## 1 Introduction  
Dielectric materials of different forms and states are inseparable parts of high voltage applications. From the point of reliability, it is the weakest part of the reliability chain of the whole device [1]. For this reason, it is necessary to take an effort to the improvement of material parameters (electrical, mechanical or physical-chemical). It leads to the reduction of the probability of failure of the entire device [2]. Current trends in the field of design and diagnostics allow the usage and study of the materials containing components up to nanometric dimensions [3]. One way to achieve the desired improvement is to use composite materials [4] where the synergy effect of individual components is utilized. In the field of insulating systems for rotating machines, there are, in particular, three component composite materials, which are composed of a matrix, filler and reinforcement [5]. This area of composite insulation systems has undergone considerable development over the past 70 years. In the case of insulation systems for rotating machines, natural fibers from materials such as silk, cellulose, and linen were initially used. Natural varnishes from trees, plants and insects, or petroleum derivatives [6] were used as an impregnants. Progress in the technology of composite insulating materials of rotating machines is dated to the time of use of mica leaves in unaltered form with cellulose reinforcement binded together with shellac or asphalt matrix [7]. This type of insulation is also known as a \"soft\" insulating system. The undisputed advantage of soft systems was their elasticity and the ability to fill the machine slot, completely without the spaces [8]. One of the main reasons for their replacements were limits in terms of the operating thermal class and voltage level or partial discharges developed in delamination due to the shrink. Another milestone in this area was the incorporation of mica paper as a part of the insulation system in the middle of the 20 th century [9]. At that time, also a development, research and application grew in the field of impregnants. Previously used bituminous varnishes being slowly replaced by synthetic resins. The most common representatives for electrotechnical applications are polyester [10], epoxy [11] and in special cases silicone [12] resins. Also natural fibers have been replaced over the time. Glass fibers, polyethylene terephthalate or polyethylene naphthalate films [13] are used today as a reinforcement. These so-called \"hard\" insulation systems can be operated at higher temperatures and voltage levels. Their disadvantage may be the fact that they do not sufficiently fill the slot of the machine, which in some cases may lead to the formation of slot partial discharges. The illustration of the stator winding with composite insulation is shown in Figure 1.  
Another significant contribution from the historical perspective on the development of composite dielectrics and their electrical properties can be attributed to [PERSON] and [PERSON] [14]. They used a nanometric fillers in corona resistant insulation at the beginning of the 1990s. As a next, [PERSON] defines the uniqueness of nanodeilectrics in his article in 1994 [15]. Then, the nanodelectric systems become an object of interest of scientists and professionals from all over the world, who are discussing other options for modifications and potential applications [16]. The most frequent studied fillers (Figure 2) are simple oxides (SiO\({}_{2}\)[17], Al\({}_{2}\)O\({}_{3}\)[18], ZnO [19], TiO\({}_{2}\)[20], MgO [21]), layered [22] or tubular [23] silicates, eventually also nonwoven nanoscale structures [24; 25; 26]. Figure 1: Illustration of stator winding with different structures of mainwall insulation. Figure 2: Different fillers used in matrices (**a**) dispersed particles; (**b**) exfoliated layered nanosilicates; (**c**) nonwoven nanofibres. With the advent of last mentioned technologies, it is possible to characterize the composites as macro-, micro- and nano-.