Design and Stability Analysis of a Three-Phase Triple-Stage Solid-State Transformer

The electrical distribution system is experiencing a profound evolution process triggered by the increasing integration of Renewable Energy Sources (RES) and Distributed Generation (DG), alongside the widespread use of Electric Vehicles (EVs), the related charging stations, and the growing adoption of Energy Storage Systems (ESSs). The behavior of such loads and sources, interfaced with the grid via an increasing number of power electronics converters and often intermittent in nature, together with a bidirectional power flow requirement, poses new challenges for the reliable and safe operation of the distribution system. In this context, the concept of Internet of Energy (IoE), or Energy Internet (EI), has emerged and is nowadays widely discussed in the literature as a new paradigm shift to address the growing demand for modernization of the current distribution network. The goal in the IoE scenario is reshaping the current distribution grid into an intelligent and flexible active network, both through a radical informatization process that involves the renewal of the grid communication infrastructure and the addition of distributed monitoring points and via the implementation of advanced energy management and control functionalities to enable the safe, robust, effective, and efficient integration of intermittent sources and loads. At the core of this future smart grid scenario, the Solid-State Transformer (SST) is envisioned as the best candidate due to its flexibility and advanced control features. This is because the SST is a power electronic-based transformer capable of providing advanced services and grid-supporting features, besides galvanic isolation and voltage adaptation, through its control system, and therefore is intended for replacing conventional Line Frequency Transformers (LFTs) at strategic nodes of the grid. Moreover, the core isolation stage of the SST operates at high frequencies and, therefore, it enables volume and weight reduction of the whole system compared to traditional and bulky LFTs. In the IoE scenario, the most suitable SST configuration is the triple-stage one, which consists of three conversion stages. Due to the large number of stages, the SST control is intrinsically complex. It has been shown in the literature how the coupling among controllers makes the design of the overall control system challenging and, additionally, multistage cascaded converters are significantly prone to instability due to interaction between converters. Moreover, even if the SST is stable as a standalone system, it may become unstable when connected to the grid because of dynamic interactions with other grid-connected converters, leading to the so-called harmonic instability phenomenon. In this context, this thesis aims to explore the SST stability issue from both the DC-link and grid-connection perspectives. To do so, in the first part of this work, the SST suitable topologies and their conversion stages are reviewed. Once the SST architecture is selected, the main ratings and parameters are designed according to the presented IoE application requirements. An average model of the converter, that enables faster simulations and physical insights into the SST dynamics, is then derived. Through it, the small-signal model of the SST can be obtained. Based on that, the SST control system is presented and designed and the related impedance model is derived. The latter is selected as assessment tool to evaluate the DC-link and grid-connection stability of the SST under investigation. The results obtained provide support during the design phase of the SST and its control strategy, with the aim to achieve a stable grid-connected operating system.The electrical distribution system is experiencing a profound evolution process triggered by the increasing integration of Renewable Energy Sources (RES) and Distributed Generation (DG), alongside the widespread use of Electric Vehicles (EVs), the related charging stations, and the growing adoption of Energy Storage Systems (ESSs). The behavior of such loads and sources, interfaced with the grid via an increasing number of power electronics converters and often intermittent in nature, together with a bidirectional power flow requirement, poses new challenges for the reliable and safe operation of the distribution system. In this context, the concept of Internet of Energy (IoE), or Energy Internet (EI), has emerged and is nowadays widely discussed in the literature as a new paradigm shift to address the growing demand for modernization of the current distribution network. The goal in the IoE scenario is reshaping the current distribution grid into an intelligent and flexible active network, both through a radical informatization process that involves the renewal of the grid communication infrastructure and the addition of distributed monitoring points and via the implementation of advanced energy management and control functionalities to enable the safe, robust, effective, and efficient integration of intermittent sources and loads. At the core of this future smart grid scenario, the Solid-State Transformer (SST) is envisioned as the best candidate due to its flexibility and advanced control features. This is because the SST is a power electronic-based transformer capable of providing advanced services and grid-supporting features, besides galvanic isolation and voltage adaptation, through its control system, and therefore is intended for replacing conventional Line Frequency Transformers (LFTs) at strategic nodes of the grid. Moreover, the core isolation stage of the SST operates at high frequencies and, therefore, it enables volume and weight reduction of the whole system compared to traditional and bulky LFTs. In the IoE scenario, the most suitable SST configuration is the triple-stage one, which consists of three conversion stages. Due to the large number of stages, the SST control is intrinsically complex. It has been shown in the literature how the coupling among controllers makes the design of the overall control system challenging and, additionally, multistage cascaded converters are significantly prone to instability due to interaction between converters. Moreover, even if the SST is stable as a standalone system, it may become unstable when connected to the grid because of dynamic interactions with other grid-connected converters, leading to the so-called harmonic instability phenomenon. In this context, this thesis aims to explore the SST stability issue from both the DC-link and grid-connection perspectives. To do so, in the first part of this work, the SST suitable topologies and their conversion stages are reviewed. Once the SST architecture is selected, the main ratings and parameters are designed according to the presented IoE application requirements. An average model of the converter, that enables faster simulations and physical insights into the SST dynamics, is then derived. Through it, the small-signal model of the SST can be obtained. Based on that, the SST control system is presented and designed and the related impedance model is derived. The latter is selected as assessment tool to evaluate the DC-link and grid-connection stability of the SST under investigation. The results obtained provide support during the design phase of the SST and its control strategy, with the aim to achieve a stable grid-connected operating system

Power Electronics Converters for the Internet of Energy: A Review

This paper presents a comprehensive review of multi-port power electronics converters used for application in AC, DC, or hybrid distribution systems in an Internet of Energy scenario. In particular, multi-port solid-state transformer (SST) topologies have been addressed and classified according to their isolation capabilities and their conversion stages configurations. Non-conventional configurations have been considered. A comparison of the most relevant features and design specifications between popular topologies has been provided through a comprehensive and effective table. Potential benefits of SSTs in distribution applications have been highlighted even with reference to a network active nodes usage. This review also highlights standards and technical regulations in force for connecting SSTs to the electrical distribution system. Finally, two case studies of multi-port topologies have been presented and discussed. The first one is an isolated multi-port bidirectional dual active bridge DC-DC converter useful in fast-charging applications. The second case of study deals with a three-port AC-AC multi-level power converter in H-Bridge configuration able to replicate a network active node and capable of routing and controlling energy under different operating conditions

Search for gravitational-lensing signatures in the full third observing run of the LIGO-Virgo network

Gravitational lensing by massive objects along the line of sight to the source causes distortions of gravitational wave-signals; such distortions may reveal information about fundamental physics, cosmology and astrophysics. In this work, we have extended the search for lensing signatures to all binary black hole events from the third observing run of the LIGO--Virgo network. We search for repeated signals from strong lensing by 1) performing targeted searches for subthreshold signals, 2) calculating the degree of overlap amongst the intrinsic parameters and sky location of pairs of signals, 3) comparing the similarities of the spectrograms amongst pairs of signals, and 4) performing dual-signal Bayesian analysis that takes into account selection effects and astrophysical knowledge. We also search for distortions to the gravitational waveform caused by 1) frequency-independent phase shifts in strongly lensed images, and 2) frequency-dependent modulation of the amplitude and phase due to point masses. None of these searches yields significant evidence for lensing. Finally, we use the non-detection of gravitational-wave lensing to constrain the lensing rate based on the latest merger-rate estimates and the fraction of dark matter composed of compact objects

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Impact of the DC-DC Stage on Grid-Connection Stability in Solid-State Transformer

This paper addresses the impact of the dc-dc stage on the grid-connection stability of a three-phase Solid-State Transformer based on the Cascaded H-Bridge and Dual Active Bridge topologies. In particular, the analysis aimed to reveal and discuss the impact of the isolated bidirectional dc-dc stage on the input dq-frame impedance matrix properties in terms of its passivity. For this purpose, the d-axis input impedance has been mainly addressed in this work. Its low-frequency simplified expression has been derived, by means of which its real part and the negative-resistance region upper limit can be analytically deduced, enabling a passivity-oriented design procedure. It is demonstrated that the dc-dc converter lowers that limit thus enhancing the grid-connected Solid-State Transformer passivity. The analysis performed in this work can be applied to a generic ac-dc-dc topology

Negative Voltage Sequence Control for an Electric Arc Furnace Power Supply based on a Multilevel AC-AC Converter

This paper proposes a novel control approach for a back to back multilevel AC-AC converter in Electric Arc Furnace (EAF) power supply applications. The study presents a feasibility analysis about the use of the chosen converter structure for EAF applications, along with considerations for control mechanisms. In particular, the effect of both positive and negative voltage sequences on the system is explored. Through comprehensive analysis, the paper introduces a novel control method utilizing negative voltage sequence, aiming to enhance the overall system performances. The results are presented using both a simplified model and the Cassie-Mayr (CM) model for the EAF

Stability Assessment Study for a Triple-Stage Three-Phase Solid-State Transformer

This paper proposes an accurate stability and performance analysis for a triple-stage three-phase modular solid-state transformer (SST), based on the cascaded H-bridge (CHB) and on the dual active bridge (DAB) topologies. The control strategy of this system consists of a complex multi-layer structure, mainly because of the need to balance the various dc-links of the converter. Such a complex system is highly prone to instability, both for the cascaded topology and for the control structure. In this paper, the small-signal modelling approach and the Middlebrook's criterion are used to address the stability issues of the converter. For this purpose, the small signal model for each converter stage is developed and through those the SST impedance transfer functions are derived. To validate the theoretical impedance equations, a numerical simulation has been carried out in order to map the frequency response of the converter stages. Then, the stability of the whole system is discussed, pointing out what parameters may cause instability in the analyzed SST system. Four relevant case studies corresponding to four different operating modes are assessed and verified via Simulink simulations