Design and Construction of Underground Cable Transmission Line

• Definition of UG Cable• Construction Principle of UG cable • Comparison between UG & O/H cable • Advantages & Disadvantages of UG line rather O/H Line • Ampacity Criterion of UG cable

• Definition of UG Cable

• Construction Principle of UG cable

• Comparison between UG & O/H cable

• Advantages & Disadvantages of UG line rather O/H Line

• Ampacity Criterion of UG cable

• Discussion about High charging current of UG cables and it’s impact of protective devices

• Different types of UG cables-Single core and three core

• Components of Single and three core cables

• What is VCV and CCV

• What is the basic different of HDPE, MDPE, and PVC compound uses in UG cables

• Different current carrying capacity of single and three core cables

• UG cable laying design procedure

• Maximum Ampacity calculation of UG cable

• Ampacity Variation due to Cable Laying Variation

• Step by step procedure of UG cable Laying

• UG cable Joint and it’s necessary jointing materials and jointing procedure

• UG cable Single Ended Bonding, Double Ended Bonding and Cross Bonding of copper screen or aluminum sheath

• UG cable Grounding materials

• Sheath Voltage Limiter

• Crossed Link connection of Screen/Sheath

• Discussion about Circulation current impact, Potential rise and Induced Voltage of UG cables

What We Discuss on this session ?

Construction Principle of UG cable

• Low pressure oil filled cables

• High pressure oil filled cables (Common for EHV)

• Paper insulated cables

• Cross linked polyethylene (XLPE) cables

An underground cable essentially

consists of one or more conductors

covered with suitable insulation and

surrounded by some protective

layer.

(Continued….)

Oil Filled Cable

 Developed in the early 1900’s;

XLPE Insulation Cable

 Cross-linked polyethylene (XLPE) through the

cross-link reaction of organic peroxides

 Cross-link method for polyethylene was

developed in the USA during the 1950’s

 Continuously developing for application to

higher voltages.

 Higher voltages 500kV XLPE cables have

already been commercialized

(Continued….)

Construction Principle of UG cable

Comparison between UG & OH Cable

01 Construction Complicated, High cost, heavy insulation

required

Easy, Low cost, no need insulation

02 Installation Easy to install Difficult to install

03 Heat dissipation limited by the layers of insulation, armoring

and sheaths

Natural cooling, surrounding

04 Size of Conductors Larger conductor sizes Smaller conductor sizes

05 Voltage carrying capacity Limited by the expensive construction & heat

dissipation

Better suited to carry higher voltages

06 Fault detection and repair Complicated and takes more time to repair Easier to detect and repair

07 Public safety More safe to the public, animals Less safe to the public, animals

08 Effect of lightning discharges not affected by the lightning strikes more prone to lightning strikes

09 Interference Does not interfere with communication lines interfere with communication lines

10 Voltage drop Less voltage drop due to larger diameter More voltage drop due to smaller diameter

11 Environmental impact more environmental and health benefits Causes human, animal intervention,

13 Life expectancy Shorter life span Longer life span

Advantages & Disadvantages of UG line rather O/H Line

Advantages

1 Less space compared to overhead lines;

2 No visual intrusion (also called visual pollution);

3 Not susceptible to atmospheric activity (wind/lightning);

4 Higher surge impedance reduces severity of switching over voltages;

5 Ideal way to transmit power across a water body (e.g., supply to an island).

Disadvantages

1 Fault location is difficult and time consuming;

2 Expensive;

3 More monitoring (for certain types of cables);

4 Jointing/termination require persons with high skill levels;

5 Joints/terminations become weak points;

6 Testing is difficult and time consuming.

Maximum Permissible Conductor Temperature

Normal Operation Emergency Operation Short Circuit

Ampacity Criterion of UG cable

Cable ampacity is determined according to IEC 60287 together with the method of installation as

documented in IEC 60364-5-52

(Continued….)

For steady state current the equilibrium (heating rate = cooling rate) temperature of the

cable heating rate principally

Installation-i type of sheath bonding,

ii mutual heating effects.

(… Continued)

cooling rate depends on the

difference between the conductor and outside ambient temperatures:

Discussion about High charging current of UG cables and it’s impact of protective devices

 Cable limitation generally scaled on higher cost;

 The limiting factor here is the capacitance of the cable has a much greater effect on performance;

 There are two main limiting effects of cable capacitance:

i The Ferranti effect, the voltage at the far end of a cable exceeding the voltage at the input

end under unloaded or lightly loaded conditions;

ii The charging current, the capacitive current flowing in the cable under loaded and unloaded

conditions;

 This will severely limit its ability to deliver power

 The charging current can also have an impact on the operation of protective devices -the settings of such devices

 Ignoring the resistance of the line and the distributed nature of the capacitance, the charging

current will be given by:

I c = V/X c = V×2πfC

where:

f = frequency

C = capacitance

Different types of UG cables-Single core and three core

Continued…

Different types of UG cables-Single core and three core

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Different types of UG cables-Single core and three core

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Different types of UG cables-Single core and three core

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Components of Single and three core cables

Conductor

copper or aluminum, stranded wires and classified into three (3)

major types of concentric, compacted circular and segmental

compacted circular

Conductor Screen

The conductor screen consists of an extruded semi-conducting

polyethylene to minimize electrical stresses due to the stranded

configuration of the conductor

Insulation

The insulation material is extruded cross-linked polyethylene The

thickness of the insulation layer is the maximum value figured out

from the design of the impulse voltage and A.C voltage

Insulation Screen

The insulation screen is provided over the insulation by extruding the

semi-conducting compound concentrically and circularly.

What is CCV and VCV

CCV or VCV actually means cable manufacturing production line type:

CCV line-catenary continuous vulcanization line

VCV line-vertical continuous vulcanization line

There are three different types of extrusion lines used for manufacturing of HV and EHV XLPE

cables:

CCV line (catenary line for continuous vulcanization)

1 The curing tube is shaped like a catenary designed to replicate the flow path of the core as it

exits the extruder.

2 To avoid sag, the conductor is rotated with suitable tension as it traverses in the tube.

3 Nitrogen gas is used to maintain pressure and cooling

VCV line (vertical line for continuous vulcanization)

1 The curing tube is vertical

2 Cable axis is controlled to be in the center by applying tension.

3 Nitrogen gas is used to maintain pressure and cooling

MDCV line (Mitsubishi-Dainichi horizontal line continuous vulcanization-Long land die)

1 The curing tube is horizontal

2 The die size is the same as the final size and the core fills the die

3 No nitrogen is used in this process.

4 Does not have the “conductor sag” issues but it becomes critical to maintain the conductor axis

Manufacturing Process in VCV Line

 The insulation has no eccentricity

 The cross-linking by use of N2 gasguarantees excellent electricalcharacteristics of the insulation

 The simultaneous extrusion of the innerand outer semi-conducting layers and theinsulation prevents treeing and otherirregularities

 Uniformity of quality is maintained of allproducts as the manufacturing processesare controlled by computer

(… Continued)

What is CCV and VCV

What is the basic different of HDPE, MDPE, and PVC

compound uses in UG cables

HDPE and MDPE are two different categories of PE, or polyethylene HDPE and MDPE are differentiatedand categorized into different classes on the basis of density and branching

Different current carrying capacity of single and three core cables

UG cable laying design procedure

Project Name: Training for Engrs’

Link Name: Demonstration

J/B #1 J/B #2

Substation-A

J/B #3 J/B #4 J/B #5 J/B #6 J/B #7 J/B #8

Complete Constructing

Typical Joint Bay for UG cable

UG cable laying design procedure

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Maximum Ampacity calculation of UG cable

Ampacity is the current that a conductor can carry continuously without exceeding its

∆Td = dielectric loss temperature rise (ºC)

Rdc = dc resistance of conductor at temperature Tc

Yc = component ac resistance resulting from skin effect and proximity effect

Rca = effective thermal resistance between conductor and surrounding ambient

It is to be noted that the dielectric loss temperature rise ∆Td is negligible for single circuit extruded dielectric cables rated below 46kV

(….Continued)

Ampacity Variation due to Cable Laying Variation

DERATING FACTORS

Step by step procedure of UG cable Laying

Cable Laying:

1 Mobilization and informing concern authorities-police/traffic control etc.;

2 Test pit-to identify the others utilities position;

3 Excavation as per section length/drum schedule;

4 Selection of cable drum placing;

5 Selection of pulling direction;

6 Cable drum shifting to site;

7 Sand bedding;

8 Set winch/roller/corner roller/wire shock/swivel

9 Pulling the power cable

10 Trefoil/tie-up cables

11 Phase/Circuit marking

12 ECC laying (if necessary)

13 Interlocked side slab laying

14 Sand filling

15 10kV sheath test

16 FOC/control cable laying

17 Cover or top slab laying

18 Sand filling/warning marker tape laying

19 Backfilling

Joint Bay & CSE Preparation:

JB:

 Excavation for JB & LBP;

 Civil work/earthing network

 Tent preparation

CSE:

 Steel structure foundation for CSE & LA;

 Steel structure for CSE & LA

 Scaffolding/tent

UG cable Joint and it’s necessary jointing materials and

jointing procedure

Why Need Cable Joint?

Category of Joint

 Straight through Long routes and repaired sections

 Branch Y joints For branching of a feeder

 T joints For branching of a feeder

 Transition joints Special joints between two different types of cables

UG cable Joint and it’s necessary jointing materials and jointing procedure

Factors for Cable Joint

 The Voltage level for which the cable is designed

 Insulation requirement will increase with voltage level

 Also different voltage level requires different level of

electrical stress control.

 Number of Phases

 3Φ and 1Φ cable have different number of cores

Therefore, Joint design is different.

 Environmental protection

 Joint design must prevent the water ingress

into cable joint from surrounding

 Mechanical protection

 Joint design must have sufficient mechanical

strength Important for cables vulnerable to

damage

(….Continued)

UG cable Single Ended Bonding, Double Ended Bonding and Cross Bonding of copper screen or aluminum sheath

UG cable grounding materials

Main grounding materials for High

Voltage and Extra High Voltage:

1 Link Box: LB type will be select

according to EBG or EBA and

single or three phase earthing

system.

i 1-1 way with SVL

ii 1-1 way without SVL

iii 3-1 way with SVL

iv 3-1 way without SVL

v 3-1 way cross bonding

2 Bonding cable

3 Earthing cable

4 ECC (earth continuity cable)

5 Earthing mesh/grounding rod

6 Cable Lug

Sheath Voltage Limiter

A sheath voltage limiter (SVL) is a surge arrester with a different name It functions as an

arrester and in most cases

Where SVL is Used in UG cable?

-at link for earthing

Where Link Box is used in UG cable?

 -at cable joint

 -at cable termination in EBA

 -at cable termination in EBG

Types of SVL:

 -without sheds use in dry environment,

 -with sheds are for outdoor application

Crossed Link connection of Screen/Sheath

Discussion about Circulation current impact, Potential rise and Induced Voltage of UG cables

When dealing with underground power cables, sheath circulating currents can be

induced.

Impact of Circulating Current:

 These currents produce power losses in the sheaths and decrease the ampacity (capacity of carrying current) of the cables.

 The circulating sheath currents generate a magnetic field that adds to the cable magnetic field.

How Reduce Circulating Current?

 Using ECC;

 Using single bonding;

 Using SVL.