Transformer Protection

A generalized macroscopic representation of electrical steels used in transformer manufacturing industry is developed. The proposed representation is specifically formulated for integration in the finite element method. Usage of the specific technique enables the accurate evaluation of electromagnetic field distribution of transformer cores under heavily saturated conditions. Advantages over conventional techniques include numerical stability, numerical accuracy, and reduction of iterations of the Newton–Raphson method.

In this paper a robust three-dimensional (3D) finite element (FE) anisotropy model is introduced based on a particular scalar potential formulation. The specific 3D FE model is suitable for the accurate evaluation of the peak flux density distribution and no load loss of one-phase and three-phase wound core distribution transformers. The accuracy of the proposed 3D FE anisotropy model is validated by local flux density and no load loss measurements.

This paper presents an analysis of the bibliography on transformers covering the period from 1990 to 2000. It contains all the transformer subjects: a) Transformer design, b) Transformer protection, c) Transformer connections, d) Transformer diagnostics, e) Transformer failures, f) Transient analysis of transformers (overvoltages, overcurrents), g) Modeling and analysis of transformer using FEM (thermal modeling, losses modeling, insulation modeling, windings modeling). Several international journals were investigated including the following: Advances in Electrical and Computer Engineering, Canadian Journal of Electrical and Computer Engineering, COMPEL (The International Journal for Computation and Mathematics in Electrical and Electronic Engineering), Electrical Engineering, Electric Power Components and Systems, Electric Power Systems Research, European Transactions on Electrical Power, IEEE Transactions on Magnetics, IEEE Transactions on Power Delivery, International Journal of ...

This paper presents an analysis of the bibliography on transformers covering the period from 1990 to 2000. It contains all the transformer subjects: a) Transformer design, b) Transformer protection, c) Transformer connections, d) Transformer diagnostics, e) Transformer failures, f) Transient analysis of transformers (overvoltages, overcurrents), g) Modeling and analysis of transformer using FEM (thermal modeling, losses modeling, insulation modeling, windings mod-eling). Several international journals were investigated including the following: Systems, and IET Generation Transmission & Distribution. Due to the high number of publication in journals, we are not considering publications of conferences and symposia. A total of 700 publications are analyzed in this paper. The research presented in this paper is important because it contains and analyzes the best research papers on transformers coming from many countries all over the world and published in top rated scientific electrical engineering journals.

Even though, the transformer is the most efficient of electrical machines, with efficiencies typically in the high 90s, it is possible to reduce transformer costs and losses by using composite magnetic cores. This paper presents a new composite magnetic core that can be used effectively for manufacturing single–phase and three–phase wound core distribution transformers. The new composite wound core concept is based on experimental evidence concerning the flux density non–uniformity of conventional single–phase and three–phase magnetic wound cores and the losses and magnetization properties of conventional and high permeability Si–Fe grain–oriented steels. A systematic experimental losses and flux distribution analysis of single phase and three–phase magnetic wound cores is undertaken as well as finite element (FE) analysis considering the bulk anisotropic characteristics of laminated wound cores.

Power Transformer Stability Test Report.
The objective of this test is to verify the correct CT arrangement, cable connection and protection relay setting for the following protections: 87T, 64REF, 50G SBEF for TR-321-C-02A, feeding from SB-321-C-01-SP-11A to SB-321-D-02A.

Power transformers are a critical and expensive component of the power system. The comprehensive transformer protection provided by combination of protection elements, with biased differential protection. Differential protection has excellent operation in most fault cases, but in the situations that a single phase to ground fault occurs near the neutral point in solidly grounded transformers, the faulted phase current increases slightly and causes differential protection not to detect the fault. For this reason, restricted earth fault (REF) relay can be used as a complementary of differential protection. REF protection uses the sufficient current that flows in this condition through the neutral conductor, for relay operation under phase to ground fault. The REF protection scheme is composed of two parts: a high impedance REF unit and a low impedance REF unit. The high impedance REF relay is defined as a relay circuit whose voltage setting is not less than its calculated maximum terminal voltage which corresponds to the maximum through fault current under transient condition. This REF protection scheme provides a high speed and absolutely stable means of protection when ground fault occurs near the windings neutral point in a solidly grounded transformer. This paper summarizes the operating principle of a high impedance protection unit, its relay sensitivity, related CT requirements, importance of a reliable protection, and the necessary setting consideration to be incorporated besides a transformer differential relay.

The losses in European Union distribution transformers are estimated at about 33 TW · h/year, whereas reactive power and harmonic losses add a further 5 TW · h/year. The reduction of distribution transformer no-load loss is particularly important as the ratio of no-load to load losses is nearly three. In this paper, the no load operation of wound-core transformers under sinusoidal and distorted supply-voltage conditions is investigated. For that purpose, a 2-D nonlinear transient finite-element analysis taking into account hysteresis has been developed. The hysteresis model is based on a modified Jiles–Atherton representation, and the proposed analysis is compared to experimental data.

Nowadays, transformers are made of conventional magnetic cores which are constructed of a single grainoriented or amorphous, magnetic steel. Even though, the transformer is the most efficient of electrical machines, with efficiencies typically above 90%, it is possible to improve transformer performance by using composite magnetic cores. Patents related to this simple and effective technique can be traced back to 1929. The specific technique can be applied to wound core distribution transformers. By using wound cores constructed with a combination of conventional and high permeability grain-oriented steel the total owing cost (TOC) of the transformer can be reduced effectively. This paper presents a brief review of patents on wound and composite magnetic cores and introduces a generalized technique for the determination of the optimum design variables of a new composite wound core design.

This paper proposes the manufacturing of distribution transformers using a novel type of magnetic core which is called composite wound core. A composite wound core is constructed of a combination of conventional and high magnetization grain-oriented steel. The main advantage of transformers assembled of composite wound cores over conventional transformers is the significant reduction of the manufacturing and operating cost. For the analysis of composite wound core transformers, a FE model considering anisotropy and high saturation conditions, and an advanced 3D hybrid FE-BE model have been developed.

Just a Case study regarding restricted earth fault (REF) a transformer protection usually used for protecting the star windings of transformer.

The transformer was frequently tripped by REF when the load increased or if there any short circuit outside the zone of transformer, specifically at load side.

This paper illustrates the case in details along with solutions.

The case study was monitored by me. And I hope this case will help those engineers who may face same issue.

This paper shows experimental results of longitudinal flux density and its harmonics at the limb, the yoke and the corner of a three phase, Si-Fe, five-legged wound transformer core. Results show that the flux density is nonuniform in the cores and there is high level of third harmonic component. Moreover, the lower flux values in the outer cores have been assessed and the simultaneous time variation of the flux in both outer cores has been demonstrated by measurements while the peak magnetic flux density values have been compared to FEM analysis. These findings enable a better understanding of the magnetic behavior of five legged wound transformer cores and their consideration is expected to achieve respective improvement of the design with respect to core losses and magnetostriction noise.

The importance of distribution transformer noload
loss on the operation of modern electrical grids is often
underestimated. Internationally, distribution transformer no load loss constitutes nearly 25% of the transmission and
distribution losses of electrical grids. The losses in European
Union distribution transformers are estimated at about 33
TWh/year whereas, reactive power and harmonic losses add a
further 5 TWh/year. In the Greek electrical grid the no-load
losses of 140,000 distribution transformers are estimated at
about 490 GWh/year. This paper has two goals: The first one is
to illustrate the significance of distribution transformer noload
loss in periods of high electric energy cost. The second
goal is the presentation of a novel numerical methodology for
wound core transformers no load loss analysis, enabling to
determine the economically and technically optimum transformer for every use, which has been developed in the
frame of the respective research project.

In recent years, researchers have proposed transformerless solutions for connecting renewable-energy power plants to the grid. Apart from lack of efficiency and increased cost and weight of the transformer, one of the reasons is the dc input current that causes transformer saturation. The purpose of this paper is the development of a finite-element computational tool that is going to aid transformer manufacturers in designing distribution transformers specifically for the renewable-energy market. It is based on a generalized macroscopic representation of electrical steels used in the transformer manufacturing industry that enables the accurate evaluation of electromagnetic field distribution of transformer cores under heavily saturated conditions. Its advantages over conventional formulations include numerical stability, numerical accuracy, and reduction of iterations of the Newton–Raphson method. An experimental verification of the proposed method is carried out.

Distribution transformer operation is sensitive to
the distortion of the supply voltage waveform. According to
Strategies for development and diffusion of Energy Efficient
Distribution Transformers (SEEDT), reactive power and
harmonic losses add a further 5 TWh/year to the losses of
European Union (EU-27) distribution transformers. In the
present paper a systematic experimental procedure is
developed in order to evaluate the effect of voltage harmonics on distribution transformer operation. Also, a theoretical analysis based on the hysteresis design tool of Matlab and a finite element code integrating the hysteresis phenomena is carried out.

Distribution transformers losses are equal to almost 2% of the electricity generated worldwide and only in the European Union are estimated at about 33 TWh/year. Approximately 75% of the total losses are due to core losses as a result of the loading characteristics of distribution transformers. Design of the joints of magnetic cores has a profound impact on core losses and transformer efficiency. The paper introduces a finite element methodology for the analysis of transformer joints. The proposed technique consists in the application of certain boundary conditions for the excitation of the joints. The main advantages of the pseudo-source technique include minimization of the computational cost and ease of implementation. The technique is combined with a number of FE formulations and a vector hysteresis model. Two-dimensional as well as 3-D FE analysis is studied. Longitudinal and normal flux measurements were carried out for the validation of the proposed technique.

This paper presents an analysis of the bibliography on transformers covering the period from 1990 to 2000. It contains all the transformer subjects: a) Transformer design, b) Transformer protection, c) Transformer connections, d) Transformer diagnostics, e) Transformer failures, f) Transient analysis of transformers (overvoltages, overcurrents), g) Modeling and analysis of transformer using FEM (thermal modeling, losses modeling, insulation modeling, windings modeling). Several international journals were investigated including the following: Advances in Electrical and Computer Engineering, Canadian Journal of Electrical and Computer Engineering, COMPEL (The International Journal for Computation and Mathematics in Electrical and Electronic Engineering), Electrical Engineering, Electric Power Components and Systems, Electric Power Systems Research, European Transactions on Electrical Power, IEEE Transactions on Magnetics, IEEE Transactions on Power Delivery, International Journal of ...

The total owning cost of a wound core distribution transformer can be reduced by constructing the wound core with a combination of conventional and high permeability grain-oriented magnetic steel. The proposed technique can be applied after the transformer’s design optimisation or it can be integrated in the design optimisation procedure. In both cases a significant reduction of the total owning cost is achieved.

Recently, an increasing concern has been raised about turn-to-turn faults (TTFs) in power transformers, because these faults can lead to severe transformer insulation failure and consequently, its outage. Generally, it is impossible to experimentally analyze the transformer behavior under such faults, since the implementation of those experiments may be substantially destructive. Therefore, computer-aided models should be developed to investigate the performance of transformer protective relays under turn-to-turn faults. So far, existing transformer models are mainly formulated to implement in the EMTP-based softwares. However, most of power system protection engineers and researchers develop their protective algorithms in other softwares, such as MATLAB. In this paper, two simplified and accurate equivalent circuits of single phase, two winding transformer (under TTF condition) are proposed. These equivalent circuits can be simply simulated in any desired software. This accurate model closely translates the state-of-art existing TTF models into a comprehensive equivalent circuit. However, precise estimation of its parameters requires a supplementary finite element analysis. This paper employs some justifiable simplifications to the accurate model and results in a simple TTF equivalent circuit. The parameters of this model can be obtained by employing just the transformer routine test datasheet. The simple equivalent circuit is simulated under several turn-to-turn fault conditions and its accuracy has been validated.

This paper introduces a novel technique for iron loss minimization of wound core transformers. The proposed technique involves the evaluation of appropriate design variables of wound cores constructed by a combination of standard and high magnetization grade steel. The evaluation of the optimum design variables of the multiple grade lamination wound core is achieved by combining a permeability tensor finite-element model and simulated annealing with restarts.

This paper proposes a three-phase five legged wound transformer core constructed of two high permeability Si-Fe wound cores and two conventional Si-Fe wound cores. The two large internal wound cores are manufactured of high permeability, grain-oriented electrical steel. The two small outer wound cores are manufactured of conventional, grain-oriented electrical steel. The specific arrangement is based on experimental evidence concerning the peak flux density non-uniformity of the typical three-phase five legged wound transformer core, constructed of the high magnetization grain-oriented steel. Since the peak flux density of the two outer cores is lower than the two internal cores, low cost, low permeability, conventional grain-oriented electrical steel can \ be used for the outer cores. Losses, excitation currents, flux waveforms, and their harmonics contents are presented in this paper. A comparison of the mixed three-phase transformer core and the typical one is also carried out.

Purpose – This paper aims to present an accurate representation of laminated wound cores with a low computational cost using 2D and 3D finite element (FE) method.
Design/methodology/approach – The authors developed an anisotropy model in order to model laminated wound cores. The anisotropy model was integrated to the 2D and 3D FE method. A comparison between 2D and 3D FE techniques was carried out. FE techniques were validated by experimental analysis.
Findings – In the case of no-load operation of wound core transformers both 2D and 3D FE techniques yield the same results. Computed and experimental local flux density distribution and no-load loss agree within 2 per cent to 6 per cent.
Originality/value – The originality of the paper consists in the development of an anisotropy model specifically formulated for laminated wound cores, and in the effective representation of electrical steels using a composite single-valued function. By using the aforementioned techniques, the FE computational cost is minimised and the 3D FE analysis of wound cores is rendered practical.

Transformer no load loss optimization is crucial for transformer manufacturers as well as for electric utilities, since it results to significant economic benefits. In this article, the three-dimensional finite element analysis is applied to power transformers in order to predict and minimize the iron loss. The proposed model is based on a particular reduced scalar potential formulation, necessitating no prior source field calculation, and employs detailed modeling of the core geometry and material, considering for manufacturing core formation process effects by convenient hysteresis phenomenological models. Comparisons between this method and test values for a number of commercial transformers, prove its validity and accuracy, rendering it a reliable tool for customized design of an industrial plant

The paper presents a three-dimensional finite element (3D FEM) anisotropy model, based on a particular scalar potential formulation, for the no load loss evaluation of wound core shell type distribution transformers. The specific 3D FEM anisotropy model is combined with a hybrid finite element-boundary element (FEM-BE) model, used for the calculation of the transformer’s short circuit impedance, and various optimization algorithms in order to minimize the total owing cost (TOC) of a distribution transformer.

The importance of distribution transformer no-load loss on the operation of modern electrical grids is often underestimated. Internationally, distribution transformer no-load loss constitutes nearly 25% of the transmission and distribution losses of electrical grids. The losses in European Union distribution transformers are estimated at about 33 TWh/year whereas, reactive power and harmonic losses add a further 5 TWh/year. In the Greek electrical grid the no-load losses of 140,000 distribution transformers are estimated at about 490 GWh/year. This paper has two goals the first one is to illustrate the significance of distribution transformer no-load loss in periods of high electric energy cost and the second goal is the presentation of a novel numerical methodology for wound core transformers no-load loss analysis, enabling to determine the economically and technically optimum transformer for every use.

Detailed analysis, simulation and hardware results of grid connected inverter with maximum power point tracker and power factor control in Malaysian climate are presented. A six-switch topology inverter with symmetrical Pulse Width Modulation (PWM) switching technique is used. A low pass filter is incorporated in the circuit to filter out unwanted harmonics and produce a sinusoidal AC current. Low total harmonic current distortion at the inverter output can be achieved. The three-phase PWM switching pattern was developed using Xilinx FPGA. The developed system is also capable of adjusting the power factor to unity. A 3kVA power transformer with a 1:2 ratio is constructed to provide galvanic isolation for better circuit performance and circuit protection. Hardware results for proposed solar photovoltaic inverter configuration interconnected to the grid are also presented. From the experimental results it is confirmed that the harmonic distortion of the inverter output waveform is within the limits laid down by the utility companies.

The iron loss and manufacturing cost of a strip-wound core can be reduced by constructing the wound core with  combination of average and high magnetization grade steel. The present paper introduces a two-dimensional finite element (2D FEM) package for the flux density distribution and iron loss computation of two grade lamination wound cores. The specific 2D FEM package is combined with deterministic and stochastic optimization algorithms in order to compute specific design variables of a two grade lamination wound core, that ensure the minimization of its iron loss and manufacturing cost.

Even though, the flux distribution at joints of stacked type transformer cores has been  investigated thoroughly many issues remain unclear in the case of wound transformer cores. The  paper addresses this lack of information by longitudinal and normal flux measurements at step-lap  joints of Si-Fe wound cores. Flux measurements are verified by an original finite element analysis  where the necessary excitation is performed by means of a pseudo-source. The advantage of the  proposed technique is the accurate estimation of the flux distribution at step-lap joints, with a two  dimensional model of simple geometry and low computational cost, by using any commercial finite  element code.

Detailed analysis, simulation and hardware results of grid connected inverter with maximum power point tracker and power factor control in Malaysian climate are presented. A six-switch topology inverter with symmetrical Pulse Width Modulation (PWM) switching technique is used. A low pass filter is incorporated in the circuit to filter out unwanted harmonics and produce a sinusoidal AC current. Low total harmonic current distortion at the inverter output can be achieved. The three-phase PWM switching pattern was developed using Xilinx FPGA. The developed system is also capable of adjusting the power factor to unity. A 3 kVA power transformer with a 1:2 ratio is constructed to provide galvanic isolation for better circuit performance and circuit protection. Hardware results for proposed solar photovoltaic inverter configuration interconnected to the grid are also presented. From the experimental results it is confirmed that the harmonic distortion of the inverter output waveform is within the limits laid down by the utility companies.

This paper shows experimental results of longitudinal flux density and its harmonics at the limb, the yoke and the corner as well as
normal flux in the step lap joint of a single phase, Si–Fe, wound transformer core. Results show that the flux density as well as the
harmonics content is higher in the inner (window) side of the core and reduces gradually towards the outer side. Variations of flux density distribution between the limb and the corner or the yoke of the core were observed. A full record of normal flux around the step lap region of the model core was also obtained. Longitudinal and normal flux findings will enable the development of more accurate numerical models that describe the magnetic behavior of magnetic cores.

Iron losses of grain-oriented electrical steels, is sensitive to the distortion of the supply voltage waveform of the excitation winding. As a result, magnetic cores of electrical machines and transformers manufactured of grain-oriented electrical steels present significant increase of iron losses when working under distorted supply voltage waveform. In the present paper, an experimental apparatus is developed in order to evaluate the effect of distorted supply voltage
waveform on iron losses of grain-oriented electrical steels. Also, a  theoretical analysis based on the hysteresis design tool of Matlab and the finite element method considering hysteresis is carried out.

The aim of the transformer design optimization is to define the dimensions of all the parts of the transformer, based on the given specification, using available materials economically in order to achieve lower cost, lower weight, reduced size, and better operating performance. In this paper, a hybrid artificial intelligence/numerical technique is proposed for the selection of winding material in power transformers. The technique uses decision trees and artificial neural networks for winding material classification, along with finite-element/ boundary element modeling of the transformer for the calculation of the performance characteristics of each considered design. The efficiency and accuracy provided by the hybrid numerical model render it particularly suitable for use with optimization algorithms. The accuracy of this method is 96% (classification success rate for the winding material on an unknown test set), which makes it very efficient for industrial use.

The paper introduces a finite element methodology for the magnetic field analysis of transformer joints. The proposed technique consists in the application of certain boundary conditions for the excitation of the joints region. The main advantages of the pseudo-source technique include minimization of the computational cost and ease of implementation. Longitudinal and normal flux measurements were carried out for the validation of the proposed technique.