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
CIGRE SC B1 Colloquium on H.V. Insulated Cables
New Delhi (India) on 13-14 Oct. 2017
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
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
Introduction
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.
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.
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 260.00) 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
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.
Key Challenges in the Câble Route-
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.
Voltage Stresses
Cable system thermal design
Cable Screen Current Stresses
Environmental Aspects Analysis for HVDC Cable Route
Key Design Consideration of Cable
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.
Voltage Stresses:
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
DC side, single pole-to-ground
DC side, pole-to-pole fault with ground contact
Transformer AC secondary side, phase-to-ground fault
Voltage Stresses:
Courtesy: Siemens
DC side, pole-to-pole fault with ground contact
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.
Courtesy: Siemens
The continuous current carrying capacity of the cable has been calculated according to the method given in
IEC 60287 "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.
Cable system thermal design
Courtesy – Geotherm Inc13-10-2017 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 Inc13-10-2017 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’).
13-10-2017 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.
13-10-2017 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.
13-10-2017 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.
Standard trench at a depth of 2 m
Courtesy: SEI
Narrow trench at a depth of 2 m.
Courtesy: SEI
Narrow trench at a depth of 1m
Courtesy: SEI
Equivalent circuit for the DC cable
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.
Courtesy: SEI
Cable Screen Current Stresses
Contribution of Converter- L G Fault
Contribution of Converter- L L Fault
Courtesy: SEI
The effects of the cables upon the environment are studies with respect to electromagnetic fields, and
chemical impact and the observations are as per below-
a) 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.
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.
Environmental Aspects Analysis for HVDC Cable Route
Courtesy: SEI
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.
Electrostatic Fields
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
Chemical Impact
Nominal Rated Power at
Receiving End
1000MW
Nominal voltage at cable
terminations, Uo:
± 320 kV
Maximum continuous voltage at
cable terminations, Um:
Uo + 5 % kV
Maximum total continuous AC
ripple superimposed on Um:
0.03 Um
(peak)
Surge withstand voltage(SIWV)
(approximately) :
740 kV
peak
Lightning Impulse withstand
voltage LIWV( approximately)
770 kV
Copper conductor size 2500 mm2 is considered for the cable rating capacity for required transmission of
1000MW under the maximum allowable conductor temperature at 90oC.
All the calculations shall be rechecked after right of way handover, detailed route survey, thermal
resistivity measurements in situ and laboratory.
9,500
Earth rod
Coffin box
"a"
Typical Joint Bay for HVDC XLPE Cable
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
Tests On HVDC Cable system
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 .
Way Forward….
Thank You
