Design and Construction of Underground Cable Transmission Line

What We Discuss on this session ? • • • • • • • • • • • • • • • • • • • • • 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 Definition of UG Cable An underground cable is a cable that is buried below the ground to distribute electrical power or telecommunications signal Such cables are an alternative to overhead cables, which are several meters above the ground Overhead cables are often replaced with underground cables Downtown areas with many tall buildings usually have few or no cables above ground, This is mainly for aesthetic purposes because underground cables can not be seen They are also less dangerous to people because they are out of the way They cost more to install, but last longer Construction Principle of UG cable An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by some protective layer Oil Filled Cable • • • • Low pressure oil filled cables High pressure oil filled cables (Common for EHV) Paper insulated cables Cross linked polyethylene (XLPE) cables XLPE Cable (Continued….) (….Continued) Construction Principle of UG cable Oil Filled Cable Developed in the early 1900’s;  Widely used due to high credibility and economic efficiency; Used for large storage transmission (66kV to 765kV) (Continued….) (Continued….) Construction Principle of UG cable 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 Comparison between UG & OH Cable Sl No Comparison Factor UG Cable 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, 12 Land use Less More 13 Life expectancy Shorter life span Longer life span Advantages & Disadvantages of UG line rather O/H Line Advantages Less space compared to overhead lines; No visual intrusion (also called visual pollution); Not susceptible to atmospheric activity (wind/lightning); Higher surge impedance reduces severity of switching over voltages; Ideal way to transmit power across a water body (e.g., supply to an island) Disadvantages Fault location is difficult and time consuming; Expensive; More monitoring (for certain types of cables); Jointing/termination require persons with high skill levels; Joints/terminations become weak points; Testing is difficult and time consuming 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 Maximum Permissible Conductor Temperature Normal Operation Emergency Operation Short Circuit 90oC 105oC 250oC 1) Normal Operation Normal operation is meant to be maintained through out a given period of time everyday or continuously, without affecting the operation 2) Emergency Load Emergency load is meant to be maintained for a short time under the condition of system breakdown or under the state of excessively loaded operation, without causing a defect 3) Short Circuit Short circuit is meant to cause no defect of the cable when an irregular current, flows for short time due to shorting or earthing (Continued….) (… Continued) Ampacity Criterion of UG cable For steady state current the equilibrium (heating rate = cooling rate) temperature of the conductor is required, 90°C for XLPE cable heating rate principally depends on: Constructioni conductor resistivity, ii loss factors of the insulation, iii Sheath, iv armor, v no of cores Installationi type of sheath bonding, ii mutual heating effects cooling rate depends on the difference between the conductor and outside ambient temperatures: i insulation, ii sheath, iii armor, iv jacket, v soil, vi earth, vii backfill, viii Concrete 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: Ic = V/Xc = V×2πfC where: f = frequency C = capacitance UG cable laying design procedure Complete Legend Constructing Project Name: Training for Engrs’ Excavation Prepare Link Name: Demonstration Substation-A 3-1T Substation-B J/B #1 3-1T OFC 48C 470m Canal J/B #4 J/B #3 J/B #2 J/B #5 Canal J/B #6 J/B #8 J/B #7 3-1T 3-1C/B 3-1C/B 3-1JB 3-1C/B 3-1C/B 3-1JB 3-1C/B 3-1C/B 3-1C/B 3-1C/B 3-1JB 3-1C/B 3-1C/B 3-1JB 3-1C/B 3-1C/B 1,510m 470m 1,190m 1,220m 470m 610m 610m 610m 3-1T 1,260m 580m 580m 580m 4,980m 5,180m Consider:  Major section  Bonding/Earthing procedure  Link box type  Termination type  Bending Radius 20D/30D (Continued… ) (….Continued) UG cable laying design procedure Typical UG Cable Trench Direct Buried 21 (Continued… ) (….Continued) UG cable laying design procedure Typical UG Cable Trench Road Crossing (Continued….) (….Continued) UG cable laying design procedure Typical Joint Bay for UG cable Maximum Ampacity calculation of UG cable Ampacity is the current that a conductor can carry continuously without exceeding its temperature rating The cable ampacity of units in amperes is: I= Tc - (Ta+∆Td) Rdc (1+Yc) Rca Where: I = cable current capacity (Amperes) Tc = conductor temperature (°C) Ta = ambient temperature (°C) ∆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 Ampacity Variation due to Cable Laying Variation DERATING FACTORS 25 (Continued…) (….Continued) Ampacity Variation due to Cable Laying Variation DERATING FACTORS 26 Step by step procedure of UG cable Laying Cable Laying: Mobilization and informing concern authorities-police/traffic control etc.; Test pit-to identify the others utilities position; Excavation as per section length/drum schedule; Selection of cable drum placing; Selection of pulling direction; Cable drum shifting to site; Sand bedding; Set winch/roller/corner roller/wire shock/swivel Pulling the power cable Joint Bay & CSE Preparation: 10 Trefoil/tie-up cables JB: 11 Phase/Circuit marking  Excavation for JB & LBP; 12 ECC laying (if necessary)  Civil work/earthing network 13 Interlocked side slab laying  Tent preparation 14 Sand filling CSE: 15 10kV sheath test  Steel structure foundation for CSE & LA; 16 FOC/control cable laying  Steel structure for CSE & LA 17 Cover or top slab laying  Scaffolding/tent 18 Sand filling/warning marker tape laying 12/25/2019 19 Backfilling UG cable Joint and it’s necessary jointing materials and jointing procedure Why Need Cable Joint? Cables are heavy and difficult to transport  Fixed drum lengths (e.g.,500m) are typical packaging norm  Longer feeder lengths need joints  Joints are needed for partial replacement (in case of a cable fault)  Joints are also needed for specific purposes (next slide) 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 (Continued…) (Oil filled/XLPE)  (….Continued) 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 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: 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 Bonding cable Earthing cable ECC (earth continuity cable) Earthing mesh/grounding rod 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       Individual section Major section Normal Joint Isolated Joint With SVL Without SVL 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 ... 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. .. cables-Single core and three core 13 (Continued…) (Continued…) Different types of UG cables-Single core and three core 14 Components of Single and three core cables Conductor copper or aluminum, stranded... Different types of UG cables-Single core and three core 11 Continued… (… Continued) Different types of UG cables-Single core and three core 12 (Continued…) (…Continued) Different types of UG cables-Single