Reaching new heights
Designing overhead transmission lines across maritime navigation routes is never easy – particularly when that route is one of the busiest shipping channels in the world. A look at the Suez Power Crossing reveals some interesting design features needed to ensure safe and efficient operation of this link which will help economic stability and development in the Middle East region.
Enhancing the economic stability of the Middle East and the area encompassing Egypt, Jordan, Lebanon and Syria in particular is an important regional objective; indeed, it can be said to be in the long-term interests of world peace. The proposed power interconnection that will run from eastern Turkey, across Asia Minor and into north Africa is seen by many as a vital part of this enhancement process.
A key part of this interconnection project is a 500 kV feeder between Egypt and Jordan – a link connecting Asia to Africa. This 290 km power line – the Suez Power Crossing – runs across the southern part of the Sinai peninsula and has to traverse the Suez Canal for connection to the main electricity distribution network of Egypt.
Egypt is a large country with considerable capacity to develop its various industries and its tourist centres outside those around the Cairo and Luxor areas. One of the keys to this development is a stable and reliable power supply. Currently there is no permanent connection between the country`s power distribution network on the west side of the Suez Canal and the centres of local industries and tourism on the Sinai peninsula.
The canal is one of Egypt`s principal sources of revenue and there is, quite naturally, considerable reluctance to embark on a venture that would present a hazard to the shipping passage – most of the vessels using the canal are large ocean-going ships with considerable freeboard and high superstructures. For this reason, and others, a cable passing under the canal was first considered for the power link. However, it was soon realised the costs of an underground cable would be prohibitive, and so a high transmission line costing only about a quarter of that of an underground cable was chosen.
The contract to construct the Suez crossing was won by German engineering firm Siemens in co-operation with the Turkish firm STFA. STFA is internationally active in transmisison projects, and has already constructed overhead transmission lines along the Sinai peninsula.
Siemens has been involved in the construction of overhead transmission lines across navigable waterways for more than 25 years. For example, It has gained considerable experience from the design and construction of the River Elbe crossing north of Hamburg employing 230 m-high suspension towers.
More recently, the Bosphorus crossing involving a span of approximately 1800 m has been undertaken by a Siemens-STFA partnership.
Crossing the canal
The design of the power link is based on Egyptian specifications for overhead transmission lines as well as accommodating the requirements of IEC and ASCE standards. In particular the overall structure has been designed to be earthquake-proof.
The 500 kV overhead transmission line has two circuits and crosses the southern section of the Suez canal about 10 km north of Port Suez. The crossing consists of four anchor towers, two for each of the two incoming single-circuit lines and two suspension towers. The span between the suspension towers and the anchor towers is 755 m and 845 m respectively. The span across the canal itself is 600 m in length.
The canal authorities specified a minimum ground clearance of 140 m for the section of the line that crosses the canal. With 140 m between the vertex point of the conductors and the water level, a conductor sag of approximately 29 m, a suspension insulator string length of 10 m and 16 m vertical distance between each of the three phases of one circuit, a tower height of 220 m was considered the optimum.
Consequently, the towers are some of the tallest of their kind in the world. The tower width is 4 m at the top and is approximately 45 m at ground level. An additional cross-arm beneath the conductors prevents the conductors from falling in the event of a failure of the suspension insulator strings which would otherwise block the canal.
Each of the two circuits comprising the overhead transmission line has been designed for a transmission capacity of 1400 MW. In accordance with the design of the incoming lines, aluminium-alloy aluminium-clad steel-reinforced triple bundle conductors are used. Each sub-conductor can thus carry a current of approximately 540 A at the assumed maximum ambient temperature of 45à‚°C taking into account the local solar radiation and a wind speed of 0.6 m/s. Based on these conditions the conductors will be heated to a maximum temperature of 100à‚°C.
The conductor materials were selected so that 33 per cent of the ultimate tensile stress of 418 N/mm2, i.e. 139 N/mm2, is not exceeded at the highest wind load. Taking the maximum permissible stress specified by IEC 826 for the maximum wind loading (viz. 70 per cent of the ultimate tensile strength) as a reference, the selected maximum working value of tensile stress is low and implies that under everyday conditions (i.e. in the absence of wind), the tensile stress is only some 13 per cent (30 kN), as compared to the usual figure of 20 per cent.
Obviously there must be an increased safety factor for a waterway that has to be operational 24 hours a day, 365 days a year. Two optical fibre earth wires (OPGW), each comprising six glass fibres, are run along the tops of the towers. These wires are fitted with aircraft warning spheres.
On the basis of experience gained by the Egyptian Electricity Authority (EEA), porcelain insulator strings with a specific leakage path of 50 mm/kV are employed. Because the insulators fracture under a load of 300 kN, it was necessary to use double insulator strings in the suspension sets and triple insulator strings in the tension sets to ensure the required level of safety.
Conventional armoured suspension grip clamps are employed for fastening the conductors to the suspension insulator sets and compression clamps at the dead-ends. Special demands were made on the fittings of the optical fibre earth wires in the crossing section. These are connected to the suspension towers and the anchor towers by means of heavily armoured dead-end grips.
In order to dampen wind-induced oscillations, spacer dampers are incorporated in the overhead conductor bundles and Stockbridge dampers in the optical fibre earth wires. All the fittings were subjected to extensive electrical and mechanical acceptance tests, which owing to the high-level forces involved, made stringent demands on test equipment.
With overhead transmission lines, electromagnetic interference is generally caused by the voltage gradient along the surface of the conductor. The height of the conductors above the canal and the selected bundle configuration reduce electrical noise emissions and electric and magnetic field strengths to values far below the maximum permitted levels.
The trouble-free passage of shipping using the canal requires that the power transmission line does not interfere with radio communications, which mostly operate using single side-band transmissions. For a transmitter field strength of 50 dB and a signal-to-noise ratio of 24 dB, short-term interference with voice radio communications is only expected to occur in rainy weather or when a vessel passes directly under the transmission line. In the case of digital control systems that are frequently used on modern ships, no interference is anticipated regardless of the weather conditions.
As would be expected with high suspension towers, wind loads greatly influence the rating of all component parts. A peak wind value of 35 m/s, which represents a gust with a reccurrence rate of once every 50 years, is generally taken as the design criterion for overhead transmission lines in Egypt, and therefore has been taken for the Suez power line crossing. With a reserve safety factor of 1.3, representing a reccurrence rate of approximately every 500 years, the base parameter at a height of 10 m is 45.5 m/s. Assuming wind distribution is a function of height, a wind velocity of 72 m/s or 260 km/h at a height of 200 m was the determined value.
On the basis of experience gained with other crossing structures, such as over the Elbe and the Bosphorus, it is known that these assumed wind loads are perhaps excessive, but in view of the high operational safety requirements, they were nonetheless incorporated in the specification.
Marked, and at times severe, earthquake activity occurs in some parts of Egypt, including the area around the Gulf of Suez. For this reason a horizontal ground acceleration of 0.3 g is written into the power link`s specification. An earthquake study with unfavourable model assumptions resulted in a peak acceleration of 0.255 g for an earthquake duration of 8 s with an occurrence rate of once in 1000 years. The investigation revealed that the proposed design value of 0.3 g for the horizontal acceleration was sufficient to cover all contingencies.
Static analysis was performed with the help of a finite element programme employing a three-dimensional bar system using the translational displacement method. The upper part of the suspension tower was computer modeled as an ideal frame system with flexible connections at the nodes. In the lower part of the tower, where the leg members and the diagonals consist of lattice box-type members, the bar ends were assumed to be restrained at the nodes.
The decisive forces for rating the leg members and most of the diagonals were derived from load case wind rectangular to the conductor and wind under 45à‚° to the line. The earthquake load case was decisive for the diagonals in the lower part of the tower. At ground level the leg member compressive forces reached a value of 12 919 kN and the uplift forces reached 9743 kN at this point. The total suspension tower weight amounts to approximately 710 t each.
Each anchor tower is capable of supporting a single circuit line comprising three single-phase conductors and two earth wires. The tower is of star (Y) shape similar to those of the incoming transmission line. The attachment points of the three conductors are at a height of 27 m and the earth wire peaks extend to 37.5 m. They are designed to accommodate a transmission route angle of up to 45à‚° for the incoming line. For this reason the overall cross-arm length is 40 m. One anchor tower for the Suez crossing was completely assembled on foundations in a test facility in order to verify the stability under load for the four major load cases. The tower passed the load tests with no damage to any part.
Investigations into the upper regions of the sub-soil revealed soft or semi-solid clay to 9 m depth, beneath which non-yielding clay was found to be 15 m interspersed with thin layers of sandy clay and below them densely packed sand with stone inclusions. The brackish ground water was close to the surface. A driven pile foundation with steel tubes, whose interior is filled with either free-flowing or reinforced concrete, was used for the suspension towers.
Sixteen piles support each leg member. Each pile extends to a depth of 26 m and has a diameter of 75 cm. The piles are anchored by means of a concrete slab. Advanced computer programmes were used for rating the piles and the associated caps.
The design and erection of overhead transmission lines across maritime navigation routes using high towers differs considerably from those applied to conventional land runs. Investigations into electromagnetic fields, conductor damping behaviour, earthquake analysis and decisions affecting wind loading require considerable detailed planning and investigative effort for preliminary designs – work far in excess of that required for conventional lines. Experience gained from previous projects was therefore indispensable in solving particular problems encountered in the Suez crossing.
Work on the foundations commenced in June 1997. Three test piles were driven to carry out tests under tensile and lateral forces. The results were successful, and a total of 73 piles were driven in at each end of the suspension tower locations and then filled with concrete.
Erection of the towers began in November 1997. The individual segments of the latticed members were pre-assembled lying on the ground and painted. A rotary tower crane was located in the centre line of the tower and grew with the height of the tower. The maximum working height of the crane was reached at 110 m.
The remaining 110 m of the tower height was constructed using a 26 m long derrick crane inside the tower body. Parts of up to 5 t could be hoisted by means of the derrick crane. Both suspension towers were completed by May 1998. Construction of the anchor towers, by means of a mobile crane, was also completed during this period.
On June 18, 1998, power flowed for the first time between Africa and Asia through the Suez crossing.
Figure 1. The 500 kV transmission line crosses the Suez canal approximately 10 km north of Port Suez
Figure 2. The 220 m-high towers supporting the transmission line are among the tallest in the world
Figure 4. An anchor tower in the test station
Figure 5. OPGW dead-end set