Low Voltage Winding Research Articles

Design Considerations for an Active–Reactive Induction Motor for Medium-Voltage Applications

The active–reactive induction motor (ARIM) has been introduced in the literature as a low-cost and high-efficiency alternative to the synchronous motor in load commutated inverter (LCI) fed drives. ARIM is a squirrel cage machine with two sets of dissimilar three-phase stator windings. While one winding is rated for high voltage and power, the other winding is rated for a lower voltage and power. The low-voltage winding fed from a voltage source inverter (VSI) supplies only reactive power to the machine such that a leading power factor appears at the high-voltage terminals, enabling LCI-fed operation at this winding. Since the ARIM is fundamentally an induction motor, the conventional induction machine design process is applicable. However, due to the nonconventional arrangement of stator windings and being intended specifically for an LCI-fed topology, this new class of induction machines requires some critical design considerations. It is shown in this article that the leakage inductances play a pivotal role in determining the ratings of the low-voltage windings and the VSI. Therefore, an iterative design procedure to obtain the correct ratings for the flux winding and the low-voltage components is proposed. It is also required to know the correct value of the leakage inductance in order to design the control loops for reliable operation. This article proposes experimental methods to determine these leakage inductances. The proposed methods are validated by designing a 3.3-kV, 75-kW prototype and experimental results are presented.

Design and Simulation of a 120 kVA Single-Phase HTS Transformer

In this paper, a 120 kVA single-phase HTS transformer operating in liquid nitrogen at 77 K with high/low voltages of 6000 V/400 V was designed and simulated in a 2-D axisymmetric model. HTS double pancakes were connected in parallel as the low-voltage (LV) windings and a HTS solenoid winding was used as the high-voltage (HV) windings. Based on finite element method (FEM), the field distribution and AC loss of the HTS transformer, also the current distribution in the double pancakes were simulated under the rated operation. According to simulation results, the winding current and AC loss in the two upper and lower double pancakes are much larger than those in other double pancakes. Then the characteristics of current distribution and AC loss of windings were discussed. The developed simulation method in this paper can provide a reference for the design and manufacture of HTS transformer.

Propagation of Overvoltages in the Form of Impulse, Chopped and Oscillating Waveforms in Transformer Windings—Time and Frequency Domain Approach

This paper describes a comparison of overvoltage propagation in transformer windings. Expanding and evolving electrical networks comprise various classes of transient waveforms, related to network reconfigurations, failure stages and switching phenomena, including new sources based on power electronics devices. In particular, the integration of renewable energy sources—mainly solar and wind—as well as expanding charging and energy storage infrastructure for electric cars in smart cities results in network flexibility manifested by switching phenomena and transients propagation, both impulse and oscillating. Those external transients, having a magnitude below the applied protection level may have still a considerable effect on winding electrical insulation in transformers, mainly due to internal resonance phenomena, which have been the root cause of many transformer failures. Such cases might occur if the frequency content of the incoming waveform matches the resonance zones of the winding frequency characteristic. Due to this coincidence, the measurements were performed both in time and frequency domain, applying various classes of transients, representing impulse, chopped (time to chopping from 1 µs to 50 µs) and oscillating overvoltages. An additional novelty was a superposition of a full lighting impulse with an oscillating component in the form of a modulated wavelet. The comparison of propagation of those waveforms along the winding length as well as a transfer case between high and low voltage windings were analyzed. The presented mapping of overvoltage prone zones along the winding length can contribute to transformer design optimization, development of novel diagnostic methodology, improved protection concepts and the proper design of modern networks.

Вектор Пойнтинга и новая теория трансформатора. Ч. 9. Размещение обмоток на разных стержнях магнитопровода

It is shown that in a transformer with windings placed on different limbs of its magnetic circuit, the load active and reactive power are transmitted only by the winding halves arranged in the transformer aperture. In the areas where the winding external parts (outside the aperture boundaries) are located, the Poynting vector is a purely reactive quantity. In view of this finding, the approach according to which the operation of power transformers is explained by considering a transformer with the high- and low-voltage windings placed on different magnetic core limbs leads to a distorted picture of the physical processes in them. For transformers with windings spaced apart from each other, it is more convenient to use universal 6T equivalent circuits means that, first, they reflect all magnetic fluxes in the iron and in the transformer aperture and, second, that they can be used to analyze both steady-state conditions and transients. The validity of all statements formulated in the article is confirmed by the vector potential and flux function distribution patterns within the transformer boundaries. This approach makes it possible to obtain a clear explanation of the occurrence of superfluxes and antifluxes (in comparison with the no-load flux) in the limbs with the short-circuited state of one of the windings, which play the key role in estimating the electrodynamic withstand capacity of transformers.

Use of Alternative Fluids in Very High-Power Transformers: Experimental and Numerical Thermal Studies

The very high-power transformer lifespan depends mainly on the temperature that cellulose insulation reaches during its operation. Traditionally, its cooling has been carried out using mineral oil as coolant. Nowadays, alternative ester-based liquids are under study as substitutes due to their better environmental and fire-safety properties. This paper compares the cooling capacity of two ester-based fluids with that of a mineral oil using a 3D numerical model of a 100 MVA low voltage winding of a power transformer with axial cooling system and ONAN cooling mode. Heat-run test results with mineral oil have been used to validate this model. As a first approximation, according to the comparison developed, ester-based fluids could replace mineral oils in this type of transformers when they are going to work in a range of powers close to the rated one.

Research on simulation calculation on short-circuit electrodynamics force of power transformer winding

The safety and reliable operation of power grid is directly related to the ability of power transformer to withstand short-circuit, therefore, it is a problem to be solved to improve the ability of large power transformer windings to withstand short-circuit. Taking a three-phase five-limb power transformer as an example, the transient electromagnetic field, short-circuit electrodynamics force of windings and mechanical strength of coils are analyzed in depth. Firstly, the three-dimensional finite element model of the prototype is established, and the magnetic flux density distribution of the three-dimensional transient electromagnetic field of transformer under short-circuit operation and the axial and radial static force magnitude of the winding are calculated by using the field-circuit coupling method, and the distribution law can be obtained. At the same time, the mechanical strength of power transformer winding in its height direction is discussed, and the modal vibration mathematical model of transformer low-voltage winding in Z-axis direction is established. The displacement change and resonance frequency of the winding wire cake in the axial direction caused by short-circuit are calculated, and the short-circuit electrodynamics force of the winding is also checked. The research in this paper provides a theoretical basis for strengthening the design of short-circuit withstanding capacity of windings, and has a certain theoretical and engineering application value.

Thermal modelling of oil-filled power transformer TM - 160/10

The results of modeling the thermal characteristics of the dry and oil-filled power transformer TM-160/10 in idle and short circuit modes are presented. The electrical, geometric and thermal characteristics of the TM-160/10 transformer are determined. Computer modeling is performed in the software package ANSYS 17.1. The 2D distributions of temperature and density of heat flows in the transformer in the longitudinal and transverse sections are determined. It is shown that the use of transformer oil for cooling the transformer significantly reduces the temperatures in the active part. The temperature distribution occupies the range of 67-91 °С. Accordingly, the temperature of the most heated part is 91 °C and also corresponds to the low voltage winding. The dependence of the most heated point of the transformer on the operating mode was studied. A formula is proposed for calculating the maximum temperature of a transformer as a function of power loss.

Numerical study on nanofluids natural convection heat transfer inside power transformer windings

As an innovative approach to improve the cooling efficiency of fluid, the use of nanofluids has attracted increasing attention in engineering applications. In this paper, the impact on the natural convective heat transfer in disc-type transformer windings due to transformer oil-based nanofluids is studied numerically. A low-voltage winding using nanofluid (SiC/oil) as the coolant is modelled two-dimensionally and simulated by computational fluid dynamics and the multi-phase mixture model. The numerical method is validated with the existing results of transformers using conventional oil cooling, and grid-independence study is carried out for the nanofluid flow. Compared with transformer oil cooling, the temperature of nanofluid cooled winding is significantly reduced, while the temperature trend along the flow direction remains essentially the same. Moreover, the effects of nanofluid on the mass flow rate and the coolant temperature have been taken into consideration in the heat transfer analysis.

Eddy Current Loss Effect in Foil Winding of Transformer Based on Magneto-Fluid-Thermal Simulation

The foil winding temperature distribution is crucial to the life of power transformer since local heating can accelerate aging of insulation materials. However, based on the traditional 3-D electromagnetic model and the principle of heat transfer, the non-uniform losses and temperature distribution of foil windings are difficult to estimate because of complex magnetic leakage flux. In this paper, the magneto-fluid-thermal coupling model has been established to calculate the temperature rise and predict the potential hotspots of a simplified dry-type power transformer rated at 2500 kVA. Moreover, the influence of insulation barrier is considered to reshape the air-flow path and to affect the radiative effect between the high-voltage and low-voltage windings. Experimental temperature rises are measured with infrared thermography to verify the simulation results, which achieved reasonable hotspot predictions and provide the theoretical method for transformer designers.

Comparative study of thermal properties of electrical windings impregnated with alternative varnish materials

The thermal design-analysis of devices with electrical windings impregnated using varnish materials is frequently challenging. This is due to difficulties in predicting theoretically the equivalent thermal properties for the composite materials, such as impregnated windings, in a reliable manner. The existing approach makes use of the impregnation ‘goodness’ factor, which allows for imperfections associated with the winding impregnation to be accounted for. This, however, requires experimentally derived data from appropriate material samples or machine assemblies. Considering relatively large range of commercially available varnishes, such data are often limited and are not readily accessible. In this investigation, a set of three alternative varnish materials commonly used in impregnation of low-voltage electrical windings has been evaluated. To provide better understanding of the material impregnating properties, data from multiple impregnations are also provided. The experimental findings have been supplemented with theoretical analysis on the impregnation ‘goodness’ and stator-winding contact thermal resistance for a selected winding construction. To highlight the importance of appropriate input thermal material data, a case study machine design is thermally analysed.

Investigation on the Resonant Oscillations in an 11 kV Distribution Transformer under Standard and Chopped Lightning Impulse Overvoltages with Different Shield Placement Configurations

This paper presents an investigation on the resonant oscillations of an 11 kV layer-type winding transformer under standard and chopped lightning impulse overvoltage conditions based on calculated parameters. The resistances, inductances and capacitances were calculated in order to develop the transformer winding equivalent circuit. The impulse overvoltages were applied to the high voltage (HV) winding and the resonant oscillations were simulated for each of the layers based on different electrostatic shield placement configurations. It is found that the placement of grounded shields between layer 13 and layer 14 results in the highest resonant oscillation and non-linear initial voltage distribution. The oscillation and linear stress distributions are at the lowest for shield placement between the HV and low voltage (LV) windings.

The influence of VFTO on SCSR windings with an equivalent PEEC model

Stepped Controlled Shunt Reactors (SCSRs) have advantages of flexible control and reliable compensation and are likely to become widely used in Ultra-High-Voltage/Extra-High-Voltage (UHV/EHV) transmission systems. Very fast transients that occur in such systems will generate a Very Fast Transient Overvoltage (VFTO) in the SCSR. This may cause damage to the SCSR and lead to system losses. In this paper, the electric field strength and magnetic induction intensity are taken as the basic physical variables when these transients occur. Maxwell’s equations are used to separate the constraint sources, such as the conduction current, magnetization current, and polarization current, from the field source. Then, an all-current Partial Element Equivalent Circuit (PEEC) model for these component elements that considers the surface charge is established. Fast and accurate calculations of the inductance parameters, which involve multiple integrals, are performed using geometrical mean distances and a complex component segmentation method. The equivalent structures and circuit equations of the SCSR windings are presented, and the voltage distribution of the SCSR windings under VFTOs of different frequencies is obtained. The results show that, the voltage waveform in the SCSR windings more rapidly attenuates as the VFTO frequency increases. The peak value of the oscillating voltage in the high-voltage winding of the SCSR is higher than that in the low-voltage winding.

Experimental evaluation of detecting power transformer internal faults using FRA polar plot and texture analysis

Power transformers are prone to short circuit faults and external harsh environmental conditions that may result in various internal winding and core deformations. Frequency response analysis (FRA) is the current technique widely used to detect such kind of faults. The technique relies on scanning each phase in a wide frequency range to detect any discrepancies in the winding FRA signature. Current interpretation process for FRA signatures relies on graphical analysis of the magnitude plot without paying much attention to the phase angle plot. This may result in inconsistent analysis for the FRA signature as so far, there is no standardized and automated technique for FRA signature classification. To overcome this limitation, a new polar plot signature based on the integration of the FRA magnitude and phase angle plots has been proposed in the literature. While this new approach has been validated through simulation analysis, its performance on real transformers has not been assessed yet. This paper is aimed at investigating the practical feasibility of replacing the conventional FRA signature with the polar plot and automate the fault diagnosis process using digital image processing techniques. In this context, various winding faults including short-circuit disks, disk space variation, and axial disk buckling have been implemented on a custom-made, Y/Y three phase, 50 Hz, 400 kVA power transformer. The low voltage winding of the investigated transformer comprises 12-turn helical type while the high voltage winding is made of composite structure comprising 10 disks of interleaved winding on the top, 10 disks winding in the middle and 10 disks of continuous winding in the bottom. Impact of various levels and locations of these faults on the FRA polar plot is investigated through extracting digital image texture analysis features. To ease the classification process, a unique classification metric namely image sum square max-min ratio error is calculated based on the extracted features.

Calculation of Leakage Reactance in Transformers With Constructive Deformations in Low Voltage Foil Windings

Foil-winding transformers with high currents in low voltage winding normally have copper/aluminum bars as leads of the coils. A practical solution for locating bar leads without increasing the size of the active part is to flatten the coil in the leads zone causing a deformation. Abnormalities in the leakage reactance have been identified in this type of transformers. In order to take this effect into account, this paper proposes a correction to the mathematical formulation for calculating the leakage reactance in transformers with deformations in the low voltage winding. Three methods for calculating the leakage reactance have been compared for four real case studies: the proposed formulation, the commonly used formulation, and detailed simulations with the finite element method in three-dimension (FEM3D). The leakage reactance calculated with the proposed method has been found to have an improved accuracy compared with the one calculated using the formulation without correction in all case studies. The main contribution of this paper is that the proposed correction in the formulation makes it possible to increase the accuracy in the calculation of leakage reactance, avoiding the need to resort to detailed simulations with FEM3D.

AC loss simulation in a HTS 3-Phase 1 MVA transformer using H formulation

One of critical issues for HTS transformers is achieving sufficiently low AC loss in the windings. Therefore, accurate prediction of AC loss is critical for the HTS transformer applications. In this work, we present AC loss simulation results employing the H-formulation for a 1 MVA 3-Phase HTS transformer. The high voltage (HV) windings are composed of 24 double pancakes per phase wound with 4 mm – wide YBCO wire. Each double pancake coil has 38 ¼ turns. The low voltage (LV) windings are 20 turn single-layer solenoid windings wound with 15/5 (15 strands of 5 mm width) Roebel cable per phase. The numerical method was first verified by comparing the numerical and experimental AC loss results for two coil assemblies composed of two and six double pancake coils (DPCs). The numerical AC loss calculated for the transformer was compared with the measured AC loss as well as the numerical result obtained using the minimum magnetic energy variation (MMEV) method. The numerical AC loss result in this work and experimental result as well as the numerical result using MMEV at the rated current agree to within 20%. Further simulations were carried out to explore the dependence of the AC loss on the gap between the turns of the LV winding. The minimum AC loss at rated current in the 1 MVA HTS transformer appears when the gap between turns is approximately 2.1 mm turn gap in the LV winding. This is due to the change of relative heights between the HV and LV windings which results in optimal radial magnetic field cancellation. The same numerical method can be applied to calculate AC loss in larger rating HTS transformers.

The New Concept of Using the Superconducting Transformers in Low- and Medium-Voltage Distribution Network

The new 2G HTS tapes with high critical current and high resistivity in the normal state allow building low-cost transformers with high short-circuit strength for the low and medium power distribution substations. The authors consider that currently the best chance of the commercial applications have transformers, which rated currents of their windings do not exceed or slightly exceed the critical current of used superconducting tapes. Such units could be the low- or medium-voltage transformers with a power less than 10 MVA, where the high-voltage winding can be made with a single HTS tape while the low-voltage winding can also be made with a single HTS tape or two-three tapes connected in a parallel. This eliminates the need of a tape transposition or an application of Roebel's cable. Superconducting transformers could stabilize the low or medium power network and increase the connectivity of renewable and dispersed energy sources. This paper presents an analysis of the possibility of using the low- and medium-voltage superconducting transformers. Such units could be, e.g., transformers working with renewable energy sources.

Diagnosis of axial displacement in transformer windings using finite element analysis

Transformer is one of the main equipment in the power system and power quality of the power system mainly depends on the working of the power transformer. Leakage reactance is one of the main parameters of the transformer. The design of the transformer has a significant impact on the leakage reactance of the transformer. The mechanical strength of the transformer mainly depends on the leakage reactance of the transformer. This paper investigates the leakage reactance of the transformer in normal conditions and during the axial displacement of the high voltage and low voltage windings using finite element analysis. Normal results of finite element analysis method are also compared with the experimental short-circuit test. Results show that leakage reactance of the transformer exponentially proportional to the displacement of the windings. It is observed that with the axial displacement of the $ 6\% $ of the winding, leakage reactance increased by $ 10\% $. Results also show that effect of the axial displacement in high voltage and low voltage winding is almost same.

Stator Winding Internal Thermal Monitoring and Analysis Using In Situ FBG Sensing Technology

This paper reports a novel thermal monitoring scheme for on-line internal temperature measurement in low-voltage random wound machine stator windings. The scheme is based on utilizing electrically nonconductive and electromagnetic interference immune fiber optic sensing technology embedded in close proximity of hot spots of interest to create a robust distributed thermal monitoring network within the machine windings. The key design and implementation features of the proposed system are presented and applied on a prototype mains-fed induction motor. The on-line thermal monitoring performance is examined in a number of typical continuous and periodic running duty tests, as defined by the relevant IEC standards for rating and performance of rotating electrical machines. It is shown that the presented scheme has the potential to provide competent on-line measurement of critical machine thermal hot spots that are largely beyond the effective reach of conventional thermal monitoring solutions. Furthermore, the proposed scheme underpins a higher fidelity understanding of the distribution and propagation of winding thermal stress, demonstrated by the experimental analysis reported in the paper.

Studying the Impulse Overloads in the Windings of a Station Service Voltage Transformer

One of the key factors determining the performance of a 110–500-kV station service voltage transformer (SSVT) is a compact insulation design that provides minimum winding sizes. Since SSVTs are new equipment that combine the operational features of a power transformer and the design features of a measuring voltage transformer, it is necessary to study the characteristic transient processes occurring under the impact of lightning-impulse test voltages and lightning and switching overloads in operation in order to ensure proper insulation design. The article presents the results of an experimental investigation of impulse transients in the windings of a full-scale mockup model of an active part of a 20-kV A SSVT with a rated voltage of 110 kV. The influence of grounding the low-voltage winding terminals on the maximum values and duration of impulse stresses on the longitudinal insulation of the high-voltage winding is revealed. It is shown that, unlike in the case of power transformers with disc windings, more significant effects on the insulation are produced when no load is applied on the low-voltage windings.

Transformer Energization From Low-Voltage Side With Limited Generation—Power System Constraints and Protection Considerations—A Case Study

A large existing delta-wye step-down transformer was proposed to be temporarily used as a step-up transformer and energized from the low-voltage wye winding by multiple small diesel generators (DGs) to accelerate commissioning activities by utilizing existing temporary generation. This paper is a case study that discusses the associated power system constraints created by this proposed modified power system configuration. The modified power system configuration resulted in an ungrounded high-voltage (HV) system created by the transformer's delta winding, which required HV system modifications to create a grounded system. Additionally, the existing power system had generation and load flow constraints and had protection and coordination constraints that needed to be maintained to supply the existing loads. Also, the inrush current resulting from the transformer energization needed to be evaluated and compared with the generator's short time current rating. Finally, modifications to the protection and coordination settings resulting from the modified power system configuration and confirmation that the arc flash incident energy levels were acceptable after implementation of modified settings were required.