and analysis of such power electronic systems. However, developing accurate AVMs for such line-commutated converters is challenging due to the presence of complicated switching patterns (operating modes), configuration of multi-phase transformers and/or rotating machines, inter-phase transformers, filters, etc. This paper presents a generalized methodology for full-order dynamic average modeling of high-pulse-count line-commutated rectifiers, which is based on the recently proposed parametric AVM approach. Extensive simulation studies are carried out considering the 18-pulse rectifier example system. The accuracy and numerical advantages of the developed generalized AVM is verified against the detailed switching model, and a previously established average model in time and frequency domains. [ABSTRACT FROM PUBLISHER]

This paper presents a dc-bus voltage control technique for a new power conversion topology feasible for parallel-integrated permanent magnet (PM) wind generation systems. This technique is based on a master–slave hysteresis control scheme in order to solve discrepancy problems that could happen between the controllers. A three-phase semicontrolled rectifier topology is proposed here as an effective interface circuit between each wind generator and the dc-bus. The proposed system provides interconnection extension ability of multiwind converter units sharing the same dc-bus and more economic utilization of the wind generator; by ensuring unity power factor operation. More advantages include voltage disturbance compensation capability, robustness: since a short circuit through a leg is not possible and high efficiency: due to the reduced number of switching elements. The rectifier topology concept, the principle of operation, the control scheme, and test results are presented. The developed technique is also implemented on a 12 kW parallel-connected PM laboratory setup in order to confirm the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

This paper deals with the design and application of eight-linear voltage controllers that provide satisfactory voltage control performance in the presence of unknown load variation. The controller synthesis is based on the synchronous generator (SG) modeling strategy and the feedback control linearization method. Two models are presented. The first one takes into account a fixed load connected to the SG. The second model considers an unknown load. This consideration permits the design of a robust controller as regards the load variation. In addition, the linearization of the feedback control is established using a single-input-single-output controller and a multi-input-single-output (MISO) controller. The MISO solution represents a linearization method without any approximations. To validate the efficiency of each solution, the controllers are developed by using the H_\infty method and implementation in a real-time domain is then performed. [ABSTRACT FROM PUBLISHER]

In order to improve the dynamic performance of the hybrid HVdc system for doubly fed induction generator wind farms, this paper presents the modeling, analysis, and control methods for its line-commutated rectifier. First, the state variable model of the rectifier subsystem on the dq reference frame is derived considering different control modes of the current source inverter subsystem. Then, the ac current dynamic of the rectifier subsystem is quantitatively analyzed based on the eigenvalue analysis method. According to the analysis results, a double loop control scheme is designed: the inner loop utilizes the inverse system control technique plus a switchable phase-lead compensator, and the outer loop implements an ordinary propotional–integral controller. Finally, the validity of the quantitative analysis method and the superiority of the proposed control scheme are verified by Simulink/SimPowerSystems simulations. [ABSTRACT FROM PUBLISHER]

This paper presents a novel ground-fault-detection technique for synchronous machines. This technique is suitable for synchronous machines with static excitation systems, whose excitation field winding is fed by rectifiers through an excitation transformer. The main contribution of this new technique is that it can detect and discriminate both ac- and dc-side ground faults in the excitation system, without the need for traditional power injection sources. This detection technique is based on the frequency analysis of the voltages or currents at a grounding impedance placed at the excitation transformer neutral terminal. This technique has been validated through computer simulations and experimental laboratory tests. [ABSTRACT FROM AUTHOR]

In this paper, the development and control for a switched-reluctance motor (SRM) drive with front-end switch-mode rectifier (SMR) are presented, and good motor driving performance and line drawn power quality are obtained. In the SMR, the dc-link voltage spikes caused by periodic pulse-type motor currents are eliminated using high-frequency small capacitor. And the bulky capacitor is designed according to the specified double-frequency voltage ripple. Then, the robust voltage and current schemes are developed to yield excellent and robust SMR control performance. Having well-regulated and adjustable dc-link volt- age source, the performance enhancements of the SRM in vibration as well as current and speed dynamic responses are performed and evaluated. The robust current and speed control schemes are designed to yield improved driving performance. In addition to voltage boosting, the commutation advanced shift is proposed to further the improve current and speed dynamic responses, the torque-generating capability, and the vibration of SRM. [ABSTRACT FROM AUTHOR]

A three-phase front-end rectifier capable of sustaining sinusoidal input currents in phase with their corresponding input phase voltages, and performing a wide dc link voltage regulation, is analyzed and presented in this paper. It makes use of three bidirectional switches between the rectifier ac side and dc link. An output power estimator, a dc link voltage regulator and a hysteresis current controller are applied in the proposed control algorithm to obtain and track the reference currents. This control algorithm, based on hysteresis current control technique, can enable the proposed rectifier-inverter ac motor drive to function within a wide output power range and large input inductance variations, yielding low current harmonics and unity power factor. Theoretical results of its analysis are verified initially through digital simulation, and confirmed by using an experimental prototype. [ABSTRACT FROM AUTHOR]

Examines the average-value modeling of an inductorless synchronous machine load commutated converter system. Synchronous machine equivalent circuit; Measurement of the values of direct current link voltage; Potential effect of the presence of fast stator electrical transients and slower mechanical and rotor electrical transients.

Line-commutated rectifiers (LCRs) are widely used in various industrial applications, wherein they are also known to be a significant source of harmonics. The dynamic phasor (DP) modeling approaches have been well studied in the literature to simulate the dynamics of power systems and their components including harmonics. In this paper, the state-of-the-art analytical DP (ADP) models of LCRs, which relate the ac and dc subsystems through complicated switch functions, are first investigated. Then, a new parametric DP (PDP) model of LCRs is proposed, wherein the DP dynamics of rectifier/dc-link are represented using a set of explicit algebraic functions that are numerically established. Rigorous computer studies demonstrate that the proposed PDP methodology is capable of accurately predicting the steady-state and transient responses of LCR systems under a wide range of loading conditions, while providing significant computational advantages over the conventional detailed model and the established ADP models. [ABSTRACT FROM PUBLISHER]

A power converter is very necessary in the system of high-speed permanent magnet generators (HSPMG), since the frequency of the output electrical energy is generally hundreds of Hertz or even more than 1000 Hz. However, the converter with switches in fast on–off transitions will have some effects on the generator and, therewith, some current harmonics will be also induced in the device. Taking a 117 kW 60 000 r/min HSPMG as an example, the harmonics in this generator were analyzed. And then, the voltage harmonic content, the current harmonic content, and the current total harmonic distortion could be obtained. In addition, stator core losses and rotor eddy current losses were analyzed comparatively, and the influence of the converter on the generator power factor was further studied. Based on the 3-D coupling field between fluid and temperature, the temperature distributions were obtained when the generator was connected to different loads, and the influence of the rectifier on the generator temperature field could be obtained, which was also compared with the test data. [ABSTRACT FROM AUTHOR]

Model predictive control has emerged as a promising approach to govern modular multilevel converters (MMCs), due to its flexibility to include multiple control objectives and simple design process. However, this control scheme presents relevant issues, such as high computational complexity and variable switching frequency. This work proposes a sequential phase-shifted model predictive control (PS-MPC) for MMCs. The key novelty of this proposal lies in the way the predictive control strategy is formulated to fully exploit a phase-shifted pulsewidth modulation technique through an appropriate choice of synchronized average models for each carrier. In this way, the proposed predictive controller obtains independent optimal modulating signals for each carrier in a sequential manner, by solving an optimization problem with reduced computational effort independent of the number of submodules. This sequential optimization allows one to formulate the optimal control problem to achieve multiple control objectives, similarly to the finite-control-set MPC (FCS-MPC). Nevertheless, the MMC governed with the proposed PS-MPC generates an output voltage with fix-spectrum and operates with an even power loss distribution among semiconductors in steady-state, outperforming the standard FCS-MPC strategy. Experimental results are provided to verify the proposed PS-MPC effectiveness when governing a three-phase MMC with four half-bridges per stack. [ABSTRACT FROM AUTHOR]

Examines the characterization of three phase flux reversal machine (FRM) as an automotive generator. Significance of loss test in obtaining the equivalent model; Construction of rectifier-chopper hardware; Viability of the FRM for automotive generator system; Improvement of the FRM designs.

Analyzes rectifier ripple current superimposed on the direct current of a 600-megawatt asynchronous bipolar link between Lithuania and Poland and a 600-megawatt asynchronous monopolar link between Poland and Sweden in evaluating possible torsional vibrations in steam turbine-generator-exciter shafts. System scaling factors; Shaft torsional phenomena.

Finite Control Set Model Predictive Control (FCS-MPC) is an alternative to conventional linear controllers in power converters due to its attractive properties such as its conceptual simplicity, flexibility, direct consideration of non linearities as well as constraints, and fast control response. This last feature imposes computational constraints and short prediction horizons are therefore needed. In fact, in many applications an horizon one FCS-MPC controller is used. However, using FCS-MPC with a short prediction horizon in topologies that exhibit non-minimum phase behavior such as active-front-end rectifiers, dc-dc boost converters, and Z-source converters, among others, may lead to control issues. This work studies the origin of these control issues using nonlinear control theory considering a dc-dc boost converter as a case study. It is shown that the difficulty to directly control the converter output voltage using short horizon FCS-MPC is the resulting unstable internal dynamic, as the short horizon FCS-MPC controller acts like an input-output linearizing controller. To solve the problem of the unstable internal dynamic, a short horizon FCS-MPC controller based on input-state linearization is proposed allowing correct operation of the system. Experimental results show the feasibility of the proposal. [ABSTRACT FROM AUTHOR]

This article presents a hybrid model predictive controller to ensure dc microgrid stability and enhance the performance of dc-dc boost converters interfaced with constant power loads (CPLs) in a hybrid system. Hybrid systems are dynamic systems with both continuous current mode and discontinuous current mode states. The main purpose in this article is to develop an advanced control technique for voltage regulation and stabilization of the converters in the presence of CPLs due to serious stability concerns, without considering the accurate modelling information of the system. In this regard, an automatic model, considering different modes of operation induced by semiconductor switches in dc-dc boost converters and highly non-linear nature of CPL is employed to design the proposed control approach. The non-linear CPL connected directly to a dc-dc boost converter is utilized to define an optimal tracking control problem by minimizing a finite-prediction horizon cost function, which is known as a finite control set MPC. The proposed controller, which is implemented in both continuous and discontinuous current modes, accounts for the regulation of output voltage within the predefined range. The effectiveness of the proposed hybrid model predictive control is verified using a comparative evaluation with discrete-time averaged model predictive control, continuous control set MPC, and the conventional PI control under experimental conditions. The results authenticate an improved dynamic performance, which can be applied to practical dc microgrids with CPLs. [ABSTRACT FROM AUTHOR]

A new DC current-voltage droop control method for a hybrid high-voltage direct-current (HVDC) system featuring a voltage-sourced converter (VSC) rectifier and a line-commutated converter (LCC) inverter is presented. The proposed method utilizes DC voltage droop control to reduce the AC voltage fluctuation at the inverter side, to exploit the hybrid HVDC system for connecting offshore wind farms to weak grids without necessitating continuous reactive power compensation facilities. Additionally, an optimal droop coefficient calculation method is proposed to keep the AC voltage constant for wind power fluctuations. To evaluate the stability of the proposed method, a small-signal state-space (SS) model of the target system is derived and the root locus analysis is given. Simulation case studies are performed using an electromagnetic transient model in PSCAD and an SS model in MATLAB. The results demonstrate that a hybrid HVDC system with the proposed method successfully integrates offshore wind energy and weak grids without additional facilities because the AC voltage can be maintained at a near constant during wind fluctuations. [ABSTRACT FROM AUTHOR]

A new brushless and permanent magnet less synchronous generator is reported in this work. Contrary to traditional brushless synchronous generators that use an extra machine along with rotating diode rectifier, the proposed machine houses the main and the exciter machine in the same frame. Thus the size, cost, and efficiency of the proposed system are expected to be more competitive. Magnetic decoupling between two windings wound for different number of poles is exploited to create such generators. Two synchronous machines are connected in cascade and housed in the same frame with the armature of the exciter machine put on the rotor and field on the stator. The main machine is a standard synchronous generator. For preliminary investigations, an experimental machine is made having four poles and six poles for main machine and exciter machine, respectively. While the armature of the exciter machine can be wound for different number of phases, a three-phase winding is considered that is fed to rotating rectifier. The output of the rectifier provides field excitation to the main synchronous machine. Extensive simulation studies are carried out using MAXWELL 2D and matlab/Simulink. Experimentations conducted on the laboratory prototype match well with the simulations confirming the effectiveness of the proposed generation system. [ABSTRACT FROM AUTHOR]

A parametric dynamic phasor (PDP) model of diode rectifiers has been recently proposed, which uses a computer-aided approach to effectively represent the rectifier dynamics including ac-side harmonics that change with system conditions. This letter extends the PDP methodology to thyristor-controlled rectifier systems where the dc-link voltage is regulated via the thyristor firing control. Steady state and large-signal transient studies demonstrate that the new/extended model is accurate even for prediction of harmonics for various operating modes. [ABSTRACT FROM AUTHOR]

The influence of passive rectification on the mechanical torque of a permanent magnet generator for a directly driven vertical axis wind turbine has been studied. Passive diode rectification introduce electromagnetic torque ripple from the generator. The conversion of electromagnetic torque ripple into mechanical torque ripple and rotational speed ripple has been modeled, analytically evaluated, and simulated. The simulations have been compared to measurements on an open site 12 kW prototype. A parameter study with the model illustrates the impact of shaft torsional spring constant, generator rotor inertia, generator inductance, and dc-link capacitance. The results show that the shaft and generator rotor can be an effective filter of electromagnetic torque ripple from diode rectification. The measured mechanical torque ripple amplitude on the prototype is less than $\pm$0.9% of nominal turbine torque. The measurements compare well with the simulations. [ABSTRACT FROM AUTHOR]

A synchronous machine model with saturation and cross saturation and an arbitrary rotor network representation that uses a voltage-behind-reactance representation for both the stator windings and the field winding of the machine is proposed. This allows the stator windings and the field winding to be represented as branches in a circuit solver, permitting straightforward simulation with connected circuitry. In particular, the model can be simulated with rectifier loads or with rectifier sources applied to the field winding. The model is validated against experimental data, and its utility is demonstrated in an excitation failure case study. [ABSTRACT FROM PUBLISHER]

This letter proposes a novel method to reject $6n \pm 1$ harmonics in the stator voltage at the point of common coupling (PCC) for a stand-alone doubly-fed induction generator (DFIG) supplying a balanced three-phase diode rectifier. These harmonics are rejected by using the rotor current controller in the fundamental synchronous reference frame. The proposed control algorithm is based on a proportional integral controller and a bank of resonant filters. In this frame, each resonant filter in the rotor current controller has the possibility of rejecting one pair of the positive and the negative stator harmonic voltages at the PCC. Experimental results with 2.2-kW DFIG are shown in this letter. [ABSTRACT FROM PUBLISHER]

A method of representing the effects of magnetic saturation in a coupled-circuit model of a claw-pole alternator is presented. In the approach considered, the airgap flux density produced by each winding is expressed as a function of magnetic operating point. A challenge in the implementation is that the airgap flux densities consist of several significant harmonics, each of which changes at a distinct rate as iron saturates. Despite this complication, it is shown that relatively simple measurements can be used to determine model parameters. The model is implemented in the analysis of several alternator/rectifier systems using a commercial state-model-based circuit analysis program. Comparisons with experimental results over a wide range of speeds and operating conditions demonstrate its accuracy in predicting both the steady state and transient behavior of the systems. [ABSTRACT FROM AUTHOR]