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LAYING OF LARGE EHV UNDERGROUND CABLE NETWORK - ISSUES OF HEAT DISSIPATION

SM Takalkar KG Gaikwad KN Velani Managing Director DGM Jr. Engineer

Takalkar Power Engineers & Consultants Pvt. Ltd. Vadodara, Gujarat, India

1. Introduction 1.1. Normally the EHV underground cables are

laid over a small distance due to the cost aspect. Laying of underground cable is preferred when there is severe crunch of right of way (ROW). The utilities serving the metropolitan and congested urban areas prefer to lay under- ground cables over longer distance.

1.2. The EHV underground cables are now being preferred even in big industrial areas, SEZs and Smart Cities. In these cases, the underground cables are also used for an aesthetic look.

1.3. However, underground cables suffer from the problem of heat dissipation. The other problem is overvoltage during less loads or no load. However, this can be overcome by providing reactors.

1.4. The presentation here below tries to bring out issues of heat dissipation being faced while designing a large EHV underground cable network for a typical smart city.

2. Details of Typical Underground EHV

Cable Network. 2.1. A developing urban area having major

industrial loads and state of art infrastructure like wide roads, airports, metro rail, sport complexes, holiday resorts, university, Public places, wild life sanctuary, hospitals, shopping malls, recreation facilities etc., will have all smart applications. Overhead power transmission and distribution is totally ruled out.

2.2. Such developing area is generally a green field project. In one such a typical project, the entire network from 400kV to 400V is laid underground and all the switching is through GIS.

2.3. The transmission cables will have voltages of 400kV, 220kV and 66kV. The distribution cables will be of 11kV and 415 volt. The entire network from 400kV to 11kV is in the form of a ring main. This will ensure 24X7 power supply.

2.4. The GIS and the RMUs will also be connected through the underground cable network.

2.5. Various options to lay the cables were explored. The direct burial was ruled out as the city is going to come up in the phased manner. Investing on cables in

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anticipation, would be disadvantageous. Since water table is very high (rising up to 1.5 m below ground) deep tunneling is also ruled out. The only option left is to make a box type RCC trench.

Cable Tunnel

Cable Trench

Cable Buried

Box Type R.C.C. Trench

2.6. It is envisaged to lay the cable in the

trenches designed to carry the number of various sizes/ voltage levels of the cables.

2.7. The soil in the area under development is marshy. It is proposed to fill the existing land before constructing the roads. The cable trenches are also expected to have atleast 600mm layer of soil above them. All the trenches shall be RCC with appropriate wall thickness to resist soil pressure. The trenches will have opening at every 100mtr length.

TYPICAL SECTION CABLE SUPPORT

8mm M.S. ROD

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2.8. The cable trenches are expected to cross

the corridors of other utilities such as, roads, water supply, drainage, ICT, Gas pipeline, strom water drain etc. The trenches and the cables will change their course in plan and elevation.

2.9. The sizes of cables selected are such that

each cable will normally loaded upto 50% only. However, in case of breakdown in the ring main any cable can be loaded upto 100%.

3. The Issue Related to Heat Dissipation. 3.1. For designing the ventilation system, heat

dissipation and subsequent rise in temperature in the cable trench/RCC tunnel, has to be worked out. This is required from the view point of working environment in the trench. If temperature in the tunnel trench goes beyond 50 deg. C, it will be difficult for the maintenance

F.G.L F.G.L

PLANFOR 2 M CABLE TRENCH

TYPICAL CROSS SECTIONFOR 2 M CABLE TRENCH

F.G.L F.G.L

PLAN2.5 M CABLE TRENCH

TYPICAL CROSS SECTIONFOR 2.5 M CABLE TRENCH

F.G.L F.G.L

PLANFOR 3 M CABLE TRENCH

TYPICAL CROSS SECTIONFOR 3 M CABLE TRENCH

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crew members to carry out any work in the tunnel.

3.2. Considering the maximum ambient temperature of 45 deg. C, one has to work out the resultant temperature in the tunnel when a design load is carried by the cables. As already indicated, the water level is very high, hence trenches are likely to be relatively in the cool condition. Simultaneous occurrence of high ambient temperature, maximum current passing through the cables and need of a crew member to enter for maintenance in the cable trench is unlikely.

3.3. Since the cables are connected in ring mains the chances of a single cable getting loaded to 100% along with the simultaneous eventualities indicated in 3.2 above, are very rare. The cable designer may therefore have to focus on load ability of the cable and resultant heat dissipation.

3.4. Referring to British Standard (BS) 7769-2-2.1-1997 & IEC 60287-2-1-1994 various calculations for thermal resistance have been given. The calculations are given for different cases as under:

a) Thermal resistance between one conductor and sheath.

b) For single core cables, belted cables, two core belted cables with circular conductors/sector shaped conductors etc.

c) Three core belted cables with different shapes and composition.

d) Oil fill cables e) Thermal resistance between sheath and

armors. 3.5. There are various empirical formulae for

calculation of temperature rise for equally loaded, unequally loaded and groups of buried cables.

3.6. There is no specific mention of any formula when the cables are laid in the RCC trench. Under the circumstances, a precise value of heat dissipation is found to be difficult.

3.7. The heat balance equation may have to be evolved for this specific case covering the installation of underground cables throughout the brand new city coming up on a barren land.

4. Means to contain heat dissipation and

increase in the temperature inside the RCC trench.

4.1. After calculation of heat dissipation taking some base load and the ambient temperature of 45 deg. C, it may become necessary to find out the ways and means to contain the inside trench temperature at around 45 deg. only. This temperature may perhaps allow the crew member to work safely.

4.2. In the case under consideration, openings have been provided at an interval of 100 M. It is necessary to carry out the study for the induced draft and the forced draft for containing the temperature inside the trench.

4.3. The provision for forced draft would mean spending energy. It is possible to monitor the heat dissipation in the trenches by use of sensors and optical fiber cables. For such a large system it will be necessary to plan the cable laying and for balancing the heat dissipation in the trench. It is felt that in the beginning, the system can be designed with induced draft and then at a later date forced draft cooling system can be developed depending upon the heat dissipation requirement.

4.4. It is also possible that the cable designs themselves may be such which can guarantee manageable temperature rise in the trenches under different loadings and ambient temperature. The manufacture of underground power cables is continuously undergoing changes with regards to material & configuration.

4.5. Heat dissipation will also depend upon the size of the cable trench and number of cables inside it. It may be pertinent to note that for such a large underground power system, cables of different voltage class and the different ampacities will be carried in a common cable trench and therefore the precise calculation of heat dissipation will be difficult. It will be futile to design the system considering all the cables simultaneously loaded to full load and dissipating heat.

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4.6. The laying of cable with different configuration (Horizontal, trefoil, buried etc.) is also likely to vary the heat dissipation and rise in temperature in the trench.

5. Anticipated Cable Network 5.1. The proposed scheme when finally

realized, is likely to have the cable lengths as follows: Cable Description Length (km) 400 kV Cable 0.3523 220 kV Cable 32.822 66 kV Cable 125.291 11 kV Cable 188.114

5.2. The extent cabling indicated above may be

laid in common trenches based on the final design of the Electrical System. However at the preliminary design stage, assumptions are made on the pattern, type and quantum of Electrical loads. Thus, the heat dissipation working done now will serve as a guide line for the final design of trenches/tunnel etc.

6. Conclusion 6.1. For a large underground power cable

network to be housed in the RCC cable trenches it is necessary to postulate proper heat balance equitation and resultant temperature rise in the trenches.

6.2. While working out heat dissipation, it is necessary to take into account various factors of land and topography, water tables, length of the cable and the trench etc. Authors

Subhashchandra M. Takalkar Born on 30th March 1948 and graduated in Electrical engineering from the MS

University of Vadodara in 1971. He has more than forty years experience in transmission and distribution of power as well as hydro power. He retired from Gujarat Energy Transmission Corporation in the cadre of chief engineer in March 2006. Presently he is a Managing Director of consultancy firm named Takalkar Power Engineers & Consultants Pvt. Ltd. in Vadodara. The firm is engaged in design, engineering and construction supervision of transmission lines and substation up to 765 kV. The firm is also actively involved in industrial electrical design and hydro power designs.

Kishor G. Gaikwad Born on 20 June 1983 and graduated from Mumbai University in Electrical Engineering in the year 2006. He has to his credit 8 years experience in the field of electrical engineering for the transmission lines and substations up to 765 kV. Presently he is working as a Deputy General Manager in the consultancy firm named Takalkar Power Engineers & Consultants Pvt. Ltd..

Keval N. Velani Born on 23rd Sep 1989 and graduated in Electrical Engineering from Birla Vishvakarma Mahavidhyalaya, VV Nagar in 2011. He has more than three years experience in substation design engineering work, Power System Protection and Distribution Network. Presently he is working as a Jr. Engineer in the consultancy firm named Takalkar Power Engineers & Consultants Pvt. Ltd..