Ensuring grid stability with increasing penetration of inverter-based resources requires robust Grid Forming (GFM) control methods. Droop control is a prevalent method, but its reliance on low-pass filters (LPF) for power calculations can limit performance and responsiveness. To address this, Virtual Oscillator Control (VOC) employing Andronov-Hopf principles offers a promising alternative while retaining the simplicity and load sharing capabilities of droop control. This paper analyzes a state of art Andronov-Hopf VOC, demonstrating its reliability when connected to a modern grid consisting of Renewable Energy Sources (RES) and Battery Energy Storage Systems (BESS). MATLAB/Simulink simulations demonstrate the controller's ability to maintain microgrid stability within acceptable limits while exhibiting droop-like characteristics during islanding operation. This paper focuses solely on islanding operation; grid-tied operation is beyond the scope of this work.

Modularity and expandability are among the most important features of modular multilevel converters (MMCs). However, these features are often constrained by the complexity of control systems. In order to leverage the advantages of modularity within the control system, this paper proposes a modular control system for MMCs based on the sub-MMC concept. In this approach, all lower-level control tasks, such as capacitor voltage balancing, differential current control, and modulation, are designed as control module that can be implemented independently for each sub-MMC. Additionally, a modular space vector modulation (SVM) technique is introduced, which can be easily implemented for each sub-MMC. The utilization of the proposed modular SVM method enhances the common DC-link voltage utilization and offers switching state redundancies that can enhance the performance of MMCs. The efficacy of the proposed method is demonstrated by simulation and experimental results.

Solid-State Transformers (SSTs) are gaining popularity in a number of applications to directly interface with the medium-voltage (MV) AC grid. As SSTs fundamentally consist of power electronic semiconductor devices, a key challenge is to design SST systems capable of withstanding the various overvoltage conditions observed on the MVAC grid such as lightning surges, switching surges and transient overvoltage conditions. This paper proposes a protection scheme that uses an AC crowbar in combination with existing MV overvoltage protection devices to offer robust protection to SSTs. The AC crowbar operation is analyzed in conjunction with MV breakers and surge protection equipment and design guidelines are presented for the case of a 13.8kV/1MVA grid-tied SST.

The concept of the Hybrid Solid State Transformer (HSST) has been introduced in earlier works – an amalgam of a large power low frequency transformer and fractional power solid state transformer. This offers an optimum compromise between cost and controllability, and thanks to its fractional approach, high efficiency. This paper presents an Input Parallel Output Series HSST, using a medium voltage two-stage isolated SST. The predicted peak efficiency of the SST is 96.8% and of the HSST is 98.5%. The HSST unit has been tested to verify the concept and operability.

This paper presents a novel three phase, three level, isolated bidirectional AC/DC converter. The topology can switch between a 400V and 800V operation whilst maintaining virtually the same efficiency for the same power. The single stage approach provides this topology a high device utilization. A set of unique modulation schemes are also presented along with a design study showing exceptionally high efficiency of close to 99% for both 400 and 800V operation.

A compact 300 kW air-cooled SiC power electronics building block (PEBB) that integrates discrete SiC power switches, advanced gate drivers, low inductance loop design, DC link capacitors, and forced air cooling heatsink is developed. This paper discusses the design consideration of the PEBB focusing on device selection and efficiency. A high-speed gate driver is designed to drive paralleled SiC devices. Direct Bonded Copper (DBC) thermal management is implemented and the junction to ambient thermal resistance for each SiC switch is experimentally extracted to be less than 1oC/W with forced air. PEBB power density of 43.9kW/L is achieved. The PEBB is tested in inverter mode and demonstrated an efficiency of 99.4% at 40 kW.

Photovoltaic (PV) systems play a critical role in renewable energy resource grid integration, and levelized cost of energy (LCOE) is commonly used to evaluate PV system feasibility in modern power grids. In this work, a revised PV system LCOE calculation model is derived to quantify the potential of LCOE reduction. Particularly, the grid support functions are valuated to offset the investment and operation costs of PV systems, which thereby reduces the LCOE. Meanwhile, PV system service lifetime is also estimated with the derived PV inverter reliability model, considering the critical components (i.e., semiconductor devices and capacitors). The case studies with field datasets are conducted to validate the effectiveness of the developed LCOE calculation model.

Advanced digital filter is important for grid voltage synchronization, harmonic information extraction, and converter control. The complex variable filter (CVF) such as the reduced order generalized integrator is one of the most prevalent filters. However, it is proven in this paper that the real coefficient CVF achieves performance identical to the real variable filter such as the second-order generalized integrator. It implies that the advantages of CVF are not fully utilized. Therefore, in this paper, the real coefficient is extended to a complex one leading to a complex coefficient CVF (CC-CVF), and the filter performance of CVF for extracting the harmonics is significantly improved. Furthermore, linear quadratic regulation is employed to design the complex coefficients to achieve a better dynamic performance. Finally, the CC-CVF is enhanced with frequency estimation capability. The proposed CC-CVF is verified by extensive simulations and experiments.

Modern civilization is related to the increased use of electric energy for industry production, human mobility, and comfortable living. Highly efficient and reliable power electronic systems, which convert and process electric energy from one form to the other, are critical for smart grid and renewable energy systems. The power semiconductor device, as the cornerstone technology in a power electronics system, plays a pivotal role in determining the system efficiency, size, and cost. Starting from the invention and commercialization of silicon bipolar junction transistor 60 years ago, a whole array of silicon power semiconductor devices have been developed and commercialized. These devices enable power electronics systems to reach ultrahigh efficiency and high-power capacity needed for various smart grid and renewable energy system applications such as photovoltaic (PV), wind, energy storage, electric vehicle (EV), flexible ac transmission system (FACTS), and high voltage dc (HVDC) transmission. In the last two decades, newer generations of power semiconductor devices based on wide bandgap (WBG) materials, such as SiC and GaN, were developed and commercialized further pushing the boundary of power semiconductor devices to higher voltages, higher frequencies, and higher temperatures. This paper reviews some of the major power semiconductor devices technologies and their potential impacts and roadmaps.

This paper discusses the advancements in the development of the medium voltage solid state transformer (SST) based on 15 kV SiC MOSFET and JBS diode. Designed for 7.2 kV single phase distribution grid applications, the medium voltage SST converts high voltage AC to low voltage 240/120V ac. The use of ultra-high voltage SiC devices allows the simplification of the power conversion circuit topology. This paper presents the characteristics of the high voltage SiC MOSFET devices as well as the topology innovations to achieve ultra-efficient SST design. Specifically, three different designs are discussed which utilize three-stage, two-stage and single stage power conversion topologies to achieve the AC to AC conversion.

Full-bridge power-factor-correction (PFC) front-end + dual-active-bridge (DAB) AC/DC topology is widely used and its small-signal model has been studied in depth. Compared to the two-stage design, the single-stage design, unfolding bridge + DAB, is attracting more attention nowadays. By eliminating the bulky DC link bus capacitor and operating the front-end with only 60Hz switching frequency, the DAB based single-stage AC/DC converter has the potential to increase the system power density and efficiency. Different from the conventional constant-switching-frequency (CSF) DAB control, a novel variable-switching-frequency (VSF) DAB control is more suitable for the single-stage design due to the 0 to peak input voltage. A full-order state-space model is proposed in this paper to describing the VSF control. The influence of DC blocking capacitor is stressed because there is always a low frequency excitation of VSF control which is not existed with CSF control. The resonance of DC blocking capacitor and transformer leakage inductor is well described by the full-order state-space model. Simulation and experimental results verified the correctness of proposed DAB full-order state-space model.

Full-bridge power-factor-correction (PFC) front-end + dual-active-bridge (DAB) AC/DC topology is widely used in industry, e.g., electrical vehicle on-board charger. Such two-stage topology limits the system efficiency, and the bulky DC link bus capacitor makes the system power density relatively low. Compared to the two-stage design, the single-stage design, unfolding bridge + DAB, eliminates the bulky DC link bus capacitor and operates the front-end with only 60Hz switching frequency, thereby has the potential to increase the system power density and efficiency. A novel variable-switching-frequency and hybrid single-dual-phase-shift (VSF-SDPS) control strategy is proposed and analyzed for the DAB based single-stage topology. The proposed VSF-SDPF control consists of two phase shifts to guarantee Zero-Voltage-Switching (ZVS) over the full range of the AC line voltage, and frequency modulation to achieve boost PFC. The conventional front-end PFC is simplified to an unfolding bridge by changing DAB control strategy to achieve PFC and ZVS at the same time. Besides, a special ZVS boundary is utilized to solve the grid current distortion problem when the switching frequency saturated, which is especially severe at light load condition. Simulation results and experimental validation are presented under 50Vrms AC line voltage and 200V DC battery voltage test condition.

Data center evolved to become large power consumers. Its supporting infrastructure, such as cooling and power distribution, consumes resources e.g. electric power and money. In fact, most money is spent on power conditioning and cooling systems. Therefore, the efficiency of data center and power quality are important topics and can be addressed by careful design. There is a need for research into application of innovative design and state-of-the-art technology for optimizing power supply in data centers. The paper investigates potential of a state-of-the-art technology such as solid state transformer (SST). The simulation proves that SST has potential to improve operational efficiency. SST shows better control of voltage, current and reactive power of the facility, eliminating need for additional devices such as power conditioners and reducing power loss. These results lead to the next stage of the project that involves developing an optimal data center design with SST and investigating data center potential in demand response applications.

A truly distributed and autonomous control strategy is proposed for the FREEDM System-a power electronics based distribution grid. The proposed control strategy requires no communication between and among all grid assets. By utilizing available local quantities (V, f) as a way to communicate among all connected devices, the strategy contains a novel dual droop control loops between the medium voltage feeder and dispatchable resources such as energy sources, storages. The proposed control is put forward to achieve intelligent power management of the FREEDM system under all modes of operation. This is a pre-requisite to use FREEDM system as a transformative platform for plug-and-play of energy (aka Energy Internet). Simulation results verify the autonomous nature of the proposed control.

Modular multilevel converter (MMC) is a relatively new and promising topology for HVDC systems. HVDC systems should remain connected during grid faults and isolate the fault. This paper studies the dynamic performance of transformer-less MMC integrated HVDC systems during unbalanced conditions and asymmetrical grid faults. It proposes a new control technique to improve unbalanced system's performance. The objective of the proposed controller is eliminating negative and zero sequence currents and to improve the overall performance. The controller calculates zero and negative sequence reference voltages and eliminates zero and negative sequence currents without using any current regulator. Therefore the controller is very fast and robust. The effectiveness of the proposed control technique has been validated by EMTDC /PSCAD simulations.

Low Voltage Ride Through (LVRT) is an important grid requirement for Voltage Source Converter (VSC) based HVDC links. This paper studies the performance of the modular multilevel converter (MMC) VSC based HVDC systems during faults or voltage dips and proposes a new control strategy to improve the LVRT performance. The proposed algorithm controls the system to generate the required active and reactive powers that are calculated mathematically based on the ratings of the MMC-HVDC system and LVRT requirements. The injected active and reactive power values obey the LVRT guidelines and are adaptable to different grid codes. The mathematical calculations are presented and EMTDC/PSCAD simulation evaluates the performance of the proposed method.

The bidirectional isolated AC/DC converter is widely used in the past decades. Most of the topologies are based on voltage source converter. However, the performance of the voltage source based converter is significantly limited by the bulky energy storage element, the range of soft switching, the leakage inductance of the transformer and so forth. A current fed isolated AC/DC converter is studied in the paper. A novel soft switching control scheme is used in this topology, which could achieve the natural clamping and the zero current commutating. Instead of causing the voltage spike problem, the leakage inductance of the transformer could be utilized to assist the soft switching. The natural voltage clamping and the soft switching could be achieved without limiting the operating range of the inductor into discontinuous current mode. The control scheme and PWM generation scheme is analyzed in this paper. Simulation results are used to validate the operation of the novel control scheme in this topology.

Three generations of medium voltage fault isolation devices (FID) for a power electronics based distribution system, the FREEDM System, are reported in this paper. In the Gen-I FID, three 6.5 kV silicon IGBTs are series connected to achieve 15 kV class distribution voltage blocking capability. Whereas in the Gen-II and the Gen-III FIDs, 15 kV Silicon Carbide (SiC) ETO devices are used. Since one single such device can meet the voltage withstand requirement, the operation losses as well as dimensions are significantly reduced. Future improvements in the Gen-III FID include the use of hybrid schemes and the development of symmetrical blocking high voltage SiC devices to further minimize the operation losses.

The isolated DC/DC stage in solid state transformer (SST) is a very critical stage in the whole system. It converts the high DC voltage to low DC voltage with the galvanic isolation after the high voltage AC/DC rectifier and before the low voltage DC/AC inverter. In previously developed solid state transformers, dual active bridge (DAB) and dual half bridge (DHB) are the most common topology for this part. In this paper, a bidirectional LLC resonant converter is presented to replace the traditional DAB or DHB in SST application. Compared with DAB, the bidirectional LLC converter can achieve higher efficiency with wider range of zero voltage switching (ZVS), zero current switching (ZCS) and simpler control strategy, which makes it highly attractive for solid state transformer applications. The converter operation modes are presented and analyzed in this paper. For achieving higher efficiency, a methodology of hardware design optimization is introduced to minimize the power losses. Analysis, simulation and experimental results based on a scaled down prototype are shown and discussed.

In this paper, the operation of a single phase medium voltage Solid State Transformer (SST) is presented. With the new development of high voltage wide band gap semiconductor devices, particularly the SiC MOSFETs and JBS diodes, SSTs that are suitable for distribution grid can be developed with simpler topology and reduced device number. Features such as renewable energy integration, grid voltage restoration, load harmonic mitigation, regenerative power flow can all be implemented at the distribution node deploying SSTs. This paper explains each of these operations, accompanied by test results carried out with a laboratory prototype.

A novel five-level DC/AC bidirectional converter is developed and applied for interfacing renewable energy into the grid. The converter has reduced switching losses, voltage stress, harmonic distortion and electromagnetic interference caused by switching operation of power devices. The bidirectional operation feature allows it to interface with parallel dc-dc PV optimizers that requires a start-up dc link voltage of 200-250Vdc. Two DC decoupled capacitors, a high-frequency three-level converter, a grid-frequency unfolding bridge and a filter compose the proposed converter. The high-frequency converter offers a three level positive voltage output, while the unfolding bridge is responsible for switching the flow direction every half grid cycle. With the proposed control strategy, the converter operates via two modes, i.e. the inverter and rectifier mode. In inverter mode, the output current of the converter generates a sinusoidal current in phase with the grid voltage, while in rectifier mode, grid voltage acts as the source and dc link supplies the PV optimizer with a threshold voltage to avoid false tripping of the optimizer. A hardware prototype is further developed to verify the performance of the topology and control strategy.

A hierarchical power management scheme is proposed in this paper for a typical DC Microgrid. Different from other Microgrids, the DC Microgrid can interface to the distribution system by Solid-State transformer (SST). The hierarchical power management strategy includes three control levels: 1) primary control for DC Microgrid to implement distributed operation 2) secondary control for the DC Microgrid bus voltage recovery to achieve seamless mode switch 3) tertiary control to manage the battery charge and discharge. The DC Microgrid can operate in islanding mode, including the individual control for distributed renewable energy source (DRER) and distributed energy storage device (DESD). In addition, the DC microgrid can operate in SST-enabled mode to interface to the distribution system. The DC Micorgrid can seamlessly switch between islanding mode and SST-enable mode. The consideration of state of charge (SOC) for battery is also involved into the tertiary control. To this end, a lab test-bed is constructed to verify the system performance. Lastly, several typical case studies are carried out and the experimental results verify the proposed power management strategy.

Modular multilevel converter is regarded as a promising technology in high voltage application, such as off-shore wind farm system. This paper proposes control technique for modular multilevel converters, aiming at suppressing the AC components in the circulating current. Specifically, an additional proportional-resonant control loop is designed to regulate the AC component of the circulating current to zero. The proposed method can effectively suppress the AC circulating current even in the unbalanced/fault condition. Simulation results for a 10MVA, 25KV DC system are provided to demonstrate the feasibility of the proposed method.

Modular multilevel converter is a next generation multilevel converters for medium to high voltage conversion applications, such as medium voltage motor drive and high voltage direct current transmission. One potential issue of this type of converter is the AC circulating current, which increases the current stress and brings additional conduction loss to the system. This paper proposes modified control architecture for modular multilevel converters, aiming at suppressing the AC components in the circulating current. Specifically, a proportional-resonant type minor loop is incorporated to regulate the most AC components of the circulating current to zero in addition to the DC regulation loop. The proposed minor loop can also be applied to single phase MMC, which is not available in previous methods. Simulation results for a three-phase MMC operating as an inverter are provided to demonstrate the feasibility of the proposed method.

This paper presents an architecture for a future electric power distribution system that is suitable for plug-and-play of distributed renewable energy and distributed energy storage devices. Motivated by the success of the (information) Internet, the architecture described in this paper was proposed by the NSF FREEDM Systems Center, Raleigh, NC, as a roadmap for a future automated and flexible electric power distribution system. In the envisioned “Energy Internet,” a system that enables flexible energy sharing is proposed for consumers in a residential distribution system. The key technologies required to achieve such a vision are presented in this paper as a result of the research partnership of the FREEDM Systems Center.

In this paper, we present the design of a high-frequency (HF) rectifier implemented in a 0.5-μm 3M/2P 5V standard CMOS process for wireless power transmission across short-range inductive links. The rectifier has been optimized for 13.56 MHz ISM band, and achieves power conversion efficiency (PCE) of ~83% and voltage conversion ratio >92% in post-layout simulations. We have successfully incorporated a dual-mode back-telemetry capability in this rectifier with little area or efficiency overhead, which tolerates wide load-fluctuations at the secondary side. With these improvements, this active back-telemetry rectifier (ABTR) is geared towards increasing the overall system efficiency, data transfer rate, and reading range in applications such as implantable microelectronic devices (IMD) and radio frequency identification (RFID).