Distributed model predictive control (DMPC) has become a focus in the energy management of shipboard power systems due to its capabilities for privacy preservation, robustness, and distributing computing burdens to local processors. DMPC determines control actions in a distributed manner based on the predictions of system statuses. However, the performance of DMPC is affected by inaccurate predictions resulting from uncertain parameters in nominal prediction models. Particularly, these inaccuracies in predicting propulsion loads and solar panel generation powers can lead to power imbalances when implementing the control actions determined by DMPC. To address this challenge, this paper proposed a novel reinforcement learning compensated DMPC (RL-C-DMPC) to distributively compensate for the control actions determined by DMPC baseline control, thereby rectifying the power imbalances caused by uncertain parameters in nominal prediction models. A value-decomposition-network-based training and distributed testing mechanism is designed for our proposed RL-C-DMPC. Furthermore, a method for range selection of compensation rate is specifically proposed for the energy management of shipboard power systems. To validate the effectiveness of our proposed RL-C-DMPC, we conduct a comprehensive case study utilizing real-life voyage data and historical solar power generation data in the area of the voyage to build the environment for training and testing. By comparing power imbalances between DMPC and RL-C-DMPC, our results indicate significant reductions in power imbalances so that frequency stability can be better ensured. Furthermore, via the case study, we also evaluate the communication robustness of RL-C-DMPC.@en

Predictive maintenance scheduling of modular-multilevel-converters (MMCs) requires estimations of the failure probabilities of MMCs during a period of time in the future. Particularly, the predicted future failure probabilities are influenced by two main factors, the mission profiles of the MMCs and the maintenance decisions on the MMCs during the prediction period. This paper proposes a failure probability prediction model (FPM) for MMCs by considering these two factors. First, the expectations of the failure probabilities of the components for all the scenarios of mission profiles are obtained. Second, in predictive maintenance scheduling problems, the decisions to perform the maintenance actions are represented by binary variables. When the number of submodules is very large, using the binomial probability form currently used in reliability engineering to express the 'r-out-of-n' failure probability of arms of the MMCs is intractable. Thus, this paper proposes a tractable form (T-form) in FPM by observing that the submodules on one arm are homogeneous. Furthermore, an approximation method, i.e., clustering and assignment (C&A), is proposed to reduce the computation times for calculating the parameters needed by the proposed T-form. Then, we perform a case study that assesses the accuracy and computation time of the C&A approach. The results show that the accuracy of the C&A approach is high and that the computation time is reduced significantly compared with the accurate method. We also show that the computation time for solving the predictive maintenance scheduling problem can be reduced hugely by using the T-form instead of the binomial probability form.

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DC microgrids can be considered as cyber-physical systems (CPSs) and they are vulnerable to cyberattacks. Therefore, it is highly recommended to have effective plans to detect and remove cyberattacks in dc microgrids. This article shows how artificial neural networks can help to detect and mitigate coordinated false data injection attacks (FDIAs) on current measurements as a type of cyberattacks in dc microgrids. FDIAs try to inject the false data into the system to disrupt the control application, which can make the dc microgrid shutdown. The proposed method to mitigate FDIAs is a decentralized approach and it has the capability to estimate the value of the false injected data. In addition, the proposed strategy can remove the FDIAs even for unfair attacks with high domains on all units at the same time. The proposed method is tested on a detailed simulated dc microgrid using the MATLAB/Simulink environment. Finally, real-time simulations by OPAL-RT on the simulated dc microgrid are implemented to evaluate the proposed strategy.

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This article aims to incorporate the reliability model of power electronic converters into power system reliability analysis. The converter reliability has widely been explored in device- and converter-levels according to physics of failure analysis. However, optimal decision-making for design, planning, operation, and maintenance of power electronic converters require system-level reliability modeling of power electronic-based power systems. Therefore, this article proposes a procedure to evaluate the reliability of power electronic-based power systems from the device-level up to the system-level. Furthermore, the impact of converter failure rates including random chance and wear-out failures on power system performance in different applications such as wind turbine and electronic transmission lines is illustrated. Moreover, because of a high calculation burden raised by the physics of failure analysis for large-scale power electronic systems, this article explores the required accuracy for reliability modeling of converters in different applications. Numerical case studies are provided employing modified versions of the Roy Billinton Test System (RBTS). The analysis shows that the converter failures may affect the overall system performance depending on its application. Therefore, an accurate converter reliability model is, in some cases, required for reliability assessment and management in modern power systems.

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Microgrids (MGs) are becoming active segments of future power systems. Frequency regulation inislanded MGs is of high importance to guarantee stable and reliable operation under different loading conditions. This paper proposes a decentralized frequency regulation of AC MGs, which relies on the local information. Comparing to the conventional approaches, the proposed scheme does not require a communication system for frequency regulation, resulting in higher reliability and lower costs. Simulation results verify the effectiveness of the proposed frequency control strategy.

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Power electronic converters are one of failure sources in energy systems, and hence drivers of downtime costs in power systems. Different approaches can be employed for converter reliability enhancement including design/control for reliability methods, condition monitoring and fault diagnosis, and maintenance strategies. This paper proposes optimal preventive maintenance strategies based on wear-out failure model of converter components. The proposed approaches employ two different performance measures at converter-level and system-level. The converter-level measures take into account planned and unplanned maintenance times or costs in a single unit or small-scale system. Moreover, the system-level measure considers not only maintenance times, but also energy losses and additional maintenance costs induced by aging of the converter components. The outcome is optimal replacement time of converter and its components, which depends on the employed performance measure. Optimal replacement scheduling is of importance for risk management and decision-making during planning of modern power electronic based power systems. The applicability of the proposed approaches is illustrated by numerical analysis in a photovoltaic system.@en

In dual active bridge (DAB) converters, there can be transformer current ringing, especially when the transformer turns ratio is high. It is induced by high dv/dt generated by fast switching as well as the low impedance of the magnetic tank at the high-frequency range. To quantify the influence of the magnetic tank, its impedance model is thoroughly modeled by considering all the parasitic components. It is found that the parasitic capacitors of the magnetics do not equally affect the current ringing, and thereby the critical one is addressed. On top of that, the design guide of the inductor is provided for the mitigation of the current ringing. Additionally, the impact of dv/dt is also studied. The models and analyses are verified on a 2.5-kW DAB prototype.

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Power electronic converters will serve as the fundamental components of modern power systems. However, they may suffer from poorer reliability if not properly designed, consequently affecting the overall performance of power systems. Accordingly, the converter reliability should be taken into account in design and planning of Power Electronic-based Power Systems (PEPSs). Optimal decision-making in planning of PEPSs requires precise reliability modeling in converters from component up to system-level. This paper proposes model-based system-level design and maintenance strategies in PEPSs based on the reliability model of converters. This will yield a reliable and economic planning of PEPSs by proper sizing of converters, cost-effective design of converter components, identifying and strengthening the converter weakest links, as well as optimal maintenance scheduling of converters. Numerical case studies demonstrate the effectiveness of the proposed design and planning strategies for modern power systems.@en

For reliability and thermal management of power devices, the most frequently used technique is to employ heatsinks. In this work, a new configuration of offset strip fin heatsink based on using the concept of curvy fins and U-turn is proposed with the aim of improving the heat transfer performance. With this aim, a three-dimensional model of heatsink with Silicon Insulated-Gate Bipolar Transistors (IGBTs) and diodes, solder, Direct Bonded Copper (DBC) substrate, baseplate and thermal grease is developed. Richardson's extrapolation is used for increasing the accuracy of the numerical simulations and to validate the simulations. To study the effectiveness of the new offset design, results are compared with conventional offset strip fin heatsink. Results show that in aspects of design of heatsinks (including heat transfer coefficient, maximum chip temperature and thermal resistance), the new introduced model has advantages compared to the conventional offset strip fin design. These enhancements are caused by the combination of the longer coolant passage in the heatsink associated with generation of disturbance and recirculation areas along the curvy fins, creation of centrifugal forces in the U-turn, and periodic breaking up boundary layers. Also, it is shown that due to narrower passage and back-and-forth route, the new introduced design can handle the hot spots better than conventional design.

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Renewable energy resources are becoming the dominating element in power systems. Along with de-carbonization, they transform power systems into a more distributed, autonomous, bottom-up style one. We speak of Smart Grid and Microgrid when distributed energy resources take over. While being a means to improve technical and financial efficiency, planning, operations, and carbon footprint, it is these new technologies that also introduce new challenges. Reliability is one of them, deserving a new way of describing and assessing system and component reliability. This paper introduces a new reliability framework that covers these new elements in modern power systems. It can be seen that reliability assessment of modern power systems also requires introducing local reliability concepts as well as incorporating different electro-magnetic/mechanical stability issues.@en

Reliability of DC-DC converters is important in photovoltaic (PV) applications like building integrated PV systems, where the module-level converter may be stressed significantly. Understanding and predicting the most life-limiting components with accurate degradation models in such systems enable the design for reliability. In this paper, a mission profile-based reliability analysis framework for PV DC-DC converters is proposed where the inputs and models of the framework can be adjusted according to the converter topology, the components and the failure mechanisms under investigation. The framework is demonstrated by comparing the influence of two yearly mission profiles on the solder joint degradation of a MOSFET in an interleaved boost converter. This is done by using an electro-thermal circuit simulation in PLECS and a finite element MOSFET model in COMSOL. This framework allows for exploring more accurate models or even simplifying parts with low sensitivity in order to obtain a thorough understanding of their accuracy and to determine the overall converter reliability.

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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.

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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.

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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.@en

A transformerless single-phase symmetric Z-source HERIC inverter featuring low leakage currents is presented in this paper. It is an attempt to utilize the Highly Efficient and Reliable Inverter Concept (HERIC) and an impedance source (Z-source) network to maintain a constant common mode voltage and thus low leakage currents in PV applications. The symmetric Z-source HERIC inverter requires two extra active switches. Nevertheless, the operation frequency of the two switches is the line frequency, leading to negligible losses. More importantly, the performance in terms of low leakage currents and harmonics is improved. Experimental tests are performed to validate the analysis and performance of the proposed system.@en

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.@en

A Dual Active Bridge (DAB) converter can achieve a wide high-efficiency range when its input and output voltages are equal, assuming a 1:1 turns ratio for its isolation transformer. If its input or output voltage is doubled, efficiency of the DAB will drop significantly, because of the introduction of hard switching and high circulating power. Thus, a new modulation scheme has been proposed, whose main idea is to introduce a voltage offset across the dc blocking capacitor connected in series with the transformer. Operational principle of the proposed modulation has been introduced, before analyzing its soft-switching area and circulating power mathematically. The final modulation scheme is not difficult to implement, but can help the DAB achieve soft switching, low circulating power, and thereby high efficiency, even with its input or output voltage doubled.
These features have been verified by experimental results obtained with a 1.2 kW prototype.@en

Due to the parasitic capacitances of the transformer and inductor in Dual Active Bridge (DAB) converters, ringing may happen in the transformer currents especially when the transformer turns ratio is high. This high frequency current ringing may lead to EMI issues. It is found in this paper that the inductor connected on the low voltage side rather than the high voltage side of a DAB converter can help to mitigate the current ringing. The impedance of the magnetic tank is modelled to analyze the current ringing. The analysis is verified on a 2.5 kW DAB prototype.

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