Power Electronics

This paper proposes a family of novel flying capacitor transformerless inverters for single-phase photovoltaic (PV) systems. Each of the new topologies proposed is based on a flying capacitor principle and requires only four power switches and/or diodes, one capacitor, and a small filter at the output stage. A simple unipolar sinusoidal pulse width modulation technique is used to modulate the inverter to minimize the switching loss, output current ripple, and the filter requirements. In general, the main advantages of the new inverter topologies are: 1) the negative polarity of the PV is directly connected to the grid, and therefore, no leakage current; 2) reactive power compensation capability; and 3) the output ac voltage peak is equal to the input dc voltage (unlike neutral-point-clamped and derivative topologies, which requires twice the magnitude of the peak ac voltage). A complete description of the operating principle with modulation techniques, design guidelines, and comprehensive comparisons is presented to reveal the properties and limitations of each topology in detail. Finally, experimental results of 1-kVA prototypes are presented to prove the concept and theoretical analysis of the proposed inverter family for practical applications.

Transformerless photovoltaic grid-connected inverters have become more and more popular in the field of distributed photovoltaic power generation systems due to the advantages on high efficiency, low cost and small size. However, common-mode currents in the transformerless photovoltaic inverters can result in serious electromagnetic interference problems and safety issues, which will reduce the reliability of the photovoltaic inverter systems. In this paper, an improved H5 topology, namely H5-D topology, is proposed, in which a clamping diode is added on the basis of H5 topology to eliminate the common-mode voltage fluctuation in H5 topology. Further, the simulation results of the H5-D topology and H5 topology are given and compared by using PSIM software, especially, on the performance of common-mode currents suppression. Finally, the experimental prototypes of the H5-D topology and H5 topology are built and tested, the experimental results validate the advantages of the H5-D topology. The proposed H5-D topology provides a new practical topology for distributed photovoltaic grid- connected power generation systems.

Soft-switching techniques of transformerless photovoltaic grid-connected inverters (TLI) can significantly reduce switching losses, as well as soften switching processes. Conventional DC-AC soft- switching configurations proposed by Dr. Divan are invalid in TLIs because of leakage current problem (LC). In order to develop soft-switching techniques in TLIs, this paper proposes a new soft-switching configuration and a procedure to guide the invention of soft-switching TLIs. First, this paper proposes two basic resonance tanks related to DC bus polarities; then uses these basic tanks to elevate four popular full bridge type TLIs according to the proposed guideline. As a result, four soft-switching TLIs are gained. Second, this paper picks obtained soft-switching highly efficient and reliable inverter concept (HERIC) as example to analyze its soft-switching operation principle and performance. As a consequence, all active switches of the gained soft-switching HERIC circuit are switched under both of zero-current turn-on and turn-off conditions; the reverse recovery problem of freewheeling diodes is alleviated owing to the zero-current turn-off of diodes; meanwhile, the common-mode voltage at the switching frequency scale is still constant. Finally, some experimental results from a 3-kW universal prototype at 50-kHz switching frequency are provided to verify the effectiveness of main contributions of this paper.

Transformerless photovoltaic (PV) inverters are more widely adopted due to high efficiency, low cost, and light weight, etc. Many novel topologies and their corresponding modulation methods have been proposed, verified, and put into use, solely focusing on active power injection without leakage current issues. However, some new grid codes require PV inverters to have the ability of injecting reactive power into the utility for grid support. In order to map the challenge, an improved hybrid modulation method is proposed and evaluated for one nonisolated H6 topology as an example. With only simple modification for switching patterns and phase shift for current reference, the variable reactive power generation ability with zero crossing distortion is achieved, while the common mode voltage is also eliminated. The operation modes with the improved hybrid modulation approach are presented in detail and some design considerations are also provided. Extensive results from a 4-kVA prototype along with the SMPLIS simulation verify the proposed hybrid modulation method.

Grid-tied photovoltaic inverters must fulfill several requirements, including high efficiency and reduced cost and complexity of the overall system. Hence, transformerless operation is advantageous in order to achieve the prior requirements. Meanwhile, such operation results in several demerits, such as the dc current component injection into the grid. This component should be effectively mitigated in order to avoid some impacts, such as the saturation of the transformers in the distribution network. On the other hand, limiting this component up to few milliamperes is a challenging issue due to the various measurement errors. Accordingly, different blocking and measurement techniques have been proposed and studied to overcome this issue, where some demerits are seen behind each technique such as the implementation complexity, the common-mode voltage problems, and the high filter requirements. Moreover, none of them measures the dc component directly, but predicts its value using different approaches. Hence, this letter proposes a new technique to measure this dc current component with high accuracy using a coupled inductor combined with a small-range Hall effect current sensor in order to achieve the lowest possible cost with the highest possible accuracy. The proposed technique is introduced, analyzed, and tested experimentally to verify its principle of operation. Also experimental measurement of the dc current component using a 5-kVA transformerless grid-tied voltage-source inverter is introduced with and without the proposed technique in order to validate its operation.

This paper presents a new diode free freewheeling and common-mode voltage (CMV) clamping branches for single phase transformerless grid connected photovoltaic (PV) inverter for complete leakage current elimination and low conduction losses. In the past, various isolation techniques have been proposed for leakage current elimination in transformerless PV inverters. However, galvanic isolation only cannot completely eliminate leakage current due to that a resonant path is created by the switch junction capacitors, which also generate leakage current. The proposed freewheeling branch consists of four MOSFETs along with a clamping branch, which consists of two MOSFETs and a capacitor divider. The divider is connected to the DC side of the converter to keep constant CMV in the freewheeling path. As a result, the improved CMV clamping has been achieved for complete leakage current elimination. The unipolar sinusoidal pulse width modulation (SPWM) technique and modified HERIC topology with AC-decoupling for galvanic isolation is adopted for lower conduction losses. The proposed topology consists of only MOSFET in the freewheeling and clamping path which provides lower conduction losses compared with diode based topologies. The performances of the proposed topology in terms of common mode characteristics, leakage current, total harmonic distortion and conversion efficiency are analyzed and compared with H5, H6, HERIC and HBZBR topologies. The detail analyses are performed using MATLAB/Simulink and PSIM.

In this paper, a modified buck–boost grid-connected three-phase photovoltaic inverter is presented. In the structure of inverter, an inductive dc link is used between the input and output. The merits of the employed inverter are soft switching and step-up/down conversion without any additional power converter stage. It is a transformerless inverter with no leakage current issue. Moreover, as a result of increased switching frequency, it offers output current total harmonic distortion within standard limits. It uses only one current sensor and there is no electrolytic capacitor in it, which leads to high reliability. The operating principle is thoroughly explained and a simple control strategy with a new maximum power point tracking (MPPT) algorithm is proposed. The simulation and experimental results are provided to verify the behavior of a modified inverter, its control strategy, and the MPPT method.

Adding the auxiliary switches to the conventional H-bridge inverter is an effective way to eliminate the leakage current for transformerless PV systems, such as H5, H6, etc. Inspired by the newly developed embedded-switch H6 topology, a novel embedded-switch inverter (ESI) is proposed in this paper for three-phase transformerless PV systems. First, the operation principle and characteristics of the proposed ESI are analyzed. Second, the common-mode model of the three-phase ESI is established, based on which the main factors that affect the leakage current are discussed. The finding reveals that the switching states with conventional modulation strategy result in high-frequency common-mode voltage, which is the source of leakage current. In order to solve the problem, a new modulation strategy is presented for ESI to eliminate the high-frequency leakage current. Finally, the time-domain simulation and the experimental tests are carried out. The results verify the effectiveness of the proposed

In this paper, a new method to calculate the five parameters of the single-diode model of a photovoltaic cell or panel is presented. This new method takes into account the intrinsic properties of the model equation and the technique of linear least-squares fitting; so, the computational complexity and costs are very low. Moreover, the proposed method, named two-step linear least-squares method, is able to work absolutely blindly with any kind of I-V curve. It does not need initial guesses at all and, consequently, it is not necessary to know previously any information of any parameter. The proposed method provides the parameters of the single-diode model just using the coordinates of N points (N≥5) of the I-V curve. The results provided by this method in a first stage have the same order of accuracy of the best documented methods in the field of parameters extraction, but, furthermore, in a second stage, the best accuracy documented until now is obtained in two important case studies usually used in the literature as well as in a large-scale I-V curve repository with more than one million of curves.

A new control approach of integrating a solar photovoltaic (PV) with a battery storage is presented to a single-phase grid for residential and electric vehicle application. The main purpose of the proposed work is to feed a continuous power to the grid, thereby enhancing the viability of the battery energy storage support connected to the system. The charging and discharging of the battery achieve power leveling and load leveling along with increased reliability of the system. The multifunctional voltage- source converter acts as an active power filter and performs the harmonics mitigation along with reactive power compensation. In the proposed system, a unique control is developed for resynchronization of the grid during reconnection of the grid after the mitigation of a failure. The overall control of the system is adaptable under various practically occurring situations such as disconnection of the PV array, the battery, and the grid from the system. The detailed design and control of the proposed system are presented. The validity of the proposed system is performed through a laboratory prototype developed for a power rating of 2.2 kW connected to the utility grid. The performance of the system is found satisfactory under various disturbance, and the recorded results have been demonstrated.

Battery storage controlled by an energy management system (EMS) becomes an enabling technique to enhance solar farm integration. In this paper, the EMS controls battery storage to shape the fluctuated photovoltaic (PV) plant output into a relatively constant power and support the peak load. The proposed integrated design method considers both battery size and EMS impacts on the utility benefits and cost. The utility benefits include power generation, peak power support, and reduced line losses. The cost of battery storage is determined by the size and lifetime based on the developed battery models. Accordingly, the utility revenue change due to the battery storage controlled by EMS can be evaluated. Therefore, the integrated design of battery size and EMS can be determined by managing the change of utility revenue to gain economic benefits for the large-scale PV power plant application. Finally, the lithium-ion phosphate (LiFePO4) battery and lead-acid battery are compared to demonstrate the proposed method on a utility system model, respectively.

A grid-connected single-phase transformerless inverter that can operate two serially connected solar photovoltaic (PV) subarrays at their respective maximum power points while each one of them is exposed to different atmospheric conditions is proposed in this paper. As two subarrays are connected in series, the number of serially connected modules within a subarray is reduced to half. Reduction in the number of serially connected PV modules within a subarray leads to an overall improvement in the magnitude of power that can be abstracted from a subarray while the modules of the subarray are exposed to varied atmospheric conditions. The topological structure of the inverter ensures that the common mode voltage does not contain high-frequency components, thereby reducing the magnitude of leakage current involved with the solar panels well within the acceptable limit. An in-depth analysis of the scheme along with the derivation of its small signal model has been carried out. Detailed simulation studies are performed to verify its effectiveness. A 1-kW laboratory prototype of the scheme has been fabricated. Detailed experimental validations have been carried out utilizing the prototype to confirm the viability of the proposed scheme.
Solar energy has been the most popular source of renewable energy for residential and semicommercial applications. Fluctuations of solar energy harvested due to atmospheric conditions can be mitigated through energy storage systems (ESS). Solar energy can also be used to charge electric vehicle batteries to reduce the dependence on the grid. One of the requirements for a converter for such applications is to have a reduced number of conversion stages and provide isolation. The Z-source inverter (ZSI) topology is able to remove multiple stages and achieve voltage boost and dc-ac power conversion in a single stage. The use of passive components also presents an opportunity to integrate ESS into them. This paper presents modeling, design, and operation of a modified ZSI integrated with a split primary isolated battery charger for dc charging of electric vehicle batteries. Simulation and experimental results have been presented for the proof of concept of the operation of the proposed converter.
Battery energy storage systems are often adopted to buffer the difference between the intermittent solar power and the load demand in power grids. The costs of such photovoltaic (PV) battery systems increase as the required energy storage increases. In this paper, a new configuration comprising the PV panels, a series dc electric spring (series ES) and a noncritical load is proposed to reduce the battery storage capacity of dc microgrids that have substantial PV installations. This arrangement forms a PV- embedded series dc ES (PVES). An optimization method considering the minimization of electricity bills of the dc microgrids is included to size the storage capacity and to determine the rating of the PV that are connected to the series ES. Experiments on a 48-V isolated dc grid and simulations on a 400- kVA grid-connected dc microgrid have been conducted to verify the storage reduction feature of the PVES. Both sets of results show that the PVES can tackle the intermittency of the solar power with a smaller storage capacity than that typically required in dc grids with PV installations.
This paper proposes a model for planning isolated microgrids. The goal of the proposed model is to minimize the investment, operational, and total costs of an isolated microgrid through the full identification of the system configuration. The model designates the optimal type of microgrid, i.e., ac, dc, or hybrid ac/dc, and the optimal sizing of both the distributed energy resources (DERs) and the electronic power converters, if needed. To render this planning approach generic, the model accommodates a wide variety of DERs, including ac and dc generators, capacitors, and energy storage systems. The detailed operational criteria of each power apparatus are taken into account in order to provide reliable operational scenarios. As a means of guaranteeing active and reactive power adequacy in isolated microgrids, the stochastic nature of generation and demand are also considered. The proposed model was developed analytically as a mixed integer nonlinear problem so that obtaining solution optima is thus possible with the use of a deterministic branch-and-bound nonlinear solver. The validity and effectiveness of the new formulation have been demonstrated through several case studies involving varied load topologies.
In the present, a power decoupling method without additional component is proposed for a dc to single- phase ac converter, which consists of a flying capacitor dc/dc converter (FCC) and the voltage source inverter (VSI). In particular, a small flying capacitor in the FCC is used for both a boost operation and a double-line-frequency power ripple reduction. Thus, the dc-link capacitor value can be minimized in order to avoid the use of a large electrolytic capacitor. In addition, component design, of, e.g., the boost inductor and the flying capacitor, is clarified when the proposed control is applied. Experiments were carried out using a 1.5-kW prototype in order to verify the validity of the proposed control. The experimental results revealed that the use of the proposed control reduced the dc-link voltage ripple by
74.5%, and the total harmonic distortion (THD) of the inverter output current was less than 5%. Moreover, a maximum system efficiency of 95.4% was achieved at a load of 1.1 kW. Finally, the high power density design is evaluated by the Pareto front optimization. The power densities of three power decoupling topologies, such as a boost topology, a buck topology, and the proposed topology are compared. As a result, the proposed topology achieves the highest power density (5.3 kW/dm3) among the topologies considered herein.
To enhance the redundancy and reliability for a distributed generation system, a grid-tied photovoltaic (PV) generation system based on series-connected module-integrated inverters (SC-MIIs) is presented in this paper. In the grid-tied SC-MII system, each PV panel is interfaced with an MII with independent maximum power point tracking to harvest maximum solar energy. The outputs of MIIs are at ac line frequency and are connected in series to fulfill the voltage requirement in the utility grid. Since the high step-up power conversion stage in the conventional microinverter is avoided, the grid-tied SC- MII system is easier to implement and features high efficiency. Meanwhile, a distributed control strategy for SC-MIIs is proposed, in which the active power sharing among the MIIs depends on the individual maximum power available from PV panels, while the reactive power of the system can be regulated by any of the MIIs according to the utility grid command. Simulation and experimental results verify the feasibility and effectiveness of the proposed system and its corresponding control strategy.
A multiphase quasi-Z-source (qZS) dc-dc converter was proposed for distributed energy generation applications. It contains single-switch qZS isolated dc-dc cells with a voltage doubler rectifier. These cells are connected in parallel at the input side and in series at the output side to increase the dc voltage gain. A dc voltage blocking capacitor in series with the isolation transformer results in resonance that could be utilized for soft-switching. Two design approaches were proposed: considering phase shedding dependent on the input voltage and without it. The former targets wide input voltage range applications, while the latter is better suited for high input current applications. An experimental prototype rated for 300 W was tested with two types of isolation transformers designed according to the two presented approaches. It could be used as a PV module integrated converter with wide input voltage regulation range. The experimental results prove efficiency improvement from the phase shedding. Resonance frequency variations caused by the utilization of the multilayer ceramic capacitors and their possible influence on switching losses are discussed.
Multi-inverter systems have been widely used for grid-connected large-scale centralized photovoltaic (LSCPV) plants. However, the problem of how time delays affect the stability of digitally controlled grid-connected LSCPV plants with multi-inverter systems has not been investigated sufficiently. This paper models a grid-connected LSCPV system as a cascade system and conducts a systematic study of the relationship between the time delay and the stability of a grid-connected LSCPV system. The analysis intuitively reveals the influence of the time delay on the stability of the grid-connected LSCPV system. The impact of the time delay on the stability range of the number of grid-connected inverters in LSCPV plants is discussed for the first time, and the stability range for a specific time delay is obtained from the root locus. In addition, considering the damping performance is negatively affected by the time delay, an improved capacitor-current-feedback active damping method is proposed to reduce the effects of the delay on the damping region and the robustness against variations in the grid impedance. Simulations and experimental results are presented to validate the theoretical analysis and the effectiveness of the proposed delay compensation method.
A single phase grid connected transformerless photovoltaic (PV) inverter, which can operate either in buck or in boost mode, and can extract maximum power simultaneously from two serially connected subarrays while each of the subarray is facing different environmental conditions, is presented in this paper. As the inverter can operate in buck as well as in boost mode, depending on the requirement, the constraint on the minimum number of serially connected solar PV modules that is required to form a subarray is greatly reduced. As a result, power yield from each of the subarray increases when they are exposed to different environmental conditions. The topological configuration of the inverter and its control strategy are designed so that the high-frequency components are not present in the common mode voltage, thereby restricting the magnitude of the leakage current associated with the PV arrays within the specified limit. Further, high operating efficiency is achieved throughout its operating range. A detailed analysis of the system leading to the development of its mathematical model is carried out. The viability of the scheme is confirmed by performing detailed simulation studies. A 1.5 kW laboratory prototype is developed, and detailed experimental studies are carried out to corroborate the validity of the scheme.
The growth in installed solar photovoltaic (PV) capacity and the ever-increasing power demand due to the use of energy-hungry appliances have caused voltage issues. In this paper, a hierarchical dispatch strategy is proposed for coordinating multiple groups of virtual energy storage systems (VESSs), i.e., residential houses with air conditioners, to regulate voltage in low-voltage (LV) grids with high solar PV penetration. Specifically, the two levels of the proposed model are: 1) in the lower level, VESSs within each intelligent residential district are controlled locally by individual aggregator; 2) in the upper level, multiple aggregators are coordinated to achieve voltage regulation through a consensus control strategy. By exchanging information through sparse communication links, each aggregator shares the required active power adjustment among all participating groups, without compromising users’ thermal comfort. Simulation result demonstrates that the proposed control scheme can effectively regulate voltage in LV grids with greater robustness and scalability.
This paper deals with the design and performance analysis of a three-phase single stage solar photovoltaic integrated unified power quality conditioner (PV-UPQC). The PV-UPQC consists of a shunt and series-connected voltage compensators connected back-to-back with common dc-link. The shunt compensator performs the dual function of extracting power from PV array apart from compensating for load current harmonics. An improved synchronous reference frame control based on moving average filter is used for extraction of load active current component for improved performance of the PV-UPQC. The series compensator compensates for the grid side power quality problems such as grid voltage sags/swells. The compensator injects voltage in-phase/out of phase with point of common coupling (PCC) voltage during sag and swell conditions, respectively. The proposed system combines both the benefits of clean energy generation along with improving power quality. The steady state and dynamic performance of the system are evaluated by simulating in MATLAB-Simulink under a nonlinear load. The system performance is then verified using a scaled down laboratory prototype under a number of disturbances such as load unbalancing, PCC voltage sags/swells, and irradiation variation.
Microinverters require high voltage gain capability for interfacing the low dc voltage output of photovoltaic (PV) module to single-phase ac grid. A two-stage nonisolated inverter is proposed in this paper, with a first boost stage and a second traditional pulse width modulated grid-tied inverter stage. The proposed boost stage consists of a coupled inductor added with a voltage multiplier to achieve the required high voltage gain at high efficiency and to operate over a wide input voltage range. Among the coupled inductor topologies, adding a clamp circuit is a common solution to limit the voltage spike caused by leakage inductance. In the proposed converter, the resonance between the coupled inductor and voltage multiplier is used to address the issue of voltage spike, thereby reducing both component count and device voltage stress. Detailed analysis and design procedure of the proposed converter is presented in this paper. Experimental results of a 250-W microinverter are presented to validate the proposed converter.
An inherent problem of solar-energy-powered-small-cell base stations (SBSs) is that the energy generation of the photovoltaic (PV) cell does not match the energy consumption of the SBS in time. In this paper, we propose optimizing the PV cell orientation angle to achieve a good match between the energy generation and consumption profiles on a daily time scale. The optimization is formulated as an integer linear programming problem. We also derive an expression for the correlation between the energy generation and consumption profiles to evaluate their general interaction independent of the exact PV cell or SBS deployment setup. The numerical evaluation of the proposed angle optimization in a business area in London in summer/winter shows that the optimal PV cell orientation in summer contradicts the conventional assumption of south facing being optimal in the northern hemisphere. Instead, a southwest orientation should be chosen in summer due to its ability to shift the energy generation peak toward the energy consumption peak in the afternoon at an SBS in central London. This is in accordance with the prediction given by our derived correlation between the solar energy generation and consumption profiles.
Photovoltaic (PV) arrays present non-linear I-V curves, which strongly depend on ambient temperature and solar insolation. This fact poses a problem in identifying the maximum PV power point. In this paper, a dc motor is powered by a PV array and an attempt is made to optimize the use of the PV energy while the DC motor is supplied with suitable current and voltage in order to match its load-speed characteristics. This is achieved through a dc/dc converter controlled by a fuzzy cognitive network (FCN), in parallel with an energy storage device (battery), so that the PV energy is always fully exploitable. The algorithm method uses an FCN and a fuzzy controller, which recognize the maximum power of the PV array very fast while at the same time, they control the speed of the dc motor, under different insolation and temperature conditions. The proposed algorithm is validated through the simulation studies and is proven to be effective.
Pollution problems caused by fossil fuels has lead more investigations on renewable energy systems. Photo-voltaic cells and fuel-cells output low level voltage than required, therefore, high gain dc-dc converters are used to boost this low voltage. The Z-source converter can be employed as dc-dc converter to boost the PV panel voltages. It also offers other advantages, such as clamped switched voltage, high voltage gain, isolation of energy source from the load and positive polarity for output voltage. Therefore, this is a good choice for high step-up applications. This paper presents analysis of a novel high stepup z-source based dc-dc converter that has higher voltage gain than the z-source converter. Furthermore, high efficiency, low device voltage stress and wide voltage gain range make it a good candidate for photo-voltaic and high voltage step-up applications. The proposed dc-dc converter is evaluated experimentally for converting 24 V DC input to 300 V DC output at 100 W and to validate the simulation results.
A photovoltaic (PV) energy harvester is proposed, and it adopts the fractional open-circuit voltage method to track the maximal power point of PV cells. The proposed harvester was designed and fabricated by using a 0.18-μm 1P6M mixed-signal process. The input voltage of the proposed harvester may range from 0.5 to 1.1 V, and its measured peak total efficiency is 93.4%. The proposed harvester is suitable for internet-of-things applications, and the maximal duty cycle it can afford for a device with a 5-mA load and 20-ms activation time is 50%. To prevent partial-shading issues in photovoltaic (PV) systems, various kinds of voltage equalizers that virtually unify characteristics of shaded and unshaded modules have been proposed. Although PV string utilization can be dramatically improved, PV systems tend to be complex and costly because, in addition to the main converter for string control, voltage equalizers are separately necessary. This paper proposes the single-switch single-magnetic pulse width modulation (PWM) converter integrating the voltage equalizer using the series-resonant voltage multiplier (SRVM) for standalone PV systems. By utilizing a square wave voltage generated across a filter inductor in a PWM buck converter for driving the SRVM, the main PWM converter and voltage equalizer can be integrated into a single unit with reducing the total switch and magnetic component counts, achieving not only system-level but also circuit-level simplifications.
To prevent partial-shading issues in photovoltaic (PV) systems, various kinds of voltage equalizers that virtually unify characteristics of shaded and unshaded modules have been proposed. Although PV string utilization can be dramatically improved, PV systems tend to be complex and costly because, in addition to the main converter for string control, voltage equalizers are separately necessary. This paper proposes the single-switch single-magnetic pulse width modulation (PWM) converter integrating the voltage equalizer using the series-resonant voltage multiplier (SRVM) for standalone PV systems. By utilizing a square wave voltage generated across a filter inductor in a PWM buck converter for driving the SRVM, the main PWM converter and voltage equalizer can be integrated into a single unit with reducing the total switch and magnetic component counts, achieving not only system-level but also circuit-level simplifications. The experimental test using the prototype for three PV modules connected in series was performed emulating a partial-shading condition. The integrated converter effectively precluded the partial-shading issues and significantly improved the power available at a load, demonstrating its efficacy
In this paper, a framework for stability analyses of a typical inverter-based islanded microgrid with two types of nonlinear loads is presented, namely ideal constant power loads (CPLs), which are the loads supplied by tightly regulated power electronics converters, and dynamic CPLs, which are used to represent motor-drive systems with large time constants. The comprehensive dynamic model of the considered microgrid is first developed, based on which a bunch of small-signal models are deduced using Taylor expansion made at different stable operating points. Afterward, eigenvalue-theorem- based stability analysis and parametric sensitivity analysis are successively performed on the obtained small-signal models to verify the stability of the system, predict the system’s unstable regions, and identify the effects of parameters on the stability boundaries. In the meantime, the impacts of different kinds of nonlinear loads on the system stability are studied. Hardware-in-the-loop (HIL) real-time simulation platform of a 30-kVA microgrid, which is mainly formed by a 10-kVA photovoltaic (PV) system, a 10-kVA wind energy conversion system, a 10-kVA lithium-ion battery energy storage system, and two CPLs, is established in Typhoon HIL 602 device. The validity of the theoretical results is verified by real-time simulation results.
Heat transfer through interfaces in nanostructures is becoming ever more important in functional nanodevices. The existence of interface between two dissimilar materials overheats nanoelectronics and impacts heat transfer greatly. It is a challenge how to modulate interface to tailor the thermal transport properties of nanodevices from the perspective of atomic level. In this brief, we consider how size and interface strain affect thermal boundary resistance (TBR) as well as thermal conductivity (TC) of Si/Ge core-shell nanowires (CSNWs) using a thermal kinetic method in terms of atomic-bond- relaxation correlation mechanism and continuum medium mechanics. We propose a theoretical model to pursue the underlying mechanism on the TBR and TC that determined on the core or shell thickness, surface roughness, and interface mismatch. Our approach provides a useful guidance to the theoretical design and experimental control of epitaxial growth in the radial CSNWs for practice applications.
In this paper, a novel framework is proposed in order to evaluate impacts of the uncertain models of the system components on the voltage regulation problem of the medium-voltage distribution systems. The investigation focuses on the model uncertainty associated with voltage dependency of loads, power factor of loads, thermal dependency of lines, shunt admittances of lines and internal resistance of substation transformer. To this end, firstly, voltage constraints are managed using a centralised voltage control algorithm (VCA) by relying on the simplified models of the system components. The system loads and lines as well as the substation transformer are then modelled with the uncertain variables which are bounded in the predefined ranges. Monte Carlo (MC) simulations are used to create wide series of scenarios that cover the possible values that the parameters of the system components can take due to their uncertain nature. The model uncertainty impacts on the voltage regulation problem are finally evaluated by the load flow calculations considering the scenarios created by the MC simulations and the set-point obtained by the VCA. The proposed investigation brings useful information regarding the possible deviations that the node voltages can have due to the uncertain models of the studied components.
Fiber Bragg gratings (FBGs) present strong advantages for temperature or strain sensing in harsh radiation environments even if their properties are affected by radiations. The amplitudes and kinetics of these radiation induced changes depend on numerous parameters, intrinsic or extrinsic to the FBGs themselves. In this paper, we characterized 40 keV X-ray radiation effects on type I FBGs inscribed in prehydrogenated SMF-28 from Corning through an ultraviolet laser exposure at 244 nm (cw). We performed a systematic study of the influence of several FBG manufacturing parameters on their radiation response up to 100 kGy (SiO2) highlighting radiation-induced Bragg wavelength shifts (RI- BWS) up to 130 pm (~13°C error for temperature measurements) but no decrease of those FBG reflectivity. Among the investigated parameters are the duration and temperature (100°C and 300°C) of the thermal treatments applied post-inscription to stabilize the FBG and to complete the H2 outgassing. For such type of FBG, the device has to be recoated after inscription; we then characterize the impact of this manufacturing step on the FBG response showing that its recoating with NOA-81 acrylate slightly degrades its radiation resistance. In addition to this study, the influence of two other parameters have also been characterized: RI-BWS increases with the dose rate in the range 1-50 Gy/s and a pre-irradiation at 1.5 MGy does not stabilize type I FBG response to a second irradiation.
In this paper a power factor correction (PFC) based Hybrid Resonance Pulse Width Modulation (HRPWM) fed brushless DC motor (BLDC) drive is proposed, analyzed and tested. In this regard, small signal and circuit analysis of HRPWM are calculated in resonance frequency, above-and below- resonance frequency. The control method for regulating DC link voltage is also presented. Furthermore, speed of BLDC motor is controlled by VSI duty cycle variations. Simulation results depict unit power factor, low THD and appropriate speed control.
This brief proposes a mechanical contact-less speed sensing approach for a pulse width modulation (PWM) operated permanent-magnet direct current (PMDC) brushed motor. A recently reported semi- analytical dynamic model which incorporates the space-domain effects, namely slotting-effect and commutation phenomenon, has been taken into consideration to apply the proposed computational speed sensing approach. This speed sensing approach is basically an indirect estimation process and discrete in manner. The proposed method is efficiently applicable at higher range of speed. Zones of estimations with varying load torque and PWM duty cycle are represented with appropriate responses. A simulation of the proposed estimation method is applied over the dynamic semi-analytical model of
24 V, 12 teeth-slots, 100 W PMDC brushed motor, and various responses are represented in this brief.
In this paper, a fault-tolerant multiphase multilevel inverter (MPMLI) configuration is proposed based on the dual inverter concept. This configuration helps to improve the performance of pole-phase modulated multiphase induction motor (PPMMIM) drive in both normal and switch failure conditions. Comparing to the existing multilevel inverter topologies for multiphase induction motor (MIM) drives, the proposed MPMLI configuration requires less number of dc sources and power electronic devices. The proposed MPMLI configuration is able to sustain under various faulty conditions, which can improve the reliability of the system. In addition, dc-link utilization (DLU) in low-pole mode is improved by using space vector pulsewidth modulation (SVPWM). In high-pole mode, carrier-phase- shifted 3-Φ SVPWM is used for reducing the torque ripple. This MPMLI configuration produces a multilevel voltage profile across the effective phase in 3-phase 12-pole mode. By maintaining the V/f ratio constant, the proposed MPMLI-fed MIM drive facilitates a wide range of load torque with half of the rated speed in faulty condition. This advantage makes it suitable for traction and electric vehicle applications where reliability and wide range of torque-speed are the major concerns. The performance of proposed MPMLI-fed 5 hp, 9-φ PPMMIM drive is simulated by finite-element method (FEM) in ANSYS Maxwell 2-D and experimentally verified by laboratory prototype under normal and faulty conditions.
This paper proposes a novel comprehensive model of a vector-controlled induction motor drive in positive sequence transient stability simulation (PSTSS) programs. The model is implemented to approximate the behavior of the point-on-wave drive model, and applied to investigate the dynamic performance of the advanced drive loads in system-level simulations. This positive-sequence drive model is developed by reducing the three-phase electrical and control representations into d-q axes positive-sequence formulations. For the positive-sequence model, the line-side rectifier is interfaced to the grid through a voltage source with separate d-q axes controls to regulate the power factor of the drive. The machine-side inverter control system is represented based on rotor flux oriented control. The dc-link of the drive converter is implemented by employing the average model of the pulse-width modulated (PWM) converter, and is utilized to integrate the line-side rectifier and machine-side inverter. The proposed motor drive model is validated by comparing the performance with the electro- magnetic transient (EMT) point-on-wave drive model. The VAr support capability of the drive load and system-level simulation are investigated by incorporating the developed model into a composite load structure in PSTSS programs.
This paper proposes an improved offset selection method for discontinuous-pulse-width-modulation (DPWM)-based back-to-back converters to reduce dc-link current ripple. DPWM is introduced to power converters to diminish the stress on power transistors and prolong their lifespan. However, when using the DPWM method, the dc-link current ripple is increased in nonswitching regions of the power transistors. Moreover, in DPWM-based back-to-back converters, the dc-link current ripple reaches its maximum when the two transistors of both inverters are clamped in opposite directions. Therefore, the dc-link capacitors endure more stress, resulting in decreased life duration. To overcome this issue, the switching method should consider the clamping periods, when the current ripple increases. This can be achieved by modifying the DPWM offset, so that the clamping states of both converters are matched. The effectiveness of the proposed method is confirmed by both simulation and experimental results.
This paper investigates robust control strategies for wind energy conversion systems with variable- speed permanent magnet synchronous generators, which are integrated into the grid to provide reliable, secure, and efficient electrical power. A three-phase grid-side converter without a grid transformer is connected to the grid using an LCL filter with low resistive losses. In these working conditions, an instantaneous power PI controller for the outer voltage-loop in the grid-side converter is used to regulate the DC-link voltage and generate the required currents for the inner current-loop in the grid- side converter. Two integral-type terminal sliding-mode (TSM) controllers are proposed to control the active and reactive powers exchanged between the converter and the grid. The switching signals in the controllers are softened to attenuate chattering. The time-varying gains in the controllers are constructed to reduce control energy wastage and avoid overestimation of the boundaries of system uncertainties. Simulated and experimental results validate the proposed method.
A speed control of sensorless induction motor (IM) drives at zero and very low frequencies is designed in this paper. A new adaptive sliding mode observer (SMO) to estimate the stator current, rotor flux, and rotor speed is proposed. To improve the robustness and accuracy of an adaptive SMO during very low frequency operation, the sliding mode flux observer uses independent gains of the correction terms. The gains of current and rotor flux SMOs are designed using Lyapunov stability theory to guarantee the stability and fast convergence of the estimated variables. A Lyapunov function candidate utilizing the error of rotor fluxes and speed estimation error is synthesized for speed estimation. Detailed simulations and experiments are given showing the operation of the sensorless speed control at very low frequency. The results prove the accuracy and robustness of the proposed adaptive SMO. Also, comparison results with the state-of-the-art methods prove that the proposed method shows excellent transient and steady-state speed estimation, particularly at very low and zero frequency operations.
Objective: Rotary biventricular assist devices (BiVAD) are mechanical pumps that are implanted in the left and right ventricles of biventricular failure patients to pump blood and provide mechanical circulatory support. The objective of this paper was to develop and test a novel sensorless control algorithm that simultaneously satisfies the objectives of providing physiologic control (BiVAD flows meet cardiac demand), preventing ventricular suction, and providing balanced left-right (systemic and pulmonary) flows without the use of implantable flow or pressure sensors in the nonlinear, time varying, and discontinuous circulatory system. Methods: The control algorithm consists of two gain- scheduled proportional-integral controllers for left and right ventricular assist devices and only requires intrinsic pump parameters (speed and power) to maintain differential pump speeds (Δ RPML and Δ RPMR) above user-defined thresholds to prevent ventricular suction, and average reference pressure heads (Δ PL, Δ PR) to provide physiologic perfusion and balance left-right-sided flow rates. A model- based approach with extended Kalman and Golay-Savitzky filters was used to estimate Δ PL and Δ PR. Efficacy and robustness of the algorithm were evaluated in silico during simulated rest and exercise test conditions for: 1) excessive Δ PL and/or Δ PR setpoints; 2) rapid threefold increase in pulmonary vascular or vena caval resistances; 3) transitions from exercise to rest; and 4) ventricular fibrillation.
Fractional-slot concentrated-winding (FSCW) permanent magnet synchronous machines, which are characterised with high power density, fault tolerance and wide constant-power speed range, are gaining more and more attention in the aircraft starter generator (SG) systems. Nevertheless, the short- circuit (SC) fault, especially the turn-to-turn SC fault, is the obstruct crux in aviation applications. This study is aimed to demonstrate the feasibility of FSCW permanent magnet SG (PMSG) in dealing with turn-to-turn SC fault. The law of FSCW-PMSG SC fault is analysed and verified by finite-element analysis (FEA), including the influence on turn-to-turn SC current by the number of the shorted turns and the coil position in the slot. The three-phase current injection control is employed to mitigate the SC fault in PMSG when a turn-to-turn SC fault occurs. A 24-slot, 16-pole FSCW-PMSG with spoke- type rotor topology is designed to confirm the ability of inhibiting turn-to-turn SC current without the risk of irreversible demagnetisation. Both FEA and experimental results are presented, verifying the effectiveness of the three-phase current injection control in restraining the turn-to-turn SC current for FSCW PMSG in an acceptable range.
PM machine or Permanent magnet synchronous motor (PMSM) is a nonlinear system with multivariable couplings. To achieve the sensorless control of a PMSM with high inertial load, a modified current observer, using PI regulator instead of sliding mode switching function, is proposed in this paper. The modified current observer can solve the chattering and phase delay problem while still maintaining the robust advantages of sliding mode system in position estimation. In addition, a new phase-locked loop (PLL) based angle switching strategy is designed to ensure the motor can smoothly switch from I-F control to closed-loop sensorless vector control in startup stage with a high inertial load. The simulation and experimental results show that the control system of PMSM with proposed ideas has fast response speed, accurate rotor position estimation, stable state switching and good system robustness under high inertia load.
This paper proposes a method for estimating two inductances and capacitance of an LCL filter, connected between a converter and a grid. Only the dc-bus voltage and converter phase currents need to be measured. An excitation signal is fed into the converter voltage reference. The fundamental and selected harmonic components are removed from the stored identification data to prevent biases in the parameter estimates. The parameters of the hold-equivalent discrete-time model are estimated recursively. The inductance and capacitance estimates are calculated from the estimated discrete-time parameters using the corresponding closed-form model. The proposed method can be added to the existing converter control algorithms. It can provide the parameter estimates for the converter control. Further, it can be run occasionally during the normal operation in order to obtain an updated grid inductance estimate. Experimental results show that the proposed method yields very good parameter estimates and that it can also detect the changes in the grid inductance.
This paper proposes a new single-phase five-level converter based on switched capacitor technique. The capacitor charging in the proposed converter is carried out in a self-balancing form which does not need closed-loop modulations or additional balancing circuits. The proposed topology is a voltage booster without using end side H-bridge for changing load voltage polarity. So, switching losses and total voltage stress of semiconductor components reduce in the proposed converter. The performing modes of the proposed topology, its modulation scheme, capacitors’ balancing analysis, capacitance and loss calculations, and also the development of the proposed converter for enhancing the quality of output voltage waveform are discussed in depth. Moreover, the comparison of the proposed structure with the other multi-level topologies shows that the proposed converter can reduce the number of semiconductor elements and the required isolated DC sources. Finally, the simulation and experimental results validate the appropriate performance of the proposed converter.
In this paper we consider a discrete-time consensus network, and assume that one of the agents acts as a leader and injects an input signal to improve the overall dynamics performances, in particular to increase the speed of convergence to consensus or to achieve finite-time consensus. Two possible control protocols are proposed and the characteristic polynomials of the resulting closed-loop systems are determined. These results allow to investigate consensus and finite-time consensus of the overall systems.
Aiming at the chattering problem of conventional sliding mode control systems for dc-dc converters, a quasi-continuous second-order sliding mode (QSOSM) control scheme for buck converters has been presented in the paper. It will be shown that the QSOSM controller is practically a continuous function of the states everywhere except the origin. And thus the chattering problem existing in conventional sliding mode can be practically solved due to the fact that the states cannot be stabilized to the origin from engineering point of view. Theoretical analysis also shows that the closed loop system of the buck converter is globally finite-time stable. Finally, the theoretical considerations have been verified by simulation and experimentation. Specifically, the comparisons between QSOSM controller and PID controller show that the former one provides a better robustness.