Conventional Power Transformer Research Articles

Design and Thermal Analysis of a 250 MVA HTS Transformer for Substation of Offshore Wind Farms

In Offshore Substations (OSs), using High Temperature Superconducting (HTS) transformers instead of conventional power transformers to reduce operational and construction limitations such as weight, capacity, marine pollution, and future planning can be considered as desirable. HTS transformers can reduce operating system costs in wind power plant development programs. However, OSs have specific operating conditions in fault mode that should be considered in the design HTS transformers.In this paper, a 250 MVA 33/220 KV HTS transformer is designed, and electromagnetic analysis is done in normal conditions to calculate AC loss and thence to design the cooling system. The second step, thermo-flow analysis is done by Finite Element Method (FEM) to calculate the temperature distribution of the windings. Increasing turbulence of inlet flow and creating a porous coating on the tapes are used in this transformer to increase the heat-transfer rate. lastly, the specifications of the HTS transformer are compared with the conventional transformer to give useful information to OS designers to investigate using novel construction ideas such as floating OS by HTS transformer.

Decision Process for Identifying Appropriate Devices for Power Transfer between Voltage Levels in Distribution Grids

During the energy transition, new types of electrical equipment, especially power electronic devices, are proposed to increase the flexibility of electricity distribution grids. One type is the solid-state transformer (SST), which offers excellent possibilities to improve the voltage quality in electricity distribution grids and integrate hybrid AC/DC grids. This paper compares SST to conventional copper-based power transformers (CPT) with and without an on-load tap changer (OLTC) and with additional downstream converters. For this purpose, a corresponding electricity distribution grid is set up in the power system analysis tool DIgSILENT PowerFactory 2022. A DC generator like a photovoltaic system, a DC load like an electric vehicle fast charging station, and an AC load are connected. Based on load flow simulations, the four power transformers are compared concerning voltage stability during a generator-based and a load-based scenario. The results of load flow simulations show that SSTs are most valuable when additional generators and loads are to be connected to the infrastructure, which would overload the existing grid equipment. The efficiency of using SSTs also depends on the parameters of the electrical grid, especially the lengths of the low-voltage (LV) lines. In addition, a flowchart-based decision process is proposed to support the decision-making process for the appropriate power transformer from an electrical perspective. Beyond these electrical properties, an evaluation matrix lists other relevant criteria like characteristics of the installation site, noise level, expected lifetime, and economic criteria that must be considered.

Solid State Transformers: A Critical Review of Projects with Relevant Prototypes and Demonstrators

The ongoing energy transition has changed the architecture of electricity networks in ways that conventional power transformers are not able to cope with the new required functionalities. For this purpose, the solid state transformer (SST), which comprises state of the art power electronics with galvanic isolation to interconnect two separate alternating current (AC) or direct current (DC) power grids, is considered to be the dominant solution. The purpose of this paper is to provide a practical, application-oriented review of the SST. In this context, the main functionalities and possible applications of the SST are presented, including smart grids (SGs), data centres, railways, offshore wind farms, etc. Furthermore, the main developed SST prototypes are analysed with special focus on the related projects, demonstrators, stakeholders and rated values, e.g., voltage, switching frequency and power. The analysis is concluded with the future trends and challenges regarding the wider implementation of SST technology in the electrical grid.

FEM-Based Power Transformer Model for Superconducting and Conventional Power Transformer Optimization

There were many promising superconducting materials discovered in the last decades that can significantly increase the efficiency of large power transformers. However, these large machines are generally custom-made and tailored to the given application. During the design process the most economical design should be selected from thousands of applicable solutions in a short design period. Due to the nonlinearity of the task, the cost-optimal transformer design, which has the smallest costs during the transformers’ planned lifetime, is usually not the design with the highest efficiency. Due to the topic’s importance, many simplified transformer models were published in the literature to resolve this problem. However, only a few papers considered this preliminary design optimization problem in the case of superconducting transformers and none of them made a comparison with a validated conventional transformer optimization model. This paper proposes a novel FEM-based two-winding transformer model, which can be used to calculate the main dimension of conventional and superconducting transformer designs. The models are stored in a unified JSON-file format, which can be easily integrated into an evolutionary or genetic algorithm-based optimization. The paper shows the used methods and their accuracy on conventional 10 MVA and superconducting 1.2 MVA transformer designs. Moreover, a simple cost optimization with the 10 MVA transformer was performed for two realistic economic scenarios. The results show that in some cases the cheaper, but less efficient, transformer can be the more economic.

Operation Risk Assessment of Hydroelectric Energy Storage Based on Data Visualization and Convolutional Neural Network

Hydroelectric energy storage, that is, pumped storage hydropower (PSH) is considered as the essential solution for grid reliability with high penetration of renewable power, due to its advantages of cost-effectiveness for grid energy storage as well as supporting ancillary services. However, the operation modes of the main transformer unit in PSH are way more complex than the conventional power transformer, which makes the condition monitoring and fault detection of PSH becoming a technical challenge. In this article, an operation status recognition model of main transformers in PSH based on artificial visualization of mechanical vibration signals and deep learning is proposed. The vibration signals on a series of 500 kV/360 MVA main transformers of PSH are monitored periodically by contacting sensor arrays. These vibration signals are processed into nephograms by using linear interpolation fitting and 1D to 2D data mapping. A deep learning method based on the convolutional neural network (CNN) is used to classify nephograms obtained under different operation modes, that is, no load, full load, DC bias, and short circuit. The proposed status prediction algorithm was trained and tested through 150 sets of vibration nephogram samples, which ensures the feasibility of the nephogram generation method and the performance of the classifier. The testing results show that the overall status prediction accuracy for the proposed algorithm achieves 89.7% when the network structure is optimized. It is indicated that the mechanical vibration of the main transformer has a pattern matching relationship with the operating state of PSH. In practice, the operating status of PSH can be diagnosed remotely by embedded IoT sensors; the health index of PSH can also be estimated by weighed analysis of the changing trend of vibration data obtained in the life cycle.

The Feasibility of Using a Conventional Power Transformer to Obtain High Surge Current

The Feasibility of Using a Conventional Power Transformer to Obtain High Surge Current

Development of a Module for Measuring Electrical Variables in Power Transformers Based in IoT, to Manage and Monitoring by Telemetry Mechanism

This work shows the development of a module that performs measurements of electrical variables in a low voltage power transformer. These variables are sent by means of the IEEE802.11 standard, connecting to a database stored in the cloud; associating with the meter IoT concepts, this to allow a client to perform an analysis, monitoring and management of their electrical network. For the construction of this module, non-invasive current sensors connected to a three-phase meter are used and a communication card is used that allows data to be extracted from the meter and sent to the cloud database. This module, to convert a conventional electrical power transformer into a smart electrical power transformer, making it a Smart Object and thus extract information from it telemetrically in real time.

Neutral Current Optimization Control for Smart Transformer-Fed Distribution System Under Unbalanced Loads

In a three-phase four-wire low voltage (LV) distribution system, unbalanced loads lead to neutral current (NC) looping resulting in increase of power losses and variation of neutral potential. Compared to the conventional power transformer, smart transformer (ST) has strict current limitations to avoid overcurrent. However, its advantages on the downstream LV grid voltage regulation can provide the capability to regulate excessive NC. This article proposes a closed-loop NC optimization control in order to, on the one hand, minimize the NC current in the normal operation satisfying the standard EN 50160 requirement, on the other hand, suppress the NC current in extreme cases to avoid the overcurrent damage of the ST. The proposed control strategies are validated by experimental tests via the hardware-in-the-loop setup and a case study based on a 350-kVA, 10-kV/400-V, ST-fed distribution network under unbalanced loading profile according to the three-phase four-wire distribution grid in the Manchester area. The results clearly prove the effectiveness and flexibility of the proposed NC optimization control strategies on the NC suppression and minimization.

Application of FRA to Improve the Design and Maintenance of Wireless Power Transfer Systems

Frequency response analysis (FRA) plays an important role in the assessment of the mechanical integrity and hence, reliability of high-voltage power transformers. Wireless power transfer (WPT) system introduced by Tesla a 100 years ago based on Faraday’s law of electromagnetic induction is similar to a conventional power transformer. Therefore, the reliability of a large WPT system can also be evaluated using FRA measurement. When the WPT system is constructed, the electrical components and the desired transfer distance need to be ascertained accurately. Furthermore, the structure of the winding and the electrical characteristic of the components in WPT systems may vary due to aging or fault events. FRA measurement can examine the discrepancy between the WPT system parameters and the reference fingerprint data to provide cost-effective condition monitoring-based maintenance scheme for the WPT systems. This paper presents a detailed circuit analysis of a two-coil inductively coupled WPT model in different compensating topologies to accomplish efficient energy transfer. Then, FRA measurement is used to detect the variation in the amplitude and the phase angle of the developed circuit input impedance in a wide frequency range. In addition, displacements due to the placement of ferrite material and the variation of the transfer distance between the transmitter and receiver units are investigated through practical measurements.

An Online Condition Assessment of Box Substation in Wind Farm Based on Hypothesis Testing

With the rapid development of wind generation, the box substation is widely applied in the wind farm. However the box substation is more prone to break down than the conventional power transformer, due to the uncertainty of wind power. It is difficult for the existing transformer condition assessment to fulfill the requirement of “few people on duty” operation model, which is promoted by both the domestic and foreign wind farms. To improve the safety and reliability of the wind generation, an on-line condition assessment of the box substation in wind farm based on hypothesis testing is proposed. Firstly, an ensemble learning based on least squares support vector regression (EL-LSSVR) is proposed. Secondly, EL-LSSVR is applied to the historical data of the normal box substation, to establish a normal hot spot temperature model. Then the normal hot spot temperature model learns from the real-time data of the evaluated box substation on-line, to establish a real-time hot spot temperature model. Thirdly, the hypothesis testing is employed to analyze the significance between the real-time hot spot temperature and the normal hot spot temperature, which are generated by the real-time hot spot temperature model and the normal hot spot temperature model respectively. The condition assessment of the box substation is implemented according to the significance. Finally, the proposed method is validated by the condition assessment experiment of the box substation, which is installed at a wind farm in South China.

Design and software implementation of solid state transformer

The work presented in this paper concerned with the analysis, design and software implementation of the Solid State Transformer as an alternative to the conventional power transformer. The proposed transformer aims to perform the same task as the conventional one with additional facilities and advantages. Three stages are considered to configure the Solid State Transformer. The first stage which is known as input stage and implemented using Vienna rectifier which converts the AC voltage of the main supply to a DC voltage. The second stage (isolation stage) step down the DC voltage to a lower level DC voltage. This stage consists of a single – phase five-level diode clamped inverter, 1 KHz step – down transformer and fully controlled bridge rectifier. The output stage (third stage) is a three-phase three-level diode clamped inverter which converts the low level DC voltage to a three-phase, 50 Hz AC voltage. Model Predictive Current Control has been employed for driving transformer’s stages. The gating signal is produced directly when the given cost function is minimized, therefore there is no need of any modulator. Behavior of the proposed structure is achieved by simulation which shows high quality power conversion with low Total Harmonic Distortion.

Hybrid AC/DC microgrid architecture with comprehensive control strategy for energy management of smart building

The permeability of distributed generations (DGs) is affected because of their uncertainty and randomness, and the traditional architecture of microgrid with conventional power transformer is unsuitable for smart building due to its uncontrollability and vulnerability to interference. In this article, a hybrid ac/dc microgrid architecture for smart building is proposed to increase the penetration of DGs and to isolate the interference to the grid. Thus, the system safety and stability can be ensured in a changeable and complicated grid environment. The advantages of this architecture include (1) the full use of the controllability of the modular multilevel converter based solid state transformer (MMC-SST), (2) the complementary characteristic of time and space of DGs, (3) and the peak shaving capacity of energy storage systems (ESSs). The characteristics of typical operating modes, i.e. grid-connected mode and off-grid mode are analyzed, and the comprehensive control strategy combining with the characteristic of each mode is designed. Finally, the effectiveness of the proposed architecture and the comprehensive control strategy is validated by a simulation platform.

Integration of Large Photovoltaic and Wind System by Means of Smart Transformer

Large renewable energy systems, e.g., photovoltaic and wind turbine, are connected to grid, which is usually weak, with long lines. This degrades power system stability and reliability. Additionally, the magnetic characteristic of a conventional power transformer (CPT) implies a slow dynamic and leads to grid current harmonic too high when wind speed is low and irradiance is weak. This paper proposes a parallel connection of a solid-state transformer-based smart transformer and a CPT to not only reduce the power rating, but also to support the voltage amplitude and improve the grid current quality. The simulation and experimental results clearly verify the correctness and feasibility of the proposed strategy.

The Smart Transformer: A solid-state transformer tailored to provide ancillary services to the distribution grid

The solid-state transformer (SST) was conceived as a replacement for the conventional power transformer, with both lower volume and weight. The smart transformer (ST) is an SST that provides ancillary services to the distribution and transmission grids to optimize their performance. Hence, the focus shifts from hardware advantages to functionalities. One of the most desired functionalities is the dc connectivity to enable a hybrid distribution system. For this reason, the ST architecture shall be composed of at least two power stages. The standard design procedure for this kind of system is to design each power stage for the maximum load. However, this design approach might limit additional services, like the reactive power compensation on the medium voltage (MV) side, and it does not consider the load regulation capability of the ST on the low voltage (LV) side. If the SST is tailored to the services that it shall provide, different stages will have different designs, so that the ST is no longer a mere application of the SST but an entirely new subject.

First 1 MVA and 10/0.4 kV HTSC transformer in Russia

This article describes results of the development, construction, and test of the first three-phase distribution power transformer prototype in Russia with windings made from second-generation HTSC wire and iron core from amorphous electrical steel. The design of its elements is described and improvement of HTSC transformer performance characteristics compared to conventional power transformers with the same power rating is shown.

Using the finite element method for calculating transformers for resistance welding machines

The method for electromagnetic and thermal calculations of transformers for resistance welding machines (RWM) is examined. The currently available methods of calculating these transformers, based on the calculation of conventional power transformers and special features typical of the RWM, are taken into account in the form of empirical corrections. It is proposed to supplement the currently available method by algorithms of numerical simulation of electromagnetic and temperature fields. A transformer for a large RWM is used as an example to show the calculation of short-circuiting impedance and temperature analysis in the repeated short-term mode. The procedural problems of numerical simulation and agreement between the results of simulation and conventional calculations are discussed.

Performance analysis of electronic power transformer based on neuro-fuzzy controller.

In recent years, electronic power transformer (EPT), which is also called solid state transformer, has attracted great interest and has been used in place of the conventional power transformers. These transformers have many important functions as high unity power factor, low harmonic distortion, constant DC bus voltage, regulated output voltage and compensation capability. In this study, proposed EPT structure contains a three-phase pulse width modulation rectifier that converts 800 Vrms AC to 2000 V DC bus at input stage, a dual active bridge converter that provides 400 V DC bus with 5:1 high frequency transformer at isolation stage and a three-phase two level inverter that is used to obtain AC output at output stage. In order to enhance dynamic performance of EPT structure, neuro fuzzy controllers which have durable and nonlinear nature are used in input and isolation stages instead of PI controllers. The main aim of EPT structure with the proposed controller is to improve the stability of power system and to provide faster response against disturbances. Moreover, a number of simulation results are carried out to verify EPT structure designed in MATLAB/Simulink environment and to analyze compensation ability for voltage harmonics, voltage flicker and voltage sag/swell conditions.

Electric field distribution in oil-pressboard insulation under AC-DC combined voltages

Converter transformers use oil-pressboard as their main dielectric medium and are operated mainly in AC-DC combined electric field. For oil-pressboard insulation, the electric field under DC voltage shows distinctive characteristics from that in AC electric field. Compared to conventional AC power transformers, the electric filed distribution in the converter transformers is much more complicated and remains a big challenge for optimization of insulation design. Existing simulation research in this field falls short of experimental verification and shows limitations in the insulation design of the converter transformers. This paper attempts to fill the gap by measuring the dynamic process and distribution of electric filed in oil-pressboard insulation under the combined field so as to provide reference for insulation design. With the established Kerr-effect electric field measurement platform, the present paper recorded the dynamic process and distribution of AC and DC electric fields at different AC/DC ratios for the double-layer overlapping oil-pressboard insulation model. The research results indicate that 1) the AC electric field distribution in oil is homogeneous and symmetric. The field strengths form linear relation with the amplitudes of applied voltages; 2) the DC electric field distribution in oil is heterogeneous and asymmetric. Under the application of positive DC voltage, the field strength around the upper electrode is much smaller than that of the lower one. The DC electric field strength shows no linear growth with the increase of the applied voltages; 3) the accumulation of interface charges forms the major cause resulting in the asymmetric distribution of the DC electric field at different positions of the insulation. The research results imply that although RC model is recognized as effective simulation method for insulation design of conventional transformers, it shows its limitations and should be enriched and adjusted according to the electric field distribution under AC-DC combined voltage and taking into full consideration of the accumulation of interface charges.

Control and Experiment of an H-Bridge-Based Three-Phase Three-Stage Modular Power Electronic Transformer

Compared with conventional power transformer, the power electronic transformer (PET) or solid-state transformer has many attractive additional features. This paper focuses on an H-bridge-based three-phase three-stage modular PET, which consists of an input stage with series-connected H-bridge converters, an isolation stage with several independent dual-active-bridge converters, and an output stage with parallel-connected H-bridge converters. This PET suffers dc-link capacitor voltage unbalancing issue, and the parallel-connected module current unbalancing sharing issue. In this paper, a system control structure is proposed for the PET to deal with these issues. Different input-stage individual module dc-link voltage balancing control methods are analyzed and compared. It is found that the one implemented by directly trimming module output voltage amplitude is most suitable for PET. Moreover, a downscaled laboratory prototype is designed, built, and tested to verify the control strategy.

Creepage discharge of oil-pressboard Insulation in AC-DC composite field: phenomenon and characteristics

Converter transformers use oil-pressboard as its main dielectric medium and are operated mainly in AC-DC composite electric field. Creepage discharge is one of the common defects for oil-pressboard insulation, especially in converter transformers. Oil-pressboard insulation shows distinctive flashover behaviors and surface electric strengths in AC-DC composite electric field from that in pure AC or DC electric field. Compared to conventional power transformers, there remain bigger challenges to realize insulation assessment and fault diagnosis for the converter transformers. Adequate knowledge of the phenomenon and characteristics of oil-pressboard insulation creepage discharge is critical for fault diagnosis of converter transformers. This paper attempts to fill the existing research gap by studying the dynamic process and phenomenon of creepage discharge in the composite field so as to provide criteria for insulation diagnosis and assessment. With the established experimental platform, the present paper observed the phenomenon of oil-pressboard insulation creepage discharge from its initiation till final flashover under composite AC-DC voltages, identified the trends of partial discharge and dissolved gas throughout the entire process, and summarized accordingly the criteria for insulation diagnosis and assessment. Results indicate that 1) for creepage discharge of oil-pressboard insulation under AC-DC superimposed voltages, abrupt changes were observed in terms of discharge repetition rate and magnitude. Discharge phases kept enlarging during the whole discharge evolution process; 2) based on the observed discharge characteristics, the evolution process of creepage discharge could be classified into three stages which imply three severity levels of the discharge; 3) compared to that in AC electric field, the creepage discharge in AC-DC composite field witnessed much less amount of and different composition of dissolved gas in oil. More C2H6 and C2H2 were observed in the AC-DC composite electric field whereas CH4 and C2H2 dominated the AC electric field. The research results indicate that as much as both PD-identification and DGA methods are recognized as effective means of fault diagnosis for converter transformers, the referential assessment criteria should be enriched and adjusted for diagnosis of creep discharge under AC-DC composite voltages, taking into full consideration of the distinctive characteristics of both discharge and gases dissolved in oil.