Water Treeing of XLPE Cables during Dynamic Mechanical Tension

Erling Ildstad and Truls A. Lindseth Department of Electric Power Engineering

Norwegian University of Science and Technology NTNU, Trondheim, Norway

Erling.ildstad@elkraft.ntnu.no

Hallvard Faremo Department of Power Technology

SINTEF Energy Research

Trondheim, Norway

Abstract- The main purpose of the work presented in this paper was to experimentally examine possible water tree enhancement caused by dynamic mechanical tension. The experiments were performed using samples of 12 kV XLPE cables exposed to static and dynamic tension. A test rig was designed allowing application of static and dynamic strain at mechanic oscillation frequencies of 0, 0.1 and 0.01 Hz at a maximum amplitude of 6 % elongation. During ageing the cable samples were soaked in tap water at 30°C and an effective 50 Hz AC voltage of 14 kV (EMax= 5.2 kV/mm) was applied across the cable insulation. The degree of water tree degradation was characterized using optical microscopy investigation of 0.4 mm thick methylene blue stained slices. Both the density and growth rate of bow-ties and vented water trees were found to increase significantly when applying mechanical tension. Compared to the non-strained cable sections the number and length of water trees were found to increase by approxi-mately 100 and 50 %, respectively. No significant difference in water tree lengths were observed between samples aged at 6% static and dynamic mechanical strain. - The results are in good agreement with the mechanical damage theory of water treeing. Keywords; XLPE Cable, mechanical tension, water treeing

I. INTRODUCTION Over the years the problems associated with premature

electrical failures caused by water treeing of XLPE insulated cables have largely been reduced. The main reason for this is application of new improved materials and use of watertight cable designs [1]. Metallic sheaths are introduced in order to prevent ingress and absorption of water into the insulation and application of new more water tree retardant insulations systems, including cleaner insulation and semiconducting materials. Nevertheless, it is considered economically favorable to use cables without metallic sheath barriers, for example in case of system voltages below approximately 36 kV and in some special off-shore application. This is parti-cularly so, when considering new design requirements for submarine power cables, intended for application as free floating grid connections for offshore oil-and gas installations

and wind-farm towers. It is very likely that during installation and service, such power cables will be subjected to high dynamic mechanical stresses; due to the weight of the cable and ocean wave movements of the installations.

The main purpose of the work presented here has therefore been to experimentally study how water tree degradation of XLPE cable insulation will be affected by static and dynamic strain.

II. MECHANICAL DAMAGE THEORY OF WATER TREEING The mechanical damage theory assumes that water

treeing is due to mechanical overstressing, caused by a combined effect of external mechanical stress and electric stress. Prev ious investigations have shown that init iation and growth of water trees in polymeric cable insulation will be enhanced by tensile mechanical tension and retarded by compressive stresses [2 and 3]. It is assumed that tensile stresses reduce the amount of energy required to cause bond scission of the polymer chains, init iating craze format ion and easing initiating of water trees.

A. Craze Formation

From fracture behavior of polymers it is known that crazing initiate cracks at stresses well below what is needed to cause bulk shear yield. Crazes are known to be localized regions of plastically deformed low density polymer, consisting of voids and polymer fibrils. Such regions are initiated when mechanical stress cause micro voids to nucleate at points of high mechanical stress concentration. Examinations of the microstructure of crazes in polyethylene have shown that the typical craze thickness of such a low density area is 0.1 - 0.5 µm, which tapers off to less than 2.5 nm at the tip of the craze [4]. Measures known to reduce stress cracking are: introduction of compressive stresses, increasing the molecular weight, annealing and addition of copolymers. In addit ion some liquid environments are known to promote crazing and crack formation, causing so-called environmental stress-cracking (ESC).

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B. Stresses Frozen-in during the Manufacturing Process In case of extruded cable insulation it is also important to consider frozen-in stresses introduced during the manufacturing process. After the extrusion and curing process the cable is rapid ly cooled from the outside. This means that the outer parts of the insulation become cold and solid at a t ime when the inner insulation parts is melted and expanded at a higher temperature. During further cooling also the internal parts will try to shrink. This introduces internal tension forces, which then causes compression of the outer surface. In addition the large thermal expansion of polymers compared to that of the conductor, will prevent the insulation to shrink to its equilibrium d imension. These mechanis ms results in a rather complex distribution of frozen-in mechanical stresses in the insulation, including both longitudinal and radial residual stress components [7]. Typical magnitudes of such longitudinal tension stresses have been measured to be in the range of 10-30 MPa, values which are comparable to or h igher than the yield strength of polyethylene [5]. – It is therefore likely that, crazes and micro cracks are present in the cable insulation. - Based upon this it is also reasonable to assume that after manufacturing the magnitude of the tensile stress will be higher close to the conductor than at the outer insulation surface, where craze formation may be prevented by frozen in compressive stresses. C. Water Tree Formation

Experience shows that initiat ion and growth of water trees is facilitated only when AC voltage is applied and the relat ive humid ity of the insulation and the semi-conductive screens are sufficiently h igh to allow format ion of liquid water within micro cracks or crazes. Generally, a tensile stress component is needed at the tip of the water filled channel to provide the driving force for creation of new craze regions. -In case of AC voltage application, such tensile stress will be generated by the highly inhomogeneous electric field and the Maxwell forces directed perpendicular to the interface between the water filled zone and the insulating material. Its magnitude per unit area is given by:

(1)

where ε1 and ε2 are the permittivity of the two reg ions and En1 and Et1 are the normal and tangential components of the electric field at the boundary, respectively. Thus in the near vicinity of the t ip, tensile Maxwell stresses will appear causing a pulsating 100 Hz compressive stress acting perpendicular to the crack surface. As schematically illustrated in Fig. 1 this will result in local tensile stresses at the tip of the water filled craze. Due to high local electric field enhancement these forces may become higher than 10 MPa, which is sufficient to create new low density crazing zones at the tip of a water filled crazing zone, into which

water are pulled by the electric fo rces [6]. In addit ion it can be shown that condensation of water is eased in this high field reg ion. Thus a situation for further growth of vented and bowtie water trees is established. It is therefore likely that the effect of apply ing mechanical tension to cable insulation is to ease micro voids and subsequently water t ree formation, while compressive forces are expected to have an opposite effect.

Figure 1: Sketch showing a mechanical model for initiation and growth of

vented water trees.

III. EXPERIMENTAL

A. Water Tree Ageing of Dynamically Stressed Cables without Conductor

All experiments were performed using 3 m long samples of a 12 kV triple ext ruded XLPE cable with copper conductor. During ageing the samples were clamped to the experimental set-up schematically shown in Fig. 2. The test-rig was designed such that cable insulation was simultaneously exposed to AC voltage, tap-water and oscillating mechanical tension. Due to the non-constrained cable samples it was not possible to apply compressive forces. Gear Alignment Cable samples in water filled tubes

Figure 2: Sketch of the experimental set-up for water treeing examination during dynamic testing.

E-field

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Prior to assembling the test objects a special technique was developed to remove the conductor from the test objects. The purpose of removing the conductor was to ease water absorption and to facilitate uniform longitudinal stretching of the insulation of the examined sample. In addition the force needed to stretch a cable without conductor will be limited to relatively low values; determined by the elasticity modulus and the effective cross section of the insulation. - Results from previous measurements of stress-strain relations have shown that the apparent elasticity modulus of XLPE cable insulation is around 80, 53 and 20 MPa at 30, 40 and 50°C, respectively. At 30°C the stress was found to be proportional to the strain up to approximately 9% at a yield limit of about 12 MPa [8].

Removal of the conductor was done by first pressing oil in between the conductor strands. This was done in order to reduce the adhesion between the copper conductor and the semiconducting screen, such that it became feasible to pull the conductor out of half the length of the 3 m long cable sample. When clamping and fixing the cable sample to the test rig, the conductor provided sufficient mechanical support on one side, while a steel tube was inserted into the conductor channel to provide mechanical support in the section without conductor strands.

Two electric motors, with different speed and gear exchange, were used to provide the longitudinal dynamic mechanical strain at a maximum strain of 6 % elongation at the mechanical oscillation frequencies of 0.1 and 1 Hz. In addition two reference test objects were mounted in the set-up, one without mechanical strain and one with a static strain of 6 %.

During ageing the cable conductors were filled with water and all cable samples were soaked in circu lating tap water at 30°C, as shown in Fig. 3. An effective 50 Hz ac voltage of 14 kV (EMax = 5.2 kV/mm) was applied across the cable insu-lation. The ageing t ime was limited to 3 weeks, due to severe mechanical wear of components used during the rapid 1 Hz testing. Mechanical Cable Clamping terminations

Figure 3: Photo showing the XLPE cable sample undergoing water tree

testing during dynamic mechanical tension.

B. Microscopy Examination of Aged Cables Each 20 cm long cable sections were helically cut into

0.40 mm thick slices using a lathe. The helicoids were then dyed according to the standard CIGRE methylene b lue pro-cedure, and investigated with an optical microscope at 25-100 times of magnification. The number and maximum leng th o f bow-t ie and vented water t rees from the conductor and ins u lat ion screens were examined in each s lice.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

A. Examples of Water Trees Micrographs of selected bow-tie and vented water trees

are shown in Fig. 4. This implies that the initiation takes place relatively quickly, and that 3 weeks of ageing was sufficient for the water trees to grow to lengths longer than the detection limit (~50 µm).

a)Vented trees at insulation screen b)Bow-tie tree

Figure 4: Micrographs showing examples of water trees after 3 weeks of ageing at static strain

Table I shows a summary of the results obtained by the microscopy examination after 3 weeks of ageing. The results are based upon microscopy examination of minimum 10 slices, showing the number of trees per cm cable and the average lengths of the longest observed water tree in each slice.

TABLE I: SUMMARY OF RESULTS

Test object

Vented (Conductor

screen)

Vented (Insulation

screen) Bow-ties

Density

n [cm-1]

Lmax average

[µm]

Density

n [cm-1]

Lmax average

[µm]

Density

n [cm-1]

Lmax average

[µm]

Reference

(no strain)

0.4

133

0

0

405

119

Static strain

6%, f=0 Hz

1.5

175

0

0

570

122

Dynamic strain

6%,f=0.1 Hz

4.1

180

1.4

109

880

122

Dynamic strain

6%, f =1 Hz

1.7

211

0

0

835

97

630

Figure 5: Average lengths of the longest observed water tree in each slice

after 3 weeks at the indicated mechanical conditions.

Figure 6: Observed number of water trees per cm cable after 3 weeks at the

indicated mechanical conditions. Results presented in Table I and the figures above, show that the effect of applying either static or oscillating mechanical tension was to sign ificant ly increase the number of water trees. Compared to the non-strained cable sections the number of bow-ties was found to increase by a factor of about 2, while the number of vented trees from the conductor screen was found to increase by about 3 to 8 times. The largest increase was observed in case of 0.1 Hz mechanical oscillations. Extremely few vented trees were found to originate from the outer insulation screen. This was somehow unexpected as the applied tension was assumed to be evenly distributed over the insulation cross-section. The observation, however, supports the assumption that frozen in compressive stresses at the outer insulation surface may balance the effect of the applied strain. On the other hand one may argue that the 1.8 times electric stress enhancement at the conductor screen may contribute to the difference. This is rather unlikely as the effective applied electric field stress at the outer insulation interface of 2.9 kV/mm is sufficiently above the critical field stress for water tree growth. Secondly, bow-t ie trees were found to be evenly distributed over the insulation cross-section, also close to the insulation screen.

In all cases the longest vented trees were found to be longer than the longest bow-tie trees and their growth rate increased by about 30 % when 6 % elongation was applied, while the average length of the longest bow-tie trees were not affected by the applied mechanical tension.

Thus, it appears that the effect of mechanical tension is to strongly increase the number of water tree initiation sites, while the rate of growth is more modestly affected. This is an observation in support of the hypothesis that when a water tree is initiated it will continue to grow at a rate mainly determined by the local electric field, mechanical stress, supply of water and the strength of the surrounding insulation. The results indicate minor difference in water tree lengths between samples aged at static and 0.1 Hz oscillating mechanical strain. The enhanced growth rate at 1 Hz oscillations indicates that the viscoelastic properties of the insulation need to be considered. It is indicated that abrupt, high frequency, elongations may become more detrimental, due to insufficient mechanical relaxation during the cycle.

V. CONCLUSIONS Application of tensile mechanical strain enhances water tree formation in XLPE cable insulation.

• This applies to both static and dynamic oscillating mechanical stress.

• Remnant compression and tension stresses, frozen-in during the production process of a cable, may also affect the number of initiation sites for vented water trees.

• Slow oscillating mechanical oscillations due to ocean wave movements at 0.1 Hz, has an impact similar to that of static mechanical load.

AKNOWLEDGEMENT

The authors would like to thank undergraduate student Malo Tanvent for his contribution to the microscopy analysis and the Research Council of Norway and the industrial partners: Nexans Norway AS, EDF R&D (France), Statoil ASA, Statnett SF, Statkraft SF and Borealis for their financial support.

REFERENCES

[l] E.Ildstad, J.Sletbak and H.Faremo: "Water Treeing and Breakdown Strength Reduction of XLPE insulation". ICSD-89. Trondheim. Norway, July 2-6.1989. [2] E. Ildstad, et al: “Influence of Mechanical Stress and Frequency on Water Treeing in XLPE Cable Insulation,” Conference Record of the 1990 IEEE International Symposium on Electrical Insulation, pp.165-168. [3] B.R. Varlow:” Electrical treeing as a mechanically driven Phenomenon”, Proc. 1998 Int. Symp. On Electrical Ins. Materials, Japan Sept. 27-30, 1998. [4] H.H. Kausch: "Polymer Fracture". 2. Ed. Springer Verlag, 1987. [5] J.W.Billing: "Examination of Mechanical Stress in Extruded Polymer Cable Insulation using Thermal Mechanical Analysis". ERA Report 89-0678R, Jan. 1990. [6] J.Sletbak, “The Mechanical Damage Theory of Water Treeing - A Status Report,” Proc.of the 3rd ICPADM, 1991, Tokyo, Japan: pp.208-213. [7] L . Olasz, P. Gudmundson: “Prediction of Residual Stresses in Cross-linked Polyethylene Cable Insulation,” Polymer Engineering & Science 2005. 45 (8): pp.1132-1139. [8] Erling Ildstad, Ståle Nordås and Truls A. Lindseth, “Water Treeing of XLPE Cables under combined mechanical and electrical stresses”, Nordic Insulation Symposium, Nord-Is 11, Tampere, Finland, June 2011.

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