DC Transformer Research Articles

Hierarchical optimal operation for bipolar DC distribution networks with remote residential communities

To facilitate the seamless integration of renewable energy, bipolar DC distribution networks (Bi-DCDNs) have been widely adopted in various applications, including the Shenzhen Future Building, the Boeing 787 aircraft, and DC LED lighting systems in Singapore. Bi-DCDNs incorporate diverse flexible devices to improve both economic efficiency and security. However, a comprehensive coordination framework considering the regulatory heterogeneity among these flexible devices remains absent. Hence, this paper proposes a hierarchical coordination framework for flexible devices in Bi-DCDNs. More specifically, the upper-level model considers the operational differences of the DC transformer (DCT) in various modes to determine the optimal switching scheme for reducing losses; In the lower-level model, the control parameters of the DCT, energy storage systems (ESSs), and DC electrical springs (DCESs) are coordinated to enhance voltage quality. Furthermore, to accurately capture the steady-state behavior of flexible devices, the hierarchical framework incorporates the Newton-Raphson power flow method. This method formulates a steady-state model for multiple flexible devices and demonstrates the impact of different control modes of DCT on power flow. Subsequently, a genetic algorithm is used to solve the proposed model, ensuring that suboptimal decisions made at the upper level are rectified at the lower level, and vice versa. The numerical results indicate that the proposed framework achieves the optimal operation for both reduced losses and enhanced voltage quality in Bi-DCDNs. Furthermore, it exhibits advantageous applications for Bi-DCDNs with additional DCTs for remote residential communities.

Multi-Terminal DC Transformer for Renewable Energy Cluster Grid Connection

An AC (alternative current) power integration of distributed energies faces multi-fold challenges such as synchronization and weak grid-induced instability. In this study, a multi-terminal DC transformer is proposed for renewable energy clusters grid connection. The DC transformer provides multiple DC input ports for renewable energy collection, while the load port is connected to the medium-voltage DC grid via a modular multilevel converter (MMC). The multi-port topology enables flexible power transfer between multiple input sources to the load without additional components. The carrier layer modulation strategy is implemented to balance the MMC module voltage; bidirectional power transmission between multiple input sources is achieved through the phase shift modulation (PSM) method. First, we provided a detailed introduction of the proposed topology and working principle. A simulation model was built using the SIMULINK, and the simulation results verified the effectiveness of the proposed converter modulation strategy and phase shift modulation method. A corresponding hardware experimental platform was designed and built, and the modulation and voltage equalization functions of modular multilevel rectifiers were presented as well as the measured results of power transmission modulation of the converter under single-input and multi-input conditions. The results indicate that the proposed transformer can achieve multiple DC inputs and has multi-channel power transmission capabilities, making it suitable for renewable energy clusters grid connection.

Closed-Form Optimal Current-Limiting Curve-Based Variable Duty-Cycle Soft Start-Up Strategy for the LLC DC Transformer

Closed-Form Optimal Current-Limiting Curve-Based Variable Duty-Cycle Soft Start-Up Strategy for the <i>LLC</i> DC Transformer

Comparative Analysis of Magnetic Field Distribution Characteristics of Two Shapes of Air-Core Bridge Arm Reactors

The application of air-core reactors in power systems is extensive and primarily aimed at enhancing system stability, limiting short-circuit currents, and providing reactive power compensation. Currently, the type most commonly used in power systems is the cylindrical-shaped air-core reactor (CAR), known for its stable mechanical structure and mature manufacturing process. However, the external magnetic field generated by this reactor propagates over a considerable distance in the air, which can interfere with the normal operation of many power electronic devices. This paper presents a comparative analysis between a novel annular-shaped air-core bridge arm reactor (AABAR) and the widely used cylindrical-shaped air-core bridge arm reactor (CABAR) within a DC transformer system. The comparison focuses on the magnetic field distribution, including magnetic flux density, magnetic field radiation range, and magnetic field energy, as well as the attenuation characteristics of these physical quantities. The concept of magnetic clearance (MC) is introduced as a quantitative metric. Through finite element simulation software (AEDT 2021 R1), it is demonstrated that the annular-shaped air-core reactor design can significantly improve spatial utilization and reduce the actual usage space of the reactors in DC transformer systems.

A Partial Power Regulated Dc Transformer Optimized by Genetic Algorithm for High Step-Up Power Conversion

A Partial Power Regulated Dc Transformer Optimized by Genetic Algorithm for High Step-Up Power Conversion

Enabling High-Power Conditioning and High-Voltage Bus Integration Using Series-Connected DC Transformers in Spacecrafts

This article proposes a photovoltaic power processor for high-voltage and high-power distribution bus, between 300 V and 900 V, to be used in future space platforms like large space stations or lunar bases. Solar arrays with voltages higher than 100 V are not available for space application, being necessary to apply power conversion techniques. The idea behind this is to use series-connected zero-voltage and zero-current unregulated and isolated DC converters to achieve high bus voltage from the existing solar arrays. Bus regulation is then achieved through low-frequency hysteretic control. Topology description, semiconductor selection, design procedure, simulation and experimental validation, including tests in vacuum and partial pressures, are presented.

Hybrid Electronic-Embedded Transformer-Based DC Transformer (EET-DCX): A New Family of Soft-Switching DCXs

Hybrid Electronic-Embedded Transformer-Based DC Transformer (EET-DCX): A New Family of Soft-Switching DCXs

Power quality improvement of unipolar-input-bipolar-output DC transmission system via load power balancing

In DC transmission and distribution systems, both unipolar and bipolar transmission modes exist, and DC transformers used in these systems are also available in either unipolar or bipolar configurations. In actual systems, due to requirements such as economy, land occupation, and reliability, there is a tendency to use a system with unipolar input and bipolar output. However, the bipolar loads, if unbalanced, will lead to increased equipment costs and voltage imbalance, causing power quality problems. This paper defines the Power Unbalance Factor (PUF) to describe the power quality of the studied DC transmission system and presents an improved DC transformer topology based on a power balancing system. This topology realizes bipolar voltage balance and improves the power quality of the DC transmission system when the load is unbalanced. The influence of the proposed solution on the power design of the DC system is demonstrated through theoretical analysis, and its effectiveness for improving the DC power quality is verified by both simulations in MATLAB/Simulink environment and physical experiments. When the power electronic transformer needs to be overloaded, the proposed topology can reduce the design power of the two branches by using the difference power, which is economical.

Quasi square wave operation of modular multilevel converter based dual active bridge DC–DC converter with inductor energy recovery

AbstractThe modular multilevel DC–DC transformer (MMDCT) provides a reliable solution to overcome the challenges of conventional dual‐active‐bridge converters in terms of power semiconductors ratings and dv/dt stress on transformer coupling windings. An alternative modulation method, quasi‐square‐wave, was proposed to reduce the cell capacitance of modular multilevel bridges. However the application of quasi‐square‐wave modulation is found to result in underdamped switching transients and losses when resetting energy stored in arm inductance. This paper presents a detailed transient analysis of MMDCT arm insertion based on an equivalent circuit, which contributes to a more accurate component sizing and gives voltage estimation for individual half‐bridge submodules. Additionally, a revised switching sequence is proposed to recover this inductor energy and lower the oscillation‐related losses. Simulated and experimental results from a scaled test rig of MMDCT are implemented and validate the proposed component sizing and switching sequence, indicating that the converter efficiency can be improved under revised switching sequence. Finally, a silicon carbide based, high‐frequency MMDCT is proposed and simulated.

DIY of dye cleaning from water using plasma and smartphone colorimetry

Plasma has been generated using high voltage DC transformer. This plasma was further utilized in dye aqueous solution. Smartphone-based colourimetry was utilized to quantify dye degradation. These handy experiments will be helpful to understand plasma, its generation and potential applications.

Multi-port DC transformer applied to large-scale DC collection systems

Abstract This paper proposes a multi-port DC transformer for boosting and flexible interconnection in large-scale DC collection systems. It can transmit power and adjust port current, while also having a relatively simple control strategy. The engineering practicality of the proposed DC transformer can be well verified through the simulation of a 4 GW DC collection system. In this application scenario, the proposed DC transformer can operate stably and safely under both dynamic and steady-state conditions.

Cascaded Stabilization for ISOP-Type DC Transformers Through Impedance Reshaping Regulator

Cascaded Stabilization for ISOP-Type DC Transformers Through Impedance Reshaping Regulator

Compact Valve Hall Layout of DC Transformer for Offshore Wind Farm All-DC Collection Platform

Abstract To solve the problems of high investment cost, difficulty in improving AC collection efficiency, and broadband resonance in flexible DC transmission of offshore wind power, researchers have proposed an all-DC collection system. To further optimize the economic efficiency of offshore wind farm engineering, this paper studies on the compact valve hall layout of DC transformer for offshore wind farm all-DC collection platforms. The minimum air clearance inside the valve hall was determined based on the insulation level of the DC transformer, and a compact valve hall layout plan for the DC transformer was proposed, taking into account factors such as the electrical main wiring scheme, weight distribution, and symmetrical balance. The proposed compact valve hall layout scheme for DC transformers can reduce the volume of offshore all-DC collection platforms, save engineering construction costs, and improve the efficiency of offshore wind farms. This article can provide reference and guidance for subsequent research on all-DC collection and economic transmission of large-scale offshore wind farms.

An Improved AC-Link Voltage Matching Control for the Multiport Modular Multilevel DC Transformer in MVDC Applications

In this paper, an improved AC-link voltage matching control (IVM) strategy is proposed for the multiport modular multilevel DC transformer (M3DCT), which comprises a single-phase modular multilevel converter (MMC) and a series of cascaded H-bridge units. The objective of the proposed IVM strategy is to address the AC-link voltage mismatch phenomenon. Distinct from existing control methods, such as various phase-shifting control methods and the conventional AC-link voltage matching control strategy, the proposed IVM strategy orchestrates the operation of the M3DCT in an innovative fashion. It allows the sum of inserted submodule (SM) numbers in the upper and lower arms to be flexible, no longer confined to a specific SM number per arm. Consequently, the AC-link voltage of the M3DCT is maintained proximate to the matched operating condition, regardless of the degree of mismatch in the DC side voltage of the M3DCT. This enables the enhancement of the M3DCT’s overall operational performance, particularly under conditions of light loads within medium-voltage DC (MVDC) distribution systems. The correctness and effectiveness of the proposed control strategy and the corresponding analysis are substantiated through simulation results.

Lower-Height-Oriented Magnetic Integration Design for Onboard Power Converter of Electric Vehicles

With the development of electric vehicles (EVs), EV onboard power converters are expected to pursue higher power density and lower height. Compared with active power devices, passive magnetics seems to be the main restriction for higher efficiency and power density. In the power distribution system, two-stage configurations of Buck+LLC converter is widely employed in various electronic equipment, which requires multiple magnetics. On this basis, the integration of two Buck+LLC converters has been studied to reduce the overall cost and improve the power density. Firstly, the two buck converters are interleaved and operate in critical mode (CRM) to reduce current ripple and realize soft switching. Hence, the two buck inductors can be negative-coupled with each other and integrated into one magnetic core to achieve lower switching frequency and smaller volume. In addition, the two LLC dc transformers are also integrated with only one switching leg and one magnetic core. By extending the effective area and canceling the high-frequency magnetic flux, the loss and volume of both the integrated buck inductors and integrated LLC transformers are reduced significantly. Detailed integration methods and parameter design processes are presented. A 700 W, 150-350 V-to-12 V dual-output dc-dc converter with 500 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> power density is demonstrated to verify the effectiveness and feasibility of the proposed methods.

A DCM-Based Non-Isolated Step-Down DC Transformer

DC transformers have emerged as essential devices for medium voltage DC (MVDC)-low voltage DC (LVDC) distribution systems. However, conventional step-down single-level converters have limits on the voltage level of the MVDC-LVDC distribution system. This paper proposes a non-isolated step-down (NISD) DC transformer based on discontinuous conduction mode (DCM). The proposed structure can withstand high voltage levels by sharing voltages between energy storage modules dividing voltage levels. The proposed NISD DC transformer determines operational modes based on energy storage modules and performs the voltage conversion process. The effectiveness of the proposed NISD DC transformer is verified based on a case study using a power system computer-aided design and electromagnetic transient simulation engine including DC (PSCAD/EMTDC™).

An MV Hybrid Isolated DC Transformer for Wide Output Voltage Range Applications

DC transformer (DCT) is the key energy conversion equipment that converts medium voltage dc to low voltage dc. Aiming at the industrial loads requiring wide output voltage range and unidirectional power flow, a hybrid isolated DC transformer (HIT) as an alternative to the traditional MMC-DCT is proposed in this paper. It is composed by thyristors, diodes, and submodule (SM) based arms. The operation principle, circuit performance, and control strategy are introduced in detail. Particularly, a discontinuous input and continuous output currents (DICO) operation mode is further proposed to suppress submodule capacitor voltage ripple. Compared with the traditional MMC-DCT, HIT can save 50% SMs, 28% semiconductor cost and 46.3% total SM capacitance. The effectiveness and the theoretical analysis have been confirmed by both simulation and experimental results.

A Novel Modular Magnetic-Coupled Converter (MMCC) for Medium-Voltage Motor Drive With Minimized DC Capacitance and Transformer Volume

A Novel Modular Magnetic-Coupled Converter (MMCC) for Medium-Voltage Motor Drive With Minimized DC Capacitance and Transformer Volume

Sequential Switching Shunt Regulation Using DC Transformers for Solar Array Power Processing in High Voltage Satellites

Sequential Switching Shunt Regulation Using DC Transformers for Solar Array Power Processing in High Voltage Satellites

Precisely Simplified Time-Varying Modeling Method Based on the Small-Signal-Waveform Assumptions for a High-Ratio MMC-Based DC Transformer

Precisely Simplified Time-Varying Modeling Method Based on the Small-Signal-Waveform Assumptions for a High-Ratio MMC-Based DC Transformer