Showing papers by "Marco Liserre published in 2021"

Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review

Abstract: In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.

A Comprehensive Assessment of Multiwinding Transformer-Based DC–DC Converters

Abstract: Multiwinding-transfomer-based (MTB) dc–dc converter did emerge in the last 25 years as an interesting possibility to connect several energy systems and/or 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 nonmodular and a modular dc–dc converter since they are themselves modular in the construction. This eventually leads to some fault-tolerant possibilities since the multiwinding transformer (MWT) connects multiples ports and if one of them is not working anymore and it can be isolated, the others might still continue operating. Unfortunately, it is exactly the MWT that creates most of the technical challenges of this class of dc–dc converters because of the cross-coupling effects among the cells, which make especially the resonant-topology very challenging to be designed. This article reviews the history of the MTB dc–dc and then provides a classification of them, comparing them with figure of merits and focusing on which is the maximum possible number of windings and which are the most suited magnetic core types. The problems coming from cross coupling and the possible fault-tolerant operation are analyzed with the help of simulation and experimental results.

Smart Transformer-Enabled Meshed Hybrid Distribution Grid

Abstract: Renewable energy sources (RES) induce problems such as voltage and current limit violations, absence of inertia and consequent stability problems, and poor power quality. Smart transformer (ST) is a promising solution for avoiding such a changing grid scenario leading to strong grid reinforcements. This article proposes an ST-enabled meshed hybrid distribution grid to achieve voltage and power flow control simultaneously with a centralized controller. In this configuration, a low-voltage dc (LVdc) line is proposed which connects the ST LVdc link with the dc bus of distributed generation (DG) converters. This introduces various active power flow paths to support the loads. The DG converters supply active power near the load points which ensures that line losses are reduced significantly while achieving improved voltage regulation as compared to conventional microgrid. Moreover, the DG converters can draw active power from ST LVdc link during absence of RES to support the load, resulting in improved utilization of these converters. The control complexity of the DG converters is reduced as the ST controls both the LVac and LVdc line voltages. Further, the newly developed power flow path allows reverse power flow from DG plants more efficiently. Performance of the proposed system is verified with simulation and experimental results.

Crossing Thyristor Branches-Based Hybrid Modular Multilevel Converters for DC Line Faults

Abstract: The modular multilevel converter (MMC) is attractive for high-voltage direct current (HVdc) applications. The dc line short-circuit fault is one of the key challenges for the HVdc system. In this article, a crossing thyristor branches (CTB)-based hybrid MMC (HMMC) with current interruption capability is proposed to protect the HVdc system under dc line short-circuit faults. In the CTB-HMMC, the thyristor branches are proposed to crossing-connect different arm inductors and each arm consists of unipolar full-bridge (UFB) submodules (SMs) and half-bridge SMs, where the CTB effectively reduces the UFB-SMs number in the arm of the HMMC with current interruption capability. The current interruption operations, including dc-side and ac-side current interruption, are also proposed for the CTB-HMMC in case of dc line short-circuit faults, where the proposed CTB-HMMC can interrupt the fault current with the advantages of fewer UFB-SMs in the arm, short current interruption time, low semiconductor cost, and low power losses. The simulation studies are conducted and a down-scale prototype is built, and the study results verify the effectiveness of the proposed CTB-HMMC and operation principles.

On The Implementation of an FRT Strategy for Grid-Forming Converters Under Symmetrical and Asymmetrical Grid Faults

Abstract: Recent studies have shown the potential benefits of grid-forming (GFM) converters and their capability of stabilizing a power system with high penetration of power electronics-based generation. A crucial aspect of any GFM converter control strategy will be the handling of fault-ride through (FRT) scenarios, which converters for generation sources will face regularly in the field. In this article, an FRT strategy for GFM converters is proposed, which respects the converter hardware limitations (i.e., current limitations) while maintaining GFM behavior even during “protective” operating modes. The FRT strategy addresses both symmetrical and asymmetrical faults and is compliant with recently proposed draft grid codes requirements published by the British system operator NGESO. The issues related to FRT of GFM converters are first discussed in detail, then a proper strategy is presented and its effectiveness is demonstrated by means of an extensive power-hardware-in-the-loop measurement campaign, showing its efficacy even against severe fault conditions.

Reviewing Thermal Monitoring Techniques for Smart Power Modules

Abstract: The increasing demand for higher power device utilization and reliability in power electronic systems is driving the integration of condition monitoring and active control in power electronic systems. With thermal heat dissipation being the limiting factor of module lifetime and performance, thermal real-time monitoring is key in this transition. However, this requires the availability of highly accurate and high-bandwidth temperature information with minimal phase lag. This paper reviews key methods for temperature extraction in power modules: temperature sensing, thermal estimators and thermal observers. While previous research has examined individual techniques of these methods in great detail, this paper presents a methodological overview that provides insight to how different technologies may contribute to next generation thermal monitoring solutions. In this context, the paper discusses how different technologies can be effectively combined to improve their overall performance. It finally discusses key challenges that must be addressed such that minimally invasive temperature sensing and thermal monitoring become an industrial practice in the future of power electronics.

Switch Open-Circuit Fault Localization Strategy for MMCs Using Sliding-Time Window Based Features Extraction Algorithm

Abstract: Fault localization is one of the most important issues for the modular multilevel converters (MMC) consisting of lots of switches. This article proposes a switch open-circuit fault localization strategy for the MMC, where a sliding-time window (STW) based features extraction algorithm (FEA) is proposed to extract the features of the MMC based on the feature relationship between neighboring STWs. Based on the extracted features, the fault in the MMC can be easily located with the 2-D convolutional neural networks. The proposed STW-FEA-based fault localization strategy can constructs concise low-data-volume features samples for the MMC in both time domain and frequency domain, and accordingly it can locate the fault with short time and high accuracy for the MMC. In addition, it does not require the creation of complex mathematical models and manual setting of empirical thresholds. Simulation and experiment are conducted, and the results confirm the effectiveness of the proposed strategy.

Research on Active Thermal Control: Actual Status and Future Trends Special Issue Commemorating 40 years of WEMPEC, 2021

Abstract: The trend towards integrating power electronic converters and pushing their device utilization towards physical limits puts the spotlight on the junction temperature of the devices. The physical limits of power density are directly tied to the maximum junction temperature and junction temperature cycles that are considered as root-cause of the aging and finally the failure of the device. Therefore, manipulating the junction temperature with smart control algorithms is a promising method to enhance the power density as well as the lifetime of future converter systems. To address these opportunities, recent research proposed methods that increase the overload capability, reduce thermal cycles, balance thermal stress and share thermal loading of power converters. This paper systematically analyzes the fundamental principle and the key objective of these methods and discusses their limitations and potential. Based on this discussion it highlights important challenges, such as thermal state variable extraction, nonlinear aging, and minimizing the impact on system operation. These challenges must be addressed by future research to make active thermal control a key enabler of smart power converter with superior power density and reliability.

Modulation for Cascaded Multilevel Converters in PV Applications With High Input Power Imbalance

Abstract: Cascaded multilevel inverters, such as the cascaded H-bridge (CHB) converter, are an attractive solution for multistring photovoltaic (PV) systems, because they enable direct connection to the medium voltage grid and maximum power point tracking (MPPT) of multiple strings. As a challenge of the topology, the operation with high power imbalance in the strings is constrained by the overmodulation. This limitation is analyzed for sinusoidal modulation and the impact on the maximum power imbalance is demonstrated. For increasing the operating range with maximum power tracking in the strings, a discontinuous modulation with extended maximum power imbalance and reduced losses is proposed. The method is analyzed in terms of maximum power imbalance, efficiency and power quality. In addition, the method is validated on an experimental test bench.

Common-Mode Voltage Mitigation of Dual Three-Phase Voltage Source Inverters in a Motor Drive Application

Abstract: Electric variable speed drives (VSDs) based on two VSDs connected to a multiphase machine are an attractive solution to replace high-power mechanic and hydraulic systems in many sectors of industry and transportation because they present high performance with reduced cost, volume and weight. Among the causes which affect the reliability of dual VSDs, the common-mode current flowing through the machine bearing is an important issue. This paper faces the mitigation of the common-mode current by reducing the common-mode voltage (CMV) generated by the operation of a dual VSD. The CMV reduction is carried out without introducing any extra device and/or passive filtering method. This CMV reduction is performed by applying a specific phase-displacement between the modulation strategies of each single inverter drive. The proposed technique has been evaluated in a down scaled experimental setup in order to test its effectiveness.

An MVDC Based Meshed Hybrid Microgrid Enabled Using Smart Transformers

Abstract: Decentralized integration of distributed generation (DG) units and loads are increasing in the modern distribution grid. Maintaining the power flow and power quality within their accepted limits is a challenging task. The formation of meshed hybrid microgrids is an effective method to improve the power system. The smart transformer (ST) is a promising solution to establish meshed hybrid microgrids in the distribution system. This paper analyzes the performance of an ST based meshed hybrid microgrid interconnected to the main grid feeder through medium voltage (MV) dc link of a second ST. The coordinated operation of interconnected ST system is proposed to explore the features of the configuration. During normal operation, the MVDC bus voltage is controlled by one ST, and this reduces the complexity of the overall control. The main grid and microgrid MVAC source failure and converter fault conditions are explored to analyze the reliability of the proposed microgrid structure. Moreover, the reactive power support capability and active power losses for the proposed system are compared with the existing solutions. Simulation and experimental results are presented to show the operation of the proposed system.

Operation and Control of the Smart Transformer in Meshed and Hybrid Grids: Choosing the Appropriate Smart Transformer Control and Operation Scheme

Abstract: The impact of low-carbon technologies, like renewable energies and electric vehicle (EV) charging stations, will be an additional strain in terms of voltage violation and current congestion on the low-voltage network, resulting in a reduced quality of supply and overheating assets. To avoid or defer the replacement of these traditional assets, the network of the future needs an intelligent solution such as smart transformers (STs) [1]. The ST is a solid-state transformer with smart control functionalities as proposed in [2].

Primary Frequency Regulation Using HVDC Terminals Controlling Voltage Dependent Loads

Abstract: HVDC can provide frequency regulation during disturbances (e.g., faults) by controlling the power flow between two remote AC areas. While this action reduces the power deviation in the area affected by the disturbance, it causes a power imbalance in the other healthy AC area, leading to a frequency variation and endangering the system stability. In this work, a HVDC primary frequency regulation controlling voltage-dependent loads (PFR-VDL) is proposed, where the HVDC terminal in the healthy area influences the grid voltage amplitude to shape (decreasing or increasing) the load consumption in order to cope with the power variation required by the fault-affected area. The PFR-VDL extracts the needed energy for the frequency support, not from the generators (with following frequency deviation) but from the voltage-dependent loads in the healthy area. This work analyzes the PFR-VDL performance, generalizing it with two possible HVDC connection cases: Asynchronous connection with single HVDC line, and embedded HVDC forming a parallel, hybrid connection with HVAC. The PFR-VDL application benefits and limitations are evaluated analytically and verified by means of PSCAD EMTDC simulations, and finally validated with a large interconnected IEEE 39 bus system.

Reduction of the Circulating Current Among Parallel NPC Inverters

Abstract: In medium/high power applications, including smart transformers, active power filters, and wind turbines, three-level neutral-point-clamped (NPC) inverters proved to be a reliable solution, providing high efficiency and low harmonic distortion. In practice, several NPCs are parallel connected and operated in interleaved manner to further increase the power handling and reduce the line filters size. However, if such configuration has a common dc-link, high-frequency zero-sequence circulating current (HF-ZSCC) arises among the inverters, increasing power losses of the switching devices and propagating the stress on the dc-capacitors. Moreover, the amplitude of the HF-ZSCC is inversely proportional to the filter inductance size, therefore in real applications, it can reach hundreds of amperes even with relatively low output currents. The research on the HF-ZSCC is mostly concentrated on two-level inverters for low voltage grids and traction applications, where the inductance size is relatively big and the HF-ZSCC does not affect the system efficiency. Differently, NPCs provide higher switching degree of freedom and more sophisticated methods can be applied to reduce the HF-ZSCC. This article investigates a double-reference pulsewidth modulation (DRPWM) as solution for diminishing the HF-ZSCC in paralleled NPCs. The performance of DRPWM method is confirmed by both simulation and experiments, performed on a 1.6-MVA system.

A Reduced Power Switches Count Multilevel Converter-Based Photovoltaic System With Integrated Energy Storage

Abstract: A multilevel topology for photovoltaic (PV) systems with integrated energy storage (ES) is presented in this article. Both PV and ES power cells are connected in series to form a dc link, which is then connected to an H-bridge to convert the dc voltage to an ac one. The main advantage of the proposed converter compared to the cascaded-H-bridge (CHB) converter, as well as compared to the available multilevel topologies, is that fewer semiconductor devices are needed here. As the output voltage levels increase, more switches are saved, which results in a more efficient, cheaper, and smaller converter. So far, there is still no modulation strategy that is designed particularly for PV-fed multilevel converters with built-in ES. The standard modulations are impractical for such an application since they suffer from deficiencies, such as polluted output signals—thus, requiring larger output filter—and overmodulation. A modified modulation strategy for PV+ES multilevel inverters is, therefore, introduced in this article. The proposal has been simulated and experimentally validated to evaluate its effectiveness, where it has been shown that the proposed topology is not exclusively feasible, but also suffers from less conduction and switching loss, achieving higher efficiency with respect to its counterpart CHB.

FS-MPC Based Thermal Stress Balancing and Reliability Analysis for NPC Converters

Abstract: Active thermal control has been introduced to regulate the steady state and reduce the transient thermal-mechanical stress in power electronic modules. Specifically, it can equally distribute the temperature among the devices, thereby better distributing the stress among a set of devices and reducing the failure probability in the most thermally-stressed devices. This is of great importance for multilevel topologies and in particular for the Neutral Point Clamped (NPC) topologies, which have an inherent thermal unbalance among devices of the same phase. In hybrid structures, whereby Si and SiC devices are mixed to achieve a better trade-off between efficiency and cost, this problem is even worse due to technology differences. This paper investigates the advantages introduced by using Finite-Set Model Predictive Control (FS-MPC) algorithms designed for achieving a balanced device junction temperature in hybrid NPC and Active-NPC converters. Moreover, a novel setup composed by a hybrid ANPC based power modules is used to experimentally validate the presented FS-MPC algorithm. A lifetime estimation is performed for the two topologies to highlight the long-term benefits of FS-MPC algorithm in these hybrid topologies for UPS applications.

Interactions Between Two Phase-Locked Loop Synchronized Grid Converters

Abstract: Grid converters synchronized by phase-locked loops (PLLs) could suffer from stability problems, especially being connected to a weak grid or high penetration of converters. The existing literature assesses the stability of the paralleled PLL-synchronized converters at the same point of common coupling (PCC) using identical control and system parameters. However, in an actual grid, the parameters of grid converters are normally different due to different manufacturers. In this regard, this article aims to study the stability indices and margins of two PLL-synchronized converters with different parameters, particularly with different PLL bandwidths and power injections. The main purpose of this article is to provide a general design guidelines of PLL bandwidths of two converters (can be extended to two wind/solar farms) for stable operation in practical grids. The state-space model of the PLL-synchronized converter based on the component connection method is developed and eigenvalue-based analysis is used to investigate the interactions between the two converters. Moreover, the stability borders of two PLL-synchronized converters using different PLL bandwidths and power setpoints are studies. Monte Carlo simulations and experimental results are provided to validate the effectiveness of the developed model and theoretical analysis.

Discontinuous-PWM Method for Multilevel $N$ -Cell Cascaded H-Bridge Converters

Abstract: The cascaded H-bridge converter (CHB) is a very suitable solution with inherent modularity for many applications such as flexible ac transmission systems and motor drives. In this article, the discontinuous pulsewidth modulation (DPWM) by clamping some power cells is used in the CHB because it permits to reduce the switching losses in the power devices. However, this provokes a high harmonic distortion (basebands and sidebands harmonics) in the CHB output voltage compared with the quality obtained avoiding to clamp cells, just applying the conventional phase-shifted PWM (PS-PWM) technique. This has been studied for the three-cell CHB topology but the extension for an $N$ -cell CHB with several clamped cells is a challenge that is addressed in this article. A generalized CHB harmonic voltage analysis applying the DPWM method is presented considering multiple clamped cells. The harmonic distortion mitigation target can be decoupled into two objectives. The proposed method cleverly groups the cells (one clamped + one or two nonclamped cells) to mitigate the basebands harmonic distortion. Simultaneously, a variable-angle PS-PWM technique is applied to the CHB in order to mitigate the sidebands harmonics content. Experimental results demonstrate the good performance of the proposed method.

The Role of Renewable Energy System in Reshaping the Electrical Grid Scenario

Abstract: Renewable Energy Systems have been in the spotlight of the academic and industrial research for more than two decades, thanks to the development of several fields related to the Electrical Engineering. More recently, with the increasing complexity of the individual renewable energy systems and the interconnection to the grid, the scientific panorama has been witnessing to a convergence of different topics, which span across several IEEE-IES thematic areas: power electronics, electrical machines, smart grids, energy storage, transportation electrification and aerospace. After a brief overview of the renewable energy technologies, this work deals with how the convergence of multiple technologies developed to provide marginal support to the grid has evolved into the foundation of the future utility grid and expanded to transportation sector. It will be shown how the design of a renewable energy system cannot prescind anymore from the electrical grid and from the ancillary services that are requested. Example of convergence are given for a smart transformer application and for a transportation application.

Neutral Current Optimization Control for Smart Transformer-Fed Distribution System Under Unbalanced Loads

Abstract: In a three-phase four-wire low voltage (LV) distribution system, unbalanced loads lead to neutral current (NC) looping resulting in increase of power losses and variation of neutral potential. Compared to the conventional power transformer, smart transformer (ST) has strict current limitations to avoid overcurrent. However, its advantages on the downstream LV grid voltage regulation can provide the capability to regulate excessive NC. This article proposes a closed-loop NC optimization control in order to, on the one hand, minimize the NC current in the normal operation satisfying the standard EN 50160 requirement, on the other hand, suppress the NC current in extreme cases to avoid the overcurrent damage of the ST. The proposed control strategies are validated by experimental tests via the hardware-in-the-loop setup and a case study based on a 350-kVA, 10-kV/400-V, ST-fed distribution network under unbalanced loading profile according to the three-phase four-wire distribution grid in the Manchester area. The results clearly prove the effectiveness and flexibility of the proposed NC optimization control strategies on the NC suppression and minimization.

Design and Analysis of an Isolated Single-Stage Resonant AC-DC Converter with PFC

Abstract: In this study, an isolated bridgeless electrolytic capacitorless single stage AC-DC converter with high power factor is proposed. The proposed topology is a combination of a series resonant circuit and a bridgeless PFC circuit. The series resonant circuit allows the output diodes to turn-off under ZCS conditions, thus reducing the reverse recovery losses. This feature, in addition to the single-stage power conversion capability, has increased the efficiency of the proposed circuit. Besides, the lack of low-frequency input bridge and large electrolytic capacitor of the DC link, makes it simple and small. Analysis and design steps of the proposed circuit are given in detail, and then the simulation results of a 900W converter are presented.

Common-Mode Voltage Mitigation Technique in Motor Drive Applications by Applying a Sampling-Time Adaptive Multi-Carrier PWM Method

Abstract: Common-mode voltage (CMV) in electric drives causes leakage current causing consequently EMI problems, loss and reduction of their components’ lifetime. Several solutions have been proposed which usually lead to higher cost because additional components are used. This paper is focused on the mitigation of the resulting CMV produced by the operation of the VSD by means of a specifically designed PWM method. The proposal is based on the analysis of the CMV harmonic spectrum using the Fourier analysis. The CMV mitigation is achieved by modifying the time-shift displacement of the carriers each sampling time considering a multi-carrier PWM technique. The resulting method has been evaluated in a down scaled experimental setup and it is easily implementable on mostly off-the-shelf mid-range micro-controller control platforms.

Synchronization of Low Voltage Grids Fed by Smart and Conventional Transformers

Abstract: The Smart Transformer (ST) is a power electronics-based transformer, which operates as grid-forming converter in the low voltage-fed grid. It synthesizes the voltage waveform with magnitude, phase and frequency independently from the main power system. If a meshed operation of the ST with a conventional transformer is required, to improve the power flow control and to control the voltage profile, the voltage waveforms between the two grids have to be synchronized. The switching under different voltage magnitude, phase or frequency, can lead to a large power in-rush. This work proposes a synchronization strategy that enables a seamless transition of the ST to parallel operations with conventional transformers. Differently from classical communication-based methods, this work addresses a more realistic implementation case with limited communication infrastructure. The ST relies only on local measurements and on its advanced control capability to determine the effective switch to parallel operations. The performance of the proposed strategy has been proved analytically and through simulations in a PLECS/MATLAB environment, and validated experimentally by means of Power-Hardware-In-Loop (PHIL) evaluation.

Multiwinding Transformer Leakage Inductance Optimization for Power Flow Decoupling in Multiport DC-DC Converters

Abstract: Isolated multiport DC-DC converters manifest some prominent advantages over usual multiple two-port DC-DC converters such as smaller holistic magnetics and higher power density. However, the operation of such a converter is tied to the power flow decoupling capability in the magnetic medium frequency transformer (MFT). This paper targets to optimize the decoupling between the multiwinding MFT ports by means of multi-objective optimization of the leakage inductance network. Three different multi-objective cost functions are proposed and solved by Genetic Algorithm (GA). The obtained results show that a winding topology where primary winding is sandwiched by the two secondaries and are tightly wound to the core, provides the minimum possible leakage inductance without interleaving the windings. The obtained topologies from solving different objectives can be used as a benchmark in design and manufacturing of mutliwinding transformers. Experimental results are provided to verify the obtained optimum design.

Impact of Optimal Control of Distributed Generation Converters in Smart Transformer Based Meshed Hybrid Distribution Network

Abstract: 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 optimal operation in such ST based meshed hybrid distribution network. A CIGRE LV residential distribution network with DG sources is connected at ST low voltage ac (LVAC) and LVDC terminals. All the DG sources are connected to LVDC line. The power management scheme and method of determining power flow solution for the considered distribution network are proposed. An optimal problem is formulated for determining the active and reactive power references of DG converters while maintaining the LVAC load bus voltages within the grid limits and considering the DG converters constraints. The minimization of energy drawn from ST medium voltage (STMV) grid is considered as objective function and solved using genetic algorithm. To know the impact of proposed optimal control of DG converters, various performance indicators i.e., energy loss, operating energy costs, voltage profile and sizing of ST converters are considered and compared with existing literature.

Enhanced Current-Type P-HIL Interface Algorithm for Smart Transformers Testing

Abstract: The energy systems are evolving towards the wide integration of power electronics-based technologies, such as electric vehicles. A promising solution to increase the grid controllability is represented by grid-forming converters, such as smart transformers (STs). Being a new technology, the ST experimental testing is a fundamental step before commercialization. Instead of performing time consuming and not flexible on-field tests, the Power Hardware In the Loop (P-HIL) offers a flexible testing environment for experimentally validating new technologies. The real-time simulation of the electrical grid offers the possibility to vary quickly the testing environment, while the power amplification stage offers the validation of the real hardware. Despite the clear testing advantages, the P-HIL stability and testing accuracy is still a matter of study. This paper introduces a new P-HIL interface approach for ST application, that can guarantee high testing accuracy in a large frequency spectrum. The proposed approach combines the tracking capability of the existing controlled Current-Type P-HIL interface algorithm, with the well-known Partial Circuit Duplication approach. The accuracy and stability analysis has been performed analytically and validated by means of extensive experimental P-HIL testing.

Evaluation of carrier-based control strategies for balancing the thermal stress of a hybrid SiC ANPC converter

Abstract: SiC devices have recently been demonstrated and utilized in many different multilevel topologies. Active neutral point clamped (ANPC) converter is one of those topologies, where the use of SiC devices can significantly increase the converter power density and reduce the output filter size. In order to achieve this, the control algorithm needs to be tailored to utilize the advantages of each power device technology. Therefore, the control strategies which could produce the optimum results for Si based ANPC might not be the optimum choice for hybrid configuration, where Si and SiC devices are mixed. In this paper, it is presented how different pulse-width-modulation (PWM) based algorithms, originally proposed for Si ANPC converters, affect the loss and junction temperature distribution of a hybrid ANPC converter. The analysis includes the inverter and rectifier operation mode, as well as a high and low modulation index operation. The device junction temperatures distribution is verified using an open module 3-level hybrid ANPC prototype.

Potentials and Challenges of Multiwinding Transformer-Based DC-DC Converters for Solid-State Transformer

Abstract: 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.

Analysis and Suppression of Zero-Sequence Circulating Current in Multi-Parallel Converters

Abstract: The use of a multi-parallel converter system has many advantages in increased scalability, better maintenance, scheduling, and improved output current quality. However, a periodic zero-sequence circulating current (ZSCC) may occur due to the asymmetry of parallel-connected converters. ZSCC produces additional losses and possible instability of the system. Therefore, proper control must be applied to suppress this harmful ZSCC. In order to design an effective controller for suppressing ZSCC, it is necessary to analyze the cause of the circulating current generation. However, most of the existing studies have applied the controller without detailed analysis. Therefore, this paper mathematically analyzes the ZSCC spectrum using the Fourier series to identify which harmonics are included in ZSCC. From the analysis results, the necessity of multi-resonant controllers to suppress the ZSCC at specific harmonics is demonstrated. Simulation and experiments are conducted to validate the analysis results and the necessity of multi-resonant controllers.