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LOAD CONTROL DEVICES ON OVERHEAD TRANSMISSION LINES
Working Group 22.06
December 2000
LOAD CONTROL DEVICES ON OVERHEAD
TRANSMISSION LINES
PREPARED BY WORKING GROUP 22.06
Members of the Working Group :
Elias GHANNOUM - Convenor (Canada) Joel ANGELINI (France), Jim FERGUSON (United Kingdom), Svein FIKKE (Norway), Edwin GOODWIN (United States), Ramon GRANADINO (Spain), Trevor JACOBS (New Zealand), Friedrich KIESSLING (Germany), Joao Felix NOLASCO (Brazil), Jan ROGIER - New Convenor (Belgium), Pavel FRONEK (Czech Republic).
Corresponding Members :
D. CHOUDHRY (India), Farid KHADRI (Algeria), Jeong Boo KIM (Korea), Tony PLOG (Netherlands), W. Neil PIERCE (Australia), Helmut STRUB (Switzerland).
LOAD CONTROL DEVICES ON OVERHEAD TRANSMISSION LINES
Working Group 22.06
TABLE OF CONTENTS
1. SUMMARY 3
2. INTRODUCTION 4
3. CLASSIFICATION OF LCD 5 3.1. Load Release Devices 5 3.2. Load Reduction Devices 5
4. TECHNICAL DATA RELATED TO AVAILABLE LCD 5 4.1. Special Elongated Fittings : Japanese Experience 6 4.1.1. Type I 6 4.1.1.1. Principle 6 4.1.1.2. Design Characteristics 8 4.1.1.3. Mechanical Performance 8 4.1.1.4. Electrical Performance 8 4.1.1.5. Service Experience 8 4.1.2. Type II 9 4.2. Controlled Sliding Clamps 10 4.2.1. French Experience 10 4.2.1.1. Principle of the Controlled Sliding Clamps 10 4.2.1.2. Calibration 11 4.2.1.3. Design of a Controlled Sliding Clamp 12 4.2.1.4. Test 13 4.2.1.5. Device Efficiency 16 4.2.2. Belgian Experience 18 4.2.3. Romanian Experience 18
5. SERVICE EXPERIENCE OF LCD IN DIFFERENT COUNTRIES 19
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LOAD CONTROL DEVICES ON OVERHEAD TRANSMISSION LINES
1. SUMMARY
Electricity utilities need to design overhead lines to withstand potential cascade failures of towers, which could occur as a result of exceptional icing or rupture of conductors. Some utilities simply design the lines with section towers at frequent intervals and other utilities use special devices called « load control devices » (LCD), the function of which is to limit the stresses applied to the towers in cases of exceptional longitudinal loads. These devices are usually based on one of the following principles:
- Releasing the conductor from the tower, - Sliding the conductor through suspension clamps to balance the load, - Elongating the conductor fitting to increase the apparent length of the conductor and decrease its tension, - Elastic deformation of the crossarms on the towers.
This report describes the characteristics and performance of elongatable fittings occasionally used in Japan and the controlled sliding clamps used in France. Some Japanese utilities have replaced conventional suspension sets with Suspension-Tension sets with elongatable fittings on existing lines under which buildings appeared after construction. These devices were occasionally used in the past in Japan and they make it possible to raise the height of conductors (in normal service) without replacing or modifying the existing suspension towers. The first type of device is based on the unfolding of the fitting, triggered by the rupture of calibrated shear pins. The loads are damped by an impact-absorbing element. The second type of device is based on sliding the shackle in a groove designed into the yoke plate. In France, EDF systematically installs controlled sliding clamps on suspension towers. Each tower of a line is designed for a conventional asymmetric ice load. The sliding load of the clamp is chosen in a range of standard values such that if the actual load from the asymmetric ice load reaches the calculated load then the conductor will slide through the clamp. The suspension tower is thus protected from failure from exceptional longitudinal loads and at the same time the tower cost is not increased.
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The results of a survey show very varied utilisation of the LCDs in different countries: either not used at all, occasionally used or systematically used. The choice of device is based on many reasons and is linked to different local practices and parameters: - Countries which systematically use LCDs, such as France, Belgium and Romania, give one or more of the following reasons : * The global cost of a line is reduced, * The efficiency of these devices has been checked in test stations and on several occasions in the field. - Countries which use these devices in a non systematic way, such as Japan, do it in specific cases, like on existing lines to avoid the replacement of the towers when buildings are built under them, - Countries, which never use LCD, give one or several of the following arguments: * The towers are systematically designed for specified longitudinal loads, * Icing is unknown or very rare in the country, * The efficiency of the devices is uncertain, * The increase of the conductor sag, generated by an LCD, is unacceptable in the view of public safety, * The use of controlled sliding clamps causes severe damage to ACSR conductors.
In conclusion, the utilities that want to go further in the field of load control devices, should concentrate on the following aspects: reliability, safety, and estimation of the cost effectiveness. With respect to this last aspect, the experience in some countries show that, for the future, the modifications and upgrading of existing lines are a potential field of application for LCDs.
2. INTRODUCTION The most serious overhead line failure, which could have severe consequences affecting the grid operation, is the cascade failure of several transmission towers.
This cascade failure is usually initiated by the failure of a single tower or by the breaking of conductors and is the result of insufficient design strength of towers to withstand high longitudinal loads.
The choice of solution to avoid such damage will depend upon a compromise between the risk and consequences of a cascade failure and the extra cost of construction specified by the designer.
The most frequent solutions to limit or avoid series tower failures are, either to use section supports at regular intervals (also called anti-cascading towers), or to provide enough longitudinal strength to the suspension towers. In the second solution, the use of LCD can make it possible to limit considerably the magnitude of the longitudinal loads and thus the cost of the towers.
After a general presentation of the different categories of LCD, this document describes some devices which are currently available. It then summarises the experience of the countries that took part in the survey conducted as part of SC22 WG 06 of CIGRE.
E:\174\Lcd1099c\LCD1099C.DOC 4 3. CLASSIFICATION OF LCD The task assigned to an LCD is to avoid major and costly line failures, which could occur in the case of certain exceptional loads, by releasing or limiting the stresses applied to a main line- component.
Practically, LCD are generally designed to protect the towers from exceptional stresses applied by the conductor in the case of unusual icing, cascading, etc.
Considering the existing devices, two categories of Load Control Device can be defined:
3.1 Load release devices In this case, the conductor load that could cause the failure of towers is totally released from the tower so that the conductor system is not controlled (or a very limited control). Different types of devices exist: -Devices in which the conductor grip within the tower attachment fitting is released, such as release type suspension clamps (no more control of the longitudinal tension), -Devices in which the conductor or the fitting itself become detached from the tower, by means of mechanical fuses such as shear pins or safety bolts, associated with a system (such as a loose jumper) to prevent the conductor from falling down to the ground.
3.2 Load reduction devices In this case, the conductor load that could cause tower failure is reduced to an acceptable value and the system maintains control of the conductors. Different types of devices exist: -Devices acting on the fastening of the conductor within the fitting, such as controlled sliding clamps, in which the conductor slides when the longitudinal load exceeds a specified value, -Devices which act on the length of the fittings (unfolding or sliding elements) so as to increase the apparent length of the conductor. -Devices which enable elastic deformation or swinging of the crossarms of the tower to absorb longitudinal load.
Dynamic damping devices constitute another category of control equipment. These act on the origin of the stress and are designed to stop or reduce dynamic phenomena such as conductor vibration and galloping: damping jumpers, Stockbridge dampers, spacer-dampers etc. These devices cannot really be considered as LCD and this report will not address them further.
4. TECHNICAL DATA RELATED TO AVAILABLE LCD This chapter gives the principles and characteristics of some important load reduction devices of two types: - Special elongating fittings - Controlled sliding clamps
E:\174\Lcd1099c\LCD1099C.DOC 5 4.1. SPECIAL ELONGATING FITTINGS: JAPANESE EXPERIENCE These devices were occasionally used in the past in Japan. This is mainly due to the increase in restrictions to the building of new lines and increasing urbanisation around the existing lines. In Japan, new buildings are often built under existing lines and Japanese utilities must find solutions to compensate for the lack of distance from the lines to the ground and the buildings without replacing or raising towers, if possible.
One solution is to replace the conventional suspension systems with Suspension-Tension Insulator Assemblies. These devices can also be used to improve the insulation level of the lines, by increasing the number of insulators without lowering the height of conductors. In these cases, the existing suspension insulator set is replaced by two tension sets suspended under a special suspension fitting, which in normal service is shorter than the conventional suspension set.
In order not to degrade the mechanical behavior of the existing towers in the case of conductor breakage in one span or unbalanced tension (unusual ice accretion or ice falling off), these devices have been designed as load reduction devices: an excessive differential tension across the tower elongates the fitting and increases the apparent conductor length so as to reduce its tension.
Two types of these fittings have been developed in Japan.
4.1.1. TYPE I 4.1.1.1 Principle: The elongation of the TYPE I device is triggered by the breaking of shear pins and is damped by the deformation of an impact load absorber . The following figures illustrate the functioning of the system in the case of conductor breaking in one span. 1) Situation in normal service
Figure 1
E:\174\Lcd1099c\LCD1099C.DOC 6 In this normal situation, no loads are applied to the shear pins or the impact load absorber.
2) Intermediate situation: after the conductor break but before the rupture of the shear pins
Figure 2
The suspension metal fitting moves to the right, due to the breaking of the conductor in the span to the left. If the dynamic load is too high for the shear pins, they break, which leads to following situation.
3) Final situation
Figure 3
The dynamic effects of conductor breaking are damped by deformation of the impact load absorber. In the final situation, the conductor tension is reduced by increasing the length of the span.
E:\174\Lcd1099c\LCD1099C.DOC 7 4.1.1.2 Design characteristics:
-The rupture strength of the shear pins is 40 to 60% of the maximum working tension of the conductor. -The permissible load of the shear pins against unbalanced conductor tension (ice drop, strong wind) is 40% of the ultimate tensile strength of the conductor. -Rupture of the conductor generates a sudden projection of the fitting, which creates a large impact load. The impact load absorber is designed to reduce this load to the same level as that existing in conventional suspension sets. -The impact load absorber is designed to control loads up to 1.5 times the maximum working tension of the conductor.
4.1.1.3 Mechanical Performance:
The performance of the device was checked on an actual scale test line. -Conductor breakage: * The final tension of the conductor in the opposite (intact) span is less than 60% of the maximum working tension (with the worst atmospheric loads). * The impact load is less than with the conventional suspension sets, because of the impact load absorber.
-Unbalanced conductor tension (ice drop, unusual ice accretion, strong wind): * The unbalanced tension applied to the tower is less than 25% of the maximum tension of the conductor in case of ice drop or strong wind. * The special suspension hardware of the device is shorter than the conventional suspension set, so that the dynamic unbalanced tension and the swing angle are larger. The tests showed the need to consider wear design aspects considering the longitudinal swing of the device.
- Wear performance * The wear calculations, assuming an expected life of 50 years, showed no problem.
4.1.1.4 Electrical performance: The tests showed that the device could withstand a power arc equal to or higher than that withstood with conventional tension sets.
4.1.1.5 Service experience: At the present time, there are around 1245 Suspension-Tension Insulator Assemblies of this type installed, of which: -170 for 66 kV lines -875 for 154 kV lines -200 for 275 kV lines No data was given about the efficiency of these devices in real faults.
E:\174\Lcd1099c\LCD1099C.DOC 8 4.1.2. TYPE II The elongation of the so-called « tension balance type » device occurs by sliding of the shackle in a groove designed into the yoke-plate; the overall elongation is shorter than with the TYPE I device, and consequently the increase in conductor sag is less.
Figure 4 illustrates the system functioning in the case of a conductor system breakage in one span, compared with a conventional device. The principle is worse for unbalanced tension due to ice drop or unusual ice accretion.
Conventional type Improved type
Figure 4
Note: After failure of the conductor in one span, the device enables the conductor tension in the opposite(intact) span to be reduced to 60% of the maximum working tension in the worst atmospheric loads.
There are limitations for the use of such devices; they are not suitable in the case of high tension differences between spans (which might cause inopportune sliding or no sliding at all) and their efficiency in reducing the tension decreases with the number of successive suspension spans in a section
It is worth mentioning the following elongating device, which is currently under consideration for use on tension towers in Japan.
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In normal condition In the condition of abnormal load
Figure 5
4.2. CONTROLLED SLIDING CLAMPS Controlled sliding clamps are used on a large scale in some countries, such as Belgium, Romania and France.
4.2.1. FRENCH EXPERIENCE In France, controlled sliding clamps have been used systematically for more than 45 years on all transmission lines, both for the phase conductors and earthwires. During this time, the technology of these devices has continuously evolved, in order to improve their performance, and to optimise their use and adapt them for new conductors.
4.2.1.1 Principle of the controlled sliding clamps
A controlled sliding clamp is made of a body in which rests the conductor and of a cover (see figure 6). This cover is installed over the conductor by means of a system the pressure of which can be adjusted to make it possible to carry out the calibration of the conductor sliding load of the clamp. The first types of clamps used torsion bars and were called « elastic tightening clamps ». EDF finally abandoned the use of torsion bars in 1980, in favour of clamps exclusively based on the controlled tightening of the bolts; these clamps are simply called « controlled tightening clamps »; they enable a more precise adjustment and allow better performance.
Figure 6: Controlled sliding clamp
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4.2.1.2 Calibration
The conductor must slide within the clamp at a specified longitudinal load, FS , that may occur as a result either of a conductor rupture or an exceptional unbalanced ice load.
FS = longitudinal load to be applied to the conductor for sliding = Sliding load due to the tightening of the conductor in the clamp + Sliding load due to the vertical load applied to the clamp by the conductor = (f1 + f2). FC + f1 .FV
f1 = friction coefficient between clamp body and conductor f2 = friction coefficient between clamp cover and conductor FC = compressive load of the clamp cover
FV = vertical load applied to the body (in the general case, this load is taken VH²²+ , to take into account the transverse load H) FS = longitudinal load at which sliding occurs
Each tower of a line is designed to resist asymmetric icing. EDF conventionally considers an ice sleeve of n cm (radial thickness) on all spans on one side of the tower and (n-2) cm on all spans on the other side; the value of n may be 2,4 or 6, depending on the area, the ice density is taken 0.6. The loads are calculated while taking into account the deflection of the insulator sets. Table 1 gives some examples of application.
Mechanical tension in
the conductors Longitudinal stress Conductors Ice
overload (daN) (daN) Tangent tower
External Weight Ultimate End End Type Section 225 kV 400 kV diameter load with ice With ice (cm) Tower (1) (2) (mm) (daN/m) (daN) n cm (n-2)cm
Conductor (4)
ASTER 570 31 1,57 18 400 2 7 800 4 420 3 380 1 200 900
PETUNIA 612 32 2,24 32 700 4 14 000 9 500 4 500 2 000 1 600
Earthwires (5)
PHLOX 116.2 14 0,63 10 800 2 4 150 1 900 2 250 1 600 (3) PHLOX 228 20 1,24 21 200 4 10 700 6 800 3 900 2 000
E:\174\Lcd1099c\LCD1099C.DOC 11 Table 1: Order of magnitude of the loads applied to the supports with unequal ice loading
(1) Length of the suspension set : 2,30 m for 225 kV (2) Length of the suspension set : 3,45 m for 400 kV (3) Conventional value (span < 1 000 m) (4) All aluminum alloy conductors (5) Aluminum alloy steel reinforced earth wires Catenary constant : 2 200 m at 45° C Length of the adjacent spans : 550 m
The sliding calibration of the suspension clamp is chosen from the following range of standard support loads : 8, 14, 20, 30 kN. For each tower, it is chosen as the value immediately higher to L-0.3. VH²²+ , where 0.3 is the assumed value of the friction coefficients, V, H and L are the vertical, transverse and longitudinal loads applied to the conductor in the clamp, in the asymmetric icing assumption. Support loads greater than 30 kN require other solutions such as the use of clamps in series, the use of suspension- tension sets or the use of a tension tower.
4.2.1.3 Design of a controlled sliding clamp
In order to make sure that the system operates correctly, the profile of the clamp body must be determined in a precise manner.
It is necessary that the conductor pressure be equally spread out over the useful length of the body to ensure a regular sliding on the basis of a longitudinal load determined by a precise adjustment. The length of the body is determined experimentally in such a way that the compression stresses do not exceed the maximum pressure allowable at the point of contact among the strands.
It is also necessary to calculate the radius of curvature of the clamp body so that the bending stresses within a strand remain acceptable.
A computer model, presented in figure 7, was developed to calculate the optimal longitudinal profile of a clamp body.
E:\174\Lcd1099c\LCD1099C.DOC 12 Ultimate pressure between two layers by point of contact The layers slide Specialization of the Self pressing tensions flexural rigidity sliding clamp Reaction of the clamp body Pressing force of the cover
Static strain induced in Ultimate load on the Minimal length of the clamp wire near clamp body suspension clamp
Calculation : - of the longitudinal profile of a conductor supported by a clamp Permanent and extreme - of the curvature ot the conductor loads on suspension clamps - of the strains induced in the wires Low temperature and storm assumption Determination of the length and the profile of the clamp body
Drawing of the longitudinal Radius of curvature profile of the conductor of the clamp body Induced strains in the wires for Length of the clamp the various loading assumptions
Figure 7- Research of the optimal longitudinal profile of a clamp body
4.2.1.4 Test
The accuracy of prediction of the load to cause conductor sliding depends on several factors: frictional coefficients, clamp and conductor manufacturing tolerances, etc.
It is necessary to reduce the dispersion as much as possible. Nevertheless, the precision on predicting loads that cause sliding, with respect to the clamp calibration load, cannot be better than 15%.
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To verify the sliding characteristics of the suspension clamps, tests are carried out with each new type of suspension clamp:
1)Measurement of friction coefficient
Test conditions This test is performed with an adjustable sagging machine. The conductor rests on the body of the suspension clamp without the cover. The winding angle is between 20 and 30 degrees and the vertical load is given by Table 2.
The measurements of the sliding efforts are performed over a length of one meter.
Test requirements The friction coefficient shall not exceed 0,35.
Suspension clamp class Conductor Vertical Load (kN)
A ASTER 75 8,5
B PHLOX 75,5 17
C PHLOX 147 24
D ASTER 570 40
E PETUNIA 612 45
F POLYGONUM 1185 45
G POLYGONUM 1185 90
Table 2: Vertical loads for sliding tests according to the type of suspension clamp
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2) Measurement of the sliding threshold forces
Test conditions • Measurements of sliding threshold values are made according to the cover pressure. • The conductor is loaded at 20% of its ultimate tensile strength in a suspension clamp with a winding angle of 15 degrees (in order to settle each wire in position). • The cover is tightened to the value given by the manufacturer and the conductor is released. • The suspension clamp is then pulled over a length of one meter. • The sliding force is the maximum tension applied to the conductor on the stressed side (the other being released).
Test requirements The value of the sliding force relative to the sliding length must remain within the allowable areas such as defined in figure 8; the force must range between the values A and B following a displacement not exceeding 1 cm. The actual curve must be shifted by a value C so as to take account of the friction due to the vertical load according to the formula C= 0.05x UTS
The breaking load of the conductor following sliding must remain higher than 95% of the ultimate tensile stress (UTS) of new conductor for homogeneous conductors, and 90% of the UTS for bimetal conductors.
E:\174\Lcd1099c\LCD1099C.DOC 15 Sliding load (kN)
NON AUTHORIZED ZONE A
B AUTHORIZED ZONE II
C
NON AUTHORIZED ZONE AUTHORISED ZONE I d Length of sliding
MAXI 1000 10
Progressive loading
RANGE (kN) A (kN) B (kN) C (kN)
8 8 6,5 3,6
14 14 11 6,3
20 20 15,5 8,9
30 30 23 13,3 NOTE : C = (A + B) /4 Figure 8
4.2.1.5 Device efficiency
1)sliding limitation:
When sliding occurs it is stopped whenever the tension difference applied to the clamp on either side of the clamp falls below the calibration value.
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Thus the sliding length depends mainly on the initial tension difference applied to the clamp, on the relaxation due to the deflection angle of the insulator set and hence on the length of this set.
The conductor sliding range observed in the overhead line network, in several cases of unequal loading, ranged from 1 to 2 m. For earthwires this value is much higher.
In case of a breakage of a conductor or a support, considerable sliding, of ten meters and more, were noted on towers located on either side of the defect.
On the other hand, the movement declines very rapidly on subsequent towers.
Finally, in some cases, in particular when sliding is considerable, the conductor can be damaged (rupture of strands, bundling and bird cage formation); this phenomenon however is very rare on conductors having an outside layer made of aluminium alloy (AAAC and AACSR conductors).
However, this inconvenience which requires the replacement of a length of conductor, is in all situations less serious than the destruction or damage of several support structures. The line downtime is reduced and the repair costs are considerably lower.
2) Action of clamps during real faults
Among the incidents, whose consequences were limited by the action of sliding clamps, we can give the following examples: -In 1949, a 40 degree angle tower fitted with suspension sets and located in the mountains, with a high level difference, was carried away by an avalanche. Two of the three conductors were broken; the conductors slid in the clamps of several suspension towers. Only one crossarm was distorted. -In 1964, after the rupture of the top of a 50 degree angle tower of a 400 kV line within the Paris area, damage was limited to 4 towers, as a result of sliding of the 6 conductors within their suspension clamps. -In 1971, an overall and persisting icing phenomenon, unusual within the area of Le Creusot, gave rise to an ice sleeve formation on the phase wires of a 400 kV line, with linear weight up to 15 kg. This overload was approximately 4 times higher than that considered in the line design calculation. The complete failure of the conductor bundle was prevented by means of the sliding of the conductors within the clamps of 5 successive towers. Only 2 of them were damaged. -In 1974, an aircraft hit 2 of the 6 twin conductor bundles made of AAAC, of a 400 kV line. The 4 conductors slid in their clamps. The 2 suspension towers supporting the damaged span, had to be replaced, as well as some elements of the two adjacent tangent towers. -In 1988, an aircraft hit the 3 twin bundles of a 400 kV line near a substation. The broken conductors slid in the clamps over more than 3 km. 2 suspension towers had to be replaced.
E:\174\Lcd1099c\LCD1099C.DOC 17 4.2.2 BELGIAN EXPERIENCE
Since 1971, controlled sliding clamps have been systematically installed on the conductors of 380 kV lines.
The first aim was to reduce the weight and cost of tangent towers, by reducing the accidental stresses considered in the design of the towers with the assumption of conductor breakage (the effect of breaking all the conductors of the bundle).
The torsion moments have traditionally been calculated by application of the maximum tension of the conductors of the phase conductor on one side of the crossarm, without taking into account the dynamic and releasing effect due to the rupture of the phase conductors and the swinging of the insulator sets. This method, if roughly applied on 380 kV lines (equipped with twin bundles), would have led to un-economic tangent towers. The use of controlled sliding clamps was estimated to offer a reduction in the weight of tangent towers of 6%.
These arguments do not apply to earthwires, because of the lower stresses and the lower potential load reduction and consequently sliding clamps are not installed on earthwires.
Two types of controlled sliding clamps are used. One type uses a torsion bar bolted in the body of the clamp and calibrated shear pins in the cover. The second type is based on the controlled tightening of the bolts.
In the lines, the conductors are systematically protected by armour-rods; the specified sliding load is determined as a function of the conductor type and is taken as a relatively high value to avoid inopportune sliding. No sliding in clamps has been seen in Belgium.
4.2.3 ROMANIAN EXPERIENCE
For the past 30 years, controlled sliding clamps have been used for the conductors of all transmission lines in Romania. These clamps are based on the controlled tightening of the bolts by a limited torque wrench. Their actual efficiency has not been measured on the network but no failure due to cascading has occurred, although most supports are guyed poles. RENEL is still working on the optimisation of the controlled sliding clamps.
Release type suspension clamps have been used, but very occasionally and only on lines without problems of conductor galloping. Their benefit has been demonstrated on several occasions.
E:\174\Lcd1099c\LCD1099C.DOC 18 5 SERVICE EXPERIENCE OF LCD IN DIFFERENT COUNTRIES Summary of the answers of the countries replying to the survey.
COUNTRY/ USE OF LCD/COMMENTS COMPANY
ARGENTINA Not used AUSTRALIA Not used Conductors are generally ACSR AUSTRIA/ Sporadic use in the past: only on a few towers on lines built before 1940 TKW BELGIUM/ Systematic use for conductors of lines equipped with bundles; TRACTEBEL see paragraph 3.2.2 BRAZIL/ Sporadic use CEMIG CANADA/ ONTARIO- Not used HYDRO
HYDRO- Sporadic use: QUEBEC 1)Controlled sliding clamps were installed for the reconstruction of 735 kV lines, after heavy icing events. No other incident has occurred since then, so the efficiency of this equipment has not been verified. 2)After erection, safety bolts for the attachment of the construction cable are used on « chainette towers ». A loose jumper is installed to retain the construction cable from falling. CZECH Not used REPUBLIC/ ENERGOVOD FINLAND/ Not used IVO FRANCE/ Systematic use on all transmission lines; EDF see paragraph 3.2.1 GERMANY/ Sporadic use in the past: BAYERNWERK Now, the German utilities consider that 1)the reliability of LCD and their effectiveness has not been adequately demonstrated 2)the increase of the conductor sag is unacceptable, in view of public safety, because of the high density of population in Germany.
GREAT Not used BRITAIN/ NGC
E:\174\Lcd1099c\LCD1099C.DOC 19 INDIA Sporadic use ITALY/ Not used ENEL 1)All towers are designed to withstand a specified longitudinal load. 2)ENEL utilises ACSR conductors and argues that the wires are considerably damaged after sliding in the clamp. 3)ENEL is not convinced of the LCD efficiency, especially in mountainous areas. JAPAN Sporadic use; development in progress; Raising the conductors on existing lines under which buildings appeared later (see paragraph 3.1) KOREA/ Not used KEPCO NETHERLANDS/ Not used KEMA NEW Not used ZEALAND NORWAY/ Sporadic use STATNETT When the sagging of the earthwire is considered less serious than a mechanical damage, sliding clamps are used for earthwires. The clamps are basically adjusted for a 30 kN sliding load. The tower tops are designed accordingly. PORTUGAL/ Not used EDP 1) all towers (suspension or tension towers) are designed to withstand, in the worst conditions and with a certain safety coefficient, the rupture of a conductor or an earthwire 2) zones with potential icing are very rare in Portugal and avoided whenever possible. ROMANIA/ Systematic use for conductors; development in progress; RENEL see paragraph 3.2.3 SPAIN/RED Not used ELECTRICA SWITZERLAND/ Sporadic use; EOS In the case of double tension sets used for twin bundles, EOS uses special yoke plates to damp the dynamic effect of the rupture of the set. USA/ Sporadic use in the past; LINDSEY since early 1970’s, two LCD systems have been used in conjunction with MANUFACTING porcelain line post insulators throughout North America : shear pins and COMPANY elastic-plastic base. These devices were a type of mechanical fuses.
Lock pin is installed in the device during the sagging operations.
E:\174\Lcd1099c\LCD1099C.DOC 20 Le CIGRÉ a apporté le plus grand soin à la réalisation de cette brochure thématique numérique afin de vous fournir une information complète et fiable.
Cependant, le CIGRÉ ne pourra en aucun cas être tenu responsable des préjudices ou dommages de quelque nature que ce soit pouvant résulter d’une mauvaise utilisation des informations contenues dans cette brochure.
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