System of Systems Strand Tilt Analysis Perspective on MediuMHigh Voltage Stator Bar and a Non-Destructive Testing Case Study

Stator copper bar is usually divided into several thin uniform strands, insulated from each other. This is done to reduce additional induced voltage due to convergence of magnetic flux from winding and rotor themselves. Flux density varies radially in density which causes different induced voltage strand to strand. To cancel out unbalanced strand voltage and minimize circulating current and resultant heating, strands are usually transposed. Transposition mechanism is fairly common, and is referred as roebelling. Manufacturing an ideal transposition is a difficult process. There is almost always some associated manufacturing defects present. Untreated issues like strand tilt leads to less utilization of material (less efficiency) & also issues like partial discharge leads to heating due to voids in the insulation. Nonconformance cost incur during the operation, if issues are not corrected. Paper presents the analysis with SoS (System of Systems) approach. In a better performing system (in this case electrical generator), each of the systems (strand, roebel coil and each phase winding) should collectively work to achieve the target of overall system (i.e. more efficient generator system). There are some other issues which arises due to the strand misalignment, e.g. void in the insulation, partial discharge and slot induced eddy voltage change and are discussed in the paper. Paper presents how important is it for system to work in coordination with other systems in order for the overall system to work efficiently. Paper also presents the complexity in evaluating strand tilt during stator coil manufacturing process. A unique solution of non-destructive testing to detect strand tilt is presented which provided excellent result. This is helpful to evaluate, how off the strand is tilted at any particular location on the generator coil.