DC Transformer Research Articles

Dynamics analysis for DC transformers integration in hybrid AC‐DC power distribution networks

AbstractIn AC/DC power distributions networks, power converters are interfacing buses to ensure DC voltage regulation and power distribution. Active front ends are used between AC and DC buses and two typical control schemes that can be implemented are voltage regulation or power regulation. On the DC side, open‐loop operated DC transformers could be used between two DC buses to provide voltage adaptation, isolation and natural power flow. In this context, power flows and DC bus voltages in the power distribution network are dependent on the combined operation of the active front ends and the DC transformer. Mainly studied from a stability perspective, this interdependency between active front ends and DC transformers has lacked investigations in terms of dynamics. Fast dynamics is nevertheless important to operate optimally a network. In this paper, the assessment of the dynamic performance for distribution systems with DC transformers is performed for different mixes of voltage regulating and power regulating active front ends. The influence of DC line length and DC transformer power reversal method on the system is further investigated. A model of the system is proposed, validated experimentally and can be used to aid system level design and analysis of future DC systems.

Over-Current Mechanism and Suppression Strategy of DC Transformer in Multiterminal Interconnected Flexible DC Distribution Network

Direct current (dc) transformers are an important component of the multiterminal interconnected flexible dc distribution network. However, it is apt to suffer from the inrush current, which can trigger the shutdown protection and threaten the reliability of the system and equipment. In this article, a 2-MW dc transformer is developed for a T-connected star type multiterminal flexible dc distribution network, where the dual active bridge (DAB) converter is one of the important components. The design of the key parameters is also presented. Based on the steady-state model of the dc transformer, the overcurrent mechanism is elaborated and a suppression strategy of H-bridge temporary blocking and DAB not blocking is proposed, which can automatically clear the fault current. Simulations and practical engineering tests are conducted to verify the effectiveness and practicability of the proposed method, which provides engineering practical value.

Time-Domain Analysis and Optimal Design of LLC-DC Transformers (LLC-DCXs) Considering Discontinuous Conduction Modes

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> -DC transformers ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> -DCXs) operate under fixed-frequency and open-loop conditions to achieve high efficiency and are promising converters that can be used in the auxiliary power units (APUs) of electric locomotives. An optimal design method of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> -DCXs is proposed in this paper. First, the time-domain expressions of the symmetric half-bridge <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter with split resonant capacitors are derived. A current increment method is proposed to determine the expressions. Second, the discontinuous conduction mode (DCM) and voltage gain limiting conditions are used to obtain the magnetizing inductance ranges. The converter losses and zero-voltage-switching (ZVS) ranges are considered to determine the final magnetizing inductance. A 10 kW prototype is used as an example, and a detailed parameter design process is implemented. Compared with conventional design methods for the magnetizing inductance, the proposed method can effectively determine the maximum magnetizing inductance for the DCM operation of the converter. The effectiveness of the proposed method is proven by the simulations and experimental results obtained for the 10 kW <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> -DCX prototype. The maximum efficiency of the designed converter is 98.12% under the full load.

Modular Multilevel Resonant DC Transformer With Inherent Balancing Capability

As the demand for dc&#x002F;dc converters in medium voltage applications is increasing in recent years, more and more attention has been paid to modular multilevel technology to reduce device stress. Derived from modular multilevel converter and LLC converter, this paper proposes a modular multilevel resonant dc transformer. Compared with the multi-stack structures, the proposed dc transformer has a simpler single-stack structure, which reduces the number and cost of sub-modules (SMs). Moreover, through the co-design of the boost inductor and the transformer, the proposed scheme can realize zero voltage switching of the primary switches and zero current switching of the secondary rectifier. With circulant modulation, not only a flexible step-down ratio is obtained, but also the SM capacitor voltages can be self-balanced without adding the control complexity. In addition, a high operating frequency is synthesized through a low switching frequency of the SM, thereby reducing the size of passive components and switching losses. The operating principle and theoretical analysis of the dc transformer are presented. A downscaled hardware prototype is built, and experimental results validate the correctness of the theoretical analysis and the feasibility of the converter.

Trajectory-Switching-Modulation-Based Fast Power Transient Control Strategy for a Bidirectional Hybrid Modular DC Transformer

The hybrid modular DC transformer (HMDCT) combining the series resonant dual active bridge (SR-DAB) and phase-shift dual active bridge (PS-DAB) has the advantages of high efficiency and bidirectional power regulation. However, due to the unregulated SR-DAB resonant tank inertia and limited PS-DAB output power, the dynamic response of HMDCT is slower than the pure PS-DAB based DC transformer, which could lead to a larger output overshoot. To address the problems, first, a novel trajectory-switching-modulation (TSM) method is proposed to control the SR-DAB resonant tank to track the desired state-trajectory rapidly by using SR-DAB switch-states. Compared with the conventional LLC trajectory control, TSM is more suitable for the SR-DAB in the bidirectional HMDCT because TSM is available under bidirectional power flow without changing the topology or increasing magnetizing losses. Then, a TSM based fast power transients control strategy is proposed to improve the bidirectional HMDCT dynamic performance by controlling the PS-DAB and SR-DAB simultaneously. Besides, PS-DAB current stress can also be reduced during the load transients. Experimental results show that the proposed strategy has better dynamic performance than the classic proportional-integral feedforward based strategy.

Adaptive Control Algorithm for Two-Stage Integrated DC Transformer in DC Microgrid Applications

This paper presents the Two-Stage integrated DC Transformer (TSDCT) for DC Micro-Grid (MG) applications based on the Adaptive Control Algorithm (ACA). The prevailing control methods and design constraints employed in the DC transformer (DCT) do not consider the influence of the circuit&#x0027;s structural elements with their parasitic components on its performance. Consequently, it devalues the control variables that fail to achieve indispensable control features in the DC micro-grid network. Therefore, the proposed ACA is employed in the TSDCT, which improves the performance of TSDCT in comparison to the other control techniques in the DC micro-grid network. The proposed ACA includes the effect of the resonant tank circuit&#x0027;s parameters and converter&#x0027;s structural elements. Thereby, it defines the design constraints that significantly improve the performance of TSDCT in terms of efficiency and a wide range of voltage regulation at different buses. The detailed modeling and analysis are carried out at each stage to describe the principle and evolution of critical components involved in the ACA. Further, the control to output transfer functions are determined towards the implementation of the proposed ACA in the TSDCT. As a result, it exhibits voltage regulation in a wide range, soft-switching operation, high control resolution, and better performance under different operating conditions. Finally, a 4&#x00A0;kW laboratory platform is built, and test results validate the performance of the proposed TSDCT based on the proposed ACA for DC micro-grid applications.

Resonant Peak Suppression Approaches for Improving the Dynamic Performance of DCX-LLC Resonant Converter Based Two-Stage DC–DC Converter

The LLC resonant converter operated with fixed switching frequency can realize the function of dc transformer (DCX) and achieve high efficiency. To regulate the output voltage, a preregulator or postregulator is required, and thus constituting the DCX-LLC resonant converter based two-stage dc-dc converters, including the preregulator&#x002B;LLC and LLC&#x002B;postregulator converters. In this paper, it is found that there are two resonant peaks in the voltage-loop gain of the preregulator&#x002B;LLC converter, which limits the crossover frequency and degrades the dynamic performance. To suppress the resonant peak, the virtual resistor introduced into output rectifier scheme (VR-ORS) for damping the resonant peak and the notch filter with specific gain inserted into voltage loop scheme (NF-VLS) for eliminating the resonant peak, are proposed, respectively. The implementation of the two approaches is derived and the design considerations of the voltage-loop compensator are presented. Finally, a 6-kW preregulator&#x002B; LLC converter is fabricated and tested in the lab. The experimental results are provided to verify the effectiveness of the proposed resonant peak suppression approaches for improving the dynamic performance.

A SOGI-based adaptive controller design for second-harmonic currents suppression of variable frequency loads in DC buildings

DC buildings with integrated DC transformers (DCTs), as a typical representative of photovoltaics, energy storage, direct current, and flexibility (PEDF) building systems, are a crucial approach to reducing carbon emissions. Heating, ventilation, and air conditioning (HVAC) systems are typical loads in DC buildings that primarily aim to enhance the air quality and thermal comfort. However, the variable frequency drives (VFDs) used in HVAC systems can generate second-harmonic currents (SHC), which decrease equipment reliability and system power efficiency. To prevent SHC on the low-voltage side from flowing to the high-voltage bus, even in the grid, a second order generalized integrator (SOGI)-based adaptive controller for suppressing the SHC of variable frequency loads in DC buildings is proposed. The controller employs a phase-locked loop (PLL) to provide feedback on the VFD output frequency to the DCT control loop's notch filter, enabling adaptive suppression of the variable frequency SHC. Firstly, an accurate small-signal model of DCT based on Fourier series is established, which considers the inductor current. Then, the formula for DCT equivalent impedance is derived based on the model, and the feasibility of suppressing the SHC by increasing the equivalent impedance at variable frequency is analyzed. To enhance the stability of DC buildings, the parameters of the controller are designed to improve dynamic performance. Finally, the simulation results and experimental results verify the effectiveness and feasibility of the proposed adaptive controller compared to other suppression strategies.© 2017 Elsevier Inc. All rights reserved.

An adaptive synchronous rectifier driving strategy for unregulated LLC resonant converter

AbstractLLC resonant converters are widely adopted in many different industrial applications due to their high efficiency operation. To further improve LLC resonant converters' efficiency, the secondary unregulated diode rectifier is replaced by the synchronous rectifier (SR). In many applications, LLC converter is designed to operate at resonant frequency to maximize the system efficiency and act as a DC transformer (DCX). The SR driving signal is set as the same as the primary driving signal to work in an open‐loop mode. However, the resonant frequency will deviate due to the tolerance, working environment, and ageing effect of circuit components, which will lead to the degradation of the system efficiency and output voltage variation. Unfortunately, most of the existing techniques cannot be simply adopted in SR LLC DCX. Therefore, an adaptive SR driving strategy that can compensate the circuit components' variations is necessary for SR LLC DCX. Inan adaptive SR strategy based on the constant voltage gain characteristic at resonant frequency point is proposed. The proposed strategy is applicable for all different scenarios with easy implementation and low cost.

The Automatization of Social Media Communication

This research paper compiles and evaluates several studies that have a particular interest in social media analysis and its applications. Studies on evaluating the impact of movies and television shows on social media platforms, using semantic knowledge graphs to analyze Covid-19 news articles and identify fake news on social media, forecasting social media data using machine learning, and the evolution of the power of central nodes in a Twitter social network are all covered in this article. The significance of sentiment analysis and opinion mining in social media, analysis of social media-based profiles, and mining serendipitous drug use from social media using machine learning are all topics covered in the paper. The paper also emphasizes the application of social media text analysis for a high-frequency-link DC transformer based on switched capacitors for medium-voltage DC power distribution applications, and urban region mining services. The research paper offers details on the state of social media analysis right now and some potential uses for it.

A Multiport DC Power Flow Controller Embedded in Modular Multilevel DC Transformer

Meshed dc or hybrid ac&#x002F;dc distribution grids have attracted increasing interest lately and their power flow control is a major topic of study. Since the dc grids implemented with existing power converters lack sufficient control freedoms for proper power flow adjustment, it is necessary to develop new dc power flow controllers (DCPFCs) to effectively mitigate power flow congestion and line losses by optimizing the power flow of the connected feeders. Unlike previously introduced solutions, this paper proposes a DCPFC directly integrated in a modular multilevel dc transformer (M²DCT). The proposed DCPFC does not require external power supply and high-voltage isolation transformer, and features a simple structure, convenience for multiport extension and bidirectional power flow adjustment capability. The power flow control strategy and its energy balance scheme based on phase-shifted modulation are elaborated. The proposed topology and its performance are verified through a scaled-down experimental platform under typical operating conditions.

Evaluation and Modeling of SiC Based Power Converter for Low Temperature Operation

Cryogenic power electronics is regarded as the next step to improve the power converter efficiency and power density. Thus, investigations on component and converter performances under low temperature operations are of great importance. Silicon carbide (SiC) devices have gradually replaced traditional Silicon (Si) devices in high voltage and high power applications. This is due to their fast switching speed and low switching loss. In this article, the performance of SiC device based power converters are investigated under low temperature operation. Firstly, the individual component performances are evaluated under low temperatures. Then, a 600 W resonant converter based DC transformer was built and tested under low temperatures. The efficiency for the tested converter with ferrite cores drops from 97.86% at room temperature (298 K) to 93.44% at 143 K. The converter power loss model under low temperature operation was developed to facilitate the understanding of converter performance at various operating temperatures.

Proposed asymmetric phase shift modulation strategy to improve zero-voltage-switch range and transmission power range for dual active bridge converter

The dual active bridge (DAB) DC–DC converter is typical in DC transformers to realize the voltage transformation and power transmission, which is determined by modulation. However, in traditional phase shift modulation strategies, the zero-voltage-switch (ZVS) range and the transmission power range of DAB are limited, and the current bias exists in light load. Therefore, a proposed asymmetric phase shift modulation strategy (PAPSM) to realize full ZVS range, larger transmission power, and non-DC bias in high frequency link (HFL) for DAB is introduced in this paper. In PAPSM, the modulation signals in adjacent periods are symmetric and in the opposite direction, and the current characteristics in the adjacent periods are still consistent. Thus, the equivalent period of DAB in PAPSM is expended, and the current bias accumulation in light load is eliminated in the equivalent period, without any extra operation. Then, the operating principles of DAB in PAPSM are introduced, to analyze the ZVS range, power delivery range, and maximum transmission power in detail. Compared with traditional phase shift modulation strategies, PAPSM can realize full ZVS range, wider power delivery range, and larger maximum transmission power by adjusting an outer phase shift ratio and an asymmetric duty ratio, and the DC bias in HFL can be eliminated. Finally, a DAB experimental setup is established based on RT-LAB, proving the theoretical analysis.

Optimal Design of a Matrix Planar Transformer in an LLC Resonant Converter for Data Center Applications

The 48-V bus converter has been gradually adopted in intermediate bus architecture. The ever-growing development of data centers consumes significant power, thereby pushing power converters to be more efficient. This article focuses on the optimization of the transformer for an LLC resonant converter used in data centers, which is the second stage operating as a dc transformer to realize the isolation and a fixed voltage step-down ratio. The front stage is a multiphase buck converter. The magnetic core geometry reported in this article for the LLC enables a single turn for both primary and secondary windings. Compared with the previous research, the improved interleaving winding scheme benefits from reduced conduction path, lower ac resistance, and smaller core volume. Ansys 3D simulation performs the design and optimization of the magnetic core. The proposed magnetic core, winding layout, and design optimization are verified in the 30-/5-V 660-W LLC resonant converter. It has higher current stress but achieves a higher efficiency of 97.7%, better thermal performance, and lower components count than the state of the art.

A Medium Voltage Input Multiport Isolated Output DC Transformer With Power Self-Balancing and Output Fault Isolation

Medium voltage dc (MVDC) converters with multiple output ports have the challenges of series input module power balancing control and fault isolation between output ports. This paper proposes a new medium voltage input multiport isolated output dc transformer topology. The proposed topology can be extended to up to 2k-1 output ports of different voltages with k modules input connected in series. Moreover, the proposed topology has the voltage self-balancing feature of input modules since they are coupled via the balancing capacitors. Current self-balancing of all MOSFETs is also achieved, as the equivalent impedance of multiple voltage sources discharging to multiple loads is the same under the influence of the split resonant inductor. Besides, the proposed topology can limit the shorted current in the output port automatically without affecting other ports, due to the impedance of the resonant tank of the faulty port being clamped. A 2kV dc input with three output ports of 410V, 380V and 40V prototype is built to verify the feasibility of the proposed topology. The results show that the proposed topology has multiple module power self-balancing feature, and can improve the reliability of the multiport power supply systems. Therefore, it has good prospects for subsea dc power distribution systems and ship electric power distribution systems.

Asymmetric Topology Design and Quasi-Zero-Loss Switching Composite Modulation for IGCT-Based High-Capacity DC Transformer

Asymmetric Topology Design and Quasi-Zero-Loss Switching Composite Modulation for IGCT-Based High-Capacity DC Transformer

DC Transformers in DC Distribution Systems

With the rapid development of power electronics technology and its successful application, many demonstration projects of medium/low-voltage DC (M-LVDC) distribution systems have been constructed. The DC transformer (DCT) is the key equipment in the M-LVDC distribution system for interconnecting the MVDC and LVDC buses. In this paper, the characteristics of DCTs are summarized. The existing topologies of DCTs are analyzed, and the relevant control strategies are researched, including steady-state control, transient control, and cascaded control. The engineering application examples of DCTs are introduced by interpreting the medium and low-voltage DC distribution system demonstration project in Wujiang City, Suzhou. Finally, the challenges faced by the DCT are given, and the future development trend is predicted. This perspective provides a constructive basis for DCTs and an important reference for M-LVDC distribution systems.

Shoot-Through Protection for an IGCT-Based ZVS Resonant DC Transformer

Integrated gate-commutated thyristors (IGCTs) have successfully demonstrated to be well-suited devices for an application in medium-voltage dc transformers based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter topology. This appli- cation enables unusually high switching frequencies in kilohertz range thanks to the zero-voltage switching while maintaining the high conversion efficiency. Moreover, the clamp circuit that is an inseparable part of every hard-switched IGCT-based converter can be completely omitted—further lowering the converter complexity and cost. Nevertheless, without the clamp circuit, the effect of shoot through becomes fatal, considering that the IGCTs have a defined mode of failure in short circuit and the desaturation effect is practically nonexistent. To prevent the shoot through, this letter proposes a simple, yet effective, protection method based on anode-voltage measurement that can be implemented locally on the IGCT gate unit. The validity of the solution is demonstrated using a custom IGCT gate unit in a medium-voltage resonant test setup.

An improved distributed secondary control for multi‐voltage‐level DC distribution networks with virtual impedance arrangement

AbstractThe distributed secondary control of the DC distribution network is becoming increasingly important in modern power systems to achieve accurate power sharing of each distributed unit. Here, an improved secondary control strategy for energy storage systems (ESSs) in the multi‐voltage‐level DC distribution network (MVL‐DCDN) is proposed. At the primary control level, the interlinked DC transformers (DCT) are controlled by difference‐of‐v2 to support bidirectional voltage making the MVL‐DCDN regarded as a single‐voltage‐level one using only the local information of DCT. At the secondary control level, it can be observed that the voltage correction term and virtual impedance correction term have a coupling relationship. In order to tackle this coupling effect, a distributed secondary control is imposed in an MVL‐DCDN based on improved droop control. Concretely, this proposed secondary control strategy contains a power sharing controller, a virtual impedance controller, an average voltage restoration controller, and a reference voltage consensus controller. With the proposed method, this coupling effect can be eliminated to ensure the steady operation of the virtual impedance. Furthermore, a steady‐state analysis is performed to discuss the unique solution of the DC distribution network and conduct hardware‐in‐loop experiments to demonstrate that this method achieves effective results.

A DAB-Based DC Transformer for Multiterminal HVdc Dynamic Simulation Platform With Short Circuit Tolerance

The dc transformer (DCT) can be used to achieve a direct connection between various dc bus voltages. A multiwinding dual active bridge (DAB) structure is proposed to be used in that occasion for a dynamic simulation platform. For high voltage side, automatic voltage balance and power-sharing balance can be achieved automatically without mandatory power sharing control. To deal with wide voltage conversion gain application, a multimode modulation with peak current value optimization is proposed. It has the tolerance for the short circuit operation under dc fault. The power transmission can be suppressed rather low during the fault. A 1 kV/1.6 kV 10 kW DCT has been fabricated and applied in a dynamic simulation platform. The bidirectional power flow capability together with the short circuit fault tolerance has been verified through the test.