Water treeing

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Water-tree or modern 3D - Lichtenberg figure in a block of pure acrylic glass with 3.81 cm on a side. It is believed that the fractal discharge pattern continues down to the molecular level.

The water treeing (WT, German "water tree formation") is one of the most important known aging phenomena of plastic-insulated high and medium voltage cables . Water trees occur when an electric field and water act simultaneously .

These tree-like structures made of water-filled micro-cavities are based on weak points in the cable insulation and lead to irreversible damage to the insulation. The growth process of water trees depends on a number of influencing factors.

Water trees can grow slowly over the entire insulation thickness without causing an immediate failure, but they weaken the cable electrically and lead to the formation of “electrochemical trees” (ECT), which then lead to failure of the cable within a few hours. Electrochemical Treeing (ECT) is the actual generic term because chemical processes are involved in the creation of these structures. However, since existing moisture is necessary for this to occur, one generally only speaks of water treeing (WT). The cause of water tree formation is the chemical degradation of polymer insulation such as XLPE or ethylene-propylene rubber , which only occurs in the presence of water and electrical stress.

history

Polycarbonate sheet with PVC cylinder, which was part of a high-voltage Trigatron spark gap switch. Corona discharges had occurred due to a design error . As a result, the PVC exposed to these discharges decomposed and hydrogen chloride gas was generated . This reacted with humidity, so that a conductive hydrochloric acid film formed on the surfaces, which led to the creation of a 2D water tree and ultimately to a premature failure of the trigatron.

Since the 1960s, the development of plastic cables led to the extensive replacement of the earth cables that had prevailed until then with polyethylene-insulated ( PE ) and later by cross-linked polyethylene-insulated (VPE) medium-voltage cables. However, after just a few years of operation, the new cables showed an increasing number of breakdowns , although the cause of the breakdown was initially unknown. The actual discovery of the phenomenon of the formation of water trees was first published in 1969 by Takao Miyashita at the Electrical Insulation Conference in Boston. In his work he examined underwater pumps whose stator was wound with polyethylene-insulated wires. In these pumps, insulation defects appeared after a very short period of operation, the cause of which was identified as tree-like structures.

In the 1970s and 1980s, many polymer cables failed after 3 to 10 years of service due to the destruction of insulation by water treeing. The largest failures occurred in North America and Northern Europe, where they quickly switched to the new technology of polymer medium voltage cables. These early cables were made almost entirely by double extrusion of the inner mesh and insulation with graphite paint and glued outer mesh. Most CV systems (English for continuous vulcanization-line) used steam hardening technology at that time.

Countries that later switched to medium voltage polymer cables, such as the United Kingdom, which made extensive use of these cables in the early 1980s, had far fewer disruptions related to water treeing. The vast majority of these cables were produced by triple extrusion on CV lines or Monosil lines. The other major change was an increased effort to ensure the cleanliness of materials during manufacture. These cables are still in use after more than 20 years without any significant aging problems due to the formation of water trees. However, different samples of serviced, aged cables from different manufacturers, covering the entire spectrum of materials, show evidence of water trees.

Types of Water Trees

A distinction is made between “Bow Tie Trees”, which only appear inside the PE insulation, and “Vented Trees”, which start from the boundary layers. The bow tie trees (size a few to a few hundred µm) stabilize in the course of time and are therefore considered to be harmless today - if they do not grow too large. However, the Vented Trees, however, can continue to grow steadily with constant moisture supply and thus in the end - as they are conductive due to the high moisture content in their structure - weaken the insulation so far at great extent that it eventually (after many years) to voltage breakdown comes the cable .

These forms of destruction through water treeing (WT) develop over the course of years in cables laid in the moist soil. They can only be seen under the microscope after coloring with a special solution.

causes

Great efforts have been made to investigate the causes of these errors and to develop preventive measures . The following main points come from the multitude of technical articles that have been written on the subject.

  • Water trees require a sufficiently high moisture content in the insulation to grow.
  • Water trees only grow when the electrical load exceeds a threshold (so they do not affect low voltage cables).
  • Water trees arise from inhomogeneities within the insulation or at the interfaces of the insulation.
  • Steam curing creates thousands of micro-cavities in the insulation, making it possible to store a higher level of moisture and also to provide places for the initiation of water trees.
  • Some additives can slow down the growth of water trees. These additives tend to be highly polar and degrade the dielectric properties of the insulation.
  • Lacquered umbrellas do not provide a perfect transition to insulation and are more of a starting point for water trees to grow.

Reduction through manufacturing processes

In order to avoid the ECT or the WT, the decisive cause of which is the presence of moisture in the cable, the cable constructions were designed in such a way that along the conductor and the outer shielding and across the sheath (PE) and the outer shielding the Advance further or the diffusion of water is hindered. With 110 kV cables , instead of the PVC or PE sheath that was previously used over the shielding , a layered sheath of closed aluminum foil with applied PE is applied, which excludes water diffusion from the ground into the cable dielectric.

As studies have shown, an " impregnation " of PE with the insulating gas SF 6 (sulfur hexafluoride) leads to an increase in the electrical strength of the insulating material. This is due to the fact that the gas SF 6 diffuses into voids in the structure of the PE and there, through the accumulation of electrons, prevents the provision of charge carriers to initiate a breakdown.

There is still a technical discussion of many water treeing problems, but four key factors in reducing water trees are generally recognized.

  • No use of steam-hardened lines from CV systems.
  • Minimizing the contamination of insulation and semiconductor shields.
  • Maintaining a smooth insulating boundary layer.
  • Triple extrusion.

The widespread use of dry cure lines with better material handling and triple extrusion has greatly diminished water treeing as the cause of early onset failure.

exams

International cable specifications do not consistently cover the topic of water treeing. Some manufacturers list the use of water tree retardant materials without any qualification or testing criteria. This is an inadequate requirement, as materials that have been classified by the suppliers as "water tree inhibiting" are not in themselves sufficient to prevent failure by water treeing. Even specifying a certain material quality is not sufficient, since defective manufacture can still lead to problems.

Accelerated aging tests have been developed that correlate well with service experience and distinguish between good and bad cables.

The principle of the test is to age the cables by applying a surge voltage while the cables are in contact with water and then to perform a breakdown test to determine the residual dielectric strength of the insulation.

There are two main tests in Europe: one requires aging for a period of two years at a normal frequency of 50 Hz, the alternative speeds up the process by using an increased frequency of 500 Hz, reducing the aging time to up to 3000 hours (125 Days) is reduced. This test was originally developed by KEMA , who are recognized as global experts in the aging of polymer cables in water and who have carried out years of intensive research on the subject. Published work by KEMA, DOW, BICC, SINTEF and others demonstrate the equivalence of the tests. Although the test equipment has special requirements, the shorter test duration makes it very attractive to both manufacturers and users of cables.

In this test, the cables are preconditioned by saturation with water and then subjected to 3000 hours in water at 2.5 times the nominal voltage with a frequency of 500 Hz in order to further accelerate the aging process.

The aged cables are then subjected to a voltage breakdown test and must meet certain threshold values ​​in order to pass the test.

It is important to note that the test is not a material test. Choosing a good quality material is critical to achieving good electrical performance and long life, but it is not enough in and of itself. The manufacturing process must also meet the required high standard. Any contamination of the insulating material or at the junctions to the insulation can lead to the potential formation of water trees and thus to premature failure. Therefore, great care must be taken to ensure that the cleanliness of the material is maintained, that the extrusion line uses a modern triple head with dry curing technology and that the process conditions are carefully controlled. These measures in combination with a strict quality assurance and control system prevent contamination or deterioration during the extrusion process. It is the combination of a high quality process with a suitable material that gives the cable the necessary properties to ensure a long service life.

Individual evidence

  1. ↑ M. Muhr, R. Strobl, R. Woschitz: WATER-TREEING - An aging phenomenon in plastic-insulated medium-voltage cables . In: e & i electrical engineering and information technology, Volume 115, Issue 5, pp 229–235 . May 11, 1998. doi : 10.1007 / BF03159576 .
  2. ↑ Boxue Du et al .: Understanding Trap Effects on Electrical Treeing Phenomena in EPDM / POSS Composites . In: Scientific Reports, Volume 8, Article number: 8481 . May 31, 2018. doi : 10.1038 / s41598-018-26773-y .
  3. ^ Takao Miyashita: Deterioration of Water-Immersed Polyethylene-Coated Wire by Treeing . In: IEEE Transactions on Electrical Insulation, Volume: EI-6, Issue: 3, Sept. 1971 . September 1971. doi : 10.1109 / TEI.1971.299145 .
  4. ↑ a b c Manfred Beyer, Wolfram Boeck, Klaus Möller, Walter Zaengl: High voltage technology: Theoretical and practical principles . In: Springer-Verlag Berlin Heidelberg . 1986. doi : 10.1007 / 978-3-642-61633-4 .
  5. ↑ a b c d e f g Water tree aging of polymeric cables. In: ducab.com, Ducab Powerplus Medium Voltage Cables. 2019, accessed August 26, 2019 .
  6. ↑ An accelerated aging test on the basis of 500 Hz for water treeing in cables. In: BANKS VAA, BICC Cables, Wrexham, UK, FAREMO H., Norwegian Electric Power Research Inst., Trondheim, Norway, STEENNIS EF, KEMA, Arnhem, Netherlands. 1995, accessed August 30, 2019 .
  7. ^ A review of the influence of frequency on accelerated aging of PE and XLPE cables. In: CRINEJ.P., Technology Consultant, Brossard, Canada, JOW J., UnionCarbide, USA. 1999, accessed August 30, 2019 .
  8. ↑ Water tree accelerated aging tests for MV XLPE cables. In: EF Steennis, WSM Geurts, and GJ Meijer, Kema, Arnhem, The Netherlands. 1999, accessed August 30, 2019 .
  9. ^ Long term wet aging of extruded dielectric cables. In: VAA Banks, RP Noyes, J. Vail, and RN Hampton, BICC Cables Ltd, Erith, United Kingdom. 1999, accessed August 30, 2019 .