Transformer Leakage Research Articles

A modelling technique to determine the high frequency transformer leakage inductance using the winding structure

The operation and efficiency of isolated DC-DC converters, critical components in solid-state transformers, are significantly impacted by leakage inductance in high-frequency transformers (HFTs). Different converters have varying demands regarding leakage inductance, ranging from precise control to minimal inductance. For instance, the performance of bidirectional isolated converters (BIDCs) and resonant converters is dependent on leakage inductance for the delivery of power. This study proposed a method for the control of leakage inductance by altering the winding configuration. The technique involved positioning the primary and secondary windings at predetermined heights to increase the separation between some winding turns, and thus, enhance leakage inductance. By not varying the average length of turns in the winding configuration, the proposed method was able to maintain a consistent copper loss. A modified mathematical model has been put forward to determine the leakage inductance effectively and precisely. The viability and accuracy of this method were validated through simulations and experiments using a three-phase HFT (3P-HFT) in a 3P-BIDC. The results showed that the leakage inductance that was calculated using the theoretical model was closely correlated to that of the simulation model, with only a variance of 3.9% and an error of 4.53% from the experimental results.

Design of a new modular‐isolated‐forward‐based active snubber cell for power switches

AbstractIn this paper, a new modular‐isolated‐forward‐based active snubber cell (SC) for power switches is designed. In the proposed new SC, the zero voltage transition (ZVT) technique is implemented with a forward converter although the current counterparts generally use a flyback converter. In the converter with the new SC, the main switch is turned on with ZVT (full zero voltage switching [ZVS]) and turned off with ZVS, the main diode is turned off with zero current switching (ZCS), the auxiliary switch is turned on with ZCS and turned off with ZVS, and the parasitic capacitor energies are recovered. In addition, thanks to the forward converter, it has been possible to minimize the transformer leakage inductance and greatly reduce the current values of devices in the new SC. The new cell is applied to a single‐phase, grid‐connected, T‐type three‐level inverter (T2‐3LI) as an example. A detailed steady‐state analysis of this inverter was made, and the theoretical analysis was confirmed with measurement results taken from a prototype with 100 kHz and 3.3 kW values. Compared to its hard switching (HS) equivalent, in the converter with soft switching (SS) cell, the total circuit loss was reduced from about 248 W to 86 W, thus achieving an increase in the total efficiency from 92.5% to 97.4%.

Application of Magnetic Composite Materials in Windings to Reduce Alternating Current Resistance in Leakage Transformers

Application of Magnetic Composite Materials in Windings to Reduce Alternating Current Resistance in Leakage Transformers

A novel distributing–collecting on-line insulation monitoring system and line selection technology for the DC supply system

At present, the power supply system of 5G base stations is a micro smart grid, it generally uses 240 V DC power supply with multiple branches, and leakage accidents will threaten personal and property safety, so it is vital to identify the fault line accurately and remove the faults rapidly. In this paper, the leakage phenomenon of transmission lines in the HVDC power supply system of a 5G communication base station is studied. To address the issue of multi-branch line leakage diagnosis and line selection in the 240 V DC system, a new distributed DC insulation monitoring and fault line selection technology and system are proposed. The distributed DC insulation monitoring line selection technology is used to collect the leakage current of each branch by setting a unique single-core DC leakage transformer and process the signal primarily. In addition, a high-resistance bridge switchgear is added to the bus bar. According to different leakage currents caused by the imbalance of the bridge before and after the switch, the insulation resistance of electrodes to the ground is calculated accurately, which solves the problem that the current technology cannot judge the fault of the positive and negative electrodes to the ground at the same time. Through both simulation and prototype experiments, the feasibility of this technology and system device in line insulation monitoring and line selection of the HVDC communication base station power supply system is verified.

Design and analysis of quasi-resonant high-gain impedance source DC–DC converter for DC microgrid

ABSTRACT This paper presents a quasi-resonant, isolated, high-gain, Y-source, DC/DC converter for a DC microgrid. Here, quasi-resonance is obtained by the LC tank during the shoot through period, resulting in natural commutation at the turn-off instant of the MOSFETs. The LC tank is formed by an additional parallel capacitor and transformer leakage inductance. The high voltage gain is obtained by including an isolation transformer with lower turns ratio, coupled inductor and voltage doubler. Here, frequency is modulated to regulate the output voltage with ON period constant. This converter’s zero current switching feature allows it to operate at a higher switching frequency and thus enhances the power density and efficiency. This converter’s continuous input current feature is an added advantage. The proposed converter inherits all the benefits of traditional impedance source converters. Finally, a 250 W hardware prototype is developed, and its performance is tested in the laboratory.

Analysis and Design Considerations of Input Parallel Output Series-Phase Shifted Full Bridge Converter for a High-Voltage Capacitor Charging Power Supply System

Capacitor charging power supply (CCPS) is used for impulse-power applications such as electromagnetic rail guns, flash lamps, medical sterilization, and rock crushing, among many other application fields. This paper describes the design and analysis of an Input Parallel Output Series-Phase Shifted Full Bridge converter for CCPS. The proposed system has a low output current ripple to improve the lifetime of the high-voltage pulsed capacitor. Design considerations of topology's electrical parameters such as HF transformer's leakage inductor and output inductor for the CCPS application are presented. The intermediate DC-link capacitance is provided as an energy storage element to minimize the disturbances (power variation) on the source for the CCPS application. Analysis and sizing of the capacitance for two different charging methods are analyzed and compared. A controller design procedure for the Active Front End Converter (AFEC) is also included to ensure near-constant power drawn, rejecting the load disturbances. Finally, a complete design procedure for the whole CCPS system is presented. Design and simulation results are presented for a rated system, followed by experimental results from a scaled-down hardware prototype to validate the design.

An Improved Grouping Transformer's Secondary Windings Current-Sharing Method for Multiphase LLC Converter

Current balancing performance of the existing grouping transformer's secondary windings (GTSW) method is determined by leakage inductance mismatches. Nevertheless, the transformer's leakage inductance parasitic parameters in practice are difficult to accurately design, especially in high-frequency occasions, causing a large current-sharing error between phases. To solve this problem, this paper proposes an improved GTSW (IGTSW) current-sharing method. By ingeniously changing the connection mode of transformer's secondary windings, the influence of secondary leakage inductance mismatch on current sharing performance is eliminated, and the current sharing performance of secondary side is significantly improved. The first harmonic approximation (FHA) analysis method is used to establish the current sharing error model of the primary side and secondary side of the system. Furthermore, the current sharing performance of the existing GTSW method and the proposed method is quantitatively compared. Then, the effectiveness of the proposed method is verified by simulation and the two-phase 480W experiment results. Finally, some great performances of the proposed method are discussed, such as advantages in high-frequency applications, no effect of parasitic capacitance on the current sharing performance, the potential scalability in the case of other DC-DC topology and high current scenarios, and modular expansion capabilities, etc.

A soft-switched flyback converter with leakage recovery snubber for class 2 LED driver

This work presents a soft-switching flyback converter with active leakage recovery snubber suitable for use in high-performance class 2 LED lamp drivers. The proposed converter enjoys the features of simple circuit structure, improved switching performance, high efficiency and high power density. Transformer leakage energy of the flyback converter is recovered using an active auxiliary circuit, which also helps in soft-switching transition of the switches and power diodes. Switching turn-on and turn-off of its main switch occurs under ZVZCS and ZVS conditions respectively, whereas ZCS turn-on and ZVZCS turn-off of the auxiliary switch are also achieved in this converter. The detailed steady-state operating principle with mathematical equations, design guidelines and loss analysis are presented in this work. Finally, the converter feasibility is thoroughly tested through laboratory experimentation on a hardware prototype. The experimental results have successfully validated the theoretical analysis and real-time behavior of the converter.

A Secondary-Side Phase-Shifted Bidirectional Soft-Switching DC–DC Converter Employing Step Voltage Switching for Eliminating Device Voltage Overshoot

This paper presents a modulation strategy for bidirectional power flow for the step voltage switched (SVS) secondary side phase-shifted (SPS) DC-DC converter that eliminates the device voltage overshoot problem. The voltage overshoot problem arises due to the resonance between transformer leakage inductance and the output capacitance of the current-fed side MOSFETs of the converter. Many snubber/clamp circuits were discussed in the literature to suppress this voltage overshoot. But the power loss in these clamp circuits will affect the converter efficiency. The bidirectional SVS SPS DC-DC converter presented in this paper eliminates the voltage overshoot problem instead of suppressing it and therefore nullifies the need for a voltage clamp circuit. And most of all switching devices undergo soft-switching (either ZVS or ZCS). These factors result in achieving a peak efficiency of 96%. The proposed topology is tested for 1.7KW at 100KHz switching frequency with voltage-fed side voltage being 72V and current-fed side voltage of 400V. The results obtained prove the feasibility of the proposed work.

A novel hybrid arc suppression device for single line-to-ground faults

Traditional passive arc suppression devices (ASDs) do not provide complete compensation for single line-to-ground (SLG) fault currents. Meanwhile, active ASDs are expensive. This study proposes a novel hybrid ASD to balance compensation and cost. The hybrid ASD consists of a passive part and an active part. To reduce the fault voltage to near zero, the passive part uses a single-phase isolating transformer and two sets of phase-selection switches. The active part, which compensates for the residual voltage caused by the transformer's leakage impedance, is connected to the secondary side of the transformer and the ground using a small-capacity inverter. A model predictive control (MPC) method is employed as the controller. We conducted simulations and experiments for various SLG fault cases, which demonstrated the effectiveness and robustness of the hybrid ASD in arc suppression. The hybrid ASD that we propose is independent of the neutral point and has several advantages, including low cost, strong arc suppression capabilities, and high robustness.

Impact of Parameter Mismatch on Three-Phase Dual-Active-Bridge Converters

Three-phase dual active bridge converters (DAB3) are a widely used topology in battery charging applications thanks to their numerous advantages, such as bidirectional power flow, galvanic isolation, low output current ripple, and inherent soft-switching. In such applications, three single-phase transformers are commonly employed as the AC-link to simplify manufacturing and reduce costs. These transformers’ leakage inductance can be utilized instead of the external leakage inductance to achieve high power density. However, the assumption of uniformity in these inductances is not always accurate as they can vary significantly during fabrication. This study presents a comprehensive analysis of the impact of transformer leakage inductance variation, which can deviate by up to 24% from the desired value. The effects of this variation are investigated from different perspectives, including power transfer, soft-switching range, root-mean-square (RMS) current, and the temperature rise of the transformer winding. Although the power transfer and total copper loss of transformers are changed insignificantly even under highly mismatched leakage inductance, the currents and thermal distribution among phases are considerably impacted. Based on statistical probability, a maximum leakage inductance variation threshold of 10–15% compared to the desired value is recommended to ensure the maximum acceptable temperature rise among phases. Experimental results are presented to validate the analysis.

A Modulation Strategy With Transformer Leakage Inductance Energy Management for a Three-Phase Matrix-Based Isolated AC–DC Converter

Three-phase matrix-based ac-dc power conversion has recently gained popularity, considering its higher conversion efficiency and power density compared to the conventional two stage solutions. In the literature, several modulation strategies have demonstrated improved total harmonic distortion (THD) of the ac current in the matrix converters through independent control of the anti-series combination of active devices that form a matrix switch (i.e., two gate drivers per matrix switch). In this paper, a matrix-based ac-dc power converter that utilizes only one gate driver per matrix switch is proposed without compromising the quality of ac current. Managing energy stored in high-frequency transformer’s leakage inductor is an issue while utilizing single driver per matrix switch. An additional short-circuit leg is proposed to circulate stored energy in the leakage inductance during zero vector periods of the proposed converter, thereby ensuring continuous path for the leakage inductor’s energy flow. Steady-state analysis of the proposed ac-dc converter along with its loss modelling are presented with necessary mathematical expressions. The presented analysis and design of the proposed converter are experimentally validated on a 2 kW hardware prototype in the laboratory environment. A pertinent comparison of the proposed matrix-based conversion methodology with the conventional matrix-based conversion strategy is also provided.

A Soft-Switched Current-Fed Dual-Input Isolated DC–DC Converter Topology

A soft-switched current-fed dual-input isolated dc-dc converter topology is proposed in this article. This converter integrates two low-voltage dc sources with similar transferred characteristics such as two PV panels or two batteries into a dc bus using a simple circuit topology and a minimum number of active and passive components. The energy transfer between the input and output ports is controlled by modulating the pulse-width of the control signals. Moreover, the soft-switching operations are discussed, and the smooth connection between the input-side inductor and the transformer leakage inductance is achieved. Furthermore, the operation of the converter in reverse power flow direction is presented to demonstrate the bidirectional operational capability of the converter. Finally, different operation modes of the proposed dual-input converter are verified by the experimental results obtained from a 250 W hardware prototype.

Analytical Hybrid Quasi-3D Transformer Leakage Inductance Model

Fast and accurate models of the transformer leakage inductance are crucial for optimization-based design of isolated converters. This article proposes an analytical hybrid Quasi-3D (HQ3D) model for E/U/ER/UR-core type transformers that combines the best suited submodels for each winding section of the transformer. A submodel is developed that directly calculates the leakage inductance contribution of curved winding sections based on the integration of the radially weighted magnetic energy density. The HQ3D model can describe arbitrary winding positions and its fully analytical nature guarantees optimal geometrical scalability. The only restriction is that the windings need to be parallel to the core edges. The HQ3D model is in the accuracy range of 3D finite-element method and verified with measurements on nine transformer prototypes with significantly different geometry ranges. The HQ3D model is rapidly executable, and therefore, well suited for optimizations of galvanically isolated converters.

Inserted-Shunt Integrated Planar Transformer With Low Secondary Leakage Inductance for LLC Resonant Converters

The leakage inductance of an integrated transformer is usually utilized as the series inductor of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> topology. However, leakage inductance exists on both primary and secondary sides of an integrated transformer and secondary leakage inductance leads the control and design of the converter to difficulty. In this article, a novel topology forinserted-shunt integrated transformers is proposed that has low secondary leakage inductance. The inserted shunt of the proposed topology is not segmental and can be located conveniently within the transformer. In addition, the inserted shunt does not require low permeability core material, simplifying its manufacture. The design and modeling of the proposed transformer topology are presented and verified by finite-element analysis and experimental implementation. The proposed topology is also compared with a recently published inserted-segmental-shunt integrated transformer. It is shown that the proposed transformer provides higher efficiency and lower ac resistance. Finally, an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter is implemented to examine the performance of the proposed integrated transformer in practice.

Wide-Frequency Range Voltage Monitoring Device for Current Transformer

To optimize the online voltage monitoring technology of the power grid and realize real-time fault warning for the power grid and capacitive devices such as current transformers (CT), this paper proposes a wide-frequency range voltage monitoring method based on current transformers according to the idea of measuring the leakage current of capacitive devices to restore the line voltage. The method measures the current transformer leakage current to invert the bus voltage and uses a wide frequency range current sensor to meet the measurement of high-frequency waveforms. This paper designs a CT wide-frequency range voltage monitoring device based on this method and conducts field tests of the device’s measurement effects. Through the fault monitoring test and voltage monitoring test, it is verified that the monitoring device is fully functional and can achieve accurate monitoring of harmonics, end shield current, relative dielectric loss, high-frequency partial discharge, and bus-voltage with high amplitude and high frequency, which can provide support for fault analysis and insulation design improvement and is of great significance to ensure safe and stable operation of the power grid.

Driving Circuit Design for Piezo Ceramics Considering Transformer Leakage Inductance

This paper presents the circuit and control method for piezo-ceramic drives. With the proposed method, a gap is imposed in the transformer core to increase the leakage inductance. This flattens the voltage gain curve of the piezo-ceramic driver over the resonant frequency range, so voltage gain changes are insensitive to frequency changes. In addition, resonant frequency tracking and power control methods are developed, while the circuit is capable of zero-voltage soft-switching such that the circuit operation efficiency can be improved. In order to solidify the practicality of the circuit design, mathematical analysis and experimental validations have been thoroughly performed. The test results help to confirm the effectiveness of the proposed method and demonstrate its practicality in industrial applications.

Improved Analytical Triple-2D Leakage Inductance Model of Cone Winding Matrix Transformers

The transformer leakage inductance is one of the limiting factors for pulse shape quality in high voltage pulsed power (HVPP) converters that are essential in applications such as cancer treatment, particle accelerators, and free electron lasers. Cone winding matrix (CWM) transformers are commonly used in HVPP converters as they offer low leakage inductance and high insulation distance. This paper proposes an improved Triple-2D (iT2D) leakage inductance model for CWM transformers. The iT2D model identifies three basic cross sections of CWM transformers and calculates their associated leakage inductance contributions. The paper proposes an accurate and compact analytical formula of the leakage inductance contribution resulting from the magnetic energy stored in between the magnetic cores. For cross sections with cone windings, an expression is presented that computes the leakage inductance per unit length of non-parallel windings. The iT2D model is verified with measurements and finite element simulations of three transformer prototypes. The analytical solutions used in the iT2D model ensure applicability to arbitrary geometric aspect ratios and designs. Furthermore, the iT2D model is rapidly executable enabling its time-efficient integration in converter optimisations.

A Coupled Inductor Based Ćuk Microinverter for Single Phase Grid Connected PV Applications

A single phase, single stage, non-isolated microinverter scheme is proposed in this paper which can address the issues that are commonly associated with flyback derived microinverters, such as i) the appearance of high voltage turn OFF spikes across the high frequency switch, ii) the requirement of lossy snubbers to control these voltage spikes, iii) the necessity for high turns ratio of the transformer and iv) poor efficiency due to the involvement of a higher number of components. The proposed topology involves only one high frequency switch. A resonance capacitor is employed which not only controls the turn OFF voltage spikes across this power switch but also ensures zero voltage turn ON (ZVS) for this switch. A coupled inductor based Ćuk configuration is incorporated in this topology wherein the energy trapped in the transformer leakage inductance is fed back to the grid, thereby improving the overall efficiency. Implementation of a dedicated boundary conduction control (BCM) strategy effectively limits the value of overall system ohmic losses and thereby ensures performance with enhanced efficiency. The inverter operating principle, the process for deriving the reference current expression, and the optimum design strategy for the inverter parameters are discussed in detail. A 220 W laboratory prototype is fabricated to validate the efficacy of the proposed scheme.

Phase-Shifted Full Bridge DC–DC Converter for Photovoltaic MVDC Power Collection Networks

The connection of photovoltaic sources to a medium voltage dc collection network requires a dc-dc converter having specific grid-connected converter capabilities. This article presents the application of a phase-shifted full bridge (PSFB) converter for medium voltage dc collection networks suited to photovoltaic power plants. The unidirectional structure of the converter is beneficial in terms of cost and simplicity of hardware implementation of the MV circuit components. The design of the PSFB converter is presented considering ratings of 250 kW power, 1.2 kV input, and 20 kV output. A converter model and a novel PSFB input voltage control regulating the dc-link voltage are developed and verified. A tuning method is given taking into account the effect of different transformer leakage inductance values. The behaviour of the PSFB under the PV power production profile and MV network faults is studied in simulation and verified against a reduced-scale prototype with ratings of 30 kW and 350 V / 600 V operating at 20 kHz. The experimental verification of the control is performed considering a real PV power profile. The experimental results under faults show the same response as in simulations. The PSFB fault response is observed to be superior compared to other dc–dc topologies with capacitive output filter only, making it suitable for grid-connected operation.