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Now the distribution cables are often used as armature winding in high voltage motor. The dielectric performance of semi-conductive layers has distinct effect on electric field distribution in the cable. On the other hand, the semi-conductive layer resistivity isn’t a constant, and it has a range or changes with temperature. We researched the rule of electric field distribution in different resistivity and dielectric constant of cable internal and external semi-conductive layers. The studied cable structure is respectively conductor, inner shield layer, insulation, outer shield layer, and sheath layer. The thickness of inner shield layer, insulation, outer shield layer, and sheath layer is respectively 1mm, 5mm, 0.8mm, and 2.1mm. The cable diameter is 38.7mm. The materials of conductor and insulation are Al and EPR (ethylene propylene rubber). The inner and outer shield layers are used as stress control layer. The stress control layer and the sheath layer are also made of semi-conductive materials. In the calculation, the dielectric constant of cable insulation is 2.7; the electric conductivity of inner and outer semi-conductive layers is 2S/m and 5S/m individually. When the operating voltage of cable is 4.5kV and the surface of cable is grounded, the electric field distribution in the cable was shown in Fig. 1. We can see from the figure that the electric potential gradually minishes from the conductor to the sheath layer and the potential of cable surface is equal to zero. The maximal electric field intensity is located at the interface between the inner shield layer and insulation and the field intensity in the inner shield layer is lower. The field intensity gradually reduces from the insulation to the sheath layer and the field intensity of cable surface is equal to zero. We studied the effect of the resistivity and dielectric constant of stress control layer on electric field distribution. The results indicate with the increase of stress control layer conductivity, the maximum field intensity in the cable gradually increases and the field intensity in the stress control layer gradually decreases. In conditions of the above calculation, the field distribution in different sheath layer conductivity is studied. We found with the increase of sheath conductivity the maximum field in the cable increases and the field intensity and voltage in the sheath layer distinctly reduce. Then we analyzed the probability of corona discharge in the winding cable using an example. A 20kV medium cable has the same structure to the above, whose main insulation is made of EPR. The field along the cable radius at the bad ground (A) and well-earthed spot was individually shown in Fig. 2. When the cable shield was not well contacted with the ground system, in other words, there is some small space between the cable and ground bushing, the electric field here was distorted and field intensity greatly increased. In addition, the different size of gaps caused different degree field distortion. In this example, when the distance from the bad ground spot to the cable core was less than or equal to 19.7mm, corona discharge will occur at the air space. The motor winding cable is often earthed by ground bushing which is inserted on the stator iron core. Figure 3 shows the electric field intensity of cable surface was distorted near the two-side outlets (O) of stator slot, from which we can clearly see the field intensity near the outlets of stator slot was high to 1.35MV/m. So corona discharge may first occur here. So the corona discharge in winding cable may first occur at the bad ground spot and the outlets of stator slot. The effect rule of semi-conductive layer on electric field provides theoretical basis for cable and anti-corona design in high voltage motor. 0 5 10 15 20

Study on Winding Cable and its Corona of High Voltage Motor using FEM