Analysis of water treeing in solid insulated cables

The physical and internal structure of water trees grown in power cables are studied and analysed. A new device for studying water trees shows that the microvoids consist of air and water voids. The tree starts from an air void. The age of the cable depends on the time required for the pressure to build up inside the air void in order to overcome the mechanical strength of the solid insulating material. At this instant water tree inception starts. This age depends also on the tip radius of the protrusion defect which has the effect of decreasing the age abruptly as it becomes small. The voltage required for this stage is the switching overvoltage. The time required after water inception is small because it propagates under the normal operating voltage of the cable