Showing papers on "Power factor published in 2019"

99% Efficient 10 kV SiC-Based 7 kV/400 V DC Transformer for Future Data Centers

Abstract: The power supply chain of data centers from the medium voltage (MV) utility grid down to the chip-level voltage consists of many series connected power conversion stages and accordingly shows a relatively low efficiency. Solid-state transformers (SSTs) could improve the efficiency by substantially reducing the number of power conversion stages and/or directly interfacing the MV ac grid to a 400 V dc bus, from where server racks with a power consumption of several tens of kilowatts could be supplied by individual SSTs. The recent development of SiC MOSFETs with a blocking voltage of 10 kV enables the realization of a simple and, hence, highly reliable two-stage SST topology, consisting of an ac/dc power factor correction rectifier and a subsequent isolated dc/dc converter. In this context, an isolated 25 kW, 48 kHz, 7 kV to 400 V series resonant dc/dc converter based on 10 kV SiC MOSFETs is realized and tested in this paper. To achieve zero voltage switching of all MOSFETs, a special modulation scheme to actively control the amount of the switched magnetizing current on the MV- and low voltage-sides is implemented. Furthermore, the design of all main components and, especially, the electrical insulation of the employed medium-frequency transformer are discussed in detail. Calorimetric efficiency measurements show that a full-load efficiency of 99.0% is achieved, while the power density reaches 3.8 kW/L ( $63~\text {W}/\mathrm {in^{3}}$ ).

Review of power electronics in vehicle-to-grid systems

Abstract: Vehicle-to-Grid (V2G) is a promising technology that allows the batteries of idle or parked electric vehicles (EVs) to operate as distributed resources, which can store or release energy at appropriate times, resulting in a bidirectional exchange of power between the ac grid and the dc EV batteries. This bidirectional exchange of power is realized using bidirectional power electronic converters that connect the grid with the EV battery. Most research on bidirectional converters for V2G applications focuses on using two dedicated power conversion stages – a bidirectional ac-dc conversion stage that helps in power factor correction, followed by a bidirectional dc-dc conversion stage that provides voltage matching. However, a single bidirectional ac-dc conversion stage can also facilitate V2G and grid-to vehicle (G2V) active power transfers. This paper reviews and compares the various bidirectional ac-dc and dc-dc converter topologies that facilitate V2G and G2V active power flows. Moreover, the paper discusses the various classes of charger/discharger systems reported for V2G applications, like on-board/off-board, integrated/non-integrated and conductive/inductive, and a comparative statement is made based on certain proposed criteria. Further, the current trends in the application of wide band-gap devices in high power-dense V2G capable converters and integration of renewable energy sources into EV charging/discharging infrastructures have also been discussed.

A Bidirectional Soft-Switched DAB-Based Single-Stage Three-Phase AC–DC Converter for V2G Application

Abstract: In vehicle-to-grid applications, the battery charger of the electric vehicle (EV) needs to have a bidirectional power flow capability. Galvanic isolation is necessary for safety. An ac–dc bidirectional power converter with high-frequency isolation results in high power density, a key requirement for an on-board charger of an EV. Dual-active-bridge (DAB) converters are preferred in medium power and high voltage isolated dc–dc converters due to high power density and better efficiency. This paper presents a DAB-based three-phase ac–dc isolated converter with a novel modulation strategy that results in: 1) single-stage power conversion with no electrolytic capacitor, improving the reliability and power density; 2) open-loop power factor correction; 3) soft-switching of all semiconductor devices; and 4) a simple linear relationship between the control variable and the transferred active power. This paper presents a detailed analysis of the proposed operation, along with simulation results and experimental verification.

Coordinated Electric Vehicle Charging With Reactive Power Support to Distribution Grids

Abstract: We develop hierarchical coordination frameworks to optimally manage active and reactive power dispatch of number of spatially distributed electric vehicles (EVs) incorporating distribution grid level constraints. The frameworks consist of detailed mathematical models, which can benefit the operation of both entities involved, i.e., the grid operations and EV charging. The first model comprises of a comprehensive optimal power flow model at the distribution grid level, while the second model represents detailed optimal EV charging with reactive power support to the grid. We demonstrate benefits of coordinated dispatch of active and reactive power from EVs using a 33-node distribution feeder with large number of EVs (more than 5000). Case studies demonstrate that, in constrained distribution grids, coordinated charging reduces the average cost of EV charging if the charging takes place at nonunity power factor mode compared to unity power factor. Similarly, the results also demonstrate that distribution grids can accommodate charging of increased number of EVs, if EV charging takes place at nonunity power factor mode compared to unity power factor.

Adaptive Reactive Power Control of PV Power Plants for Improved Power Transfer Capability Under Ultra-Weak Grid Conditions

Abstract: This paper analyzes the power transfer limitation of the photovoltaic (PV) power plant under the ultra-weak grid condition, i.e., when the short-circuit ratio (SCR) is close to 1. It explicitly identifies that a minimum SCR of 2 is required for the PV power plant to deliver the rated active power when operating with the unity power factor. Then, considering the reactive power compensation from PV inverters, the minimum SCR in respect to power factor (PF) is derived, and the optimized coordination of the active and reactive power is exploited. It is revealed that the power transfer capability of PV power plant under the ultra-weak grid is significantly improved with the low PF operation. An adaptive reactive power droop control is next proposed to effectively distribute the reactive power demands to the individual inverters, and meanwhile, maximize the power transfer capacity of the PV power plant. Simulation results of a 200-MW PV power plant demonstrate that the proposed method can ensure the rated power transfer of PV power plant with the SCR of 1.25, provided that the PV inverters are operated with the minimal PF=0.9.

99.1% Efficient 10 kV SiC-Based Medium-Voltage ZVS Bidirectional Single-Phase PFC AC/DC Stage

Abstract: Due to their extremely high energy demand, data centers are directly supplied from a medium-voltage (MV) grid. However, a significant part of this energy is dissipated in the power supply chain since the MV is reduced step-by-step through multiple power conversion stages down to the chip-voltage level. In order to increase the efficiency of the power supply chain, the number of conversion stages must be substantially reduced. In this context, solid-state transformers (SSTs) are considered as a possible solution, as they could directly interface the MV AC grid to a 400 V DC bus, whereby server racks with a power consumption of several tens of kilowatts could be directly supplied from an individual SST. With a focus on the lowest system complexity, the SST, ideally, should be built as simple two-stage system consisting of an MV AC/DC power factor correction (PFC) rectifier stage followed by an isolated DC/DC converter. Accordingly, this paper focuses on the design and realization of a 25 kW, 3.8 kV single-phase AC to 7 kV DC PFC rectifier unit based on the 10 kV SiC MOSFETs. By simply adding an $LC$ circuit between the switch nodes of the well-known full-bridge-based pulse width modulated AC/DC rectifier, the integrated triangular current-mode concept is implemented, which only internally superimposes a large triangular current ripple on the AC mains current and, therefore, enables zero-voltage switching over the entire AC mains period. Special attention is paid to the realization of the MV inductors and their electrical insulation, the AC-input $LCL$ filter to limit electromagnetic interference emissions, and the challenges arising due to cable resonances when connecting the SST to the MV grid via an MV cable. Despite the large insulation distances required for MV, the realized 25 kW MV PFC rectifier achieves an unprecedented power density of 3.28 kW/L (54 W/ $\mathrm {in}^{3}$ ) and a full-load efficiency of 99.1%, determined using a calorimetric measurement setup, which is discussed in detail in the Appendix.

An IPT Battery Charger With Near Unity Power Factor and Load-Independent Constant Output Combating Design Constraints of Input Voltage and Transformer Parameters

Abstract: Inductive power transfer (IPT) techniques are becoming popular in battery charging applications due to some unique advantages compared to the conventional plug-in systems. A high-performance IPT charger should provide the battery with an efficient charging profile consisting of constant charging current and constant charging voltage. However, with a wide load range, it is hard to realize the initial load-independent constant current (CC) and the subsequent load-independent constant voltage (CV) using a single IPT converter while maintaining nearly unity power factor and soft switching of power switches simultaneously. This paper systematically analyzed the characteristics of an LCC – LCC compensated IPT converter and proposed a design method to realize the required load-independent CC and CV outputs at two zero-phase angle frequencies. The design also combats the constraints of an IPT transformer and input voltage, thus facilitating the use of a simple duty cycle control operating at two fixed frequencies for both CC and CV operations. The design criteria, control logic, and sensitivities of compensation parameters to the input impedance and load-independent output are discussed. Finally, an IPT battery charger prototype with 1 A charging current and 24 V battery voltage is built to verify the analysis.

Power Quality Management of PV Power Plant With Transformer Integrated Filtering Method

Abstract: This paper presents a new solution of power quality management for the photovoltaic (PV) power plant with the transformer integrated filtering method. The innovation of the PV power plant is that it contains two-stage filtering station, which consists of a box-type transformer with integrated filter and a 110 kV grid-connected transformer based on inductive filtering method. The filtering reactor can be integrated into the box-type transformer as a decoupled winding, so that the modular design of the box-type transformer and the passive filter is realized. Inductive filtering method applied in the grid-connected transformer not only improves the power quality at the point of common coupling, but dampens the harmonic resonance between the passive filter and grid. The hierarchical structure of the PV power plant is introduced firstly. Then, the equivalent circuit model and the mathematical model of the primary and secondary filtering stations are established, respectively. Moreover, the influence of the weak coupling of integrated reactor on the filtering performance is analyzed. The antijamming capabilities of the inductive filtering method and the traditional filtering method are compared under the condition of parameter disturbance. At last, the site tests of the PV power plant with transformer integrated filtering method are made, which show that the PV grid-connected system comprised of the two-stage filtering station has the characteristics of low harmonic emission, high power factor, and stable operation.

Comparison Between Linear Induction Motor and Linear Flux-Switching Permanent-Magnet Motor for Railway Transportation

Abstract: Linear induction motors (LIMs) have been extensively adopted in urban railway transportation given their nonadhesion thrust and simple stator structure. However, these LIMs undergo unfavorable power factor and efficiency considering their copper and eddy losses. Recently, linear flux-switching permanent-magnet motors (LFSPMs) have attracted the attention of researchers due to their high power density, simple structure, easy heat dissipation, high efficiency, and high power factor. However, so far, no quantitative comparison exists between LFSPMs and LIMs of the size 1:1 to prove the feasibility of LFSPMs to be used in railway transit systems. Therefore, a comparative analysis between the two motors is conducted in this paper. First, this paper investigates the LIMs for railway transit using the finite element method (FEM). Second, the LFSPMs for railway transit is designed and optimized. Third, the electromagnetic performance of the two motors is compared and analyzed through the FEM. Finally, a small-sized prototype of the LFSPMs is constructed to validate the FEM results. It is concluded that LFSPMs offer a favorable thrust force, have high efficiency, and high power factor but have large normal force and force ripple. Therefore, applying LFSPM requires a reliable supporting device and a shock absorber.

Fuzzy Logic Inverter Controller in Photovoltaic Applications: Issues and Recommendations

Abstract: Fossil fuels and other conventional energy sources used to generate electricity are finite. Therefore, alternative energy sources should be pursued to meet present and future energy demands. The photovoltaic (PV) is a promising renewable energy source, especially for the remote areas. The PV is a DC power source that needs to be converted into usable AC power using an inverter. However, its nonlinearity and output fluctuation pose challenges in the design of PV based inverter. In this paper, a PV inverter controller system with the fundamentals of a fuzzy logic controller (FLC) and its applications and execution are reviewed. The different fuzzy controllers, inverter control algorithms, and switching techniques are studied. The findings indicate that the fuzzy logic controls have been gaining attention in the area of power control engineering, especially in inverter controller design for PV applications and generation. The FLC has a flexible and intelligent design, expedient user interface, easy computation and learning system, and combinations of different control algorithms. The FLC is also verifiable for completeness, redundancy, and consistency. However, finding the boundaries of membership functions and other rules of FLC requires manual tuning, long computation time, and considerable effort. This paper comprehensively reviews the FLC-based inverter control system to minimize PV output fluctuations, which cause inverter issues related to output harmonics, power factor, switching schemes, losses, and system implementation. The inverter system and its control strategy for future PV applications and generation require further research and development. Consequently, this review focuses on many factors and challenges and provides recommendations for designing capable and efficient inverter control systems for converting PV power to usable AC power. All the highlighted insights of this review will hopefully lead to increased efforts toward the development of the advanced inverter control systems for PV applications for AC loads and the utility grid.

Leaky-Least-Logarithmic-Absolute-Difference-Based Control Algorithm and Learning-Based InC MPPT Technique for Grid-Integrated PV System

Abstract: This paper introduces a novel leaky least logarithmic absolute difference (LLLAD)-based control algorithm and a learning-based incremental conductance (LIC) maximum power point tracking algorithm for a grid-integrated solar photovoltaic (PV) system. Here, a three-phase topology of the grid-integrated PV system is implemented, with the nonlinear/linear loads. The proposed LIC technique is an improved form of an incremental conductance (InC) algorithm, where inherent problems of the traditional InC technique, such as steady-state oscillations, slow dynamic responses, and fixed-step-size issues, are successfully mitigated. The prime objective of the proposed LLLAD control is to meet the active power requirement of the loads from the generated solar PV power, and after satisfying the load demand, the excess power is supplied to the grid. However, when the generated solar power is less than the load demand, then LLLAD meets the load by taking extra required power from the grid. During these power management processes, on the grid side, the power quality is maintained. During daytime, the proposed control technique provides load balancing, power factor correction, and harmonic filtering. Moreover, when solar irradiation is zero, then the dc-link capacitor and a voltage-source converter act as a distribution static compensator, which enhances the utilization factor of the system. The proposed techniques are modeled, and their performances are verified experimentally on a developed prototype in solar insolation variation conditions, unbalanced loading, and in different grid disturbances such as over- and undervoltage, phase imbalance, harmonics distortion in the grid voltage, etc. Test results have met the objectives of the proposed paper, and parameters are under the permissible limit, according to the IEEE-519 standard.

Harmonic Overloading Minimization of Frequency-Dependent Components in Harmonics Polluted Distribution Systems Using Harris Hawks Optimization Algorithm

Abstract: This paper presents a novel approach to optimal planning of a resonance-free C-type harmonic filter to minimize the harmonic overloading level of frequency-dependent components in a non-sinusoidal distribution system. In the studied system, the non-sinusoidal conditions are represented by the utility side's background voltage distortion and the load side's current distortion in addition to the harmonic characteristics of the utility, power cable, distribution transformer, and hybrid linear and nonlinear loads. A constrained optimization problem is formulated to find the optimal filter design that can enhance the power quality performance of the system while complying with the harmonic limits reported in the IEEE Standard 519, filter operation limits reported in the IEEE Standard 18, and other sets of operational ranges to maintain voltage and power factors within their acceptable limits, in addition to diminishing harmonic resonance hazards that may arise due to the filter connection. The problem is solved using a recent swarm intelligence optimization algorithm called the Harris hawks optimization (HHO) algorithm. The results obtained by the conventional methods presented in the literature, namely loss-based and adjusted power factor expressions, are compared with the results obtained by the proposed methodology for validation of the solution. Besides, the problem is solved using other swarm intelligence methods and these methods are compared with the HHO algorithm. The results obtained show the effectiveness of the approach proposed using HHO in finding the minimum power loss and harmonic overloading level of the frequency-dependent components compared to the other optimizers.

Grid Interactive Solar PV-Based Water Pumping Using BLDC Motor Drive

Abstract: This paper proposes bidirectional power flow control of a grid interactive solar photovoltaic (PV)-fed water pumping system. A brushless DC (BLDC) motor drive without phase current sensors is used to run a water pump. This system enables a consumer to operate the water pump at its full capacity for 24 hours regardless of the climatic condition and to feed a single-phase utility grid when water pumping is not required. The full utilization of a PV array and motor pump is made possible in addition to an enhanced reliability of the pumping system. A single-phase voltage source converter with a unit vector template generation technique accomplishes a bidirectional power flow control between the grid and the dc bus of the voltage source inverter (VSI), which feeds a BLDC motor. The VSI is operated at fundamental frequency, which minimizes the switching loss. The maximum power point operation of a PV array, and power quality improvements, such as power factor correction and reduction of total harmonic distortion of grid, are achieved in this system. Its applicability and reliability are demonstrated by various simulated results using MATLAB/Simulink platform and hardware implementation.

Mission Profile Based Reliability Evaluation of Capacitor Banks in Wind Power Converters

Abstract: With the increasing penetration of wind power, reliable and cost-effective wind energy production is of more and more importance. The doubly-fed-induction-generator-based turbine system is widely used and dominates the wind market. In this paper, an analytical approach to assess reliability for power capacitors, both the dc-link capacitor bank and ac-side filter capacitor bank, is presented considering the annual mission profile. Based on the electrical behavior at various loading conditions, the lifecycle of the single power capacitor can be predicted through its electrothermal stresses. This percentile lifetime can be translated to the Weibull lifetime distribution of the power capacitor by considering the parameter uncertainties and tolerance variations. Thereafter, a reliability block diagram is used to bridge the reliability curves from the component-level of the individual capacitor to the system-level of the capacitor bank. A case study of a 2-MW wind turbine shows that the lifecycle is significantly reduced from the individual capacitor to the capacitor bank, where the dc-link capacitor bank dominates the lifetime consumption. Furthermore, the electrical stresses of the power capacitors are experimentally verified at a down-scaled 7.5 kW prototype.

Design and Performance Comparisons of Brushless Doubly Fed Generators With Different Rotor Structures

Abstract: The brushless doubly fed generator (BDFG) shows the great potential for use in large variable speed wind turbines due to its high reliability and cost benefits of a partially rated power electronics converter. However, it suffers from the compromised efficiency and power factor in comparison with conventional doubly fed induction or synchronous generators. Therefore, optimizing the BDFG, especially the rotor, is necessary for enhancing its torque density and market competitiveness. In this paper, a novel cage-assisted magnetic barrier rotor, called the hybrid rotor, is proposed and analyzed. The detailed analytical design approaches based on the magnetic field modulation theory are investigated. In addition, the machine losses and mutual inductance values using the proposed rotor designs are calculated and their performance implications evaluated. Finally, the comparative experimental results for two BDFG prototypes are presented to verify the accuracy and effectiveness of the theoretical studies.

A High-Power-Density Power Factor Correction Front End Based on Seven-Level Flying Capacitor Multilevel Converter

Abstract: This paper presents a power factor correction (PFC) front end based on a seven-level flying capacitor multilevel (FCML) boost converter. Compared to the conventional two-level boost converter, the proposed seven-level FCML converter features the use of low-voltage-rated transistors, reduced voltage stress, and high effective switching frequency on the filter inductor. These characteristics of the FCML converter lead to drastic reduction in the filter inductor size while maintaining high efficiency and, therefore, significantly improve the power density of the PFC front end compared to conventional solutions. On the other hand, the small inductance imposes challenges on the PFC control. The dynamics of the seven-level FCML converter has been analyzed, and a feedforward control has been implemented to overcome these challenges. A hardware prototype is designed for universal ac input (90 to 265 Vac), 400-V dc output, and 1.5-kW power rating. Compared to existing solutions, the hardware prototype demonstrates improved efficiency and power density while maintaining high power factor and low THD. A power density of 219 W/in3 (490 W/in3 for the power stage) has been achieved, and a peak efficiency of 99.07% has been experimentally verified.

A Modified Neutral Point Balancing Space Vector Modulation for Three-Level Neutral Point Clamped Converters in High-Speed Drives

Abstract: This paper describes a high-performance neutral point voltage balancing technique for a neutral point clamped (NPC) converter. Conventional neutral point voltage balancing methods do not function well under low power factor, low pulse ratio, and near-unity modulation index operation conditions. These conditions are essentially dominant operation conditions for aircraft starter/generator systems. This paper introduces an alternative space vector modulation technique for three-level NPC converters in an aircraft starter generator system. The selection of voltage space vectors is optimized for high modulation index and low power factor operation. Disturbances caused by a low pulse ratio are also compensated. The proposed method maintains neutral point voltage balance and ripple minimization over the full range of operating conditions. This paper also provides a detailed analysis of the sources of neutral point voltage imbalances and ripples in high-speed drives with deep flux weakening. Simulation results obtained from a Simulink/PLECS model and experimental results obtained from a 45 kVA, 32 krpm aircraft starter generator test rig prove that the proposed method eliminates the neutral point voltage imbalance and significantly reduces the neutral point voltage ripple.

Autotuning Technique for the Cost Function Weight Factors in Model Predictive Control for Power Electronic Interfaces

Abstract: This paper presents an autotuning technique for the online selection of the cost function weight factors in model predictive control (MPC) The weight factors in the cost function with multiple control objectives directly affect the performance and robustness of the MPC The proposed method in this paper determines the optimum weight factors of the cost function for each sampling time; the optimization of the weight factors is done based on the prediction of the absolute tracking error of the control objectives and the corresponding constraints The proposed method eliminates the need of the trial-and-error approach to determine a fixed weight factor in the cost function The application considered is a capacitor-less static synchronous compensator based on the MPC of a direct matrix converter This technique compensates lagging power factor loads using inductive energy storage elements instead of electrolytic capacitors The result demonstrates that the proposed autotuning approach of cost function weights makes the control algorithm robust to parameter variation and other uncertainties in the system The proposed capacitor-less reactive power compensator based on the autotuned MPC cost function weight factor is verified experimentally

Optimal Design Method of Interleaved Boost PFC for Improving Efficiency from Switching Frequency, Boost Inductor, and Output Voltage

Abstract: Many research works have been conducted on increasing the efficiency of interleaved boost converters (IBCs) applied for power factor correction (PFC) by using high-performance power electronic devices and soft switching circuits, which are not helpful in reducing the hardware cost and systematic complexity. In this paper, three intrinsic parameters affecting the efficiency of IBC PFC are found and analyzed, including the switching frequency value, boost inductor value, and output voltage value. Based on the findings, considering the limitations of current ripples, power factor, maximum magnetic flux density, and volume of a boost inductor, an optimization design method for high-efficiency IBC PFC is proposed. A 3.3-kW IBC PFC prototype is developed based on Si devices. The experimental result verifies that the proposed method can improve the efficiency of IBC PFC with low-cost low-performance devices in the full-load range.

An Active Modulation Scheme to Boost Voltage Utilization of the Dual Converter With a Floating Bridge

Abstract: This paper proposes a capacitor voltage regulation method for the dual converter with a floating bridge for aerospace applications. This topology has previously been reported, but with a constrained voltage utilization factor due to the need for capacitor voltage regulations. In this paper, the effect of switching states on the voltage variation of capacitor is quantitatively modeled and an enhanced space vector modulation scheme with current feedback is proposed to achieve an active control of the floating capacitor voltages. This proposed method also allows further exploitation and utilization of converter voltage. The relationship between the allowed modulation index of dual converter and load power factor is obtained and expressed using a fitted polynomial equation. The advantages of the proposed method include boosted voltage utilization and superior performance in term of capacitor voltage balance. These advantages have been proven through simulation and experimental results on RL loads as well as with an open-end winding induction motor. The proposed modulation scheme can boost the converter voltage utilization by at least 10% while achieving full four-level operation. More importantly, the higher available voltage allows extending the constant torque region of the motor, the further beginning of field weakening operation could be postponed.

Design Optimization of a Spoke-Type Permanent-Magnet Vernier Machine for Torque Density and Power Factor Improvement

Abstract: Permanent-magnet vernier machine has an excellent potential for electric vehicles due to its desirable features of a simple mechanical structure and high torque density, whereas the poor power factor is its main drawback. In this paper, a spoke-type permanent-magnet vernier machine is designed and optimized for torque density and power factor improvement. First, the machine structure, operation principle, and sizing equation are introduced. Then, the machine is optimized by a multi-objective genetic algorithm. To enhance the optimization efficiency and accuracy, multi-level, sensitivity analysis, and approximate model technologies are employed in the optimization. It is shown that the improved torque density and power factor are achieved in the optimized machine as compared to the initial one. Finally, a prototype machine is built, and the experiments on the prototype machine are carried out to verify the design optimization.

An Adaptive PR Controller for Synchronizing Grid-Connected Inverters

Abstract: In this paper, an adaptive method for tuning a proportional-resonance controller for synchronization of the grid-connected inverters is presented. In the proposed approach, the grid frequency is obtained by minimizing the error signal using a frequency-locked loop mechanism that consists of a resonant adaptive filter and a perturbation-based extremum seeking algorithm. Simulations and experimental studies are presented to demonstrate performance of the proposed controller in face of the frequency variations of an emulated grid voltage waveform. The results are compared with conventional nonadaptive methods, which indicate that performance characteristics for voltage/frequency tracking and power factor can be achieved based on the IEEE 1547 standard.

Stability Analysis for the Grid-Connected Single-Phase Asymmetrical Cascaded Multilevel Inverter With SRF-PI Current Control Under Weak Grid Conditions

Abstract: This paper analyzes the influence of phase-locked loop (PLL) on the stability of LCL -type single-phase grid-connected asymmetrical cascaded H-bridge multilevel inverter (ACHMI) with synchronous reference frame proportional-integral (SRF-PI) grid current control under weak grid scenarios The ACHMI system is composed of power stage circuit and control system, where the control system contains the dual-loop current control strategy established in the hybrid reference frame, the SRF-PLL, and the hybrid modulation method employed to synthesize the multilevel output voltage The small-signal model of the whole ACHMI system is first established by using a simple step-by-step derivation method, and then, the small-signal analysis method is adopted to linearize the ACHMI, which is then utilized to derive the impedance model of the ACHMI system Furthermore, an improved impedance stability criterion is derived, which is then employed to analyze the system stability By using this criterion, the stability of the ACHMI can be evaluated with the variation of the bandwidth of PLL, the output power factor angle of the ACHMI, and the amplitude of the grid current reference signal under weak grid conditions In this paper, a systematic design procedure for the optimal selection of the PI controller of the PLL is presented, which guarantees the steady-state performance and dynamic response of the ACHMI system With this design method, the dual-loop current control and PLL can be taken into account simultaneously when analyzing the stability margin of the ACHMI Finally, the simulation and experimental results from a down-scaled grid-connected ACHMI prototype system are provided to confirm the validity of theoretical analysis

A Generalized Discontinuous PWM Scheme for Three-Level NPC Traction Inverter With Minimum Switching Loss for Electric Vehicles

Abstract: A discontinuous pulsewidth modulation (DPWM) strategy for three-level neutral-point-clamped (NPC) traction inverter drive for electric vehicles (EVs) with minimum switching loss for the full power factor (PF) range is developed and presented in this paper. Considering the dynamic loading conditions in an EV propelled by an induction motor (IM), the discontinuous modulation strategy is generalized to obtain minimum switching loss for a wide variations in load PF and modulation depth. The dynamic operating conditions in EV may render frequent unbalances in dc-link capacitor voltages in NPC inverter. It is demonstrated that the addition of an appropriate offset signal with the reference discontinuous signals generates compensating neutral current in the right direction, which can be used to mitigate existing unbalance in two dc-link capacitor voltages. Minimum switching loss at variable PF is attained by positioning the nonswitching clamped regions of the reference auxiliary signals according to the load PF. The effectiveness of the proposed scheme for a wide range of torque/speed variations is studied through simulation by developing a MATLAB/SIMULINK model of an IM traction inverter drive with field-oriented control strategy for closed loop control. Finally, the modulation scheme is validated through experimentation in a small-scale prototype NPC inverter with motor loads.

Probabilistic Hosting Capacity Enhancement in Non-Sinusoidal Power Distribution Systems Using a Hybrid PSOGSA Optimization Algorithm

Abstract: The high penetration of distributed generation (DG) units with their power-electronic interfaces may lead to various power quality problems, such as excessive harmonic distortions and increased non-sinusoidal power losses. In this paper, the probabilistic hosting capacity (PHC) due to the high penetration of photovoltaic units in a non-sinusoidal power distribution system is investigated. A C-type harmonic filter is proposed, to maximize the harmonic-constrained PHC. An optimization problem is formulated by using a Monte Carlo simulation, taking into account various uncertain parameters, such as the intermittent output power of the DGs, background voltage harmonics, load alteration, and the filter parameters’ variations. In addition, different operational constraints have been considered, such as the bus voltage, line thermal capacity, power factor, and individual and total harmonic distortion limits. A swarm-based, meta-heuristic optimization algorithm known as the hybrid particle swarm optimization and gravitational search algorithm (PSOGSA) has been examined for the optimal design of the proposed filter. Besides, other optimization algorithms were examined for validation of the solution. The PHC results obtained are compared with the conventional deterministic HC (DHC) results, and it is found that the PHC levels are higher than those obtained by conservative HC procedures, practical rules of thumb, and the DHC approaches.

Integration of Solar PV With Low-Voltage Weak Grid System: Using Normalized Laplacian Kernel Adaptive Kalman Filter and Learning Based InC Algorithm

Abstract: This paper proposes a novel normalized Laplacian kernel adaptive Kalman filter (NLKAKF) based control technique and learning based incremental conductance (LIC) maximum power point tracking (MPPT) algorithm, for low-voltage weak grid-integrated solar photovoltaic (PV) system. Here, a two-stage topology of three-phase grid integrated solar PV system is implemented, where the loads are connected at the point of common coupling. Proposed LIC is the improved form of incremental conductance (InC) algorithm, where inherent problems of traditional InC technique, such as steady-state oscillation, slow dynamic responses, and fixed step size issues, are successfully mitigated. The prime objective of proposed NLKAKF control is to meet the active power requirement of the loads from generated solar PV power, and after feeding load, excess power is fed to the grid. However, when generated PV power is less than the required load power, then NLKAKF control meets the load by taking extra required power from the grid. During this process, power quality is improved at the grid. The controller action provides reactive power compensation, power factor correction, and harmonics filtering and mitigation of other power quality issues. Moreover, when the solar irradiation is zero than voltage source converter acts as a distribution static compensator (DSTATCOM), which enhances the utilization factor of the system. The proposed techniques are modeled and their performances are verified experimentally on a developed prototype, in solar insolation variation conditions, unbalanced loading, as well as in different grid disturbances such as overvoltage, undervoltage, phase imbalance, harmonics distortion in the grid voltage, etc.

A Review on Flicker-Free AC–DC LED Drivers for Single-Phase and Three-Phase AC Power Grids

Abstract: Light-emitting diodes (LEDs) are coming strongly into the lighting market due to their advantages over conventional lighting solutions: energy efficient, controllable in both light and color, long lifetime, lack of a warm-up period, and high power density. Some of these advantages will make LED light sources to be more than just a light bulb, being able to transmit data, control light color, hue, and intensity or even detect people in indoor environments. Nevertheless, these advantages attributed to LED capabilities are, in reality, achieved thanks to the LED driver. This paper reviews the current state-of-the-art strategies to drive LEDs from ac power grids with special emphasis into removing the most limiting component from the point of view of the lifetime, which is the electrolytic capacitor, while achieving a flicker-free performance of the light output of the LED driver. Moreover, it focuses on analyzing the required regulations, challenges, and applicability of LED drivers in both single-phase and three-phase ac power grids.