Transformerless Inverter

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.

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.

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.

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 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 deals with various transformerless PV Inverter topologies with MPPT and low leakage current. Transformerless inverters are widely used in grid-tied photovoltaic (PV) generation systems, due to the benefits of achieving high efficiency and low cost. For PV panels, due to parasitic capacitance there will be some leakage current. So there is a strong trend in the photovoltaic inverter technology to use transformerless topologies in order to acquire higher efficiencies combining with very low ground leakage current. The major task of this work is the investigation and verification of transformerless topologies and control strategies to minimize the leakage current of PV inverter topologies in order to comply with the standard requirements and make them safe for human interaction. Simulations were done using Matlab/Simulink. High conversion efficiency and low leakage current are demonstrated.

The photovoltaic inverter is one of the most important elements of the solar photovoltaic systems. In the last years the transformerless grid connected inverters are preferred for photovoltaic applications. The current paper aims to present an analysis of a new topology of this type of inverters, as well as to prove its operating ability and control of the process of injecting of solar power from photovoltaic panels into utility grid. The topology is based on theoretical model which is not investigated in details till now. This topology is symmetric with flying inductor and can be implemented in inverters that are able to operate in all modes – buck, buck-boost and boost in both half-waves, as well as in some combinations of them. Also it has a possibility to change the behaviour in dependence of the grid voltage.

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 aim of the current paper is to present simulations models and results for a new symmetric grid tied transformerless inverter topology with flying inductor. The topology is the plant of the automatic system, and its control is very important for the quality of the produced electric power. The simulations will be used also to get the total harmonics distortions. Simple PI regulator will be used to control the process. The presented simulations results refer to the quality of the output current sinusoid at different working modes. The study shows that the above mentioned topology is suitable for direct connecting to the grid if the photovoltaic voltage is higher or smaller than the amplitude of the grid voltage. The analysis of the results indicates that the advantages are smooth change of the operating modes, lighter control of the circuit because of his symmetry in both half cycles, as well as better efficiency and smaller disturbances because of the three level work. The change of working modes allows the operation with a smaller duty cycle, which is favorable for a current mode control inverters.

Transformer-less inverters are necessary parts for grid-connected renewable energy resources. Owing to its cost effectiveness, downsize and less weight, great attention has been paid to these inverters development. With these aforementioned advantages, these inverters have limitations like the flow of leakage current through photovoltaic arrays, high total harmonic distortion (THD) at inverter's output and DC current injection to the grid. This study presents coupled inductor-based single-phase transformer-less semi-Z-source inverter topology to lessen those limitations. Since the DC input and AC output voltage share a common ground, the presented inverter system is categorised under doubly grounded topologies. For the purpose of handling, the non-linearity of the voltage gain of semi-Z-source inverter, a non-linear sinusoidal pulse-width modulation technique has been employed. The prototype of the suggested inverter has been constructed. The performance and compatibility of modulation technique are verified under different loading conditions. The feasibility of the configuration is ensured based on the mitigated common-mode leakage current, the substantially lower THD as well as DC current injected to the grid. Moreover, the presence of coupled inductor significantly contributes in reducing input current ripple, installation area of the inverter and enhancing the efficiency. Finally, this topology exhibits appreciable performance to operate synchronously and transfer power to the grid.

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http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=7014454&tag=1 
http://digital-library.theiet.org/content/journals/10.1049/iet-pel.2013.0486

The advent of multi-level Statcom technology with high power density provides the platform for a cost
effective and practical relocatable unit. The RXPE SVG range of Medium Voltage Statcoms is just that
where it is possible to have a transformerless direct connection of a Statcom at voltage levels from 3.3kV up to 35kV. These can be easily containerised into standard units so that relocation is straight-forward using conventional transport options. Other capacities can be optimised using internal dry-type transformers to provide the most appropriate package for the specific requirement.

The applications of this technology for capacity enhancement serves both developed networks such as the UK where there is the need to maximise capacity both due to regulatory requirements as well as way leave restrictions, as well as developing networks where there is the need to stabilise voltage on long feeders with fluctuating loads with poor power-factor and limited capital spend capability.

One particularly case-study for Southern Africa will be shown, where an existing 110kV/33kV substation has reached a maximum transfer capability. Additional reactive power will provide much needed MW transfer capability, however load growth is such that this benefit will only last for 5 years, by which time the second transmission line will have been constructed. Once the transmission line is constructed, then the Statcom will become redundant at that location. The use of a relocatable statcom therefore is of great technical and commercial benefit since it can be rapidly deployed in a temporary configuration until the major reinforcement arrives, then it can be placed on the back of a truck and transported to another site where it can be redeployed in a similar manner.

Reactive power in the right location can provide significant active power benefit if it is coordinated as part of the overall voltage control strategy. The Statcom can also have additional benefits including voltage dip reduction, flicker mitigation and active harmonic filtering.

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.

This paper presents the simulation performance of control of a standalone photovoltaic (PV) maximum power point tracking (MPPT) system with transformerless inverter. The tested control strategy for PV module is based on Perturb and Observe (P&O) MPPT algorithm, and current and voltage are employed to enhance controllability of the control scheme applied into a dc-dc boost type power converter. A single-phase HERIC (Highly Efficient and Reliable Inverter Concept) transformerless inverter connected as an output to the converter is applied and controlled to feed ac loads.

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.

Recently, reduced common-mode voltage (CMV) pulsewidth modulation (RCMV-PWM) methods have been proposed to reduce the leakage current in three-phase transformerless photovoltaic (PV) systems. However, most of these studies only focus on leakage current elimination and neglect the overall performance of the PV systems on issues such as cost, voltage linearity, dc-link current ripples, and harmonic distortion. In this paper, a three-phase transformerless inverter, adapted from the single-phase H5 topology, is investigated. Since the H5 topology has been conventionally developed for a single-phase system, its adaptation to the three-phase system requires the development of corresponding three-phase modulation techniques. Hence, modulation techniques are proposed based on conventional PWM. The performances of the proposed PWM, in terms of CMV, leakage current, voltage linearity, output current ripples, dc-link current ripples, and harmonic distortion are studied and discussed via simulation and experiment. It is proven that the proposed topology is able reduce the leakage current without sacrificing the overall performance of the system.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6824178&tag=1 
http://umexpert.um.edu.my/file/publication/00003228_115096.pdf 
http://repository.um.edu.my/95094/1/Modulation%20Techniques%20to%20Reduce%20Leakage%20Current%20in%20Three-Phase%20Transformerless%20H7%20Photovoltaic%20Inverter.pdf

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.

A single phase transformerless semi-Z-source inverter topology is presented in this paper in order to incorporate distributed photovoltaic generators to the grid with reduced total harmonic distortion (THD) and DC current injection. These proposed inverter topologies are capable of achieving the sinusoidal output voltage by using only two active switches whereas the traditional inverters require four. A modified form of nonlinear SPWM technique is employed here to overcome the nonlinear voltage gain problem of semi-Z-source inverter for providing desired duty cycle which generates the sinusoidal voltage at the output. This inverter topology shows less DC current injection to the grid. Furthermore, common ground is being shared by both DC input and AC output voltage; hence common mode leakage current gets mitigated. Finally, a prototype rated 50W is constructed and a verification of theoretical outcome with the results of the experimental setup are presented. The results of the experiments satisfy the effectiveness of the aforementioned topology. © 2013 IEEE.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6579281

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.