STUDY ON PERFORMANCE OF 220 kV M/C MA TOWER DUE TO WIND

STUDY ON PERFORMANCE OF 220 kV

M/C MA

TOWER DUE TO WIND

V. LAKSHMI

Assistant Professor, Department of Civil Engineering, JNT University Kakinada Kakinada, Andhra Pradesh, India

M.V.R. SATYANARAYANA

Assistant Engineer, PRI Division, Panchayat Raj Department, Kakinada, Andhra Pradesh, India

V.RAVINDRA

Professor, Department of Civil Engineering, JNT University Kakinada Kakinada, Andhra Pradesh, India

Abstract

Electric Power is today playing an increasingly important role in the life of the community. In the electric power system the production and transmission of power are two predominant factors. For the purpose of transmission of electricity towers are the main medium with some wires at required distances and altitudes. The remote hydroelectric power plants have given rise to the need for extra high voltage. Prior to 1950, 150 kV electric transmission lines were considered and still higher voltages are being considered these days. Hence it has given rise to the need for relative tall structures such as towers. Thus the study of designing and erection of steel towers has become a challenging task. Transmission line tower normally comprise of several hundred - angle members eccentrically connected. Structural analysis of this type of structure requires extensive data generation. Conventional process of data generation in describing the topology, geometry, load and support conditions are very tedious, time consuming and susceptible to error. In general, most towers may be idealized as statically determinate and analyzed for forces. However, the computations are very lengthy and there are many chances of making errors in such cases. Further if a member is to be redesigned then the entire wind load computations have to be repeated. The computational problem in tower is more acute and tedious as the load depends on the member sizes. A rational and economical design can be made with the help of software’s like STADD Pro 2003, SAP-2000.In this dissertation an attempt is made to develop a software for load calculations on transmission line towers of capacity 220 kV, only as per new code i.e., IS – 802 (Part- I / Sec-1):1995 by considering reliability, security, safety and anti-cascading conditions. In MS-office the EXCEL is used to develop program for wind load calculations on the tower and other loads like wind load on conductor, conductor weights are calculated manually. Then these loads are applied on the model done in the STADD Pro for analysis. Different types of failure conditions are modeled and their effects on the performance of the tower are studied.

1. Introduction

The performance of the intact structure under medium wind load of basic wind speed 38.8 m/sec is presented. The configuration of tower consists of 33.935 m height arranged in 13 height panels at different levels. The structure has 131nodes and 225 members. The tower is to transmit tower voltage of 220 kV. The details of load calculation, modeling and analysis are discussed. The wind intensity converted into point loads and loads are applied at panel joints.

2. Formulation of the Problem

TABLE 1 Joint Co-ordinates of the node numbers

Tier Node Number

CO-ORDINATES OF NODE POINT

DISTANCES in m

X Z Y

1

11 -4.520 0.000 -4.520

12 4.520 0.000 -4.520

13 4.520 0.000 4.520

14 -4.520 0.000 4.520

2

21 -3.3986 5.6800 -3.3986

22 3.3986 5.6800 -3.3986

23 3.3986 5.6800 3.3986

24 -3.3986 5.6800 3.3986

3

31 -2.297 11.260 -2.297

32 2.297 11.260 -2.297

33 2.297 11.260 2.297

34 -2.297 11.260 2.297

4

41 -1.6514 14.530 -1.6514

42 1.6514 14.530 -1.6514

43 1.6514 14.530 1.6514

44 -1.6514 14.530 1.6514

5

51 -1.300 16.310 -1.300

52 1.300 16.310 -1.300

53 1.300 16.310 1.300

54 -1.300 16.310 1.300

55 0.000 16.310 -1.300

56 1.300 16.310 0.000

57 0.000 16.310 1.300

58 -1.300 16.310 0.000

59 -5.155 16.310 0.000

60 5.155 16.310 0.000

6

61 -1.2667 17.571 -1.2667

62 1.2667 17.571 -1.2667

63 1.2667 17.571 1.2667

64 -1.2667 17.571 1.2667

65 0.000 17.571 -1.2667

66 1.2667 17.571 0.000

68 -1.2667 17.571 0.000

7

71 -1.2352 18.760 -1.2352

72 1.2352 18.760 -1.2352

73 1.2352 18.760 1.2352

74 -1.2352 18.760 1.2352

8

81 -1.1475 21.36 -1.1475

82 1.1475 21.36 -1.1475

83 1.1475 21.36 1.1475

84 -1.1475 21.36 1.1475

85 0.000 21.36 -1.1475

86 1.1475 21.36 0.000

87 0.000 21.36 1.1475

88 -1.1475 21.36 0.000 89 -4.630 21.36 0.000

90 4.630 21.36 0.000

9

91 -11368 22.485 -1.1368

92 1.1368 22.485 -1.1368

93 1.1368 22.485 1.1368

94 -1.1368 22.485 -1.1368

95 0.000 22.485 -1.1368

96 1.1368 22.485 0.000

97 0.000 22.485 1.1368

98 -1.1368 22.485 0.000

10

101 -1.1088 23.545 -1.1088

102 1.1088 23.545 -1.1088

103 1.1088 23.545 1.1088

104 -1.1088 23.545 1.1088

11

111 -1.000 26.260 -1.000

112 1.000 26.260 -1.000

113 1.000 26.260 1.000

114 -1.000 26.260 1.000

115 0.000 26.260 -1.000

116 1.000 26.260 0.000

117 0.000 26.260 1.000

118 -1.000 26.260 0.000

119 -4.430 26.260 0.000

120 4.430 26.260 0.000

121 -1.000 27.660 -1.000

12 122 1.000 27.660 -1.000

123 1.000 27.660 1.000

124 -1.000 27.660 1.000

13 131 0.000 33.935 0.000

3. Calculation of Co-efficient of drag

For panel – I

Base width (a) = 9.04 m

Top width (b) = 6.78 m

Height (h) = 5.68 m

Members faced along wind direction

Leg members (130 x 12) – 2Nos,

Diagonal members (75 x 6) – 2Nos.

In between members (50 x 6) R1 to R8 – 2Nos.

Length of the members

Leg = 6.550 m, Diagonal = 9.116 m, R1 = 1.042 m

R2 = 1.644 m, R3 = 1.965 m, R4 = 1.251 m

R5 = 2.422 m, R6 = 2.653 m, R7 = 2.076 m

R8 = 1.564 m

Center of Gravity of this panel









C =

3 68 . 5 78 . 6 04 . 9 78 . 6 2 04 . 9    

= 2.71 m4

Exposed area of the member for one leg = 6.550 2 1000

130



 = 1.703 m2

Total exposed area for all members panel – I = 4.532 m2 Projected solid area of the panel –I

= abh 2

= 5.68

2 78 . 6 04 . 9  

= 44.98 m2

Solidity Ratio =

a ojectedare

a Exposedare

Pr =44.98 532 . 4

= 0.10076

Table 2 Drag Co-efficient

C

Ratio

Drag Co-efficient

C

0.1 3.4 0.2 2.9 0.3 2.5 0.4 2.2 0.5 2.0

For this solidity ratio the coefficient drag values given are interpolated from table 2

0.1  3.4

0.2  2.9

For the value the solidity ratio

0.10076  3.396

4.Calculation of Wind Load

Basic Wind Speed Vb = 38.8 m / sec

Risk factor k1 = 1.00 for 132kV & 220 kV

Table 3 Terrain Roughness coefficient

Terrain Category 1 2 3

Terrain roughness Co-efficientK₂ 1.08 1.00 0.85

Terrain Roughness Co-efficient k2 = 0.85 from table 3

Reference wind speed

375 . 1

b V R

V  =

375 . 1

8 . 38

= 28.21 m / sec

For panel – I

Solidity ratio = 3.396

Center of gravity = 2.70 m4

Table 4 Gust Response Factor

G

G

the Ground Values of

G

G

1 2 3

Up to 10 1.70 1.92 2.55

20 1.85 2.20 2.82

30 1.96 2.30 2.98

40 2.07 2.40 3.12

50 2.13 2.48 3.24

60 2.20 2.55 3.34

70 2.26 2.63 3.46

80 2.31 2.69 3.58

Note: Intermediate values may be linearly interpolated.

Gust factor GT = 1.70 (This value is taken from table 4)

Design Wind Speed R V k k d

V  ₁ ₂ = 1.000.8528.21= 23.98 m / sec

Design wind pressure 2 6 . 0 V_d d

P   = 0.623.982 = 345.02 m / sec

Force for Panel – I T G e A dt C d P wt

F     =345.023.3964.5322.55 = 13540.728

5. Parameters for Study

The Parameters involved in this study are axial deflections, axial forces in various members of the transmission tower under basic wind speed is 38.8 m/sec. The tower is 3D space frame with the deflections are in three planes viz. UXY Deflection in XY plane i.e. opposite to Z-axis, UYZ deflection in YZ plane i.e. opposite to X-axis and Uxz deflection in XZ plane i.e. opposite to Y-axis are considered.

The deflections of nodes in three planes are presented in Table 5, Axial forces of vertical members are presented in Table 6. Axial forces of horizontal members are presented in Table 7. Axial forces of diagonal members are presented in Table 8. The details of deletions at various nodes are presented in figure 2, 3, and 4. Similarly the axial forces in various vertical, horizontal and diagonal members are furnished in figure 5, 6 and 7.

Table 5 Nodal Deflections in Tower for Wind Load

Tier Node Number

Nodal Displacements

X Y Z

1 11 0.000 0.000 0.000

13 0.000 0.000 0.000

2 21 0.092 0.032 0.041

23 0.092 -0.032 0.041

3 31 0.220 0.062 0.007

33 0.220 -0.062 0.007

4 41 0.333 0.065 0.020

43 0.333 -0.065 0.020

5

51 0.413 0.074 -0.012

55 0.413 0.000 0.000

60 0.417 -0.367 0.000

6

61 0.505 0.095 0.000

65 0.504 0.000 0.000

67 0.504 0.000 0.000

7 71 0.629 0.113 0.056

73 0.629 -0.113 0.056

8 81 0.961 0.149 -0.001

85 0.961 0.000 0.000

90 0.961 -0.677 0.001

9

91 1.128 0.160 0.001

93 1.128 -0.160 0.001

96 -0.063 -0.157 0.000

98 -0.063 0.157 0.000

10 101 1.302 0.167 0.044

103 1.302 -0.167 0.044

11

111 1.779 0.179 0.000

113 1.779 -0.179 0.000

119 1.779 0.835 0.000

12 121 2.044 0.186 0.002

123 2.044 -0.186 0.002

13 131 3.350 0.000 0.000

Table 5 (Continued)

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0

-0 .2 5 0 .0 0 0 .2 5 0 .5 0 0 .7 5 1 .0 0 1 .2 5 1 .5 0 1 .7 5 2 .0 0 2 .2 5 2 .5 0 2 .7 5 3 .0 0 3 .2 5 3 .5 0 3 .7 5

DIS

P

LACE

M

E

N

T

, m

m

N O D E N U M B E R

Fig. 1 NODAL DISPLACEMENT in X - DIRECTION

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

D

ISP

LACEM

ENT

, m

m

NO DE NUM BER

6. Performance of Tower Based on Deflections

The members in 11th, 12th and 13th tier had largest deformation with node 131 recording peak deformation in the direction of wind flow in YZ plane. The deformations are maximum positive values and the 4th and 9th tier nodes having negative values, the maximum negative value at node 43 and all nodes subsequent tiers had deformation bending the tower in one direction as shown in figure.1.

The bent shape can be clearly felt along XZ plane with least deformation at all nodes near the base and as we approach to top of the tower having largest deformation. Symmetric deformation among all the nodes of one plane is observed from the figure 2.

Both the nodes facing the wind and nodes away from wind direction in one plane had same deflection of the figure 3.

7. Performance of Tower Based on Member Forces

After determination of the external loads acting on the tower is calculated, the deflections in different planes are observed in previous section. In this section the truss is analyzed for lateral wind loads for the forces in various members with a view to fixing up their sizes. Since axial force for a truss element, the member has to be designed for either compression or tension. When there are multiple load conditions certain members may be subjected to both compression and tensile forces under different loading conditions. Reversal of loads may induce alternate nature of forces. The total compression or tensile forces in horizontal, vertical and diagonal members are given in table 6, 7, and 8.

To critical elements are also observed from the figures indicating both deflections and member forces. The graphs are drawn between node numbers and deflections in figure 1, 2 and 3 and member numbers and axial forces in figure 5,.6 and 7.

20 40 60 80 100 120

-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06

DISPLACE

ME

NT,

m

m

NODE NUMBER

Table 6 Performance of Vertical Members for Intact Condition

Tier

No Vertical Member Number

Axial Force, kN

Intact condition

1 14 -593.54

2 24 -508.42

3 34 -490.81

4 44 -478.26

5 54 -416.36

6 64 -416.36

7 74 -315.38

8 84 -210.82

9 94 -56.95

10 104 -131.28

11 114 0

12 124 0

0 10 20 30 40 50 60 70 80 90 100 110 120 130

-600 -500 -400 -300 -200 -100 0

AX

IA

L FOR

C

E,

kN

MEMBER NUMBER

Table 6 Performance of Horizontal Members for Intact Condition

Tier No

Horizontal Member Number

Axial Force, kN Intact condition

4 415 0

416 0

5 515 -7.89

516 7.89

7 715 0

716 0

8 815 -9.78

816 9.78

10 1015 0

1016 0

11 1114 -57.11

4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0

-6 0 -5 0 -4 0 -3 0 -2 0 -1 0 0 1 0

AX

IA

L FORCE,

kN

M E M B E R N U M B E R

Table 7 Performance of Diagonal Members for Intact Condition

Tier No Diagonal _Member Number

Axial Force, kN

Intact Condition 1 18 -73.45 2 28 -41.60

3 38 -5.65

4 48 15.72 5 58 -87.31 6 68 76.04 7 78 -69.60 8 88 -71.34 9 98 56.95

10 108 -56.45

11 118 0.67

8. Performance of Tower Due to Member Forces

In vertical members as shown figure 4 A uniform reduction in axial compressive forces is observed as we approach top tier from bottom tier vertical members.

As shown in figure 5 the horizontal members in 4th, 7th and 10th tier members had no axial forces, the maximum axial forces in the 11th tier member. At 5th and 8th tiers the axial force is more as compared to its successive tiers, which accommodate of conductors. It is observed that in horizontal members in tiers attached to cross arms (5, 8, 11) large values of axial tensile forces are observed and in horizontal member in tiers immediately below the tiers where cross arms (4, 7, 10) are connected the axial forces are minimum due to redistribution of axial forces.

The diagonal members as shown figure 6 had largest axial forces in all tiers and the 3rd and 11th tier members are very low axial forces compare with other tiers. Axial forces are present in all diagonal members in all tiers and there is reduction in axial force in tiers, in which horizontal members have significant contribution.

2 0 4 0 6 0 8 0 1 0 0 1 2 0

-1 0 0 -8 0 -6 0 -4 0 -2 0 0 2 0 4 0 6 0 8 0

AX

IA

L

FORC

E,

kN

M E M B E R N U M B E R

Considering the above diagram the members in the bottom two tiers are critical in taking the cable load under medium wind intensity. More over the vertical members in the bottom ties are likely to have larger contribution of the loads and are vital in stability of the tower under any loading conditions. Any abnormality in the cross sectional properties or the material properties are likely to result in abnormal functioning of the entire structure.

9. Conclusions

1. It is observed that the effect of broken conductor & ground wire is not significant in lower tier members. As we approach the top tiers, the effect is felt near the panels connected to cross arm due to redistribution of forces in members.

2. In case of the tower subjected to wind load maximum positive deformation is observed at peak in the direction of wind flow and maximum negative deformations are observed in 4th tier leading to formation of contra-flexure point between 4th and top tier.

3. A uniform reduction in axial compressive forces is observed as we approach top tier from bottom tier vertical members.

4. It is observed that in horizontal members in tiers attached to cross arms (5, 8, and 11) large values of axial tensile forces are observed. The member in tiers immediately below the tiers where cross arms are connected (4, 7, and 10) the axial forces reduced due to redistribution of axial forces.

5.

.

10. References

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