The MICALASTIC Insulation in ITAIPU™

MICALASTIC is the registered trademark for Siemens insulation systems for high-voltage windings of rotating electrical machines. These systems use mica, a material capable of withstanding high electrical and thermal loads, together with curable, elastic epoxy resins as bonding material. Since the early days of electrical machine construction, the naturally occurring, inorganic mineral mica has been an indispensable constituent of high voltage insulation systems. The most important criterion for the use of mica is its ability to durably withstand the partial electrical discharges which can occur inside the insulation due to high electrical stresses.

16.2.1 Manufacturing and Design

As early as 1957, Siemens-Dynamo werk in Berlin manufactured the first stator windings that made use of mica tape and a vacuum-pressure impregnation process. With this method, single coils and Roebel bars for hydroelectric generators are continuously wrapped with mica tape in the slot and end sections. The taped winding elements are then dried out and degassed in a vacuum impregnation tank, and flooded with low-viscosity, curable synthetic resin. High nitrogen pressure applied to the impregnating bath completely impregnates the mica tape. After being placed in accurately sized, portable pressing molds, the insulation is cured at high temperatures in large chamber ovens. Continued development of this insulation technology ultimately led to the use of a film of ground mica on mechanically strong glass fabric as the carrier material with epoxy resin as the impregnant, which produced a very durable electrically,  thermally and mechanically),modern insulation system. Long duration tests in a slot model were unnecessary, since the desired voltage endurance had already been achieved in the previous development stages (Fig. 2) using lower-quality carrier materials. Short-duration tests were performed, however, for verification.

16.2.2 Fitting of Roebel Bars into Slots:

      Winding elements with cured MICALASTIC insulation are secured in the slots by filling up the tolerances between the slot wall and the conductive surface (coil side corona shielding) of the bar insulation. Initially, Siemens used graphite-treated paper as filler material. Since about 1969, however a special bar fitting procedure has been used for hydroelectric generators. The main features of this procedure are U-shaped slot liners made of polyester fleece impregnated with a conductive material, and a conductive, curable synthetic resin paste between the surface of the bar insulation and the slot liner (Fig. 3). Therefore, the insulation does not stick to the stator core, and the option of removing the bars, even though seldom required, is retained. In the radial direction, the slot portion of the winding elements is secured by means of various packing strips or ripple springs, and slot wedges. Bracing the end windings and jumpers by using glass fiber reinforced spacers and epoxy-resin impregnated cording makes the winding resistant to electro dynamic forces during operation and to possible short-circuit faults. This resistance is also aided considerably by the mechanical stiffness of the MICALASTIC insulation, which is also cured within the end winding.

       

16.2.3 Thermal Stability

        The MICALASTIC insulation system was developed strictly for a continuous load in accordance with temperature class F (155°C). Nevertheless, generator design engineers generally guarantee compliance with class B (130°C) temperature limits for nominal operating conditions, as is also required in most invitations to tender. In practice, the stator windings of hydroelectric generators are frequently dimensioned for even lower operating temperatures, because the stators will usually be optimized for good   efficiency by adding electrically active material (winding copper and core lamination). Particularly low operating temperatures can be expected in the case of stator windings with direct water cooling. With an appropriately   dimensioned de mineralized-water cooling system, the maximum winding temperature can be reduced to 70°C and lower. Thermal aging of the insulation is therefore essentially eliminated, and thermo mechanical stresses are also substantially reduced. The resulting increase in operational reliability makes a real difference in the case of hydroelectric generators which are essential to safe grid operation

17. CONCLUSION:

Hence Vacuum-Pressure Impregnation technology can be used in a wide range of applications from insulating electrical coil windings to sealing porous metal castings. It normally produces better work in less time and at a lower cost than other available procedures.

Our VPI systems can be configured in a variety of ways, depending on the size and form of the product to be impregnated, the type of impregnant used and other production factors. System packages include all necessary valves, gauges, instruments and piping. These systems can be large or small, simple or highly sophisticated and equipped with manual, semi-automatic or automatic controls.

              Vacuum Pressure Impregnation (VPI) yields superior results with better insulating properties, combined with “flexible” rigidity, resulting in greater overall reliability and longer life. VPI reduces coil vibration by serving as an adhesive between coil wires, coil insulation, and by bonding coils to their slots.