Multilevel Active Bridge (AB)-based converters are potential candidates for DC applications due to their ability to integrate multiple sources while minimizing volume. This paper presents an analytical and simulation-based investigation of circulating current behavior and power factor performance in AB-based multilevel converter structures. A High-Frequency Link (HFL) analysis framework is developed to characterize voltage, current, and power transfer waveforms, providing insight into reactive power generation and its impact on overall efficiency. By introducing a modulation-based control approach, the proposed converters significantly reduce circulating currents and enhance the power factor, particularly under varying phase shift conditions. Compared to conventional Quadruple Active Bridge (QAB) topologies, the discussed multilevel architectures offer reduced transformer complexity and improved power quality, making them suitable for demanding applications such as electric vehicles and aerospace systems.

This article provides a systematic overview of the basics of talkative power conversion (TPC). TPC is an emerging technique for simultaneous information and power transmission, in which data modulation is integrated into a switched-mode power converter. The data sequence is embedded in the ripple voltage, which is superimposing the output voltage of the converter. In contrast to conventional power line communication (PLC), TPC can be used universally, not only in grid applications. Aspects of power electronics (PE) and digital communication are presented in a structured form, including new perspectives such as multiple-input multiple-output (MIMO) techniques applied to TPC, adaptive modulation and channel coding, and advanced receiver design with adaptive channel and load estimation. The new aspects aim to mitigate the inherent shortcomings of TPC.

By integrating standard power electronic modules like the half-bridge and bidirectional unit, it is possible to create a T-type circuit that is versatile for use in both DC-AC and AC-DC applications. Using these, various arrangements of bidirectional DC-AC converters, including mid-point-clamped-based multilevel inverters (MLIs), can be constructed. However, realization of such T-type circuit in mid point-clamped MLIs with limited voltage blocking capability of the available wide-band-gap devices is challenging as these MLIs suffer from half dc-link voltage utilization at the ac output. In this paper, new opportunities of bidirectional and T-cell branch in MLIs with Gallium Nitride-high electron mobility transistors (GaN-HEMTs) are outlined. Thanks to the contribution of a front-end T-cell branch, the half dc-link voltage utilization factor of the conventional mid point-clamped-based MLIs is enhanced, whilst more number of output voltage levels can be generated facilitating incorporation of standard commercially available 650 V GaN-HEMTs. This results in the creation of new MLIs that feature smaller grid-interfaced filters, reduced overall losses, and improved power density. Moreover, with additional bidirectional GaN-based cell, application of such converters can be further broadened within new inductor-less dual-mode MLIs topologies operating over a wide range of input dc voltage changes. Theoretical analysis with several simulation and experimental results extracted from a 3 kW, 99.2% efficiency, 1.92 kW/L GaN-based prototype are given to corroborate the effectiveness and feasibility of the proposed solution.

Communication-based coordination between power converters in a PV park aims at faster response to voltage and frequency events and improved transient response. However, communication introduces significant delays that reduce the ability for fast respond, decrease stability margin and degrade performance. Accurate analysis of its impact on stability becomes increasingly complicated as the number of converters increases. This paper presents a method for accurate and computationally efficient delay-dependent stability analysis for high-dimensional model of multi-converter PV parks in time-domain, based on the reduced-order Discretized Lyapunov Functional (DLF) method, a variant of Lyapunov-Krasovskii Functionals.

Grid-forming static synchronous compensator is an effective solution for inverter-based resources for reactive power support at the point of common coupling. However, the alternating voltage controller design is not fully investigated, and the potential high-frequency resonance may be triggered due to the control delay and the grid capacitive compensation. This paper mainly focuses on the virtual admittance-based voltage control, and the dissipative region of the converter output impedance in the high-frequency range is first discussed. Afterward, a passivity-based damping method is proposed using capacitor current feedforward and capacitor voltage feedforward, and the dissipative region can be optimized to the Nyquist frequency. Finally, the effectiveness of the proposed method is validated through the case study.

In dc interconnections such as dc microgrids or electric vehicle charging stations, a flexible management of the power flow is of greater importance. For that purpose, the exchange of information beside the power transfer is necessary. The dc output voltage of switching dc converters contains already information – the switching frequency itself which can be seen in the voltage ripple. This ripple is always there and and can be used to transfer further information. Modulating information onto the voltage ripple is called talkative power conversion (TPC). For that purpose, classical modulation techniques such as frequency-shift keying (FSK) and phase-shift keying (PSK) are used and can be modulated on top of the power modulation through pulse-width modulation (PWM). A suitable converter in dc microgrids or charging stations is the dual-active bridge (DAB) converter due to its bi-directionality and its high power density. The selection of a suitable TPC information modulation scheme depends on the requirement of plug-and-play capabilities and easy adoption.Thus, PSK is selected and analysed as a TPC strategy that can be adapted for TPC in DAB converters. In this work, the emerging challenges of PSK-TPC in DAB converters and the necessary modifications in the control and PWM of the thus talkative DAB converter are shown and explained.

Hybrid-bridge based dual-active-bridge (DAB) converter comprised of three-level (3L) and two-level (2L) full-bridge (FB) is promising to be a good candidate for medium voltage DC (MVDC) grids. By applying a corresponding modulation strategy proposed in this paper with multiple working patterns, a wide voltage conversion gain can be obtained. However, given that the conventional neutral-point-clamped (NPC) DAB converter has potential risk losing the control of the voltage conversion gain in backward and forward modes, an asymmetric active-neutral-clamped (A-ANPC) 3L structure is employed in the proposed converter to overcome this issue. Afterwards, to ensure a smooth working pattern transition, a simplified control strategy is proposed. The backflow power remains constant regardless of working conditions, and the zero-voltage-switching (ZVS) can be achieved for all power switches under the proposed modulation strategy. The operation principle, characteristics, and performances of the proposed converter are analyzed in detail and experimentally verified.

Talkative power conversion (TPC), which embeds information into the switching ripple to achieve the simultaneous transmission of power and data without installation of additional hardware, can reduce the additional communication infrastructure in cyber-physical systems of smart grid and renewable power plants. The TPC in DC/DC converters has been the subject of numerous investigations, but comparatively few studies have focused on the DC/AC converters. In this paper, the space-vector pulse width modulation (SVPWM)-based TPC techniques including both variable zero vector width modulation (VZVWM) and variable zero vector position modulation (VZVPM) for three-phase DC/AC converters are proposed. The DC/AC converters can transmit information to the receiver, crossing the LCL filter and power cable, with a good power quality. The sliding FFT method is employed for demodulation, and the bit rate is analyzed. Finally, simulations and experiments prove the viability of the SVPWM-based three-phase DC/AC talkative power converters with a bit rate of 100 bps and 1-blt length time delay.

Talkative power conversion (TPC), which can simultaneously transfer the energy and data by the power electronics converters and avoid using extra communication infrastructures, is an emerging technique for the digital power and digitalization of power system. In this paper, the TPC technology is used in the dual-active bridge (DAB) converter, in which the switching frequency and common phase are used as the degrees of control freedom respectively to modulate the data and power simultaneously. The transient performances of the DAB converters modulated by the PWM-frequency shift keying (PWM-FSK), which is a simple and effective technique for the TPC. The constraints on carrier frequency are derived, and the influence of data communication on power conversion is analyzed. The demodulation strategy based on the fast Fourier transform (FFT) algorithm is used to acquire the sent data. The simulation and experimental results clearly validate the effectiveness and correctness of the theoretical analysis.

Renewable energy sources such as wind and solar are increasingly being integrated into the grid, replacing conventional synchronous generators. This demands their participation in the provision of grid services such as frequency regulation. The potential of variable speed wind turbines (VSWT) in this respect has been well established. The transient power exchange during the frequency support (FS) phase causes the rotor speed of a VSWT to move in a direction away from its maximum power point (MPP), resulting in a reduction in wind energy capture potential. This, together with the subsequent need to recover the rotor speed after the FS phase, could result in a significant reduction in the power delivered from the wind farm (WF) to the grid compared to pre-disturbance conditions, leading to a secondary frequency drop (SFD) in the power system if not properly managed. A power system model with VSWTs providing frequency support is developed in this work. The root causes for the occurrence of an SFD are identified and possible solutions to mitigate or minimise its severity are explained. Finally, a power system level dynamic tuning method for the FS controller gains of the VSWTs is proposed as a measure to minimise the severity of a potential SFD in the power system during an under-frequency event, and its effectiveness is proven by time domain simulation results.

Power systems are experiencing a rapid and dramatic transformation driven by the massive integration of nondispatchable renewable energy sources, such as wind and solar, and highly variable loads, such as electric vehicles and air conditioning. This challenges existing grid assets, eventually leading to updating them, which, in turn, increases significantly the costs of sustainable technologies. Power electronics is a pivotal technology for electrical power processing for renewable energies and sustainable transportation. By means of “smart” functionalities, power electronics converters already embedded in such applications can also contribute to guaranteeing the overall system’s stable operation. Anyway, this cooperative contribution from distributed devices may be not enough leading to the need for the voltage transformation and power transmission of “system-level” power electronics solutions. In the case of large charging stations, a smart transformer (ST), while, in the case of large solar and wind parks, integration medium- or high-voltage direct current (HVdc) transmissions are system-level solutions. This article wants to review the potential of using such infrastructures to increase the capacity of existing grid assets, avoiding or deferring their upgrade and, hence, reducing the overall costs of renewables integration and the electrification of the transport sector. In fact, the power converters embedded in ST and HVdc can provide fast frequency and voltage response, and precise control of power flow acting at the system level much more effectively and feasibly for system operators as the distributed power converters embedded in several small sources and users. This article reviews, for the first time, these two key power electronics “system-level” solutions together—ST and HVdc—starting from their basic functionalities and showings how they can go beyond them, showing how, with grid-forming functionalities, they can offer new “smart grid” tools to enhance the capability of the existing electric grid infrastructures.

The trend towards high power density and high reliability of electric drive systems in mobility applications pushes the use of high-speed machine in combination with Wide Band Gap (WBG) power semiconductor technology. Replacement of Si power technology with WBG without mitigating high voltage slew rates dv/dt degrades machine winding insulation. Of the different mitigation techniques employed, especially the passive LCR filter at the output of the inverter cannot be optimally designed without consideration of the inherent low impedance of the high speed machines. This paper presents the analytical techniques used for LCR filter design for motor drives and introduces the technique to incorporate machine impedance Zdm for optimal design of the parameters to achieve high efficiency. An analytical technique based optimization algorithm is introduced for the reduction of filter inductor volume to achieve high power density in these applications. The proposed design methodology is evaluated in simulations and experiments with Gallium Nitride (GaN) based inverter technology.

In distribution networks integrated distributed generation, the forward and reverse power of power electronic transformers may be unequal. For this regard, this paper proposes a bidirectional asymmetric-power-flow isolated DC/DC (BAI-DC/DC) converter topology, which not only meets the requirements of bidirectional asymmetric power transmission, but also reduces the active device rating and increases the number of power paths, which can decrease system cost and improve system reliability. Simulation and preliminary experimental results verify the feasibility of the proposed topology.

The dual-active-bridge is a DC/DC converter that is widely used due to its design flexibility. Diverse modulations have been used to extend the soft-switching range, reducing the RMS current or improve the overall performance. In this paper, the analysis of different quasi-discontinuous conduction modulations for the DAB converter is made. The analysis considers the RMS current, the displacement angles calculation, and the softswitching conditions to establish a design guide for the modulations analyzed. Four modulations were considered. Simulation and experimental results are presented.

The dual-active-bridge (DAB) converter has extended its use due to the important features that make it suitable for a wide range of applications. The Solid-State Transformer (SST) in Smart Transformer (ST) application uses the DAB converter as the core of the system. One of the most important features is reactive power management because it allows the system to control the input voltage amplitude. In this paper, an ac/dc converter based on the DAB converter with reactive power management is described. The first stage is comprised of a synchronous rectifier, reducing the system complexity in comparison with its PWM rectifier counterpart. The DAB converter comprises the second stage and it draws or delivers current, allowing a low input current THD and variable power factor. The design procedure is shown as well as simulation results.

Multiwinding-Transfomer-based (MTB) DC-DC converter is an interesting possibility to interconnect several energy systems and to offer higher power density because of the reduction of transformer core material and reduction of power converter stages. MTB DC-DC converters can be considered as an interesting compromise between non-modular and modular DC-DC converters since they are themselves modular in the construction. This eventually leads to some fault-tolerant capabilities since the multiwinding transformer (MWT) couples multiples cells to a common core and in case of a failure the faulty cell can be isolated and the healthy ones can still keep operating. However, it is exactly the MWT that creates most of the technical issues of the MTB DC-DC converters because of cross-coupling effects among the cells, a maximum number of windings, and stray component deviations, which might imply a challenging design. Thus, this paper evaluates the MTB DC-DC converters for Solid-State Transformer (SST) applications, comparing them by means of Figure of Merits (FOM) and with the support of simulation and experimental results. Finally, the outcomes are gathered in a qualitative comparison among the MTB topologies in terms of their potentials and challenges.

Voltage controlled voltage source converters (VSCs) have been widely applied in microgrids, uninterruptible power sources, smart transformer and 400 Hz ground power units for airplanes, etc. An LC filter is generally adopted to attenuate high frequency switching harmonic and to improve the qualities of output voltage and current for grid or loads. Nevertheless, VSCs have to trade-off between the stability characteristic and the ability of switching harmonics suppression when a single-loop voltage control method is adopted. In general, the resonant frequency ωrof LC filter should be less than 1/4 of sampling frequency ωsto ensure sufficient attenuation of the switching harmonic. However, ωrshould be higher than 1/3 of ωsfor the system stability when a proportional-resonant (PR) controller with a positive proportional gain is implemented. This paper proposes a feedback of modulation voltage (FMV) control design method for single-loop control to ensure stability condition in a higher frequency range and good switching harmonics suppression at the same time. Finally, simulation results are provided to verify the effectiveness of the proposed method.

A smart transformer (ST) based meshed hybrid distribution network is realized by extending ST low voltage dc (LVDC) link to form a LVDC line which connects dc buses of existing distributed generation (DG) converters. This paper proposes a method for power loss minimization in such ST based meshed hybrid distribution network. A CIGRE LV residential distribution network with DG sources is connected at ST LVAC and LVDC terminals. An online optimal problem is formulated for determining the active and reactive power references of DG converters for minimized power losses in line. The total power loss minimization at each time is chosen as the objective function and solved using genetic algorithm. The total loss in the system is the sum of both LVAC and LVDC line losses. This total loss is calculated using the backward forward sweep power flow method. The proposed method is tested in MATLAB.

As the modular multilevel converter (MMC) becomes more used in medium voltage applications, its usage along with wide band gap devices, with low conduction losses and high switching frequency capability, seems promising. Currently it is not rare to see these converters fully equipped with SiC-MOSFETS in their structure, even tough the costs still can be prohibitive. To investigate this issue, this paper presents an analysis and discussion of the application of different devices technology for the cells of a Modular Multilevel Converter, e.g. SiC MOSFETS and IGBTs. The study addresses the different loading of the devices and how to better apply and select them considering different MMC applications. To further increase the efficiency of the topology, an analytical comparison is realized to evaluate the losses in different technology devices in parallel. At the end, a cost/benefit analysis is presented.

The Modular Multilevel Converter represents a really promising solution for several high-power applications. Nevertheless, to furtherly enhance the penetration of such conversion structure and to fully exploit its advantages, some issues related to its operation need to be addressed. In particular, this converter features some circulating currents that deeply affect both the conduction losses occurring in the semiconductor devices and the voltage ripple across the capacitors of this conversion structure, which in turn influences the converter efficiency. In order to minimize the converter losses, this work proposes innovative control strategies of the circulating currents by calculating the optimal value of their reference. Alongside the increase of the converter efficiency, the proposed control strategies allow additional advantages in terms of enhancing the converter reliability and extending the overall system lifetime.

The stringent regulations on the power quality declared in the standard IEEE 519-2014 push the companies and the power producers to install active filters to compensate the voltage harmonics distortion in the point of common coupling (PCC). However, in the case of a Photovoltaic (PV) park, the cost for an additional active filter converter can be saved by using the PV converters themselves as active filter. This solution is very attractive, but reserves several challenges. In fact, the harmonic current injection by the PV converters can generate ripples in the DC link which increases the stress on the converter components and affects the MPPT. Moreover, the overcurrent protection of the PV converter must be taken into account. In this paper a centralized optimized strategy to share the harmonic current injection among all the converters in a PV park is investigated. The optimization is formulated as a quadratic programming (QP) problem: the active power consumed by the PV park for the active filtering and the DC link ripple of the PV converters caused by the harmonic currents injection are minimized. The limit on the maximum injectable harmonic current by each PV converter are respected.

The ongoing electrification of the transport sector raises demands for new power electronic solutions. As a consequence, modular converter structures are state of the art for fast charging, since high power and short charging times are required. This work presents a modular multiport DC-DC converter which has the capability to decrease the cost and size of the required charging station. Furthermore, design guidelines for the investigated topology are presented. The analysis is validated with a three port isolated DC-DC converter with separated loads.

In medium/high power applications including smart transformers, active power filters and wind turbines, parallel converters are a reliable solution to achieve low harmonic distortion, high efficiency and increased power handling. However, if such topology has a common dc-link source, the circulating current arises between the converters, increasing power losses of the switching components and propagating the stress of the dc-capacitors. This paper provides comparison of four most commonly used pulse-width modulation methods and their influence on the circulating current and the quality of the output current. The efficacy of the investigated method is validated by experiments.

The quasi discontinuous conduction mode (QDCM) of the double active bridge (DAB) is addressed in this paper. The DAB converter usually is operated turning on always two semiconductors per bridge, and this leads to a continuous inductor current: However, a similar operation to dc/dc converters can be implemented, for this, less than two switches are turned on, at certain time, in one of the bridges; this leads to the quasi-DCM. With this mode of operation, a natural soft-switching is performed during the whole range of operation, but also low current stress is performed compared to the square modulation, not only at low power range. The modulation is relatively simple compared to other techniques. The proposed technique is described, analyzed, numerically simulated, and experimentally tested.

A modular Solid-State- Transformer (SST), typically consisting of cascaded H-Bridge (CHB) cells and isolated dc-dc converters, is considered as one of the most promising candidates for grid integration of large-scale PV systems (from MW to GW). However, the power imbalance among different modules and phases limits the operation range of SST. In this paper, a new common-dc-bus SST architecture, which consists of boost dc-de converters, isolated dc-dc converters and cascaded H-bridge cells, is proposed to solve the power imbalance issue. Power losses in the conventional and proposed common-dc-bus SST architectures are compared. Considering the characteristics of the proposed architecture, Zero Voltage Switching (ZVS) and the reactive current ratio of the isolated dc-dc converters are studied in detail. Simulation results of a three-phase 7-level large-scale PV system verify the effectiveness and feasibility of the proposed architecture.

In Electric Vehicle (EV) application, the in-wheel motors are driven by the same dc-link voltage. To reduce the space, the Voltage Source Inverters (VSIs) can share one dc-link capacitor. Long-term experiences and experimental data demonstrate that the capacitors are one of the major failures in the motor drive system. The current profile of the dc-link capacitors is the main factor for this degradation, therefore, by reducing the current harmonic components of the dc-link capacitors can increase the lifetime. In this paper, a dc-link capacitor current harmonic reduction method for dual Permanent Magnet Synchronous Motor (PMSM) drive system in EV application is proposed. The Phase Shift Pulse Width Modulation (PSPWM) is employed in the dual inverters to reduce the harmonic components of dc-link capacitor. To verify the proposed method, a experimental platform is established and the experimental results are presented.

The power electronics-based Smart Transformer (ST) can increase the hosting capacity of Distributed Generators (DGs) as well as electric vehicle charging stations and potentially reduce the reinforcements in the distribution grid by providing the DC connectivities and optimizing the capability in the secondary substations. Due to the asynchronous behaviour between the power generation in DGs and load consumption, the ST will operate in asymmetrically bidirectional power flow conditions. The power electronics in the ST can be fully used under the forward power flow condition, while only be partially used under the reverse power flow condition. This paper proposes a Bidirectional-Asymmetrical DC/DC converter in ST to reduce the cost of the power semiconductors and to satisfy the ST operating in bidirectional requirement. Under forward operation mode, the power sharing between two paralleled full bridge rectifiers in the secondary side of the proposed converter is studied and then the output impedance reshaping solution is presented to improve the dynamic performance of the output voltage under load disturbances. The simulation results clearly validate the effectiveness and feasibility of the proposed converter.

In high power applications such as wind energy systems, smart transformers and active power filters, parallel inverters are usually utilized in order to obtain high control bandwidths for achieving low harmonic distortion or for providing grid services. This paper provides analysis of the maximum frequency of the current that can be injected by parallel neutralpoint clamped (NPC) inverters based on proportional resonant control without facing stability problems. A lead compensation is proposed to increase the control bandwidth for improved harmonic control and for a wide frequency range, which can be controlled by the parallel inverter system. The analysis is validated by simulations and experiments.

In the More Electric Aircraft framework, the objective is to increase the efficiency and the reliability of the aircraft by gradually substituting the existing subsystems with electrical ones. Newer aircrafts already incorporate a great share of electric power, making the on-board electrical power distribution system (EPDS) a complex structure composed of generators, actuators and storage. Because of the importance of power electronics in this scenario, the control system is critical to ensure the stability of the EPDS. This paper analyzes the sensitivity to the control parameters of the EPDS stabiltiy. A state-space model is derived and the root loci are used to show in a graphical way the effects of the design parameters.

The increasing penetration of Distributed Generators (DG) in the modern electric distribution network poses high priority on the problem of the stability. In this article the Harmonic Stability of a Smart Transformer-fed microgrid is investigated under different control strategies. The considered microgrid is composed by a Smart Transformer and three Distributed Generators, considering the bandwidth of the DGs unknown. The robustness is evaluated analysing the eigenvalues as a consequence of a variation of the DGs bandwidth. The system is modelled as a Multi Input Multi Output System (MIMO); the eigenvalue based analysis is carried out to assess the stability and compare the robustness of the traditional double-loop PI and a state-feedback (SF) integral controller. The results show that the SF controller ensures a higher robustness than the traditional PI controller with respect to increasing bandwidths of the DGs.

Considering the increase in renewable generation and the consequent reduction in power system inertia, the Virtual Synchronous Machine (VSM) control method has been proposed to control power converters to emulate the inertia and other the characteristics of the synchronous machine. However, to achieve the function of VSM control, an extra energy base, typically storage, is required to connect to the controlled converter. In this work we investigate the application of the VSM control to the distribution system demand through the use of a VSM controlled smart transformer. Through control of the demand in this way, the demand itself can be used to emulate inertia and provide frequency support. This paper presents the details of the flexible demand control applied to a smart transformer supplying a low voltage distribution grid. The operation of the control is validated on scaled hardware using real time simulation with hardware in the loop. Simulations on a 400 kVA, 400 V distribution network are used to quantify the demand flexible. IEEE 39 bus is used to verify the benefit of the proposed control in terms of voltage and frequency in the power system.

The aim of reducing charging duration and the expected growth of the number of electric cars leads to a variable and high power demand on the current and near future fast charging stations (FCS). Currently, the voltage adaption from the MV level to the FCS is provided by a bulky 50/60 Hz transformer, leading to limited possibilities in the scalability in terms of charging power of the FCS. Therefore, in this paper three different modular possibilities to realize these systems by directly connecting a power electronic based solution to the medium voltage (MV) grid are investigated. The necessary isolation is realized in the DC-DC stage by the medium frequency transformer of the quadruple active bridge, which, as it will be shown, can be connected in different configurations. The prospective best solution, the cascaded H-bridge + interphase quadruple active bridge, is analyzed in more detail. The benefit of this architecture is the direct MV connection possibility and simultaneously natural power balancing among the three phases. Furthermore, the architecture provides modularity and scalability with respect to the desired power level of the charging station.

This paper highlights the specific design challenges faced while implementing a GaN based phase shifted full bridge (PSFB) converter for battery charger applications. Inherent with the PSFB is the voltage ringing across the secondary side devices. This effect becomes even more severe, when fast switching GaN devices are used. Especially the effects of parasitics in the circuit become a major problem and need to be considered. Effective solutions to mitigate the overvoltages on the secondary side devices are theoretically derived and practically implemented in a 1.4kW GaN based PSFB. The effectiveness of the different solutions is presented.

Microgrids (MGs) with large penetration of photovoltaic (PV) based distributed generation (DG) sources require improved power flow control methods due to the intermittent nature of renewable energy production. In this paper, a flexible power distribution system is proposed which introduces a medium voltage (MV) dc link connection between two smart transformers (ST) with a common battery energy storage system (BESS) configuration. The MVDC connection is achieved without extra investment in converters and this facilitates a flexible way of transferring active power between MGs. The dc power transfer offers a solution with lower power loss in comparison with three phase ac system. This also improves the overall reliability of the system by ensuring the power supply to the loads during ST MV converter failure. PSCAD simulation results are discussed to show the merits of the proposed configuration.

In the last years, the electrical grid has seen a deep change in the power production share between conventional generators, such as coal and gas turbines, to renewable ones, like wind and photovoltaic power plants. These resources are characterized by a power electronics-based interface to the grid. However, these resources do not offer any rotational inertia to the grid, that, in case of large disturbances, can suffer of large frequency deviation. Several strategies have been proposed in literature for damping the frequency deviation, mostly involving HVDC systems. The HVDC can request active power to the DC-connected AC areas in order to provide the needed energy to damp the frequency variation. However, all these strategies do not investigate the impact of the power variation on the frequency in the other DC-connected AC areas, that may result large if these system's inertia is low. This paper proposes a control strategy for minimizing the impact of the power request in these DC-connected AC areas varying the power consumption of near voltage-sensitive loads by means of a controlled voltage variation. As demonstrated in this work, the controlled voltage variation is able to reduce the frequency swing in these AC areas, matching the power variation request for frequency support with the power variation achieved from the voltage-sensitive loads.

Nowadays, the design of power converters has been focused on the optimization in terms of reliability, fault-tolerant capability and power density. The use of modular converters leads to natural fault-tolerant capability because the power system can be still operating even when some module has failed. In this paper, the reliability of modular converters, which is high-dependent of several factors like as thermal stress, is studied. In the paper, the power devices aging of dc/dc interleaved power converters is managed. The proposed method is based on sharing conveniently the total managed power between the power modules. This can be done without losing performance in the power system. The proposed power routing technique has been tested in a three-module interleaved boost converter which is used in multiple applications.

A simultaneous wireless information and power transfer system employing separated frequency bands for energy and data, dubbed dual-band SWIPT, is investigated. Elementary circuit elements are optimized numerically. An experimental testbed based on a gallium nitride (GaN) full-bridge converter demonstrates data rates in excess of 450 kbps employing on-off keying in conjunction with a simple diode detector for data recovery.

DCIDC battery chargers used in industrial electric vehicle applications are designed to be size, weight and power efficient. Wide bandgap (WBG) devices which enable high frequency operations help reduce size of magnetics as well as the converter volume achieving better efficiencies compared to Si based solutions. It has not been often discussed in literature but in case of battery chargers the overall saved energy becomes a better performance indicator than the efficiency at rated power alone. This is due to their peculiar charge/discharge characteristics. In this paper, the design of a GaN (Gallium nitride) converter for an industrial battery charger application is studied. An optimal design procedure aiming at optimizing the converter energy utilization efficiency (EUE) over the complete state of charge (SoC) as well as minimizing BOM (Bill of Material) costs due to magnetics is presented in order to highlight the energy perspective. The results show considerable energy savings compared to conventional design methods.

In this paper the Feedback Linearization (FL) control theory is applied to a grid-connected Cascaded H-Bridge (CHB) converter. A DC voltage control is designed taking into account the nonlinear model of the converter and its performances are compared with the performances obtained in case of a conventional PI controller. With the aim to provide a fair comparison, the results are evaluated in terms of: settling time in case of a reference voltage variation, voltage overshoot in case of a load variation and total harmonic distortion of the grid current. Simulation and experimental results demonstrate that the FL-based control provides higher power quality and faster response to reference variations than the PI control. Comparing the performances in terms of responses to the load variations, better dynamics performance can be achieved also with the FL-based control increasing the settling time while maintaining an admissible grid current total harmonic distortion index.

Modular power converters offer reduced dv/dt, improved power quality and higher redundancy. Normally, the power is shared equally by the individual cells of a modular system. However, differences in electrical and thermal parameters of the individual cells result in a non-optimal efficiency and reliability characteristics with equal power sharing. Therefore, an unequal internal power sharing strategy, called power routing, has been proposed to improve primarily the system reliability. Nevertheless, a unified approach to route the power in modular converter architectures is missing. This work uses graph theory to represent various modular topologies for easier identification of power paths. Graph theory simplifies the power flow in complex modular architectures to a network flow problem, focusing on the reliability and efficiency optimization. The paper presents a power routing algorithm based on convex cost flow optimization for improving the reliability without sacrificing the efficiency. The proposed method is validated through case studies and the proof of concept is demonstrated experimentally.

Virtual Synchronous Machines (VSMs) have been proposed during the last decade as possible solutions for integrating the increasing amount of power electronics-based grid connected converters in the utility grid. One of their main characteristics is the ability to self-synchronize themselves to the main grid without the need of a dedicated unit, since they can reproduce the power synchronization mechanism of a synchronous machine (SM). Different control strategies have been proposed in the literature based on the aforementioned principle. The aim of this work is to provide a generalized formulation of a power synchronization based (PSB) control algorithm, outlining the different loops constituting the control and to give an overview of the solutions proposed in the literature. Similarities along with advantages and drawbacks of each formulation are highlighted in the paper and time-domain simulation in MATLAB/SimulinkIPLECS are performed to compare the performance of the different solutions.

During the last decade virtual synchronous machines (VSMs) have been intensively investigated and plenty of control strategies have been proposed as possible implementations. Some of the proposed control strategies aim to reproduce the inertia of a synchronous machine (SM), but rely on a dedicated unit for the synchronization to the main grid. However, one of the most appealing features of a SM is its ability to self-synchronize itself to the grid thanks to the intrinsic power synchronization mechanism. The Synchronous Power Control (SPC) is among the control strategies proposed in the literature based on the power synchronization mechanism. In this paper, the aforementioned control is deeply investigated. Its concept is first introduced and a small-signal model of an SPC-based grid connected converter is developed. Subsequently, a comprehensive parameter sensitivity analysis is performed in order to identify the effects of control parameters variations on the eigenvalues of the system under different grid conditions. Time-domain simulations in MATLAB/Simulink/PLECS confirm the validity of the developed frequency-domain model.

The Cascaded H-Bridge (CHB) converter is an appealing solution for large-scale grid-connected photovoltaic (PV) systems. However, the phase power imbalance is one of the main challenges due to the non-uniform irradiance, unequal temperature and parameter mismatch, which may result in imbalanced grid current. The interphase Quadruple Active Bridges (QAB)-based CHB (CHB-QAB) converter is one of the promising converter architectures to overcome the issue of the phase power imbalance. Nevertheless, the existing control strategy for the interphase CHB-QAB PV systems can not ensure the phase power balancing when the mismatch exists in the CHB stage. If the sinusoidal zero-sequence voltage injection (SZVI), the conventional control strategy can exchange the power among three phases to keep grid currents balanced. However, in the condition of high mismatch, the control strategy with SZVI may cause over-modulation. In this paper, a control strategy for the interphase CHB-QAB PV systems is proposed, which can ensure the phase power balance as well as individual MPPT, even in the condition of high mismatch. Based on the derived average and small-signal models of the overall system, the control system is designed. As a consequence of the proposed control strategy, the bandwidth of the dc-link voltage controller can be increased for reducing the dc-link voltage ripple of H-bridge cells. This enables to reduce the dc-link capacitance. Simulation results validate the effectiveness and superiority of the proposed control strategy.

The converter on the low voltage side of a Smart Transformer often has to supply unbalanced loads. The consequent neutral current flows through the DC link electrolytic capacitors in a DC-split configuration and hence represents the main cause of their overheating and of their premature failure. Since this kind of unbalanced operation is a very frequent condition to be faced, one way to extend the capacitor lifetime and the system reliability is to limit the neutral current through a zero-sequence current control. Nevertheless, this technique produces a large deviation of the PCC voltages, which no longer meet the limits set by the standards. To this purpose, the present paper proposes a zero-sequence injection technique that can achieve the best trade-off between the capacitor lifetime extension and the quality of the voltages at the PCC.

Unlike the conventional Uninterruptible Power Supply (UPS), which generally is connected to the scheduled critical loads (data centers, chemical plants and hospitals), the Smart Transformer (ST) experiences a complicated distribution grid with high penetration of Distributed Generator (DG) and nonlinear loads, leading to ST-fed distribution grid suffering from more power quality issues, due to harmonic interaction among DG, nonlinear loads and ST. Based on the CIGRE LV 50Hz distribution grid model, this paper systematically studies the impact of distribution grid integrated with DGs and nonlinear loads on grid voltage power quality by comparing various voltage FeedForward (FF) (reference and measured voltage FF), various current FeedBack (FB) (filter inductor and capacitor current FB), and various current controller (proportional and proportional plus resonant controller). In order to satisfy the standard EN50160 requirement on voltage quality, an improved voltage control strategy is proposed to improve grid voltage quality. The simulation results based on the CIGRE model clearly validate the effectiveness and feasibility of the proposed voltage control strategy.

The more electric aircraft concept is pushing towards a DC distribution system for the on-board electrical appliances. In the scenario where multiple high-voltage and low-voltage buses are present, a power electronics converter interface is mandatory. In order to implement a resilient distribution system, the power exchange between different buses is envisaged, to ensure that a failure in a supply bus does not impair the rest of the distribution system. In this paper, two power converter structures able to achieve this goal are analyzed and compared in view of their contribution to improve the overall system resiliency. A control structure is investigated to manage the load supply with the desired priority. The variable virtual resistor control allows shifting the load priority on demand and, as a consequence, is found suitable for achieving the desired goals. Theoretical analysis and experiments are reported to prove the performance of the chosen solution, which is based on a QAB controlled with virtual resistors.

The focus of this paper is on power flow control in the asymmetric quad-active-bridge (QAB) converter and its limitations imposed by uneven values of parasitic elements in the isolation transformer. The QAB converter under study is intended for solid-state transformer applications in asymmetric configuration, where one of the ports is placed at the low voltage side and rated for the threefold power of the medium voltage ports. Significant imbalance between leakage inductance could be observed between them. The steady-state analysis of the asymmetric QAB converter and assessment of power losses using numerical simulation are provided. Experimental waveforms were obtained using a prototype rated for cumulative MV dc link of 2400 V and power of 30 kW.

This paper presents a three-phase converter architecture with a reduced common mode voltage to be used in photovoltaic power systems. The full-bridge architecture is modified by adding two additional switches in the DC path that allow decoupling the load from the source during the freewheeling phases. Optimized modulation strategies are analyzed and compared against the space vector modulation for the full-bridge. Extensive measurements show an improved efficiency when SiC devices are used, and a theoretical analysis links this efficiency to a marked extension of the converter lifetime.

Reliability of power electronics is a critical issue, as most of the electrical energy is processed by power electronics. Various stressors impact the safe operation of the systems, including harsh environments, temperature variations, humidity, vibration and radiation. Physics of failure analysis uses models that describe how failure mechanisms evolve over time and induce failures. Active methods help to increase the reliability during operation. These methods rely on intelligent control that help to avoid operation conditions that affect stress that lead to failures. This paper provides an overview on recent active methods to increase the reliability of power electronics and categorizes them according to their impact on reliability, invasiveness to system operation and open research opportunities. An industry perspective taken from a survey in the end of 2016 is included to rate how promising the different methods are considered.

With increased penetration of renewable energy sources in electric grid, maintaining quality voltage at the load terminal has become a challenge. Voltage disturbances like sag, swell, harmonics, etc., can propagate from one section to another section of electric grid and degrade the performance of sensitive loads. This paper explores the capability of smart transformer (ST) to improve the voltage quality in medium voltage (MV) grid, by replacing one conventional power transformer (CPT) with an ST in a multifeeder electric grid. In addition to supplying ST low voltage loads, the voltage regulation in MV grid is achieved by controlling the reactive current injection to the MV grid. Harmonic damping capability is integrated with the control loop to damp out the grid voltage harmonics. An improved stability of MV grid voltage regulation control is achieved in the presence of harmonic damping loop and this is verified with theoretical analysis. Extensive simulation study is carried out to validate the ST features.

Integration of charging stations for electric vehicles (EVs) will result in severe stress on electric grid. The EVs connected to the grid can also act as a source of energy and this demands greater control as well as coordination for better utilization. Conventional power transformers (CPTs) utilized in the EV charging stations are not well equipped for meeting these challenges. Recently, smart transformer (ST) has been proposed as an alternative to CPT. The ST connects medium voltage (MV) grid to low voltage (LV) grid with two intermediate MV and LV dc links. This paper utilizes the availability of the dc links of the ST to integrate the EVs charging station to electric grid rather than connecting these through the CPTs to the grid. A comprehensive analysis of sizing of the power converters utilized in the considered system has been presented. A comparison of efficiency for the different configurations is carried out and the possible more efficient scenario is suggested. Infact, the ST based EV charging system is found to be more efficient in comparison to CPT based EV charging station. The performance of ST based EV charging station is verified through simulation studies.

Harmonics and load unbalance create several adverse issues in power distribution system, which are generally mitigated by bulky shunt active power filter (SAPF). This paper explores load compensation features of smart transformer (ST) in a two-feeder city center, where conventional power transformer (CPT) in one feeder is replaced by the ST whereas other feeder is continued to be supplied through the CPT. The ST is controlled not only to supply its own loads connected at the low voltage side, but also to compensate reactive, unbalance and nonlinear loads of the other feeder. This makes total MV grid currents of the combined city center balanced, sinusoidal, and in unity power factor at the point of common coupling (PCC) voltage. Therefore, in addition to providing continuous and reliable support to the ST based loads, the ST eliminates need of bulky SAPF and isolation transformer from the city center which are used for achieving reactive and harmonic current compensation. A switching model of the system is developed in power system computer aided design (PSCAD) software and implemented on a developed experimental prototype to show the capability of the ST.

Interactions between power electronic converters are typically investigated by means of impedance-based model analysis. The effects of the PLL are usually neglected, but recent studies have shown the importance of including those effects for stability analysis purposes. This paper focuses on the investigation of interactions between synchronization units of converters operating in parallel. The impedance-based approach is used for the investigation in the frequency-domain. The converters are supposed to share the same point of common coupling (PCC) and the characteristic of the grid at the connection point is crucial for determining the effects of the interactions between the operating units. In order to isolate the effects of the synchronization, the converters is modeled as a current sources, whose reference currents are calculated using the angle detected by their PLLs. Time-domain simulations are performed as well as experimental laboratory tests in order to validate the presented analysis.

Distribution grids unbalance challenges the DC-link of the Smart Transformer leading to higher ripples and then the lifetime of the DC aluminium electrolytic capacitors (AEC) is reduced. Additionally, dead time and nonlinearity characteristic of IGBT also may introduce ripple current in DC link, potentially decreasing the lifetime of AEC. Based on physical characteristic of AEC, the lifetime estimation of AEC is investigated considering aforementioned impacts. To suppress these impacts, an improved voltage control strategy that can reduce AEC current ripple and then preserve AEC lifetime is proposed. The theoretical analysis has been demonstrated by the simulation results.

Parallel Neutral-Point Clamped (NPC) inverters are commonly employed in medium voltage high power applications, such as wind energy system, smart transformer, and power conditioners. In literature, it is shown that the Phase Disposition modulation (PD) strategy provides better performance in terms of harmonic profile and current ripple for single NPC application. Therefore, the PD has been commonly employed as an optimal solution and used for optimized Space Vector Modulation (SVM). However, in case of interleaved operation and common dc-link, one of the main challenges is: the circulating current between inverters. With this consideration, comparisons between the PD and another carrier-based modulation - Alternative Phase Opposite Disposition (APOD) are carried out based on the space vector representation. The analysis shows that the APOD modulation offers much lower circulating current compared to the PD modulation, maintaining similar current ripple as well as spectrum quality of the total current. As a result, an APOD-based SVM technique is proposed in this paper, in order to reduce the circulating current and meanwhile maintain other superior characteristics. Full switching sequences as well as design criteria are presented for all sectors. Simulation and experimental results are provided to validate the effectiveness of the proposed modulation technique.

The Smart Transformer (ST) is a solid-state transformer performing not only a voltage step-down function but also energy and information management. With the help of control and communication technology, the ST can increase hosting capacity of renewable energies and provide ancillary services to utility and customers. To better exploit the potential of a ST-fed grid, a proper design of architecture, control scenarios and its corresponding communication network has to be done. This paper aims at giving a better understanding of the functionality of the ST-fed grid and of the associated communication challenges.

Power Semiconductors undergo thermal stress during operation, which is caused by thermal cycling. This refers to heating up and cooling down of the junction temperature and leads to aging and finally failures. Active thermal control can reduce the thermal stress of power semiconductors by regulating the power losses. Most algorithms proposed so far are only applicable to hard switching power converters and implement a regulation of the switching frequency, which is not applicable for the isolated DC/DC converters. This work proposes to regulate the duty cycle of the DC/DC converter to control the semiconductor losses, aiming at influencing the consequent thermal stress. The proposed algorithm is analyzed analytically and a thermal controller is designed, which is capable of reducing thermal cycles during operation without prior knowledge about the mission profile. The capability and the limitations of the proposed algorithm are demonstrated with simulations and the effectiveness is validated on a laboratory prototype with junction temperature measurement.

The Smart Transformer (ST) represents an enabling technology for providing new services to Low Voltage (LV) and Medium Voltage (MV) grid, overcoming the problems that are limiting the widespread diffusion of distributed renewable sources in conventional distribution systems. The ST 3-stage configuration includes a DC/AC 4-wires converter at the LV side, that must be able to cope with the equipment already installed in the LV grid in which ST wants to substitute conventional distribution transformer. Grid converters with capacitance towards ground, when connected to ST-fed LV grid can cause common mode current problems not yet discussed in the literature. This paper aims to fill this gap through an in-depth analysis of the problem and indicating possible solutions.

The Smart Transformer (ST) offers ancillary services in the low voltage grids. These new services in a real distribution grid can be demonstrated by means of simulation software, but it is difficult to prove them in lab due to the complexity and heterogeneity of the distribution grids. The Power-Hardware-In-Loop (PHIL) evaluation is an interesting solution for emulating the interaction between the real grid, simulated in a Real Time Digital Simulator (RTDS), and the ST. However the PHIL evaluation requires careful analysis. Depending on the interface topology the stability and the accuracy of the loop can be affected. This is mostly true when the interface converter is not a linear power amplifier, but just a simple DC/AC switching converter. Actually, no considerations have been made in the literature about the accuracy and stability of the PHIL in case of non-linear interface converter. This paper focuses on evaluating the harmonic behavior of the PHIL in case of current harmonic content in the grid. Depending on the bandwidth of the interface converter, the accuracy of the PHIL in simulating harmonic currents can be affected. The analytical considerations on the accuracy of the loop have been verified experimentally in a complex LV grid.

The massive integration of decentralized energy resources (DERs) poses a big challenge on the grid management. The resulting problems often require advanced power electronics based solutions like active filters or STATCOMs or even more futuristic system level solutions like smart transformers. The interaction of all the mentioned systems through grid connected converters originate stability problems. The identification of the system impedance or directly of the possible resonance and the consequent implementation of flexible and tunable damping solutions like notch filters, is discussed in this paper with reference to an already present measurement campaign. The results of an already developed 1MVA power converter are shown. Meanwhile, a project is carrying to realize a multi megawatt medium voltage grid impedance analyzer. In this paper different scenarios are analyzed in simulation and experimentally.

The system resonance frequency of a grid connected inverter with LCL filter depends on the filter parameters and the time varying grid impedance. To obtain stability and to preserve good dynamic behavior of the system, grid impedance adaptation of the current controller is proposed. The design of the current controller is described and the adaptation strategy to prevent the excitation of the system resonance frequency during grid-impedance changes is introduced. The actual grid impedance is estimated by an extended Kalman filter (EKF) and a Luenberger observer allows to use the same number of voltage and current sensors as in conventional PI controllers. The proposed control method provides the possibility to tune the controller dynamics independently from the grid impedance. The theoretical analysis is validated with laboratory measurements.

Thermal cycling is one of the main sources of aging and failures in power electronics. A possibility to reduce the stress to semiconductors is to control the amount of losses that occur in the device during operation. This work presents an active thermal controller that aims at reducing the junction temperature variations in the case of variable power profile. The switching frequency of the converter is the parameter that is affected by the active thermal control, while the operation of the converter remains unchanged. The novelty of the approach is that the switching frequency variation is exploited to prevent excessive cooling down of the semiconductor during a power reduction. A thermal model is used to estimate the losses, so the prior knowledge of the mission profile is not needed. The results of the proposed solution are validated with an experimental prototype and a wide-bandwidth temperature measurement system directly applied to the semiconductor chip. Finally, the impacts of the controller on the module's lifetime is estimated.

With the significant increase in renewable energy sources (RES) and storage elements, power distribution system needs new services for reliable and continuous operation. Smart transformer (ST) based on power electronics, control, and communication has been proposed for improving the performance of power system. However, operational capabilities and applications of ST with RES and energy storage in distribution system are still not explored in detail. In this paper, a new prospective of ST application, dual microgrid (DMG) operation, is proposed. In normal medium voltage (MV) grid operation, the ST supplies power to loads connected to its low voltage side and injects into the MV grid depending upon the available RES and load demand. During the voltage disturbances, the ST feeds power to the sensitive loads connected to the other feeders equipped with the conventional power transformers. In this case, the distribution loads are isolated from the MV utility grid. This ability of the smart transformer to isolate from the utility grid and provide continuous power to the sensitive loads of other feeders has potential to significantly improve the performance of power system as well as ST. With simulation results based on PSCAD/EMTDC software, this paper introduces for the first time an unexplored feature of the smart transformer.

Compared with the traditional microgrid, a smart transformer (ST)-based microgrid shows advantages in terms of higher efficiency, increased hosting capacity, and enhanced reliability. However, this novel microgrid presents challenges because of the complex resonances within the system and the interactions between ST and power converter-based distributed energy resources (DERs). Both the ST and DERs might be severely affected by each other and leads to performance degradation and system instability. To solve these problems, this paper first develops equivalent circuits of a ST-based microgrid. Then, the resonance and interaction problems are investigated based on the equivalent circuits. The active damping method is used to fully address the resonance issue and mitigate the coupling effects. To guarantee superior performance, this paper also designs high-performance control strategies for both the ST and DERs. Simulation and experimental results are provided to verify the validity of the proposed control strategy.

Smart grids have attracted the attention of the scientific community for years, since the ability to better manage distributed resources in a dynamic grid scenario is of paramount importance. Among the solutions that have been investigated, there is the smart transformer (ST) distribution concept, where an AC/AC power converter substitutes the traditional distribution transformer while enabling additional services. Considering that the ST can generate arbitrary voltage waveforms, power distribution is not restricted anymore to the fundamental frequency, but additional harmonics can be used. This paper analyzes different control possibilities for multi-frequency converters, comparing the results in terms of dynamic performance, control complexity, and communication requirements.

This paper discusses the effect of Constant Power Loads (CPLs) on the stability of a Smart Transformer (ST). The main characteristic of a CPL is that its current decreases when its voltage increases and vice versa. Therefore, in small signal analysis a CPL behaves as a negative impedance, and as a consequence it can impact the system stability. The aim of this work is to establish how it is possible to stabilize the system by acting on the filter parameters and the control strategy. A distributed power generating systems supplied by a ST has been considered as case study. The analysis and the simulation results show that it is not possible to stabilize the system by using standard controllers without an appropriate selection of the output filter components.

The future concept of universal operation embraces those distributed power generation systems (DPGS) capable of getting disconnected from the main grid to start operating within an island. Once the cause of disconnection is solved, the DPGS is able to resynchronize and reconnect with the grid. In these conditions, the universal operation can be successfully implemented through a voltage control. Voltage control is manly adopted in uninterruptible power supply (UPS) and island applications. However, a study that solely focused on the universal operation is missing. The goal of this work is to help in the selection of the most suitable control structure as function of: 1) power dynamics, power quality, and robust stability in grid-connected; 2) voltage regulation and performance under local load steps in island mode; and 3) behavior during the reconnection with the grid. For that purpose, five architectures have been reviewed within a general mathematical framework and with experimental results.

The smart transformer, a solid-state transformer with control and communication functionalities should provide services to the grid. This paper proposes to use a different frequency respect to the fundamental frequency, to provide such services to the distribution grid. The approach has the main benefit to transfer energy from point to point of the grid, exploiting the lower impedance path that multi-frequency converters offer. This paper describes the control strategy of the multi-frequency converters, and verifies their impact on distribution grid.

Transformerless (TL) topologies are employed in 1φ PV inverter topologies due to their small size and low weight. Avoiding the grid side transformer requires the modulation technique and the basis topology to be accordingly changed in order to mitigate dc current components in the grid side and the leakage current to ground. This paper carries out a sensitivity analysis of selected TL topologies. This analysis investigates the impact of parameter variations of the employed semiconductor devices and detects the device which affects the most the overall efficiency. As a result, relevant info for engineers selecting the most suitable power devices for the implementation of a certain TL topology is provided.

Due to the possibility to drastically reduce the Solid State Transformer (SST) volume and weight, its use is becoming a reality in traction and wind power plant applications, while, in the electric distribution system, it is still considered futuristic. A SST, with managerial role in the electric distribution grid, is generally called Smart Transformer (ST). Unfortunately the low efficiency, the low reliability and the high cost still act as barriers for its widespread use in the real world. This paper focuses on the impact of a modular design, by benchmarking different ST topologies. Moreover, the paper provides guidelines on how to choose the semiconductor modules and assessments on how the choice affects the efficiency of the ST.

Grid-connected converters employ LCL-filters, instead of simple inductors, because they allow lower inductances while reducing cost and size. Active damping, without dissipative elements, is preferred to passive damping for solving the associated stability problems. However, large variations in the grid inductance may compromise system stability, and this problem is more severe for parallel converters. This situation, typical of rural areas with solar and wind resources, calls for robust LCL-filter design. This paper proposes a design procedure with remarkable results under severe grid inductance variation. The procedure considers active damping using lead-lag network and capacitor current feedback. Passive damping is also discussed. The design flow, with little iteration and no complex algorithms, selects the proper ratios between the switching and resonance frequency, the grid and converter inductance, and the filter capacitance and total inductance. An estimation for the grid current total harmonic distortion (THD) is also proposed. Simulation and experiments validate the proposals.

Three-phase active rectifiers guarantee sinusoidal input currents and unity power factor at the price of a high switching frequency ripple. To adopt an LCL-filter, instead of an L-filter, allows using reduced values for the inductances and so preserving dynamics. However, stability problems can arise in the current control loop if the present resonance is not properly damped. Passive damping simply adds resistors in series with the LCL-filter capacitors. This simplicity is at the expense of increased losses and encumbrances. Active damping modifies the control algorithm to attain stability without using dissipative elements but, sometimes, needing additional sensors. This solution has been addressed in many publications. The lead-lag network method is one of the first reported procedures and continues being in use. However, neither there is a direct tuning procedure (without trial and error) nor its rationale has been explained. Thus, in this paper a straightforward procedure is developed to tune the lead-lag network with the help of software tools. The rationale of this procedure, based on the capacitor current feedback, is elucidated. Stability is studied by means of the root locus analysis in z-plane. Selecting the lead-lag network for the maximum damping in the closed-loop poles uses a simple optimization algorithm. The robustness against the grid inductance variation is also analyzed. Simulations and experiments confirm the validity of the proposed design flow.

The temperature of a power semiconductor device is important for both its optimal operation and reliability. If the temperature is known during the operation of a converter, it can be used to monitor the health of power modules: a measurement of aging, scheduling of maintenance, or even implementation of active thermal control to reduce losses and increase lifetime can be performed given an accurate knowledge of temperature. Temperature measurements via thermo-sensitive electrical parameters (TSEP) are one way to carry out immediate temperature readings on fully packaged devices. However, successful implementation of these techniques during the actual operation of a device has not yet been achieved. This paper provides an overview of literature where the usage of TSEPs has been hypothesised or realised in realistic power electronic converter setups. Barriers and limitations preventing wider scale implementation of these methods are discussed. Their potential use in the aforementioned goals in condition monitoring and active thermal control is also described.

Active damping methods are used to guarantee the stability of LCL-filter-based grid converters. Many methods have been proposed in literature but a deep comparison of their dynamic performance is missing. The aim of this paper is to fill this gap and to provide a criterion to choose the best trade-off between dynamic performances and stability.

Grid converters require a simple inductor or an LCL-filter to limit the current ripples. The LCL-filter is nowadays the preferred solution as it allows lower inductance values. In order to solve the stability concerns, active damping is preferred to passive damping since it does not use dissipative elements. However, large variations in the grid inductance and resonances arising from parallel converters may still compromise the system stability. This calls for a robust design of LCL-filters with active damping. This paper proposes a design flow with little iteration for two well-known methods, namely lead-lag network and current capacitor feedback. The proposed formulas for the resonance frequency, grid and converter inductance ratio, and capacitance of the LCL-filter allow calculating all the LCL-filter parameters. An estimation for the achieved Total Harmonic Distortion (THD) of the grid current is also provided. Experimental results show very robust designs to the parameter variations.

The dual active bridge topology allows bidirectional power flow and galvanic isolation for DC/DC energy conversion. These features have made it the possible backbone of the future smart transformer for distribution. The different voltage drops and commutation dead-times of the semiconductor switches result in DC-voltage at the transformer terminals. Even small DC-voltage components produce large DC-bias currents as they are only limited by the transformer resistances. The DC-bias degrades the transformer performance by increasing the losses. If the core saturates the resulting current pulses can damage the converter. A typical approach to avoid the DC-bias is placing a capacitor in series with the transformer. This capacitor suffers large current variations, reducing its reliability, and complicates the control. The dual active bridge usually handles the power flow by modifying the phase-shift of the converter square waveforms. In this paper the duty-cycle of the converter waveforms is controlled to cancel the current DC-bias in both transformer sides. A formula relating the reference power and phase-shift is provided for the case of varying duty-cycle waveforms. The implementation with constant sampling frequency and its limitations are explained.

During last years an increasing attention has been paid to the efficiency of grid-connected PV inverters. They are manufactured from a number of discrete components and by using a certain topology and control strategy. Hence, the performance of a certain PV inverter not only depends on the selected topology and control strategy but also on the characteristics of the employed components. The aim of this paper is evaluate the effect of physical variations associated to the main components on the overall efficiency of PV inverters. It is concluded that a statistical evaluation of the power converter provides a better understanding of the PV inverter performance and, in this sense, the definition of the European Efficiency must be reviewed in order to show the quality of the manufactured product.

Wind speed variations make the power of wind turbine system to fluctuate, which could increase the thermal stress of the power converter and reduce its lifetime. In order to relieve this problem, short-term energy storage technologies are applied to improve the thermal performance of a 3 MW grid side wind power converter. The cost, weight and cycle life of the energy storage technologies are evaluated based on a typical low speed high turbulence wind profile. In detail, a wind turbine system model is established and its control strategy is illustrated, which is followed by the power control method of the energy storage system. Then the conventional thermal evaluation approach is simplified for evaluation with long term wind profile. The case studies are done to address the optimal power size and capacity of the energy storage system by comparing the improvement of the thermal performance. Also, the two promising candidates, ultracapacitors and batteries, are compared.

Modular multilevel converter (MMC) is the latest converter topology suitable for the transformerless applications in HVDC transmission. HVDC systems are required to remain connected during grid faults, provide grid support and completely decouple the healthy side from the faulty one. The MMC converter weak points are challenged by this particular condition and by these demands. This paper demonstrates the effect of negative and zero sequence current control in MMC-HVDC during asymmetric grid faults. A current limitation strategy for MMC is derived and verified through simulations.

Reliability is one of the key issues for the application of Insulated Gate Bipolar Transistors (IGBTs) in power electronic converters. Many efforts have been devoted to the reduction of IGBT wear out failure induced by accumulated degradation and catastrophic failure triggered by single-event overstress. The wear out failure under field operation could be mitigated by scheduled maintenances based on lifetime prediction and condition monitoring. However, the catastrophic failure is difficult to be predicted and thus may lead to serious consequence of power electronic converters. To obtain a better understanding of catastrophic failure of IGBTs, the state-of-the-art research on their failure behaviors and failure mechanisms is presented in this paper. Moreover, various fault-tolerant design methods, to prevent converter level malfunctions in the event of IGBT failure, are also reviewed.

In the field of wind energy generation particular interest has been focused in recent years on distributed generation through small wind-turbines (power unit 200 kW) because of their limited size and lower environmental impact. The field of small generation was dominated by the use of asynchronous generators directly connected to the grid, while recently permanent magnet synchronous generators (PMSG) with power converter, either partially or fully controlled, became popular. This paper reviews the control issues related to these small wind-turbine systems: generator torque control, speed/position estimation, pitch control, braking chopper control, dc/dc converter control, and grid converter control. Specific issues for small wind-turbines arise in the wind energy extraction optimization and limitation and in the innovative concept of “universal” wind-turbine operation, that leads these system to operate grid-connected, standalone or in load supporting mode.

This paper is focused on the control of small wind turbines, rated less than 200 kW, in grid-interactive microgrids. The considered system is based on a back-to-back power conversion stage. The management of the active and reactive power exchange in the different operating conditions is the key point to allow a practical integration of wind turbine systems in the main electrical grid. This issue lies in a proper control system design achieving high performance in grid-connection as in stand-alone operation modes. A cascade control loops structure is proposed in the paper. Some simulation and experimental results validate the control algorithm.

The future electric grid concept will cover some small parts to be disconnected and work in an autonomous way isolated from the main utility. Control of microgrids - composed by the couple distributed sources-local loads - with the competence of operating in grid-connected and island mode is a trending research area. The presence of an efficient algorithm for synchronizing the microgrid with the main grid every time the reclosure is allowed is crucial for assuring a safe operation. The synchronization system presented in this work is compounded by two elements: Dual Second Order Generalized Integrator (DSOGI) and stationary reference frames phase-locked loop (SRF-PLL). Besides, the voltage control of the microgrid plays a great role in the synchronization system performance.

The control of grid voltage and current source converters (VSCs and CSCs) in stationary reference frames by means of resonant controllers is a well-known issue. The corresponding sinusoidal control references should be also limited under anomalous conditions to assure the safe operation of the converter. Present saturators, designed for synchronous reference frames, limit the reference but not its amplitude or phase which distorts sinusoidal waveforms and changes the a priori active and reactive power references. This work is focused on developing different novel techniques achieving different grades of distortion and bandwidth in function of the control loop structure and system requirements. Tools like Kalman Filter (KF), second order generalized integrator (SOGI) and phase-locked loop (PLL) or vector calculus are employed to develop these saturators. Simulation and experimental results are provided to verify the saturators performance in several control systems.

Fast change of the irradiance conditions entails failure of the maximum power point tracking (MPPT) algorithm in a photovoltaic system (PVS). In this paper it is proposed the combination of a modified Perturb and Observe (P&O) distributed MPPT and an adaptive algorithm. The MPPT has been tested in case of hard mismatch conditions through simulation showing high performance in terms of efficiency.

Photovoltaic Systems (PVS) can be easily integrated in residential buildings hence they will be the main responsible of making low-voltage grid power flow bidirectional. Control issues on both the PV side and on the grid side have received much attention from manufacturers, competing for efficiency and low distortion and academia proposing new ideas soon become state-of-the-art. This paper aims at reviewing part of these topics (MPPT, current and voltage control) leaving to a future paper to complete the scenario. Implementation issues on Digital Signal Processor (DSP), the mandatory choice in this market segment, are discussed.