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Solving the Corrosion Problems on Armour Clamps on 110 kV Single-core Submarine Cables

SREĆKO ALJINOVIĆ MARIO GUDELJ Hrvatski operator prijenosnog sustava d.o.o. Croatian Transmission System Operator Ltd. Croatia

SUMMARY After laying single-core XLPE cables between Pelješac and the island of Korčula we have become aware of chemical processes and induced currents occurring in the armour of flat laid cables. To achieve vector reduction of currents through the armour on the places where the cables leave the flat formation and transform into a trefoil formation, the rings that are the clamps have been installed to equalize the armour potential because conductors interconnect them. However, the original design of the clamps has not taken into account a combination of different materials present in seawater. Formation of a galvanic cell caused corrosion of the cable armour, which was accelerated by the current of the submarine cable armour. Since on some places the cable armour was completely damaged in the length more than 1 metre, the same type of clamps could no longer be used i.e. installed. In cooperation with the submarine cable producer, the clamp design has been modified by making clamps from the same kind of materials. The first new clamps were installed some two years ago so this paper reviews first experiences related thereto. KEYWORDS submarine power cable, armour grounding ring/clamp, corrosion, current

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SEERC First South East European Regional CIGRÉ Conference, Portoroz 2016

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1. INTRODUCTION 1.1. Reasons for installing armour grounding clamps Recently laid single-core cables, like the one been laid between Pelješac and Korčula [1], [7] on the Croatian coast of the Adriatic, Figure 1, are facing a problem of induced currents in the cable armour made from aluminium alloy on the cables lying outside the coastal belt.

The cable cores outside the coastal belt are several dozens of meters away from each other and there is no possibility of vector cancellation of magnetic flow as in the case of trefoil configuration of cores the result of which is current induction in the cable armour. The consequence of current induction is cable heating and considerably lower transmission capacity of the cable.

Figure 1: Design of 110 kV cable between Pelješac and Korčula

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To prevent negative thermal influence of induced current in the cable armour, at the places where the cable crosses over from the coastal belt to high sea, armour grounding clamps have been installed [2], [7], Figure 2, on every core of the high-voltage cable. They have been galvanically interconnected as shown in Figure 3, which resulted in vector cancelling that is reduction of the armour-induced currents.

Figure 2: Original armour grounding clamps installed on the submarine cable in Split

Transmission Area Disadvantage of these armour grounding clamps is that its component parts are made from different materials the result of which is formation of a galvanic cell causing corrosion. 1.2. Reference to the term “armour grounding clamp” In Croatia, the translation of armour grounding clamp is “a clamp for potential equalization” or “a ring for potential equalization” [7] because of its ring shape. Neither of those terms is incorrect and it is used. The first one suggests that the clamps installed on each phase are interconnected thus equalizing the potential and the other term implies their ring shape although the ring is split for installation purposes. However, the most accurate term would be “interphase clamps for vector cancelling of armour current” but it is too long and ungainly. Another English translation of this term is armour grounding connection but in the Croatian language and generally in technical papers, the preference is given to the armour grounding clamp which is also used herein.

1. Thrust ring 2. Insulation insert 3. Phase Interconnection point 4. Clamp for connecting Al armour 5. Al wire armour Ø 4 mm 6. Bolt M12

Cross-section A-A

Birds-eye view

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Figure 3: Method of interconnecting armour grounding clamps (110 kV Lora – Kaštel Sućurac cable in the vicinity of Split)

2. THE ORIGINAL ARMOUR GROUNDING CLAMPS 2.1. Use of armour grounding clamps Years-long operation of HV cables resulted in noticeable damage both of the armour grounding clamps and, which is particularly disturbing, of the aluminium alloy cable armour at the places where the clamps have been installed.

Because of the resultant damage, armour grounding clamps lost their primary function of potential equalization because the galvanic continuity between them was disrupted. This kind of damage brings about additional thermal stress of the cable and further electrochemical damage of the cable armour by “sweeping away” the material caused by circulation of electric current between the cable sheath and the sea as shown in Figure 4 [4].

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Figure 4: The principle of the galvanic corrosion development

2.2. Electrochemical basics of damage of the armour grounding clamps The reason for decomposition of the cable armour lies in the fact that every metal submerged into electrolyte establishes the potential as shown in Table [7]. The more negative the metal potential, less noble the material is. Turbulences, flow rate, pressure, higher temperature and salinity of seawater accelerate the corrosion process.

Table I : Standard ionization potential of some metals (T=298.15 K; p=101 325 Pa) Metal Ionization, Oxidation Standard

potential, V

Sodium Na → Na+ + e- -2,712

Magnesium Mg → Mg++ + 2e- -2,34

Beryllium Be → Be++ + 2e- -1,7

Aluminium Al → Al+++ + 3e- -1,67

Manganese Mn → Mn++ + 2e- -1,05

Zink Zn → Zn++ + 2e- -0,762

Chromium Cr → Cr+++ + 3e- -0,71

Iron Fe → Fe+++ + 3e- -0,44

Cadmium Cd → Cd++ + 2e- -0,402

Cobalt Co → Co++ + 2e- -0,277

Nickel Ni → Ni++ + 2e- -0,225

Tin Sn → Sn++ + 2e- -0,136

Lead Pb → Pb++ + 2e- -0,126

Hydrogen H → 2H+ + 2e- 0

Copper Cu → Cu++ + 2e- 0,345

Copper Cu → Cu+ + e- 0,522

Silver Ag → Ag+ + e- 0,8

Platinum Pt → Pt++ + 2e- 1,2

Gold Au → Al+++ + 3e- 1,42

Non

-nob

le m

etal

s C

OR

RO

DE

Nob

le m

etal

s

D

O N

OT

CO

RR

OD

E

water

Positively charged metal ions

Neutral hydrogen bubbles

Less noble metal Noble matal

Electric current

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Except for negative impact of salt water itself, the reasons for deterioration is also mechanical damage of galvanic interconnects among the cable cores. Cable divers inspected several places and found the galvanic interconnects mechanically broken, which was followed by electrochemical reaction on the rings and finally decomposition of the cable armour as shown in Figure 5.

Figure 5: Damaged armour of the submarine power cable

3. MODIFIED ARMOUR GROUNDING CLAMPS In order to minimize the impact of galvanic corrosion, vicinity of the materials with differing standard potential of ionization should be avoided. Since the cable armour has been made from aluminium alloy, in agreement with the cable producer, we have decided to have all the elements of the new armour grounding clamps, including bolts and connections, made from aluminium [3], [7]. 3.1. Process of replacing armour grounding clamps The following steps were required to replace the original armour grounding clamps with the modified ones [7]: - Disassembling of existing armour grounding clamps on all three phases; - Removal of damaged part of the aluminium armour; - Replacement of old clamps with two new aluminium one on each core (Figure 6), one on the cable armour towards open sea and the other towards the mainland, - Galvanic connection of both rings/clamps on the same core by means of two aluminium tapes, - Galvanic interconnection of rings/clamps on all three high voltage cores by means of two aluminium tapes.

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The method of connecting modified armour grounding clamps is shown in Figure 7 whereas Figure 8 shows as-built clamps at the location of CS (Cable Station) Deda on the island of Pag.

Figure 6: Design of new armour grounding clamps

Figure 7: Method of connecting new armour grounding clamps

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Figure 8: As-built new armour grounding clamps (middle phase) 4. EXPERIENCE WITH RECONSTRUCTED ARMOUR GROUNDING CLAMPS Except for 110 kV Pelješac – Korčula cable, the process of gradual replacement of original armour grounding clamps with the modified ones was completed in the middle of last year, so some experience has already been gained. While inspecting the armour grounding clamps replaced first, the cable divers noticed hardly any loss of material from the submarine cable armour [6], [7]. In Kaštela Bay, they noticed traces of aluminium corrosion on the rings (Figure 9). Likewise, on the clamps near Lora, the north coast of the town of Split (and in Kaštela Bay), one interphase connection between the middle and west cable (Figure 10) was found missing. As regards the cable armour, near CS Deda on the island of Pag (Figure 11), minor corrosion damage was noticed on two wires only. More attention should be paid during the next diving inspection to see if it is really a question of corrosion or some growth/fouling and if corrosion is spreading. At CS Toreta, on the island of Pag, no anomalies have been noticed. An in-depth analysis could demonstrate the regularities why the corrosion traces are noticed on some clamps and on some not. At this stage, it could be assumed that the behaviour of such kind could be a consequence of different depths, salinity and particularly pollution of seawater.

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Likewise, the possibilities for installation of cathodic protection should be examined by placing a sacrificial anode on the clamps. With respect to standard potential, the materials that could be used for sacrificial anodes are magnesium or specially made and activated aluminium alloy.

Figure 9: CS Kaštel Sućurac, Corrosion traces on the clamps

Figure 10: CS Lora, Corrosion affected and broken interphase connection

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Figure 11: CS Deda, Limited traces of corrosion still possible on the cable armour

5. CONCLUSION What we achieved by replacing the original armour grounding clamps on single-core cables produced from a combination of different materials with the modified ones made from the same kind of material, was that no metal is being swept away from the cable armour any longer [7]. However, corrosion occurrences could still be noticed on the clamps themselves, mostly on interphase connections. The consequence of corrosion on modified armour grounding clamps is more frequent checks and replacements of the new clamps, the interphase connections in particular, which will also involve higher maintenance costs. However, the maintenance process should not be of larger concern, because in the long run, cable service life is extended because the corrosion influence is redirected from the cable armour to the replaceable clamps. In the future, an additional cathodic protection could also be installed using a sacrificial anode to be installed on the modified clamps. The kind of material for making the sacrificial anode should still be determined [7].

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BIBLIOGRAPHY [1] „KB 110 kV Pelješac – Korčula“, Glavni Projekt, Dalekovod-Projekt d.o.o- Zagreb,

Zagreb, listopad 2010. [2] „KB veza 110 kV TS Dobri – TS Kaštela, Podmorska dionica: Lora – K. Sućurac“,

Izvedbeni projekt, Eting d.o.o – Split, Split, rujan 2005 [3] „Zamjena prstenova za izjednačavanje potencijala u plaštovima različitih faza

podmorskog 110 kV kabela“, Tehničko rješenje, ABB, listopad 2012. god [4] L. Pomenić, „Zaštita materijala“, Tehnički fakultet Sveučilišta u Rijeci, Zavod za

materijale Katedra za strukturu i svojstva materijala [5] „Održavanje i remont broda“, Tehnički fakultet Sveučilišta u Rijeci [6] Izviješće ronilačkog pregleda i snimanja spojnica za izjednačavanje potencijala

podmorskih 110 kV kabela: Pag: Deda i Toreta, Pelješac – Korčula i Lora – Kaštel Sućurac, Obrt za podvodne radove Blatnjak, Split 5. 11.2014

[7] S. Aljinović: “Saznanja vezana za modificirane spojnice za izjednačavanje potencijala na 110 kV podmorskim jednožilnim kabelima”, 12. savjetovanje HRO CIGRE, Šibenik 8.-11. studenog 2015.