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POWER ELECTRONICS 2015

Category Archives

Maximizing the Power Conversion Efficiency of Ultra-Low-Voltage CMOS Multi-Stage Rectifiers

Abstract:

This paper describes an efficient method to explore the design space of AC/DC converters for energy harvesting circuits. A simple analytical model of the rectifier circuit valid down to ultralowvoltage operation (input voltage below the thermal voltage) is proposed. Based on the Shockley diode equation, the use of an optimization equation that relates the number of stages of the rectifier to the saturation current of the diodes is demonstrated. A set of universal normalized curves to guide the designer regarding the minimum available power from the AC power source is presented. Three rectifiers with different numbers of stages and saturation currents along with matching networks integrated in a 130 nm CMOS technology demonstrated a conversion efficiency of around 10% for an available power of around -20 dBm and DC load voltage and current of 1 V and 1 μA, respectively.


Study on the Single-stage Forward-flyback PFC Converter with QR Control

Abstract:

The singlestage forwardflyback power factor correction converter with quasi-resonant (QR) control is studied in this paper. This converter merges flyback converter and forward converter through a common transformer. Only the flyback subconverter works when the input voltage is lower than the reflected output voltage while both the flyback subconverter and the forward subconverter operate to share the output power in the rest region. The dead zones which exist in the ac input current of traditional forward converter are eliminated and high power factor can be achieved. Two different QR control schemes have been studied. Detailed analysis, optimal design considerations, and comparison for these two QR control modes are provided. Finally, two 120 W experimental prototypes for LED driver were built up to verify the theoretical analysis.


Burst Mode Elimination in High Power LLC Resonant Battery Charger for Electric Vehicles

Abstract:

In order to recover and fully charge batteries in Electric Vehicles, smart battery chargers should not only work under different loading conditions and output voltage regulations (close to zero to 1.5 times the nominal output voltage), but also provide a ripple-free charging current for battery packs and a noise-free environment for the Battery Management System (BMS). In this paper, an advanced LLC design procedure is investigated to provide advantageous extreme regulation and eliminate detrimental burst mode operation. A modified, special LLC tank driven by both Variable Frequency (VF) and Phase Shift (PS) proves to be a successful solution to achieve all the regulation requirements for battery charging (from recovery, bulk, equalization, to finish). The proposed solution can eliminate the negative impact of burst mode noises on the Battery Management System, provide a free-ripple charging current for batteries in different States of Charge, reduce the switching frequency variation, and facilitate the EMI filter and magnetic components designs procedure. In order to fully consider the characteristics of the full bridge LLC resonant converter, especially the output voltage regulation range and soft transitions of the MOSFETs in the Fixed Frequency Phase Shift mode, a new set of analytical equations is obtained for the LLC resonant converter with consideration of separated primary and secondary leakage inductances of the high frequency transformer. Based on the proposed strategy and analytical equations, multivariate statistical design methodology is employed to design and optimize a 120VDC, 3kW battery charger. The experimental results exhibit the excellent performance of the resulting converter, which has a peak efficiency of 96.5% with extreme regulation capability


Hybrid PWM-Resonant Converter for Electric Vehicle On-Board Battery Chargers

Abstract:

A novel hybrid PWMresonant converter is presented in this paper for electric vehicle (EV) 3.3 kW on-board battery chargers (OBCs). While the proposed converter has all the benefit of the before developed hybrid converters for OBCs, the proposed converter has fewer components and achieves much lower voltage stress in the rectifying diodes compared to the before hybrid converters. As a result, it is possible to employ superior diodes such as schottky barrier diodes below 300 V featuring low forward-voltage drop and better reverse-recovery for EV 3.3 kW OBC applications. In addition, the proposed converter achieves much better transformer utilization compared to the before hybrid converters. Due to this, the proposed converter can achieve more optimal efficiency over the overall battery charging profile. The effectiveness of the proposed converter has been verified with the experimental results under an output voltage range of 250-420 V dc at 3.3 kW.


The Hybrid-Cascaded DC–DC Converters Suitable for HVdc Applications

Abstract:

A dcdc converter is essential for interconnecting HVdc networks of different voltage levels. This letter introduces a new family of dcdc converters consisting of semiconductor switches and stack of submodules, which is named as hybridcascaded dcdc converter (HCDC). This concept shows very attractive capital costs and low power losses. In this letter, the detailed operating principle of the HCDC is analyzed. Simulation results are obtained from PSCAD/EMTDC to verify the feasibility of the converters. The performances of the converters are evaluated in detail.


High Gain Zero Voltage Switching Bidirectional converter with reduced number of switches

Abstract:

A nonisolated bidirectional dc-dc converter has been proposed in this brief for charging and discharging the battery bank through a single circuit in applications of uninterruptible power supplies and hybrid electric vehicles. The proposed bidirectional converter operates under a zerovoltage switching condition and provides large voltage diversity in both modes of operation. This enables the circuit to step up the low-battery bank voltage to high dc-link voltage, and vice versa. The bidirectional operation of the converter is achieved by employing only three active switches, a coupled inductor, and an additional voltage clamped circuit. A complete description of the operation principle of the circuit is explained, and the design procedure of the converter has been discussed. The experimental results of a 300-W prototype of the proposed converter confirmed the validity of the circuit. The maximum efficiency of 96% is obtained at half load for boost operation mode and 92% for buck mode of operation.


A New Hybrid Boosting Converter (HBC) for Renewable Energy Applications

Abstract:

A Hybrid Boosting Converter (HBC) with collective advantages of regulation capability from its boost structure and gain enhancement functionality from its voltage multiplier structure is proposed in this paper. The new converter incorporates a Bipolar Voltage Multiplier (BVM), featuring in symmetrical configuration, single inductor and single switch, high gain capability with a wide regulation range, low component stress, small output ripple and flexible extension, which makes it suitable for front-end PV system and some other renewable energy applications. The operation principal, component stress, and voltage ripple are analyzed in this paper. Performance comparison and evaluation with a number of previous single-switch single-inductor converters are provided. A 200W 35V to 380V second-order HBC prototype was built with peak efficiency at 95.44%. The simulation and experimental results both confirm the feasibility of the proposed converter.


Analysis of CLL Resonant Converter with Semi Bridgeless Active Rectifier and Hybrid Control

Abstract:
A novel CLL resonant converter with a semi bridgeless active rectifier (S-BAR) is analyzed and its hybrid control strategy is proposed in this paper. This topology is developed by replacing rectifier lower diodes with synchronous switches and regulation by a phase-shifted PWM signal. Theoretical and simulation results show that the performance of the proposed S-BAR is appropriate for resonant converters and secondary side controlled infrastructure requiring a wide voltage range control at the secondary side. The hybrid control algorithm is obtained with the phase-shift and frequency tuning considering the voltage at the load terminal. A high efficiency can be maintained by this proposed control reducing conduction losses and providing ZVS soft switching condition with a wide output voltage range. To confirm the performance of the proposed converter, experimental results are provided for which the output voltage / current range from 0 to 80 V / 0 to 8.75 A with an input of 120 V and an average efficiency of 93 %.


Hybrid Three-Level and Half-Bridge DC–DC Converter With Reduced Circulating Loss and Output Filter Inductance

Abstract:

A hybrid threelevel (TL) and halfbridge (HB) dcdc converter is proposed in this paper. The TL dcdc converter and HB converter have their own transformers, respectively. Compared with conventional TL dcdc converters, the proposed one has no additional switch at the primary side of the transformer, where the TL converter shares the lagging switches with the HB converter. In order to reduce the circulating current in the primary side, a blocking capacitor is used to reset the primary winding current of the TL converter. Moreover, the rectifier stage is composed of four diodes in the center-tap rectification, forcing the circulating current at the primary side to stay zero during the freewheeling period. The magnetizing inductor of the HB transformer can extend the zero voltage switching operation range of the lagging switches even at light loads. Furthermore, the proposed converter can reduce the output filter inductance. Due to the advantages mentioned above, the efficiency of the converter is improved dramatically. The features and design guidelines of the proposed converter are given in the paper. Finally, the performance of the converter is verified by a 1-kW experimental prototype.


Design, Operation, and Control of S3 Inverter for Single-Phase Microgrid Applications

Abstract:

A singlephase voltage source inverter with a front-end dc-dc conversion stage followed by a synchronized push-pull configuration operating at a desired fundamental frequency (FF) is presented. The duty cycle of the dc-dc conversion stage is varied in the form of a unidirectional sine wave to produce a similar output voltage across the dc-link capacitor. The unidirectional voltage is made into an alternating voltage by the synchronized push-pull configuration. This inverter employs three semiconductor switches, in which one is operating at a high frequency and the rest are operating at an FF. Hence, it is named as the S3 inverter. Furthermore, simple and cost-effective analog circuits are presented for the generation of switching pulses and the control of the amount of power fed to the grid. The hardware prototype of the S3 inverter has been built in a laboratory, and its performance during the stand-alone and grid-connected modes of operation is validated.


Hybrid IPT Topologies With Constant Current or Constant Voltage Output for Battery Charging Applications

Abstract:

The inductive power transfer (IPT) technique in battery charging applications has many advantages compared to conventional plug-in systems. Due to the dependencies on transformer characteristics, loading profile, and operating frequency of an IPT system, it is not a trivial design task to provide the battery the required constant charging current (CC) or constant battery charging voltage (CV) efficiently under the condition of a wide load range possibly defined by the charging profile. This paper analyzes four basic IPT circuits with series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP) compensations systematically to identify conditions for realizing load-independent output current or voltage, as well as resistive input impedance. Specifically, one load-independent current output circuit and one load-independent voltage output circuit having the same transformer, compensating capacitors, and operating frequency can be readily combined into a hybrid topology with fewest additional switches to facilitate the transition from CC to CV. Finally, hybrid topologies using either SS and PS compensation or SP and PP compensation are proposed for battery charging. Fixed-frequency duty cycle control can be easily implemented for the converters.


Hybrid Modulation Scheme for High Frequency AC Link Inverter

Abstract:

This paper describes a hybrid modulation scheme for a highfrequency aclink (HFACL) multistage inverter comprising a front-end dc/ac converter, followed by isolation transformers, an ac/pulsating-dc converter, and a pulsating-dc/ac converter. The hybrid modulation scheme enables 1) removal of the dc-link filter evident in conventional fixed dc-link (FDCL) inverters placed after the ac/pulsating-dc converter stage and before an end stage voltage source inverter and 2) significant reduction in switching loss of the inverter by reducing the highfrequency switching requirement of the pulsating-dc/ac converter by two-third yielding higher efficiency, improved voltage utilization, and reduced current stress. Unlike the FDCL approach, in the HFACL approach, hybrid modulation enables the retention of the sine-wave-modulated switching information at the output of the ac/pulsating-dc converter rather than filtering it to yield a fixed dc thereby reducing the highfrequency switching requirement for the pulsating-dc/ac converter. Overall, the following is outlined: 1) hybrid modulation scheme and its uniqueness, 2) operation of the HFACL inverter using the hybrid modulation scheme, 3) comparison of the efficiency and losses, current stress, and harmonic distortion between the hybridmodulation-based HFACL inverter and the FDCL inverter, and 4) scaled experimental validation. It is noted that the term hybrid modulation has no similarity with the modulation scheme for a hybrid converter (which are conjugation of two types of converters based on a slow and fast device) reported in the literature. The term hybrid modulation scheme is simply chosen because at any given time only one leg of the inverter output stage (i.e., pulsating-dc/ac converter) switch under high frequency, while the other two legs do not switch. The outlined hybrid modulation scheme is unlike all reported discontinuous modulation schemes where the input is a dc and not a pulsating modulated dc, and at most only one leg stays on or off permanently in a 60° or 120° cycle.


A Hybrid Estimator for Active/Reactive Power Control of Single-Phase Distributed Generation Systems with Energy Storage

Abstract:

This paper presents a new active/reactive power closed-loop control system for a hybrid renewable energy generation system used for singlephase residential/commercial applications. The proposed active/reactive control method includes a hybrid estimator, which is able to quickly and accurately estimate the active/reactive power values. The proposed control system enables the hybrid renewable energy generation system to be able to perform real-time grid interconnection services such as active voltage regulation, active power control, and fault ride-through. Simulation and experimental results demonstrate the superior performance of the proposed closed-loop control system.


Flying-Capacitor Based Hybrid LLC Converters with Input Voltage Auto-Balance Ability for High Voltage Applications

Abstract:

An advanced hybrid LLC series resonant converter with integrated flyingcapacitor cell is proposed in this paper to enable the high step-down conversion in the high input voltage applications. The inherent flyingcapacitor branch in the primary side can effectively halve the primary switch voltage stress compared with the half-bridge LLC converters. And the input voltage can be shared equally and automatically between the two series half-bridge modules without additional balance circuit or control strategies due to the built-in flyingcapacitor cell. Moreover, the inherent soft switching performance during wide load range that exists in the LLC converters is still kept to reduce the switching losses, which ensures the high efficiency. Besides, the proposed converter can be extended to further decrease the switch voltage stress by employing stacked connection. Finally, a 500V~640V –input 48V -output 1kW prototype is built and tested to verify the effectiveness of the proposed converter. The results prove that the proposed converter is an excellent candidate for the high input voltage and high step-down DC/DC conversion systems.


Performance Analysis of Bidirectional DC–DC Converters for Electric Vehicles

Abstract:

This paper presents the performance analysis and comparison of two types of bidirectional dcdc converters-cascaded buck-boost capacitor in the middle and cascaded buck-boost inductor in the middle for use in plug-in electric and hybrid electric vehicles. The comparison of the two converters is based on device requirements, rating of switches and components, control strategy, and performance. Each of the converter topologies has some advantages over the other in certain aspects. Efficiency analysis has been carried out for specific scenarios in vehicle applications. The simulation and experimental results are provided for both converter types.


Shunt Active Power Filter With Open-End Winding Transformer and SeriesConnected Converters

Abstract:

This paper presents a shunt active power filter based on the concept of openend winding (OEW) transformer for three-phase three-wire systems. The proposed configuration is constituted of two converters, with three legs in each one, series-connected throughout a three-phase OEW transformer. Model, pulsewidth modulation (PWM) strategy, and control scheme are presented as well. The main advantage of the proposed system lies on the multilevel waveform generation, which allows reduction of either harmonic distortions or switching losses. This proposed topology is suitable for medium-voltage distribution systems. Simulation and experimental results validate the theoretical study.


Implementation of Hybrid Filter for 12-Pulse Thyristor Rectifier Supplying HighCurrent Variable-Voltage DC Load

Abstract:

This paper presents a parallel combination of a shunt-connected passive filter and a distribution static compensator (DSTATCOM) for a 12pulse thyristor rectifier feeding a high-current variablevoltage electrolyzer load. Since the process requires variation of voltage and current over a wide range, the reactive power consumed by the thyristor rectifier also varies enormously. As the passive filter is only able to provide a fixed compensation, the DSTATCOM is used to meet variable reactive power demand to keep the input power factor high even at partial load. In order to keep the DSTATCOM rating small, the bulk of reactive power is supplied by the dominant harmonic passive filter. Moreover, DSTATCOM does not provide any harmonic compensation, which is also taken care by the passive filter. Design and control of this system are discussed briefly, and more emphasis is laid on the implementation of the hybrid filter. An experimental system is tested rigorously, and detailed results are presented.


Control strategy for Single-phase Transformerless Three-leg Unified Power Quality Conditioner Based on Space Vector Modulation

Abstract:

The unified power quality conditioner (UPQC) is known as an effective compensation device to improve power quality for sensitive end-users. This paper investigates the operation and control of singlephase threeleg UPQC (TL-UPQC), where a novel space vector modulation method is proposed for naturally solving the coupling problem introduced by the common switching leg. The modulation method is similar to the well-known space vector modulation widely used with threephase voltage source converters, which thus brings extra flexibility to the TL-UPQC system. Two optimized modulation modes with either reduced switching loss or harmonic distortion are derived, evaluated and discussed, in order to demonstrate the flexibility brought by the space vector modulated TL-UPQC. Simulations and experimental results are presented to verify the feasibility and effectiveness of the proposed space vector modulation approach as well as two mentioned modulation modes.


Dynamic Voltage Restorer Based on Three-Phases Inverters Cascaded Through an Open-End Winding Transformer

Abstract:

This paper investigates a dynamic voltage restorer (DVR) composed of two conventional three-phase inverters series cascaded through an openend winding (OEW) transformer, denominated here DVR-OEW. The DVR-OEW operating with either equal or different dc-link voltages is examined. The proposed topology aims to regulate the voltage at the load side in the case of voltage sags/swells, distortion, or unbalance at the grid voltage. A suitable control strategy is developed, including space-vector analysis, level-shifted PWM and its equivalent optimized single-carrier PWM, as well as the operating principles and characteristics of the DVR. Comparisons among the DVR-OEW and conventional configurations, including a neutral-point clamped converter-based DVR, are furnished. The main advantages of the DVR-OEW compared to the conventional topologies lie on: 1) reduced harmonic distortion, 2) reduced converter losses, and 3) reduced voltage rating of the power switches. Simulated and experimental results are presented to validate the theoretical studies.


An Integrated Dynamic Voltage Restorer-Ultracapacitor Design for Improving Power Quality of the Distribution Grid

Abstract:

Cost of various energy storage technologies is decreasing rapidly and the integration of these technologies into the power grid is becoming a reality with the advent of smart grid. Dynamic voltage restorer (DVR) is one product that can provide improved voltage sag and swell compensation with energy storage integration. Ultracapacitors (UCAP) have low-energy density and high-power density ideal characteristics for compensation of voltage sags and voltage swells, which are both events that require high power for short spans of time. The novel contribution of this paper lies in the integration of rechargeable UCAP-based energy storage into the DVR topology. With this integration, the UCAP-DVR system will have active power capability and will be able to independently compensate temporary voltage sags and swells without relying on the grid to compensate for faults on the grid like in the past. UCAP is integrated into dc-link of the DVR through a bidirectional dc-dc converter, which helps in providing a stiff dc-link voltage, and the integrated UCAP-DVR system helps in compensating temporary voltage sags and voltage swells, which last from 3 s to 1 min. Complexities involved in the design and control of both the dc-ac inverter and the dc-dc converter are discussed. The simulation model of the overall system is developed and compared to the experimental hardware setup.


Reduced Capacitance Thin-Film H-Bridge Multilevel STATCOM Control Utilizing an Analytic Filtering Scheme

Abstract:

In this paper, an analytic capacitor voltage filtering scheme for a cascaded Hbridge (CHB) multilevel converter-based static synchronous compensator (STATCOM) is proposed. The scheme is able to filter out a low-frequency ripple of any magnitude and introduces a negligible delay to the control system. Therefore, it facilitates using high-reliability film capacitors with low capacitance instead of the conventional electrolytic capacitors. The proposed analytic filtering scheme is implemented and verified on a single-phase seven-level CHB multilevel converter-based STATCOM. The parameters’ sensitivity evaluation is provided to demonstrate practicability of the proposed scheme. Simulation and experimental results are provided to compare the performance of the proposed filtering scheme with the performance of the most commonly used second-order filter-based approach to demonstrate its effectiveness.


Application of PI and Super Twisting Drivers to Voltage Regulation of Wind farm via StatCom

Abstract:

In this paper, the steady-state and dynamic behavior of a wind farm with reactive power compensation using a StatCom devices is analyzed. An algorithm and a method for calculating the steady-state of the wind farm with SCIG and the random wind speed is proposed. A StatCom is used to minimize the effects of the wind speed variation. Two controllers are analyzed in the StatCom; one PI lineal type and the other is a non-lineal robust type known as SuperTwisting, to regulate the voltage magnitude in the connection node.


Online Reference Limitation Method of Shunt-Connected Converters to the Grid to Avoid Exceeding Voltage and Current Limits Under Unbalanced Operation

Abstract:

This paper focuses the analysis around a gridconnected converter that operates under unbalanced voltage and current conditions. Two different scenarios have been selected to validate the proposed algorithms. In the first scenario, the converter operates as a reactive power compensator, exchanging reactive power with the grid, and minimizing the dc bus voltage oscillations. The second scenario is more challenging, since it operates as a load balancer. In this application, the unbalanced current consumption of the load is compensated by the converter, exchanging positive and negative sequence currents with the grid. Another objective in this second scenario is to compensate the reactive power of the unbalanced load. It is important to highlight that in both scenarios, any maximum limit of the converter can be exceeded due to the specific unbalanced voltage and current conditions. Three variables are considered critical for the converter: 1) output ac current limit; 2) output ac voltage limit; and 3) dc bus voltage oscillation limit. Thus, this paper proposes a control method, which limits online the reactive power reference in the first scenario and the exchanged current references in the second scenario in order not to exceed any of the mentioned critical variables. The corresponding experimental results are shown in Part II of this paper so as to validate the limitation algorithm obtained by means of the mathematical analysis carried out in Part I.


Individual Phase Current Control Based on Optimal Zero Sequence Current Separation for a Star-Connected Cascade STATCOM under Unbalanced Conditions

Abstract:

This paper proposes an individual phase current control method based on optimal zero sequence current separation for starconnected cascade STATCOMs under unbalanced grid voltage. The Individual phase current control (IPCC) is adopted to get better adaptation to unbalanced conditions. Each cluster of the cascade STATCOM has its own phase current control loop and dc voltage feedback loop, and it can generate the necessary negative sequence current automatically to rebalance power. However the zero sequence current which is unnecessary and harmful in star configuration is introduced in the process. An optimal zero sequence current separation (OZSCS) is presented to remove the zero sequence current, by reconstructing the reactive current command. This improved IPCC enables starconnected STATCOMs adapted to unbalanced conditions without the need for any power balancing algorithms. Also the operation range of the proposed control method is analyzed. Simulation researches are performed to verify the operation range analysis, and experimental operation is tested on a 25-level ±10Mvar/10kV STATCOM product installed at a wind farm.


A Fast LP Approach for Enhanced Utilization of Variable Impedance Based FACTS Devices

Abstract:

Transmission systems are under stress and need to be upgraded. Better utilization of the existing grid provides a fast and cheap alternative to building new transmission. One way to improve the utilization of the transmission network is power flow control via flexible ac transmission system (FACTS) devices. While FACTS devices are used today, the utilization of these devices is limited; traditional dispatch models (e.g., security constrained economic dispatch) assume a fixed, static transmission grid even though it is rather flexible. The primary barrier is the complexity that is added to the power flow problem. The mathematical representation of the DC optimal power flow, with the added modeling of FACTS devices, is a nonlinear program (NLP). This paper presents a method to convert this NLP into a mixed-integer linear program (MILP). The MILP is reformulated as a two-stage linear program, which enforces the same sign for the voltage angle differences for the lines equipped with FACTS. While this approximation does not guarantee optimality, more than 98% of the presented empirical results, based on the IEEE 118-bus and Polish systems, achieved global optimality. In the case of suboptimal solutions, the savings were still significant and the solution time was dramatically reduced.


FACTS Devices Allocation via Sparse Optimization

Abstract:

Although there are vast potential locations to install FACTS devices in a power system, the actual installation number is very limited due to economical consideration. Therefore the allocation strategy exhibits strong sparsity. This paper formulates FACTS device allocation problem as a general sparsity-constrained OPF problem and employs 28174 FACTS Devices Allocation via Sparse Optimization norms to enforce sparsity on FACTS devices setting values to achieve solutions with desirable device numbers and sites. An algorithm based on alternating direction method of multipliers is proposed to solve the sparsity-constrained OPF problem. The algorithm exploits the separability structure and decomposes the original problem into an NLP subproblem, an 28175 FACTS Devices Allocation via Sparse Optimization regularization subproblem, and a simple dual variable update step. The NLP subproblem is solved by the interior point method. The 28175 FACTS Devices Allocation via Sparse Optimization regularization subproblem has a closed-form solution expressed by shrinkage-threholding operators. The convergence of the proposed method is theoretically analyzed and discussed. The proposed method is successfully tested on allocation of SVC, TCSC, and TCPS on IEEE 30-, 118-, and 300-bus systems. Case studies are presented and discussed for both single-type and multiple-type FACTS devices allocation problems, which demonstrates the effectiveness and efficiency of the proposed formulation and algorithm.


A New Control Strategy for Distributed Static Compensators Considering Transmission Reactive Flow Constraints

Abstract:

This paper presents a new control strategy for a distributed static compensator (also known as distributed STATCOM or DSTATCOM), configured to regulate the reactive (VAr) flow at a point in a transmission system. This new control strategy takes into account the operating VAr limits of that reactive flow in determining the steady-state output of the DSTATCOM. The new control strategy applies a slow reset regulator (SRR) to slowly bias the VAr set point of the DSTATCOM master controller to maintain its steady-state output within a target bandwidth. The operating result maintains an appropriate VAr reserve level from the DSTATCOM for dynamic events in the system. This paper also presents a new algorithm to calculate the operating constraints of the SRR that reflect the VAr flow at the local or remote point in the transmission system and the allowable VAr thresholds for that flow. These allowable thresholds can be utilized to the full extent to lower the steady-state output of the DSTATCOM, maximize its VAr reserve for dynamic events and reduce equipment and associated system operating losses. Modeling, implementation, and simulation of an engineering project show that the new control strategy and algorithm are functioning properly as expected.


Reinforcement Learning-Based Intelligent Maximum Power Point Tracking Control for Wind Energy Conversion Systems

Abstract:

This paper proposes an intelligent maximum power point tracking (MPPT) algorithm for variable-speed wind energy conversion systems (WECSs) based on the reinforcement learning (RL) method. The model-free Q-learning algorithm is used by the controller of the WECS to learn a map from states to optimal control actions online by updating the action values according to the received rewards. The experienced action values are stored in a Q-table, based on which the maximum power points (MPPs) are obtained after a certain period of online learning. The learned MPPs are then used to generate an optimum speed–power curve for fast MPPT control of the WECS. Since RL enables the WECS to learn by directly interacting with the environment, knowledge of wind turbine parameters or wind speed information is not required. The proposed MPPT control algorithm is validated by simulation studies for a 1.5-MW doubly-fed induction generator-based WECS and experimental results for a 200-W permanent-magnet synchronous generator-based WECS emulator.


Maximum Power Point Tracking Strategy for Large-Scale Wind Generation Systems Considering Wind Turbine Dynamics

Abstract:

Under the global trend of renewable energy development, various advanced techniques such as forecasting algorithm, intelligent computation, and optimal control are expected to make the complex and uncertain renewable energy system stable and profitable in the near future. This paper presents a new control strategy for large-scale wind energy conversion systems to achieve a balance between power output maximization and operating cost minimization. First, an intelligent maximum power point tracking (IMPPT) algorithm is proposed such that short-term wind speed prediction, wind turbine dynamics, and MPPT are collectively considered to improve system efficiency. Second, in view of a spatial and temporal distribution of wind speed disturbances, a box uncertainty set is embedded in the forecast wind speed, which is likely more realistic for practicing engineers. Then, the IMPPT and box uncertainties are applied to the wind energy conversion system (WECS) control strategy, which is formulated as a min-max optimization problem and efficiently solved with semi-definite programming (SDP). Finally, a comparison with the conventional MPPT control method demonstrates that the proposed approach can obtain higher efficiency, which validates this paper.


On the Design and Capacity of Grounding Systems for Grid-Connected DGUs

Abstract:

Established standards and industrial codes, for interconnecting distributed generation units (DGUs), address voltage and frequency changes, levels and quality of injected power, islanding conditions, and protection. However, these standards and codes do not clearly address the grounding configurations for gridconnected DGUs. One of the challenges related to the grounding configuration for DGUs is the harmonics generated by power electronic converters employed in DGUs. These harmonics can raise the ground potentials and disrupt the function of ground fault protection. This paper develops and tests a frequency selective grounding configuration for gridconnected DGUs. The developed grounding configuration offers limiting ground potentials and ground fault currents, while imposing negligible impacts on the function of ground fault protective devices. The frequency selective grounding configuration is designed and tested for two wind energy conversion systems and a photovoltaic system. Test results show that the developed grounding configuration achieves its objectives with negligible impacts on the grounded DGU and its ground fault protection.


Using Improved Power Electronics Modeling and Turbine Control to Improve Wind Turbine Reliability

Abstract:

Improving offshore wind turbine reliability is a key industry goal to improve the availability of this renewable energy generation source. The semiconductor devices in the wind turbine power converter are traditionally considered as the most sensitive and important components to achieve this and managing their thermomechanical stressing is vital, since this is one of their principal long-term aging mechanisms. Conventional deterministic reliability prediction methods used in industrial applications are not suitable for wind turbine applications, due to the stochastic nature of the wind speed. This paper develops an electrothermal model of the power devices, which is integrated with a wind turbine system model for the investigation of power converter thermal cycling under various operating conditions. The model has been developed to eliminate the problems of pulse width modulation switching, substantially reducing simulation time. The model is used to improve the current controller tuning method to reduce thermal stresses suffered by the converter during a grid fault. The model is finally used to design a control method to alleviate a key problem of the doubly fed induction generator-severe thermal cycling caused during operation near synchronous speed.


Wound Rotor Machine With Single-Phase Stator and Three-Phase RotorWindings Controlled by Isolated Three-Phase Inverter

Abstract:

This paper proposes a singlephase grid-connected wound rotor machine that has the singlephase stator and threephase rotor windings. The machine has no auxiliary winding or capacitor unlike conventional singlephase machines. Nevertheless, it can operate in all four quadrants of the torque-speed plane with a threephase inverter. For adjustable speed drives, an isolated threephase inverter is applied to the rotor windings while the stator winding is directly connected to a singlephase source. The grid filter and rectifier of the conventional system are eliminated and the rotor-side slip rings can be also removed by the inverter integration. So the overall structure of the proposed drive system is simple and cost effective. In this paper, the proposed machine is modeled into a modified d-q model considering the absence of q-axis stator coil. Its characteristics are analyzed and vector control methods of the grid power factor, dc-link voltage, and speed are proposed. For more efficient control, the optimal rotor current set is calculated from the minimum copper loss condition. The system has wider operating areas than other singlephase drive systems. The feasibility of the proposed system is verified by experiments.


A Single Sensor Based MPPT Controller for Wind-Driven Induction Generators Supplying DC Microgrid

Abstract:

In this paper, a simple method of tracking the maximum power (MP) available in the wind energy conversion system (WECS) for dc microgrid application is proposed. A three-phase diode bridge rectifier along with a dcdc converter has been employed between the terminals of winddriven induction generator and dc microgrid. Induction generator is being operated in self-excited mode with excitation capacitor at stator. The output current of the dcdc converter i.e., dc grid current is considered as a control variable to track the MP in the proposed WECS. Thus, the proposed algorithm for maximum power point tracking (MPPT) is independent of the machine and wind-turbine parameters. This algorithm has been implemented using dsPIC30F4011 controller. Further, a method has been developed for determining the duty ratio of the dcdc converter for operating the proposed system in MPPT condition using wind turbine characteristics, steady-state equivalent circuit of induction generator and power balance in power converters. Circuit simplicity and simple control algorithm are the major advantages of the proposed configuration for supplying power to the dc microgrid from the proposed small scale WECS. The successful working of the proposed algorithm for MPPT has been demonstrated with extensive experimental results along with the simulated values.


A Discrete-Time Direct Torque Control for Direct-Drive PMSGBased WindEnergy Conversion Systems

Abstract:

This paper proposes a novel flux-space-vector-based direct torque control (DTC) scheme for permanent-magnet synchronous generators (PMSGs) used in variable-speed directdrive wind energy conversion systems (WECSs). The discretetime control law, which is derived from the perspective of flux space vectors and load angle, predicts the desired stator flux vector for the next time-step with the torque and stator flux information only. The space vector modulation (SVM) is then employed to generate the reference voltage vector, leading to a fixed switching frequency, as well as lower flux and torque ripples, when compared to the conventional DTC. Compared with other SVM-based DTC methods in the literature, the proposed DTC scheme eliminates the use of proportional-integral regulators and is less dependent on machine parameters, e.g., stator inductances and permanent-magnet flux linkage, while the main advantages of the DTC, e.g., fast dynamic response and no need of coordinate transform, are preserved. The proposed DTC scheme is applicable for both nonsalient-pole and salient-pole PMSGs. The overall control scheme is simple to implement and is robust to parameter uncertainties and variations of the PMSGs. The effectiveness of the proposed discretetime DTC scheme is verified by simulation and experimental results on a 180-W salient-pole PMSG and a 2.4-kW nonsalient-pole PMSG used in variable-speed directdrive WECSs.


Contribution of VSC-HVDC to Frequency Regulation of Power Systems With Offshore Wind Generation

Abstract:

Modern large wind farms are required to provide frequency regulation service like conventional synchronous generation units. The frequency support capability of modern wind farms has been widely investigated and implemented. Remotely located large offshore wind farms are probably connected to the onshore system grid through voltage-source converter-based-high voltage direct current (VSCHVdc) transmission systems. Due to the decoupling of VSCHVdc and signal transmission delay, offshore wind farms may not be able to respond to the onshore grid frequency excursion in time and, consequently, the stability and security of the power system will be put at risk, especially for those with high wind penetration. This paper proposes a coordinated control scheme to allow VSCHVdc link to contribute to the system frequency regulation by adjusting its dc-link voltage. By means of this approach, the dc capacitors of VSCHVdc are controlled to absorb or release energy so as to provide frequency support. To further enhance the system frequency response, the frequency support from VSCHVdc is also finely coordinated with that from offshore wind farm according to the latency of offshore wind farm responding to onshore grid frequency excursion. The control scheme is evaluated for both underfrequency and overfrequency events, and results are presented to demonstrate its effectiveness.


Split Converter-Fed SRM Drive for Flexible Charging in EV/HEV Applications

Abstract:

Electric vehicles (EVs) and hybrid EVs are the way forward for green transportation and for establishing low-carbon economy. This paper presents a split converterfed four-phase switched reluctance motor (SRM) drive to realize flexible integrated charging functions (dc and ac sources). The machine is featured with a central-tapped winding node, eight stator slots, and six rotor poles (8/6). In the driving mode, the developed topology has the same characteristics as the traditional asymmetric bridge topology but better fault tolerance. The proposed system supports battery energy balance and on-board dc and ac charging. When connecting with an ac power grid, the proposed topology has a merit of the multilevel converter; the charging current control can be achieved by the improved hysteresis control. The energy flow between the two batteries is balanced by the hysteresis control based on their state-of-charge conditions. Simulation results in MATLAB/Simulink and experiments on a 150-W prototype SRM validate the effectiveness of the proposed technologies, which may provide a solution to EV charging issues associated with significant infrastructure requirements.


A Zero-Voltage-Transition Bidirectional DC/DC Converter

Abstract:

A three-level (TL) bidirectional dc/dc converter is a suitable choice for power electronic systems with a high-voltage dc link, as the voltage stress on the switches is half and inductor current ripple frequency is twice the converter’s switching frequency. This study proposes a zerovoltage transition (ZVT) TL dc/dc converter to enable operation with higher switching frequency in order to achieve higher power density and enhance efficiency. Two identical ZVT cells, each one composed of two resonant inductors, a capacitor, and an auxiliary switch, are integrated with the conventional TL topology to enable soft switching in all four switches in both buck and boost operation modes. In addition, a variable dead-time control is proposed to increase the effective duty ratio at heavy loads. The proposed soft-switching feature has been demonstrated under different loading conditions. A 650-W prototype is designed and fabricated, which exhibits 95.5% at full load.


Analysis and Parameter Optimization of Start-up Process for LLC Resonant Converter

Abstract:

There is a current spike in LLC resonant converter during startup and frequency modulation method is adopted to limit the resonant current usually, which controls switching frequency decreasing from specific high frequency to steady-state frequency in exponential pattern with a time constant tau. However, it is hard to determine the crucial parameter of time constant of the exponential switching frequency sweep curve, due to the complexity in analyzing dynamic process of the converter. In this paper, the operation modes and characteristics under continued conductive mode and discontinued conductive mode are analyzed based on state-plane analysis and the boundary of two conductive modes is figured out. Then, the iteration calculation model of the LLC converter is proposed to predict the resonant current during startup progress, and time constant of the existing soft start method is optimized using iteration algorithm. Using the proposed iterative design method, the current stress of LLC resonant converter is limited, as well as guaranteeing the response speed of output voltage.


Operating Conditions Monitoring for High Power Density and CostEffective Resonant Power Converters

Abstract:

Resonant power conversion is a key enabling technology for many high-performance and high-efficiency systems. Induction heating is an example of such systems, which outperforms conventional heating technologies, leading to a wide spectrum of industrial, domestic, and medical applications. In order to ensure the proper operation of the resonant converter, accurate converter operation monitoring techniques are required. This paper proposes an effective monitoring technique based on the measurement of the resonant capacitor voltage. The proposed monitoring system provides essential information regarding the output power, soft-switching conditions, and input voltage, while featuring a cost-effective implementation. The proposed technique is validated through an FPGA-controlled resonant power converter applied to induction heating, proving the feasibility of this proposal.


DCM-based Zero-Voltage Switching Control of a Bidirectional DC-DC Converter With Variable Switching Frequency

Abstract:

This paper investigates a control approach for achieving reliable zerovoltage switching transitions within the entire operating range of a conventional nonisolated bidirectional dcdc converter that utilizes synchronous rectification. The approach is based on operation in the discontinuous conduction mode with a constant reversed current of sufficient amplitude, which is achieved by load-dependent variation of the switching frequency. This paper focuses on the obtained resonant voltage transitions and provides analytical models for determining the reversed current and timing parameters that would ensure safe, reliable and highly efficient operation of the converter. In addition, the proposed approach solves the synchronous transistor’s spurious turn-on and body diode reverse recovery induced issues, does not require any additional components or circuitry for its realization, and can be entirely implemented within a digital signal controller. The effectiveness and performance of the presented control approach was confirmed in a 1-kW experimental bidirectional dcdc converter that achieved 97% efficiency over a wide range of output powers at switching frequencies above 100 kHz.


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