Ground wire, optical ground-wire cable.

Defining the tension (manually and explanations for the programs)

The distances between the wires and cords are defined based on their sag, corresponding overall span; at the same time, the sag of the cord shouldn’t be more than that of the wire.

In respect to the conditions of lightning-proofness, the vertical distance in the span between the ground wire, as well as optical ground-wire cable, and the wires of an overhead line should not be less than the values defined by EIR for +15ºС f(see the table); and not less than the vertical distance between the wire and the cord on support.

 

Span length, m

100

150

200

300

400

500

600

700

800

900

1000

1200

1500

Minimum vertical distance between the cord and the wire, m

2.0

3.2

4.0

5.5

7.0

8.5

10

11.5

13

14.5

16

18

21

 

The intermediate values of table are interpolated.

The sag of the cord based on the values in the figure:

The distance between the wire and the cord on the support is defined accounting the weight of suspension insulator strings.

To input the distance between the wire and the cord into the program, use an average distance between the wire and the cord at neighboring supports.

After defining the sag of the cord at +15ºС, define the corresponding tension in the cord.

The calculated value is inserted into the equation of the wire state (see Theory and the example of mechanical calculation) and the tension for maximum loads (ice, lowest temperature) and average operating conditions is defined. The calculated are compared to other permissible tension from 2.5.7 table of EIR or National Standard. The tension should not exceed the permitted values.

Most often, the calculated tensions are lower that permissible ones, which lowers the load on supports as compared to the case when the cord is drawn to the full extent. The calculated tensions are taken for further calculations of mounting stresses and sag.

 

Example of calculation.

It is necessary to replace the existing ground wire with ОКГТ-ц-1-24-(G.652)-12,6/56 24 on a power transmission line of 110 kV with АС150/19 wire.

Based on the measurements of stresses and sag of overhead lines, the mechanical calculation by LineMech program defines that the values for permissible tension in the wire are: maximum 12.2 daN/mm2, 8.1 daN/mm2..

The calculation of the optical ground-wire cable is performed based on the manufacturer’s permissible tension.

The program for defining the tension in the ground wire calculates for known spans of the overhead lines (calculation in DOS) the following:

 

SOURCE DATA FOR CALCULATION:
OWN-WEIGHT-OF-WIRE LOAD = 0.003339
OWN-WEIGHT-OF-CORD LOAD = 0.004345
MAXIMUM LOAD FOR CORD= 0.026330
LOWEST TEMPERATURE=-35.00000
TEMPERATURE FOR MAXIMUM LOAD=-5.000000
DISTANCE BETWEEN WIRE AND CORD= 3.600000
LENGTH OF SPECIFIC SPAN =310.00000
TENSION IN WIRE AT=+15 degrees С = 2.700000
LENGTH OF EQUIVALENT SPAN  =263.00000
THERMAL EXPANSION COEFFICIENT FOR CORD = 0.000017
MODULUS OF ELASTICITY =9911.0000

 

CALCULATION RESULTS:

PERMISSIBLE TENSION IN CORD AT LOWEST TEMPERATURE, degrees C =  4.678400
PERMISSIBLE TENSION IN CORD AT ATMOSPHERIC OVERLOADS= 4.075943
PERMISSIBLE TENSION IN CORD AT MAXIMUM LOAD=20.791965

 

The maximum permissible tension in optical ground-wire cable accepted for the project is 21 daN/mm2 according to the calculation results (lower than the manufacturer’s data). To verify, mechanical calculation of the optical ground-wire cable is performed for the accepted tension.

The tension in the ground wire could be sorted out with the help of LineMech, LineMount, provided the sag and the distance between the wire and the cord in different modes and different spans are verified. This is cannot be done manually.

 The source data for spans and supports, АС150/19 wire, and optical ground-wire cable as “cord”, as well as the defined permissible tension, are input  to LineMount to calculate mounting stresses and sag. The calculation results reflect the entire picture of the correlation between the sag of the wire and that of the cord.