Underground Raceway Systems - UGS.pdf · Cable Sizing Determines the minimum size for each cable...

© 1996-2009 Operation Technology, Inc. - Workshop Notes: Underground Raceway Systems

Underground Raceway

Systems


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.


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


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


Cable Ampacity

FundamentalsVarious thermal barriers:

1. Conductor insulation

2. Air inside a duct

3. Duct wall

4. Soil surrounding an underground duct

5. Additional thermal insulation applied such as

polyurethane


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


Heat Flow Model


Heat Flow Model

Installation under an isolated

condition

Installation of groups of three

or six circuits

RHO of Soil = 90

Ta = 20 oC

(Generalized)


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)


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 conditions

F = Cable Ampacity Adjustment Factor

I = 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


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


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


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 effects

Rca’ = Thermal resistance between conductor & ambience (Ω-ft)


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


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)


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.


Cable Sizing Example

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

(Study Case)


Cable Sizing Based Voltage Drop

Set Voltage Drop = 2%

Operating Current = 140 A

Optimal Size is Calculated

One Size Smaller is Displayed


Cable Sizing Based on Ampacity

Operating Current = 140 A

Optimal Size is Calculated

One Size Smaller is Displayed


New UGS Presentations

• Project Editor – Presentation – Underground

Raceways - Right-Click – Create New

• Select UGS Mode – Click ‘New Presentation’

Double-click

to change

presentation

properties


UGS Presentation

• UGS presentation isconceptually a cross-sectionof cable raceways.

• Each UGS presentation is a

different 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.


UGS Edit Toolbar

New Heat Sources

New Cables

New Duct Bank RWs

New Direct Buried RWs

New Locations for

Direct Buried RWs

Existing Heat Sources

Existing Cables

Existing Duct Bank RWs

Existing Direct Buried RWs

New Conduits for

Duct Banks RWs

Display Options


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


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


Cable Representation

3 Conductor / Cable and

3 Conductor / Phase

Symbol: 1, 2 and 3

1 Conductor / Cable and

1 Conductor / Phase

Symbol: 1A, 1B, 1C

Single Phase Cable

Symbol: 1F, 1R

DC Cable

Symbol: 1P, 1N


UGS Example

Duct BankX and Y = 30

Width = 15 Height = 8

ConduitConduit Size = 4

Y = 3.35

Pump CableFrom OLV

New Cable5 kV Kerite 1/C

Operating Load = 200 Amps

Run Steady-State Temp Calc


UGS Large Example


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.


Alarms and Warnings

Calculated 109 C is greater

Calculated 88.3 C is greater


Multiple Presentations

Same Cables and Heat Source


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.


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 the

same temperature. When this situation occurs, the temperature of the hottest conductor in this cable

branch 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.


• 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 within

the cable library for each selected

conductor will be considered.

• Cables may be excluded if the potential

size of the cable cannot vary.

• The calculation is an iterative process;

adjusting the cable size and then calculating

cable 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


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.


Example From NEC


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


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


(Detail 2) in ETAP


Results for Detail 2


NEC (Detail 3) in ETAP


Results for Detail 3

Underground Raceway Systems - UGS.pdf · Cable Sizing Determines the minimum size for each cable...

Documents

Transcript of Underground Raceway Systems - UGS.pdf · Cable Sizing Determines the minimum size for each cable...