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Smart Grid Capabilities
POWER Engineers, Inc.Transmission Cable System Design
Energypetrol
Thermal Design of Underground Cables
Basic Principles
Cable System Thermal DesignCurrent rating dependent on: amount of heat generatedtransmission effectiveness of heat to cable surface and dissipation to surrounding environmentmaximum allowable conductor temperatures
Cable Design
Cable System Thermal Design
Cable System Thermal DesignExternal thermal resistance (T4) is the most influential parameter in cable ampacity.
Soil Thermal PerformanceSoil and backfill thermal performance is critical in determining cable ratingFor given transmission line, native soil samples should be tested every 150 meters for both in-situ (existing conditions) and also dry-out valuesThe thermal resistivity testing is normally performed during geotechnical investigation and soil surveyA controlled backfill design could be developed depending on the results of the thermal testsEarth ambient temperature readings should also be measured if possible, which is a key factor in heat flowElectrical DesignReasons for grounding/bondingLimit sheath voltagesReduce or eliminate sheath lossesMaintain a continuous ground path to permit fault-current return and adequate lightning and switching surge protectionSplice Design
Sheath InterruptSheath Bonding/Grounding LeadsSectionalized Joint
Splice Design
Sheath InterruptSheath Bonding/Grounding LeadsSectionalized Joint
Solid Bonding (Multi Point Bonding)Simplest solution to dealing with induced sheath voltages, and most common bonding method used for underground distribution circuits (but not common on transmission)Induced voltages creates current flow in the sheath (circulating currents) which may not be inappropriate for high ampacity applicationsEquipment ground connection design may also be affected due to high circulating current
Splice DesignStraight Joint
Sheath Bonding/Grounding LeadSolid BondingI2R Circulating Currents in Sheaths
Solid Bonding
Single Point Bonding (Open Point Bonding)Eliminates circulating currents, resulting in higher ampacityCreates voltage rise (standing voltage) at the ungrounded (unearthed) end of the sheathRequires an appropriately sized ground continuity conductor (GCC) to carry the fault current (under short circuit conditions)Geometric placement of GCC is important factor for managing sheath voltage rise during faultTypically used on shorter lengths
Single Point BondingNo I2R Circulating Currents in Sheaths
Single Point Bonding
Cross BondingNo circulating currents when cross bonding is balancedRequires dividing the circuit into groups of three minor sections, to bond and ground all three sheaths at the ends of the triad without significant current flow since the closing voltage will generally approach zeroStanding voltages still occur at the sheath end of each minor sectionGCC is not required if cable sheath is sized to carry fault currents
Cross Bonding
Cross Bonding
Total voltage over sheath is zero
Hybrid Bonding
22Electrical Design
Special bonding versus solid bonding
Link Boxes
Grounding Link Box
Sheath bonding leadsGround connectionLinks (removable)Link BoxLink Box with Sheath Voltage Limiters (SVLs)
Sheath bonding leadsGround connectionSVLsSheath Voltage LimitersLimit transient overvoltages that might damage the cable jacket or shield interruption within the spliceAt each splice, interrupted shield gaps experience unacceptably high transient voltages during circuit switching and lightning strikes, and must be protected by connecting across them suitably rated SVLsVoltage sensitive and short the insulated gap to limit the magnitude of the transient voltage. When the transient has passed, the SVL returns to high resistance and effectively becomes an open-circuit
SVL Sizing
GroundingGround continuity conductorSized to carry current back to the ground connections at a terminationAt least one end of the cable shield conductor and ground conductor must always be connected to ground, or these conductors will rise to an appreciable voltage, damaging the cable and forming a serious safety risk to personnelBelow grade connections should be exothermic welded connections or irreversible compressionAbove grade connections can be boltedIndependent ground connections should be made at terminal structures for GCC, link boxes, and lightning arrestersCable System Design ConclusionsThermal circuit and cable rating is heavily contingent upon native earth environmentBonding design needs to take into consideration the:Cable ratingSystem operation during transient conditionsSafety29