What is Water Treeing? Causes and Measures

Water treeing occurs in Cross-linked Polyethylene (XLPE) cables--commonly used for high-voltage transmission--due to the combined effects of moisture and AC electrical stress. The phenomenon is named for the branching, tree-like pattern of the degradation.

While both "water trees" and "electrical trees" exhibit dendritic patterns, their origins differ. Electrical trees typically stem from localized high electrical stress and partial discharge. In contrast, water trees originate from moisture ingress and electrochemical stress without initial partial discharge.

A water tree consists of micro-voids filled with water, typically 0.1 to 1 μm in size. Because these water-filled paths are more conductive than the surrounding insulation, they compromise the cable's dielectric performance. Although water trees grow slowly, they can bridge the insulation layer over time, triggering a complete ground fault.

Since this issue often affects medium-to-high voltage cables (e.g., 6.6 kV or higher), a failure can result in significant facility downtime and power outages affecting surrounding areas. Therefore, understanding and preventing water treeing is critical for maintaining power system reliability.

Water trees are classified into three types based on their initiation point:

Inner Vented Trees:

Originate from the interface between the internal semiconducting layer (conductor shield) and the insulation, growing outward. These are often caused by protrusions or irregularities on the semiconducting screen.

Outer Vented Trees:

Originate from the interface between the external semiconducting layer (insulation shield) and the insulation, growing inward. Like inner vented trees, these start from surface imperfections or contact with water at the outer boundary.

Bowtie Trees:

Originate within the bulk of the insulation material, growing in two opposite directions like a bowtie. These are typically initiated by contaminants (impurities) or micro-voids trapped inside the XLPE during manufacturing.

While vented trees are often linked to interface irregularities, bowtie trees are associated with material purity. Modern manufacturing techniques, such as triple-layer co-extrusion, have significantly reduced the occurrence of vented trees by ensuring smoother interfaces.

Three primary factors contribute to the formation of water trees:

• Moisture: The presence of water or high humidity is a prerequisite. Water ingress into the cable insulation or voids facilitates the process.
• Electric Field: Continuous AC voltage stress drives the movement of water and ions into the insulation (dielectrophoresis).
• Defects (Stress Concentrators): Voids, contaminants, or protrusions in the semiconducting layers concentrate the electric field, initiating the degradation process.

Unlike electrical trees, water trees can grow at operating voltages lower than the breakdown voltage. However, surge voltages from switching operations or lightning can accelerate the transition from a water tree to a fatal electrical tree.

To prevent water treeing, it is essential to eliminate the contributing factors--moisture entry and insulation defects. Effective measures include:

• Cable Selection: Use cables with water-blocking structures or Tree-Retardant XLPE (TR-XLPE) insulation designed to resist water tree growth. Cables with moisture-impervious sheaths are highly effective in wet environments.
• Environmental Control: Avoid submerging non-submersible cables in water. Ensure ducts and conduits are drained and sealed to prevent long-term moisture exposure.
• Operational Care: Minimize switching surges where possible, as voltage spikes can accelerate insulation aging.

Detection and Maintenance
Prevention is ideal, but detection is necessary for aging infrastructure. Diagnostic methods such as insulation resistance testing, dielectric loss tangent (tan δ) measurement, and residual charge measurement are commonly used to assess the condition of cables and detect water treeing before failure occurs.