OUTDOOR STRUCTURE FOR HIGH VOLTAGE POWER CABLE

Fig-A Typical Outdoor Grid Substation

Outdoor Steel Structure Design and Installation for HV & EHV Power Cable

The structures shall be designed to carry cable sealing ends, cables and cleats, earthing bars, earth disconnecting links, sunshields and other fittings  should be followed an adequate design manner.

Each sealing end should be provide with a separate structure which be positioned so as to align with the phase center distances of the substation equipment but not less than the minimum electrical and safety clearness as international standard like specified in BS 162.

All structures should be equipped with a suitable framework mounted immediately below the sealing ends to accommodate a phase plate in a conspicuous position. All phase plates shall be manufactured from mild steel sheet with vitreous enameled finish.

Don’t compromise to provide for fixing and bonding copper fittings as may be necessary to accommodate all apparatus affixed thereto and to secure the structures to their foundations.

Factors of Safety on Outdoor Structure for 132kV Power Cable

The factor of safety for each complete structure and foundation should not be less than 2.0, based upon the maximum working loadings specified and described below.

In calculations of the factor of safety the strength of compression members should be based on the crippling loads as International Standard formula.

The strength of both tension and compression members should be based on the elastic limit and the rate of unsupported length to their least radius of gyration should not be more than:

120 - for main members

200 - for braces

250 - for redundant members

350 - for members loaded in tension only

Maximum Simultaneous Working Loads on Outdoor Structure

The assumed maximum simultaneous working loads on the structures should be reckoned as follows:

Wind Loading of Structure

i) A wind pressure of 1427 N/m2 applied at right angles to the lines on the whole projected areas of phase conductors of 1686 n/m2 in the case of earth wires. A wind pressure of 1427 N/m2 shall also be applied to the projected area of each insulator set.

ii) A wind pressure of 4280 N/m2 on the whole projected area of any one face of the structure.

Vertical Loading on Structure

Vertical loadings should include dead weight of all conductors insulator and equipment and the self weight of the structure. 

All structures also should be capable of supporting a loading of 150 kg per phase to allow for imposed loads of men and tools during maintenance however this loading is not to be combined with other loadings in checking the overall factor of safety.

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Design of Outdoor Steel Structure for 132 kV Power Cable Installation

Fig-CAD drawing for steel structure

The compression members shall consist of rolled steel sections and the tension members of rolled steel sections or flats. The design shall be such as to keep the number of different parts as small as 

Pockets or depressions likely to hold water shall be avoided and if not avoidable shall be properly drained.

Steel structures should be fabricated from either mild steel BS4360 grade 43A or high tensile steel BS 4360 part 2 grade 50C. Notable that all material should be free from blisters, scale or other defects.

All members should be stamped or marked for erection purposes.

All members, fittings, bolts, washers, screwed rods, nuts etc., should be galvanized.

All parts shall be secured by means of bolts and nuts and screwed rods whose minimum diameter shall be 12.5 mm. All nuts and pins shall be locked in position by means of lock washers or other approved devices. Taper washers should be provided where necessary.

When in position, all bolts or screwed rods shall project through the corresponding nuts but by no more than 10 mm. Bolt heads rather than nuts shall be on the outer faces of members.

No bolts hole shall be more than 1.5 mm larger than the diameter of the bolt, but after galvanizing sufficient clearance shall be left for insertion of the bolts.

During erection care shall be taken to ensure that the structures are not strained or damaged in any way and that the treated surfaces are not injured. Rust streaks and foreign matter deposited on galvanized surfaces during transport and storage shall be removed.

Workmanship of Steel Structure

All members shall be cut to jig and all holes shall be drilled or punched to jig so that when the members are in position the holes will be truly opposite each other before being bolted up. Drifting or reaming of holes will not be allowed.

The drilling, punching, cutting and bending of all steelwork and the removal of sharp edges and burrs shall take place before galvanizing.

Approved steel gauges of the stud type shall be provided to enable checking of members as may be considered necessary by the qualified Engineer.

Built members shall when finished be true and free from al kinks, twists and open joints and the material shall not be defective or strained in any way.

In order to check the workmanship, not less than 1% of the members corresponding to each type of support shall if required be selected at random and assembled to form complete supports in the presence of the qualified Engineer at the manufacture’s works.

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Sun-shields on Structure for Power Cables

At places where power cables are brought above ground level e.g. sealing end structures, proper sun shields shall be provided and fitted. These shall comprise of perforated cable trays galvanized and fitted externally to each. Sealing end structure such as to shield the cable from sunlight and allow air flow over the cable.

Marking on Steel Structure Sections

All steel sections, fish-plates and gusset plates should be indelibly marked with an assembly code number or letter, or a combination of both, to facilitate quick recognition and correct assembly. If this identification is stamped on to the member, it’s may  applied before galvanizing.