Cathodic Protection For Tower Foundations Using ...

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Cathodic Protection For Tower Foundations Using Induction From The Transmission Line Electric Field José Maurílio da Silva and João Nelson Hoffmann

Abstract - This paper presents a new method for cathodic protection of transmission line grillage foundations, developed by COPEL (Electric Utility of Paraná, Brazil) and LACTEC (Institute Of Technology). This method uses induction from the electric field generated by the transmission line conductors as the primary source of the required DC power for cathodic protection. Index Terms - Transmission lines, towers, foundations, corrosion, cathodic protection, electrostatic induction, electric fields

I. INTRODUCTION

T

RANSMISSION line lattice towers are designed for long periods of operation, and they should present an anticorrosion protection system that should be compatible to the environment. Steel structures are provided with a protective coating obtained by immersion in a bathing of hot zinc (hot galvanizing), and in the field it will be subjected to the aggressiveness of two different environments - atmosphere and soil. In the atmosphere, metals are not corroded to a great amount of electrolytes and for this reason, most of the atmospheric corrosion proceeds slowly, so that the whole metallic surface behaves as cathode and anode at the same time. However, in the soil environment, the presence of ions to a certain depth can initiate the formation of an ionic gradient along foundations, causing the richest areas in salts to become anodic and, after a certain period of time, it can totally corrode the grillage foundations. Thus, sandy soils (with good aeration and low humidity) would be indicative of little corrosion, while pasturage and agricultural soils would facilitate, in principle, the corrosion process. Also, the formation of high oxygen gradients along foundations (differential aeration) leads to accelerated corrosion in areas where this element lacks.

J. M. da Silva is with LACTEC - Instituto de Tecnologia para o Desenvolvimento, Address: Centro Politécnico, Curitiba, Paraná, Brazil, fax: +55-41-266-3582, phone: +55-41-366-2020, e-mail: [email protected] J. N. Hoffmann is with COPEL - Companhia Paranaense de Energia Address: Rua José Izidoro Biazetto, 158, Curitiba, Paraná, Brazil, fax: +5541-331-3959, phone: +55-41-310-5635, e-mail: [email protected]

Most of transmission lines in COPEL are located in soils that facilitate the corrosion process on grillage foundations. After examining several towers, partial corrosion and even total corrosion of the galvanized coating was found. In some cases, accelerated metal loss of the steel members was also found. On new towers it was verified that the tower/soil interface area is more affected by corrosion. Following some procedures suggested by [1],[2],[3] it was also possible to conclude that, for old towers, corrosion is practically present on the whole surface of grillage foundations. Besides the physical-chemical issue, there exist other factors that affect the corrosion process, as for example, the existence of different materials such like buried copperweld counterpoises. In this case, it has been studied the possibility of using aluminum as an alternative material for counterpoises [4],[5]. For stub foundations, excavation and reconstruction close to the tower/soil interface area have been used as a remedial procedure. Concrete is broken and rebuilt to about 50 cm above the soil, including painting of tower legs. Approximate costs to these procedures are: − US$ 500 for a simple excavation around the four feet of tower − US$ 650 for installation of aluminum counterpoises − US$ 2,000 for the substitution of all grillage foundations in a 138 kV suspension tower, which can reach about US$ 4,000 depending on tower dimensions − US$ 3,000 for refurbishing stub foundations of 138 kV towers, which can reach even US$ 50,000 depending on voltage level, tower dimensions and soil characteristics Considering these costs, a research program for alternative means of corrosion control may be attractive. Cathodic protection is a technique that allows the control of corrosion for buried metallic structures, reducing maintenance costs. Cathodic protection using impressed current is a method which is commonly used for the protection of pipelines, storage tanks, marine vessels, transit systems, bridges, etc. The application of such system to transmission line tower foundations did not revealed much practical due to some particular problems, for example, the need of a permanent source of DC power supply close to each tower. Thus, the use

2 of conventional sources, such like a low voltage distribution system, is only possible close to urban regions. Also, some utilities in Brazil were motivated to investigate and develop their own system of cathodic protection, since accelerated corrosion in grillage foundations has been found in most towers. Some alternative systems were developed, as for example, by using solar panels as the source for DC power [6],[7]. Considering that this problem involves a combination of different expertises, COPEL and LACTEC joined their specialists in underground corrosion, cathodic protection, tower grounding and high voltage electric fields. As a result, it was found an alternative system for cathodic protection of grillage foundations that uses the electric field generated by the transmission line itself as the source of the necessary energy. This method presents the advantages of low cost and easiness of installation, both in rural and urban regions. II. DESCRIPTION OF

THE NEW METHOD

The new method developed for cathodic protection of grillage foundations is composed by three basic sets (Fig. 1): − Coupling device : component which receives the induced energy from the transmission line electric field − Electronic device: component that converts the energy received by the coupling device to a form of energy that meets the requirements of cathodic protection

Fig. 1 – Assembly Of Components

− Grounding system: anode bed powered by the electronic device and designed in such a way that the anodes will corrode instead of the tower grillage foundation A. Coupling Device The coupling device is an aluminum tube installed in the tower and positioned in parallel to the transmission line conductors (usually closer to the lowest conductor), and connected to the electronic device by a rigid aluminum wire. The coupling device is connected to the tower by means of two steel bars (Fig. 2), each one with a ceramic insulator and a clamp for the connection of the support to the tower leg.

Fig. 2 – Support Of The Coupling Device

B. Electronic Device Basically it is composed by an amplifier-rectifier of the induced current and surge protection devices. The electronic device is completely encapsulated by an impermeable polymer (Fig. 3), and is located close to the support of the coupling device, usually at an average height of about 20 m above ground. This component has three terminals: − − −

High-voltage terminal, composed by an insulator and connected to the coupling device Low-voltage terminal, connected to the anode bed Ground terminal, connected to the tower

Fig. 3 – Electronic Device

C. Grounding System The grounding system or anode bed is usually designed with several steel rods buried in the ground close to the tower legs

3 (Fig. 4). The number and location of rods are defined as a function of soil resistivity and the rate of corrosion of the installation to be protected.

2 R max = 1.5 × 10 5 / I DC − RT ,

Where, IDC = electric current necessary for cathodic protection (mA), RT = tower grounding resistance (Ω). A negative value for Rmax in this expression indicates that an additional set of coupling & electronic devices should be necessary . The value of Rmax leads to the estimated number "n" of anodes that should be necessary to the anode bed, by means of:

n ≥ [ρ av × ln (4L/d )] / 2 πLR max

Where, ρav = average soil resistivity (Ω.m) L,d = lenght and diameter of each anode (m), according to Fig. 4 .

Fig. 4 - Anode Design

A 1 kV insulated copper wire with 2.5 mm2 section does the interconnection between the low-voltage terminal of the electronic device and the anode bed. This wire runs down to the soil in a conduit through the internal side of one the tower legs. A measuring terminal for the generated DC current (“cathodic current”) and surge protecting devices are installed to this wire at a height of about 3 m from ground. III. INSTALLATION OF COMPONENTS The steps necessary to the installation of the cathodic protection system are described below: A. Evaluation Of The Rate Of Corrosion By using a battery or like, a DC current is injected into the foundation, and the required current for cathodic protection (IDC) is determined [3]. For assessment of the rate of corrosion of buried metallic materials, it is often used an approach that the potential soil-tower (measured with a copper-copper sulphate reference electrode) should be more negative than 850 mV [8]. There is also a different approach [9] for obtaining IDC by means of a tower polarization: the measured "off" potential (Eoff) should be 100 mV more negative than the potential soiltower (the “100 mV Potential Decay Protection Criterion” [1]). B. Anode Bed The design of the anode bed is of fundamental importance for the success of the installation. Depending on the abovedetermined value of IDC, an approximate value for the maximum allowable grounding resistance of the anode bed is given by

In summary, design of the anode bed results from a combination of the required current for cathodic protection ρm) and anode geometry (L,d). (IDC), soil resistivity (ρ C. Design Of The Coupling Device The coupling device is designed once for each family of structures. Dimensions and positioning are defined in such a way to ensure that the induced energy is sufficient for cathodic protection, and the final design should not violate the necessary electric gaps on the transmission line tower head. Definition of the coupling device for many structures used by COPEL was made experimentally. The generated cathodic current was measured while varying the position (distance from tower and distance from conductors) and dimensions (diameter and length) of the aluminum tube. Moreover, the measured values of the AC current induced by the transmission line conductors in the coupling device were approximately the following: − − − −

150 µA at 69 kV 300 µA at 138 kV 450 µA at 230 kV 600 µA at 500 kV

It is concluded that, for the towers used by COPEL, the coupling device could be standardized for application in the voltage levels of 69 kV to 500 kV. Only distance to the nearest conductor should be variable. The following parameters were defined: − − − −

Length of the aluminum tube: 6 m Diameter: 33 mm Distance to tower leg: 0,8 m Minimum distance to the nearest conductor: - 1,25 m at 69 kV - 1,75 m at 138 kV

4 - 3,0 m at 230 kV - 6,0 m at 500 kV These distances are larger enough for not compromising the electric insulation of the tower. Also, for 750 kV towers a distance of 9,0 m can be considered, with 750 µA of AC current induced in the coupling device. As alternative for other tower series, designing of the coupling device can also be made by computacional simulations, using a software developed by COPEL and based on the methodology described by [10]. In this software all tower members are simulated in a three-dimensional model, including the coupling device and all transmission line conductors. Currents and voltages induced in the coupling device by the transmission line conductors are determined. The values measured experimentally confirmed computed values to a satisfactory precision. This software is also used for predicting the optimum design of the coupling device. IV. RESULTS OF PRACTICAL APPLICATIONS The steps necessary for the installation of cathodic protection to a 138 kV transmission line are described below, as well as other results of the same application to 69 kV, 138 kV, 230 kV, 500 kV and 750 kV towers. A. 138 kV Transmission Line (Guarapuava - Vila Carli) This transmission line is composed by 63 steel structures. Towers 1 to 50 are new and double circuit, while towers 51 to 63 are existing ones and single circuit, already in operation for about 20 years at 69 kV, and now upgraded to 138 kV. The system of cathodic protection using line induction was designed for the final voltage of operation (138 kV), in spite of the initial operation at 69 kV. Then, data and measurements presently available refer to the transmission line operating at 69 kV. 1) Evaluation Of The Rate Of Corrosion On Existing Towers Fig. 5 shows the values of current density IP (mA/m2) measured for some towers. IP refers to the cathodic current (IDC) that is necessary to allow the potential Eoff as above mentioned, divided by the area of contact of the grillage foundation with soil. This figure shows a clear difference between the values of IP necessary for new foundations and for the existing ones. It is concluded that for the installation of cathodic protection to new towers with copperweld counterpoises, a current density of about IP ≅1 mA/m2 can be considered. However, for existing foundations IP should be determined for each tower, or by means of a statistical approach taken from a tower population. 2) Design Of Anode Bed Based on the necessary values for IDC (from 10 to 100 mA), and based on the average soil resistivity (300 Ω.m to 2,800 Ω.m), the anode bed was designed with 4 interconnected rods for new towers, and 8 interconnected rods for the existing towers (Fig. 6).

Fig. 5 – Measured Current Density (Ip)

Since measurements indicated that soil resistivity increase with depth, it was decided to install the anodes closer to surface in order to obtain a lower grounding resistance for the anode bed. 3) Design Of The Coupling Device Two sets of coupling/electronic devices were used in the existing towers. Also two sets were used in the first ten new towers closer to one substation, since it was suspected that the lower grounding resistance of the substation grounding grid could absorb part of the cathodic current generated at the closer structures. The remaining 40 new towers were equipped with just one set of coupling/electronic device.

Fig. 6 – Anode Bed

4) Results Even considering that the transmission line was energized at 69 kV while the whole cathodic protection system was designed for the final operation at 138 kV, it was already found sufficient cathodic protection current at many towers, as Fig. 7 and Fig. 8 demonstrate.

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Fig. 7 – Measurements On New Towers

Fig. 9 – Electric Potentials Ecorr, Eoff And Eon on Existing Towers

B. 138 kV Transmission Line (Francisco Beltrão – Pato Branco)

Fig. 8 – Measurements On Existing Towers

In these Figures Ii refers to the current density injected by the cathodic protection system with the transmission line operating at 69 kV, being indicated the estimated values for 138 kV. In the existing section of line, 3 towers still do not present cathodic protection at 69 kV, while in the new section 5 towers are still unprotected, due to the presence of copperweld counterpoises that tends to corrode the grillage foundations [7]. Even after energizing the transmission line at 138 kV, towers 1, 2 (new towers) and 61 (existing tower) should still not be totally protected. Considering the electric potential soil-tower (Ecorr), it was verified values in the range -400 mV to -600 mV for the existing towers (Fig. 9). The "on" electric potentials (Eon) were measured for cathodic polarization [1],[5],[6] until the complete protection of the grillage foundations were reached. The values found were somewhat disperse, suggesting that the method of [1] can indicate that the tower is protected even having Eon not reached -850 mV.

Tower 91 of this transmission line presented Ecorr ≅ -430 mV, being this value decreased to -892 mV with a cathodic current of 80,5 mA injected by the new cathodic protection system. In this particular case, the transmission line ground wires (Petrel ACSR) and also the copperweld grounding counterpoises were isolated from the structure. It was verified that the ground wire deviated part of the injected cathodic current to neighboring towers, which indicated the need of cathodic protection to all or most structures of the transmission line for an effective protection. On the other hand, copperweld counterpoises increases Ecorr (to more positive values) due to the presence of copper, suggesting the use of aluminum wire [4],[5] or galvanized steel wire as counterpoises. C. 230 kV Transmission Line (Umbará - Metalurgica Guaíra) Cathodic protection was applied at tower 11 of this transmission line. Low soil resistivity and steel ground wires allowed a cathodic current of about 100 mA, decreasing Ecorr from -580 mV to -1000 mV. D. 500 kV Transmission Line (Areia – Fóz do Areia) Some tests were made at a double circuit 500 kV tower, and the system generated 68 mA of cathodic current with the coupling device located to about 9 m below the lowest transmission line conductor. When the coupling device was positioned at 6 m from the lowest conductor the system generated 105 mA. E. Number of Towers Protected Table I below shows the total number of towers where the proposed new system of cathodic protection is in operation.

TABLE I

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Utility (State)

COPEL (Paraná)

CPFL (São Paulo) ELETROSUL (Santa Catarina) FURNAS (Rio de Janeiro) ESCELSA (Espírito Santo) COELBA (Bahia) KLABIN (Paraná)

APPLICATION OF CATHODIC PROTECTION WITH INDUCTION FROM CONDUCTORS Number Transmission Line of towers 230 kV (Umbará – SIG) 1 138 kV (Pato Branco – Francisco 1 Beltrão) 69/138 kV (Vila Carli – 63 Guarapuava) 138 kV (Lapa – Piên) 133 138 kV (Sabará – Ponta Grossa 23 Norte) 138 kV (Alto Paraná – Paranavaí) 69 138 kV Iratí – Sabará 144 230 kV (Salto Osório – Cascavel) 20 138 kV (Fóz do Chopim – 31 Realeza) 69 kV (Telêmaco Borba – Klabin) 3

significant improvement for the control of corrosion in transmission line grillage foundations. Years in operation

4 4 3 3 3 3 2 2 2

138 kV

88

1

500 kV

9

1

500/750 kV

4

1

138 kV

4

3

69 kV

63

1

69 kV

2

5

V. COST EVALUATION By using cathodic protection for grillage foundations, a significant reduction in maintenance costs can be predicted, considering excavations, tower leg painting or substitution of corroded foundations. These costs change from about US$ 600 to US$ 2,000 for 138 kV towers, and from about US$ 1,200 to US$ 4,400 for 230 kV, while the installation of cathodic protection can be evaluated from Table II. TABLE II RELATIVE COST EVALUATION

Substitution of grillage foundations and counterpoises Substitution of grillage foundations Conventional cathodic protection (127/220V) Cathodic protection with solar panels Cathodic protection with induction from conductors

VII. REFERENCES

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% 138 100 100 80 38

It is concluded that the new method proposed (cathodic protection by using the energy induced by the transmission line electric field) is economically attractive. VI. CONCLUSION Considering the experience obtained from above practical applications, and also based of the costs presented, it is concluded that the cathodic protection using energy induced by the transmission line electric field is technically feasible and economically attractive. As a result it is expected that the new method of cathodic protection, developed and presented in this paper, represents a

[1]

G.K.Glass, “The 100-mV Potencial Decay Cathodic Protection Criterion, ” NACE, March, 1999. [2] W.B.Holtsbaum, “Consideration of IR Drops in “on”Potential CP Criteria,” Materials Performance, July , 2000. [3] M.G.Ali and Rasheeduzzafar, “Polarization Period, Current Density and the Cathodic Protection Criteria,” ACI Materials Journal, May-June, 1992. [4] J. M. da Silva and C. V. D'Alkaine, “Use Of Aluminum Wires As Grounding Counterpoises For Transmission Lines (in Portuguese),” II SEMEL, 1990, Brazil [5] J. M. da Silva, J. N. Hoffmann and A. Soncin, “Viability Of The Use Of Aluminum As Grounding Counterpoises For Transmission Lines (in Portuguese,” VI ERLAC, 1995, Brazil [6] E. T. Serra, M. Z. Sebrão and B. C. Dutra, “Application of Photovoltaic Solar Panels For The Cathodic Protection Of Transmission Line Tower Foundations (in Portuguese),” XII SNPTEE, 1993, Brazil [7] M. Z. Sebrão, B. C. Dutra, M. M. Araújo and L. A. R. Assumption, “Study of Corrosion and Anticorrosion Protection For The Foundations Of The Transmission Line ±600 kV Fóz-Ibiuna I (in Portuguese),” XIII SNPTEE, 1995, Brazil [8] D. Hughnes and P. Davies, “Monitoring Steel Tower Foundation Corrosion,” Transmission & Distribution World - August, 1996 [9] Nace RP0169 (1992) - Control de Corrosion Externa de Sistemas de Tuberias Metálicas Subterráneas o Sumergida (in Spanish) [10] J. N. Hoffmann and P. Pulino, “New Developments On The Combined Application Of Charge Simulation And Numerical Methods For The Computation Of Electric Fields,” IEEE Transactions on Power Delivery, April, 1995

VIII. BIOGRAPHIES José Maurílio da Silva received the B.S. degree in Chemistry in 1983 and the M.S. degree in Chemistry in 1987, from Universidade Federal de São Carlos. He received his PhD degree in Physical Chemistry in 1993 from Universitá Degli Roma “La Pienza”, working with Iron Corrosion. From 1986 to 1999 he worked with corrosion researches for Companhia Paranaense de Energia (COPEL). Since 1999 he is with LACTEC (Instituto de Tecnologia Para O Desenvolvimento) as senior researcher, working specially with underground corrosion, having developed the patents “Cathodic Protection For Tower Grillage Foundations” and “Cathodic Protection For Tower Foundations By Transmission Line Leakage Current”. João Nelson Hoffmann received the B.S. degree in Electrical Engineering from Universidade Federal do Paraná in 1981, and the M.S. degree in Applied And Computational Mathematics from Universidade Estadual de Campinas (UNICAMP) in 1993. In 1982 and 1983 he worked with design of transmission lines and substations. From 1984 to 1987 he worked with technical specifications for high voltage equipments. Since 1988 he is with Companhia Paranaense de Energia (COPEL), Curitiba, Paraná, Brazil, working with transmission line researches, specially for overhead urban and compact lines, corona and earthing problems, having developed a patent for “Cathodic Protection For Tower Grillage Foundations”.