High Temperature Superconducting Transformer Research Articles

Thermal analysis of the HTS transformer in the fault current condition considering the tape roughness effect

High temperature superconducting (HTS) transformers have many advantages over conventional transformers in terms of volume, weight, and total efficiency. However, fault conditions limit the electro-thermal performance of an HTS transformer. Based on this, we investigate the thermal modeling of the HTS transformer in the fault-current state. For thermal modeling, heat transfer and parameters affecting it such as surface roughness and bubble behavior have been considered. At first, an accurate thermal model for a 10 kVA HTS transformer has been prepared. Experimental results show that the penalty of this model is about 2.5 K in the hot-spot point (HSP) temperature in fault-current conditions. In a second step, the impact of a roughness on the heat transfer and mass transfer rate in nucleated boiling mode is investigated. Results show that surface roughness decreases the HSP temperature by more than 10 K in fault-current conditions. These results have significant impacts on protecting HTS windings against quenching.

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.

AC loss study on a 3-phase HTS 1 MVA transformer coupled with a three-limb iron core

High-temperature superconducting (HTS) technology provides an alternative approach to achieve compact transformers. Addressing AC loss in the HTS winding is crucial for HTS transformer applications. Most numerical AC loss studies on HTS transformers have neglected the influence of iron cores. This work carries out an AC loss study to explore the impact of an iron core on the HTS windings in a 3-phase HTS 1 MVA transformer coupled with it. AC loss simulations for the transformer winding both with and without the iron core are conducted by adopting the three-dimensional (3D) T-A homogenization method. When the iron core is incorporated, the saturation magnetic fields of iron materials, flux diverters (FDs) with different geometries, and variations in turn spacings in the LV winding composed of Roebel cables are considered to investigate their influence on the AC loss of the transformer winding. The inclusion of the iron core leads to a 1.2% increase in AC loss for the transformer winding while simulating at the rated current. We attribute this slight difference to the non-inductive winding structure of the transformer winding, where a strong magnetic field generated in the space between the LV and HV windings effectively shields the influence of the iron core.

Modeling and Experimental Study of the Inrush Current of a High-Temperature Superconducting Transformer

Inrush current in high-temperature superconducting (HTS) transformers is a little-studied phenomenon. After connecting to the power grid, a current flow through the windings which exceeding the critical current value of the superconducting tape. It may cause significant overheating and thermal damage of winding. The purpose of the study is to develop a mathematical model for calculating inrush current pulses in a HTS transformer and its verification by physical experiments. To achieve the goal of the study, a mathematical model has been developed that accurately represents the electromagnetic and thermal transient processes after HTS transformer is turned on at idling or under load. The model considers the critical parameters of the HTS tapes, the process of heating and cooling of the windings, quench characteristics, and the electrical and magnetic parameters of the transformer. Good compliance of the experimental results and mathematical modeling with a deviation of 1.99 % allowed us to verify the model. The most important result is the creation of a mathematical model of the HTS transformer at the moment of connecting to power grid. This model represents the temperature changing of the windings during the loss of superconductivity. The developed model can be used in the analysis and modeling of inrush current in designed and operating HTS transformers for any power. The obtained results are significant for determine the optimal starting characteristics, geometric and electrical parameters of HTS transformers. The proposed methods for reducing the inrush current ensure safe and reliable operation of the HTS transformer when switched on at idling or under load.

Temperature reduction of an HTS transformer under short circuit fault by modifying the cryostat structure: Impact of Perlator and valve location

High Temperature Superconducting (HTS) transformers are one of the potential technologies for power systems connected to offshore wind farms and stand-alone and bulk power grids. In such systems, proper fault performance of any electric device including HTS transformer is a vital factor to ensure a safe, and reliable delivery of electric power as well as power quality in electric grid. Short circuits can increase the risk of developing hot spots in superconducting tapes and as a consequence burning the windings in severe fault current cases. One important way to limit the temperature increase of the superconducting winding during short circuit is to increase the heat transfer of the liquid nitrogen (LN2) during fault. In this paper, the impact of increasing the turbulence of the inlet fluid on the Hot Spot Point (HSP) temperature of superconducting windings of a 120 kVA HTS transformer was investigated during a short circuit fault. To increase turbulence and consequently, heat transfer, a device known as Perlator was used. Then, the impact of the Perlator structure and the location and angle of inlet valves were investigated on the HSP temperature of an HTS transformer, under 65 K and 77 K operating temperatures. The results indicated that by using a Perlator and adjusting valve number and location in the cryostat structure, the HSP temperature of the HTS transformer under the fault current was significantly reduced by about 46.2 K which can be vital to save the transformer from failure.

Insulation characteristics of polyimide film used in liquid nitrogen of compact HTS transformers

AbstractSince compact high temperature superconducting (HTS) transformer has a small size and high efficiency, it can meet the requirements of high‐voltage transformers for vehicle‐mounted mobile substation. However, due to the small internal insulation size of the compact HTS transformers, it is necessary to improve the insulation strength to ensure the insulation safety of the transformer. Reasonable main insulation structure can greatly improve the insulation level of transformer. Based on the design theory of main insulation structure, this paper designs a main insulation structure model “small liquid nitrogen gap—barrier” suitable for compact HTS transformer. The insulation withstand voltage experiment of the main insulation structure shows that when the liquid nitrogen gap distance is less than 8 mm, the insulation strength of liquid nitrogen in the structure is improved. When the liquid nitrogen gap distance is too small, the size and thickness of the barrier will affect the insulation strength of the structure. According to the insulation strength of main insulation structures with different liquid nitrogen gaps, the feasibility of “small liquid nitrogen gap—barrier” main insulation structure is verified. The small liquid nitrogen gap structure can be adopted for the main insulation of compact HTS transformer.

Impact of Superconductor Technologies on Network Design and Energy Cost- a Case Study in Libya

Several studies and investigations haveattempted to introduce a new design for the conventionaldistribution networks. This proposed design aims to offer a reduction in substation installation cost and in power losses by eliminating some voltage levels in networks. Sine, this is difficult to achieve with conventional distribution networks, when the conventional cables and transformers are used, where the impedance of conventional equipment is high. Due to High-Temperature Superconductor (HTS) materials provide extremely low impedance to AC when cooled to the boiling point of liquid nitrogen (77K), the interest in using HTS materials to produce power cables and transformers is increased. The present paper investigates the practical impacts of installing HTS cables and transformers and seeks to introduce a future network design which leads to the best implementation for superconducting networks, thus, a drop in the substation installation cost and energy losses cost by eliminating some voltage levels in the traditional networks.

High Temperature Superconducting Transformer Operating at an Increased Frequency of Alternating Current

The study's aim is to substantiate and employ materials with high technical characteristics for the active elements of a transformer, particularly in the context of increased operating frequencies. To accomplish this objective, the following tasks were addressed: An analysis and synthesis of the properties of a magnetic core composed of amorphous iron and superconducting windings in a high-temperature superconducting transformer (HST) were conducted. Dependencies and graphs illustrating the impact of frequency increase on heat losses related to hysteresis and eddy currents were derived. These losses occur within the magnetic circuit. The study also demonstrates the relationship between reducing size and weight parameters and material consumption of windings in high-temperature superconducting (HST) tapes. Current densities in these tapes can reach up to 500 A/mm². The most noteworthy outcome, taking advantage of superconductivity in the HST, is that HST acts as an ideal diamagnetic material. Additionally, the windings exhibit high electrical conductivity. This resolves issues related to current displacement to the conductor's surface, commonly known as the "skin effect," and effectively addresses concerns about increased resistance in transformers and electrical machines. The absence of the skin effect in HST conductors has been rigorously demonstrated using Bessel functions. The significance of these results is in enhancing the efficiency of transformers operating at high frequencies. This is achieved by synthesizing the properties of the transformer's active elements, which include an amorphous iron magnetic core, HST windings, and a dielectric medium utilizing liquid nitrogen.

Influence of Interturn Contact Stress on the Quench Behavior of Insulated YBCO Pancake Magnets: A Study Based on Numerical Simulations

Insulated high temperature superconducting (HTS) magnets are still widely used in low field applications, such as HTS motors and generators. In this paper, a two-dimensional/three-dimensional (2D/3D) mixed electrothermal model and a homogeneous 3D electrothermal model are employed to study the influence of inter-turn contact stress on the minimum quench energy (MQE), longitudinal and transverse normal zone propagation velocity (NZPV) of insulated YBCO magnets for the temperature range 30 K – 60 K. This temperature range is of interest usually in applications, such as HTS motors, generators and transformers, because it allows the use of low cost cooling system. It is found that MQE increases significantly with increasing inter-turn contact stress. The inter-turn contact stress has small influence on the longitudinal NZPV of insulated YBCO pancake magnet. However, the transverse NZPV rises significantly with the increase of inter-turn contact stress. The transverse NZPV is nearly proportional to the inter-turn contact stress. The 3D homogeneous model gives results that are in agreement with those of 2D/3D mixed model, especially for the temperature range 45 K – 60 K; however, the MQE and transverse NZPV given by the two models are different for the temperature range 30 K – 45 K. That means for insulated HTS YBCO pancake magnets operating at the temperature range 45 K – 60 K, the 3D homogeneous electro-thermal model can be a good replacement of the 2D/3D mixed model to study the quench behavior, which can achieve better computational efficiency without significant loss of accuracy.

Study of Electromagnetic and Thermal Transients in a High-temperature Superconducting Transformer during a Short Circuit

Today, high-temperature superconducting (HTS) current limiters and transformers allow to limit the surge short circuit current during failure without negatively affecting on the power grid complex at the normal operation mode. However, the transition of a superconductor to a resistive state at the moment of current limitation can cause significant heat generation, which can destroy the transformer windings. The research goal is to provide optimal technical characteristics of the HTS transformer to achieve effective short circuit current limitation and prevent thermal breakdown of its windings. To achieve this goal, a mathematical model of a HTS transformer was developed. The presented method considers the material type and geometry of the superconducting tape, the critical parameters of the superconductor (current and temperature), the parameters of the cryogenic liquid, dependence of the resistance and heat capacity of the HTS tape layers on temperature. The simulation model was created in the Matlab/Simulink software. The most important result is the possibility of obtaining optimal electrical and thermal parameters of the HTS transformer windings during the short circuit current limitation, as well as ensuring the thermal stability of the superconducting tape at the quench moment. The obtained results are significant in the design and operation of HTS transformers. For efficient and safe operation in the current-limiting mode, it is necessary to take into account heat generation on the transformer windings. It is important for the superconductor returning to the superconducting state without causing significant overheating of the windings.

Methodology for determining the parameters of high-temperature superconducting power transformers with current limiting function

<span lang="EN-US">This paper substantiates a new adaptive method for determining parameters of high-temperature superconducting power transformers with current limiting function. The main focus is the design of current-limiting superconducting windings in the light of new restrictions on current density, magnetic induction, critical current and critical temperature. The presented method considers the nature of alternating current (AC) losses in a superconductor under nominal operating conditions, features of the dielectric medium (liquid nitrogen), as well as the reduced values of the short-circuit voltage (0.5 to 1.5%). The main design features of high-temperature superconducting (HTS) transformers are specified, and a prototype of a three-phase HTS transformer of 63 kVA with a short-circuit current limiting function is developed. It is shown that HTS units have some advantages over conventional transformers: a 90 to 95% active losses reduction, short-circuit current limitation function, explosion and fire safety, a 60% reduction in weight and size, and increased efficiency (up to 99.8%). Experimental studies confirm that the short-circuit current limitation function is safe and efficient. It is demonstrated that during the short-circuit current limitation, significant heat flows occur on the windings, which should not exceed the critical value above which the superconductor could not return to the superconducting state by itself.</span>

Role of Insulation Materials and Cryogenic Coolants on Fault Performance of MW-Scale Fault-Tolerant Current-Limiting Superconducting Transformers

Fault-tolerant current-limiting (FTCL) high-temperature superconducting (HTS) transformers are promising components for playing a role in renewable energy integrated modern power systems. In this article, the impact of different insulation materials and cryogenic coolant fluids on the electro-thermal characteristic of an MW-scale FTCL-HTS transformer is investigated. For this purpose, an equivalent circuit model (ECM) is established to characterize the temperature, recovery time, and fault tolerability of FTCL-HTS transformer under different conditions. The proposed ECM is firstly validated using experimental results of a typical HTS transformer. After that the model is developed for a 50 MVA, 132 kV/13.8 kV HTS transformer. In order to add fault tolerance capability to the 50 MVA HTS transformer, three strategies were considered in this article. In the first strategy, the effect of three insulation materials (including Kapton tapes, Nomex papers, and Acrylated Urethane solid insulation) covering HTS tapes was investigated on fault performance. The second strategy was changing cryogenic fluid to liquid hydrogen instead of liquid nitrogen. As for the last strategy, the impacts of thickness and material properties of stabilizer in HTS tape were investigated on the maximum temperature and recovery time of HTS transformer. Results show that by using these strategies, the maximum temperature of HTS tape of transformer winding under a short circuit fault was reduced.

Impact of Perlator on the cooling liquid flow and hottest point temperature of superconducting windings in HTS transformer

The generated heat by the superconducting windings and the other parts such as current leads in transformer increases the hottest point temperature (HPT) and causes the high temperature superconducting (HTS) windings to quench. Due to the properties of superconducting windings, reducing the HPT is of critical importance for the stable operation of the HTS transformer. The cooling system of HTS transformers, not only provides the cryogenic temperature for the proper operation of the superconductors but also is responsible for dissipating the generated heat by the windings. In this paper, the effect of the angle of inlet pipes in cooling system was investigated. This was a simple and effective solution which increases the heat transfer in liquid nitrogen. It was shown that inlet angle has a significant effect on the flow turbulence and the windings temperature. The Perlator is used as a lattice sheet which is installed inside the inlet valve and increases the turbulence of inlet flow of liquid nitrogen to increase heat transfer and reduce HPT. The thermal analysis is obtained by finite element method using ANSYS Fluent software. The influence of changing the inlet pipe angle and different structure of Perlator on heat transfer was investigated.

Impact of the radius of double pancake windings on the electromagnetic behavior of high temperature superconducting transformer

PurposeIt has constantly been important to investigate the distribution of magnetic fields in high temperature superconducting (HTS) transformers because the high magnetic field applied to the HTS tapes reduces the critical current and increases the ac losses. The purpose of this study is investigation of the impact of the radius of double pancake windings on the electromagnetic behavior of HTS transformer. In this paper, by changing the radius of the windings in a step-by-step manner in two modes, the electromagnetic behaviors in double pancakes (DPs) of a single-phase HTS transformer have been investigated.Design/methodology/approachIn this paper a 15.4 kVA single-phase HTS transformer has been designed and simulated using the finite element method, using COMSOL multiphysics software. The effect of changing the radius of the low-voltage (LV) and high-voltage (HV) windings on the electromagnetic parameters such as distribution of circulating currents and magnetic field in the LV DP windings has been investigated.FindingsAccording to the results, by increasing the radius of the LV winding, the electromagnetic behavior of the highest and lowest DPs becomes highly undesirable, while in other DPs, it becomes desirable. The same thing happens by increasing the radius of the LV and HV windings, but with much less intensity. Therefore, according to Ce, the most optimal case is when the two windings (HV and LV) are close to each other and to the core, and if the radius needs to be increased, it is better to increase the radius of both windings.Originality/valueFor the first time, the impact of the radius of DP windings on the electromagnetic behavior of HTS transformer has been investigated.

Электротепловые переходные процессы в сети с высокотемпературным сверхпроводящим трансформатором с функцией токоограничения

The results of a study of thermal and electromagnetic transients in an electric power system containing a high-temperature superconducting (HTS) transformer with a current limiting function are presented. For carrying out the research work, an experimental HTS transformer sample with windings made of Y1Bа2Cu3O7 superconductor and with liquid nitrogen at an operating temperature of 77 K serving as dielectric medium was developed. It has been demonstrated, taking the experimental sample as an example, that transformers with HTS windings can be used to limit short-circuit currents in electrical systems. By using the developed mathematical model of a three-phase single-machine network containing an HTS transformer, it became possible to perform an in-depth analysis of electromagnetic transients in the current limiting mode and to study the thermal and electrodynamic effects produced by the short-circuit current in varying the short-circuit fault kind (single-phase, two-phase and three-phase short-circuit faults) and the type of connected load (active, active-inductive, and active-capacitive) on the current limiting degree. It has been determined that significant heat flows arise at the current limitation moment, which also should be limited. A positive effect of HTS transformers on the electric power system operating modes is shown. The obtained results of theoretical and experimental studies of HTS transformers prove a high efficiency of their current limiting function.

Application of Flux Diverters in High Temperature Superconducting Transformer Windings for AC Loss Reduction

Flux diverters (FDs) are used in High Temperature Superconducting (HTS) transformers for AC loss reduction and flux optimization. In this paper, a 2D axial symmetric superconducting winding model is proposed and two designs of flux diverters are applied to the windings. A homogenization approach is used to analyze the windings with large turn numbers. The key parameters including the number, width, height and the spatial positions of the FDs are adjusted for AC loss and magnetic flux analysis. The means of obtaining optimum designs of the FDs is provided and can be used to develop new winding designs with FDs, which contributes to better electromagnetic performance and higher efficiency of HTS transformers.

Precise appraisement of the harmonic loads impact on hysteresis losses in a 3-phase HTS transformer

Nowadays, the application of non-linear loads such as, rectifiers, electronic phase control, PWM drives, gives rise to a vastly increased level of harmonics in the power network. As such, the transport currents of transformers are non-sinusoidal alternating currents. On the other hand, AC losses are one of the most critical characteristics for the design and operation of high-temperature superconducting (HTS) transformers. The amount of hysteresis losses is a key parameter for the design of HTS windings affecting equipment, cooling cost, efficiency, and transformer economic feasibility. The computation of AC losses by traditional techniques, is suitable for the sinusoidal current condition. While, non-linear loads brings about different number of peaks and amplitudes in the current waveform passing through the HTS transformer windings. So that, in addition to the major hysteresis loop, a number of minor hysteresis loops are also formed changing the hysterical nature of the AC losses.Hence, this paper proposes an accurate, yet efficient analytical algorithm, for calculating hysteresis losses in a three-phase HTS transformer's windings in the presence of the harmonic loads accounting for the AC losses estimation. This is accomplished by performing a numerical Finite Element Method to verify the analytical results. The analysis of the hysteresis losses in an exemplary 315 kVA three-phase HTS distribution transformer for different harmonic loads, is another object of this paper.

Inductance Calculation of HTS Transformers with Multi-segment Windings Considering Insulation Constraints

High-temperature superconducting (HTS) transformers with multi-segment windings have been proposed in earlier literatures for the hysteresis loss reduction and short circuit electromagnetic force mitigation. The optimum distributive ratios for these multi-segment windings have been determined in earlier literatures. Asymmetrical seven segment winding with its optimum distributive ratio (k = 1/12) results in minimization of short circuit electromagnetic forces under normal tap and any adjusted tap. However, insulation design and leakage inductance calculation for the multi-segment winding of an HTS transformer have not been addressed in past literatures. Insulation design is one of the most crucial aspects in the transformer design, especially for high voltage power transformers. The analysis and optimization of the electric field distribution over the winding’s segments insulation in these HTS transformers provide the required dimensions for precise determination of the leakage inductances. In this paper, for the first time, winding insulation design and leakage inductance calculation of a 200 MVA, 230/20 kV HTS transformer with asymmetrical seven segment winding (and k of 1/12), are carried out. The analytical method besides a two-dimensional (2D) finite element method (FEM), through COMSOL Multiphysics software, is utilized for insulation design and calculation of leakage inductances of this transformer.

Characterizations of a Novel Structure of Fault-Tolerant HTS Cable

This paper presents a type of high temperature superconducting (HTS) cable, which is fabricated by winding the second generation (2G) HTS tapes on round former consisting of stranded stainless steel (SS) wires. It has the advantages of strong flexibility, high mechanical strength, and high engineering current density. With taking the resistivity of SS much larger than that of copper or aluminum, the cable has higher resistance and strength than the conventional HTS cable with former made from copper strands at condition of fault accident. After the configuration of the cable is designed in detail, the quenching resistance and fault current limiting are numerically analyzed. We fabricate a model cable and carry out the experiment of fault current. The experimental results show that the quenching resistance of this cable is much higher than the resistance of conventional underground XLPE cable and overhead conductor, which indicates that the proposed cable has a strong ability of fault current limiting and can be applicable in HTS devices with functions of fault current limiting such HTS cable, transformer, motor/generator, and current limiter.

Influence of field-dependent critical current on harmonic AC loss analysis in HTS coils for superconducting transformers supplying non-linear loads

There are two main obstacles in front of the development of high temperature superconducting (HTS) technology for electric power network applications; tape price and cooling cost. In order to reduce cooling cost, it is vital to evaluate AC transport current loss of the tapes precisely and then reduce it by some design innovative approaches. In addition, AC transport current loss in HTS material is a critical design variable for large-scale power network applications such as HTS transformers, superconducting fault current limiters, and power cables, since they are continuously carrying the network/load current during their operating life. In existing power networks, harmonic production sources are commonly used and thus, currents are distorted. Therefore, the effect of nonsinusoidal current on the critical apparatus in the network such as transformer must be studied. In this paper, AC transport current loss of a single-turn 2G YBCO HTS coil was modeled and numerically calculated under nonsinusoidal transport current using finite element method. Furthermore, influence of dependency of critical current density to magnetic field on the AC transport current loss of HTS coil when carries distorted currents was considered. It was observed that nonsinusoidal current causes excessive losses in HTS coil. On the other hand, a case study on an HTS transformer supplying non-linear load was considered to study the loss increment as well as heat load change. It was observed that current harmonics increases the AC loss, and heat load of transformer and decreases the efficiency, consequently.