© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Underground Raceway
Systems
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Slide 2© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Raceway Systems
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Slide 3© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Derating Analysis
• Determines the proper size of cables to carry the specified loads for new systems.
• Calculates maximum cable ampacities for specific scenarios.
• Examines cable temperatures and ampacities for existing systems to determine operating and emergency limits.
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Slide 4© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Derating Analysis
Steady-state temperature calculation
Uniform-ampacity cable ampacity calculation
Uniform-temperature cable ampacity calculation
Cable sizing
Transient temperature calculation
•NEC Accepted -
Neher-McGrath Method
•IEC 287 Method
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Slide 5© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Ampacity
FundamentalsCable Ampacity is the current a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
Heat is generated when current is carried by a conductor since it must pass through the electrical resistance of the conductor.
Watts = I2R
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Slide 6© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Ampacity
FundamentalsVarious thermal barriers:
1. Conductor insulation2. Air inside a duct3. Duct wall4. Soil surrounding an underground duct5. Additional thermal insulation applied such as
polyurethane
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Slide 7© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Ampacity
Fundamentals
Heat Transfer Equation
The rate of heat transfer is directly dependent on the difference in temperature between the conductor (Tc) and the ambient temperature (Ta)
RHO is thermal resistance in degrees Centigrade-cm/watt
Rearranging the terms for I:
R).RHO2(ITaTc
ITC TA( )
R RHO( )
TC
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Slide 8© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Heat Flow Model
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Slide 9© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Heat Flow Model
Installation under an isolated condition
Installation of groups of three or six circuits
RHO of Soil = 90
Ta = 20 oC
(Generalized)
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Slide 10© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Heat Transfer Problem
Ultimate Unchanged Surrounding Environment
In actual practice, the surrounding medium in which the cables are to be installed rarely match those conditions under which the stated ampacities apply.
Adjustment Factor
Heat Flow
Immediate Surrounding Environment (Actual Installation Conditions)
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Slide 11© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Adjustment Factor
Cable Derating is based on a concept of an adjustment (multiplying) factor that is applied against base ampacity.
The multiplying factor takes into account the differences in the cable’s actual installation conditions from the base conditions.
I x FI'
I’ = Allowable cable ampacity for the actual installation conditionsF = Cable Ampacity Adjustment FactorI = Base Ampacity specified by cable manufacturer or NEC under an
isolated condition with a soil thermal resistively (RHO) of 90 and a specified ambient temperature
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Slide 12© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Adjustment Factor
Composition
Ft = Adjustment factor to account for the differences in the ambient and conductor temperatures from the base case
Fth = Adjustment factor to account for the difference in the soil thermal resistivity from RHO of 90
Fg = Adjustment factor to account for cable grouping
gF x th
Fx t
FF
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Slide 13© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Duct Bank Example
I = 375 Amps 350 MCM
I = 450 Amps 500 MCM
Ft = 0.82 Ta from 20 C to 30 C
Tc from 90 C to 75 C
Fth = 0.9 RHO of 90 to 120
Fg = 0.479 350 MCM Cable
Fg = 0.478 500 MCM Cable
350 MCM
F = 0.82 x 0.90 x 0.479 = 0.354
500 MCM
F = 0.82 x 0.90 x 0.478 = 0.354
I’ = 375 x 0.354 = 133 Amps
I’ = 450 x 0.353 = 159 Amps
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Slide 14© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Neher-McGrath Equation
(1+Tc) is a multiplier used to convert direct current resistance (Rdc) to alternating current resistance or impedance. For wire sizes smaller than No. 2, this term becomes insignificant.Δ TD compensates for heat generated in the jacket and insulation for higher voltages. It is insignificant for voltages below 2kV.
Rca' Tc)(1 Rdc
ΔTd)(TaTcII = Ampacity (kA)
Tc = Conductor temperature (Deg C)Ta = Ambient Temperature (Deg C)Δ Td = Conductor temperature rise due to dielectric loss (Deg C)Rdc = Conductor dc resistance (μΩ/ft)Tc = Loss increment due to conductor skin & proximity effectsRca’ = Thermal resistance between conductor & ambience (Ω-ft)
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Slide 15© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Neher-McGrath Example
Calculate ampacity of 3/C concentric stranded XHHW insulated copper cable enclosed in a 1 inch steel conduit. Ta = 40 C
DC 0.292 [NEC Table 8, Chapter 9]
DI 0.09 0.292
DI 0.382
Ri 0.012 400 logDI
DC
Ri 0.56
t = insulation thickness 2t = 2 x 0.045 in. = 0.09 in. [NEC Table 310-13]
From N-M Table VIIFrom N-M Table VII
a 3.2
b 0.19
1 Inch Rigid Steel Conduit ID = 1.049 in.
OD 1.315
From N-M Table VII
Ds 2.16 DI
Ds 0.825
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Slide 16© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Neher-McGrath Example
Rsdn a( )
Ds b
Rsd 9.457
EmissivityE 0.95
Ds2 1.315
Emissivity
Emissivity
Conduit OD
RE9.5 n( )
1 1.7 Ds2 E 0.41( )[ ]
RE 7.054
Rca Ri Rsd RE
Rca 17.071
Rdc75 194
Rdc90 Rdc75234.5 90( )
234.5 75( )
Rdc90 203.402
I90 40( )
203.5 Rca( )
I 0.12 kA with Ta = 30, I = 131 Amps
(Table 310-16 lists 130 Amps, Ta=30)
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Slide 17© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Sizing
Determines the minimum size for each cable that will carry the specified load current without violating the cable temperature limit.
The sizing calculation is an iterative process involving adjustment of the cable size and temperature.
Able to ‘lock-in’ specific cable sizes that cannot be changed.
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Slide 18© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Sizing Example
1. Load WKSHOP-EX42. Run Load Flow3. Update Cable Load Amp
(Study Case)
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Slide 19© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Sizing Based Voltage Drop
Set Voltage Drop = 2%
Operating Current = 140 A
Optimal Size is Calculated
One Size Smaller is Displayed
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Slide 20© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Sizing Based on Ampacity
Operating Current = 140 A
Optimal Size is Calculated
One Size Smaller is Displayed
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Slide 21© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
New UGS Presentations
• Project Editor – Presentation – Underground Raceways - Right-Click – Create New
• Select UGS Mode – Click ‘New Presentation’
Double-click to change presentation properties
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Slide 22© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
UGS Presentation
• UGS presentation isconceptually a cross-sectionof cable raceways.
• Each UGS presentation is adifferent cross-section of theunderground system.
• If you delete a raceway from a UGS presentation into the Dumpster, the raceway can be added to other UGS presentations as an existing raceway.
• In UGS, each presentation acts independently from each other.
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Slide 23© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
UGS Edit Toolbar
New Heat Sources
New Cables
New Duct Bank RWs
New Direct Buried RWsNew Locations forDirect Buried RWs
Existing Heat Sources
Existing Cables
Existing Duct Bank RWs
Existing Direct Buried RWsNew Conduits forDuct Banks RWsDisplay Options
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Slide 24© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
UGS Components
Heat Source
New Duct Bank – RW1
Existing Cable - Pump Cable
Cable 5 cannot fit inside this conduit and is placed outside the conduit
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Slide 25© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Inserting Cables
• Three main methods for adding cables to the existing conduits:
1. Drag the cable from OLV using Ctrl+Shift Key
2. Use the Existing Cable button from the UGS Toolbar
3. Use the Routing Page from the Cable Editor
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Slide 26© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Cable Representation
3 Conductor / Cable and 3 Conductor / PhaseSymbol: 1, 2 and 3
1 Conductor / Cable and 1 Conductor / PhaseSymbol: 1A, 1B, 1C
Single Phase CableSymbol: 1F, 1R
DC CableSymbol: 1P, 1N
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Slide 27© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
UGS Example
Duct BankX and Y = 30Width = 15 Height = 8
ConduitConduit Size = 4Y = 3.35
Pump CableFrom OLV
New Cable5 kV Kerite 1/C Operating Load = 200 Amps
Run Steady-State Temp Calc
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Slide 28© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
UGS Large Example
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Slide 29© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Steady-State Calculation
Calculation Pre-Requisite: All cables have been carrying the specified load long enough that the heat flow has reached its steady-state and no more changes of temperature will occur throughout the raceway system.
The cable temperature calculated is dependent on raceway system configuration, cable loading, and the location of each particular cable.
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Slide 30© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Alarms and Warnings
Calculated 109 C is greater
Calculated 88.3 C is greater
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Slide 31© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Multiple Presentations
Same Cables and Heat Source
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Slide 32© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Uniform Ampacity -
Ampacity CalculationApproach is based on the equal loading criterion for ampacity calculations.
Calculations determine the maximum allowable load currents when all the cables in the system are equally loaded to the same percentage of their base loading.
The cable allowable current is updated by the calculated ampacity.
Calculation Procedure
1. Determine initial loading level based on base ampacity.
2 Calculate cable temperature as in steady-state temperature calculation.
3. Check cable temperature values against the cable temperature limit.
If the temperature of the hottest cable is within close range of the temperature limit, the solution has been reached. If not, adjust the cable loading uniformly at the same percentage, either increasing or decreasing the loading in order to make the highest cable temperature come closer to the temperature limit. Then go to back to step 2 to recalculate cable temperature.
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Slide 33© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Uniform Temperature -
Ampacity CalculationApproach is based on the equal temperature criterion for ampacity calculations.
Determines the maximum allowable load currents when all the cables in the system have their temperature within a small range of the temperature limit.
In the case where these conductors are not located in the same conduit/location, they may not have thesame temperature. When this situation occurs, the temperature of the hottest conductor in this cablebranch will be used to represent this cable branch.
Calculation Procedure
1. Determine an initial loading level based on the base ampacity from the Cable Library and using cable derating factors for the given configuration.
2. Calculate cable temperature as in the steady-state temperature calculation.
3. Check cable temperature values against the cable temperature limit.
If temperature values of all cables are within close range of temperature limit, the solution has been reached. If not, load change required for the cable temperature to approach the temperature limit based on the gradient of cable temperature change is determined.
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Slide 34© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
• The Cable Sizing Calculation determines the minimum size of each cable that will carry the specified load current without violating the cable temperature limit.
• Only the ‘available’ cable sizes withinthe cable library for each selectedconductor will be considered.
• Cables may be excluded if the potentialsize of the cable cannot vary.
• The calculation is an iterative process; adjusting the cable size and then calculatingcable temperatures.
• Once a solution is reached, calculation results will be reported in the output report. Cables will automatically be changed to the new sizes if the Update Size option is checked in the Study Case.
Cable Sizing Calculation
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Slide 35© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Transient Temperature Calculation
Calculates and then plots cable temperature variations as a function of time in accordance to load changes.(Table of Ampacity versus Time)
Provides a tool to verify operation conditions of the raceway systems against the cable short-time or emergency temperature limits.Transient temperature calculations can be used to determine the cable peak temperatures during a short-time interval (usually less than a day), and compare them against maximum allowable temperatures, resulting in a more flexible and economical design of your raceway systems.
The transient temperature calculations are based upon a dynamic thermal model of the raceway system,constructed mainly from thermal resistance, thermal capacitance, and heat sources.
Thermal resistance is used to represent different thermal layers from cable conductor to ambient soil.
Thermal capacitance is used to represent the capability of each layer to absorb heat. ETAP مرجع آموزش فارسی
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Slide 36© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Example From NEC
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Slide 37© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
NEC Duct Bank (Detail 2)
Depth= 30 in
Fill RHO = 60
1kV NEC Rubber2
1/C CU 3-phase
Magnetic
Class = 100%
Size = 350 AWG
Load = 284.5 Amps
per phase
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Slide 38© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
NEC Duct Bank (Detail 3)
Depth = 30 in
Fill RHO = 60
1kVNEC Rubber2
1/C CU 3-phase
Magnetic
Class = 100%
Size = 750 AWG
Load = 334.9Amps
per phase
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Slide 39© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
(Detail 2) in ETAP
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Slide 40© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Results for Detail 2
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Slide 41© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
NEC (Detail 3) in ETAP
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Slide 42© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems
Results for Detail 3
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