GB/T 31235-2024Technical specification for fittings of +/-800 kV DC overhead transmission line (English PDF)

1 Scope

This document specifies the general requirements, the technical requirements, the criteria for judgement, the requirements for marking and accompanying documents and the packaging requirements for the fittings of +/-800 kV direct current overhead transmission lines, and describes the corresponding test methods.

This document applies to the design, manufacture, testing and acceptance of the fittings of +/-800 kV direct current overhead transmission lines.

4 General requirements

4.1 Appearance and dimensions. 4.1.1 The appearance quality and the dimensional tolerances of the fittings shall comply with GB/T 2314. 4.1.2 The nominal failing load and the connection type and dimensions of the connecting fittings shall comply with GB/T 2315.

4.2 Materials. 4.2.1 The material of the fittings shall reduce the hysteresis and eddy current losses of the product. 4.2.2 For steel and iron parts with a nominal failing load of 160 kN and above, the tensile strength of the material shall be not lower than 500 MPa. Apart from Q550, 35CrMo and 40Cr, the yield-to-tensile ratio shall be not greater than 0.8. 4.2.3 The material of compression fittings shall be able to withstand cold deformation. 4.2.4 The non-metallic materials used in the parts of the fittings shall resist ageing, shall have sufficient resistance to ozone, to ultraviolet radiation and to atmospheric pollution at the operating temperature, and shall not induce corrosion in the materials they touch.

4.3 Manufacturing process and quality control. 4.3.1 Forged parts of the fittings shall comply with DL/T 768.2. 4.3.2 Pressed parts shall comply with DL/T 768.3. 4.3.3 Aluminium parts shall comply with DL/T 768.5. 4.3.4 Welded and thermally cut parts shall comply with DL/T 768.6. 4.3.5 Hot dip galvanized parts shall comply with DL/T 768.7.

4.4 Other requirements. 4.4.1 The model designation of the fittings shall comply with DL/T 683. 4.4.2 At 1.1 times the highest operating voltage of the conductor, the fittings shall show no visible corona, the radio interference voltage shall be not greater than 1 000 µV, and the requirements of Annex A shall be met. 4.4.3 The current density over the electrical contact area of the fittings shall be not greater than 0.15 A/mm2.

5 Technical requirements

5.1 Spacers. 5.1.1 Spacers shall comply with DL/T 1098. 5.1.2 Spacers shall be easy to install and remove, shall not damage the conductor under normal operating conditions and shall keep the sub-conductor spacing at the position where they are installed. 5.1.3 The spacer clamp shall be able to take up the slack caused by creep of the conductor and shall grip it firmly without damaging it. 5.1.4 The surface of the spacer clamp shall be free from obvious sharp edges, burrs and similar defects, the transition from arc to arc shall be natural and smooth, and shot blasting may be applied. 5.1.5 Spacers shall withstand the mechanical loads arising under installation, operation (including short circuit) and maintenance conditions. 5.1.6 The longitudinal grip of the spacer clamp shall be not less than 2.5 kN, the failing load of the clamp body shall be not less than 6 kN, the torsional grip torque of the clamp shall meet Table 1 and the tensile and compressive forces of the clamp shall meet Table 2.

5.1.6 Table 1, torsional grip torque of the spacer clamp, in newton metres, against the outside diameter D of the conductor in millimetres, for the hinged clamp and bolted cap clamp on one hand and the preformed clamp on the other: for D not greater than 34.3, 40 and 40; for D greater than 34.3 and not greater than 36.3, 50 and 50; for D greater than 36.3 and not greater than 38.4, 60 and 60; for D greater than 38.4 and not greater than 40.6, 70 and 63; for D greater than 40.6 and not greater than 42.8, 90 and 72; for D greater than 42.8 and not greater than 48.0, 120 and 96. Table 2, tensile and compressive force between the spacer clamps, in kilonewtons, in heavy ice areas and in areas that are not heavy ice areas: for D not greater than 28.0, 6 and 4; for D greater than 28.0, 8 and 6.

5.1.7 A spacer used within the span shall be of flexible construction and shall have damping properties. The joint of the clamp shall be free to move: the clamp shall move not less than +/- 12.5 mm forwards and backwards in the direction of the conductor, not less than +/- 25 mm in the direction perpendicular to the conductor and not less than +/- 12.5 mm in the horizontal direction, and not less than 10 degrees in the conical direction. 5.1.8 The centripetal force of a spacer under short-circuit current may be calculated in accordance with DL/T 1098, and its overall mechanical strength shall meet the requirement of that centripetal force. 5.1.9 The fatigue performance of spacers shall meet DL/T 1098. 5.1.10 The logarithmic decrement of a damping spacer system on a bundled conductor shall be not less than 0.04. 5.1.11 The clamp joints of a rubber-damped spacer shall all be filled with rubber, the clamp shall be lined with a rubber pad, and DL/T 1098 shall be met. 5.1.12 The body frame and the clamps of the spacer should be produced by solid die forging, pressure casting or low pressure casting, and shall be heat treated and surface finished as required. 5.1.13 Measures shall be taken to prevent the bolts joining the assembled frame and the clamps from working loose.

5.2 Suspension clamps. 5.2.1 Suspension clamps shall comply with DL/T 756 and DL/T 763. 5.2.2 Suspension clamps should be designed to reduce the effect of aeolian vibration on the conductor; the clamp shall have good dynamic characteristics so that its body can rotate freely and easily. 5.2.3 The radius of curvature of the groove in the body of the suspension clamp shall be not less than eight times the diameter of the conductor. 5.2.4 The groove of the suspension clamp, the keeper and the other surfaces in contact with the conductor shall be even and smooth and free from burrs, projections and any other defect that could wear the conductor. 5.2.5 Measures shall be taken to prevent the bolts of the keeper assembly of a conductor suspension clamp from working loose. 5.2.6 The clevis of the suspension clamp shall have adequate wear resistance. 5.2.7 Where armour rods are used when the conductor is installed in the suspension clamp, the ends of the armour rods shall be of the duck-bill form and shall comply with DL/T 763. 5.2.8 The ratio of the grip of the suspension clamp to the rated tensile strength of the conductor shall comply with Table 3.

5.2.8 Table 3, ratio of the grip of the suspension clamp to the rated tensile strength of the conductor, in per cent, against the ratio of the aluminium area to the steel area for aluminium conductor steel reinforced, aluminium alloy conductor steel reinforced, aluminium conductor aluminium-clad steel reinforced and aluminium alloy conductor aluminium-clad steel reinforced: for a ratio not greater than 2.3, 14; for a ratio greater than 2.3 and not greater than 3.9, 16; for a ratio greater than 3.9 and not greater than 4.9, 18; for a ratio greater than 4.9 and not greater than 6.9, 20; for a ratio greater than 6.9 and not greater than 11.0, 22; and for a ratio greater than 11.0, 24. For all-aluminium alloy stranded conductor and aluminium conductor alloy reinforced no area ratio applies and the value is 24.

5.2.9 At the mouth of the suspension clamp (including inside the clamp), the bending stress and the crushing stress should also be considered; the exit angle on one side is generally between 0 degrees and 25 degrees, and may be increased suitably to meet a particular requirement. 5.2.10 The clevis and the other fittings of the suspension clamp shall be made of steel with a tensile strength not lower than 500 MPa, and all the fittings and fasteners made of steel shall be protected against corrosion by hot dip galvanizing. 5.2.11 The body and the keeper of the suspension clamp shall be made of aluminium alloy, and the clamp body should be produced by solid die forging or by casting.

5.3 Tension clamps and joints. 5.3.1 Tension clamps and joints shall comply with DL/T 757 and DL/T 758. 5.3.2 The surface of the aluminium tube of tension clamps and joints shall be smooth and even and free from cracks, delamination and similar defects. 5.3.3 The tapered length of the compression tube of tension clamps and joints shall be not less than one times the diameter of the conductor. 5.3.4 The flat end of the tension clamp and of the jumper clamp should make contact on both faces; the assembly gap shall be not greater than 0.8 mm, the flatness of the contact face shall be not greater than 30 µm, the surface roughness Ra shall be not greater than 3.2 µm, and the outer face shall be even, free from scoring and smooth around its edges. 5.3.5 The coaxiality tolerance between the centre of the steel anchor of the tension clamp and the centre of the steel tube of the joint sleeve shall be not greater than 0.5 mm, and the mouth of the steel tube shall be deburred and radiused. 5.3.6 The grip of tension clamps and joints on the conductor or earth wire shall be not less than 95 % of the rated tensile strength of that conductor or earth wire. 5.3.7 The bodies of tension clamps and joints follow GB/T 3190. The aluminium tube may be a hot extruded or drawn aluminium tube of aluminium purity not lower than 99.5 %, with a tensile strength of not less than 80 MPa, an elongation after fracture of not less than 12 % and a Brinell hardness of not more than 25. 5.3.8 The body material of the tension clamps and joints used with aluminium alloy conductors follows GB/T 3190, with a tensile strength in the range 130 MPa to 150 MPa, an elongation after fracture of not less than 12 %. 5.3.9 The assembly of the aluminium tube of the tension clamp with the sleeve of the jumper plate shall use an effective process to give a tight fit and shall be firmly welded. The material of the jumper plate follows GB/T 3880 (all parts), machined from section. 5.3.10 The steel anchor of the tension clamp and the steel tube of the joint shall comply with GB/T 699 or GB/T 700, with a Brinell hardness of not more than 156. 5.3.11 The steel anchor of the tension clamp shall be machined by a forging process; the steel anchor of the tension clamp and the steel tube of the joint shall not be machined from tube and shall be free from cracks, delamination and scale. The steel anchor is protected against corrosion by hot dip galvanizing, and the bore of the steel tube shall be cleared of zinc after galvanizing.

5.4 Grading rings and shielding rings. 5.4.1 Grading rings and shielding rings shall comply with DL/T 760.3. 5.4.2 The connection of the grading ring and the shielding ring to the fittings of the insulator string shall be convenient to install and reliable in service. 5.4.3 Measures shall be taken to prevent the bolts of the grading ring and shielding ring assembly from working loose. 5.4.4 Grading rings and shielding rings shall have sufficient mechanical strength to withstand a static mechanical load of not less than 2 000 N. Under normal operating conditions the ring body, the connecting bracket and the fasteners shall be free from fatigue damage, loosening and similar defects. 5.4.5 Grading rings and shielding rings should be made of aluminium tube. 5.4.6 During manufacture the ring body shall be free from splitting of the tube, severe distortion and similar defects; the radial deformation of the tube diameter shall be kept within +/- 3 % of the diameter of the aluminium tube, and the tolerance on the bending radius of the ring body shall not exceed +/- 1 % of the radius of the ring.

5.5 Connecting fittings. 5.5.1 Connecting fittings shall comply with DL/T 759. 5.5.2 The bodies of tower connecting fittings, ball eyes, socket clevises, U-shaped shackles and right-angle hangers shall be forged and heat treated, and the strength grade of the bolts used with tower connecting fittings shall be not lower than 6.8. 5.5.3 A fitting connected to a yoke plate shall not knock against the yoke plate in service. 5.5.4 The attachment holes of the yoke plate shall not be machined by welding in a sleeve. 5.5.5 Where the lowest ambient operating temperature is below minus 30 °C, forged connecting fittings and the bolts that carry shear force shall be made of alloy structural steel and shall be quenched and tempered; the impact test temperature shall be lower than the lowest ambient operating temperature and the impact energy shall be not lower than 27 J.

5.6 Jumper fittings. 5.6.1 Jumper fittings shall comply with DL/T 1372. 5.6.2 Jumper fittings shall withstand the various mechanical loads arising during installation, operation and maintenance and shall withstand the operating temperature under the working current, including short-circuit current. 5.6.3 Where a cage type jumper is used, spacers shall be used in the middle section of the jumper to hold the multiple sub-conductors of the jumper. 5.6.4 Where an aluminium tube type jumper is used, an aluminium tube shall be used as the current-carrying body in the middle section of the jumper, and a flexible conductor is used to connect the terminal clamp of the aluminium tube to the tension clamp of the conductor. Where the aluminium tube is joined in sections, reliable measures against loosening shall be taken at the joint. 5.6.5 The aluminium tube used for an aluminium tube rigid jumper shall comply with GB/T 4437.1. 5.6.6 The jumper plate parts connecting the two ends of the aluminium tube to the flexible jumper shall be hot extruded plate of aluminium purity not lower than 99.5 %.

5.7 Vibration dampers. 5.7.1 Vibration dampers shall comply with DL/T 1099. 5.7.2 Vibration dampers shall have good mechanical and fatigue properties; in service they shall not slip, the damper weights shall not come off and the parts shall not work loose. 5.7.3 The weights of the vibration damper are machined from cast steel, forged steel or forged aluminium, and steel parts shall be hot dip galvanized on the surface. 5.7.4 The clamp body is made of aluminium alloy, the material complying with GB/T 1173; the construction should be of the hinged type, but a preformed construction may also be used. The preformed rods are of aluminium-clad steel wire, which shall comply with YB/T 123. 5.7.5 The stranded steel wire shall meet the requirements of YB/T 4165; the tensile strength of the individual wires of the strand shall be not lower than 1 570 MPa, the lay ratio shall be not greater than 11, and the wire shall not birdcage or corrode.

5.8 Connection and fastening. 5.8.1 The bolts and nuts used for connecting fittings shall comply with DL/T 764, and the strength grade of the bolts used with fittings of 160 kN and above shall be not lower than 6.8. 5.8.2 Fastening bolts shall comply with DL/T 284, their strength grade shall be not lower than 6.8, and after installation the thread projecting beyond the nut shall be two to four pitches. Measures shall at the same time be taken to prevent the bolts from working loose. 5.8.3 Nuts shall comply with DL/T 284. 5.8.4 Spring washers shall comply with GB/T 93, plain washers with GB/T 95 and split pins with DL/T 1343.

6 Test methods and criteria for judgement

6.1 The testing of the fittings includes type tests, routine tests and sampling tests.

6.2 The test items and quantities for the fittings shall follow GB/T 2317.4.

6.3 The test methods and the criteria for judgement for the general requirements of the fittings shall comply with the product standards listed in Clause 4.

6.4 The test methods and criteria for judgement for spacers shall comply with DL/T 1098.

6.5 The test methods and criteria for judgement for suspension clamps shall comply with DL/T 756 or DL/T 763.

6.6 The test methods and criteria for judgement for tension clamps shall comply with DL/T 757.

6.7 The test methods and criteria for judgement for joints shall comply with DL/T 758.

6.8 The test methods and criteria for judgement for grading rings and shielding rings shall comply with DL/T 760.3.

6.9 The test methods and criteria for judgement for connecting fittings shall comply with DL/T 759, and the method and criteria for the low temperature impact test shall comply with GB/T 229.

6.10 The test methods and criteria for judgement for jumper fittings shall comply with DL/T 1372.

6.11 The test methods and criteria for judgement for vibration dampers shall comply with DL/T 1099.

6.12 The test methods and criteria for judgement for connection and fastening shall comply with the product standards listed in 5.8.

7 Requirements for marking and accompanying documents

7.1 The marking of the fittings shall meet GB/T 2314.

7.2 The fittings shall carry a clear and permanent mark that is easy to identify, including the type and rating, the mark of the manufacturer and any other unique identifier, or a two-dimensional code where the purchaser so requires.

7.3 The marking of conductor compression fittings shall include the rating of the conductor for which they are suitable, the compression position and the direction of compression.

7.4 For fittings made up of several parts joined together, installation instructions shall be supplied.

7.5 For fittings fastened with bolts, a statement of the bolt installation torque shall be supplied.

8 Packaging requirements

8.1 The packaging of the fittings shall meet GB/T 2314.

8.2 The packaging of the fittings shall have suitable padding, protective filling, boards or dividers, so that it can withstand accidental impact during handling, storage and transhipment.

8.3 Effective protective measures shall be taken for the electrical contact faces of the fittings, to prevent scoring of the surface, oxidation and corrosion.

8.4 All the packaging shall be marked clearly and correctly, and the marking on each package shall not be easily erased.

8.5 The marking on the package shall state the name of the supplier and the name of the purchaser, the name of the project, the contract number, the kind and quantity of the goods, the net mass and the gross mass and any other necessary information.

Annex A Test methods for corona and radio interference (normative)

A.1 Test arrangement. The arrangement of the corona and radio interference test shall meet the following requirements. a) The arrangement and assembly of the test object shall be close to actual practice on the line, and the clearance from the live parts to earth shall be not less than 10 m. b) When several fittings are tested horizontally, the distance between the fittings shall be not less than 6 m. c) An ultraviolet imager is used for the visible corona voltage test in daylight; the test steps and method follow the naked eye (telescope) observation method. Photographs shall be taken after the test, and the photographs shall show the part of the test object at which corona started.

A.2 Method of calculating the specified test voltage. In areas at an altitude of 1 000 m and below, the specified test voltage is calculated by formula (A.1) as the product of 1.1, the correction factors k1, k2 and k3 and 816 kV. The legend is as follows: U0, the specified test voltage for the visible corona test, in kilovolts (kV); k1, the correction factor for the position of the test object, settled according to the position in which the fitting is used on the line, generally taken as 1.0 for suspension fittings and mid-span fittings and as 1.1 for tension fittings; k2, the correction factor for the suspension height of the test object, equal to one plus the height h of the live part above earth, in metres, less 10, multiplied by 0.025; and k3, the meteorological correction factor, which under conditions of an ambient temperature of 10 °C to 40 °C and a relative humidity of 20 % to 70 % corrects for air density alone and is calculated by formula (A.2). The legend of formula (A.2) gives p1, the atmospheric pressure at the time of the test, in kilopascals (kPa); p2, the standard atmospheric pressure, in kilopascals (kPa); t0, the standard temperature, 20 °C; and t1, the temperature at the time of the test, in degrees Celsius. Where the fittings are used in areas at an altitude higher than 1 000 m but not higher than 4 000 m, the test voltage shall be corrected for altitude by formula (A.3), whose legend gives H, the altitude, in kilometres (km). Formulae (A.2) and (A.3) have come through the digitization as broken fractions and the equations themselves are not reproduced here.