What is Water Trees & Electrical Trees in cables?: Exploring the Causes and Consequences of Cable Insulation Failure

What is “Treeing” in High-Voltage (HV)?

Treeing is the result of damage or weakness to a material that has dielectric properties.

• Vacancies,
• Gaps,
• Air bubbles, or
• Contaminants

are often the underlying reason.

Insulation that has defects is more likely to have partial discharges.

Longevity is another factor that may contribute to treeing in high-voltage and medium-voltage cables, as well as cables made of cross-linked polyethylene (XLPE).

What is meant by Water Tree?

Insulation on cables may develop a fault that resembles a tree and is filled with water. This defect is known as a “water tree.” In most conditions, the faults are caused by

• Gaps,
• Impurities, or
• Other imperfections.

The trees have the potential to cause the insulation to break prematurely. In most cases, the direction in which water trees spread is parallel to the electric field.

They can only take place when there is water present within the insulation. When conditions are dry, it is typically impossible to see them with the direct contact. There are specialised dying methods available, which may be used to make them more apparent.

It is more common to find water trees in areas of cables that are in a condition of strain, such as bends in the wire.

Even if it is feasible to define the conditions that might lead to the development of water trees, the precise mechanism or the chemical processes that are involved in developing them. The cable’s breaking strength is decreased as a result of the presence of water trees.

What is the water-tree phenomena in cables?

The water treeing phenomena can take place in cables operating at high voltage as well as in cables operating at medium voltage.

This phenomenon causes an imminent risk since, in the long run, it will inevitably result in damaged cables.

An advanced stage of arborealization is characterised by

• The breakdown of the insulation and
• The establishment of contact between the vases in the wire.

What causes this phenomenon?

This phenomenon is characterized by extremely tiny growths that are empty inside the insulator along with filled with water molecules as a result of moisture leakage.

Whether the insulation is made of PVC or XLPE, this can result in the emergence of the afforestation phenomenon, particularly if the conductor’s surface is uneven and the electric field is irregular.

The irregularity of the electric field may contribute in the presence of high-value concentration points of the field in areas where the conductor and the insulation have protrusions, therefore, the insulator or its edges develop a thin, thread-like crack, and this crack’s branches grow in all directions to resemble a tree, giving rise to the phenomenon.

This phenomenon ultimately results in the insulator collapsing and the cable being destroyed.

What is meant by Electrical Tree?

Electrical Trees are developed in the lack of water in dry environments. They are caused by

• Empty spaces,
• Contaminants and
• Faults in the insulation.

High electrical tension which changes direction such as AC wires can potentially intensify the phenomenon. Occasionally, water trees can develop into electrical trees.

These trees are associated with partial discharge which can accelerate insulation breakdown. Electrical trees are plainly apparent to the observer.

Types of Electrical Trees

In general terms, trees can be divided into two categories:

• Vented trees and
• Bow-tie trees.

Vented Trees

Vented trees are trees that have their roots in one electrode but extend their branches to another electrode. Because they have exposure to air, which helps partial discharges, these trees have a higher rate of growth.

Bow-tie Trees

Bow-tie trees are ones that start out inside the insulation and grow outward. Due to the reason that they are generated inside the insulation, they are unable to reach air, which results in a restricted amount of partial discharge.

They advance at a more gradual pace than vented trees.

What is electrical treeing partial discharge?

Electrical trees develop when there is an immense amount of electrical stress on them, and they are linked to phenomenon known as partial discharge (PD), which is the localized breakdown of the dielectric properties of a segment of an insulating system.

Detection and identification of electrical trees

Electrical trees may be noticed and found using partial discharge monitoring.

Because the measurement results of this method do not allow for absolute interpretation, data acquired during the operation is compared with measurement values of the exact same cable recorded during the test.

This enables for the easy and rapid categorization of the dielectric state of the cable under test (fresh, highly aged, defective).

A 50-60 Hz (or) a sinusoidal 0.1 Hz VLF (very low frequency) voltage can be used to evaluate the degree of partial discharges.

The turn-on voltage, a major measurement criterion, can vary by more than 100% between 50-60 Hz measurements when compared to a 0.1 Hz VLF (very low frequency). 

Sinusoidal alternating current (AC) source at the power frequency (50-60 Hz) as required by IEEE standards 48, 404, 386, and ICEA standards S-97-682, S-94-649, and S-108-720.

Modern PD-detection systems use digital signal processing software to analyse and show test findings.

The majority of insulation faults may be identified by analysing the PD signals acquired during the test using the appropriate equipment. They are usually shown in a partial discharge mappings format.

A phase-related representation of partial discharges may provide additional important information regarding the device under examination.

A suitable measurement report includes:

• Calibration pulse (in line with IEC 60270) & end detection
  • The measuring setup’s background noise
• PDIV (partial discharge inception voltage)
• Partial discharge level of 1.7 V_o
• PDEV (partial discharge extinction voltage)
• PRPD (phase-resolved partial discharge pattern) for advanced understanding of partial discharge phenomena (optional)

How does a tree affect cables?

It is a destructive process caused by partial discharges that spreads through the strained dielectric insulation in the shape of tree branches. A typical breakdown process and cause of electrical failures in underground power lines is treeing of solid high-voltage (HV) cable insulation.

What is the difference between Water Treeing and Electrical Treeing?

Water Treeing	Electrical Treeing
Water treeing has been possible at low field strength (1kV/mm).	Electrical treeing is with high local strength
Water treeing has no visibility	Electrical Treeing is an apparent indication of an electrical failure.
Water Treeing characterized by an exceptionally low rate of tree discharge recognizable	Partial discharge accompanied by electrical treeing

Can Water Treeing be repaired once it occurs?

The problem with the water trees can been fixed, and the growth of water trees in newly manufactured cables, as much as decreased as a direct result of

• Improvements in the design,  
• Manufacturing process, and
• Insulation compound.

Can Electrical Treeing be repaired once it occurs?

There are ways to reduce or repair electrical treeing impairment:

• Root cause identification and treatment: Determine the cause of electrical treeing. Voltage spikes, dampness, and pollutants are common causes. Fixing the root cause can prevent tree-like channels.
• Remove the affected area: Localized electrical treeing may be prevented by isolating the damaged insulation. This may need cutting or replacing the afflicted area.
• Reinforce (or) replace the insulation: Damaged insulation may need to be reinforced or replaced. This can repair the insulation and avoid further difficulties.
• Maintain the insulating system: Regular maintenance & monitoring can detect and fix faults before electrical treeing. Keep insulation dry, clean, & contaminant-free.

Conclusion

Water entering the high voltage cable causes the formation of water trees, which travels through the insulation through microcracks to the core via the insulation shield. Trees produce anything like a needle in isolation, disrupting the electric field distribution in the cable. Electrical trees are formed in a similar manner. The process is local deterioration of the material, which becomes conductive & affects the distribution of the electric field in the insulation.