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Water Tree Degradation And How to Manage It Author: Keith Lanan, Member IEEE – International Project Coordinator, UtilX®Corporation Presenter: Jack Stel, BSc-Eng, C-Eng, FCMI, MCIBS – V.P. International Business Development, UtilX® Corporation
Overview • XLPE insulated cables gradually become less reliable through water tree degradation. • The resulting reduction of system reliability eventually requires action. • Circuits are prioritized for action based on criticality and risk of failure. • Replacement results in a high quality system. But, it is slow and can exhaust resources. • Restoration’s lower cost and rapid application can make it a an important part of system management.
Water Tree Growth Bowtie water tree growing from dust particle in 6-year old XLPE insulation. • Defects in insulation focus AC stress. • Water diffuses into the cable from the environment. • With high AC stress and water, water trees grow slowly across the insulation. • As the water tree grows longer, the cable becomes more susceptible to failure.
Water Tree Growth (continued) • Unless protected with a full-metal sheath (such as extruded lead), water vapor will diffuse into the XLPE until it reaches equilibrium with the surrounding environment – usually near saturation. • PE or PVC Jackets, water-swellable compounds, strand-filling materials, or water blocked accessories have little effect on eventual water concentration. • Cables built and operated to industry standards are vulnerable to water tree degradation.
Transition to Electric Tree • The XLPE within the water tree is still a good insulator, however it has a lower resistance to discharge than the surrounding insulation. • If the stress across the insulation exceeds the reduced discharge resistance, a partial discharge is initiated. • This discharge will burn an open channel in the insulation called and electric tree. • The discharge will continue and the channel will lengthen until the cable faults or the stress falls below the extinction voltage. Small bowtie water trees and large electric tree several centimeters from cable fault
Progression to Failure • Once established, an electric tree will re-initiate discharge at much lower stress. • A small electric tree may exist in a cable for some time – as long as the extinction voltage is well above operating voltage. • If the extinction voltage drops below the operating voltage, the discharge will continue and lead to failure within hours. • As water trees make a cable progressively weaker, eventually minor transient voltage events, such as switching or grounding will initiate an electric tree.
Detecting Water Trees • Water trees have minute electrical signatures. • Detection systems generally rely on the cumulative property of a characteristic from many water trees. • Water trees can be indirectly measured with an AC breakdown or withstand tests. However this can destroy old cables. • The use of PD equipment to detect the discharge and then immediately shut the test down leaves small electrical trees behind. • High voltage DC may be valuable for testing new cables, but it is not appropriate for commissioning craftwork on aged cables as it makes water trees more susceptible to discharge when placed back in service. • The only definitive way to directly measure water trees is to send a cable sample to the laboratory for water tree inspection. • Insulation failures must be analyzed and monitored to assess the extent of water tree effects on the system.
Managing System Reliability • Managing the effect of water tree degradation is a matter of balancing the value of system reliability against the cost of the remedy. • As the underground systems ages, the frequency of outages stemming from water trees increases and can eventually become the leading cause of outages. • For the cables in the ground the options are limited to replacement and restoration.
All cables Review record Young Age No action Old Insufficient Capacity Replace OK Highest Critical Action Lower Low Risk No action High Test Good Condition No action Bad Moderate Monitor Action Prioritization Prioritization
The Restoration Option • At the point where the water tree problem becomes more pervasive, restoration is more favorable because its lower cost and faster application allows more circuits to be addressed with the same resources.
Treatment Process • Preparation • Pneumatic Testing • Joints • Energize or not • Vacuum • Fluid Injection • Completion
Preventing Faults • After treatment the restoration fluid diffuses into the insulation where it consumes the water in the water trees. • The water is replaced with a high dielectric strength silicone polymer that has particular qualities that prevent discharges. • The prevention of discharges blocks the usual progression from water tree to electric tree and failure. • As a consequence, the cable can withstand higher transient voltages without the initiation of electric trees.
Conclusions • The frequency of cable failure today is a result of the quality of the cables that were installed decades ago. • To maintain an acceptable level of system reliability, managers must assess and prioritize the cables within the system and apply the available tools accordingly. • For cables in the ground the options are limited to replacement or restoration.