A Novel Approach to Design Cathodic Protection ... - IEEE Xplore

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A Novel Approach to Design Cathodic Protection System for High Voltage Transmission Cables Davide Lauria University of Naples Federico II Via Claudio, 21 Naples, 80125, Italy [email protected]

Mario Pagano

Carlo Petrarca

Cosimo Pisani

Member, IEEE University of Naples Federico II Via Claudio, 21 Naples, 80125, Italy [email protected]

University of Naples Federico II Via Claudio, 21 Naples, 80125, Italy [email protected]

University of Naples Federico II Via Claudio, 21 Naples, 80125, Italy [email protected]

Abstract -- Corrosion is an electrochemical process involving electrical current. It consists of the destruction of a metal through interaction with the environment. The phenomenon is observed in underground and buried structures, where the aging can be strongly influenced by electrical currents flowing in the earth. Methods of protection (i.e. ‘cathodic protection’) are related to the use of sacrificial anodes or more sophisticated systems, which impose a negative voltage to the metal structures to be protected. The corrosion phenomenon can be relevant also in electric power systems applications. Underground or undersea electrical cables need to be protected as consequence of stray currents flowing in the soil. The paper studies the problem of cathodic protection design for high voltage transmission cables in undersea applications. The aim is to propose a desing procedure which can be implemented by not-expert electrical designers. A practical approach, which characterizes the distribution of the voltage on the outer layer of power cable, is formalized on the basis of common simplified assumptions. Numerical simulations give evidence to the formulation according to the features of environmental conditions. Index Terms— Analytical modeling, corrosion, cathodic protection, high voltage power cable, sacrificial anode, undersea.

I.

thin jacket of polyethylene or, more frequently, of polyethylene/semiconductor material. Many technical issues have to be carefully considered when the designed solution is represented by underground/undersea power cables transmission line. Among these concerns, corrosion can represent a crucial problem, especially in undersea installations where the corrosion is accelerated by several factors (i.e stray electrical currents and electrolyte medium). In this case, the cable has to be protected by the detrimental effects of external currents action. Corrosion phenomenon occurs in metallic structures, where a basic ‘corrosion cell’ consisting of anode, cathode, electrolyte, and connecting conductor can be observed [2], [3]. When corrosion occurs, an electric current moves through the electrolytes, so that positive ions enter the electrolyte from the anode, while electrons move from the anode to the cathode via the metallic connection, Fig.1a. In simple way, this corrosion mechanism can be represented by an equivalent electrical circuit, fig.1b.

INTRODUCTION

Whilst the majority of the electric power transmission around the world is still achieved through Overhead transmission lines, cables undergrounding is recently becoming an attractive solution to face with the cogent regulatory and ecological issues [1]. Insulated power cables consist of two basic components: a conductor to carry the current and an insulation to support the line-to-ground voltage. Cable ampacity determines conductor size, whereas insulation thickness is related to the designed highest line-to-ground voltage, although voltage class is designated on the basis of the rated line-to-line system voltage. In underground applications, electrical cables are equipped by an armour, which is able to provide the reinforcement for the installation and operation. In undersea applications, an important requirement for power cables is limiting moisture and water ingress. Thus, the cable has to be even equipped by an outer barrier. This barrier is usually applied over the insulation, near the outer layer of the armour. The barrier consists of extruded lead, usually covered with a

electrolyte

C

A a) RE

ic electrolyte A

C

metallic connection b) Fig.1 – Corrosion of metallic structures: a) basic phenomena; b) equivalent electrical cell

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At the interface between metal electrodes (i.e. anode A and cathode C) and electrolyte, there is an electromotive force (EMF), called the electrode ‘standard potential’. The corrosion current ic moving through the cell is limited by the electrolyte equivalent resistance RE . There are several methods in which cathodic protection for metallic structures can be accomplished [4]. These methods are based alternatively on the growth of the cell equivalent resistance (for instance using a bitumen layer) either on the construction of a new electrochemical cell, where the protected part can be assumed as a cathode with respect to a sacrificial material assumed as anode (e.g. magnesium or zinc electrode), Fig.s 2a and 2b. Bond anode/ structure

As sacrificial anode

electrolyte

C1

C2 a) RE

ic As C1

C2

metallic connection b) Fig.2 – Cathodic protection: a) basic phenomenon; b) equivalent electrical cell

In power cable applications, the common methods currently employed for protecting metallic parts are based on using: anode beds, active cathodic protection and dynamic cathodic protection [5], [6], [7] and [8]. In undersea applications, because current can flow to metallic anode by the surrounding electrolyte, the realization of suitable cells with the use of sacrificial anodes is frequently adopted. Anode beds deal with the installation in the ground near the power cable of a material, which has an higher potential voltage than the metallic sheath that has to be protected. The standard practice is to have these beds at, or near, the power cables, where the circuit terminates. In a strict sense, the use

of anode bed is a passive cathodic protection. Sometimes, the cell system can be managed by means of an impressed current forced by a rectifier between the cathode and the sacrificial anode. The value of the current is chosen to protect metallic elements of the cable. Finally, if necessary, when some power cable installations are in environments, where periodic ground stray currents can cause degradation, a controlled current protection system can be used. This is the case, for instance, of power cables installed in proximity of electric powered transportation systems such as substation and/or light rail systems which can inject large amounts of dc current into the earth. In this case a variable current protection system can be used. The difference in stray current sensing alerts anode/rectifier units to self adjust. The type of setup is done along the length of the circuit to ensure proper protection. By investigating the specialized literature, approaches to model cathodic protection for electrical cables are few and spare. In particular, [5] analyzed the corrosion on a underground cable. The approach, which is applied to bare concentric neutral cable, is developed considering the cable as direct buried and a lumped representation is used. The work aimed to calculate the cathodic protection current. Even in [3] a lumped model to represent the cathodic protection as an equivalent electrical circuit is proposed. The model was formulated for underground metallic pipelines and buried structures and it aimed to help corrosion engineer, which adds a quantitative dimension to corrosion control. In more recent years, [6] and [7] evidenced the problem of corrosion for a submarine cable application in New York. The work pointed out the corrosion issue for an undersea cable subject to stray current and focused the attention on the configuration of the system realized for the cathodic protection. Details on the analytic model used to control the dynamic cathodic protection current were reported in [8]. In this work the indication of the assumed hypothesis were discussed and the model was developed according to a distributed model approach. This paper proposes a design approach, which in a practical way, re-organize models and results reported in the related literature. The aim is to propose a procedure which gives evidence to the main design variables basing itself on simplified hypotheses. This procedure presents the advantage to be implemented by not-expert electrical designers. It represents a first basic framework from which further studies, based on boundary elements and/or finite elements method, will start in the near future. This paper presents a procedure available for corrosion control and its applications in the field of electric power systems. A novel approach aiming at characterizing the distribution of the voltage on the outer layer of the high voltage power cables in undersea application is formalized. To overcome the limits related to the application of a modeling with lumped parameters, the approach defines the distribution of the voltage by means of a



distributed parameters model. The model is formalized taking into account also the features of the operating environment. The approach aims to determine the current which needs to be applied to the protected system, according to the assigned threshold of the current density distribution function. By considering a real-life application [9], some numerical analyses devoted to validate the designed modeling approach are presented in the last part of the paper. II.

from the surrounding medium, (ii) to protect the armor from corrosion, (iii) to offer a further mechanical protection. Bitumen, polymers like high-density polyethylene (HDPE) are generally used for the encapsulation of the cable.

HIGH VOLTAGE SUBMARINE CABLES

In submarine environment, high electric power transfers can be achieved via high voltage alternating current (HVAC) or high voltage direct current (HVDC) transmission systems. Technical and economic issues drive the more suitable choice between HVAC e HVDC solution design. At the present, typical power transmission involves the employment of cross-linked polyethylene-XLPE insulated submarine power cables [9], [10], [11] Fig.3, which consists of a series of layers: • Conductor core: the layer is the one specifically devoted to the power transfer. Conductor design can be solid, keystone shaped, stranded-compacted or stranded. It is made by copper, or, in case of higher sections, by stranded aluminum. • Conductor shield: the purpose of the shielding is twofold: 1) it ensures radial symmetry to the electric field and thus unidirectional stress to the electrical insulation; b) it ensures close contact with the insulation and the smoothing of the electric field at the conductor core by preventing the formation of air cavities. Either thermosetting or thermoplastic semiconductive polyolefin tapes may be used. • Insulation: this layer is devoted to withstand the various voltage field stresses, such as rated voltage, lightning and switching over-voltages. • Insulation shield: this layer is devoted to ensure close contact between the insulation and the metallic sheath. It prevents concentration of electric field at the interface between the insulation and the external semiconductor. • Metallic sheath: this layer is devoted to nullify the electric field outside the cable and to provide a radial waterproofing. Materials used to realize such a layer are lead alloys. The screen may also be used to carry the faulty current in the case of a line-to-ground fault. • External jacket: this jacket provides electrical insulation from the armor and from the sea. It is used to prevent the lead sheath from corrosion. The layer is either polyethylene sheath or is made of semi-conducting polymer co-extruded with the outer jacket [12]. The presence of semiconductor material layers is advantageous in reducing the thickness of layer. • Armor: this layer gives mechanical protection to the cable against external damages such as abrasions, cuts, anchors, dredges etc. Since the armor is subject to corrosion, it is often realized with galvanized copper wires or steel/stainless steel wires [12]. • Serving: this layer is devoted to (i) insulate the armor

Fig.3 – Arrangement of a single core HV XLPE submarine power cable

III. APPLICATION A model to design cathodic protection for undersea HV power cable is proposed. The model is developed under the assumption that only one specific layer of the cable is subject to corrosion. The model aims to find the relationship among the protecting current and the parameters of the system cableenvironment for any acceptable corrosion protection criterion. The following analysis takes into account that the most common radial water barrier used today for submarine cables consists of covering the extruded sheath (e.g. lead alloy) by an extruded anti-corrosion jacket [12]. Thus, focusing the proposed model on assessing the distribution of the current in the sea electrolyte, the external jacket made of semiconductive polymer will be considered as the external layer of the cable. It will be protected by using an active cathodic protection system. A.

Proposed model The aim of the proposal is to control the potential of the semi-conductive jacket so that it assumes a negative potential with respect to a sacrificial bed. This is obtained by applying an external DC electrical power source between the sacrificial anode and the metal screen. In order to design a reliable anti-corrosion system, the electric field distribution for areas inside (i.e. between metal screen and semi-conductor) and outside the cable (i.e. though the sea water) needs to described. The results of the model give indications on the feasibility of the protection approach and on its limitations as a function of cable geometrical parameters, such as length or diameter, or physical parameters like the electrical conductivity of surrounding water or outer sheath, etc. The schematic set-up used for the formalization of the model is shown in Fig. 4. It represents an unitary length L of power cable submerged in sea water (1) along the z-axis. The



inner layer of the cable (i.e. (4)) is covered by the metal screen layer (3) and anti-corrosion jacket (2). A DC voltage source is connected between the sacrificial anode (5) and the metal screen. R2 and R3 are the external radii of the metal and semi-conductor layers, respectively.

data used for the analysis are reported in Tab.I. TABLE I POWER CABLE RELEVANT GEOMETRICAL DATA Layer Lead screen Semi-conductive jacket

Outer Radius 3 cm 3.5 cm

The main data for the applications, collected in [8] and [13], are reported Tab. II . Fig.4 – Schematic set-up for analysis

The cable of length L can be represented by a cascade of elementary sections of length dz, as shown in Fig.5. Any section is characterized by transversal and longitudinal equivalent parameters. In particular, rw ([Ω/m]) is the equivalent resistance per unit of length of the electrolyte which depends on the geometrical characteristics of the anode and on the boundary conditions, it can be determinate on the basis of the field force lines; rs1 ([Ω/m]) is the transversal leakage resistance per unit of length of the semi-conductive polymer layer; rs2 ([Ω/m]) is the longitudinal resistance per unit of length of the semi-conductive polymer layer; rm ([Ω/m]) is the longitudinal resistance per unit of length of the metal screen layer. The following order relations hold true: (1) rs 2 >> rs1 >> rw >> rm The values of these parameters can be obtained from the system geometry and the electro-physical characteristics of the environment and of materials. Anode rwdz

Epc

rs2dz

Semi-cond layer

rs1dz rmdz

metal screen layer z Fig.5 – Equivalent representation of the system under test

The potential distribution with respect to any layers can be assessed by means of a distributed model. The model is usually solved by assuming appropriate boundary conditions, neglecting the mutual influence of cable terminals, and eventually, considering the action of stray currents as source of disturbance. The results can be used to determine the value of the applied cathodic protection voltage, Epc, for any assumed corrosion protection threshold.

TABLE II TYPICAL VOLUME RESISTIVITY AT 283 K Volume Layer

resistivity [Ω⋅m]

Sea water Lead Semi-conductive jacket

0.35 2.2 10-7 1 102-1 106

The approach to determine the cathodic protection voltage consists of two successive steps. The first step calculates the action of the stray currents as source of disturbance on the potential distribution. Later, the results of step #1 are used for simplifying the formulation of the cathodic protection problem. A.

Step #1 Stray currents, flowing though the sea-water, are able to perturb the potential distribution on the cable. The influence of stray currents can be calculated according to the approach proposed in [8] by making a proper characterization for the present application. More specifically, by making reference to the equivalent representation in Fig. 6, the potential distribution can be calculated according to: d 2U m ( z ) − k 2U m ( z ) = −k 2U 0 ( z ) dz 2 (2) r k= m rsw where Um(z) is the potential of the metal screen; U0(z) is the potential of the soil as a function of the stray currents; rsw=rs1+rw is the linear transition resistance of the cable surface (caused by polarization and by jacket resistance). It is worth to note that, due to (1), the potential of the metal screen can be assumed as equal as to the potential of the semiconductive layer, Us(z).

IV. NUMERICAL APPLICATIONS The approach proposed in Sec.III was applied to a 50 m length of undersea HV power cable. The cable geometrical



ρw 0.35 = = 0.185 Ω 2πr 2π0.3 1 GE = = 5.38 S RE RE =

Soil rswdzr rmdz

Metal screen layer

(4)

By assigning the semi-conductive jacket resistivity value equal to 2*104 Ωm, the conductance of the semi-conductive layer can be evaluated as:

z

Fig.6 – Equivalent representation in case of stray currents

Therefore, by imposing: U 0 ( z ) = E0 z

ρ s  r2  2 10 4  3.5  ln ln  =  = 9.8 Ω 2πl  r1  2π50  3  1 Gs = = 0.1 S Rs Rs =

I0 = 0 the potential distribution is: E0 U m ( z )= U s ( z )= exp(−k z ) + U 0 ( z ) (3) k In the following Fig.7, the distribution of the potential along the cable is reported as a function of the stray current electric field. The values of E0 vary in the interval [0.1, 0.3] mV/m, as discussed in [8].

(5)

Anode Epc

Gw Semi-cond layer

Ipc

Gs Metal screen layer

10

9

Fig.8 – Equivalent representation in case of absence of stray current Potential distribution [ V ]

8

By looking at Fig.8, the protection current, Ipc, can be derived as:

7

I pc =

3.2486

6 3.2485

Zoom

3.2484

5

3.2483 3.2482 3.2481

4

3.248 0

10

20

30

40

50

z, m

3

0

5

10

15

20

25

30

35

40

45

50

Cable length [ m ]

Fig.7 – Potential distribution as a function of stray current value action

These results, which give evidence to the opportunity to assume as equipotential the cable surfaces under stray current field, can be extended to any external electrical field. This is a strong consequence of (1). B.

Step #2 Assuming that the semi-conductive layer is equipotential, the system in Fig.5 can be solved according to the representation of Fig.8, where Gw and Gs are, respectively, the equivalent electrolyte conductance and semi-conductive layer linear leakage conductance. In case of achieving a galvanic cell by installing a single anode bed, consisting of an electrode of hemispherical shape with a radius of 0.3 m, the current field lines in the electrolyte can be considered as radial [14], under a rough approximation; thus:

E pc 1 1 + Gw Gs

=

E pc Gs ≅ E pc Gs Gs +1 Gw

(6)

Imposing a current density on the jacket equal to 0.2 A/m2, Ipc and Epc can be calculated as functions of the number of anodes. In Fig.9 the current density distributions are calculated in three representative cases, which consider, respectively, the presence of 61, 41 and 29 uniformly spaced zinc anodes. The presence of a large number Na of anodes ensures the protection of the whole cable surface, since the protection current density of 0.2 A/m2 is uniformly distributed on the entire cable length. By decreasing Na, at an almost constant current Ipc, the level of protection is sensibly reduced. In Tab. III the details about the latter are reported. The outcomes related to the voltage magnitude represent the voltages that has to be applied in an active cathodic protection system to obtain the assigned current density threshold. However, these values have to be increased by the offset values related to both the standard potential of the anode bed and, eventually, the stray currents electrical field.



TABLE III RELEVANT DATA FOR THE CALULATION OF THE CURRENT DENSITY

REFERENCES [1]

DISTRIBUTIONS

#1

Numbers of anodes [-] 61

Protection current, Ipc [A] 19.5

Applied cathodic protection voltage [V] 191.1

#2

41

19.5

191.1

#3

29

18.4

180.3

Case

[2] [3]

[4] [5]

[6] 0.22

Ipc = 19.5 A Ipc = 19.5 A Ipc = 18.4 A

0.2

[7]

0.18

J [ A/m2 ]

[8] 0.16

[9] [10]

0.14

[11] 0.12

0.1

0

5

10

15

20

25

30

35

40

45

50

Cable length [ m ]

Fig.9 – Current density distribution on the cable as a function of the anode number

V.

CONCLUSIONS

[12]

[13]

[14]

R. Benato, C. Di Mario, A. Lorenzoni, “Lines versus cables: Consider all factors,” Transm. Distrib. World, vol. 59, No. 11, Nov. 2007, pp. 26– 32. E. S. Ibrahim, “Corrosion control in electric power systems”, Electric Power Systems Research, Vol. 52 (1999) pp: 9–17. R.A. Corbett, “Cathodic Protection as an Equivalent Electrical Circuit”, IEEE Tr. on Industry Applications, Vol. IA-21, No. 6, November/December 1985, pp: 1533-1537. L. Lazzari, P. Pedeferri, M. Ormellese: Protezione catodica, Polipress ed., 2006. O.W. Zastrow, “Estimating Cathodic Protection Requirements for Electric Cables in Contact with Soil”, IEEE Tr. On Industry Applications, Vol. 24, No. 2, March/April, 1988, pp. 356-360. E.C. Bascom, Y. Iossel, J.F. Troisi, “Construction features and environmental factors influencing corrosion on a self-contained fluidfilled submarine cable circuit in long island sound”, IEEE Tr. On. Power Delivery, Vol.13, No.3, July 1998, pp.677-681 E.C. Bascom, R.J. Schwabe, J.F. Troisi, A.V. Poliakov, S. Zelingher, S.A. Dave, “Submarine cable cathodic protection”, IEEE Computer Applications in Power, Vol. 14, No. 1, Jan 2001, pp. 39-43. EPRI final report on Summary of Diagnostic Evaluation for Submarine Cable Cathodic Protection System Performance. Feasibility Assessment of Technologies, n° 1001931, July 2001. R. Donaghy, “HV Submarine Cable Systems Design, Testing and Installation”, CIGRE Ireland Technical Seminar, October 2010. B.C.Hydro report on DC l and DC 2 Submarine Cable Condition and Life Assessment, March 1999, available on line. K. S. Suh, C. R. Lee, J. S. Noh, J. Tanaka, and D. H. Damon “Electrical Conduction in Polyethylene with Semiconductive Electrodes”, IEEE Tr. on Dielectrics and Electrical Insulation, Vol. 1 No. 2, April 1994, pp: 224-230. Cigré WG B1.27 tutorial on Recommendations for testing of long AC submarine cables with extruded insulation for system voltage above > 30 (36) to 500 (550) kV. K.Y. Lee, J.C. Nam, D. H. Park, D.H. Park, “Electrical and Thermal Properties of Semiconductive Shield for 154KV Power Cable”, IEEE Proceedings of 2005 lntemational Symposium on Electrical Insulating Materials, June 5-9, 2004, Kitakyushu, Japan, pp.616-619. V. Carrescia: La Sicurezza Elettrica, TNE ed.

This paper investigated the corrosion phenomenon in HV power cables. A distributed model to design cathodic protection system in undersea environment was proposed. The model was developed under the assumption that only the external layer of the cable is subject to corrosion. The approach was formulated in case of protecting cable layer by means of using an active cathodic protection system. The modeling found the relationship between the protection voltage and the parameters of the system cable-environment. The performed analyses highlighted the opportunity to further investigations aimed at modeling the electric current field as a function of the anode bed shape and number. They will be able to verify the stiffness of the presented preliminary procedure.
