No cable is made to last forever, which is why damage and failure will inevitably affect even the most rugged of cable designs that any cable supplier in Singapore has to offer. One of the most well-known examples of these is the water treeing phenomenon, which is characterised by the occurrence of dendritic cracks in the insulation of high-voltage cabling. Below, we take a closer look at what causes this type of damage, how it can lead to insulation breakdown and major accidents, and the countermeasures to prevent it in the first place.
Understanding Water Treeing in Electrical Cables
Water treeing is a degradation process that occurs in cross-linked polyethylene (XLPE) insulation, a thermoset insulating material that is commonly used in high-voltage power cables. When these cables are exposed to both moisture and an alternating current (AC) electric field, microscopic cracks can develop and branch out in a tree-like pattern, hence the name “water treeing.” This phenomenon weakens the insulation’s integrity over time and, if unchecked, can lead to serious electrical failures.
There are two primary types of treeing in cable insulation: water trees and electrical trees. While both cause dendritic, or branch-like, cracking, their sources differ. Water treeing originates from moisture in areas where cables come into contact with water, such as underground or underwater settings. In contrast, electric trees typically develop due to high local electric fields and do not necessarily involve moisture.
Water trees consist of microscopic water droplets, typically 0.1 to 1 micrometre in size, which increases the conductivity within the insulated material. This heightened conductivity reduces the insulation’s overall effectiveness, eventually leading to insulation breakdown if not addressed. High-voltage cables operating at 6kV and above are the ones that tend to be especially vulnerable. If water treeing leads to a breakdown, it can result in a high-voltage ground fault, potentially causing power outages in entire facilities or even larger grid areas.
The impact of such failures is significant. Beyond power outages, they can cause costly equipment damage, disrupt operations, and may even lead to legal liabilities if surrounding areas are affected. To mitigate these risks, regular inspections and preventive measures are essential to detect early signs of water treeing and preserve the reliability and safety of high-voltage power cables.
Types of Water Treeing in Electrical Cables
Water treeing in high-voltage power cables is categorised into three types based on growth origin and direction, each affecting insulation differently.
- Inside Vented Tree
Originating in the inner semiconducting layer between the conductor and insulation, this treeing type forms from tiny protrusions that create cracks. These cracks extend outward, gradually weakening the insulation from the inside.
- Outside Vented Tree
Beginning at the outer semiconducting layer, this type grows inward, with cracks moving toward the insulation core, initiated by outer protrusions.
- Bowtie Water Tree
This type, named for its shape, forms within the insulation material, often due to air pockets or contaminants. It spreads from a central point in multiple directions, causing progressive damage.
All types of water treeing degrade insulation over time. If left unaddressed, they may lead to pinholes, electrical faults, and, ultimately, cable failure by creating conductive paths from the conductor to the shield.
Key Causes of Water Treeing in Cables
Water treeing in power cables stems from several factors that disrupt the cable’s insulation integrity over time. This degradation is primarily attributed to gaps, hydrothermal effects, and the influence of electric fields, which together accelerate the water-treeing process.
1. Gaps in Insulation and Layers
Gaps are small spaces within the insulation layer caused by various factors such as the presence of air bubbles, impurities introduced during manufacturing, or physical stress during installation.
2. Hydrothermal Effects
Exposure to moisture is a significant contributor to water treeing. Cables that operate in wet or submerged conditions are especially vulnerable, as water can gradually seep into the insulation.
3. Electric Field Influence
Water treeing can also be triggered by the presence of high-voltage electric fields, especially during unusual electrical events like lightning surges and circuit switching. These surges create localised voltage spikes, which in turn cause partial discharges or corona effects within the insulation. These discharges exert additional stress on weak areas in the insulation, accelerating erosion and fostering the growth of water trees.
Over time, as these conditions interact, the electric field near any gaps or impurities becomes distorted, intensifying the stress on the insulation material. This creates a synergistic effect, where the combined presence of moisture and electric stress causes the water trees to extend further, forming characteristic branching structures that weaken the cable’s insulation.
Measures to Prevent Water Treeing in Cables
Preventing water treeing in cables involves reducing insulation gaps, moisture exposure, and high-voltage surges. Key preventive steps include:
1. High-Quality Insulation Materials (EE Cable)
Using high-quality, water-resistant EE-rated cables can greatly reduce water tree formation. These cables minimise entry points for particles or air bubbles, decreasing micro-gaps where water trees originate and improving insulation uniformity.
2. Limiting Moisture Exposure
Avoid prolonged exposure of cables to water or high humidity. Keeping cables dry prevents moisture buildup in insulation gaps, which is critical in high-voltage and wet environments, like marine installations.
3. Managing Power Interruptions
Reducing abrupt shutdowns limits switching surges that stress insulation. By carefully controlling power interruptions, voltage surges—and thus water tree risk—are mitigated.
Inspection and Detection of Water Trees
Regular inspections can identify water trees early, preventing insulation breakdown. Common detection methods:
- DC Leakage Method: Detects irregularities in leakage current that indicate weak insulation points caused by water trees.
- AC Superimposition Method: Uses a modified frequency to detect signal variations in affected areas, allowing real-time detection of potential insulation issues.
Routine monitoring and maintenance are vital to ensure the reliability of high-voltage cables. These preventive measures help utilities manage water treeing risks and ensure dependable cable performance.
Conclusion
Water treeing in cables, once a significant reliability issue, has become much more manageable thanks to advancements in cable manufacturing and materials. Techniques like co-extrusion, which form insulation and semiconductive layers simultaneously, have minimised voids, creating a more resilient cable structure. The development of tree-retardant cross-linked polyethylene (TR-XLPE) has also been instrumental in extending cable lifespan, particularly in high-voltage and underground distribution systems. With these improvements, newer cables now offer enhanced durability and reliability, ensuring that water treeing is far less of a threat to modern infrastructure.
To get the latest high-quality cables for your application, don’t hesitate to reach out to us at Cable Source today. As one of Singapore’s most trusted cable suppliers, we boast a supply, reach, and expertise that only a few other cable stockists can equal. Check out our extensive instrumentation cable catalogue today, or speak to our experts about your specific requirements and we’ll guarantee to deliver exactly what you need.