9th INSUCON International Electrical Insulation Conference, Berlin 2002 NEW MATERIALS AND TECHNIQUES FOR THE PRODUCTION OF COILS AND ROEBEL BARS Rudolf Bruetsch, Roger Schwander, Franz Wolf and Marco Naegelin Von Roll Isola, CH-4226 Breitenbach, Switzerland INTRODUCTION The driving force of material and process development in the insulation technology of rotating high voltage machines has changed in the last ten to fifteen years. Being mainly technology driven in the past, cost has become the determining factor in the industry. Cost reduction programmes are very popular in the machine industry and any improvements in products or processing are only accepted if they reduce overall cost. On the other hand, high voltage motors and generators are long-life capital goods and a reduction in reliability cannot be tolerated. To be successful in the market any development of new insulating materials has to reflect these facts. In the manufacturing process of rotating high voltage machines a substantial part of the total insulating cost is related to labour. We estimate that for large machines the contribution of labour to the total insulating cost is between 30 and 60 %, the higher percentages being related to machines where press cure or single bar VPI technology is applied [1]. In the manufacturing process of large machines the production and insulation of Roebel transposed bars is an essential part. Roebel bars are manufactured according to the following steps: 1 Conductors - usually enamelled and/or single or double Daglas covered – are cut to the appropriate length and crimping of the wire is done to form the crossovers on top and at the bottom of the stack. After stripping off the insulation at the ends the conductors are laid together to constitute a half–bar. this purpose. Figure 1 shows where the materials are being placed in the Roebel bar. Insulating or conductive mastic Insulated conductor Stack separator Conductor crossover Figure 1 Placement of insulating or conductive mastic and stack separator in the Roebel bar 5 Finally both ends of the Roebel bar have to be bent carefully to form the end winding (Figure 2). This step is critical for two reasons: Bending has to be done precisely so that the bars fit perfectly in the stator and it also has to be done carefully to avoid any damage of the conductor insulation. 2 A separator is used to bond two half–bars together. The separator material is a fleece or a fabric impregnated with a thermosetting epoxy resin. Curing time needed to ensure mechanical stability of the bar for further handling is typically ½ hour at 160 °C. 3 At the crossovers of the Daglas-insulated conductors damage of the conductor insulation may occur during the mechanical bending of the wire. Therefore mica chips have to be placed between adjacent conductors at the position of the crossovers to ensure a reliable insulation. This is usually done by hand. 4 The gaps on top and at the bottom of the Roebel bars have to be filled to avoid partial discharge. Mostly a mastic – non conductive or conductive is applied for Figure 2 Roebel bar end windings of a hydro generator All these process steps are labour-intensive and therefore a significant cost factor in the production of Roebel bars. In order to respond to the demand of cost reduction a new range of fast curing products for the consolidation of Roebel bars and double layer coils was developed. The main feature of the new products is a short curing cycle at relatively low temperatures, which is sufficient to provide mechanical and dimensional stability of the bar or coil for further handling. After application and curing of the main wall insulation the fast curing materials meet class F requirements. The application of the fast curing materials can be automated using new types of equipment for the production and consolidation of Roebel bars. This equipment eliminates the application of insulating chips at the Roebel crossovers. The application of the mastic is also automated and optimal edge radii are generated eliminating all finishing work. FAST CURING MATERIALS FOR CONSOLIDATION OF ROEBEL BARS THE Specifications of the new range of fast curing electrical insulating materials were chosen to meet the demand for cost reduction and the technical requirements related to handling and reliability. Requirements are: - All materials based on epoxy binder resin chemistry to be compatible with all insulating materials used in practice. - Shelf life of ≥ 4 months at room temperature. - Fast curing conditions 15 min. at 120 – 130°C to achieve sufficient mechanical stability. - Mechanical stability after fast curing defined by a glass transition temperature of the material of ≥ 120°C to allow the next steps of manufacturing to take place. - Thermal class F after full curing together with the main wall insulation. The range of the newly developed fast curing products covers Glasoflex® stack separator materials for the slot part and a more flexible version for the end parts and two versions of mastic to fill the gaps in Roebel bars at crossovers. An overview on the product range is given in table 1. TABLE 1 Properties of fast curing electrical insulating materials for Roebel bar consolidation Separator Materials Glasoflex® Nominal Thickness Total Weight Glass Fabric Glass Fleece Resin Thickness after Consolidation Thermal Class Mastics Density Resistivity Thermal class Unit For Slot Part For End Part mm 0.35 0.70 1.00 0.40 g/m2 500 480 855 290 g/m2 g/m2 230 - 80 270 2 x 50 50 g/m2 mm 270 0.23 400 0.22 485 0.4 240 0.14 F F F F Unit g/m3 Ωm Non Conductive 1.55 ∞ F Conductive 1.55 < 50’000 F pation curve. The glass transition temperature of the separator materials was determined as a function of curing time and temperature applied. Results are shown in figure 3. Glass Transition Temperature (°C) 180 170 160 150 140 130 120 Test results The mechanical stability of the separator material can be expressed by the mechanical dissipation factor and the glass transition temperature. Dynamic mechanical analysis (DMA) was used to measure the mechanical dissipation factor at a frequency of 1 Hz. According to IEC 61006 the glass transition temperature is defined as the temperature of the peak of the mechanical dissi- 0 1 2 3 4 Curing Time (hours) Curing at 120°C 5 6 Curing at 160°C Figure 3 Glass transition temperature of fast curing stack separator materials in function of curing time and temperature according to IEC 61006 The results show that after an initial curing of 15 min. at 120°C a glass transition temperature of 153°C is obtained which is sufficient to ensure mechanical stability for further handling of the Roebel bars. During the curing cycle of the main wall insulation – which is usually done at 160°C – the separator material will fully cure; the corresponding values of the glass transition temperatures are 168°C after ½ hour and 169°C after 1 hour at 160°C. To ensure reliable longterm thermal ageing properties the temperature index of the separator materials was determined according to IEC 60216. Measurements of weight loss and flexural strength were made at temperatures of 180, 200, 220 and 240°C. The temperature index was determined for 10 % weight loss of organic matter and 50 % of loss of flexural strength. Results are given in figure 4. The manufacturing centre consists of the following five main units: Time (h) 100’000 Flex.Strength Temp. Index = 159 °C Figure 5 Manufacturing centre for automated production of Roebel bars Weight Loss Temp. Index = 168 °C Straightening-, stripping- and cutting- machine The machine is capable to process single solid or hollow conductors. Conductors are automatically cut and straightened and the insulation is stripped off at the conductor ends. 10’000 1’000 Crimping unit 100 0.24 0.23 0.22 0.21 0.20 Temperature (10-2/K) 0.19 Figure 4 Determination of the temperature index of fast curing separator materials according to IEC 60216 In the crimping unit the individual conductors are shaped at one or both sides. Crimping is done 3dimensionally so that the conductor insulation remains intact and no mica chips have to be placed between conductors at the position of crossovers. This unit is also supporting the trend from 360°- towards 540°- and 720°-Roebel bars to minimize electromagnetic losses in the stator. Buffer unit and assembling table NEW PROCESS EQUIPMENT FOR CONSOLIDATION OF ROEBEL BARS THE To meet the demand for cost saving and to use the potential of fast curing materials in the production of Roebel bars a manufacturing line was developed which enables automated production of Roebel bars (Figure 5). After the forming process the conductors are laid in a magazine with a tilting system to constitute a half-bar. According to the chosen option, the half-bars are either manually unloaded on an assembling table or automatically unloaded on a storage conveyor. The manual braiding of the conductors to half-bars is a very easy operation due to the 3-dimensional crimping. Two half-bars are then joined and a fast curing stack separator is inserted to form the complete Roebel bar (Figure 6). bars are ready for the application of the mainwall insulation. Forming the end windings with the bar forming machine The last step in Roebel bar production before applying the main wall insulation is the forming of the end windings at both ends of the bar. End windings are automatically bent by the new bar forming machine (Figure 8). It is equipped with 21 numerical axes resulting in a tolerance of the bending of ± 0.2 mm. Time needed for forming both ends of one Roebel bar is approx. 2½ minutes. Figure 6 To half-bars on the assembling table before joining to a Roebel bar Protection taping and insertion of fast curing mastic The fast curing mastic is applied in tape form. The dimensions of the cross-section of the mastic tape have to be selected to give an optimal filling of the gaps at the Roebel crossovers. The mastic tape is covered by a release film on the outer side and after application on top and bottom of the Roebel bar it is fixed by a shrink film taped around the bar. All these process steps are done automatically by this unit (Figure 7). Figure 8 Bar forming machine, version for hydro bars CONCLUSIONS Significant cost reduction can be achieved in the production of coils and Roebel bars for rotating high voltage machines by applying new insulating materials and new techniques. The combination of a new range of fast curing insulating materials for the consolidation of bars together with a new highly automated production line which eliminates all labour–intensive manufacturing steps results in a considerable saving of time and cost. Large original equipment manufacturers have installed this equipment and time reduction realized in the production of Roebel bars is up to 80 %. Figure 7 Unit for applying insulating or conductive mastic and protection taping After that the Roebel bar is ready for curing the consolidating materials. Since the stack separator and the mastic are both fast curing, the curing cycle needs only a holding time of 15 minutes at 120°C. Then the taped shrink film and the release film can easily be removed. The faces and edges of the cured mastic need no finishing, after forming the evolvents the Roebel References [1] Allison, J., Bruetsch, R. and Thaler, T. ”Factors Determining Cost and Quality of the Electrical Insulation in the VPI Process”, IEEE International Symposium on Electrical Insulation, Montreal, Canada, June 1996, pp. 259-262.
