Showing papers on "Power factor published in 2015"
TL;DR: This review paper is the first of its kind with the aim of providing a “one-stop” information source and a selection guide on impedance-source networks for power conversion for researchers, designers, and application engineers.
Abstract: Impedance networks cover the entire of electric power conversion from dc (converter, rectifier), ac (inverter), to phase and frequency conversion (ac-ac) in a wide range of applications. Various converter topologies have been reported in the literature to overcome the limitations and problems of the traditional voltage source, current source as well as various classical buck-boost, unidirectional, and bidirectional converter topologies. Proper implementation of the impedance-source network with appropriate switching configurations and topologies reduces the number of power conversion stages in the system power chain, which may improve the reliability and performance of the power system. The first part of this paper provides a comprehensive review of the various impedance-source-networks-based power converters and discusses the main topologies from an application point of view. This review paper is the first of its kind with the aim of providing a “one-stop” information source and a selection guide on impedance-source networks for power conversion for researchers, designers, and application engineers. A comprehensive review of various modeling, control, and modulation techniques for the impedance-source converters/inverters will be presented in Part II.
601 citations
TL;DR: In this paper, a critical comparison of IPT and CPT for small gap applications is provided, wherein the theoretical and empirical limitations of each approach are established, and guidelines for selecting IPT or CPT in small gap systems are presented.
Abstract: Inductive power transfer (IPT) and capacitive power transfer (CPT) are the two most pervasive methods of wireless power transfer (WPT). IPT is the most common and is applicable to many power levels and gap distances. Conversely, CPT is only applicable for power transfer applications with inherently small gap distances due to constraints on the developed voltage. Despite limitations on gap distance, CPT has been shown to be viable in kilowatt power level applications. This paper provides a critical comparison of IPT and CPT for small gap applications, wherein the theoretical and empirical limitations of each approach are established. A survey of empirical WPT data across diverse applications in the last decade using IPT and CPT technology graphically compares the two approaches in power level, gap distance, operational frequency, and efficiency, among other aspects. The coupler volumetric power density constrained to small gap sizes is analytically established through theoretical physical limitations of IPT and CPT. Finally, guidelines for selecting IPT or CPT in small gap systems are presented.
411 citations
TL;DR: In this article, the authors compared the performance of the modular multilevel dc converter (M2DC) and the three-phase dual-active bridge converter (DAB) in terms of efficiency, amount of semiconductor devices, and expense on capacitive storage and magnetic components.
Abstract: It is expected that in the near future the use of high-voltage dc (HVDC) transmission and medium-voltage dc (MVDC) distribution technology will expand. This development is driven by the growing share of electrical power generation by renewable energy sources that are located far from load centers and the increased use of distributed power generators in the distribution grid. Power converters that transfer the electric energy between voltage levels and control the power flow in dc grids will be key components in these systems. The recently presented modular multilevel dc converter (M2DC) and the three-phase dual-active bridge converter (DAB) are benchmarked for this task. Three scenarios are examined: a 15 MW converter for power conversion from an HVDC grid to an MVDC grid of a university campus, a gigawatt converter for feeding the energy from an MVDC collector grid of a wind farm into the HVDC grid, and a converter that acts as a power controller between two HVDC grids with the same nominal voltage level. The operation and degrees of freedom of the M2DC are investigated in detail aiming for an optimal design of this converter. The M2DC and the DAB converter are thoroughly compared for the given scenarios in terms of efficiency, amount of semiconductor devices, and expense on capacitive storage and magnetic components.
382 citations
TL;DR: In this paper, a relative more advanced approach is proposed, which is based on the loading and strength analysis of devices and takes into account different time constants of the thermal behaviors in power converter.
Abstract: As a key component in the wind turbine system, the power electronic converter and its power semiconductors suffer from complicated power loadings related to environment, and are proven to have high failure rates. Therefore, correct lifetime estimation of wind power converter is crucial for the reliability improvement and also for cost reduction of wind power technology. Unfortunately, the existing lifetime estimation methods for the power electronic converter are not yet suitable in the wind power application, because the comprehensive mission profiles are not well specified and included. Consequently, a relative more advanced approach is proposed in this paper, which is based on the loading and strength analysis of devices and takes into account different time constants of the thermal behaviors in power converter. With the established methods for loading and lifetime estimation for power devices, more detailed information of the lifetime-related performance in wind power converter can be obtained. Some experimental results are also included to validate the thermal behavior of power device under different mission profiles.
342 citations
TL;DR: In this paper, a double-sided LCLC -compensated capacitive power transfer (CPT) system is proposed for the electric vehicle charging application, where two pairs of metal plates are utilized to form two coupling capacitors to transfer power wirelessly.
Abstract: A double-sided LCLC -compensated capacitive power transfer (CPT) system is proposed for the electric vehicle charging application. Two pairs of metal plates are utilized to form two coupling capacitors to transfer power wirelessly. The LCLC -compensated structure can dramatically reduce the voltage stress on the coupling capacitors and maintain unity power factor at both the input and output. A 2.4-kW CPT system is designed with four 610-mm × 610-mm copper plates and an air gap distance of 150 mm. The experimental prototype reaches a dc–dc efficiency of 90.8% at 2.4-kW output power. At 300-mm misalignment case, the output power drops to 2.1 kW with 90.7% efficiency. With a 300-mm air gap distance, the output power drops to 1.6 kW with 89.1% efficiency.
320 citations
TL;DR: In this article, a series-series-compensated topology with dual-side power control and a corresponding control strategy is proposed to significantly increase the overall efficiency, especially for systems with large coupling factor variations and in partial load mode.
Abstract: In this study, a 3-kW inductive power transfer system is investigated, specifically intended for contactless vehicle charging. A series-series-compensated topology with dual-side power control and a corresponding control strategy is proposed to significantly increase the overall efficiency, especially for systems with large coupling factor variations and in partial load mode. The topology, which is closely related to the dual-active bridge converter, enables the dual-side power control without adding additional dc/dc converters to the system, and thus keeping the additional hardware effort minimal. A detailed analysis of the proposed topology is provided, and the benefits of the dual-side control are demonstrated both theoretically and experimentally. A hardware prototype is built and a peak dc-to-dc efficiency of 95.8% at 100 mm air gap and a minimal efficiency of 92.1% at 170 mm air gap is measured, including the power electronic components. The partial load efficiency at 500 W output power is still as high as 90.6% at 135 mm air gap. Overall, the proposed topology provides a practical method to overcome the main drawback of most single-side controlled inductive power transfer systems, which is a significant efficiency drop outside the nominal operating point.
319 citations
TL;DR: The design and implementation of a single-phase on-board bidirectional plug-in electric vehicle (PEV) charger that can provide reactive power support to the utility grid in addition to charging the vehicle battery is presented.
Abstract: This paper presents the design and implementation of a single-phase on-board bidirectional plug-in electric vehicle (PEV) charger that can provide reactive power support to the utility grid in addition to charging the vehicle battery. The topology consists of two-stages: a full-bridge ac-dc boost converter; and a half-bridge bidirectional dc-dc converter. The charger operates in two quadrants in the active-reactive power (PQ) power plane with five different operation modes (i.e., charging-only, charging-capacitive, charging-inductive, capacitive-only, and inductive-only). This paper also presents a unified controller to follow utility PQ commands in a smart grid environment. The cascaded two-stage system controller receives active and reactive power commands from the grid, and results in line current and battery charging current references while also providing a stable dynamic response. The vehicle's battery is not affected during reactive power operation in any of the operation modes. Testing the unified system controller with a 1.44 kVA experimental charger design demonstrates the successful implementation of reactive power support functionality of PEVs for future smart grid applications.
288 citations
TL;DR: A unified energy management scheme is proposed for renewable grid integrated systems with battery-supercapacitor hybrid storage that enables the real power transfer along with ancillary services such as current harmonic mitigation, reactive power support, and power factor improvement at the point of common coupling.
Abstract: In this paper, a unified energy management scheme is proposed for renewable grid integrated systems with battery–supercapacitor hybrid storage. The intermittent nature of renewable-energy resources (RES), coupled with the unpredictable changes in the load, demands high-power and high-energy-density storage systems to coexist in today's microgrid environment. The proposed scheme dynamically changes the modes of renewable integrated systems based on the availability of RES power and changes in load as well. The participation of battery–supercapacitor storage to handle sudden/average changes in power surges results in fast dc link voltage regulation, effective energy management, and reduced current stress on battery. In addition, the proposed energy management scheme enables the real power transfer along with ancillary services such as current harmonic mitigation, reactive power support, and power factor improvement at the point of common coupling. The proposed scheme is validated through both simulation and experimental studies.
264 citations
TL;DR: In this paper, a perturbation-and-observation-based tracking system is developed through additional hardware such as a cascaded boost-buck dc-dc converter, an efficiency sensing system, and a controller.
Abstract: All the wireless power transfer (WPT) systems share a similar configuration including a power source, a coupling system, a rectifying circuit, a power regulating, and charging management circuit and a load. For such a system, both a circuit- and a system-level analyses are important to derive requirements for a high overall system efficiency. Besides, unavoidable uncertainties in a real WPT system require a feedback mechanism to improve the robustness of the performance. Based on the above basic considerations, this paper first provides a detailed analysis on the efficiency of a WPT system at both circuit and system levels. Under a specific mutual inductance between the emitting and receiving coils, an optimal load resistance is shown to exist for a maximum overall system efficiency. Then, a perturbation-and-observation-based tracking system is developed through additional hardware such as a cascaded boost-buck dc-dc converter, an efficiency sensing system, and a controller. Finally, a 13.56-MHz WPT system is demonstrated experimentally to validate the efficiency analysis and the tracking of the optimal load resistances. At a power level of 40 W, the overall efficiency from the power source to the final load is maintained about 70% under various load resistances and relative positions of coils.
264 citations
TL;DR: Convergence to the configuration of minimum losses and feasible voltages is proved analytically for both a synchronous and an asynchronous version of the algorithm, where agents update their state independently one from the other.
Abstract: We consider the problem of exploiting the microgenerators dispersed in the power distribution network in order to provide distributed reactive power compensation for power losses minimization and voltage regulation. In the proposed strategy, microgenerators are smart agents that can measure their phasorial voltage, share these data with the other agents on a cyber layer, and adjust the amount of reactive power injected into the grid, according to a feedback control law that descends from duality-based methods applied to the optimal reactive power flow problem. Convergence to the configuration of minimum losses and feasible voltages is proved analytically for both a synchronous and an asynchronous version of the algorithm, where agents update their state independently one from the other. Simulations are provided in order to illustrate the performance and the robustness of the algorithm, and the innovative feedback nature of such strategy is discussed.
247 citations
TL;DR: In this paper, the authors present a method to maximize the efficiency and increase the amount of extractable power of a WPT system operating in non-resonant operation, which is based on actively modifying the equivalent secondary-side load impedance by controlling the phase shift of the active rectifier and its output voltage level.
Abstract: Wireless power transfer (WPT) is an emerging technology with an increasing number of potential applications to transfer power from a transmitter to a mobile receiver over a relatively large air gap. However, its widespread application is hampered due to the relatively low efficiency of current Wireless power transfer (WPT) systems. This study presents a concept to maximize the efficiency as well as to increase the amount of extractable power of a WPT system operating in nonresonant operation. The proposed method is based on actively modifying the equivalent secondary-side load impedance by controlling the phase-shift of the active rectifier and its output voltage level. The presented hardware prototype represents a complete wireless charging system, including a dc–dc converter which is used to charge a battery at the output of the system. Experimental results are shown for the proposed concept in comparison to a conventional synchronous rectification approach. The presented optimization method clearly outperforms state-of-the-art solutions in terms of efficiency and extractable power.
TL;DR: In this article, an improved droop control method was proposed to improve the reactive power sharing accuracy, which mainly includes two important operations: error reduction operation and voltage recovery operation, which is activated by the low-bandwidth synchronization signals.
Abstract: For microgrid in islanded operation, due to the effects of mismatched line impedance, the reactive power could not be shared accurately with the conventional droop method. To improve the reactive power sharing accuracy, this paper proposes an improved droop control method. The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease. The needed communication in this method is very simple, and the plug-and-play is reserved. Simulations and experimental results show that the improved droop controller can share load active and reactive power, enhance the power quality of the microgrid, and also have good dynamic performance.
01 Jan 2015
TL;DR: In this paper, a directional control method for power flows on a set of interface lines between two regions of power system considering static voltage stability margin is developed, where a surface approximation approach is firstly used to obtain the relationship between the interface flow solution and the generation direction of generator (the portion of generation variation in each participating generator to satisfy the desired power increase on the interface and the system loss).
Abstract: A directional control method (DCM) for power flows on a set of interface lines between two regions of power system considering static voltage stability margin is developed in this paper. A surface approximation approach is firstly used to obtain the relationship between the interface flow solution and the generation direction of generator (the portion of generation variation in each participating generator to satisfy the desired power increase on the interface and the system loss). Then, an optimization model is built to determine the optimum dispatching scheme of generators. This method not only can control the total power on the interface to satisfy the power demand but also can realize the directional control of power on each interface line based on the needs of operation. The proposed DCM is further extended to determine the optimum dispatching scheme of generators for maximizing the interface flow margin (IFM), which is the active power margin of the key transmission lines between two regions of power system constrained by static voltage stability. A modified continuation power flow (MCPF) is used to show and evaluate the impacts of the DCM on the IFM. The New England 39-bus system and the IEEE 300-bus system have been employed to verify the effectiveness of the DCM.
TL;DR: In this paper, a robust optimal power management system (ROPMS) is developed for a hybrid ac/dc micro-grid, where the power flow in the microgrid is supervised based on solving an optimization problem, satisfying demanded power with maximum utilization of renewable resources, minimum usage of fuel-based generator, extending batteries lifetime, and limited utilization of the main power converter between the ac and dc micro-grids.
Abstract: Hybrid ac/dc micro-grid is a new concept decoupling dc sources with dc loads and ac sources with ac loads, while power is exchanged between both sides using a bidirectional converter/inverter. This necessitates a supervisory control system to split power between its different resources, which has sparked attention on the development of power management systems (PMSs). In this paper, a robust optimal PMS (ROPMS) is developed for a hybrid ac/dc micro-grid, where the power flow in the micro-grid is supervised based on solving an optimization problem. Satisfying demanded power with maximum utilization of renewable resources, minimum usage of fuel-based generator, extending batteries lifetime, and limited utilization of the main power converter between the ac and dc micro-grids are important factors that are considered in this approach. Uncertainties in the resources output power and generation forecast errors, along with static and dynamic constraints of the resources, are taken into account. Furthermore, since uncertainties in the resources output power may result in fluctuations in the dc bus voltage, a two-level controller is used to regulate charge/discharge power of the battery banks. Effectiveness of the proposed supervisory system is evaluated through extensive simulation runs based on dynamical models of the power resources.
TL;DR: A design guideline for the CF-DAB converter applied to PV systems, as well as other applications with a wide input voltage variation, and an optimized operating mode is proposed to achieve the minimum root-mean-square transformer current.
Abstract: The current-fed dual active bridge (CF-DAB) dc–dc converter gains growing applications in photovoltaic (PV) and energy storage systems due to its advantages, e.g., a wide input voltage range, a high step-up ratio, a low input current ripple, and a multiport interface capability. In addition, the direct input current controllability and extra control freedom of the CF-DAB converter make it possible to buffer the double-line-frequency energy in grid-interactive PV systems without using electrolytic capacitors in the dc link. Therefore, a PV system achieves high reliability and highly efficient maximum power point tracking. This paper studies the optimized operation of a CF-DAB converter for a PV application in order to improve the system efficiency. The operating principle and soft-switching conditions over the wide operating range are thoroughly analyzed with phase-shift control and duty-cycle control, and an optimized operating mode is proposed to achieve the minimum root-mean-square transformer current. The proposed operating mode can extend the soft-switching region and reduce the power loss, particularly under a heavy load and a high input voltage. Moreover, the efficiency can be further improved with a higher dc-link voltage. A 5-kW hardware prototype was built in the laboratory, and experimental results are provided for verification. This paper provides a design guideline for the CF-DAB converter applied to PV systems, as well as other applications with a wide input voltage variation.
TL;DR: In this paper, a universal steady-state model was developed to simply and accurately describe the analytical expressions for the highfrequency-link (HFL) electrical quantities of isolated dual-active-bridge (DAB) dc-dc converter under PWM plus phase shift control.
Abstract: This letter first develops a universal steady-state model to simply and accurately describe the analytical expressions for the high-frequency-link (HFL) electrical quantities of isolated dual-active-bridge (DAB) dc–dc converter under PWM plus phase-shift control. Second, a universal reactive power interaction among the HFL electrical quantities is present; using this interaction, the circulating current characteristic of DAB can be described accurately by HFL power factor. On this basis, a practical HFL fundamental-optimal strategy is proposed to decrease the circulating current and increase the efficiency. At last, experimental results verify the correctness of the universal model and the effectiveness of the fundamental-optimal strategy.
TL;DR: It is emphasized that a high power factor (PF) is equivalently important for high power generation, in addition to high efficiency, because power is determined by (Th − Tc)2(S2σ)/L, where Th, Tc, and L are the hot and cold side temperatures, and leg length, respectively.
Abstract: Thermoelectric power generation is one of the most promising techniques to use the huge amount of waste heat and solar energy. Traditionally, high thermoelectric figure-of-merit, ZT, has been the only parameter pursued for high conversion efficiency. Here, we emphasize that a high power factor (PF) is equivalently important for high power generation, in addition to high efficiency. A new n-type Mg2Sn-based material, Mg2Sn0.75Ge0.25, is a good example to meet the dual requirements in efficiency and output power. It was found that Mg2Sn0.75Ge0.25 has an average ZT of 0.9 and PF of 52 μW⋅cm−1⋅K−2 over the temperature range of 25–450 °C, a peak ZT of 1.4 at 450 °C, and peak PF of 55 μW⋅cm−1⋅K−2 at 350 °C. By using the energy balance of one-dimensional heat flow equation, leg efficiency and output power were calculated with Th = 400 °C and Tc = 50 °C to be of 10.5% and 6.6 W⋅cm−2 under a temperature gradient of 150 °C⋅mm−1, respectively.
TL;DR: The main objective of this proposed strategy is to control the state of charge of the battery bank limiting the voltage on its terminals by controlling the power generated by the energy sources.
Abstract: This paper presents a new strategy to control the generated power from energy sources existing in autonomous and isolated microgrids. In this particular study, the power system consists of a power electronic converter supplied by a battery bank, which is used to form the ac grid (grid former converter), an energy source based on a wind turbine with its respective power electronic converter (grid supplier converter), and the power consumers (loads). The main objective of this proposed strategy is to control the state of charge of the battery bank limiting the voltage on its terminals by controlling the power generated by the energy sources. This is done without using dump loads or any physical communication among the power electronic converters or the individual energy source controllers. The electrical frequency of the microgrid is used to inform the power sources and their respective converters about the amount of power that they need to generate in order to maintain the battery-bank charging voltage below or equal its maximum allowable limit. Experimental results are presented to show the feasibility of the proposed control strategy.
TL;DR: The control and modulation method for B3C has been proposed for realizing maximum power point tracking (MPPT), battery management, and bus voltage regulation simultaneously, and can be transited between conductance mode and MPPT mode automatically according to the load power.
Abstract: In order to interface one photovoltaic (PV) port, one bidirectional battery port, and one load port of a PV-battery dc power system, a novel nonisolated three-port dc/dc converter named boost bidirectional buck converter (B3C) and its control method based on three-domain control are proposed in this paper. The power flow and operating principles of the proposed B3C are analyzed in detail, and then, the dc voltage relation between three ports is deduced. The proposed converter features high integration and single-stage power conversion from both PV and battery ports to the load port, thus leading to high efficiency. The current of all three ports is continuous; hence, the electromagnetic noise can be reduced. Furthermore, the control and modulation method for B3C has been proposed for realizing maximum power point tracking (MPPT), battery management, and bus voltage regulation simultaneously. The operation can be transited between conductance mode and MPPT mode automatically according to the load power. Finally, experimental verifications are given to illustrate the feasibility and effectiveness of the proposed topology and control method.
TL;DR: In this paper, an open-circuit voltage measure is performed during the pseudonormal operation of the interfacing power electronic converter. And the proposed MPPT technique is supported by theoretical analysis and used to control a synchronous Buck-Boost converter.
Abstract: Thermoelectric generators (TEGs) convert heat energy into electricity in a quantity dependent on the temperature difference across them and the electrical load applied. It is critical to track the optimum electrical operating point through the use of power electronic converters controlled by a maximum power point tracking (MPPT) algorithm. The MPPT method based on the open-circuit voltage is arguably the most suitable for the linear electrical characteristic of TEGs. This paper presents an innovative way to perform the open-circuit voltage measure during the pseudonormal operation of the interfacing power electronic converter. The proposed MPPT technique is supported by theoretical analysis and used to control a synchronous Buck-Boost converter. The prototype MPPT converter is controlled by an inexpensive microcontroller, and a lead-acid battery is used to accumulate the harvested energy. Experimental results using commercial TEG devices prove that the converter accurately tracks the maximum power point during thermal transients. Precise measurements in the steady state show that the converter finds the maximum power point with a tracking efficiency of 99.85%.
TL;DR: In this article, the authors proposed a mathematical model which estimates the energy deviation for the stacks of both the MMC and the AAC during steady-state operation under any power factor and for AC voltage magnitude fluctuation of up to ±10%.
Abstract: Multilevel converters, such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), rely on charged capacitors in their cells to generate their AC voltage waveform. Since the cell capacitors are physically large and occupy approximately half the cell volume, their capacitance must be kept minimal while limiting the voltage fluctuation caused by the current passing periodically through these capacitors. This study proposes a mathematical model which estimates the energy deviation for the stacks of both the MMC and the AAC during steady-state operation under any power factor and for AC voltage magnitude fluctuation of up to ±10%. The analysis is then used to calculate the minimum size for the cell capacitors in order to keep their voltage fluctuation within set boundaries for both topologies. The results show that the MMC requires 39 kJ/MVA of capacitive energy storage under sinusoidal modulation but this reduces with triplen injection modulation. The AAC has a lower requirement for storage in its cells of 11 kJ/MVA but the AAC has a six-pulse DC current ripple which requires a filter estimated to have a further 33% capacitive storage.
TL;DR: In this paper, an optimized reactive power compensation algorithm (RPCA) is proposed to improve the system operation stability and reliability, and facilitate MPPT implementation for each converter module simultaneously.
Abstract: Cascaded multilevel converter structure can be appealing for high-power solar photovoltaic (PV) systems thanks to its modularity, scalability, and distributed maximum power point tracking (MPPT). However, the power mismatch from cascaded individual PV converter modules can bring in voltage and system operation issues. This paper addresses these issues, explores the effects of reactive power compensation and optimization on system reliability and power quality, and proposes coordinated active and reactive power distribution to mitigate this issue. A vector method is first developed to illustrate the principle of power distribution. Accordingly, the relationship between power and voltage is analyzed with a wide operation range. Then, an optimized reactive power compensation algorithm (RPCA) is proposed to improve the system operation stability and reliability, and facilitate MPPT implementation for each converter module simultaneously. Furthermore, a comprehensive control system with the RPCA is designed to achieve effective power distribution and dynamic voltage regulation. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed reactive power compensation approach in grid-interactive cascaded PV systems.
TL;DR: An improved MPDPC for PWM rectifiers is proposed, which is able to operate under both balanced and unbalanced grid voltages, and can obtain sinusoidal grid currents and eliminate twice grid-frequency oscillation in both active power and the new reactive power.
Abstract: Model predictive direct power control (MPDPC) has been proposed as an effective alternative to conventional direct power control for pulsewidth-modulation (PWM) rectifiers. However, the sampling frequency of MPDPC still has to be high to achieve satisfactory performance. Furthermore, the grid currents of MPDPC would become highly distorted under unbalanced network conditions. To cope with the problems above, this paper proposes an improved MPDPC for PWM rectifiers, which is able to operate under both balanced and unbalanced grid voltages. By using a new definition of instantaneous reactive power in the predefined cost function, the proposed MPDPC can obtain sinusoidal grid currents and eliminate twice grid-frequency oscillation in both active power and the new reactive power. Neither complicated positive/negative-sequence extraction of grid voltage/current nor power compensation is required. Depending on the desired performance, two variants of the improved MPDPC are proposed and comparatively studied. At the same sampling frequency, the first approach achieves relatively low switching frequency, whereas the second approach obtains lower power ripples. Both simulation and experimental results are presented to confirm the effectiveness of the proposed methods.
TL;DR: In this article, the authors proposed a simple DPC-SVM that is effective under both ideal and unbalanced grid voltage conditions by using an extension of original instantaneous power theory, which does not require the extraction of complex positive/negative sequence from the grid voltage/current or power compensation.
Abstract: Compared with conventional table-based direct power control (DPC), DPC using space vector modulation (DPC-SVM) exhibits several specific features, such as a constant switching frequency and small ripples in both active power and reactive power. However, conventional DPC-SVM exhibits highly distorted grid currents when the grid voltages are unbalanced. In this study, we propose a novel and simple DPC-SVM that is effective under both ideal and unbalanced grid voltage conditions by using an extension of original instantaneous power theory. After deducing the power slopes of both active power and reactive power, the suitable converter voltage reference to nullify the errors of active power and reactive power is analytically derived, which is subsequently synthesized by SVM. The proposed DPC-SVM does not require the extraction of complex positive/negative sequence from the grid voltage/current or power compensation. Compared to prior DPC-SVM using original imaginary power, the proposed method exhibits much better performance by obtaining highly sinusoidal line currents and eliminating twice grid-frequency ripples in both active power and the reactive power under unbalanced conditions. Simulations and experimental results supported the theoretical study and confirmed the effectiveness of the proposed method.
TL;DR: In this article, the authors evaluate the effects of the simple voltage-balancing differential power processing (DPP) control approach on the sub-module-level maximum power point (MPP) efficiency and show that the submodule MPP efficiency of voltage balancing DPP converters exceeds 98% in the presence of worstcase MPP voltage variations due to irradiance or temperature mismatches.
Abstract: Differential power processing (DPP) architectures employ distributed, low power processing, submodule-integrated converters to mitigate mismatches in photovoltaic (PV) power systems, while introducing no insertion losses. This paper evaluates the effects of the simple voltage-balancing DPP control approach on the submodule-level maximum power point (MPP) efficiency. It is shown that the submodule MPP efficiency of voltage-balancing DPP converters exceeds 98% in the presence of worst-case MPP voltage variations due to irradiance or temperature mismatches. Furthermore, the effects of reduced converter power rating in the isolated-port DPP architecture are investigated by long-term, high-granularity simulations of five representative PV system scenarios. For partially shaded systems, it is shown that the isolated-port DPP architecture offers about two times larger energy yield improvements compared to full power processing (FPP) module-level converters, and that it outperforms module-level FPP approaches even when the power rating of DPP converters is only 20-30% of the PV system peak power. In the cases of aging-related mismatches, more than 90% of the energy yield improvements are obtained with DPP converters rated at only 10% of the PV peak power.
TL;DR: In this paper, a new series of control strategies which utilize the zero-sequence components are proposed to enhance the power control ability under this adverse condition, and the power converter can enable more flexible control targets, achieving better performances in the delivered power and the load current when suffering from the unbalanced ac voltage.
Abstract: Three-phase dc-ac power converters suffer from power oscillation and overcurrent problems in case of the unbalanced ac source voltage that can be caused by grid/generator faults. Existing solutions to handle these problems are properly selecting and controlling the positive- and negative-sequence currents. In this paper, a new series of control strategies which utilize the zero-sequence components are proposed to enhance the power control ability under this adverse condition. It is concluded that by introducing proper zero-sequence current controls and corresponding circuit configurations, the power converter can enable more flexible control targets, achieving better performances in the delivered power and the load current when suffering from the unbalanced ac voltage.
TL;DR: An integrated onboard charger for electric vehicles that incorporates an asymmetrical nine-phase machine and an inverter into the charging process that operates with a unity power factor and is capable of vehicle to grid (V2G) operation as well.
Abstract: This paper considers an integrated onboard charger for electric vehicles that incorporates an asymmetrical nine-phase machine and an inverter into the charging process. The charging is from three-phase mains, and it employs exclusively the power electronic components that already exist on board the vehicle and that are mandatory for the propulsion. No new elements are introduced. Moreover, the charging is achieved without any hardware reconfiguration since the existing elements and their connections are not altered during the transfer from propulsion to the charging mode. Instead, the operating principle is based on additional degrees of freedom that exist in nine-phase machines. These degrees of freedom are employed to avoid electromagnetic torque production in the machine during the charging process, although currents flow through its stator windings. The configuration operates with a unity power factor and is capable of vehicle to grid (V2G) operation as well. A detailed theoretical analysis is given, and the control for the charging/V2G and propulsion modes is discussed. Theoretical analysis is validated by experiments for charging, V2G, and propulsion operating regimes.
TL;DR: In this paper, a multilevel topology with medium-frequency ac link for medium-voltage grid integration of utility photovoltaic (PV) plants is discussed.
Abstract: A multilevel topology with medium-frequency ac link for medium-voltage grid integration of utility photovoltaic (PV) plants is discussed in this paper. A megawatt-scale PV plant is divided into many zones, each comprising of two series-connected arrays. Each zone employs a medium-frequency transformer with three secondaries, which interface with the three phases of the medium voltage grid. An insulated-gate bipolar transistor full-bridge inverter feeds the MF transformer. The voltages at the transformer secondaries are then converted to three-phase line frequency ac by three full-bridge ac-ac converters. Second line frequency harmonic power does not appear in the dc bus, thereby reducing the dc capacitor size. Cascading several such cells, a high-quality multilevel medium-voltage output is generated. A new control method is proposed for the cascaded multilevel converter during partial shading while minimizing the switch ratings. The proposed topology eliminates the need for line frequency transformer isolation and reduces the dc bus capacitor size, while improving the power factor and energy yield. This paper presents the analysis, design example, and operation of a 10-MW utility PV system with experimental results on a scaled-down laboratory prototype.
TL;DR: An active method for double-frequency power ripple decoupling in single-phase inverters is presented, exhibiting the main advantage of not using additional power semiconductors besides the H-bridge, and its operating principle and control analysis are detailed.
Abstract: An active method for double-frequency power ripple decoupling in single-phase inverters is presented in this paper, exhibiting the main advantage of not using additional power semiconductors besides the H-bridge. The proposed method requires only two capacitors placed between the midpoint and one end of each inverter leg. An original control solution of the inverter ensures the power ripple transfer toward the two decoupling capacitors without affecting the inverter output voltage. The simple design makes the proposed solution easy to adapt for single-phase inverters in H-bridge configuration. This paper focuses on the autonomous operation mode of the inverter, detailing its operating principle and the control analysis. The system performances, including the impact of the decoupling circuit on the inverter efficiency, are assessed by means of experimental results.