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The 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. [ABSTRACT FROM AUTHOR]

Dual active bridge (DAB) is an isolated DC‐DC converter gaining wider attention in power electronics applications. The high frequency (HF) transformer is an integral part of the DAB which is prone to saturation. Silicon carbide (SiC) based DAB are generally preferred for highly efficient power conversions, calling for an extremely low DC resistance of the transformer. This aggravates the DC bias issue significantly. The DC bias current generally flows due to the mismatch in static and transient switching of active devices, leading to eventual saturation of the transformer. This paper proposes an active method to precisely control the dead time of the devices (8–100 ns) without sacrificing the voltage utilization of the converter. This method does not require a sophisticated DC offset current measurement technique. The field programmable gate array (FPGA) based control platform on the gate driver side executes the proposed algorithm. The proposed control is experimentally verified in a 5 kW SiC based converter. The control implementation methodology is discussed with the support of necessary experimental results. [ABSTRACT FROM AUTHOR]

A modular system of cascaded converters based on model predictive control (MPC) is proposed to meet the application requirements of multiple voltage levels and electrical isolation in renewable energy generation systems. The system consists of a Buck/Boost + CLLLC cascaded converter as a submodule, which is combined in series and parallel on the input and output sides to achieve direct-current (DC) voltage transformation, bidirectional energy flow, and electrical isolation. The CLLLC converter operates in DC transformer mode in the submodule, while the Buck/Boost converter participates in voltage regulation. This article establishes a suitable mathematical model for the proposed system topology, and uses MPC to control the system based on this mathematical model. Module parameters are designed and calculated, and simulation is built in MATLAB/Simulink to complete the simulation comparison experiment between MPC and traditional proportional integral (PI) control. Finally, a physical experimental platform is built to complete the physical comparison experiment. The simulation and physical experimental results prove that the control accuracy and response speed of MPC are better than traditional PI control strategy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract The 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.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

LLC 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Metro stray currents flowing into transformer-neutral points cause the high neutral DC and a transformer to operate in the DC bias state.Because neutral DC caused by stray current varies with time, the neutral DC value cannot be used as the only characteristic indicator to evaluate the DC bias risk level. Thus, unified characteristic extraction and assessment methods are proposed to evaluate the DC bias risk of a transformer caused by stray current, considering the signals of transformer-neutral DC and vibration. In the characteristic extraction method, the primary characteristics are obtained by comparing the magnitude and frequency distributions of transformer-neutral DC and vibration with and without metro stray current invasion. By analyzing the correlation coefficients, the final characteristics are obtained by clustering the primary characteristics with high correlation. Then, the magnitude and frequency characteristics are extracted and used as indicators to evaluate the DC bias risk. Moreover, to avoid the influence of manual experience on indicator weights, the entropy weight method (EWM) is used to establish the assessment model. Finally, the proposed methods are applied based on the neutral DC and vibration test data of a certain transformer. The results show that the characteristic indicators can be extracted, and the transformer DC bias risk can be evaluated by using the proposed methods. [ABSTRACT FROM AUTHOR]

Metro stray currents flowing into neutral-point-grounded transformers can cause serious direct current (DC) bias. Affected by both metro train and urban power grid operations, transformer neutral-point DC caused by metro stray current is complex, dynamic, and time-varying, which changes the DC bias risk level of transformers. Thus, just installing blocking devices (BDs) at transformer neutral points with high instantaneous DC may make it difficult to comprehensively mitigate DC bias and lead to increased BD installation. To solve this, through optimizing BD installation placements, a mitigation method for transformer DC bias is proposed. In the proposed method, the DC bias risk level and BD installation number are included in the constraint and objective functions. To evaluate the risk level of transformer DC bias, four indicators are proposed considering the distribution characteristics of neutral DC. To optimize the BD installation placements, the effects of both the metro train dynamic operation and the operation topology of the urban power grid on neutral DCs are considered. The Monte Carlo method is used to sample the train operation conditions and a relation matrix among transformers is established. Applying the method to a certain power system, the result of BD installation placements shows that the transformers supplying the metro system must have BDs installed at their neutral points. [ABSTRACT FROM AUTHOR]

Abstract Currently, the digital transformation of the power grid is underway, and the intelligent health management technology for power transformers is rapidly advancing. However, there are issues in the operation and maintenance process, such as weak information correlation and low decision‐making efficiency. Knowledge graphs have been applied in other industrial fields, such as spacecraft maintenance, to significantly improve knowledge query efficiency. However, there is a lack of literature on knowledge graph construction in the field of power transformer operation and maintenance. Additionally, there is limited publicly available data and difficulties in effectively mining operation and maintenance knowledge in this field. A method for constructing a knowledge graph for power transformer operation and maintenance based on ALBERT is proposed. Firstly, publicly available literature in the field of power transformers is collected, and a sample enhancement method using regular matching is used to enrich the semi‐structured corpora, such as power system accident investigation reports, to construct a training dataset for power transformer operation and maintenance. Then, the ALBERT‐BiLSTM‐CRF deep learning algorithm is applied to extract power transformer operation and maintenance entities from the relevant literature and accident investigation reports, and this method is compared with traditional deep learning algorithms to demonstrate its superiority. Subsequently, the ALBERT‐BiLSTM‐Attention deep learning algorithm, which incorporates ALBERT and attention mechanism, is utilised to extract relationships between power transformer operation and maintenance entities. Compared to other deep learning algorithms, this algorithm demonstrates better performance in the domain‐specific texts of power transformer operation and maintenance. Finally, the Neo4j graph database is used to visualise and present the knowledge graph, enabling decision support based on the power transformer operation and maintenance knowledge graph.

Currently, the digital transformation of the power grid is underway, and the intelligent health management technology for power transformers is rapidly advancing. However, there are issues in the operation and maintenance process, such as weak information correlation and low decision‐making efficiency. Knowledge graphs have been applied in other industrial fields, such as spacecraft maintenance, to significantly improve knowledge query efficiency. However, there is a lack of literature on knowledge graph construction in the field of power transformer operation and maintenance. Additionally, there is limited publicly available data and difficulties in effectively mining operation and maintenance knowledge in this field. A method for constructing a knowledge graph for power transformer operation and maintenance based on ALBERT is proposed. Firstly, publicly available literature in the field of power transformers is collected, and a sample enhancement method using regular matching is used to enrich the semi‐structured corpora, such as power system accident investigation reports, to construct a training dataset for power transformer operation and maintenance. Then, the ALBERT‐BiLSTM‐CRF deep learning algorithm is applied to extract power transformer operation and maintenance entities from the relevant literature and accident investigation reports, and this method is compared with traditional deep learning algorithms to demonstrate its superiority. Subsequently, the ALBERT‐BiLSTM‐Attention deep learning algorithm, which incorporates ALBERT and attention mechanism, is utilised to extract relationships between power transformer operation and maintenance entities. Compared to other deep learning algorithms, this algorithm demonstrates better performance in the domain‐specific texts of power transformer operation and maintenance. Finally, the Neo4j graph database is used to visualise and present the knowledge graph, enabling decision support based on the power transformer operation and maintenance knowledge graph. [ABSTRACT FROM AUTHOR]

Modular multilevel converter-based solid-state transformers (MMC-SST) usually use the input series output parallel structure or the dual active bridge (DAB) structure. These two structures suffer from low power supply reliability and high economic costs, respectively. Therefore, based on hybrid frequency modulation, this paper proposes an arm integrated double capacitor submodule (AIDCSM)-type MMC-SST topology. This topology effectively reduces the number of switching devices by integrating the submodule arms, while enabling the topology to possess DC short-circuit fault ride-through capability. When compared to the DAB-type MMC-SST with half-bridge submodules, the proposed AIDCSM-type MMC-SST saves 2/5 switching devices and 1/2 high-frequency transformers, and enables uninterrupted operation under DC short-circuit faults. At the same time, the control strategy of the proposed AIDCSM-type MMC-SST is thoroughly investigated under both normal operating and DC short-circuit conditions. Simulation and experimental results demonstrate the correctness and effectiveness of the proposed topology and control method. [ABSTRACT FROM AUTHOR]

A DC-DC converter that can deliver high voltage gain and isolation with dual input is presented. The proposed converter incorporates a quasi-switched boost topology with an inductor in series with the main source. A switched capacitor cell and an auxiliary input source are connected before a push-pull switching stage to improve voltage boosting and the utilisation of dual input source. The high-frequency transformer provides galvanic isolation and improved voltage gain. The voltage doubler circuit (VDR) preceding the load has the ability to convert the transformer output to a DC supply and double the boost factor. Detailed operating waveform, theoretical and simulation results of the converter are presented with component selection and comparison with relevant converter topologies. A 250 W converter is implemented for an output of 400 V, when fed from two energy sources of 24 V and 12 V. The experimental results obtained satisfy the performance of the proposed converter. The benefits of the converter are (i) utilisation of hybrid energy sources, (ii) near to continuous input current, (iii) galvanic isolation and (iv) very high boost voltage. [ABSTRACT FROM AUTHOR]

Optical direct current (DC) transformer has become a hot spot of research with its high accuracy, wide bandwidth, and high voltage isolation characteristics, but has the technical difficulties of low signal-to-noise ratio of ODCT signal and poor temperature stability. For this reason, the study proposes a new type of optical DC transformer integrating material science and signal processing. The study introduces an improved terbium gallium garnet crystal sensing unit material and a signal processing algorithm with a plus-window dual correlation detection algorithm, and constructs an optical DC transformer model. Simulation results show that the temperature compensation method can effectively weaken the influence of temperature change on the measurement accuracy under both warming and cooling conditions, and higher accuracy can be obtained by using the whole period window for measurement. The system applying terbium gallium garnet crystals helps to enhance the measuring system's output signal-to-noise ratio and sensitivity. Terbium gallium garnet crystals as a sensing material can further decrease the measuring error compared with other magneto-optical glasses. Taken together, DC measurement system using terbium gallium garnet crystals and dual correlation detecting algorithm can control the error to about 0.3 s. Simulation experiments verify the validity and feasibility of the research methodology, which can guide the research and application of optical DC transformers in the future. [ABSTRACT FROM AUTHOR]

The DC voltage ratio standard device is an important tool for calibrating DC voltage transformers. At the 1100 kV voltage level, an increase in electric field intensity will increase the local heat generated inside the device, affecting the accuracy of its measurement. Using a suitable grading ring can even out the electric field intensity and reduce the maximum field strength to improve its measurement accuracy. This article mainly optimizes the design of the grading-ring structure of the 1100 kV DC voltage ratio standard device. First, a finite-element model of the 1100 kV DC voltage ratio standard device was built based on ANSYS; the electric field distribution around the voltage divider was calculated and analyzed, and a data set was constructed based on the calculation results. Secondly, for the optimization of electric field strength, this article presents the design of the nation standard device neural network, which learns the relationship between the structural parameters of the toroidal core and the field strength under the PyTorch 1.8 deep learning framework. Due to the strong convergence performance, few parameters, and ease of implementation of the grey wolf optimization algorithm, this study selected this algorithm to optimize the structural parameters of the grading ring. Finally, simulation examples are established in Python for validation. The experimental results indicate that the maximum field strength of the grading ring decreased from 12,161.1348 V/cm to 10,009.2881 V/cm, a reduction of 17.69%. The optimized structural parameters of the grading ring effectively reduced the electric field intensity around the 1100 kV DC voltage proportional standard device, providing a reliable and practical design approach for the selection of the DC voltage ratio standard device. [ABSTRACT FROM AUTHOR]

Abstract The temperature distribution of ultrahigh voltage (UHV) converter transformer is the key to its service life, allowable load and safe operation. The influence of a high‐proportion and large‐value harmonic current on the magnetic field, loss and temperature distribution of a UHV converter transformer is studied. A new calculation method is proposed to determine the winding temperature under the combined actions of multiple key factors. First, the skin effect of UHV converter transformer winding under high proportion and high current harmonics is analysed extensively. It is found that the increase of harmonic current frequency leads to exponential increase of winding loss and temperature by changing skin depth. Second, based on the superposition principle, a calculation method for winding loss considering harmonic current and different load rates is developed. The temperature distribution under different harmonic current frequencies and contents is obtained. The winding losses and temperature under different harmonic currents are quantified. Finally, a new calculation method is proposed for the converter transformer winding temperature, considering the combined actions of factors such as load rate, cooling oil inlet rate and temperature, harmonic current frequency and content. Experimental verification showed an error of only 0.58 K in the actual transformer hotspot temperature, confirming the effectiveness of this method. This method is of great significance for temperature control and safe operation of UHV converter transformers.

Abstract The isolated DC/DC converter plays an irreplaceable role in the development of the DC microgrid. However, the multiple discrete high‐frequency transformers occupy considerable volume in power converters due to the connection of multiple isolated converters. In order to reduce the number of discrete transformers and increase the power density as well as the reliability of the power converter, a three‐port isolated DC/DC converter with a decoupling magnetic integrated high‐frequency transformer (DMIHFT) is proposed for the DC microgrid in this paper. The semiconductor count of the proposed converter is reduced by combining one DAB converter and two LLC converters. Three discrete transformers are integrated into a single common core, and magnetic decoupling is achieved among each transformer by rational design at the same time. In addition, the magnetic circuit model derivation process of the proposed DMIHFT and the operation principles of the converter are described in detail. The design guidance of the proposed DMIHFT is present. And then, a 15 kW/20 kHz DMIHFT is designed based on the design guideline, and the simulation is carried out. Finally, a 15 kW/20 kHz DMIHFT prototype is fabricated, and the effectiveness of the proposed DMIHFT is verified by experimental results.

Dual-phase composite magnetic materials have magnetic and permanent magnetic properties. They can realize the dual-phase conversion of soft magnetic and permanent magnetic composites with a small amount of excitation energy. They have the advantages of good control and conversion characteristics and save energy, and they have a wide range of application scenarios in regard to power equipment. In this paper, the magnetization modeling of dual-phase composite magnetic materials is carried out based on micromagnetic theory, and a specific mathematical expression is given. Secondly, the preparation process of the dual-phase composite magnetic material is studied, the dual-phase composite magnetic material is prepared, and the demagnetization curve of the dual-phase composite magnetic material is measured. Finally, the application of dual-phase composite magnetic materials in power equipment is carried out. Using the soft magnetic and permanent magnetic characteristics of dual-phase composite magnetic materials, their impact on DC bias suppression in transformers is assessed. Magnetic circuit reluctance theory is used to develop the structure and electromagnetic design of a transformer. A transformer prototype with DC bias suppression ability based on dual-phase composite magnetic materials is manufactured, and simulation and experimental research are carried out. The simulation and experimental results verify the correctness of the proposed scheme. Although this scheme requires a more complex core structure, the energy-saving effect is remarkable without changing the transformer's neutral grounding. The indicators meet the actual requirements of the project. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The injection of the Direct Current (DC) bias component will lead to oversaturation of the transformer core and affect its safe operation. In this paper, the vector magnetic potential A and winding current I are used as the variables solved in the time-step finite element method to simulate the nonlinear transient magnetization characteristics of the transformer under DC bias, and the fixed-point reluctivity is introduced to deal with the nonlinear iterative problem. The field-circuit coupling relationship is established through the electromagnetic induction law, and the two-dimensional time-step finite element model of the transformer core is found. The dynamic J–A hysteresis model is used to deal with the hysteresis effect of iron cores. The fixed-point reluctivity of difference form is introduced to solve the problem of uneven transition and multi-value of magnetization curve with hysteresis in the iterative process. The determination scheme for the equivalent thickness of the core model and the initial iterative value are discussed. A laminated core transformer is made for experiments, and the accuracy and applicability of the model proposed in this paper are verified by comparing the calculation and experimental results. The influence of different voltages and different DC bias components on the electromagnetic characteristics of the transformer is analyzed. [ABSTRACT FROM AUTHOR]

A modular Sigma DC/DC converter with wide input voltage range is proposed in this paper. The proposed converter is combined with a traditional LLC converter and two multi-resonant converters via Sigma architecture. Among them, the traditional LLC converter operates as a DC transformer (DCX) at resonant frequency to achieve maximum efficiency. Meanwhile, one of the multi-resonant converters, the DC to DC (D2D) part of the Sigma structure, is responsible for voltage regulation over a wide input voltage range. In addition, the other multi-resonant converter has two operation modes, including DCX and D2D. When it operates in the DCX mode, the Sigma converter has better performance and higher efficiency. When it operates in the D2D mode, the Sigma converter can handle a wider input voltage range. For each submodule, the operation principle and characteristics of the proposed Sigma converter are analyzed in detail. In addition, some key points of the parameter design for each part are also demonstrated. Finally, an experimental prototype with an input voltage of 540~1100 V is built to verify the effectiveness of the proposed converter. [ABSTRACT FROM AUTHOR]

The series‐type direct current (DC) hybrid circuit breaker (S‐HCB) concept was previously reported to offer better performance than solid‐state circuit breakers (SSCB) and hybrid circuit breakers (HCB). S‐HCB offers low conduction power loss like an HCB and μs$\umu \text{s}$‐scale interruption time, which is even faster than an SSCB. It uses a pulse transformer to isolate the lower‐voltage high‐inductance power electronic circuit from the high‐voltage, low‐inductance main power loop. This paper provides analysis of the impact of the S‐HCB circuit components on the overall system performance and a scalable S‐HCB design guide for different DC system voltage and current ratings. In addition, system energy flow analysis is performed in the time domain to provide an understanding of how energy is delivered, dissipated, and released throughout the entire fault interruption process. The S‐HCB prototype was experimentally tested at 3 kV/30 A and 6 kV/150A with the results showing the interruption of the low fault current of 30 A and the high fault current of 150 A within 8 μs$\umu \text{s}$ and maintaining the fault current at a near zero value for 300μs$\nobreakspace \umu \text{s}$ to enable an arcless opening of a series mechanical switch. The key design challenges of S‐HCB at high voltage and high current ratings were discussed and possible solutions to mitigate those challenges were introduced. [ABSTRACT FROM AUTHOR]

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™). [ABSTRACT FROM AUTHOR]

The temperature distribution of ultrahigh voltage (UHV) converter transformer is the key to its service life, allowable load and safe operation. The influence of a high‐proportion and large‐value harmonic current on the magnetic field, loss and temperature distribution of a UHV converter transformer is studied. A new calculation method is proposed to determine the winding temperature under the combined actions of multiple key factors. First, the skin effect of UHV converter transformer winding under high proportion and high current harmonics is analysed extensively. It is found that the increase of harmonic current frequency leads to exponential increase of winding loss and temperature by changing skin depth. Second, based on the superposition principle, a calculation method for winding loss considering harmonic current and different load rates is developed. The temperature distribution under different harmonic current frequencies and contents is obtained. The winding losses and temperature under different harmonic currents are quantified. Finally, a new calculation method is proposed for the converter transformer winding temperature, considering the combined actions of factors such as load rate, cooling oil inlet rate and temperature, harmonic current frequency and content. Experimental verification showed an error of only 0.58 K in the actual transformer hotspot temperature, confirming the effectiveness of this method. This method is of great significance for temperature control and safe operation of UHV converter transformers. [ABSTRACT FROM AUTHOR]

This document provides a historical overview of the development and use of transformers in electrical systems. It highlights the contributions of various inventors and the significance of transformers in transforming electrical energy. The document also discusses the design problems and requirements for instrument transformers, power transformers, and special transformers. It explores the challenges faced in achieving accuracy and reliability in transformer designs. Additionally, it mentions the emergence of alternative transformer designs, such as electronic and optoelectronic transformers. The document concludes by summarizing the specific research contributions included in the special issue of Energies, which cover topics related to power transformers, measurement transformers, and special transformers. [Extracted from the article]

Aiming at the operational stability of DC-biased transformer, a multi-parameter correlation method based on electromagnetic coupling is presented in this paper. The mode-state analysis scheme is designed, and the feature parameters of electromagnetic, mechanical, and acoustic fields in internal components are simulated and analyzed. The electromagnetic properties under DC bias are simulated by the electromagnetic model, thus the winding current and magnetic flux density are extracted as the feature parameters. Then the vibration and stress distribution can be solved by the mechanical model, which are treated as the feature parameters of the mechanical field. By utilizing the computed mechanical information as excitation, the spatial-temporal distribution of sound pressure can be obtained in the acoustic model. Taking three-phase three-limb transformer and three-phase group transformer as examples, the electromagnetic, mechanical, and acoustic parameters of components are analyzed under different conditions. The variations of feature parameters are summarized and contrasted. Furthermore, actual vibration and noise parameters are measured through dynamic experimental platforms. The effectiveness of the multi-parameter correlation method is verified by the consistency between simulation and experiments, and the unobservable abnormal physical features can be represented by observable electrical information. [ABSTRACT FROM AUTHOR]

The isolated DC/DC converter plays an irreplaceable role in the development of the DC microgrid. However, the multiple discrete high‐frequency transformers occupy considerable volume in power converters due to the connection of multiple isolated converters. In order to reduce the number of discrete transformers and increase the power density as well as the reliability of the power converter, a three‐port isolated DC/DC converter with a decoupling magnetic integrated high‐frequency transformer (DMIHFT) is proposed for the DC microgrid in this paper. The semiconductor count of the proposed converter is reduced by combining one DAB converter and two LLC converters. Three discrete transformers are integrated into a single common core, and magnetic decoupling is achieved among each transformer by rational design at the same time. In addition, the magnetic circuit model derivation process of the proposed DMIHFT and the operation principles of the converter are described in detail. The design guidance of the proposed DMIHFT is present. And then, a 15 kW/20 kHz DMIHFT is designed based on the design guideline, and the simulation is carried out. Finally, a 15 kW/20 kHz DMIHFT prototype is fabricated, and the effectiveness of the proposed DMIHFT is verified by experimental results. [ABSTRACT FROM AUTHOR]

DC line protection is the key to the development of flexible DC grids, and fault identification within 3 ms is required. Existing scholars have proposed a digital twinning based flexible DC line protection method with high speed and sensitivity advantages. However, its reliability is easily affected by transformer measurements, which may lead to protection misoperation. The requirements of DC control protection equipment can be hardly meted by existing bad data detection methods with insufficient accuracy and rapidity. Therefore, in order to improve the reliability of this protection method, a self-correcting method of bad data based on the moving average method is proposed in this paper. The predicted value of the measured data is obtained by using the moving average method according to the time sequence characteristics of the steady change of the measured data, and the bad data is detected and corrected by comparing the predicted error and the actual error, without iterative calculation and pre-trained model. The results of a simulation test using the four-terminal flexible DC grid show that the proposed method has higher accuracy and rapidity with good error correction performance than any existing method does, and it can be adapted to the protection method and improve the anti-interference capability as well as the reliability of protection in an effective manner.

Abstract DC short‐circuit fault has become one of the major concerns in bipolar DC system operation under complicated conditions. Compared with conventional two‐port DC‐DC converter, fault‐tolerant DC‐DC converter with multiple external ports is evaluated as a preferential choice for high reliability, availability and flexibility required bipolar DC system due to its capability of fault reconfiguration. This article focuses on a type of fault‐tolerant DC‐DC converter interconnected with DC bus and integrated energy storage devices applied in bipolar DC system and theoretical analyses on transmitted power characteristics under bipolar mode and monopolar mode are also carried out in this article. For bipolar mode, the minimized‐peak‐current control strategy is proposed to reduce current stress on switching devices. As for monopolar mode and source‐port fault mode, the fault reconfiguration method and constant‐transmitted‐power control strategy are designed to eliminate fault impaction and achieve the constant transmitted power of output port connected with critical load, by means of power sharing between input port and normal output port or between the normal output ports. Furthermore, intersection of zero‐voltage‐switching (ZVS) operation area under different working modes is depicted with consideration of two optimizing control. Finally, the performance of the proposed fault reconfiguration method and optimizing control have been tested in PLECS environment and experimental prototype to verify the reliability and flexibility.

The integration of renewable energy sources in Medium-/High-voltage DC grids has called up continuous research to develop DC-DC conversion systems. This paper presents a hybrid modular multilevel DC-DC converter topology which successfully overcomes the issue of capacitors voltage imbalance during DC-DC conversion. In the proposed topology, single MMC leg is employed with half-bridge submodules in the upper arm and full-bridge submodules in the lower arm along with soft-switched high-voltage valves. The proposed hybrid modular multilevel converter has inherited arm energy equalization with the help of the parallel connectivity of involved arms' capacitors. During the equalization period, the series-connected upper arm capacitors are connected in parallel to the series-connected lower arm capacitors to transfer the energy between the involved arms which results in operating with balanced capacitors voltages. The proposed modular DC-DC converter is a bidirectional converter which allows power flow from high-voltage side to low-voltage side and vice versa, i.e., it can be operated successfully as a DC transformer. The operational concept of the proposed configuration, control strategy, and parameters design are presented. Simulation results are presented along with experimental results of a scaled-down prototype to validate the proposed approach. Simulation and experimental results show the viability of the suggested approach. [ABSTRACT FROM AUTHOR]

The concept of bipolar DC distribution has been shown to possess superior flexibility, efficiency, and quality when compared to the conventional unipolar approach. However, the control dynamics of the power conversion circuit for bipolar DC distribution are typically limited by the presence of DC-side capacitors. To enhance the stability and the reliability of DC power distribution systems, it is essential to ensure stable responses to both load dynamic characteristics and DC faults. This paper proposes a bipolar configuration and control strategy to improve the DC-link response characteristics of DC distribution systems. The proposed circuit utilizes a zig-zag transformer to enhance the DC voltage control dynamics of the converter. The zero-sequence impedance along the common mode path of the converter is effectively reduced by the zig-zag transformer, with an impedance range equivalent to the leakage components of the transformer. The converter of the bipolar structure can independently control the voltages of each DC pole, which enables faster dynamic responses of the voltage controls. Additionally, this structure facilitates the handling of asymmetric loads on each pole. The validity and the operation of the proposed configuration are verified through simulations and experimental results. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Engineering Technology is the property of Editorial Department of Electric Power Engineering Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

DC short‐circuit fault has become one of the major concerns in bipolar DC system operation under complicated conditions. Compared with conventional two‐port DC‐DC converter, fault‐tolerant DC‐DC converter with multiple external ports is evaluated as a preferential choice for high reliability, availability and flexibility required bipolar DC system due to its capability of fault reconfiguration. This article focuses on a type of fault‐tolerant DC‐DC converter interconnected with DC bus and integrated energy storage devices applied in bipolar DC system and theoretical analyses on transmitted power characteristics under bipolar mode and monopolar mode are also carried out in this article. For bipolar mode, the minimized‐peak‐current control strategy is proposed to reduce current stress on switching devices. As for monopolar mode and source‐port fault mode, the fault reconfiguration method and constant‐transmitted‐power control strategy are designed to eliminate fault impaction and achieve the constant transmitted power of output port connected with critical load, by means of power sharing between input port and normal output port or between the normal output ports. Furthermore, intersection of zero‐voltage‐switching (ZVS) operation area under different working modes is depicted with consideration of two optimizing control. Finally, the performance of the proposed fault reconfiguration method and optimizing control have been tested in PLECS environment and experimental prototype to verify the reliability and flexibility. [ABSTRACT FROM AUTHOR]

In 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. [ABSTRACT FROM AUTHOR]

Abstract The 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 p−v2$p-v^2$ 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.

Aiming at the stability of a medium-voltage DC network based on a modular multilevel converter (MMC), this paper proposes an admittance stability criterion considering the influence of current-limiting inductors at the medium voltage side, which prevents the complex products and matrix calculations of traditional criteria. The DC admittance model DC transformers (DCTs) under different working modes are then established based on Thevenin/Norton equivalent circuit methods to analyze the stability of the DC system based on the proposed admittance stability criterion, which proves that the voltage resonance problem at the medium voltage side can be improved by adding active damping control strategies on DCTs also proves the effectiveness of the proposed stability criterion. The time-domain simulation and the hardware-in-loop simulation are then built in PLECS and RT Box to further verify the correctness of the system stability analysis and the effectiveness of the proposed admittance criterion, which provides a theoretical basis and technical reserve for the stable operation of the DC distribution power system. [ABSTRACT FROM AUTHOR]

DC electronic current transformer is the data source of the system control and protection device, so its measurement delay has always been concerned. In this paper, the DC current transformer is modelled equivalently and the key factors affecting the delay of shunt and the remote module are analyzed. A field test method for the delay characteristics of the DC electronic current transformer is proposed. Moreover, a complex multi-state large current generator and integrated test system are developed. On the basis of the IEC 60044-8 standard transmission protocol, the accuracy of standard signal acquisition with the shunt is better than 0.2%. The maximum output current of the established testing system is 600 A. Based on the field test in an actual HVDC project, transient step and various frequency signal components are applied for analyzing. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes a transformer type dc power flow controller (DCPFC) based on stacks composed of sub‐modules (SMs). The converter can perform dc power flow control independently on point‐to‐point as well as multi‐terminal connection, which can be applied to either high voltage or medium voltage dc grids. In order to explain the operation principle, different operating states of the proposed DCPFC are analysed. With phase‐shift modulation and the closed‐loop current control, the proposed DCPFC can achieve bidirectional power flow regulation. Following the theoretical analysis of the proposed DCPFC, simulation results have verified the feasibility of bidirectional power flow control. A downscaled experimental prototype is also built to demonstrate the operation principle and performance of the proposed DCPFC.

Copyright of Zhejiang Electric Power is the property of Zhejiang Electric Power Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Zhejiang Electric Power is the property of Zhejiang Electric Power Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Zhejiang Electric Power is the property of Zhejiang Electric Power Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Direct current (DC) bias leads to increased vibration and noise in transformers. One of the main causes is the magnetostrictive effect of the transformer core. To address this phenomenon of magnetostriction, firstly, a transmission line model (TLM) of a single-phase transformer under DC bias is developed using transmission line theory and Jiles–Atherton (J–A) ferromagnetic hysteresis theory, taking into account the winding copper loss, core eddy current loss, and leakage effect. Secondly, the time-domain simulation of the single-phase transformer based on the Newton–Raphson iterative method is carried out, and the magnetostriction characteristics of the transformer under different DC and its variation law are analyzed. Finally, the results show that the DC bias results in magnetostrictive distortion and vibration acceleration curve distortion, the left and right wings of the magnetostrictive butterfly curve are no longer symmetrical, the slope of the vibration acceleration image increases significantly, and the degree of distortion is positively correlated with the magnitude of the DC. In addition, the peak values of the magnetostrictive deformation and vibration acceleration become larger under DC bias, leading to an increase in the vibration and noise of the transformer. The research object of this paper is the single-phase transformer, and the research method can also be applied to the study of three-phase transformers. [ABSTRACT FROM AUTHOR]

Direct current transformer (DCT) is a key piece of equipment in direct current (DC) microgrids, and the mainstream topologies mainly include LLC resonant converter (LLC) and dual active bridge (DAB). In this paper, a novel bi-directional buck/boost + CLLLC cascade topology is proposed for the input-series-output-parallel cascade converter system of a DC microgrid. To solve the problem that frequency variation causes the converter to deviate from the optimal operating point, resulting in low efficiency, and the inability to achieve a soft switching function. The CLLLC converter operates near the resonant frequency point as a DCT, only providing electrical isolation and voltage matching, while the buck/boost converter controls the output voltage and the voltage and current sharing of each module. Compared to other cascaded converter systems, the cascaded converter proposed in this paper has high efficiency, simplifies the parameter design, and is suitable for wide input and wide output operating conditions. The system adopts a three-loop control strategy, establishes the small-signal modeling of the system, and its stability is verified by theoretical analysis and simulation. The simulation and experimental results verify the correctness of the proposed cascaded converter based on buck/boost + CLLLC and the effectiveness of the control strategy. [ABSTRACT FROM AUTHOR]

The 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 p−v2$p-v^2$ 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. [ABSTRACT FROM AUTHOR]

Seriously abnormal data exist in the synchronous monitoring data of transformer DC bias, which causes serious data feature contamination and even affects the identification of transformer DC bias. For this reason, this paper aims to ensure the reliability and validity of synchronous monitoring data. This paper proposes an identification of abnormal data for the synchronous monitoring of transformer DC bias based on multiple criteria. By analyzing the abnormal data of different types, the characteristics of abnormal data are obtained. Based on this, the abnormal data identification indexes are introduced, including gradient, sliding kurtosis and Pearson correlation coefficient. Firstly, the Pauta criterion is used to determine the threshold of the gradient index. Then, gradient is used to identify the suspected abnormal data. Finally, the sliding kurtosis and Pearson correlation coefficient are used to identify the abnormal data. Data for synchronous monitoring of transformer DC bias in a certain power grid are used to verify the proposed method. The results show that the accuracy of the proposed method in identifying mutated abnormal data and zero-value abnormal data is claimed to be 100%. Compared with traditional abnormal data identification methods, the accuracy of the proposed method is significantly improved. [ABSTRACT FROM AUTHOR]

This paper proposes an LLC‐DAB integrated high‐frequency isolated DC–DC converter topology structure to reduce the loss and cost of the DC power distribution system. Furthermore, the complexity and efficiency of the system also can be improved. The proposed converter consists of a DAB converter and two input‐parallel‐output‐parallel half‐bridge LLC resonant converters. It provides two DC voltage level buses, namely the HVDC bus and the LVDC bus. Compared with the typical converter topology of the DC power distribution system, the number of power semiconductors of the proposed converter topology is reduced by sharing the power semiconductors of the DAB converter and LLC resonant converter. As a result, with the number of power semiconductors decreasing, the cost and loss of the system are reduced. Moreover, the proposed converter can control the flow direction of power conversion and supply power to the loads. The single‐phase‐shift modulation method is adopted for the DAB converter, and two LLC resonant converters operate as the DC transformer under resonant frequency with open‐loop control. The operation principles and characteristics of the proposed converter are discussed. Finally, the simulation is carried out and an experimental prototype is developed to verify the effectiveness of the proposed topology. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]