Photoblog - Water Treeing in High Voltage Cables : A 230kV HPPT Termination Repair

By Chris Dodds on 4th August, 2015

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Photoblog - Water Treeing in High Voltage Cables :  A 230kV HPPT Termination Repair

This series of excellent photographs taken from site have been kindly provided by : 

  • Douglas Page
  • Instructor & Cable Splicer at Hydro One
  • Training and Development Services

As Ontario’s largest transmission and distribution electricity provider, Hydro One is committed to providing homes and businesses across the Canadian province with safe, affordable and reliable electricity.

Pictured: Water Treeing

Water Treeing

Water Treeing

Water treeing is one of the major causes of premature failure of medium, high and extra high voltage power cables. Water-trees increase cable insulation losses and in some cases can initiate electrical trees that lead to breakdown. High voltage cables are exposed to water and moisture in general condition. Water action on cable insulation systems causes changes in their properties and creates such water-trees.

The electrical degradation process of ‘treeing’ in cable polymers occurs in the presence of electrical stress and moisture. This is acknowledged as a serious cause of non-mechanical insulation degradation and cable failures.

The water tree issue is considered by most people as solved and indeed due to improvements in the design, the manufacturing process and the insulation compound the growth of water trees in new cables has been drastically reduced.

Water tree retardant cross-linked polyethylene power cables (TR-XLPE) have extended medium voltage primary distribution cable reliability and lifetime. Longer-life TR-XLPE cables have been successfully installed by North American utilities for over 25 years using advanced performance XLPE insulations on ‘wet’ designs for underground distribution systems. Further reading : TRXLPE Medium Voltage Cables (Transmission & Distribution World).

Water-treeing and the expected remaining lifetime of aged cables on high voltage power networks provides serious challenges and requires a high-skilled response by HV cable splicers - asset managers often have to maintain and service kilometres of first generation cable requiring traditional cable jointing and repair skills that are often no longer taught in today's modern utility jointing schools.

Accelerated aged tests conducted on new cables in laboratory conditions by CIRED have concluded, " bow-tie trees seem to represent an image of contaminants present in the insulation and vented trees seem to be related to the impurity content of the semi-conductive layer." Further reading : New Test Results With 3kHz Accelerated Growth Of Water Trees In Medium Voltage Power Cables (2011).

An Example Of Water-Treeing. Images Courtesy : Douglas Page

The following sequence of images convey water-treeing or arcing inside a high voltage cable termination. This a 230kV HPPT (High Pressure Pipe Type) Cable Termination or (Pothead). These images show the importance of routine oil sampling, which detected acetylene in the samples, which in turn indicates arcing or burning.

The cable splicers froze the cable below the pothead, removed the porcelain and found a high voltage cable very near failure. Then the cable splicers basically removed everything that been damaged and rebuilt the cable first, then rebuilt the stress cone. It was later found that a rubber gasket had a small imperfection that allowed contaminants (moisture debris) into the pothead, which as it accumulated had created a conductive path on and in the paper insulation, which would eventually have lead to cable failure.

The following images portray the process described above.


HV Cable 1


HV Cable 2


HV Cable 3


HV Cable 4


HV Cable 5


HV Cable 6


HV Cable 7


HV Cable 8


HV Cable 9



HV Cable 10


HV Cable 11


  • Further reading :

EHV Cable Splicing - 115kV Stop Joint (Taped) Flickr Album

Masterclass : 230kV Pipe Type Pothead Replacement /Cable Freeze 

What Is Electrical Stress?


Thorne & Derrick Specialist Electrical Distributor

Established since 1985, T&D distribute the most extensive range of LV, MV & HV Cable Jointing, Terminating, Pulling & Installation Equipment - contact us today for a competitive quotation.

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Category:  Cable Jointing - PhotoBlog

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