Underground Raceway Systems - dl.etap.irdl.etap.ir/pdf/OTI/14 - UGS.pdf · Neher-McGrath Method ... Underground Raceway Systems. Slide 15. Neher-McGrath Example Calculate ampacity

© 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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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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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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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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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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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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Heat Flow Model


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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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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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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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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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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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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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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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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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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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Cable Sizing Example

1. Load WKSHOP-EX42. Run Load Flow3. Update Cable Load Amp

(Study Case)


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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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Cable Sizing Based on Ampacity

Operating Current = 140 A

Optimal Size is Calculated

One Size Smaller is Displayed


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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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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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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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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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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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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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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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UGS Large Example


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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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Alarms and Warnings

Calculated 109 C is greater

Calculated 88.3 C is greater


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Multiple Presentations

Same Cables and Heat Source


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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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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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• 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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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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Example From NEC


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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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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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(Detail 2) in ETAP


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Results for Detail 2


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NEC (Detail 3) in ETAP


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Results for Detail 3


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Documents

Underground Raceway Systems - dl.etap.irdl.etap.ir/pdf/OTI/14 - UGS.pdf · Neher-McGrath Method ... Underground Raceway Systems. Slide 15. Neher-McGrath Example Calculate ampacity