World's Longest Overhead Transmission Line.............

This is about the World's Longest Overhead Transmission Line as of today. It is around 1700 km in length, transmitting power of 560 MW. Interestingly enough it its transmission voltage is 500 kV dc, mainly due its relatively long distance.

The Inga-Kolwezi Link, formerly known as the Inga-Kolwezi link has the distinction of being the World's Longest Transmission Lines. It is a line linking the Inga hydroelectric complex at the mouth of the Congo River to mineral fields in Shaba (Katanga) in the Democratic Republic of Congo.

The world's longest transmission lines were completed and dedicated in 1982, following an almost 10 years of planning and construction. It is believed that it cost around $1.3 billion, duly exceeding its initial budget of $250 million.

The variation in the cost could be attributed to the difficulty in terrain and other technical aspects of the project. According to an ABB paper, due to the extreme length of the transmission lines and the difficult logistics along the route, it was decided to build two mono polar lines with four switching stations. The converter stations were built so that the two converter poles can be operated in parallel with ground return, in case of a mono polar line outage.

There were insinuations though that a large part of the variance in cost could be explained by suggestions that well-placed officials in the Mobutu government (in the former Zaire) may have accepted gratuities at various critical junctures during the construction phase. But is another issue though, and most if not all of transmission lines engineers who’ve been in projects most likely know on the probability and reality of these types of scenario. :)

Hydroelectric Dam on the Congo River at Inga Falls

This remarkable project is not only significant to the Country of Congo, but for the continent of Africa as a whole. Sure it gave them the distinction of having the world’s longest transmission lines to date. But more importantly, the fact an energy source is harnessed and an availability of substantial amount of power truly comes in handy. Construction of the Inga–Shaba Project provided the Ministry of Energy and the Société nationale d'électricité (SNEL), with the means to promote further development activity throughout Zaire, by attracting potential investors and overseas firms.

Finally, in a world energy report in 2007 by A. Clerici of Italy, it is found out that the continent ofAfrica has 14% of the world’s population. Yet, it only accounts for 3% of the world’s energy consumption. With only 7% of its hydro-potential of 230GW being explored, much is still needed to be done. Much is still needed to be done by us, Engineers. :)

TIA ( This Is Africa )

LONGEST SINGLE SPAN IN A TRANSMISSION LINE

AMERALIK SPAN - LONGEST SINGLE SPAN IN TRANSMISSION

                    

The longest single span in a transmission line in the world is found in Greeanland. We've already talked about the longest transmission line in terms of circuit kilometers on this blog entry. This is for the distinction in a single span.

Ameralik Span. This is what it is called. It is a 132 kV powerline from Nuuk to Buksefjord Hydroelectric Power Plant over Ameralik fjord with a span of 5376 meters. It was built in 1993 by a Norwegian company Statnett. It is part of a single-circuit 132 kV powerline running from Buksefjord hydroelectric power plant to Nuuk.

It is made up of of 4 steel conductors with a diameter of 40 mm. One of the conductor acts as a backup. It is unique in the sense that each of the conductor has its own tower. Each tower at its North End, is estimated to have an elevation of 444. Its other end (South End) is estimated at 1013 metres above sea level.

Some of the photos of this amazing accomplishment:

                              

                              

                                  

                              

                                      

Sag & Tension

Sag and tension calculations for conductor earth wire are done for the river crossing by following steps :

1. Determination of Equivalent Span :Based on anchor spans L₁ & L₃ and crossing span L₂, the equivalent span for river crossing portion is determined by the following formula :
   Eq. Span = ÖL₁³ + L₂³ + L₃³ / L₁   + L₂   + L₃ (Refer Figure 10-II)
    
2. Sag and tension calculation for conductor/earth wire for above equivalent span is done from the following formula :T² (T - K + aEa (q₂-q₁)) = W² L² aE / 24 x q²where,
   T = Tension at temperature q₂ (kg)
   K= Constant
   a = Area of conductor/Earth wire (mm²)
   E = Modulus of Elasticity kg/mm²a = Linear Coeft. of expansion (per degree celcius)
   W = weight of conductor (kg/m)
   L = Equivalent span (m)
   q = Wind load factor = P² + W² / W² = 1 (At no wind condition)
   q₁ = Initial condition temperature
   
   
3. Calculation for conductor

1. Initial condition for conductor is taken as 32°C and No wind and T₀ tension under these conditions is taken as 22% of ultimate tensile strength of conductor.
2. From the above value of T₀, we calculate the constant 'K which is fixed for all further sag-tension calculations.
3. Considering the calculated value of K, the tension at 0°C under No-wind & full wind and 32°C full wind and 75°C No-wind is determined from the above formula by hit & trial method.
4. Sag at various tension :
   = WL_A² / 8T  where L_A is actual span

4. Calculation for Earth wire

1. The earth wire sag at 0°C and no-wind should be 90% of the Conductor sag at 0°C and No-wind. Value of tension at 0° and no wind is determined by the following formula
   T = {WL² / 0.9 x Sag of Conductor at 0⁰N/W}
   for equivalent span
   where L = Equivalent span in meters.
2. As in the case of conductor, the tensions at 0°C (No-wind & full wind Condition), 32"C full wind conditions and 75°C no-wind condition are determined.
3. Sag = [WL_A² / 8T_A] where L_A = Actual span.

Ruling Span

The Ruling Span is defined as the assumed uniform span that most likely represents actual spans that are in any particular section of the line. In the absence of finite element analysis tools or software (ex. PLS CADD), the ruling span is used to calculate sag and clearances on the plan profile drawing, and it is necessary in structure spotting.

Ruling Span Tip: When stringing the line, the general rule is that the spans in the line should not be more than twice the ruling span, or less than half of the ruling span.

Ruling Span is one of the most used yet misunderstood and misused terms in the design, staking, and construction of overhead lines. “Ruling span” is loosely used with several different meanings.

Theoretical Ruling Span: It is the equation derived from the conductor length equation and by making certain assumptions, approximations, and formula substitutions. This formula must be used if the actual spans are already known.

 

The theoretical ruling span equation is not exact because of the assumptions made. Since

its accuracy is sufficient for most line designs, it is the equation used most often to calculate the ruling span for new overhead distribution lines.

Estimated Ruling Span: If the actual spans are not yet determined but knowledge gained from a reconnaissance and previous surveys of the proposed line are known, it is possible to estimate a ruling span. A traditional “rule of thumb” equation that may be helpful in the estimation of a ruling span is:

Se = Average Span + 2/3 (Maximum Span – Average Span)

Use this rule for estimating the ruling span with caution.  Use only this formula if the actual spans are not yet known.

What would happen if my ruling span is different from the actual design?

• If the design sag is greater than the theoretical sag, then the actual sag of the installed conductors will be less than the predicted sag. This condition will lead to increased conductor tensions, which may exceed the permitted loads of support structures and guying assemblies.

• If the design sag is less than the theoretical sag, then the actual sag of the installed conductors will be greater than the predicted sag. This condition may result in inadequate ground clearances.

Codes Used For Design & Fabrication Of Mono-Pole

Steel poles is fast becoming the pole of choice in construction of power lines. Most of the replacement of wooden poles have been to steel poles.

Steel poles has a distinct advantage over wood poles, primarily its durability and longer life span (if properly treated, like galvanizing).

This article is basically a guide on the standards and codes used in the design of steel poles. The standards presented herein are that of the US Rural Utilities Services.

Codes, standards, or other documents referred to in this specification shall be considered as part of its specification. The following codes and standards are referenced when designing and fabricating steel poles to be used as transmission or distribution poles in the areas within the US Rural Utilities Services.

1. American Institute of Steel Construction (AISC), Specification for the Design, Fabrication and Erection of Structural Steel for Buildings.

2. American Society of Civil Engineers (ASCE) Standard, Design of Steel Transmission Pole Structures, Manual 72, latest edition.

3. American Society for Testing and Materials (ASTM), various standards, latest revision.

4. American Concrete Institute (ACI), Building Code Requirements for Reinforced Concrete, ACI 318, latest edition.

5. American Welding Society (AWS), Structural Welding Code, AWS D1.1, latest edition.

6. American National Standards Institute (ANSI), National Electrical Safety Code, ANSI C2, latest edition.

7. Steel Structure Painting Council (SSPC), Surface Preparation Specification, SPCC-SP6, latest edition.

The above codes can be used for any country unless appropriate data ( ex. weather related ) for that country is used.

 

Wind Load Calculation as per IEC 60826

Some of the data given by Customer is

dynamic wind pressure (q0 ) = 53.13 Kg / m^2

Reliability level = 2   (Return period of design load=150 yrs)

Ground roughness = B

q0  = 53.13 Kg/m^2

      = 521.20 N/m^2

As per IEC 826,

q0  = 0.5 X 1.225 X Vr^2

Therefore Vr = 29.17 m/s.

This "Vr" is inclusive of ground roughness.

Again, High wind velocity Vr = Vm X ground roughness coefficient.

For terrain class B, ground roughness coefficient = 1.0

Therefore, Vm = 29.18 m/s. we call it high wind velocity.

Now this is wind speed for that particular location.

But the wind pressure will vary depending upon height. It depends upon Drag coefficient & Gust response factor.

Drag Coefficient will remain constant at all height but Gust response factor will vary depending upon height.

Hence gust is main factor which will change the pressure at that height.

Assume we have divided pole shaft in three parts to apply wind loads as per our standard test procedure.
Let pole height = 30m
Assuming Drag Coefficient = 1.2 ( Same as insulator )

01) WIND PRESSURE AT 10m ABOVE GROUND.

    At Pole ht = 10m
    Gust response factor = 1.92
    As per IEC 826,
    Final wind pressure    = pressure x Drag x Gust
                                    = 521.2 x 1.2 x 1.92
                                    = 1201 N/m^2
                                    = 122.5 kg/m^2

02) WIND PRESSURE AT 20m ABOVE GROUND.

    At Pole ht = 20m
    Gust response factor = 2.2
    As per IEC 826,
    Final wind pressure    = pressure x Drag x Gust
                                    = 521.2 x 1.2 x 2.2
                                    = 1376 N/m^2
                                    = 140 kg/m^2

03) WIND PRESSURE AT 30m ABOVE GROUND.

    At Pole ht = 30m
    Gust response factor = 2.3
    As per IEC 826,
    Final wind pressure    = pressure x Drag x Gust
                                    = 521.2 x 1.2 x 2.3
                                    = 1439 N/m^2
                                    = 147 kg/m^2

Hence this is maximum wind pressure is at height of 30m.

To do the design at higher side we can refer max pressure for the entire height & for economical design we can use different pressure at different height as described above.

 

Gust Response factor for Pole & Insulator
Height	Terrain cat
	A	B	C	D
10	1.70	1.92	2.55	3.30
20	1.85	2.20	2.82	3.65
30	1.96	2.30	2.98	3.85
40	2.07	2.40	3.12	4.15
50	2.13	2.48	3.24	4.30
60	2.20	2.55	3.34	4.50
70	2.26	2.63	3.46
80	2.31	2.69	3.58

 

I hope the above information will help you.