A comparison between partial discharge propagation in multiple-/spl alpha/ and single-/spl alpha/ transformer winding

Power transformers are in service in different environmental, electrical and mechanical conditions. Many experiences have proved that the inner insulation system degradation may pose the transformer to fail while in service. On the other hand, partial discharges (PD) are recognized as the main cause of insulation deterioration process. Therefore, reaching the optimum inner insulation system is one of the challenges a transformer designer is faced with. The solution lies in the correct and accurate modelling of different types of transformer windings. Transformer strength especially during transient conditions is a criterion for transformer insulation designers. This challenge has made designers switch from ordinary layer and disc windings to multiple-alpha windings. Multiple-alpha windings have more complicated structure and comprise various parts with different physical structure and electrical characteristic. These kinds of windings have usually more wire length. These characteristics make partial discharge measurements be more hassling. Partial discharges that take place inside the winding propagate along the winding to reach the measuring terminals. Typical partial discharge signals cover a wide frequency range from DC up to hundreds of MHz and different frequency components propagate through the winding depending upon the winding structure in different modes. In this paper a comparison has been made between the results gained when the winding is single-alpha and those of multiple-alpha. A 66 kV/25 MVA interleaved winding, which has 19 fully interleaved discs plays the role of a single-alpha winding. When this main winding is connected to the tap winding with different structure and magnitude response, a multiple-alpha winding is constructed. In the experimental measurements in the laboratory, the line and neutral-end current signals are detected by two home-made high frequency current transformers (HF-CT) and recorded with a 500 MHz digital oscilloscope. Home-made sensors are designed to provide maximum sensitivity in the desired frequency range