Within a solid-state transformer, the isolated dc-dc converter and in particular its medium-frequency transformer are one of the critical components, as it provides the required isolation between primary and secondary sides and the voltage conversion typically necessary for the operation of the system. A comprehensive optimization procedure is required to find a transformer design that maximizes power density and efficiency within the available degrees of freedom while complying with material limits, such as temperature, flux density, and dielectric strength as well as outer dimension limits. This paper presents an optimization routine and its underlying loss and thermal models, which are used to design a 166 kW/20 kHz transformer prototype achieving 99.4% efficiency at a power density of 44 kW/dm
 3
. Extensive measurements are performed on the constructed prototype in order to measure core and winding losses and to investigate the current distribution within the litz wire and the flux sharing between the cores.

Design and Experimental Analysis of a Medium-Frequency Transformer for Solid-State Transformer Applications

Online partial discharge (PD) measurements have long been used as an effective means to assess the condition of the stator windings of large generators. An increase in the use of PD online measurement systems during the last decade is evident. Improvements in the detection capabilities are partly the reason for the increased popularity. Another reason has been the development of digital signal processing techniques. In addition, rapid progress is being made in automated single PD source classification. However, there are still some factors hindering wider application of the system, such as the complex PD mechanism and PD pulse propagation in stator windings, the presence of detrimental noise and disturbances on-site, and multiple PD sources occurring simultaneously. To avoid repetition of past work and to provide an overview for fresh researchers in this area, this paper presents a comprehensive survey of the state-of-the-art knowledge on PD mechanism, PD pulse propagation in stator windings, PD signal detection methods and signal processing techniques. Areas for further research are also presented.

A Review of Online Partial Discharge Measurement of Large Generators

The investigation of mica and mica/epoxy-composites has always been of high importance and has gained increased attention in recent years due to their significant role as insulation material in the electrical industry. Electrical insulation represents a key factor regarding the reliability and lifespan of high voltage rotating machines. As the demand for generating power plants is increasing, rotating machines are of intrinsic importance to the electrical energy supply. Therefore, impeccable functioning is of immense importance for both the producers of high voltage machines as well as the energy suppliers. Thus, cost reduction caused by shorter maintenance times and higher operational lifespan has become the focus of attention. Besides the electrical properties, composites should offer compatible chemical and mechanical, as well as thermal characteristics for their usage in insulating systems. Furthermore, knowledge of several occurring stresses leading to the final breakdown of the whole insulation is required. This review aims to give an overview of the properties of pure components, the composite, and the possible occurring failure mechanisms which can lead to a full understanding of insulation materials for high voltage rotating machines and therefore establish a basis for prospective optimizations.

Mica/Epoxy-Composites in the Electrical Industry: Applications, Composites for Insulation, and Investigations on Failure Mechanisms for Prospective Optimizations

This is a study of potential improvements of diagnostic methods used on high-voltage generators and motors.  It considers offline electrical measurements on the main insulation of stator windings, ...

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Dielectric response and partial discharge measurements on stator insulation at varied low frequency

Reliability is a critical consideration for wind turbine generator systems as failures contribute directly to operation and maintenance costs and hence the cost of energy. Improving reliability hinges on an understanding of the mechanisms of failures that occur. This paper discusses the major failure mechanisms in the WT generator system, particularly the converter and the generator, from the point of view of their use in wind turbines. It also looks at the opportunities for improving reliability.

A review of failure mechanisms in wind turbine generator systems

This article discusses about the abnormal or faulty operating condition of premature power generator insulation failures that can be classified as: design failures, material failures, manufacturing failures and maintenance failures. The final failure of the mica insulation is usually caused by electric breakdown through a tree channel. Formation and propagation of trees in the insulation therefore reflect the aging process of the insulation.

Insulation Failure Mechanisms of Power Generators [Feature Article]

Due to an increasing demand for new, or the refurbishment of power stations, high-voltage rotating machines continue to play a significant role in generating electrical energy. An important component of these machines is their electrical insulation. Although much research has been done to improve the material, about a quarter of all failures still are related to insulation problems. Research to improve winding insulation is very complex because a large number of factors influence insulation life, among them the raw materials, size of the mica tapes, geometrical characteristics of the bar and taping- or manufacturing techniques. The results show that electrical tree propagation is the main electrical degradation mechanism that leads to breakdown of the main wall insulation and that poor impregnation of the insulation and type of taping can reduce time to breakdown significantly.

Electrical breakdown in high-voltage winding insulations of different manufacturing qualities

For various types of epoxy resin and various kinds of ion-exchange on the mica surface the electrical tree propagation in the mica epoxy interface has been investigated. Apart from statistical scattering a significant spread by more than one order of magnitude in tree propagation time was found. Furthermore, the optical appearance of the corresponding trees was observed. Branch- and bush-type trees were found yielding shorter or longer tree propagation times, respectively. The observed spread is ascribed to the combination of resin and mica. However, the composition of the resin has the strongest influence. Thus, for further improvements of mica epoxy insulation it is more promising to study the components of the resin compared to an ion-exchange on the mica surface.

R. Vogelsang

Papers

Insulation Failure Mechanisms of Power Generators [Feature Article]

Electrical breakdown in high-voltage winding insulations of different manufacturing qualities

Electrical tree propagation along mica barriers in dependence on the resin components