The rapid development of renewable energy systems (RES), especially photovoltaic (PV) energy and wind energy, poses increasing requirements for highpower, low-loss, fast-switching, and reliable semiconductor devices to improve system power capacity, efficiency, power density and reliability. The recent commercialization of wide bandgap (WBG) devices, specifically Silicon Carbide (SiC) and Gallium Nitride (GaN) devices, provides very promising opportunities for meeting such requirements with their attractive features of high voltage blocking capability, ultra-low switching losses, fast switching speed, and high allowable operating temperatures. This paper analyzed the performance benefits and application challenges of using SiC or GaN devices in both PV and wind energy conversion systems. Solutions to these challenges of using WBG devices in various RES were reviewed and proposed, and the benefits of using such emerging devices were confirmed in simulation based on a 250 kW commercial-scale PV inverter and a 250 kW doubly fed induction generator wind turbine system.

Application of wide bandgap devices in renewable energy systems — Benefits and challenges

Solid-state transformers (SSTs) are an emerging technology that has been developed to improve the stability, reliability, and economic operation of distribution systems. These new transformers are composed of a medium ac voltage (MV) stage, a dc stage, and a low ac voltage (LV) stage. Passive and active dc links are used to construct the dc stage in SSTs in order to offer new functionalities, including hybrid (ac and dc) distribution, reactive power compensation, voltage/frequency regulation, power quality improvement, and distributed generation utilization. On one hand, a distribution SST has its ac stage connected to an MV level, which mandates specific power electronic converter (PEC) topologies, switching element capabilities, and filtering circuits. On the other hand, the dc-link stage has to provide isolation between the MV and LV levels, which requires the employment of isolated dc PECs. Part I of this work provides a review of SST designs and constructions (for deployment in distribution systems), in terms of the required technology, supported functionalities, and construction features.

https://scholar.ptuk.edu.ps/bitstream/123456789/723/1/S.pdf

Solid-State Transformers for Distribution Systems–Part I: Technology and Construction

This paper presents an inductive power transfer (IPT) system targeting at electric vehicles (EVs) and hybrid EVs. IPT systems provide significant benefits over the conventional plug-in chargers. However, in order for IPT to be adopted for EV charging, efficiency is a key figure of merit, which needs to be achieved. This paper presents a high-frequency inverter using gallium nitride (GaN) power transistors that have the benefit of low on-resistance and gate charge to reduce the switching and conduction loss. The switching characteristics of the GaN GIT are studied, and the inverter is designed to ensure low switching losses while keeping overshoot and slew rates under control. An efficiency centric mode of operation is proposed to improve the efficiency of the system while ensuring sufficient power transfer. The system efficiency peaks at 95% at 100-kHz operation and 92% at 250-kHz operation for a coil gap of 80 mm at 2-kW output power. At a coil gap of 150 mm, the system obtains above 90% efficiency at 1.3 kW. The IPT system is compared with a similar system using SiC power transistors and outperforms it by 1% at 2 kW.

https://dr.ntu.edu.sg/bitstream/10356/83228/1/A%202-kW%2c%2095%20Efficiency%20Inductive%20Power%20Transfer%20System.pdf

A 2-kW, 95% Efficiency Inductive Power Transfer System Using Gallium Nitride Gate Injection Transistors

In this paper electromagnetic interference issues of power electronics systems with Wide Band Gap devices have been studied. The main advantages of these power switches are fast switching transient and high switching frequency capabilities which can improve efficiency of power electronics systems and reduce sizes of passive elements such as inductors and capacitors. Several conducted and radiated emission tests and efficiency measurement have been carried out to analyze the switching frequency and transient effects on harmonic emission and switching losses. The results have been compared with the same drive configuration, layout and enclosure but with conventional power switches (Si switches).

Electromagnetic interference issues of power, electronics systems with wide band gap, semiconductor devices

This article proposes a new single-stage high-frequency-link based single-phase solid-state transformer (SST) topology. Switching tube multiplexing is carried out to reduce the number of switching devices and the corresponding modulation strategy is given. An optimized hybrid control analysis for reducing the average conduction current of insulated gate bipolar transistor (IGBT) and inductor was performed. By switching different control modes, the average current value of the SST inductor is kept being the minimum so as to reduce the conduction losses of IGBTs in the SST. As the core unit of the SST is the dual active bridge converter, the proposed optimal control is based on the extended phase-shift control, instead of triple-phase-shift control, that makes the implementation method very simple. And the feasibility of topology and optimized hybrid control scheme is verified through experiments.

Research on Single-Stage High-Frequency-Link SST Topology and Its Optimization Control

The growing interest in integrating distributed generation into the existing power distribution grid, the increase in the penetration levels of renewables, as well as the need to achieve a more efficient, reliable, and sustainable grid is leading to the development of new grid-interfaced power converters such as the solid-state transformer (SST). As current and voltage ratings of commercially available power semiconductor devices are normally below power ratings required in distribution systems (e.g., 13.8 kVrms), multiple modules must be connected in cascade configuration at the high-voltage (HV) side to reach higher voltage ratings as well as in parallel at the low-voltage (LV) side to achieve high current levels. A new SST topology consisting of modular boost-based three-level ac–dc converters, medium-frequency transformers with two secondary windings, and four-leg ac–dc converters is presented in this paper. When compared to similar approaches, the proposed topology comprises fewer power conversion stages, lower voltage across the semiconductor devices on the HV side, and lower current flowing through each device on the LV side. These characteristics reduce the number of series-connected modules in the HV side and parallel-connected devices in the LV side. The feasibility of the proposed topology is experimentally validated on a 500 W, 120 Vac/48 Vdc scaled-down prototype.

A New SST Topology Comprising Boost Three-Level AC/DC Converters for Applications in Electric Power Distribution Systems

Medium-voltage converters, especially those making use of SiC devices, require high common-mode voltage immunity and resilience against associated high dv/dt across multiple isolation barriers. A truly modular and common-mode immune switching position could be beneficial for these applications. The design of a modular switching position is presented here for a series connection of power semiconductors with voltage-balancing and self-powered-gate capabilities. The designs of the voltage-balancing and self-powered circuits are described, followed by simulations and testing of a 3.3-kV switching position formed by two 1.7-kV SiC MOSFETs in series. Testing results demonstrate the ability of the proposed switching position to balance the voltage across series-connected MOSFETs even if the gate signals of the series-connected devices are not perfectly synchronized while powering themselves directly from the OFF-state voltage across them. In addition, a startup circuit for the switching position is proposed and experimentally confirmed.

A Modular Switching Position With Voltage-Balancing and Self-Powering for Series Device Connection

Photovoltaic (PV) systems based on microinverters harvest more sun energy than traditional central or string inverters because shading of a PV panel within an array affects only that panel. However, cost is still an issue when competing not only with fossil-based energy sources but also with central or string inverters. The main cost of a microinverter is most likely not related to the semiconductor devices but to the heat management elements. Increasing efficiency should reduce heat removal requirements and thus lowering costs to a level where grid parity could be reached. Gallium nitride (GaN) transistors are wide bandgap devices with lower switching losses than silicon (Si) devices, and thus, reduced heat management requirements. A complete design of a dual-stage microinverter based entirely on GaN technology and a comparison with a silicon design are presented to evaluate the impact on system efficiency. Experimental results from a 200W prototype demonstrate the benefits of the proposed GaN-based design, in particular, a 4% improvement in the efficiency representing 8W of less dissipated heat in the active devices when the converter operates at 200W.

Design of a GaN-based microinverter for photovoltaic systems

This paper presents the analysis and control of a current imbalance compensator designed to eliminate upstream negative- and zero- sequence current components caused by single-phase loads in three-phase four-wire systems. The proposed compensator significantly reduces the fundamental-frequency current component in the neutral conductor and can potentially be used to eliminate harmonic currents. The Matlab/Simulink™ simulations are based on a 3-MVA radial feeder case study and a scaled-down 10-kVA test setup used to validate the proposed ideas. Three TMS320F28335 DSPs are used to implement the control system and the results of both the simulations and the prototype testing are shown and discussed. Results from testing of the prototype demonstrate the ability of the compensator to reduce the current in the neutral conductor.

/pdf/correcting-current-imbalances-in-three-phase-four-wire-fmq5tkv6iv.pdf

Correcting current imbalances in three-phase four-wire distribution systems

Research into microgrids has become more promising during recent years. It has the potential of leading to a more resilient electric power delivery system in the face of system disturbances. Low pass ac filters are frequently adopted in microgrid power electronic interfaces that convert dc power to ac power because most of today's distribution grids have ac voltages. Compared to a simple L filter, higher order filters, such as LC or LCL filters, are preferred due to their more effective reduction of switching-frequency harmonics and smaller sizes. However, the resonance problems caused by paralleling multiple converters with LCL filters, especially those converters having large power ratings (e.g., in the MVA range), must be accounted for when designing the entire microgrid. The analyses and designs should consider not only the resonant stability of a single converter but also the resonance propagation between multiple converters. A microgrid resonance propagation circuit model is built and investigated in this paper. Experimental waveforms validate the proposed analyses.

Vinson Jones

Papers

A New SST Topology Comprising Boost Three-Level AC/DC Converters for Applications in Electric Power Distribution Systems

A Modular Switching Position With Voltage-Balancing and Self-Powering for Series Device Connection

Design of a GaN-based microinverter for photovoltaic systems

Correcting current imbalances in three-phase four-wire distribution systems

Resonance propagation of ac filters in a large-scale microgrid