design, testing and installation of innovative 380 kv
TRANSCRIPT
Design, testing and installation of innovative 380 kV Dutton–Rosental towers
P. Berardi*, M. Forteleoni, M. Marzinotto, A. Piccinin, A. Posati, M. Rebolini
TERNA RETE ITALIA S.p.A.
Italy
SUMMARY Overhead power lines, since their conception and first installations, were designed as industrial
products, built only to achieve the excellence from both the technical and economical point of view.
Such kind of structures let to build up the main framework of the Italian grid.
Traditional latticed steel towers allowed the growth of an efficient and boost network in a territory like
the Italian one, full of difficult routes and with frequent mountain crossings (especially across Alps
and Apennines), extreme weather conditions (high wind, heavy ice and snow loads), high seismicity
risk and to face increase of energy request by the Italian energy market during the last century.
However, in the last fifteen years, Transmission System Operators have increasingly developed a new
vision of electrical installations, with the aim to merge technical needs with the request of a deep
integration with the surrounding environment.
The combination of the technical requirements with the reduction of environmental impacts is a goal
started by Terna Rete Italia since the late 90s [1].
The solution of monotubolar steel poles was the beginning of a new approach for a more “visually
friendly” tower design; likewise a first experience of a “beauty contest” was held in 2003 with a
competition open to famous architects to design environmentally friendly supports, won by Norman
Foster.
After the installation of Foster Towers on the single circuit 380 kV “Tavarnuzze – S. Barbara –
Casellina” overhead line in 2007, Terna Rete Italia called for a new competition in 2009, won by
Hugh Dutton and Giorgio Rosental architects.
The design of the winner new tower takes its inspiration from the behaviour of trees, able to follow the
wind flow and the shape of the landscape: in the same way the so called “Germoglio” towers are able
to join the environment with its innovative profile.
The aim of this paper is to describe the design phases of the new “Germoglio” pylons project and the
tests performed on the prototypes of all the components.
Many topics have been studied and evaluated to ensure the feasibility of the architectural choice, in
particular the behaviour of the whole system made up by the tower and the insulators closely
connected during the stringing and sagging operations.
21, rue d’Artois, F-75008 PARIS B2-309 CIGRE 2016
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A real insulating tensostructure has been designed using standard components: two different kind of
insulators (both composite and ceramic), counterweights and special suspension clamps and fittings
have been considered.
All prototypes of new components have been subjected to type tests, reproducing the mechanical and
electrical configuration of this such innovative structure, under real line conditions.
After all the type tests, six “Germoglio” towers have been produced and installed on the new double
circuit 380 kV “Trino – Lacchiarella” overhead line, along the highway between Milano and Genova.
KEYWORDS
Overhead line structures, innovation, environmental integration, full scale tests.
1. THE BEAUTY CONTEST
Following his continuous research to find new innovative components, Terna Rete Italia launched a
new tower design contest [2] to evaluate proposals of innovative structures for 380kV OHLs.
The aim of the competition was to develop structures that can merge both the technical requirements
used for the design of standard latticed steel towers (mechanical and electrical features, flexibility, use
of conventional materials, construction techniques suitable for an industrial production, easy
installation, possibility to make ordinary maintenance operations) and the reduced environmental
impact (integration with the landscape, reduced land use, innovative visual appearance).
The projects, made in compliance with the Italian regulation [3] and with the European standards [4]
and [5], were judged by and independent commission according to the creativity and to the
architectural quality of the proposals, after the verification of the technical compliance by Terna Rete
Italia engineering department.
Famous architects, as Giorgetto Giugiaro and Enrico Frigerio, took part in the competition, which was
awarded to Hugh Dutton and Giorgio Rosental with the “Germoglio”.
Figure 1. a) “Soluzione B” by Giugiaro – b) “Dinamico Caos” by Frigerio – c) “Germoglio” by Dutton – Rosental
The winner, the “Germoglio”, was developed by Terna Rete Italia starting from a preliminary
architectural proposal until an engineered project, installed on the 380kV Italian grid.
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2. A NEW CONCEPT FOR OHL TOWERS: THE “GERMOGLIO”
The inspiration of the “Germoglio” comes from the trees which naturally grow up in the environment
and are easily able to adapt to all the external conditions of wind, snow, temperature. In the same way
this OHL tower can be considered as a complex system made by the connection of the main body, the
insulating equipment, the counterweights and the conductors.
Its name means “newborn tree” which grows up from the ground with its leaves.
Figure 2. “Germoglio” preliminary sketch
The visual aspect of such structure and consequently its insertion in the environment is strongly
influenced by the light. The main body is made with sharp edges: the lower part is a single square
cross section which splits in two arms with pentagonal sections which become two triangular sections
on the top (see Fig. 3): from any position an observer can see only one surface of the tower illuminated
by the natural light.
Figure 3. Main body cross sections
The other important feature of the “Germoglio” is the connection between conductors, insulating
equipment and tower body. Each phase is linked to the others with composite insulators while the
connections to the tower are made in following ways:
the higher phase is connected to the top of the tower with a composite insulator and to the
main body with porcelain post insulators,
the middle phase is connected to the main body with porcelain post insulators,
the lower phase is connected to the main body with porcelain post insulators and to the bottom
of the tower, though the counterweights system, with a composite insulator.
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The counterweights system regulates the load applied by the insulating equipment on the top of the
tower, preventing the overloading of the arms in case of high transversal loads form the conductors
due to the combination of line deflection angle and high wind speed.
The counterweight vertical displacement is also studied in order to not exceed the maximum allowable
conductor sag, both in the front and in the rear spans, considering all the external load conditions,
including the hypothesis of broken conductors.
Figure 4. FEM model of line section stability
3. STRUCTURAL DESIGN
The structural design follows the same standards approach used for latticed steel towers, in accordance
to [3], considering the load conditions of the Italian territory divided in two zones, A and B, with
different values of wind speed, ice loads and temperature.
In the first installation on the Trino – Lacchiarella OHL a “N”-type “Germoglio”, for a straight line
route, is used.
The design load are calculated using Terna Rete Italia standard conductors and OPGW:
– triple bundle ACSR Ø31,5mm conductors,
– two separate ground wires on each arm (OPGW Ø17,9mm).
Italian territory Horizontal conductor load – Every Day Stress (EDS)
3 x ACSR Ø31,5mm OPGW Ø17,9mm
A 21% RTS = 3540 daN 15% RTS = 1590 daN
B 20% RTS = 3370 daN 14% RTS = 1480 daN
Table I. Conductors and OPGW
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The structural design takes into account both the global architectural vision of the new structure and
the technical needs to ensure the reliability level requested for a standard OHL tower.
The definition of the construction sequence is a very challenging part of all the project: the geometry
of the metal sheets cutting, all the full penetration welds and the position of the internal reinforcement
are studied to respect the ideal external appearance also considering the feasibility of an industrial
production.
Every component is studied using Finite Element Models before testing full scale prototypes.
Figure 5. FEM analysis and reduced scale (1:10) prototype
The research of innovative constructive solutions allows to reach an important goal: the external
appearance is not influenced by the internal structural reinforcements and by the counterweighs
system designed for the mechanical stability of tensostructure and conductor spans.
The connection between all the sections is realised with internal flanges, which do not affect the
external profiles too.
Before building full scale prototypes for load tests a reduced scale prototype (1:10) was produced to
check the feasibility of the welded connections (in particular for the construction of the upper part of
the tower, where there are three changes of cross sections from square section, to pentagonal section,
to triangular section, in order to avoid weld overlaps) and the general stability of the insulating
equipment connected with the counterweighs system.
A group of steel cables is installed between the two tower arms to balance the load applied on the two
sides of the structure.
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4. TESTING ON INSULATING EQUIPMENT
The design of the insulating equipment, including the counterweighs system, were tested to verify both
the electrical and the mechanical performances. Testing on both composite and porcelain post
insulators were performed, mainly according to [6], [7] and [8].
Figure 6. Insulating equipment and real scale sample for RIV Corona and dry switching impulse withstand test
Due to the unconventional and complex shape of the tower, the inference of the performances under
standard switching impulse of insulators and tower clearances, the critical electrical gradient of
fittings, clamps, etc. and the radio interference noise level were very difficult to estimate. Laboratory
tests were then an obliged step in order to ascertain if the tower design needed some refinement.
Furthermore, a real scale model with two phases and a structure representing the external surface of
the tower was produced to perform electrical tests.
The following tests were successfully performed according to Terna Rete Italia technical
specifications and to [9]:
– radio interference voltage (RIV) test,
– visible corona test,
– dry switching impulse withstand test.
Figure 7. Flashover along porcelain post insulators
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The counterweighs system is designed to adjust the load applied on the composite insulators in order
to not exceed their allowable mechanical load and to not overload the top of the tower. At the same
time it prevents the violations of external clearances in case of broken conductors.
This system is composed by a mobile weight connected with a chain to a steel cable directly linked to
the lower composite insulator. A special device is added between the steel cable and the chain to
prevent chain torsion.
A system of three fixed pulleys is installed inside the “Germoglio” in order to reduce the dimension of
the counterweighs, instead of connecting directly the weights to the tensostructure.
Figure 8. Counterweighs system
5. PRODUCTION AND LOAD TESTS ON REAL SCALE STRUCTURE
The full scale load tests are the last check before the production of the “Germoglio” for the Trino –
Lacchiarella OHL: all the components tested in the Tower Test Station, except composite and
porcelain post insulators replaced by steel elements, are presently installed on the grid.
During the tests all the load cases required in [1] were applied according to [10]. Load cases
considered are:
a) Every Day Stress (EDS), no broken conductor or ground wire + top deflection check;
b) Low Wind (CVS3), no broken conductor or ground wire + top deflection check;
c) Wind & Ice (MSB), 2 broken conductors (on different phases) and no broken ground wire;
d) Wind & Ice (MSB), no broken conductor or ground wire;
e) High Wind (MSA), 1 broken conductor and 1broken ground wire;
f) High Wind (MSA), 2 broken conductors (on different phases) and no broken ground wire;
g) High Wind (MSA), no broken conductor or ground wire.
The following standard test devices for the application of external loads on testing structures are
available at Tower Test Station:
2 main latticed structures to apply external loads on the tower during the tests (one structure
for the transversal loads, one for the vertical and longitudinal loads);
a foundation system for the tower placing;
24 winches to apply equivalent loads from conductors, ground wires, wind on structure;
a system of pulleys and steel wires to properly reduce the loads requested by the test and the
winches capability.
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The control of the winches and the simultaneous recording of loads on tower, measured by load cells,
were performed in the main control room. An auxiliary control room hosts the strain gauges data
acquisition system to compare stress and strain measured to the theoretical values obtained with FEM
models.
Figure 9. “Germoglio” assembly sequence at Tower Test Station
Figure 10.Full scale load tests of double circuit and single circuit “Germoglio” towers
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6. OPERATION AND MAINTENANCE EQUIPMENT
During the full scale load tests, operation and maintenance equipment was tested as well. Every
“Germoglio” is equipped with an innovative external rail, which replaces the common climbing
devices; in this configuration an innovative semi-automatic lifter can be used.
Figure 11. Operation and maintenance equipment
Special devices for the maintenance tool connections are welded on the upper part of tower arms to
allow operators climbing without changing the external tower profile.
The door on the bottom of the “Germoglio” allows internal inspections of the bolted connections
between the tower sections and the counterweighs system.
7. INSTALLATION ON 380kV DOUBLE CIRCUIT OHL TRINO-LACCHIARELLA
The first six “N”-type “Germoglio” are installed on the 380kV double circuit Trino – Lacchiarella
OHL.
As the line route of this innovative OHL is mainly located in the countryside, Terna Rete Italia has
widely installed environmental friendly structures, as monopoles, in order to reduce its visual impact.
“Germoglio” towers were selected as special components for the most visible section of the whole
line, along the A7 highway.
Figure 12. “Germoglio” on the Trino – Lacchiarella OHL
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Special deep foundations have been designed for the six “Germoglio” in order to guarantee the
structural stability of the line section, which are built upon rice fields (low resistance terrains).
Towers installation and stringing and sagging operations were performed according to the procedures
defined during the full scale load tests.
Figure 13. Installation and stringing operations
Figure 14. Trino – Lacchiarella “Germoglio” towers in operation
The Trino – Lacchiarella OHL is in regular operation since January 2014.
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8. CONCLUSIONS
The continuous research of innovative 380kV towers by Terna Rete Italia has the aim to combine
standard latticed steel towers with new structures for a better acceptance of the power transmission
grid.
Every new installation increases the number of instruments available for OHL designers to fulfill a
better environmental integration of transmission infrastructures, improving safety and reliability of the
Italian grid.
The “Germoglio” project is a successful example of applied research: a combination of architectural
study, environmental inspiration and advanced engineering design.
BIBLIOGRAPHY
[1] P.Berardi, A.Posati, M.Rebolini, R.Rendina. L’energia Elettrica n. 4 – volume 88, Luglio-Agosto
2011:“Innovazione nelle linee elettriche aeree”
[2] Bando di concorso di progettazione – Servizi di progettazione di strutture portanti 2007/S 170 –
209940 del 05/09/2007
[3] CEI 11-4 1998-09: “Esecuzione delle linee elettriche aeree esterne”
[4] EN 50341-1 Overhead electrical lines exceeding AC 1 kV - Part 1: General requirements –
Common specifications (December 2012)
[5] UNI ENV 1993-1-1 Eurocode 3-Part 1-1: “Design of steel structures”
[6] IEC 61109:2008 – “Insulators for overhead lines – Composite suspension and tension insulators
for a.c. systems with a nominal voltage greater than 1 000 V - Definitions, test methods and
acceptance criteria”
[7] IEC 62217:2012 – “Polymeric HV insulators for indoor and outdoor use – General definitions,
test methods and acceptance criteria”
[8] IEC 61467:2008 “Insulators for overhead lines – Insulator strings and sets for lines with a
nominal voltage greater than 1000 V – AC power arc tests”
[9] CEI EN 61284 (09.1997) “Requirements and tests for fittings”
[10] IEC 60652 2002–06 "Loading tests on overhead line structure"