What Is Trees - Insulation?

Trees, in the context of electrical insulation, are branching channels of degradation that form within solid dielectric materials under sustained high electric stress. The term describes the visual resemblance of these channels to the branching structure of a natural tree when viewed under microscopy. Two principal varieties are recognized: electrical trees, which form through localized partial discharges and physical erosion of the polymer matrix, and water trees, which form through electrochemical degradation in the presence of moisture. Both types represent progressive failure mechanisms in polymeric insulation and are among the primary causes of premature breakdown in high-voltage power cables, transformers, and electrical equipment operating over extended service lives.

The phenomenon was first systematically studied in cross-linked polyethylene (XLPE) cable insulation during the 1960s and 1970s as utilities began deploying solid-dielectric cables at transmission voltages. Unlike gas-filled or oil-filled insulation systems, solid polymers confine degradation locally and allow it to propagate for months or years before causing complete failure. This slow, progressive nature makes treeing one of the central concerns in estimating insulation life and scheduling cable replacement or remediation.

Electrical Tree Initiation and Propagation

Electrical trees initiate at points of elevated electric field intensity: voids within the dielectric, contaminants, conductor irregularities, or geometric defects at interfaces. When the local field exceeds the partial discharge inception voltage, repetitive microdischarges erode the polymer, creating hollow, carbonized channels that extend through the material in a branching pattern. Channel diameters typically range from several to tens of microns. As the electrical tree grows, it progressively reduces the effective insulation thickness, lowers the partial discharge inception voltage of the surrounding material, and reduces electrical resistance, accelerating failure. Research published in High Voltage by the IET on electrical tree degradation in high-voltage cable insulation reviews how tree morphology, categorized as bush-like, branch-like, or filamentary, is governed by the applied voltage waveform, frequency, and the crystalline structure of the polymer.

Water Trees and the Role of Humidity

Water trees differ from electrical trees in that they do not require partial discharge activity and do not leave carbonized channels. Instead, they form through the combined action of electric stress and moisture diffusion into the polymer. In XLPE cables, water infiltrates from the environment or from absorbed moisture in the polymer itself and accumulates at micro-voids and defects under the influence of the alternating electric field. According to SINTEF's research on water treeing in power cables, initiation theories include mechanical void formation under repeated stress cycles and electrochemical degradation driven by galvanic action between conductor and metallic screen. Humidity and water ingress are therefore direct precursors to water tree formation. Dry cables with metallic moisture barriers are significantly more resistant, while cables designed without water barriers can develop water trees within a few years of installation in wet environments.

Tree Inhibition and Insulation Life Assessment

Engineering responses to treeing include voltage stabilizer additives blended into the polymer compound, which dissipate localized energy and suppress partial discharge activity. IEEE Xplore documents research on electrical tree inhibition by voltage stabilizers that demonstrates additives extending time-to-breakdown by factors of three to ten in laboratory specimens. Insulation life is assessed through accelerated aging tests in which cables are subjected to elevated voltage and temperature for thousands of hours, allowing extrapolation to service conditions through inverse power-law and exponential degradation models. Online partial discharge monitoring during service is a standard diagnostic method for detecting active electrical trees before complete insulation failure occurs.

Applications

Trees in electrical insulation are a critical concern in a range of fields, including:

• High-voltage underground power cable design and life assessment
• Transformer and switchgear solid insulation systems
• Power electronics modules where polymer-encapsulated devices operate under repetitive voltage stress
• Offshore and submarine cable installations where humidity and water ingress are unavoidable
• Nuclear power plant cable aging management programs