CIGRE SC B1 Colloquium on H.V. Insulated Cables New Delhi (India) on Oct. 2017

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1 CIGRE SC B1 Colloquium on H.V. Insulated Cables New Delhi (India) on Oct Challenges in Planning and Design of Cable Transmission System of Pugalur North Trichur HVDC System- Users Perspective R. K. CHAUHAN, M.M. GOSWAMI, B.B. MUKHERJEE, S. BHATTACHARYA, PUNEET TYAGI,P. CHAKRABORTY Power Grid Corporation of India India

2 Introduction About 6000 MW power from the new IPP generation in Raigarh(Kotra), Champa, Raigarh(Tamnar) and Raipur area is earmarked for transfer to Southern region (SR) as the target region and balance power is to be transferred to the Western/ Northern regions. Out of this 6000 MW power, Kerala has a share of 2000 MW. This Transmission system is realized by ± 800kV, 6000MW HVDC Bipole between Raigarh (Near kotra) Pooling Station and Pugalur ±320 kv,2 x 1000 MW Voltage Source Converters (VSC) symmetrical monopoles HVDC systems from Pugalur to North Trichur

3 Introduction The two symmetrical monopoles converter stations will be connected by a combination of HVDC OH line and 320 kv DC XLPE underground cables. The length of the OHL is approximately 143 ± 15 Km and the length of the XLPE cable is approximately 32 ± 05 Km. A transition station is planned to connect HVDC overhead line with HVDC underground cable

4 Key Challenges in the Câble Route- Contraints of the Utility Corridor Along NH 544: ROW is a major contraint in Kerala After exploration of varions options, utility corridor of National highway 544 from Wadaakanchary to North Trichur was finalised for the Cable Corridor. The Utility corridor was having a width of 2 meters For crossings of culverts, rivers /streams and other impediments in the given ROW, HDD or other suitable method as required may be adopted for laying the cables.

5 Key Challenges in the Câble Route- The Extension of NH544 At the time of finalisation of the corridor for the Cable, the National Highway 544 was under expansion from four lane highway to 6 lane highways. The cable corridor was allotted considering the adjacency to 6 lane high way and during the bidding stage due to on-going construction work of national Highway, the clear corridor of the Cable route was not available for detailed route survey and soil investigation. The same was a key challenge in finalising the bill of quantity of the cable supply, laying and installation of the same.

6 Key Challenges in the Câble Route- Cable Laying /crossing of the existing CCK pipeline. During preliminary cable route survey a petroleum product pipeline was found to be under operation from Kochi to Karur for transporting petroleum product. The pipeline is under high pressure and is catholically protected against corrosion. The pipe line was crossing the NH 544 from Vellanikkara(km ) to Vadakkenchery (km 237) at the following locations- Before the Kuthiran Hills ( near Hotel Mekattil) Just before Kuthiran Temple Immediately after the Kuthiran temple Below the Irimpupalam At Kombazha after Peechi Catchment Area

7 Key Challenges in the Câble Route- Future LPG Pipeline During the preliminary route survey it was found a new LPG pipeline between Cochin to Palakkad is also likely to be in close proximity with the cable route. The pipeline is of 12 "diameter along and the length of this parallel section is approximately 1.2 km. Top of pipe is at a depth of 1.2 m (approx.) from ground level. The pipeline will be catholically protected against corrosion. It is required to provide adequate mitigation measures for example an appropriate polarization cell or any other suitable method to prevent corrosion of the existing pipeline.

8 Key Challenges in the Câble Route- Cable Bridge for Canal Crossing Another major impediment in the Cable route is a canal in the wild life area Required study of various options for Cable Crossing of the Canal including options like HDD,Cable Bridge etc. It was decided after careful consideration of all the factors that the optimum solution for the Canal Crossing was a Cable Bridge with Cable encapsulated in Concrete duct.

9 Key Design Consideration of Cable Voltage Stresses Cable system thermal design Cable Screen Current Stresses Environmental Aspects Analysis for HVDC Cable Route

10 Voltage Stresses: Voltage stress which govern the insulation thickness of cable are the steady state, transient & dynamic stresses. Transient and dynamic stresses imposed on the DC cable system are caused by faults within the HVDC system and overvoltage emanating from lightning strikes to the overhead lines which propagate into the cables. Three different fault scenarios are seen as relevant for the cable design in this project because they lead to maximum transient voltage stresses for the cable system DC side, single pole-to-ground DC side, pole-to-pole fault with ground contact Transformer AC secondary side, phase-to-ground fault

11 Voltage Stresses: DC side, single pole-to-ground DC side, pole-to-pole fault with ground contact Courtesy: Siemens Transformer AC secondary side, phase-to-ground fault

12 Current Stresses: DC side, pole-to-pole fault with ground contact results in highest DC side Current. After the ac breaker is opened fault current will decay depending on resistance of converter, converter reactor and DC line. DC side, pole-to-pole fault with ground contact Courtesy: Siemens

13 Cable system thermal design The continuous current carrying capacity of the cable has been calculated according to the method given in IEC "ELECTRIC CABLES - CALCULATION OF THE CURRENT RATING ". However, thermal resistance of surrounding medium (T4) in IEC does not cover the plural soil condition (mixture of native soil, thermal backfill and concrete duct), In order to provide optimum design Finite Element Method (FEM) analysis, which shows conductor temperature at the given heat loss and soil thermal resistivity. The design cases based on initial survey and soil investigation are based on different ROW which is available for the cable route. The following aspects are investigated. Standard trench at a depth of 2 m Narrow trench at a depth of 2 m. Narrow trench at a depth of 1m.

14 Factors Affecting Thermal resistivity Heat flows through a soil mainly by conduction along mineral particles, and by conduction and convection through the moisture or air that occupies the pore space between solid particles. Thermal resistivity depends on : 1 Moisture content 2 Dry Density 3 Structural Composition 4 Soil particle shape/texture Courtesy Geotherm Inc

15 Variation of soil thermal resistivity with moisture content In a dry state the pore spaces are filled with air (~4500 Ccm/W). As water (~165 C-cm/W) replaces air, the soil resistivity is substantially lowered (as much as 3 to 7 times) as the good heat conduction paths are expanded ( thermal bridges ) Courtesy Geotherm Inc

16 Variation of soil thermal resistivity with Dry density Soil densification (or compaction) increases mineral grain contacts and displaces air (ie. lowers porosity), therefore reducing the soil resistivity, most notably at low moisture contents. Well-graded soils are potentially more dense because smaller grains can efficiently fill the spaces between the larger particles. Dry density is expressed as the ratio of the dry weight of the soil solids to the total volume. The total volume is taken as the initial volume of the undisturbed moist soil in a sample tube

17 Variation of soil thermal resistivity with Soil Composition Soil is a composite consisting of solid mineral grains, typically only making point-to-point contact, and pore space filled with water and air. The thermal resistivity of a given soil mass is a function of the intrinsic resistivities of its components

18 Variation of soil thermal resistivity with Texture This refers to soil grain size, shape, and particle size gradation. Since most of the heat is conducted through the solid particles and their contacts, the resistivity is minimized for soils that maximize these contacts. Hence thermal resistivity varies for gravel, sand, silt, clay etc

19 Courtesy: SEI Standard trench at a depth of 2 m

20 Courtesy: SEI Narrow trench at a depth of 2 m.

21 Courtesy: SEI Narrow trench at a depth of 1m

22 Cable Screen Current Stresses Another key design study for the cable design is the Cable screen Current Stress. This also becomes important in the subject project which is having a mix of overhead line and the HVDC XLPE Cable as there may be a contribution to these stresses from the Converter Side. Equivalent circuit for the DC cable Courtesy: SEI

23 Cable Screen Current Stresses Contribution of Converter- L G Fault Contribution of Converter- L L Fault Courtesy: SEI

24 Environmental Aspects Analysis for HVDC Cable Route electromagnetic fields, and The effects of the cables upon the environment are studies with respect to chemical impact and the observations are as per belowa) Electro magnetic Fields The magnetic fields surrounding electrical conductors are produced by, and dependent on, the DC current flowing in them. For the subject project the cables operate as a positive and negative pole system, the current in each positive and negative cable runs in opposite directions, leading to a significant cancellation of the external magnetic field.

25 Environmental Aspects Analysis for HVDC Cable Route DC power transmission in the underground cables generates static magnetic fields in the range of ~28 micro-tesla (μt) at 1.0 m above ground and ~54 micro-tesla at the ground surface with both symmetrical monopoles operating at full load These levels are in the same order of magnitude as the earth s natural static magnetic field. Courtesy: SEI

26 Electrostatic Fields The electrostatic fields surrounding conductors are produced by, and dependent on, the voltage applied to the conductor. However, the DC cable conductors are surrounded by XLPE insulation and a metallic screen (or shield), of which the metallic screen is grounded at earth potential. Thus, the metallic screen confines the electric field to the cable insulation, and there is virtually no electrostatic field outside the cable surface. This is valid for any shielded power cable, regardless of whether the voltage is AC or DC.

27 Chemical Impact Under normal operation conditions, land power cables do not release chemicals, consumables, or other agents into the environment. I n case of damage or rupture, cables with solid insulation, such as the XLPE used for this project, do not contain fluids that can leak into the environment

28 Nominal Rated Power at Receiving End Nominal voltage at cable terminations, Uo: Maximum continuous voltage at cable terminations, Um: Maximum total continuous AC ripple superimposed on Um: Surge withstand voltage(siwv) (approximately) : Lightning Impulse withstand voltage LIWV( approximately) 1000MW ± 320 kv Uo + 5 % kv 0.03 Um (peak) 740 kv peak 770 kv Copper conductor size 2500 mm 2 is considered for the cable rating capacity for required transmission of 1000MW under the maximum allowable conductor temperature at 90 o C. All the calculations shall be rechecked after right of way handover, detailed route survey, thermal resistivity measurements in situ and laboratory.

29 Typical Joint Bay for HVDC XLPE Cable Coffin box 9,500 Earth rod "a"

30 Tests On HVDC Cable system The cable system is defined as being comprised of the HVDC land cables, land cable joints, land/ohl transition station joints and all auxiliary equipment. The cables supplied in this project are already covered by range of approval of completed pre-qualification test of another size according to CIGRE TB 496. Since this is a critical item of this project also a first time usage in our country all the type tests shall be performed on the cable system being supplied in this project as per CIGRE Technical Brochure496, IEC62067 ( as applicable). Since HVDC cable systems are exposed are interface a to overhead lines, the superimposed lightning impulses may be of the same and opposite polarities. Hence as a suppliment to the tests already a part of CIGRE Technical Brochure496, lightning impulse tests shall be done with superimposed lightning impulses may be of the same and opposite polarities. Routine and sample tests shall be performed the routine tests described in CIGRE Technical Brochure496 and IEC62067

31 Way Forward. The first VSC HVDC interconnection in India offers a technology driven option for power transfer between two load centers in Southern parts of India. The HVDC cable and transmission line mix and a cable route through a city offers challenges in design and execution. The detailed engineering of the subject project is under progress and the successful execution of the project will be a bench mark for some more HVDC project utilising the Cable Technology to mitigate the ROW issues for city infeeds and in submarine applications involving off shore wind farms and cross border transmission system.

32 Thank You