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To derive an analytical model for a dc‐ac‐dc parallel resonant converter operating in lagging power factor mode based on the steady‐state operation conditions and considering the effects of a high‐frequency transformer.

A range of published works relevant to dc‐ac‐dc converters and their control methods based on pulse‐width‐modulation technique are evaluated and their limitations in output measurement of higher output voltage converters are indicated. The circuit diagram of the converter is described and the general mathematical model of the system is obtained by deriving and combining the mathematical models of the different converter blocks existing in the system. The derived mathematical model is used to study the steady‐state and transient performance of the converter. The deriving procedure of the analytical model for a parallel resonant converter is extensively given and the analytical model obtained is verified by simulation results achieved using MATLAB/SIMULINK and the program written by the authors.

The paper suggests an analytical model for dc‐ac‐dc parallel resonant converters. The model can be used in the output voltage estimation of a converter in terms of its phase‐shift angle and the dc‐link voltage.

The resources in the library of the authors' university and also the English resources relative to dc‐ac‐dc converters reachable through the internet were researched.

The analytical model suggested can be used in estimating the output voltage of the converters used in high‐voltage applications or where there are difficulties in employing sensors in measurement of the output voltage due to high price or implementation problems.

The originality of the paper is to present an analytical model for dc‐ac‐dc parallel resonant converters. Using this model makes it possible to estimate the output voltage of the converter using the dc‐link voltage and the phase‐shift angle. The proposed model provides researchers to regulate the output voltage of the converters using feed‐forward control technique.

Improving reliability is a key factor in reducing the cost of wind energy, which is strongly influenced by the cost of maintenance operations. In this context, this paper aims to propose a degradation model that describes the phenomenon of fault propagation to apply proactive maintenance that will act on the cause of failure to prevent its reoccurrence as well as to improve future system designs.

The methodology adopted consists in identifying the different components of a wind turbine, their causes and failure modes, and then, classifying these components according to their causes of failure.

The result is a classification of the different components of a wind turbine according to their failure causes. From the obtained classification, the authors observed that the failure modes for one component are a failure cause for another component, which describes the phenomenon of failure propagation.

The different classifications existing in the literature depend on the nature, position and function of the different components. The classification of this study consists in grouping the components of a wind turbine according to their failure causes to develop a degradation model considering the propagation of failure in the field of wind turbines.

To analyze the operating performance of an ac‐dc‐ac‐dc PWM parallel resonant converter operating at lagging power factor mode controlled based on fuzzy logic control method.

A range of published works relevant to dc‐ac‐dc converters and their control methods based on PWM technique are evaluated and their limitations in converter output voltage control are indicated in the first section of this paper. The Simulink model and different stages of the converter are described in the second section. In Section 3, the general mathematical model of the system is derived and the phase‐shift PWM switching technique is explained. The equivalent circuit of the high‐voltage high‐frequency transformer used in the converter and the effects of the transformer parameters on the converter operation are presented in Section 4. In Section 5, fuzzy logic control and the basic concepts of this method are described and its application to the proposed converter output voltage control is explained. In Section 6, the Simulink simulation results of the fuzzy logic control application are given for different operating conditions. In Section 7, an overview of the hardware used in this study is presented and the experimental results are given to show the performance of the controller. Finally, Section 8 gives the conclusions of the study.

The fuzzy logic control which is a suitable method for nonlinear systems such as the converter proposed in this paper, is successfully applied for output voltage control of the converter. The controller performance is satisfied. The phase‐shift angle of the converter is used as the control parameter. The paper also presents how the parasitic parameters of the transformer used in high‐voltage applications can be used as the circuit resonant elements.

In preparing this paper, the resources books and periodic journals existing in our university library and also the English resources relative to dc‐ac‐dc converters reachable through the internet were researched.

The suggested control method can be used in the control of linear and nonlinear systems. The study carried out in this paper is also a very good approach to be used in high‐voltage high‐frequency converters output voltage control.

Since, the control approach proposed in this paper does not require the information on converter and transformer parameters that affect the converter output voltage, so it can effectively be used in applications where there are parameter variation problems. The design of the transformer for the required load, finding an optimum operating frequency for the converter, and using the transformer parameters as resonant elements of the circuit to decrease the switching losses are the other contributions of this paper.

For direct torque controlled induction motor drives, an effective solution to eliminate harmonics is the use of multipulse alternating current (AC)-direct current (DC) converters. Many researchers have used different configurations based on 24- and 30-pulse rectifications for improved power quality. However, the total harmonic distortion (THD) of AC mains current with these topology is more than 4 per cent when operating at a light load. For mitigating the THD problems observed in the input currents, Abdollahi propose 40-, 72- and 88-pulse AC-DC converters, while the power quality enhancement was the main concern. It is known that by increasing the number of pulses further results in reduction in current harmonics, but this is accompanied by an increase in cost and complexity. In this context, the purpose of this paper is to design a new delta/hexagon transformer based 36-pulse AC-DC converter for harmonic reduction without increasing the cost and complexity.

The proposed converter consists of two paralleled 18-pulse AC-DC converters involving a nine-phase shifted uncontrolled diode bridges with an interphase transformer circuit.

In this paper, the proposed scheme is simulated by matrix laboratory (MATLAB)/SIMULINK considering different loading scenarios. The simulation results show that the proposed scheme improves the power quality indices and satisfies the The Institute of Electrical and Electronics Engineers (IEEE)-519 requirements at the point of common coupling. Also, a laboratory prototype is implemented using the proposed design, and the experimental results confirm the simulation results under different loading conditions.

The proposed solution is a tradeoff among the pulse number, the transformer platform, the complexity of the scheme and the cost. The proposed scheme has an optimized configuration in this regard.

The paper describes the simulation of a short circuit of one diode in a three‐phase convertor set connected to a 2 winding transformer. The forcing currents are computed with the circuit simulation method. The circuit — field model is solved with the finite‐element method. In the paper is presented the distribution of flux lines and values of short circuit forces (strains) solved during one period every 15 degrees in the window of the convertor transformer. This approach to dynamic phenomena using the method presented has not yet been applied to short circuit research in transformers.

Conventional isolated dc–dc converters offer an efficient solution for performing voltage conversion with a large improved voltage gain. However, the small-signal analysis of these converters shows that a right-half-plane (RHP) zero appears in their control-to-output transfer function, exhibiting a nonminimum-phase stability. This RHP zero can limit the frequency response and dynamic specifications of the converters; therefore, the output voltage response is sluggish. To overcome these problems, the purpose of this study is to analyze, model and design a new isolated forward single-ended primary-inductor converter (IFSEPIC) through RHP zero alleviation.

At first, the normal operation of the suggested IFSEPIC is studied. Then, its average model and control-to-output transfer function are derived. Based on the obtained model and Routh–Hurwitz criterion, the components are suitably designed for the proposed IFSEPIC, such that the derived dynamic model can eliminate the RHP zero.

The advantages of the proposed IFSEPIC can be summarized as: This converter can provide conditions to achieve fast dynamic behavior and minimum-phase stability, owing to the RHP zero cancellation; with respect to conventional isolated converters, a larger gain can be realized using the proposed topology; thus, it is possible to attain a smaller operating duty cycle; for conventional isolated converters, transformer core saturation is a major concern, owing to a large magnetizing current. However, the average value of the magnetizing current becomes zero for the proposed IFSEPIC, thereby avoiding core saturation, particularly at high frequencies; and the input current of the proposed converter is continuous, reducing input current ripple.

The key benefits of the proposed IFSEPIC are shown via comparisons. To validate the design method and theoretical findings, a practical implementation is presented.

The purpose of this paper is to develop a numerical simulation method based on the transient upstream finite element method (FEM) and Schottky emission theory to reveal the distribution characteristics of space charge in oil-paper insulation.

The main insulation medium of the converter transformer in high voltage direct current transmission is oil-paper insulation. However, the influence of space charge is difficult to be fully considered in the insulation design and simulation of converter transformers. To reveal the influence characteristics of the space charge, this paper proposes a numerical simulation method based on Schottky emission theory and the transient upstream FEM. This method considers the influence of factors, such as carrier mobility, carrier recombination coefficient, trap capture coefficient and diffusion coefficient on the basis of multi-physics field coupling calculation of the electric field and fluid field.

A numerical simulation method considering multiple charge states is proposed for the space charge problem in oil-paper insulation. Meanwhile, a space charge measurement platform based on the electrostatic capacitance probe method for oil-paper insulation structure is built, and the effectiveness and accuracy of the numerical simulation method is verified.

A variety of models are calculated and analyzed by the numerical simulation method in this paper, and the distribution characteristics of the space charge and total electric field in oil-paper insulation medium with single-layer, polarity reversal of plate voltage and double-layer are obtained. The research results of this paper have the guiding significance for the engineering application of oil-paper insulation and the optimal design of converter transformer insulation.

This paper aims to present how to use the difference equations for analysis of flyback converter circuit.

Switching circuits have variable structural topologies. In every switched-mode, they have different dynamics and different equations. First, the exact equivalent circuit of flyback converter, then, set of difference equations are obtained. The flyback converter has a nonlinear structure; however, the presented technique allows the circuit equations to be linear. The transient-state and steady-state analysis of flyback converter, one of popular switching circuits, are carried out by using difference-equations.

The proposed analysis method does not contain any numerical approximation and the results are in the form of exact solution. Another superiority of the method is that the desired instantaneous values can be obtained directly, the simulation does not need to be started from the beginning. Numerical results agree well with the theoretical results of flyback converter. The simulation results obtained by using the proposed method and Matlab-based results are compared.

This paper contributes a different mathematical background for analysis of switching converters to the literature.

Power‐electronic systems have been playing a significant role in the integration of large‐scale wind turbines into power systems due to the fact that during the past three decades power‐electronic technology has experienced a dramatic evolution. This second part of the paper aims to focus on a comprehensive survey of power converters and their associated control systems for high‐power wind energy generation applications.

Advanced control strategies, i.e. field‐oriented vector control and direct power control, are initially reviewed for wind‐turbine driven doubly fed induction generator (DFIG) systems. Various topologies of power converters, comprising back‐to‐back (BTB) connected two‐ and multi‐level voltage source converters (VSCs), BTB current source converters (CSCs) and matrix converters, are identified for high‐power wind‐turbine driven PMSG systems, with their respective features and challenges outlined. Finally, several control issues, viz., basic control targets, active damping control and sensorless control schemes, are elaborated for the machine‐ and grid‐side converters of PMSG wind generation systems.

For high‐power PMSG‐based wind turbines ranging from 3 MW to 5 MW, parallel‐connected 2‐level LV BTB VSCs are the most cost‐effective converter topology with mature commercial products, particularly for dual 3‐phase stator‐winding PMSG generation systems. For higher‐capacity wind‐turbine driven PMSGs rated from 5 MW to 10 MW, medium voltage multi‐level converters, such as 5‐level regenerative CHB, 3‐ and 4‐level FC BTB VSC, and 3‐level BTB VSC, are preferred. Among them, 3‐level BTB NPC topology is the favorite with well‐proven technology and industrial applications, which can also be extensively applicable with open‐end winding and dual stator‐winding PMSGs so as to create even higher voltage/power wind generation systems. Sensorless control algorithms based on fundamental voltages/currents are suggested to be employed in the basic VC/DPC schemes for enhancing the robustness in the entire PMSG‐based wind power generation system, due to that the problems related with electromagnetic interferences in the position signals and the failures in the mechanical encoders can be avoided.

This second part of the paper for the first time systematically reviews the latest state of arts with regard to power converters and their associated advanced control strategies for high‐power wind energy generation applications. It summarizes a variety of converter topologies with pros and cons highlighted for different power ratings of wind turbines.

This paper aims to propose a new design for high-power compact solid-state transformers (SSTs) made with grain-oriented electrical steel (GOES) wound cores that benefit from the natural reduction of iron losses at high temperatures.

An experimental approach, coupled with numerical and analytical investigations, is widely used for proving the validity of the proposed concept.

With cores much hotter than coils, the new design of medium frequency transformers can be used for building compact SSTs that rated powers and common-mode insulation voltages much higher than existing ones with similar efficiencies.

The thermal design must provide a large difference between core and coil temperatures in a reasonable volume.

The increasing number of intermittent renewable sources place electric grid stability at risk. Smart nodes, made of SSTs, improve the global grid stability because they are able to provide real-time control of energy fluxes at critical points. In railway applications, high-power SST cells can be distributed along the train providing a larger volume for passengers.

The increasing part of electricity in a flexible grid requires performant and high-power SSTs made with components that have an environmental footprint as low as possible.

This paper proves that the design of high-power transformers with GOES wound cores much hotter than coils is possible. It proposes also a thermal equivalent circuit that helps the design.