Article CIGRE 22_206E.PDF

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Belgian experience on initiatives to improvethe capability of existing overhead lines

by

J. LAMSOUL J. ROGIER P. COUNESON A.VAN OVERMEERE

(Tractebel*) (Tractebel) (Tractebel) (Laborelec)

BELGIUM

* Avenue Ariane 5-7, B-1200 BRUSSELS (BELGIUM)

1. Introduction - The benefit of technical audits

The concern in Belgium to improve or at least to

maintain in good condition the existing overhead

transmission lines has been expressed in various papers

published in recent years. This concern originates on the

one hand from the nature of the high voltage network,

very dense, strongly meshed and relatively old, and onthe other hand from the persisting difficulty to obtain

the permits to build new overhead transmission lines

needed to meet the increasing demand of electricity.

When specific old line components near the end of their

useful life and need a total or partial replacement, thequestion arises whether it would be advisable to take the

opportunity to proceed to a more extensive intervention

on the line concerned, to avoid spreading the work in

time without any long-term vision.

In Belgium, the decisions on integrated interventions on

overhead transmission lines are taken only after a

complete technical audit [1].

2. Methodology and Process of a technical audit

2.1. Methodology of a technical audit

One could summarise the methodology of an audit or an

expert assessment of an old line by the sequence :

measure - understand - assess – decide –guarantee.

• The first action corresponds to the measuring of the

degradation of the defective elements (see § 2.2.).

• The analysis of the defect allows to understand thedeterioration mechanism: the internal cause

(corrosion, wear) usually depends on a local and/or

variable sources (wind, ice, humidity). The cause

may also be external (lightning, tree fall) (see § 5).

• The risk (defined in § 3) assessment in relation to

the condition of the defective elements allows tocompare different intervention solutions.

• The decision on actions to be taken can be based

only on quantifiable criteria (see § 3).

• The actions have to be accompanied by a

maintenance program appropriate to guarantee thenew capability of the line and its components (see

§ 2.4.).

2.2. Degradations leading to high risks

The audit has to detect above all the degradations that

may lead to high risks such as :

• the corrosion of the conductor core if it is of steel;

• the corrosion of the conductors near the joints;

• the broken strands inside the suspension clamps;

• the loosening of the insulator rods and the security

clip wear;

• the advanced wear or corrosion of the insulator

string attachments to the towers in the case of point

contacts.

• the abnormal deformations and generally all

advanced degradations which could not be normally

eliminated by replacement or repair.

21, rue d'Artois, F-75008 Parishttp://www.cigre.org

Session 2000© CIGRÉ22-206


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In presence of aggravating circumstances certain risks

may also be very high :

• the fusion of broken strands on conductors with a

small cross-section due to a lightning arc or fault

conditions;• the poor surface condition of the insulators in the

presence of pollution;

• the cross-arm ends of an old design (for instance :

attachments out of the axis resulting in cross-arm

torsion);

• the deformed or missing steel angles, in case of an

accidental loading that determines the angle;

• the poor earthing in case of a fault.

2.3. Process of an audit

The auditing process can be outlined as follows [1] :

• Collection of all design data, inspection and

modification reports.

• Scheduling a general inspection visit by a

"reinforced" patrol detecting defects without

climbing the supports.

• Identifying hot points by infrared thermography.

• Scheduled outage, detailed examination of towersand their line equipment; sampling, dismantling and

replacement of complete insulator strings, including

the attachments if possible; radio frequency

inspection of conductors on suspension strings.

• Drawing up a first report including a description of

the line condition.• Supply of a second report with the comparison of the

action programs proposed to the utility to increase

the line capability.

2.4. Capability improvement

The capability of a line defined as the product of its

transit capacity and availability :

[capability] = [transit capacity] x [availability]

can be improved in different ways of which some are

described hereunder :

[transit capacity] = [current] x [voltage]

The electric current can be improved (uprating) either

by the operation of conductors at a higher temperature

or by using a conductor with a larger cross-section or a

higher electrical conductivity; the voltage level by

larger insulating distances and consequently by the

replacement of cross-arms and possibly the tower top.

[availability] = [reliability] x [maintainability]

The specific reliability of a tower or a foundation can be

improved (upgrading) by its structural strengthening;

the maintainability by safety equipment, training of line

erection personnel, etc.

3. General criterium for actions required

The strategy of actions to be taken is based on the

absolute criterium of minimising the net present value

of the sum of the following two costs:

• the investment costs to improve the capability of the

system (including the operation and maintenance

costs);

• the failure risks or the product of the failure

probability and the consequences in financial terms.

The proposed action costs must thus be at leastcompensated by the reduction of the failure risks. This

general approach is examined more in detail by the

Working Group 13 "Management of existing overhead

lines" of the CIGRE Study Committee 22.

The practical approach applied in Belgium is described below.

• If the risk is too low, there may be no action

necessary and the risk will be tolerated.

• If the risk is too high, the overhead line will be put

out of service or the line dismantled.

However, if one wants to manage the risk, one of the

four following options with increasing investment costs

can be justified.

• If the risk is identified and localised we can

envisage:

- elimitation of recurrent causes;

- corrective maintenance at a local scale;- repair or possibly partial renovation (life

extension) of the elements of the line

equipment.

• If the risk is generalised but if the towers and

foundations are still in good condition, one can

consider a renovation of the conductors as well as of

the line equipment (refurbishment or extensiverenovation to restore the intended design working

life of the line component).

• The transit capacity can be improved by installing

new conductors with a higher electrical conductivity,

providing the towers and foundations have a

sufficient structural strength (uprating or

improvement of the transit capacity).

• Generally, the installation of conductors with a

larger cross-section requires reinforcement of towersand foundations (upgrading or improvement of the

structural reliability) to cope with the higher loads.


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All integrated interventions on existing overheadtransmission lines must be consistent with the network

development plan.

4. Type of inspected network and itsparticularities

4.1. Type of network

The technical audits in Belgium are applied only to

overhead transmission lines built 40 years ago or more.

As the 380 kV network has been developed only since

the late 1960s, the Belgian overhead network, for which

a complete audit is generally required, is essentially that

with the 70 or 150 kV voltage levels.

This particular network dates mostly back to the years

1930-1940. So it is generally over 50 years old. All the

supports concerned are towers with a bolted, riveted or

welded steel lattice, not galvanised, but with regularly

painted angles.

As the 70 and 150 kV overhead transmission lines built

after 1960 have a more modern design, the results

obtained by technical audits performed on the on

average 50 year old lines cannot be extrapolated to the

more recent lines.

4.2. Particularities of the overhead network

Generally speaking, all drawings related to the old lines,

at least the plan views and the longitudinal profiles, arerare and obsolete. If they exist they have not been

updated.

Moreover, as no detailed history of those lines is

available, it is difficult to know the exact situation and

condition of the whole of their constituent elements.

The conductors of the old 70 kV network have a small

cross-section, generally of copper; the earth wire is

made of bronze of a very small cross-section. One of the

particularities of this network is its evolution in time.

Initially designed for a lower voltage level (50 kV), it

was later adapted to 70 kV. Adaptations included thelengthening of the cross-arms and in some cases the

transition from a double circuit to a single circuit.

The conductors of the old 150 kV network are mostly of

the aluminium conductor steel reinforced type (ACSR),

with a cross-section of about 200 mm². The initial earth

wire was of steel.

Like the 70 kV network, the 150 kV network evolvedvia one or sometimes even two intermediate voltage

levels (70 and 110 kV). This situation has led to tower

top modifications, sometimes in many different ways.

In the 150 kV network the initial steel earth wire has been modified and replaced by an earth wire with a

more substantial cross-section, generally an all

aluminium alloy conductor (AAAC), in order to

increase the short-circuit current withstand. This

modification led to replacing the earth wire peak, and

heightening it to compensate the greater sag of the newearth wire.

Fig.1 Typical modifications on 150 and 70 kV lines

5. Synthesis of the audits carried out

5.1. Maps

In order to allow the verification of the regulatory

clearances of the line with regard to the obstacles

crossed, new plan views and longitudinal profiles have been established.

Those drawings have been produced by means of aerial

photogrammetry, which considerably reduces the time

and costs compared to a survey from ground level [2].

They allow :

• a more sound operation of the line concerned due to

the fact that all the obstacles will be very well

defined thanks to the aerial photographs taken at a

scale of 1/7000 and to the restitution of the profile

and the plan view on land registry maps;• to detect critical points : clearances to new buildings,

trees to prune and/or to cut down;

• to examine the possibility of operating the line at a

higher temperature (increased from 40°C to 60°C

and even 75°C as allowed by Belgian regulations).

5.2. Conductors

5.2.1. General condition

The investigation of the general condition and the

internal corrosion in particular of the conductors is

based on the metallurgical analysis of conductor samples taken either from the jumpers of the dead-end

strings or from the spans which recently needed repair.


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It has been observed that the galvanised steel core of the jumpers deteriorates faster than that of the conductors in

the span, which can be explained by a lower tightening

due to the lack of mechanical tension and consequently

by easier access of atmospheric pollutants to the steel

wires. Accordingly, the conductors may be expected to

be in good condition if the jumpers are.

In case of doubt about the condition of ACSR

conductors, an investigation with the Overhead LineCorrosion Detector (OHLCD) can be carried out on the

spans most representative for the environment [1], [4].

By measuring the remaining thickness of the zinc layer

of the galvanised steel wires, one can detect indirectly,

by comparison with the method of Foucault currents,

the presence of important galvanic corrosion between

the steel core and the aluminium layers. This process is

initiated automatically if zinc is lacking, and is crucial

to the design working life of a the ACSR conductor.The portion of the span affected by a definite rate of

degradation is registered.

As the absence of galvanisation inevitably leads to the

degradation in time of the conductor condition, its

remaining working life is no longer foreseeable.

5.2.2. The 70 kV network

Statistical analysis has shown that 70 % of the

conductor ruptures occur on old overhead transmission

lines equipped with copper or similar conductors of a

small cross-section, up to 50 mm².

Short-circuit currents or mechanical overloads (wind)

can cause a rupture if the conductor with a small cross-

section has been previously damaged. This weak point

has mostly arisen from a direct electrical arc on the

conductor. Another possible cause of rupture is the

exaggerated heating of the conductor during short-

circuits in corroded places in the clamps or near the

clamps.

The solutions recommended to remedy those defects

are:

• installation of arcing devices, if absent;• protection of the conductor in a clamp by an armor-

rod;

• transition from an A insulator set to a single

suspension insulator set (to increase the insulating

distance to the conductor);

• replacement of the existing conductors.

However, the low cross-section and the nature of the

conductors make it very expensive to replace them. For

instance, replacing them by all aluminium alloy

conductors (AAAC) with a larger cross-section can

make it necessary to reinforce the towers.

Therefore, the solution of replacement by conductors of the same cross-section and the same nature is fully

acceptable providing that after some modifications the

line may operate at a higher temperature.

It should be borne in mind that although the lines were

designed initially for operation at 40°C, it is feasible atlittle cost to operate at 75°C as authorised by the present

Belgian regulations. This is possible at very low cost

because the spans are generally very short (≤200 m). The transition from 40°C to 75°C allows to

increase the transit capacity by a factor 2.6.

5.2.3. The 150 kV network

The ACSR conductors show generally a severe

galvanisation loss of the steel wires, leading toextensive galvanic corrosion of the aluminium strands

and finally to a significant reduction in the ultimate

mechanical strength of the conductor.

In this case, the conductors are replaced by compact and

more aerodynamical conductors of the type AERO-Z

[3] with the same external diameter. This technique

avoids any extensive checking and/or reinforcement of

towers, while the transit capacity increases with 17 %.

As far as the earth wire is concerned, it has been

replaced twenty years ago. Its condition is quite

acceptable and does not need any intervention, apart

sometimes from readjusting the sag.

5.3. Insulator strings

The examinations of both 70 and 150 kV insulator strings reveal extended wear of certain elements,

especially on U-bolts and on eyes providing the

attachment of suspension strings to towers.

Also, the line equipment is strongly corroded (arcing

ring, ball and socket joints, bolts, etc.).

As in the majority of cases the conductors have to be

replaced, it would be logical in the context of the

strength coordination to also replace the insulator

strings and modify them. For instance :

• attachment of the strings to the tower cross-arms

with hinges in order to avoid point contact;

• installation of a suspension clamp with a triple

articulation designed for conductors protected witharmor-rods, to decrease bending moments in the

conductor;

• installation of toughened glass insulators instead of

porcelain.


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5.4. Towers

The towers have been painted regularly, so that their

condition with regard to angle steel corrosion is

satisfactory.

Some profiles are bent, twisted or buckled and have to

be replaced. A few cases, relatively rare, present very

extensive corrosion needing the complete replacement

of angles; namely the concrete-stub interface area is

generally the most critical one.

As said previously, certain lines have undergone some

modifications during changes made to their voltage

level. Certain of theses adaptations were implemented

correctly and need only a few small interventions.

However, some were made in an incongruous way and

necessitated serious interventions or even full

replacement of the tower top as this option is the most

reliable and even the least expensive.

5.5. Foundations

The observed degradations on the old foundations are

essentially due to poor in-situ execution of the concrete:

incomplete mixing of the concrete, gravel pockets and

presence of cavities in the concrete due to insufficient

vibration during concrete placing, and due to low

cement dosage. This situation has led to an initial poor

mechanical resistance.

On some foundations we can observe the erosion in

time of the sand and the fine granules in the concrete.

The cement that must bind them chemically hasgradually disappeared by the chemical attack of sulfates

and chlorides from the soil and/or the organic attack of

the possibly unwashed sands used in the concrete. In

certain circumstances the formation of expansive salts

and the alkali reactions cause expansion of the concrete

which may result in internal cracking and explosion. At

present, only appropriate cements are used so as to

avoid degradations that were not yet known at the time.

One can also notice fortunately that the general

condition of the backfilled concrete is better than the

condition of the visible concrete. Probably the water in

the concrete evaporated quicker in the upper partexposed to the air during the concrete curing, while the

lower backfilled part is situated in a more moist

environment less exposed to the weather. At present the

upper part of the foundations is always protected

(watering, curing compound) during the curing process.

The upper level of certain chimneys is often not enough

raised, and sometimes buried with regard to the soil. As

a result the stubs may get overgrown by vegetation,

which may cause a preliminary corrosion of the stubs

due to their being permanently exposed to humidity.

Therefore it is recommended to heighten the chimneysconcerned to avoid frequent and costly maintenance.

Other chimneys simply need clearing the covering soil

to the level corresponding to the surrounding terrain.

However, the advanced deteriorated condition of the

greater part of the top of the chimneys rising above thesoil necessitates their repair. This is the more

indispensable that their degradation is going to continue

due their exposure to weather conditions.

6. Practical experiences in increasing the linecapacity

6.1. Synthesis of uprating analysis

Taking into account the conclusions the audits generally

lead to, i.e. the replacement of conductors, and hence

significant investment costs, it may be advisable to

conduct additional evaluations to identify the difference

in cost for at the same time increasing substantially thetransit capacity of the line concerned (uprating).

If the outside diameter of the new conductors is larger

than that of the existing conductors and that as a

consequence the loading on supports is increased, the

Belgian Regulations on Electrical Installations impose

to verify the structural strength of the supports,

including the foundations of the line concerned

(upgrading).

This check up has to be performed consistent with the

stipulations of the current regulations, i.e. a normal

maximum wind velocity of 126 km/h and an exceptionalmaximum wind velocity of 178 km/h (double dynamic

pressure of the former).

The second hypothesis is of course very severe for the

existing lines and particularly for the steel lattice towers

of the 70 kV lines.

However, we notice that the impact of the secondhypothesis on towers and foundations is somewhat

mitigated when using compact conductors of the

AERO-Z type [3] for which the corresponding

aerodynamical coefficient is reduced with a factor 0.83

for a diameter smaller than 27.65 mm and a factor 0.62for a diameter between 28 and 30.65 mm.

6.2. The 70 kV network

The checks required by the regulations lead to

considerable reinforcements of supports and even moreso as the distances between the tower’s footings are

very small.

As a result the majority of supports necessitate doubling

of the main legs from the lower cross-arms to the level

of the foundations. In this case it seems more

economical to replace the existing towers by new towerswith a similar outline but using a higher steel quality.


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Also, the foundations that are the monoblock type needto be reinforced, be it to a lesser extent, providing the

monolithic character of the foundation can be

maintained. This assumes that the concrete does not

deteriorate, which may not always be the case.

In the hypothesis that the foundations present a goodquality, it is estimated that the cost difference,

characterised by a multiplication factor with regard to

the basic solution that is – let us repeat – thereplacement of the conductors by conductors of the

same outside diameter, is situated between:

• 2.3 and 3.2 in the case of tower reinforcement;

• 2.0 and 2.2 in the case of tower replacement.

If the foundations are of poor quality, thus implying

intervention on every foundation, these factors become

respectively 4.7 and 4.3.

6.3. The 150 kV network

For the 150 kV network the situation is generally

simpler and easier to manage.

This is essentially due to the fact that the towers have

another outline than that of the 70 kV lines, allowing

simpler reinforcements: either replacement of angles

using provisional angles, or doubling of angles, or

addition of a secondary bracing to reduce the

slenderness ratio of members that have to be

strengthened. Furthermore, the pad and chimney

foundations for each foot have a concrete of higher

quality and allow easier reinforcement.

The problem generally encountered on foundations is

uplift stability.

From the point of view of costs, the difference with

respect to the basic solution (replacement of the existing

conductors by conductors of the same outside diameter)

is the following:

• for an increasing of the transit capacity by 45 %, the basic price is multiplied by a factor 1.45 to 1.7;

• for an increasing of the transit capacity by 80 %, the

basic price has to be multiplied by a factor 1.65 to1.90.

Generally speaking, we can conclude that for the

150 kV network, the multiplication factor for the costs

is obviously equal to the multiplication factor of the

transit capacity of the line.

7. Conclusion

A better knowledge of the actual capability or the real

capacity and availability of the overhead transmissionline obtained by on the one side the standard

methodology of a technical audit and on the other side

the historical data based on performance statistics andinspection reports, allows us to take the appropriate

decisions to adapt their performance. The examples

supplied in the present report prove this assertion.

Acknowledgment :

We thank for their contribution the authors of the

different audit reports : J.L. Berlemont, B. Brijs, A.

Bruneau, E. Celens, B. De Waele, D. François, A. Gille,

Y. Le Roy, N. Michalakis, R. Sverzutti and E. Vilret.

References

[1] Delrée X., Rogier J., Van Overmeere A. : Inspection policy of existing overhead lines and assessment methodology based on practical experience.

Report CIGRE 22-302; Paris, 1994.

[2] Rogier J., Goossens L., Mazoin M., Robberechts

W., De Clerck E., Jadot A. : Visualization of overhead

line project. Report CIGRE 22-209, Paris, 1996.

[3] Couneson P., Lamsoul J., Delplanque D., Capelle

T., Havaux M., Guéry D., Delrée X. : Improvement of

existing high voltage overhead lines performance by

using fully locked conductors and ground wires. Report

CIGRE 22-209, Paris, 1998.

[4] Rogier J., Goossens L., Lilien J.L., Wolfs M., VanOvermeere A., Lugentz L. : Experience with occasional

and permanent measurements on Belgian overhead

lines. Report CIGRE 22-104, Paris, 1998.

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