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