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The deployment of solid-state transformers (SSTs) in medium-voltage distribution systems is facing various challenges, especially the challenge of insulation coordination design against grid-originated lightning impulses. In this paper, two challenges in existing insulation coordination designs for grid-connected SSTs are identified. One challenge is the mismatch between metal-oxide varistor (MOV) protective levels and SST insulation strength, the other challenge is the incompatibility of standard impulse test on SST protective structures. To address the MOV selection challenge, a novel lightning protection scheme is designed to protect a single-stage SST where the semiconductor modules are directly exposed to external lightning impulses. The in-lab lightning impulse tests are performed to verify the overvoltage attenuation performance of the prototyped lightning protection scheme. To address the impulse test challenge, the surge withstand capability of the protected SST is comprehensively evaluated with a complete set of insulation coordination design procedures beyond the BIL test. After these two challenges are addressed, a discussion is presented on substituting conventional transformers with the protected SSTs into insulation-coordinated distribution systems to facilitate the field deployment of SSTs.

Solid-state dc transformer to integrate low-voltage dc (LVdc) microgrid, wind turbine (WT) generator, photovoltaic (PV), and energy storage (ES) into medium-voltage (MV) direct-current (MVdc) distribution grids is attractive. This article proposes current-source dc solid-state transformer (SST) for MVdc collection system in WT, PV, and ES farms or as an interface between the MVdc grid and the LVdc microgrid. Compared to conventional current-source converter (CSC) based SSTs, a switch reduction scheme on reverse-blocking device bridges is proposed to reduce device count and the number of devices on the dc-link current path. Importantly, the proposed switch reduction scheme is generic and can be applied to the dc ports of dc–ac, ac–dc, or dc–dc hard-switching or soft-switching CSC-based SSTs. Based on this scheme, the proposed current-source dc SSTs are derived, which have reduced electrolytic-capacitor-less dc-link. The proposed dc SSTs also achieve single-stage isolated dc–dc or dc–ac conversion, full-range zero-voltage switching (ZVS) for main switches, zero-current switching (ZCS) for resonant switches, and controlled $ dv/dt $ . The proposed dc SSTs, operating principles, predictive control method, the ZVS, and the controlled $ dv/dt $ under voltage buck-boost ranges are verified with MV simulations and an experimental prototype based on SiC mosfets , diodes, and a nanocrystalline transformer.

This paper discusses the use of soft switching techniques in isolated and non-isolated converters, showing that the new range of grid connected applications require higher performance power conversion than traditional voltage source converters can provide. The paper begins with a review of soft switching techniques, discusses the challenges of achieving soft switching in four quadrant converters, and shows the impact that soft switching can have on the next generation of multi-port converter applications, such as energy storage, PV/battery/grid systems and microgrids.

This article presents the analysis and implementation of a predictive control method for dc-link regulation and voltage balance in a cascaded modular reduced dc-link solid-state transformer (SST). Passive components like bulky dc links limit the power density of power converters, especially medium-voltage (MV) SST. Reduced dc-link or low-inertia converters can dramatically reduce the size, cost, and weight by tolerating larger dc-link ripples and improve the reliability with electrolytic capacitor-less dc link. However, a small dc link leads to tight coupling between the input and the output stages, which is a challenge for control design. In stacked low-inertia converters (SLIC), the low-inertia converter modules are stacked for MV applications, resulting in coupling between the modules and making the control more challenging. A new model predictive control method that can achieve deadbeat regulation on the dc link without weighting factors has been proposed to address this novel problem. This article focuses on analyzing the condition of the low-inertia dc link up to 80% ripple, the robustness of the control under parameter mismatches, high-order terms, and important implementation issues, such as model-based sampling and computation delay compensation. Significantly, the high-order terms are introduced because of the large dc-link ripple. These high-order terms are unique to the SLIC and negligible in conventional high-inertia converters. A discrete-time large-signal model is built to capture the dc-link's nonlinear dynamics, and the eigenvalues of a small-signal Jacobian matrix are analyzed with Floquet theory to evaluate stability, using the modular soft-switching SST (M-S4T) as an example of the SLIC. Simulation and experimental results of an MVDC M-S4T verify the analysis and the predictive control method. Finally, the general application of the predictive control to low-inertia converters is compared against a conventional PI controller using a reduced dc-link active-front-end rectifier as an example.

Turn-to-turn faults between primary windings due to insulation degradation are a major cause of distribution transformer failure, and occur due to high levels of stress such as overloading and overheating. An additional consequence of these faults is an increased voltage on the transformer secondary due to effective change in turns ratio. This paper develops a novel method for early detection of insulation degradation and subsequent inter-turn winding failure by monitoring the transformer secondary voltage. The algorithm is based on a cumulative sum (CUSUM) statistic and compares voltages on neighbouring transformers to flag degrading assets. Results obtained from simulation as well as experimental data show that smart meter measurements can be utilized to achieve very high detection accuracy while keeping costs low. The paper also demonstrates the validity of the algorithm in the presence of measurement noise, residential solar power injection etc.

Medium-voltage dc (MVdc) grids are attractive for electric aircraft and ship power systems, battery energy storage system (BESS), fast charging electric vehicle (EV), etc. Such EV or BESS applications need isolated bidirectional MVdc to low-voltage dc (LVdc) or LVac converters. However, the existing Si-based solutions cannot fulfill the requirements of a high-efficiency and robust converter for MVdc grids. This article presents a 5-kV SiC-based universal modular solid-state transformer (SST). This universal current-source SST can interface either an LVac or LVdc grid with an MVdc grid in single-stage power conversion, while the conventional dual-active bridge (DAB) converter needs an additional inverter. The proposed SST module using 3.3-kV SiC MOSFET s and diodes is bidirectional and can serve as a building block in series or parallel for higher voltage higher power systems. The topology of each module is based on the soft-switching solid-state transformer (S4T) with reduced conduction loss, which features reduced electromagnetic interference electromagnetic interference (EMI) through controlled dv/dt, and high efficiency with full-range zero-voltage switching for main devices and zero-current switching for auxiliary devices. Operation principle of the modular S4T (M-S4T), capacitor voltage balancing control between the cascaded modules, design of components including a medium-voltage (MV) medium-frequency transformer (MFT) to realize a 50-kVA, 5-kV dc to 600 V dc or 480 V ac M-S4T are presented. Importantly, the MV MFT prototype achieves very low leakage inductance (0.13%) and 15-kV insulation with coaxial cables and nanocrystalline cores. The proposed universal modular SST is compared against the DAB solution and verified with dc–dc and dc–ac simulation and 4-kV experimental results. Significantly, the MV experimental results of a modular dc transformer with each module at MVdc are rarely covered in the literature and reported for the first time.

This article presents control challenges of stacked low-inertia converter (SLIC) or cascaded reduced dc-link solid-state transformer (SST) and proposes a novel model predictive priority-shifting (MPPS) control with implicit modulator and a discrete-time large-signal model for voltage balancing and dc-link regulation. Low-inertia converters, featuring small electrolytic capacitor-less dc links, dramatically reduce cost, size, and weight compared to conventional solutions. However, without a large dc-link buffer, the input and output are tightly coupled, leading to significant control challenges. The control becomes even more challenging with these converters stacked input-series output-parallel (ISOP) for medium-voltage (MV) grid, which causes coupling between the modules besides the coupling within each module. This article analyzes the multiobjective, multidegree of freedom control problem, using the modular soft-switching solid-state transformer (M-S4T) as an example of the SLIC. First, distribution of control efforts under controller saturation is critical because multiple control objectives can be conflicting, especially when the module voltages are unbalanced and are being restored. The MPPS can shift the priorities to address this issue. Second, due to the low inertia and high dc-link ripple, classic space vector pulsewidth modulation, average model with small-ripple assumption, and control design based on small-signal model cannot accurately modulate, model, and control the nonlinear reduced dc link. Therefore, a discrete-time large-signal model of the M-S4T is established to derive the predictive control in the MPPS. The MPPS and the PI control are compared in MV simulations to show the issue of applying the PI to the SLIC and the effectiveness of the MPPS for voltage balancing and dc-link regulation in a deadbeat manner. Finally, the proposed control is tested on a 5-kV ISOP SiC SST prototype to verify priority shifting to address controller saturation issue and fast and robust voltage balancing.

Three billion people around the world live in severe energy poverty, including 1.1 billion who live completely off-grid. Providing affordable energy access to them can dramatically impact their living standard, health, education, productivity, and ability to be a part of modern society. In developing economies, the utilization of energy is also limited to some extent. Many programs and initiatives (e.g., the IEEE Empower a Billion Lives and the IEEE Smart Village) have been doing stellar work in tackling energy poverty in developing economies. However, much remains to be done to crowdsource relevant innovation to accelerate the deployment of energy access solutions in the affected areas, including the developing economies. According to the previous programs and initiatives, it has been identified that power electronics is one of the key technologies to address real-life issues in energy access and utilization, water supply, rural transportation, etc., for developing economies. In this context, it calls for innovative and scalable solutions that should be for emerging applications of power electronics with an emphasis on technical innovation and business viability to rapidly and sustainably scale to a wide range of customers in developing economies. Hence, we organized this Special Issue on Emerging Applications of Power Electronics in Developing Economies , aiming to bring together researchers, experts, policy makers, and stakeholders to tackle energy poverty issues in developing economies.

Solid-state transformers (SSTs) are a promising solution photovoltaic (PV), wind, traction, data center, battery energy storage system (BESS), and fast charging electric vehicle (EV) applications. The traditional SSTs are typically three-stage, i.e., hard-switching cascaded multilevel rectifiers and inverters with dual active bridge (DAB) converters, which leads to bulky passives, low efficiency, and high electromagnetic interference (EMI). This article proposes a new soft-switching solid-state transformer (S4T). The S4T has full-range zero-voltage switching (ZVS), electrolytic capacitor-less dc link, and controlled dv/dt , which reduces EMI. The S4T comprises two reverse-blocking current-source inverter (CSI) bridges, auxiliary branches for ZVS, and transformer magnetizing inductor as a reduced dc link with 60% ripple. Compared with the prior S4T, an effective change on the leakage inductance diode is made to reduce the number of the devices on the main power path by 20% for significant conduction loss saving and retain the same functionality of damping the resonance between the leakage and resonant capacitors and recycling trapped leakage energy. The conduction loss saving is crucial, being the dominating loss mechanism in SSTs. Importantly, the proposed single-stage SST not only holds the potential for high power density and high efficiency but also has full functionality, e.g., multiport dc loads integration, voltage regulation, and reactive power compensation, unlike the traditional single-stage matrix SST. The S4T can achieve single-stage isolated bidirectional dc–dc, ac–dc, dc–ac, or ac–ac conversion. It can also be configured input-series output-parallel (ISOP) in a modular way for medium-voltage (MV) grids. Hence, the S4T is a promising candidate for the SST. The full functionality, e.g., voltage buck–boost, multiport, etc., and the universality of the S4T for the dc–dc, dc–ac, and ac–ac conversion are verified through the simulations and experiments of two-port and three-port MV prototypes based on 3.3 kV SiC mosfet s in dc–dc, dc–ac, and ac–ac modes at 2 kV.

Rogowski coil current sensors are popular solutions for current sensing having several advantages over current transformers (CTs), including size, bandwidth and dynamic range. These sensors have been used by electric utilities for gaining visibility into the electric grid directly helping in grid operations. This paper presents an overview of the design principles and test results for a PCB-based Rogowski coil that can be retrofitted around any existing conductor, resulting in a sensor that can be rapidly installed in the field. Due to the absence of any magnetic components in the coil, Rogowski coils cannot saturate and thus have a very high dynamic range, only limited by the accompanying signal conditioning or data converter circuits. In order to prevent saturation of the signal conditioning stage at higher current levels, a method to auto-tune the sensor has been presented, resulting in a dynamic range that is orders of magnitude higher than state of the art. The method, called dynamic range correction (DRC), uses edge intelligence to detect fault current scenarios and adjust the analog circuit to prevent saturation or distortion, while capturing the fault current waveforms. This feature enables post-event diagnostics for electric utilities at a very low cost.

Silicon carbide (SiC) MOSFETs with ratings of 3.3 to 15 kV represent a dramatic improvement over available silicon devices, enabling the realization of medium-voltage converters in demanding applications such as solid-state transformers. Typical voltage-source converter configurations realize significant reduction in switching loss with SiC devices, but high dv/dt of 30–50 kV/ μ s generates high levels of electromagnetic interference (EMI) and displacement currents. Current-source based zero-voltage switching (CS-ZVS) converters such as the soft-switching solid-state transformer (S4T), dramatically reduce switching loss using ZVS, thus realizing both controlled dv/dt and low EMI. CS-ZVS converters require reverse blocking (RB) modules that are realized using series connection of a diode and a MOSFET. However, no data are available from manufacturers or from literature on RB module characterization under ZVS conditions. This article presents detailed characterization results and model extraction for a 3.3 kV 45 A and a 1.7 kV 10 A SiC RB module, under both hard switching and ZVS modes. A novel double-pulse testbed is designed and built for this characterization of RB devices under both hard switching and ZVS conditions. Significantly, it is shown that when the RB modules are used in CS-ZVS converters, the dynamic voltage sharing between the RB modules in a phase leg and within the RB module (between the diode and the switch) results in a unique voltage stress. Modulation strategies to address this unique voltage stress are proposed and verified through experiment results, using a S4T prototype rated at 1.5 kV, 10 kVA .