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Battery fault diagnosis is crucial for stable, reliable, and safe operation of electric vehicles, especially the thermal runaway early warning. Developing methods for early failure detection and reducing safety risks from failing high energy lithium-ion batteries has become a major challenge for industry. In this article, a real-time early fault diagnosis scheme for lithium-ion batteries is proposed. By applying both the discrete Fréchet distance and local outlier factor to the voltage and temperature data of the battery cell/module that measured in real time, the battery cell that will have thermal runaway is detected before thermal runaway happens. Compared with the widely used single parameter based diagnosis approach, the proposed one considerably improve the reliability of the fault diagnosis and reduce the false diagnosis rate. The effectiveness of the proposed method is validated with the operational data from electric vehicles with/without thermal runaway in daily use.

For electrolyzer applications, traditional solutions using line frequency transformers plus rectifiers are bulky, heavy and have low controllability. The Solid State Transformer (SST) could be a promising solution to solve the mentioned issues. This paper compares the semiconductor ratings and capacitance of five different Modular Multilevel Converter (MMC) based Solid State Transformer (SST) topologies. The results show that the DRU-MMC based topologies have the lowest semiconductor ratings and capacitance. Because of the unidirectional power flow requirement, the source side MMC of Back-to-Back (BtB) MMC based SST could be replaced by DRU, thus the cost is drastically saved. Another interesting finding is that the DRU-MMC energy ripple is much lower than half of the energy ripple in BtB MMC, which is different from HVDC MMC.

The emphasis on clean and green technologies to curtail greenhouse gas emissions due to fossil fuel-based economies has originated the shift towards electric mobility. As on-road electric vehicles (EVs) have shown exponential growth over the last decade, so have the charging demands. The provision of charging facilities from the low-voltage network will not only increase the distribution system's complexity and dynamics but will also challenge its operational capabilities, and large-scale upgrades will be required to meet the inevitably increasing charging demands. An ultra-fast (UF) charging infrastructure that replicates the gasoline refueling network is urgently needed to facilitate a seamless transition to EVs and ensure smooth operation. This paper presents a review of state-of-the-art DC fast chargers, the charging infrastructure's current status, motivation, and challenges for medium-voltage (MV) UF charging stations (UFCS). Furthermore, we consider the possible UFCS architectures and suitable power electronics topologies for UF charging applications. To address the peak formation issues in the daily load profile and high operational expenses of UFCSs, integration of renewable energy sources and energy storage systems due to their technological and economic benefits is being considered. The benefits of line frequency transformer (LFT) replacement with a solid-state transformer (SST), SST models, SST-based UF chargers, and MV SST-based UFCS architectures, as well as related MV active front-end and back-end power electronics topologies, are presented. Finally, the application of microgrids' hierarchical control architecture is considered for chargers and system-level control and management of UFCSs.

To accurately simulate electric vehicle DC fast chargers' (DCFCs') harmonic emission, a small time step, i.e., typically smaller than 10 μs, is required owing to switching dynamics. However, in practice, harmonics should be continuously assessed with a long duration, e.g., a day. A trade-off between accuracy and time efficiency thus exists. To address this issue, a multi-time scale modeling framework of fast-charging stations (FCSs) is proposed. In the presented framework, the DCFCs' input impedance and harmonic current emission in the ideal grid condition, that is, zero grid impedance and no background harmonic voltage, are obtained based on a converter switching model with a small timescale simulation. Since a DCFC's input impedance and harmonic current source are functions of the DCFC's load, the input impedance and harmonic emission at different loads are obtained. Thereafter, they are used in the fast-charging charging station modeling, where the DCFCs are simplified as Norton equivalent circuits. In the station level simulation, a large time step, i.e., one minute, is used because the DCFCs' operating power can be assumed as a constant over a minute. With this co-simulation, the FCSs' long-term power quality performance can be assessed time-efficiently, without losing much accuracy.

Smart grid steady control relies heavily on the communication infrastructure among sensors, actuators, and control systems, which makes it vulnerable to cyber-attacks. Accurate acquisition of dynamic state information is deemed vital for efficient detection of these cyber-attacks on a smart grid. However, several popular state estimation methods at the present stage are restricted in practical use and require some assumptions. In this paper, we investigate the security of smart grid systems. We (1) identify and define the security problem in the smart grid, (2) compare the performance of several state estimate methods including Least Square, Kalman filter, Extend Kalman filter, in identifying smart grid dynamic information using measurements, and (3) investigate the Chi-square detector, Euclidean Distance, and Cosine similarity matching approaches for attack detection.

Triple active bridge (TAB) as an isolated multiport converter is a promising integrated energy system for smart grids or electric vehicles. This article aims to derive and analyze zero voltage switching (ZVS) regions of TAB, in which both switching losses are reduced, and electromagnetic interference issues are mitigated. In the proposed closed-form solution of ZVS criteria, parameters such as the parasitic capacitance of the switches, the leakage inductance of the transformer, the switching frequency, the port voltage, the phase-shift inside and between the full-bridges are all taken into account. The analysis shows how the five degrees of freedom can be used to maintain ZVS operation in various operating points. The analysis and derived closed-form ZVS criteria are experimentally verified using a laboratory prototype. The derived analytical ZVS criteria are a powerful tool to study and optimize the operation of TAB converters.

Multiactive bridge converters (MAB) have become a widely-researched candidate for the integration of multiple renewable sources, storage, and loads for a variety of applications, from robust smart grids to more-electric aircraft. Connecting multiple dc ports reduces power conversion stress, improves efficiency, reduces material billing, and increases power density. However, the power flows between the ports of an MAB converter are magnetically coupled via the high-frequency (HF) transformer, making it difficult to control. This article presents an MAB converter configuration with a rigid voltage source on the magnetizing inductance of the transformer resulting in inherently decoupled power flows. As a result, the configuration allows independent power flow control tuning of the rest of the ports. The theory behind the power flow decoupling of the proposed MAB configuration is analyzed in detail using a reduced-order model. A 2-kW, 100-kHz Si-C-based four-port MAB converter laboratory prototype is built and tested, showing completely decoupled control loops with fast transient response regardless of their control bandwidths. The proposed configuration therefore makes the operation and design of the MAB family of converters much more feasible for any number of ports and precludes the need for a high-performance dynamic decoupling controller.

This paper proposes an improved analytical stray capacitance model for inductors. It considers the capacitances between the winding and the central limb, side limb, and yoke of the core. The latter two account for a significant proportion of the total capacitance with the increase of the core window utilization factor. The potential of the floating core/shield is derived analytically, which enables the model to apply not only for the grounded core/shield, but also for the floating core/shield cases. On the basis of the improved model, an analytical optimization method for the stray capacitance in inductors is proposed. Moreover, a global Pareto optimization is carried out to identify the tradeoffs between the stray capacitance and ac resistance in the winding design. Finally, the analysis and design are verified by finite element method simulations and experimental results on a 100-kHz dual active bridge converter.

This paper proposes a new series resonant DC-DC converter with four configurable operation states depending on the input voltage and output voltage levels. It suits well for the DC-DC stage of grid-connected photovoltaic (PV) systems with a wide-input voltage range and different grid voltage levels, i.e., 110/120 V and 220/230/240 V. The proposed converter consists of a dual-bridge structure on the primary side and a configurable half- or full-bridge rectifier on the secondary side. The root-mean-square (RMS) currents are kept low over a fourfold voltage-gain range; The primary-side MOSFETs and secondary-side diodes can achieve zero-voltage switching (ZVS) on and zero-current switching (ZCS) off, respectively. Therefore, the converter can maintain high efficiencies over a wide voltage gain range. A fixed-frequency pulse width modulated (PWM) control scheme is applied to the proposed converter, which makes the gain characteristics independent of the magnetizing inductance and thereby simplifies the design optimization of the resonant tank. The converter topology and operation principle are first described. Then the characteristics, i.e., the dc voltage gain, soft-switching, and RMS currents, are detailed before a performance comparison with conventional resonant topologies is carried out. Furthermore, the design guidelines of the proposed converter are also presented. Finally, the experimental results from a 500-W converter prototype verify feasibility of the proposed converter.

This paper analyses the fault current characteristics of a flexible DC grid with a power flow controller (PFC) and proposes a coordinated control method for suppression of fault currents between the PFC and the DC circuit breaker (DCCB). The equivalent circuits of the PFC during different stages of the DC fault are analyzed. Moreover, the influence of the output voltage of the PFC on the fault current is studied. After that, the current suppression characteristics of the current limiting reactor (CLR) and PFC are investigated. Due to the output voltage of the PFC can be flexibly adjusted during the DC fault, the fault current can be suppressed significantly by adjusting the output voltage of the PFC when the DC fault is detected. This paper adopts a bang-bang control for PFC during the fault. Finally, a simulation model was built in Matlab/Simulink to verify the fault current characteristics of the DC grid containing the PFC, and the coordinated control of the PFC was verified.

For a bipolar meshed DC distribution grid, the number of DC transmission lines is greater than converter stations. Therefore, the power flow control (PFC) needs to be introduced to increase the control flexibility of the DC grid. Besides, the unbalanced power between the positive and negative poles in bipolar DC girds may result in a large unbalanced current in the neutral line, causing the pole voltages to deviate from the rated value and increasing the power losses in the bipolar network. This paper aims to realize the PFC of DC distribution grids while suppressing the unbalanced voltage/power of the positive and negative poles. The application of a series parallel-connected PFC in bipolar DC distribution systems is investigated. Subsequently, the relationship between the output voltage of the PFC and the unbalanced power of the bipolar DC distribution grids is derived under the two different control modes of PFC. The simulation model is built in MATLAB/Simulink. The results show that when constant voltage control is applied, the PFC can suppress the unbalanced current in the transmission line, and it can also realize flexible control of the DC power flow.

To improve access to electricity, decentralized, solar-based off-grid solutions like Solar Home Systems (SHSs) and rural micro-grids have recently seen a prolific growth. However, electrical load profiles, usually the first step in determining the electrical sizing of off-grid energy systems, are often non-existent or unreliable, especially when looking at the hitherto un(der)-electrified communities. This paper aims to construct load profiles at the household level for each tier of electricity access as set forth by the multi-tier framework (MTF) for measuring household electricity access. The loads comprise dedicated off-grid appliances, including the so-called super-efficient ones that are increasingly being used by SHSs, reflecting the off-grid appliance market’s remarkable evolution in terms of efficiency and price. This study culminated in devising a stochastic, bottom-up load profile construction methodology with sample load profiles constructed for each tier of the MTF. The methodology exhibits several advantages like scalability and adaptability for specific regions and communities based on community-specific measured or desired electricity usage data. The resulting load profiles for different tiers shed significant light on the technical design directions that current and future off-grid systems must take to satisfy the growing energy demands of the un(der)-electrified regions. Finally, a constructed load profile was also compared with a measured load profile from an SHS active in the field in Rwanda, demonstrating the usability of the methodology.

In this paper, the output impedance of a three-phase inverter based on a dual current control (also called double synchronous reference frame current control) is modelled, which includes: the current loop gain, the control delay, and the Phase-Locked Loop (PLL). The impact of these parameters on the impedance is then analysed by a sensitivity study. The model is derived using transfer matrices and complex transfer functions, and it results in a compact impedance formulation that can be used in harmonic small-signal stability studies and system-wide steady-state harmonic calculations.

This paper presents a multilevel, cascaded converter topology that allows for integrations of combined photovoltaic and battery storage units. The modular features of the system enable the series connection of PV panels that, in turn, have a parallel connection to a specified voltage level. These voltage levels stem from a series string of power electronic submodules. Likewise, low voltage energy storage units are interfaced to the upper arm of the system via identical converter modules and provide various directions of dc and ac active power flow, depending on the mode of operation. By implementing additional, nestled current loops, guided by passive filters, the system controls the power flow between the photo-voltaic string, batteries and the low voltage dc terminal. The enhanced level of power flow allows the converter to achieve both a controlled energy exchange as well as various auxiliary services in the DC system. This paper demonstrates three distinct types of power flow that are achievable with by combining the multilevel topology along with multifrequency operations. Simulation results from a representative system model are utilised to demonstrate the three possible modes of operation.

DC Microgrid with integrated photo-voltaics (PV) and battery storage system is a promising technology for future smart grid applications. This paper compares three battery storage technologies namely: lithium-ion (Li-ion), lead-acid, and vanadium redox flow battery (VRFB) in the residential low-voltage dc grids, to increase the penetration of PV to improve self-sufficiency and grid independence. A multi-objective optimisation method is proposed to visualise the trade-offs between four objective functions: cost of electricity, energy autonomy, peak power reduction, and lifetime capital cost. The method is applied to a near-future scenario, based on a real residential feeder. Pareto fronts of the dominant designs are used to compare different battery technologies for this PV-battery integrated dc microgrid application. Additionally, the results provide insight into the dimensioning the battery, the PV, and the front-end ac-dc converter.

Solar Home Systems (SHS) have proven to be an effective means to tackle the global energy poverty that still affects around 1 billion people. However, present-day SHS (which are standalone systems with usually a purely dc architecture) have a limited power rating (usually up to 100 Wp). To enable higher power levels of electricity access in an economically viable way, energy sharing between these individual SHS through interconnectivity is a logical progression. The interconnectivity has to be implemented at a higher voltage level in order to reduce the conduction losses and cable costs. Existing control schemes do not take into account the multi-voltage dc microgrids. In this paper, the state of charge (SOC) balancing in such an interconnected SHS-based dc microgrid is addressed. In particular, the adaptive droop-based SOC control is extended for multiple voltage levels in a dc microgrid without any means of active communication. This is achieved through the creation of a voltage dead-band, SOC-based droop resistances, and the use of voltage ratios in dc-dc converters.

The transition from the conventional electrical power distribution system to ones that incorporate more stochastic electrical sources is resulting in a need for more robust and dynamic forms of power delivery. One solution to the various issues of power balancing and control is the utilization of additional frequencies within the distribution network. By having a greater variety of operational frequencies, the resulting decoupled power flow enables the system to have a higher degree of flexibility and functionality. This paper presents a controller for load-interfacing power electronic converters that operate within a system consisting of several ac frequencies as well as a superposed dc offset. The power exchange capabilities of this multifrequency converter, such as the decoupled power transfer between different operating frequencies are analyzed and demonstrated. Likewise, the underlying control structure is easily modifiable for a wide range of frequency combinations. The resulting increased level of active and reactive power control can then be utilized by the distribution network to ensure a more enhanced and stable transition to an electrically diverse and smarter microgrid. Simulation results for several different operating points are presented.

Medium voltage dc microgrids seeks to compete with conventional ac systems by providing various benefits regarding electrical power availability and efficiency. One method of achieving this goal is to provide a more effective and reliable interface to renewable energy sources and es pecially electrical storage devices. This paper presents a multilevel, cascaded converter topology that allows for the integration of various low voltage power sources and storage units into a single string of series connected half-bridge submodules. The resulting topology allows the system to function as medium-voltage dc voltage source. By implementing an additional, nestled current loop, guided by passive tuned filters, the converter controls the power flow between the submodules and the system, thus achieving power balance in all operating modes. This paper presents the underlying converter control principle for configurations containing electrical battery interfacing devices. Simulation and experimental results from a scaled prototype demonstrate the converters capabilities for a wide range of output power delivery.

The two main challenges of meshed low voltage DC grids today are the flexible control of power flow and the short-circuit protection. The conventional approach to deal with both problems is to incorporate galvanically isolated DC-DC converters with integrated short-circuit protection, which are rated for the full power rating of the grid. In this paper, we describe an approach based on the combination of a converter which has a partial power rating with respect to the grids power rating and a circuit breaker with full rating with respect to the grid. Based on the review of abnormal operating conditions of the power flow control converter and its requirements on protection. We propose a new protection strategy for the power flow control converter and evaluate the applicable circuit breaker technologies based on the review of the protection requirements.

Flexible power flow control is one of the main challenges in the development of the meshed low voltage direct current distribution system. The most widely adopted approach to achieve flexible power control in the network is to use various solid-state based solutions which are rated for the peak power of the grid. In this paper, we propose a solution based on a converter which has several times smaller power rating than the grid power rating. Due to the multi-port nature of the proposed solution, it is well suited for the bipolar networks.

The concern for power quality has been on the rise in recent years for all kinds of electric power systems. Harmonic analysis is especially interesting in the case of Offshore Wind Power Plants (OWPPs) due to the special susceptibility of these systems to harmonic issues and to the inherent differences in their behavior with respect to other electric networks. The aim of this article is to provide an overview of the basic concepts for harmonic analysis in OWPPs and to outline the typical assessment procedure used to this day. Some of the limitations of current industry practice will be outlined, as well as some possible complementary approaches for its improvement.

Back-to-back Modular multilevel converters (MMC) for medium voltage dc (MVDC) distribution link applications offer exciting opportunities due to its superior harmonic performance and high efficiency. Based on the steady state equations, it is shown that under specific active and reactive power operation, an increase in dc link voltage can be achieved without necessarily increasing the voltage seen by the MMC submodule components. Using steady state loss model, it is proved that the converter operating efficiency can be improved if this concept is applied.

Solar Home Systems (SHS) have recently gained prominence as the most promising solution for increasing energy access in remote, off-grid communities. However, the higher than standard testing conditions (STC) temperatures have a significant impact on the SHS components like PhotoVoltaic (PV) module and battery. A modeling methodology is described in this study for quantifying the temperature impact on SHS. For a particular location with high irradiation and temperatures and a given load profile, an SHS model was simulated, and the temperature-impact was analyzed on the performance and lifetime of the SHS components. Different PV module temperature estimation models were applied, and the corresponding dynamic PV outputs were compared. The nominal operating cell temperature (NOCT) model was found inadequate for estimating PV module temperatures under high irradiance conditions. The PV yield was found to be affected by almost 10% due to thermally induced losses. When different levels of temperature variations were considered, the battery lifetime was seen to be up to 33% less than that at 25 ° C. The modeling methodology presented in this paper can be used to include the thermal losses in SHS for rural electrification, which can further help accordingly in system sizing.

The ac resistance and parasitic capacitance of the inductor are the primary considerations in the winding design for the dual-active bridge converter (DAB). They are dependent of up to four independent structure variables. The interactive restrictions between those variables makes the design difficult. In this paper, the core-related capacitances between the central limb, side limb, yoke and winding are derived, and a local optimization for it is proposed. Moreover, a total winding capacitance design method is proposed by mapping the four dimensional problem into two dimensions. The analysis and design are verified by finite element method simulations and experimental results on a 100 kHz prototype are performed.

This paper proposes an improved analytical turn-on power loss model for 650-V GaN eHEMTs. The static characteristics, i.e., the parasitic capacitances and transconductance, are firstly modeled. Then the turn-on process is divided into multiple stages and analyzed in detail; as results, the time-domain solutions to the drain-source voltage and drain current are obtained. Finally, double-pulse tests are conducted to verify the proposed power loss model. This analytical model enables an accurate and fast switching behavior characterization and power loss prediction.

In meshed low voltage DC distribution grids, one of the main challenges is achieving controllability of the power flow. The most common approach to achieve the full control over the power flow in the meshed DC grid is to deploy DC-DC converters rated for the full power rating of the grid. The drawbacks of this solution are costs and losses of such DC-DC converters. To reduce the costs and the losses from the system standpoint, we introduce a partially rated power flow control converter. This paper presents the control modes of the partially rated power flow control converter and experimentally demonstrates its functionality. The proposed power flow control converter consists of two cascaded converters. On one side the converter is rated for the maximum grid voltage but only a fraction of the grid current, and on the other side, it is rated for the maximum grid current but only a fraction of the grid voltage. The proposed converter can operate in three modes and demonstrates the power flow control capability.

Power fluctuation caused by wind speed variations may be harmful for the stability of the power system as well as the reliability of the wind power converter, since it may induce thermal excursions in the solder joints of the power modules. Using the wind turbine rotor inertia energy for power leveling has been studied before, but no quantified analysis or generic design method have been found. In this paper, the transfer functions from the wind speed to electrical power, electromagnetic torque, and rotating speed are built based on which the rotating speed controller is designed in the frequency domain for power leveling. Moreover, the impact of other parameters on power leveling, including the time constant of maximum power point tracking (MPPT) and the rotor inertia, are also studied. With the proposed optimal design, the power fluctuations are mitigated as much as possible, while the stability of the rotating speed is still guaranteed. Moreover, the oscillation of the electromagnetic torque is also reduced, and the performance of the MPPT is only weakened slightly.

This chapter discusses the modeling and control methods for basic DC-DC converters by three specific case studies. It covers both the frequency-domain control methods and time-domain control methods. Specifically, the voltage mode control of a Buck converter, current mode control of a Boost converter, and a unified second-order boundary control of a Buck-Boost converter are presented. The analysis methods and control schemes can be extended to other DC-DC converter applications.

A voltage doubler circuit is realized to extend the soft-switching range of Dual Active Bridge (DAB) converters. No extra hardware is added to the DAB to form this circuit, since it is composed of the dc blocking capacitor and the low side full bridge converter, which already exist in DAB. With the voltage doubler, the DAB converter can achieve soft switching and high efficiency when the low side dc voltage is close to 2 pu (1 pu is the high side dc voltage divided by the transformer turn ratio), which can be realized only when the low side dc voltage is close to 1 pu by using the conventional phase shift modulation in DAB. Thus the soft switching range is extended. The soft switching boundary conditions are derived. A map to show the soft switching or hard switching in the full load and voltage range is obtained. The feasibility and effectiveness of the proposed method is finally verified by experiments.

Off-grid solar home systems (SHSs) currently constitute a major source of providing basic electricity needs in un(der)-electrified regions of the world, with around 73 million households having benefited from off-grid solar solutions by 2017. However, in and of itself, state-of-the-art SHSs can only provide electricity access with adequate power supply availability up to tier 2, and to some extent, tier 3 levels of the Multi-tier Framework (MTF) for measuring household electricity access. When considering system metrics of loss of load probability (LLP) and battery size, meeting the electricity needs of tiers 4 and 5 is untenable through SHSs alone. Alternatively, a bottom-up microgrid composed of interconnected SHSs is proposed. Such an approach can enable the so-called climb up the rural electrification ladder. The impact of the microgrid size on the system metrics like LLP and energy deficit is evaluated. Finally, it is found that the interconnected SHS-based microgrid can provide more than 40% and 30% gains in battery sizing for the same LLP level as compared to the standalone SHSs sizes for tiers 4 and 5 of the MTF, respectively, thus quantifying the definite gains of an SHS-based microgrid over standalone SHSs. This study paves the way for visualizing SHS-based rural DC microgrids that can not only enable electricity access to the higher tiers of the MTF with lower battery storage needs but also make use of existing SHS infrastructure, thus enabling a technologically easy climb up the rural electrification ladder.

In this paper, a three phase cycloconverter type high frequency AC link inverter is discussed. The configuration consists of a high frequency full-bridge inverter and a high frequency transformer followed by a three phase cycloconverter whose switch is composed of anti-series connected MOSFETs with a common source. A simple digital control strategy based on space vector modulation (SVM) and repetitive control for the cycloconverter is proposed although its input voltage is a high frequency AC pulse. The operation principle of the proposed control strategy is analyzed and the equivalent working modes during one interval are also presented. The effectiveness of the proposed control strategy is verified through Matlab/Simulink simulations and experiments on a 1.45kW prototype.

This paper presents the benchmark study of ac and dc active power decoupling circuits for second-order harmonic mitigation in kW-scale single-phase inverters. First of all, the best solutions of active power decoupling to achieve high efficiency and power density are identified and comprehensively studied, where the commercially available film capacitors, circuit topologies, and control strategies for active power decoupling are all taken into account. Then, an adaptive decoupling voltage control method is proposed to further improve the performance of dc decoupling in terms of efficiency and reliability. The feasibility and superiority of the identified solution for active power decoupling together with the proposed adaptive decoupling voltage control method are finally verified by both the experimental results obtained on a 2 kW single-phase inverter.

Three-phase back-to-back converters have a wide range of applications (e.g. wind turbines) in which the reliability and cost-effectiveness are of great concern. Among other com- ponents and interconnections, DC-link capacitors are one of the weak links influenced by environmental stresses (e.g. ambient temperature, humidity, etc.) and operating stresses (e.g. voltage, ripple current). This paper serves to investigate the ways of reducing ripple current stresses of DC-link capacitors in back-to- back converters. The outcome could benefit to achieve either an extended lifetime for a designed DC-link or a reduced DC-link size for fulfilling a specified lifetime target. The proposed control strategies have been demonstrated on a study case of a 1.5 kW converter prototype. The experimental verifications are in well agreement with the theoretical analyses.

Energy storage can be used to suppress the power fluctuations in wind power systems, and thereby reduce the thermal excursion and improve the reliability. Since the cost of the energy storage in large power application is high, it is crucial to have a better understanding of the relationship between the size of the energy storage and the reliability benefit it can generate. Therefore, a reliability-oriented energy storage sizing approach is proposed for the wind power systems, where the power, energy, cost and the control strategy of the energy storage are all taken into account. With the proposed approach, the computational effort is reduced and the impact of the energy storage system on the reliability of the wind power converter can be quantified.

Wind turbines are subjected to wind speed variations that cause a power profile that will stress the overall system. This stress is tranfered to the power converter, resulting in temperature variations of the power devices and, hence, causing the reduction of the lifetime. The lifetime expectation changes depending on the real wind speed once the wind turbine is operating. Usually, the real wind speed profiles are employed to evaluate this stress but they do not consider all possible operation conditions and require intensive computations. To solve these issues, this paper proposes the generation of random wind speed profiles, based on the measured ones, in order to evaluate the thermal stress of the power devices based on a simplified statistical approach. Wind turbines are subjected to wind speed variations that cause a power profile that will stress the overall system. This stress is tranfered to the power converter, resulting in temperature variations of the power devices and, hence, causing the reduction of the lifetime. The lifetime expectation changes depending on the real wind speed once the wind turbine is operating. Usually, the real wind speed profiles are employed to evaluate this stress but they do not consider all possible operation conditions and require intensive computations. To solve these issues, this paper proposes the generation of random wind speed profiles, based on the measured ones, in order to evaluate the thermal stress of the power devices based on a simplified statistical approach.

In this paper, a synchronous reference frame (SRF) controlled three-phase boost-type grid-connected inverter with Current Space Vector Modulation (CSVM) is proposed for alternative energy sources regenerations. The averaged model of the equivalent boost-type inverter on SRF is derived in this paper. With the proposed averaged model, a SRF-PI compensator is analyzed with Bode diagram in frequency domain. Hence, the steady-state error tracking of the current can be effectively eliminated and power factor of the inverter can be well controlled under the decoupled d-q SRF control strategies. The feasibility of the proposed control scheme is verified by experimental results of a 700VA laboratory prototype.

This paper presents the implementation of a wireless controlled three-phase parallel-connected inverter system which supplies auxiliary power for railway facilities. In order to implement droop method without the use of output currents sensors, a power computation scheme based on the virtual output currents establishment is proposed in this paper. The overall control scheme consists of an inner loop for three-phase output-voltage regulation and an outer loop for the automatic loads sharing. For the outer loop, the Proportional-Derivative droop controller is adopted in order to deal with trade-off between the good dynamic performance and small deviation of frequency and amplitude. For the inner loop regulation, repetitive controller is employed in the system under d-q frame supplying power to nonlinear loads. The whole system is described, and the simulations as well as experimental results of a two 15kVA-parallel-connected inverters prototype are presented to demonstrate the validity of the proposed system.

Inductive power transfer (IPT) is becoming increasingly popular in stationary electric vehicle charging systems. In this paper, the influence of the different IPT coupler geometries on the performance factors efficiency, power density, misalignment tolerance, and stray field is studied. Five different cou-pler topologies namely the circular, rectangular, double-D (DD-DD) and the double-D transmitter with double-D-Quadrature receiver (DD-DDQ) are considered in this study. The electromagnetic behavior of the couplers is modeled using three-dimensional finite element analysis. To ensure a fair quantitative comparison, a multi-objective optimization framework is developed to analyze the Pareto trade-offs between conflicting performance metrics like power densities, efficiencies, and misalignment tolerance for all the considered coupler topologies.

Wind energy science : WES 7(1), 387-411 (2022). doi:10.5194/wes-7-387-2022, Published by Copernicus Publications, Go��ttingen

To accurately simulate the harmonic emission of EV DC fast chargers (DCFCs) and the harmonic voltage of the power grid to which the chargers are connected, a small time-step, i.e., typically smaller than 10μs, is required. However, for harmonic assessment, a long timescale, typically a day, is required. A conflict between accuracy and time efficiency exists. To address this issue, a multitimescale modeling framework of fast charging stations (FCSs) is proposed in this paper. In the presented framework, the DCFCs’ input impedance and harmonic current emission in the ideal grid condition, i.e., the grid impedance is zero and there are no background harmonic voltages, is obtained firstly through a converter switch model with a small timescale. Since the DCFC’s input impedance and harmonic current source change in the charging course, the input impedance and harmonic emission at different input power should be obtained. Then, the DCFCs’ input impedance and harmonic emission will be used in the fast-charging station modeling, where the DCFCs are simplified as their Norton equivalent circuits. In the station level modeling, a bigger time step, i.e., 1 minute, is used, since the DCFCs’ operating power can be assumed as a constant in one minute. With this framework, the FCSs’ long-term power quality performance can be assessed efficiently without neglecting the DCFCs’ small timescale dynamics.

In this decade, a significant amount of additional wind and solar power will be integrated in the Dutch high voltage grid. The 383 MW nearshore Wind Park Fryslân is foressen to be connected to the Dutch 110kV grid in 2020 through two 55km cable circuits. To quantify the risks of exceeding emission levels for both the TSO and the wind farm developer, harmonic voltage distortion studies must already be performed at an early phase of the project. Relevant data of wind farm components may not yet be available then and must therefore be estimated including data of the wind turbine generator (WTG). The objective of this paper is to assess the impact of a WTG converter impedance model on the harmonic impedance of the windfarm and subsequent harmonic voltage amplification. For this purpose, an analytical converter harmonic model with tunable parameters has been developed. A sensitivity study was performed in a case study of Wind Park Fryslân to analyse the impact of these parameters on the harmonic amplification at the Point of Connection with the 110 kV grid. The length and amount of array cables were also included in the study. The paper follows a step-wise approach where subsequently wind farm impedance, grid impedance and amplification factor have been calculated. It has been found that the WTG converter impedance model has a significant impact on the harmonic amplification of low harmonic orders (h=2 to h=10). Notably, not taking the converter impedance into account (infinite impedance) will result in both over- as well as underestimation of the harmonic amplification over the mentioned frequency range. It is shown that, when no WTG manufacturer has been selected yet, the analytical converter model provides a valid alternative for performing early-stage harmonic studies.