A modular DC solid state transformer for future onboard DC grid

A Solid-State Transformer as Enabling Technology for Microgrids for Integration into the Power Distribution System: An Overview

The solid state transformer (SST) is a power electronic transformer that combines power electronic conversion technology with high frequency power conversion technology based on the principle of electromagnetic induction. Modulation strategies based on the input stage of modular multilevel solid state transformers currently have the following types: nearest level modulation (NLM), carrier phase shift modulation (CPS) and hybrid modulation. This paper focuses on the advantages and disadvantages of the nearest level modulation and hybrid modulation. Considering comprehensively, the hybrid modulation method is adopted in the input stage of the modular multilevel solid state transformer to realize the control of the solid state transformer rectifier stage. The simulation results show that the hybrid modulation method reduced the harmonic content of the grid side output current and the arm circulation to a certain extent.

Power electronic transformers (PETs) are promising for the high-speed traction industry owing advantages such as high power density, high efficiency, and environmental friendliness. The cascade H-bridge (CHB) rectifier at the front end of a PET constitutes a significant proportion of the overall volume and mass of the PET. Modulation strategies for CHBs are divided into three main categories: staircase, carrier phase-shift (CPS), and hybrid modulations. Within these different modulation strategies, there is a tradeoff between the DC-link capacitance and switching loss, leading in turn to a tradeoff between the volume and mass of the DC-link capacitor and heat sink. In this study, the DC-link capacitance was calculated based on a restriction set on the DC-link voltage ripple. For CPS and hybrid modulations, the capacitance was calculated using the amplitude of the second-order harmonic in the DC voltage, whereas for staircase modulation, the worst-case method was proposed to calculate the required DC capacitance. The switching and conduction losses were calculated from the loss function in the HITACHI 6500V IGBT datasheet and the switching characteristics of the different modulation methods. The volume and mass of the heat sink were calculated using the thermal model of the switching device and the switching loss. Among the three considered modulations, hybrid modulation exhibited the highest power density. The accuracy of the calculations was confirmed through experiments and real-scale simulations.

Solid-state transformer (SST) technology has emerged as a promising alternative in distribution networks providing transformation between voltage levels through the employment of power electronic converters and a high-frequency isolation transformer. SST operation is employed in either medium or high-frequency ranges, leading to a considerable reduction in overall weight and volume. This paper proposes a modified single shift modulation for Modular Multilevel Converter Based Solid State Transformer (MMC-SST) operation for high power wind-turbine integration. The power controller is designed such that the power requirement of the medium-voltage DC (MVDC) grid is achieved using a proportional-integral (PI) controller along with traditional MMC voltage balancing which further enhances the overall system performance. A MATLAB/SIMULINK model is constructed for a 4 MW DC microgrid to validate the proposed control and modulation scheme. Different scenarios for normal and fault conditions are investigated where the efficiency of the proposed system under balanced power distribution reaches 96%.

The recent advancement in the technology of SiC MOSFETs has spurred interest in designing compact and high switching frequency (10–20 kHz) power converters. However, grid-integration of these power converters at medium voltage (MV) scale would require a conventional transformer. With the development of new high voltage (HV) 10 kV and 15 kV SiC MOSFETs, these converters can directly interface with medium voltage (MV) grids without the need for line-frequency transformers, using simple two-level and three-level topologies. The application of these devices is currently being explored in all MV Applications (8 kV to 30 kV) like Solid State Transformer, MV Drives, Power Conditioning Systems, and MVDC isolators. This paper discusses application of 10 kV SiC MOSFETs and JBS Diodes for enabling Asynchronous Microgrid Power Conditioning System (AMPCS). This medium voltage power converter is enabled by series-connection of devices, in a Three-Level Neutral Point Clamped (3L-NPC) configuration. The voltage balancing of these series-connected devices is achieved by using R C-snubbers. This paper addresses the different conduction modes and switching sequences of a 3L-NPC pole, which is used as building block for the three-phase converter. The switching loss analysis, for various snubber values, is presented for the MOSFETs and the clamping diodes along with experimental results. This research helps in providing an overview of switching losses that are disspated through the device (and heatsink) and through the snubber resistor in a 3L-NPC convertor pole.

Solid State Transformer (SST) is a new type of power transformer based on power electronic converters and high frequency transformers. The SST realizes voltage transformation, galvanic isolation, and power quality improvements in a single device. In the literature, a number of topologies have been introduced for the SST. In this work, employing a modular multilevel converter, a new SST topology is introduced which provides not only high-voltage AC (HVAC), low-voltage AC (LVAC) and low-voltage DC (LVDC) ports, but also high-voltage DC (HVDC) port. Besides, the proposed topology is easily scalable to higher voltage levels.

Implementing medium-voltage DC (MVDC) grids in ship power systems has many intricacies, such as larger fault currents, the inability to include conventional protection systems, etc.; however, this has not been deeply investigated. To circumvent these intricacies, it is important to understand the characteristics of MVDC grids installed in a ship power system environment. A feasible method to achieve this is to model the grid components in terms of their equivalent circuit and study their typical behaviour under fault conditions. Being a DC circuit, the steady-state grid parameters of the ship power system are predominantly resistive in nature; therefore, the pertinent circuit model approach remains theoretically adequate. Pertinent results and information gathered from such studies might provide a clear insight into the typical behaviour of MVDC grids under faults and help to resolve their diagnostic integrity. In this context, a simulation study that aims to understand the influence of inherent circuit parameters of MVDC grid components under normal and fault conditions is initiated. First, a circuit model that adequately represents a 2 MW MVDC grid is selected, and its transient and steady-state behaviours under normal and fault conditions are studied. The grid components (i.e., generator, converters, etc.) are modelled with r, l, and c parameters in such a way that the steady-state and transient nature of the grid elements are theoretically represented. Later, the influence of these circuit representations during short-circuit faults in different sections of the DC grid is studied. Pertinent results reveal that the circuit representations of grid components manifest significant impact under fault conditions and induce damped-out oscillations superimposed on grid voltage and currents. This information is essential to developing smart fault protection systems operating independently of the main grid control algorithm and might help to enhance safety features along with grid resilience and reliability.

Power electronic transformer (PET) which has a big potential application value in smart grid is an electrical power transformer device adopting power electronic converter and high frequency switch transformer. A new PET with power factor correctors (PFC) is proposed in this paper. Due to high power level of PET, PFC should have a high power level as well. Therefore, the multi-phase interleaved PFC is employed. The paper describes the one cycle control principle, proposes a current synthesis method based on IGBT current, and then analyses the relationship between ripple current and duty cycle of IGBT. In addition, the whole PFC system is simulated completely by means of Matlab/Simulink. In order to verify the theoretical analysis and simulation analysis, a four-phase interleaved PFC with a rated output power of 8.0kW is designed and implemented based on the an analog control chip. The obtained results show that the interleaved PFC by means of one cycle control and current synthesis is feasible, capable of reaching a good suppression effect of ripple current.

This paper presents the design of new highfrequency transformer isolated bidirectional dc-dc converter modules connected in module-cascaded solid state transformer (SST). A phase-shift dual active bridge (DAB) converter is employed to achieve high-frequency galvanic isolation, bidirectional power flow, and zero voltage switching (ZVS) of all switching devices, which leads to low switching losses even with high-frequency operation. Bidirectional DC-DC converter is crucial to the power transmission in SST. The proposed DAB converter consists of two three-leg bridges and a high-frequency transformer with winding shunting taps. Furthermore, the commutation inductance connected in series with the transformer in the conventional DAB converter is integrated into the transformer windings, thus, enjoying smaller volume and higher power density. By changing the number of primary winding turns, leakage inductance, which is the key parameter in the energy transfer process of DAB converter, can be adjusted on a large scale, enabling the possibility of reducing loss over wide load range. Additionally, time-sharing circuit topology and operation mode are adopted to adapt to the output power by detecting the output current as the feedback. As a result, the efficiency at both light and heavy load can be significantly improved compared with the conventional DAB converter, and, therefore, high efficiency over wide load range and high power density can be achieved. Besides, the additional bridge arm structure along with a flexible control scheme provides possible high-level fault tolerance. Finally, the simulation results on a 2-kW DAB converter module switching at 10kHz are presented to validate the theoretical analysis.

The modern trend towards high-speed trains (HST) with distributed propulsion systems, demands high efficiency and high-power density traction systems. Line frequency transformers (LFTs) in railway traction systems are heavy and bulky, quite often necessitating power density to be compromised to achieve maximum efficiency of typically 90-92%. The advancements in power converter topologies, power switching devices and magnetic materials makes it possible to substitute massive LFTs with a new technology called solid-state transformers (SST) (also known as power electronic transformers (PET) or medium frequency transformers (MFT)) traction technology. This technology enables high power density systems with comparatively lower noise emissions which provide essential functionality without compromising efficiency. However, there are still major challenges to overcome associated with power converter connection on the high-voltage (HV) side, architecture modification and the compactness of the transformer design. This paper reviews the existing architectures and also introduces the new research possibilities especially in the power conversion stages, and the power switching devices. Finally, the design guide lines for high-power converters are presented.

Power electronic transformers, also known as solid state transformers, are drawing more and more attention of the researchers. One of its key components is high frequency power transformer (HFPT) subjected to electrical stress of several kilohertz. Voltage frequency has a significant impact on discharge behaviors of insulation material, or even brings about accelerated insulation aging and premature failure. In order to investigate the influence of voltage frequency on surface discharge behaviors and aging process, an experimental platform with variable applied voltage for electrical aging tests was set up. Presented aging tests in this paper were conducted on polyimide films subjected to surface discharges under sinusoidal voltage at different frequencies ranging from 5 kHz to 30 kHz. In the paper special focus is given to the lifetime of specimens and discharge characteristics, including maximum and average discharge amplitude, total discharge number and amplitude per cycle or per second. In terms of these data, insulation lifetime plot relevant to voltage frequency is computed, and the correlation between lifetime and discharge characteristics is built, considering partial discharge as the core cause of insulation failure. Experimental results reveal the accelerated aging mechanism from high frequency electrical stresses based on the analyses of lifetime and discharge characteristics of specimens under different frequencies.

Compared with the line-frequency transformer (LFT), the emerging power electronic transformers (PETs) have gained wide concerns due to the significant merits of higher power density, higher reliability, more flexibility, and multiple functions. However, the need for bulky energy storage elements, multi-stage power conversion and reduced conversion efficiency, and the intrinsic twice-frequency pulsating power issue are the main disadvantages of the conventional single-phase PETs. To overcome the above shortcomings of conventional single-phase PETs, this paper develops a matrix-type single-phase AC-AC PET without bulky energy storage elements. The proposed PET consists of a line-frequency commutated rectifier, a half-bridge LLC resonant converter with a fixed switching frequency, a boost converter, and a line-frequency commutated inverter. The LLC operates efficiently with unity voltage gain and acts as a high-frequency isolated DC transformer (DCX). The boost converter provides AC output voltage regulation function and the line-frequency commutated inverter unfolds the output voltage of the boost converter to generate the sinusoidal AC output voltage. As a result, high power density, reduced power conversion stages, direct AC-AC power conversion without twice-frequency pulsating power, high conversion efficiency, and high reliability are achieved. The experimental results on a 1kW PET prototype show that sinusoidal input current and output voltage, ZVS of the LLC stage, and output voltage regulation capability are realized. The experimental results verify the correctness and feasibility of the presented methods.

This paper investigates the impact of several design parameters on the electro-thermal performance of a solid-state circuit breaker (CB) for medium voltage DC (MVDC) grids. The parameters studied are the cooling system design along with the ambient temperature under steady-state. The impact of the current limiting inductor and the threshold current that trips the breaker on the CB operation under short circuit condition was also investigated. The performance of the breaker is evaluated in terms of conduction power losses, dissipated power and junction temperature in the power electronic devices used. Four electrothermal models for heat flux and junction temperature estimation have been developed and presented. The simulations highlighted the superiority of the “RC” equivalent thermal network in terms of required computational time and accuracy. Finally, the importance of designing the breaker according to the electrical and thermal limitations of the power electronic devices used is also highlighted.

Aiming at fulfilling the desirable features of compact structure, high efficiency, high reliability, and multifunctionality for the solid state transformers (SSTs), a single-cell matrix-type SST as well as its control method is proposed as the interface between the medium-voltage (MV) and the low-voltage (LV) levels for direct MV AC to LV AC conversion. To achieve zero voltage switching (ZVS) at both the MV and LV sides, a modulation scheme to construct the special resonant currents during the dead time by actively controlling the MV and LV side gate signals is implemented. Furthermore, the ZVS conditions of the MV and LV side bridges are analyzed, and system parameters are properly designed to ensure full range ZVS operation. A 1-kW SST prototype is developed to verify the functionality and effectiveness of the proposed methods. The experiments show that sinusoidal input and output, bidirectional power flow capability, and ZVS at both sides over wide input voltage and load ranges are achieved with the nominal efficiency of 96.1%. The experimental results match the theoretical analysis and the simulation results well.

As the key equipment in the modern power system, the power electronic transformer's performance and reliability play a vital role in the stable operation of the power system. This paper studies the soft switching technology, control technology and fault protection method of power electronic transformer. First, the soft switching technology of power electronic transformers is introduced, including zero voltage switching, zero current switching and so on. These soft switching technologies can effectively reduce the switching loss of the transformer, improve efficiency, and reduce electromagnetic interference and harmonic generation. Secondly, the control technology of power electronic transformer is introduced, including traditional proportional, integral, and derivative control (PID) control, model predictive control and direct power control. These control technologies can realize the precise control of the output voltage and current of the power electronic transformer, to meet the requirements of the power system for power quality and stability. In addition, this paper also introduces the fault protection methods of power electronic transformers, including short-circuit protection, over-voltage protection and over-medium current protection. These fault protection methods can effectively protect power electronic transformers from various faults and ensure their normal operation and safety.