Showing papers on "Isolation transformer published in 2022"

Principle and Control Design of a Novel Hybrid Arc Suppression Device in Distribution Networks

Abstract: A single line-to-ground (SLG) fault may lead to a more severe line-to-line fault and power supply interruption if the ground-fault current exceeds a certain value. Arc suppression device (ASD) is a good solution to eliminate the ground-fault current. A novel hybrid ASD is proposed in this article, which consists of a passive device and an active one. The passive device utilizes multiterminal breakers and an isolation transformer to couple a secondary voltage of a zig-zag grounding transformer to the neutral point to compensate the majority of the ground-fault current. The active device uses a single-phase voltage source inverter to eliminate the residual fault current due to the leakage inductance of the zig-zag grounding transformer in the passive device. A dual-loop voltage and current control method for the active device is designed for the accurate residual current compensation. Results of simulation and prototype experiment validate the effectiveness of the proposed hybrid ASD. The proposed hybrid ASD does not need to detect distributed line-to-ground parameters, and it has lower cost, less control complexity, higher reliability, and better performance, compared to other ASDs.

Design and Experimental Analysis of 166 kW Medium-Voltage Medium-Frequency Air-Core Transformer for 1:1-DCX Applications

Abstract: The galvanic isolation of solid-state transformers (SSTs) is typically realized with a medium-frequency (MF) magnetic-core transformer (MCT). Previous demonstrations indicate that achieving highly power-dense and lightweight MCTs imposes several challenges on the design because of stringent requirements related to insulation and cooling. This work investigates the achievable efficiency of an optimized 166 kW/7 kV air-core transformer (ACT), which is a core part of a dc transformer (DCX), i.e., an unregulated dc–dc SST with a voltage scaling defined by the transformer turns ratio. The ACT features relatively low complexity of the construction, comparably high coupling values, and high efficiency. Modeling, optimization, and construction of the realized ACT are explained and guidelines regarding insulation, cooling, and shielding of the magnetic stray flux are discussed in detail. Furthermore, the prototype is experimentally validated to demonstrate its full functionality. In the investigated DCX, which is based on a series resonant converter (SRC) topology, the realized ACT is found to achieve a full-load efficiency of 99.5% and an unprecedented gravimetric power density of 16.5 kW/kg. With the use of 10 kV SiC MOSFETs, the complete DCX is estimated to reach an efficiency of 99% at 166 kW output power.

A New Technique for Improvement Differential Relay Performance in Power Transformers

Abstract: Transformer protection devices are often used to identify internal or external transformer problems and act to either prevent damage or unnecessarily disconnect power transformers. This study proposes a new differential element that combines harmonic restraint, security, and reliability with harmonic blocking speed to improve the relay performance in a power transformer. Under high load, a negative-sequence differential element adds more sensitivity for internal turn-to-turn failures. External fault detection monitoring enhances security in an external problem involving current transformer (CT) saturation. Furthermore, overcurrent elements may be configured to vary dynamically in operation is provided. This element enhances protection coordination for various operating conditions without requiring modifications to the transformer group settings. The balance of the paper discusses the use of an under-load tap changer using a time-synchronized phasor monitoring system to reduce loop current and losses in parallel transformer applications.

High Frequency High Power Integrated Transformer Design for Resonant Converters with SiC Devices

Abstract: Resonant converters are used extensively for voltage regulation and isolation purposes in electric vehicles. The power level and frequency of the converters is increasing with the ongoing development of the wide-band-gap devices, which brings more challenges to the design of magnetic components. The first challenge is that such high power and frequency make it hard to make proper Litz wire. In this paper, a new current sharing method for paralleled Litz wire transformers is introduced. Using this method, the transformer can stand much higher current without increasing the cost or volume. And the power level can be further increased by integrating more matrix transformers. The second challenge is the resonant inductance integration. Different resonant inductance integration methods are summarized and compared. And controlling the leakage inductance by properly breaking the interleaving structure is proposed in this paper. Using this method, the desired leakage inductance can be reached without sacrificing too much winding loss. Comprehensive transformer design and optimization procedure is also presented in this paper. A 30 kW 100 kHz Serial Half Bridge DC-DC converter prototype is designed to verify the methods at last.

Common Mode Noise Analysis for Inductive Power Transfer System Based on Distributed Stray Capacitance Model

Abstract: This article reveals the significant stray capacitances in inductive power transfer (IPT) coil, and offers two methods for modeling the stray capacitance effect: concentrated model and distributed model. Both analytic formulas and FEM simulation are introduced to estimate the value of parasitic capacitances, and experimental measurements on an 11 kW IPT coil prototype are conducted to verify the correctness of both methods. The common-mode noise propagation circuit of the IPT system is derived and simulated based on these two models. The results show that parasitic capacitance between Litz wire winding and shielding metal on the transmitter side plays a dominant role in CM noise propagation and the distributed model has higher fidelity because of its asymmetric structure. Besides, to mitigate conductive CM noise, an EMI filter, and HF isolation transformer are introduced. Finally, the conductive EMI tests are performed in an 11kW IPT prototype. The experimental results validate the accuracy of the proposed CM noise model with stray capacitances of the IPT coil. And the combination of EMI filter and HF isolation transformer can suppress CM noise greatly and render the CM noise spectrum below the CISPR 32 Class-B limit.

Decoupling Capacitor Minimization of an MMC-Based Photovoltaic System With Three-Winding Power Channel

Abstract: This article proposed a grid-connected photovoltaic (PV) converter based on modular multilevel converter (MMC), which can realize long-distance dc transmission while realizing the local consumption of PV. In addition to the advantage that there is no PV power mismatch between the upper and lower arms, the isolation transformer couples the two submodules of the upper and lower arms on a three-port transformer, and the differential-mode components of the capacitor voltage ripples in the upper and lower arms can be canceled between each other. Besides, through the double fundamental frequency circulating current control, the power exchange among MMC phase units can be realized, thereby eliminating the common-mode component of the capacitor voltage ripple. Through the combination of topology and control strategy, the low-frequency components of the voltage ripple can be completely canceled. Since the decoupling capacitance is reduced dozens of times, the volume and cost of the system are significantly reduced. Therefore, the life and reliability of the system are increased. Simulation and experimental results verify the effectiveness of the proposed topology and its control strategy.

Equivalent Circuit Model of a Pulse Planar Transformer and Endurance to Abrupt dv/dt

Abstract: Wide-bandgap (WBG) semiconductor materials offer faster and more reliable power electronic components in electric energy conversion systems. However, their faster switching speed and abilities to operate at higher frequency than silicium devices have brought new challenges, such as electromagnetic interference (EMI) issues. In gate driver applications, EMI issues must be tackled given the close proximity between gate driver systems and WBG power modules. This article focuses on planar pulse transformers for gate drivers in high power applications (3.3 kV, 500 A SiC module). This article tries to give a standard procedure to design, then, simulate pulse transformers with their electrostatic shielding. First, a design guideline using Altium designer is proposed to respect the European standards. A method to extract the transformer design from Altium to Ansys is also proposed. Finally, a frequency analysis is discussed to use in Ansys simulations and parameters extraction. Tests have been performed to check the proposed transformer’s electromagnetic compatibility immunity under a 125 kV μs⁻¹ common mode transient immunity test.

Multi-terminal Detection and Localization of Partial Discharge in Transformer

Abstract: The transformer is the key equipment in the power system, and partial discharge is the main reason for the deterioration of the insulation of the transformer. The existing partial discharge detection method cannot determine whether the discharge comes from the transformer body or the peripheral equipment of the transformer when the discharge signal is detected. In order to study the difference between the internal and external partial discharges of the transformer received signals at the detection end and form an effective positioning method, this paper injects high frequency small signals into the external leads, inside the windings and surrounding screens of the 110kV three-phase double-winding transformer to simulate the external discharge of the transformer, the discharge inside and outside of the windings. The research shows that the first wave polarity of the external discharge and the internal non-winding discharge of the transformer is the same at each detection terminal, and the first wave polarity of the winding discharge signal at each detection terminal is different. The polarity information can be used to judge whether the winding discharges; the external discharge is the same as the transformer internal discharge. Non-winding discharge has different proportions of high-frequency and low-frequency components in each detection terminal signal. The signal energy of each detection terminal of external discharge is concentrated in low frequency, and the signal energy of each detection terminal of non-winding discharge inside the transformer is concentrated in high frequency, which can be judged by frequency domain information. The external discharge and the internal non-winding discharge of the transformer provide a reference for the research on the multi-terminal detection and positioning of the transformer.

A Circulating Power Suppression Structure for Three-Winding-Transformer-Based Converter

Abstract: A circulating power suppression structure for a three-winding-transformer-based converter is proposed in this letter. The equivalent circuit models of the dual-transformer-based and three-winding-transformer-based converter are derived, which illustrates their merits and demerits. With the proposed structure, the drawbacks of these two transformer-type converters can be overcome, and their advantages can be combined. Useless circulating loss in three-winding-transformer can be reduced to mW level and can be omitted. The complex and coupled dual-transformer parameter design problem can be solved. The analysis and performance of the proposed structure are fully validated by experimental results.

Gate Driver Power Supply with Air-gapped Transformer for Medium Voltage Converters

Abstract: In this paper, a gate driver power supply (GDPS) for SiC MOSFETs in medium-voltage power converters is presented. The main design considerations of the proposed GDPS are high voltage isolation, low coupling capacitance, and regulated output voltage. We present a design procedure for the transformer with an air gap to achieve the design objectives. Due to the air gap, the coupling factor of the proposed transformer is significantly lower than that of transformers without air gaps. Therefore, a series-series compensation network is designed to drive the loosely coupled magnetic link to achieve high-efficiency power transfer and stable output voltage. The input and output voltages of the proposed power supply are designed to be 24 VDC, and the rated output power is 20 W. The proposed transformer exceeds a partial discharge inception voltage of 20 kV RMS and has a coupling capacitance of 2.1 pF. The enclosure of the power supply is characterized by a creepage distance of 152 mm and a clearance distance of 80 mm between the input and output terminals of the GDPS. Compared with the commercially available high voltage gate driver power supply ISO5125I-120, the output power of the developed power supply is increased by four times, and the isolation strength doubled.

Design of an Isolated Gate Driver for Medium Voltage Cascaded H-Bridge (CHB) Based Solid State Transformer (SST)

Abstract: The design of an isolated gate driver for a Medium Voltage (MV) Cascaded H-bridge (CHB) converter is proposed in this paper. Isolation with proper electrical insulation and a low common-mode coupling capacitance are considered as the primary design criteria while designing the gate driver. A detailed circuit analysis has been carried out to find the isolation issues in the MV CHB converters, which necessitates a 2 ^nd stage of isolation in the gate driver circuit. Apart from improving the isolation voltage level, the additional isolation stage helps to reduce the coupling capacitance in the common mode path. As the device count for an MV CHB converter is significantly high, the complementary pulses have been generated locally in a CPLD-based delay card, which reduces the count of the optical cables and the associated components. It helps to make the system simple and cost effective. Finally, the design of the proposed gate drivers and associated delay cards are verified in a hardware prototype of 1.65kV/300V, 10kVA Solid State Transformer (SST).

Switching on Device of a Two-Transformer Power Substation

Abstract: This paper considers the study of the physical processes of a two-transformer substation with alternate switching on of their loads. It is known that at the transformer substation, when the load is turned on, automatic switches are used, as a result of which there are current surges, voltage drops, electric arcs and additional losses in electrical equipment and in the network. To eliminate the above disadvantages, a device called a diode-reactor starter is proposed, which ensures the inclusion of a power transformer under load, limits inrush currents, voltage drops and uneven distribution of magnetic fluxes in the transformer windings, and also eliminates the occurrence of an electric arc on switching devices. The proposed device is to make a bumpless switching on of a power transformer using an electronic device, which, after the soft start is completed, is shunted by a contactor. In the form of an electronic starter with a smooth and symmetrical increase in three-phase current in the transformer windings, a diode rectifier from reactors in the rectified current circuit is used, and a short circuiter is used as a bypass contactor at the input of the diode bridge. After turning on the short circuit, the electronic part of the launcher will be removed from the power supply system. The results of the study of physical processes in the Matlab/Simulink environment during the start-up of a power transformer with subsequent transition to a stationary mode of operation are presented.

Research on Suppression of Transformer DC Bias Caused by Metro Stray Current under Different Grounding Modes of the Grid

Abstract: The stray current leaked during the operation of urban metro will cause DC bias of transformers along the line. In order to reveal the influence of stray current leaked during actual operation of metro on the main transformer of nearby substation, the effectiveness of transformer DC bias suppression under different grounding modes of the grid was studied, and the dynamic distribution features of stray current was considered. The simulation model of DC bias suppression of main transformer in 110kV substation of urban power grid was established by using PSCAD / EMTDC platform. Through external controllable voltage source equivalent to DC potential difference between different substations, the phenomenon of DC bias hysteresis saturation of transformer was simulated. Further adjusting the grounding mode of transformer neutral point through small resistor or capacitor, the suppression effect of transformer DC bias caused by metro stray current under different grounding mode of power grid was analyzed. The simulation results show that the distortion rate of excitation current waveform and hysteresis loop is reduced, and the DC bias is suppressed when the transformer is grounded through small resistor. The distortion rate of hysteresis loop is further reduced compared with that of series small resistor, and the suppression effect of DC bias is better when the transformer is grounded by capacitor.

Design Topology of Sustainable Overload Protection of Transformer

Abstract: Overload on transformer has been a principal cause of overheating in its windings, reduction of efficiency, lowering of lifespan and sometimes leads to burn. This can be attributed to a large quantity of current drawn by the loads in the circumstances. In this paper, a design topology capable of providing sustainable protection to transformer from overload is presented. The designed system detects current drawn when being loaded and able to protect the transformer by isolation while displaying the overload current immediately a load is placed resulting in drawing a higher amount of current than the rating of the transformer. This was verified using simulation and validated by experiment on a prototype design. The simulation and experimental results achieved show good correlation, and exhibited the cogency of the design topology.

Research and analysis on suppression and influence of transformer direct current bias

Abstract: DC bias generates a large DC component in the windings of the transformer in operation, thereby generating harmonics, causing vibration, noise, and overheating. In severe cases, it can cause damage to the transformer, which will affect the safe and stable operation of the AC system. This article takes the DC bias of the main transformer of a 220kV substation as an example, and uses the capacitance isolation method to suppress the DC bias. Due to the addition of capacitor equipment at the neutral point, the zero-sequence impedance of the main transformer and the change of the zero-sequence impedance of the power grid are changed, which has an impact on the zero-sequence (directional) current protection and ground distance protection. Through theoretical analysis and simulation research verification, for general 220kV substations, the capacitor DC isolation method (1.2Ω) can be used to suppress DC bias, which has a small impact on the relay protection and does not need to adjust the related protection settings.

Microprocessor Implementation of Means for Diagnosing Winds of Power Oil Transformer

Abstract: Power oil transformers are an important component of the power infrastructure. Monitoring the condition of the windings of power oil transformers helps to prevent their premature failure and significant damage to the windings, the repair of which is expensive and leads to significant financial losses. Methods for periodic inspection of the state of the windings of the power transformer are not effective enough. This paper focuses on approaches to the analysis of the assessment of the insulation life of the power transformer windings. The mathematical model proposed by the author allows to estimate the state of insulation of windings of the power transformer in the conditions of short-term and long-term influences on a condition of isolation. This study is aimed at developing a microprocessor tool for diagnosing the windings of a power oil transformer by thermal aging in accordance with the proposed mathematical model. Microprocessor implementation simplifies the use of this mathematical model, as all calculations are automatic. The proposed approach does not require significant investment and meets the requirements of speed. The developed tool can be recommended for use by power companies in diagnosing power oil transformers, which can be integrated into the monitoring system of all power equipment of the switchgear of a power plant or substation, which reduces in some way the cost of maintenance of power equipment.

Simulation of Short Circuit to Ground of Transformer Winding Based on MTL Model

Abstract: Internal overvoltage of transformer caused by VFTO can threaten the security of the transformer. When the pulse wave invades the transformer with short-circuit fault to ground, the extremely uneven voltage distribution in the internal windings will not only endanger the safety of windings but also affect the stability of power system. The internal distribution parameters of the transformer winding model is necessary for the analysis of the short-circuit fault to ground based on MTL theory. By establishing a distributed parameter model of the transformer winding model with 16 turns, this paper analyzes the influence of the variation of distribution parameters in case of short-circuit to ground fault, then the modified short-circuit model to ground is applied to a transformer winding model with 180 turns. The results show that the modified model can well simulate the change of short-circuit fault to ground. The voltage waveform distribution between turns is extremely uneven.

Design and optimization of three-winding planar transformer for three-port high-frequency resonant converter

Abstract: To improve the power density of three-port high-frequency resonant converter using wide bandgap devices, a three-winding planar transformer is proposed. By analysing the topology and working conditions of the converter, the design requirements for the three-winding transformer are clarified. Based on the equivalent circuit of the three-winding transformer, the magnetizing inductance of transformer is directly and accurately controlled by the air gap of the core. The factors affecting the accuracy of air gap reluctance and transformer loss are analysed and some improved schemes are proposed. According to the optimized design process of the transformer, the method of selecting the optimal core size is given.

Parameter Robustness Analysis and optimization Design of CLLC High Frequency Isolation Circuit Applied to Power Electronic Transformer

Abstract: Power electronic transformer is the key equipment to realize the flexible access of distributed energy and new load/energy storage in distribution network. In this paper, the robustness analysis and optimal design of circuit parameters are carried out for CLLC DC transformer used in high frequency isolation link of SST. CLLC DC transformer can use open loop control to simplify control complexity, realize nearly constant bidirectional voltage gain in a wide range, and use the leakage inductance of high-frequency transformer as resonant inductance to reduce the difficulty of circuit design. In this paper, a parameter design method for CLLC DC transformer is proposed. Using this method, the designed CLLC DC transformer has good voltage regulation ability and wide range soft switching characteristics even when the inductance and capacitance value have design deviation. The influence of parasitic parameters of high-frequency isolation transformer and device on the voltage gain of DC transformer under light load is analyzed, Finally, a typical DC transformer is designed and verified by simulation in PSIM software.

Research on Radial Stability of Power Transformer Windings

Abstract: In this paper, the influencing factors of the radial strength of transformer windings are studied based on the inner winding of the single windings, and the elastic-plastic material properties are introduced to simulate the change process of the stiffness, defects and quantity of the transformer struts on the radial strength of the windings. Taking the experimental transformer as the research object, a two-dimensional finite element calculation model is established to analyze the variation law of the axial leakage magnetic field and radial electrodynamic force of the inner winding of the transformer. The radial force deformation process of the inner windings of the three-dimensional transformer is further studied, and the stability of the transformer winding under various working conditions is compared. The research on the winding stability of the transformer strut performance in this paper lays the foundation for the follow-up research on the cumulative deformation of the transformer winding.

A High-Power-Density Active-Clamp Converter with Integrated Planar Transformer

Abstract: This paper proposes an active-clamp forward-flyback (ACFF) converter with an integrated planar transformer for wide-input voltage and high-output current applications, such as low-voltage direct-current (LDC) converters in electric vehicles. An integrated planar transformer that consists of a forward-flyback transformer, single primary winding, and efficient structure of secondary windings is adopted for the proposed converter, and since this transformer is implemented with a common four-layer printed circuit board (PCB) winding, a high power density and low cost of the proposed converter can be achieved. In addition, due to the low leakage inductance induced by the planar transformer, a reduced commutation period can be achieved, and it is possible to increase the switching frequency resulting in low volume of transformer. Although the integrated planar transformer has relatively high conduction loss, the active-clamp topology can significantly reduce the conduction loss on switches compared with widely used full-bridge (FB) converters because it only utilizes two switches and shows the low circulating current. As a result, the proposed converter with an integrated planar transformer has strengths in high power density and cost competitiveness without degraded efficiency, and it is a very attractive topology for LDC converters and other applications that require wide-input voltage and high-output current.

Simulation Research on Electrodynamic Force and Deformation of Transformer Windings under Short-circuit Condition

Abstract: As an important equipment for electrical energy transformation, the operating condition of power transformer has a great influence on the normal operation of power system. When a short-circuit fault occurs outside the transformer, the electrodynamic short-circuit force generated by the interaction between the internal short-circuit current and the strong magnetic field acts on the windings, resulting in the deformation of the windings, thus causing transformer damage. Therefore, it is important to study the distribution of leakage magnetic field inside the transformer, the distribution of electrodynamic force and the deformation on the windings after short-circuit to ensure the stable operation of the transformer. In this paper, a three-dimensional model of the transformer is established based on an actual large transformer for simulation. The simulation results show that the leakage flux after short-circuit is mainly distributed in the channel between high voltage windings (HVW) and low voltage windings (LVW). Both HVW and LVW are subjected to large force in the middle, small force at both ends and compression force in axial direction. The HVW and LVW are subjected to expansion force and extrusion force respectively in radial direction. The deformation displacement direction of windings is consistent with the electrodynamic force direction, and the deformation in the middle is the most obvious. The research results provide guidance for improving the stable operation of transformers.

Research on Axial Stability of Power Transformer Windings

Abstract: In this paper, the distribution of magnetic flux leakage and force in transformer short circuit are calculated by using two-dimensional finite element model of transformer. Based on the calculation of magnetic field and force, the blocks and disks of Transformer Winding models of three-dimensional single-coil winding are further analyzed. The factors affecting the axial stability of transformer are studied. Through finite element simulation software, the influence of three working conditions on the axial stability of winding are calculated, including the accumulation of multiple short-circuit impact, assembly defects and different design of supporting structure. This paper lays a foundation for the subsequent study of the axial stability of transformer and the cumulative effect of short circuit.

Automatic Load Sharing of Transformer

Abstract: Transformer plays a major role in the power system. It works 24 hours a day and provides power to the load. The transformer is excessive full, its windings are overheated which leads to the judgment of the transformer installation which leads to disruption of the power supply to the load. It takes a lot of time to repair and involves a lot of costs. This project deals with transformer protection under conditions of overcrowding. The transformer could be protected by reducing the additional the transformer's load by connecting and using another transformer in conjunction the primary transformer using a microcontroller and a switch relay. The load on the first transformer is compared to the reference value by the Arduino. When the load exceeds the reference value, the slave transformer is automatically attached to the first transformer and the extra load is shared. Therefore, the number of transformers works well under conditions of overcrowding and damage could be prevented. In this project, slave transformers share the master transformer's duty in the event of overcrowding and overheating. Sensor circuit with Arduino, current transformer, and other components is designed to collect data from the master transformer, and if it is determined to be overloaded, the slave transformer is immediately attached to the master transformer and the load is shared. The Arduino keeps track of the transformer's current volume and displays it. If loads are introduced to the current transformer's second side in the second side riser. As the current volume exceeds the estimated current value of the transformer, so the microcontroller sends a travel signal to the relay, thus opening the slave transformer. Initially when we open the load, it will be shared by the first transformer. Once the load on the first transformer has been raised above its maximum capacity, the standby transformer will automatically share the load.

Research on Photovoltaic Grid Connected Inverter Without Isolation Transformer

Abstract: AbstractTraditional photovoltaic grid connected inverter usually has power frequency transformer or high frequency transformer, which brings many inconvenience. Due to the existence of equivalent parasitic capacitance of photovoltaic cell board to ground, it will lead to the generation of ground drain current in the photovoltaic grid connected inverter system without isolation transformer. In this paper, a new NPC topology of two-stage non-isolation transformer is adopted, the topology can completely eliminate the leakage current, suppress the pulsation of neutral point potential. The former stage adopts single current loop control to realize the maximum power point tracking of photovoltaic cell board; The second stage adopts the DC voltage outer loop and the double closed-loop control of the grid connected current inner loop. The advantages and disadvantages of PI and quasi PR are compared and analyzed. It is pointed out that the quasi PR controller is more suitable for the control of single-phase photovoltaic grid connected inverter. The experimental prototype of 5 kW new type double stage non-isolated transformer NPC grid connected inverter is built. The experiment of the new NPC topology is carried out. The results verify the feasibility and superiority of the new topology.KeywordsPhotovoltaic power generationTransformerlessLeakage currentPR controllerGird-connected inverter

Design of Medium-Voltage and Medium-Frequency Transformer for Solid State Transformers

Abstract: This paper presents a design guideline of medium-voltage (MV) and medium-frequency (MF) transformer for the isolated dc/ dc stage in the solid state transformer (SST). A dedicated leakage inductance can be integrated inside the transformer to get rid of the extra auxiliary inductor for phase-shift based power control. Therefore, the circuit prototype could be more compact. The design guideline includes transformer topologies comparison, core material selection, winding type considerations, and power loss & thermal calculations. Some of the design parameters are further illustrated by finite element analysis (FEA). This transformer was fabricated and measured. Finally, the transformer was demonstrated in a 7-kV 10-kW 60-kHz dc/dc converter.