John G. Buchan PhD.pdf - OpenAIR @ RGU - Robert Gordon ...

velocities and attenuation .Tests were performed on a number of highvoltage machines and results of experiments on a 6.6 kV ; form woundmotor and on a 22 kV generator winding were discussed.Two basic modes of pulse propagation are identified.In the seriesmode the winding behaves like a simple transmission line in which thea pulse has a finite propagation velocity and is subject toattenuation. In the second mode ,a portion of the pulse travelsthrough the winding virtually unhindered and without appreciable timedelay. The path of propagation is through the endwinding conductorsby electromagnetic coupling .This mode occurs at higher pulsefrequencies, above a certain cut-off frequency. These results aresimilar to the. findings of -the study on the propagationcharacteristics of partial discharge pulses in motor windingsdescribed in the following sections .2.2 Stator Winding Test RigThe model phase winding used for experimentation is constructed from agroup of fourteen,7-turn coils supported in a wooden cradle whichhas the slotted profile of a stator core as shown in the photograph infigure 2.2.1.A sheet of copper is formed over the teeth of thewooden frame to represent the iron core of a machine . This model isvalid , since the depth of penetration of flux at a frequency of 100kHz is53 um( conductivity of sheet steel is taken to be 3.6 Krim ).Therefore at higher frequencies, the ferromagnetic properties of theiron of a stator core has negligible effect on the propagation ofelectromagnetic signals within the stator winding . The inductance ofthe slot portion of a stator coil becomes independent of frequencyabove100 kHz.The rotor of the machine is considered to have negligible influence onthe pulse propagation behaviour at the frequencies encountered inpartialdischarge pulses due to the skin effect and the fact that anappreciable air gap exists between the stator and rotor surfacesTherefore the rotor is not simulated by the model .Each coil in the model is of a form wound construction in which 2'parallel conductors are wound to form a7 turn coil in a double-stackconductor arrangement . By connecting the parallel conductors at thecoil ends the coil is made to behave electrically as a7 turn coil .Page 68