Basic data on mechanical calculation of wires

Loads, climatic conditions Reference combinations of conditions

The calculation is performed based on permissible tension method, provided there is some margin of safety.

 – margin of safety

– tension in the wire material

 

The calculation is performed for normal mode (all the wires working) and breakdown mode (one wire is broken).

The operating mode of the wires is variable. The stress in the wires is a variable. The impact of the external influences is uneven, but the calculation is performed for even distribution of the loads from the atmospheric phenomenon.

The most convenient form for accounting the loads would be in the form of specific loads. The loads will be expressed in: ; kg/m*mm2.

There are definite types of loads for mechanical calculation, seven all in all:

            - load from own weight of the wire;

- from the weight of ice;

- summary load, from own weight of the wire and ice, ;

- from the wind pressure on the wires not covered with ice (horizontal load);

- from the wind pressure on wires covered with ice;

- summary load, from own weight of the wire and the wind pressure on the wires not covered with ice, .

- summary load, from own weight of the wire and the wind pressure on the wires covered with ice, .

The mechanical calculation is performed for a definite combination of climatic conditions.

For 35 kV – once in 5 years.

For overhead lines of 110-330 kV – once in 10 years (return period for climatic conditions).

For the lines of 500 kV – once in 15 years.

(According to the effective EIR, the return period for climatic conditions is once in 25 years.)

 

The earthmarking charts of the given area are available for wind pressure (seven districts); for ice (five districts).

The data for calculation:

1.      Wind pressure.

2.      Ice.

3.      Temperature.

 

Wind velocity, m/s

I

21

25

30

II

 

 

 

III

 

 

 

IV

Wind velocity

V

 

 

 

VI

 

 

 

VII

40

43

45

 

5 years

10 years

15 years

 

Wind pressure, kg/m2

I

0

40

55

II

 

 

 

III

 

 

 

IV

Wind pressure

V

 

 

 

VI

 

 

 

VII

100

115

125

 

5 years

10 years

15 years

 

All the values in the table are given for the height of 15m. If the height is bigger, the coefficient of correction is applied. Besides, a coefficient of correction is introduced for wind unevenness (from 1 to 0.7).

 

The ice is accounted for its thickness, and applied based on the return periods for 5 and 10 years.

I

5

5

II

5

10

III

10

15

IV

15

20

V (special)

20 and more

22 and more

 

5 years

10 years

 

Temperature.

The reference values are:

- the lowest temperature. The maximum contraction of the wire (maximum tension);

- the highest temperature. The maximum elongation of the wire (maximum sag).

The reference values are:

= -40ºС; = +40ºС.

The reference temperature for ice accumulation is = - 5ºС.

Average annual temperature.

Accepted as = +5ºС (0ºС).

 

The calculation is performed based on seven combinations of climatic conditions:

 

 

Combinations of conditions

I

Ice on the wires; = - 5ºС; q1=0.25 q

II

Ice on the wires, no wind;  = - 5ºС; q=0

III

Wind pressure – q;  = - 5ºС; no ice

IV

No ice or wind; average annual temperature -

V

 = +15ºС; no wind or ice

VI

(lowest temperature mode); neither wind nor ice

VII

(highest temperature mode); neither wind nor ice.