Cable Ampacity

USER'S GUIDE ADVANCED POWER CABLE AMPACITY PROGRAM Based on NEHER-McGRATH AND IEC 287 METHODS

EDSA MICRO CORPORATION

11440 West Bernardo Court, Suite 370 San Diego, CA 92127 U.S.A. © Copyright 2005 All Rights Reserved

Version 5.10.00

Advanced Power Cable Ampacity Program

Dec 2005

Advanced Power Cable Ampacity Program EDSA MICRO CORPORATION WARRANTY INFORMATION There is no warranty, implied or otherwise, on EDSA software. EDSA software is licensed to you as is. This program license provides a ninety (90) day limited warranty on the diskette that contains the program. This, the EDSA User’s Guide, is not meant to alter the warranty situation described above. That is, the content of this document is not intended to, and does not, constitute a warranty of any sort, including warranty of merchantability or fitness for any particular purpose on your EDSA software package. EDSA Micro Corporation reserves the right to revise and make changes to this User's Guide and to the EDSA software without obligation to notify any person of, or provide any person with, such revision or change. EDSA programs come with verification and validation of methodology of calculation based on EDSA Micro Corporation's inhouse software development standards. EDSA performs longhand calculation and checks the programs’ results against published samples. However, we do not guarantee, or warranty, any program outputs, results, or conclusions reached from data generated by any programs which are all sold "as is". Since the meaning of QA/QC and the verification and validation of a program methodology are domains of vast interpretation, users are encouraged to perform their own inhouse verification and validation based on their own inhouse quality assurance, quality control policies and standards. Such operations - performed at the user's expense will meet the user's specific needs. EDSA Micro Corporation does not accept, or acknowledge, purchase instructions based on a buyer's QA/QC and/or a buyer's verification and validation standards. Therefore, purchase orders instructions are considered to be uniquely based on EDSA's own QA/QC verification and validation standards and test systems.

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Please accept and respect the fact that EDSA Micro Corporation has enabled you to make an authorized disk as a backup to prevent losing the contents that might occur to your original disk drive. DO NOT sell, lend, lease, give, rent or otherwise distribute EDSA programs / User's Guides to anyone without prior written permission from EDSA Micro Corporation. All Rights Reserved. No part of this publication may be reproduced without prior written consent from EDSA Micro Corporation.

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Advanced Power Cable Ampacity Program USERS GUIDE TABLE OF CONTENTS Foreword:.......................................................................................................................................................................................... 5 What's New in this Release of the Cable Ampacity Program…………………………………………………………. ……5 Overview........................................................................................................................................................................................... 6 Upcoming Features in the Next Program Release ........................................................................................................................... 6 Background....................................................................................................................................................................................... 7 General Data ................................................................................................................................................................................ 8 Cable in Air ................................................................................................................................................................................... 8 Solar Radiation............................................................................................................................................................................. 9 Heat Source/Sink........................................................................................................................................................................ 10 Installation .................................................................................................................................................................................. 10 Possible Cable Installation Conditions ....................................................................................................................................... 10 Backfill ........................................................................................................................................................................................ 11 Conductor Material ..................................................................................................................................................................... 11 Conductor Construction.............................................................................................................................................................. 11 Dried And Impregnated .............................................................................................................................................................. 12 Conductor Losses....................................................................................................................................................................... 12 Cable Insulation.......................................................................................................................................................................... 12 Skid/Concentric Neutral Material ................................................................................................................................................ 14 Bonding Arrangement................................................................................................................................................................. 14 Loss Factor Constant ................................................................................................................................................................. 14 Jacket/Pipe Coating Material...................................................................................................................................................... 14 Armor/Reinforcement Material.................................................................................................................................................... 14 Armor Bedding, Serving Material................................................................................................................................................ 14 Armor Permeability ..................................................................................................................................................................... 15 Insulation Shielding .................................................................................................................................................................... 15 Sheath/Reinforcing Material ....................................................................................................................................................... 15 Cable Transposition.................................................................................................................................................................... 16 Pipe Material and Configuration ................................................................................................................................................. 16 Material and Construction of the Ductbank or Duct.................................................................................................................... 16 Cable Dimensions ...................................................................................................................................................................... 17 Running Cable Ampacity Program.................................................................................................................................................. 21 Loading Sample Cable/Project Library ........................................................................................................................................... 21 Provided Sample Projects............................................................................................................................................................... 22 Step by Step Instructions for Adding a New Cable Type................................................................................................................ 23 Using the Drop-Down Menu............................................................................................................................................................ 24 Adding/Creating a New Cable Type Using the Wizard ................................................................................................................... 26 How to Add/Create a Project/Study ................................................................................................................................................ 40 How to Run a Simulation ................................................................................................................................................................ 48 Program Validation and Verification................................................................................................................................................ 51 References – Bibliography.............................................................................................................................................................. 52 APPENDIX I: IEC & Neher-Mcgrath Cable Ampacity Calculations Methodology ........................................................................... 54 APPENDIX II: Some Useful Diagrams and Figures........................................................................................................................ 57 APPENDIX III: Tables of Material Properties.................................................................................................................................. 69

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LIST OF TABLES Table 1: Comparing Results of Neher McGrath and EDSA’s Cable Ampacity Program Table 2: Specific Inductive Capacitance of Insulation Table 3: Thermal Resistivity of Various Materials Table 4: Pipe Constants Table 5: Conductor Material Table 6: Dielectric Loss Table 7: Resistivities of Materials Table 8: Absorption Coefficients of Solar Radiation Table 9: Constants For Ducts Or Pipes

52 69 69 69 70 70 70 71 71

LIST OF FIGURES Figure 1: Main Menu of Cable Ampacity program .......................................................................................................................... 21 Figure 2: Opening a cable/project library........................................................................................................................................ 21 Figure 3: Main Cable Menu after a cable/project library is loaded.................................................................................................. 22 Figure 4: Cable Type Dialogs ......................................................................................................................................................... 23 Figure 5: Cable Conductor Dialog................................................................................................................................................... 24 Figure 6: Selecting an Item from the Dropdown Menus.................................................................................................................. 24 Figure 7: Cable General Data Dialog.............................................................................................................................................. 26 Figure 8: Conductor Data Dialog .................................................................................................................................................... 27 Figure 9: Conductor Dimension Data Dialog .................................................................................................................................. 28 Figure 10: Cable Insulation Data Dialog ......................................................................................................................................... 29 Figure 11: Cable Insulation Dimension Data Dialog ....................................................................................................................... 30 Figure 12: Cable Sheath Data Dialog ............................................................................................................................................. 31 Figure 13: Reinforcing Tape Data Dialog........................................................................................................................................ 32 Figure 14: Reinforcing Tape Dimension Data Dialog...................................................................................................................... 33 Figure 15: Concentric Neutral/Skid Wire Data Dialog..................................................................................................................... 35 Figure 16: Jacket/Pipe Coating Data Dialog ................................................................................................................................... 36 Figure 17: Armor/Serving/Bedding Data Dialog.............................................................................................................................. 37 Figure 18: Overall Cable Dimension Data Dialog ........................................................................................................................... 38 Figure 19: Project/Study General Data Dialog................................................................................................................................ 42 Figure 20: Adding a Cable to a Study/Project................................................................................................................................. 44 Figure 21: Selecting a Cable from the Cable Type Library ............................................................................................................. 45 Figure 22: Report Browser Window Showing the Result of Amapcity Calculation ......................................................................... 49 Figure 23: 138 kV, 2000 MCM high pressure oil-filled, 3-conductor, pipe type cable..................................................................... 51 Figure 24: Basic Thermal Circuit .................................................................................................................................................... 54 Figure 25: Definition of Thermal-Ohm Units ................................................................................................................................... 55 Figure 26: Mathematical Model of a Cable Thermal Circuit............................................................................................................ 55 Figure 27: Cable Topology / General Parameters Self Contained Cables ..................................................................................... 67 Figure 28: Typical Pipe Cable Cross-Section ................................................................................................................................. 67 Figure 29: Configuration of Cable in the Duct/Conduit ................................................................................................................... 68 Figure 30: Ductbank Gb Factor ....................................................................................................................................................... 68

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Foreword: This manual assumes that the user is a Professional Engineer familiar with the concepts of cable ampacity calculation. Determination of the validity of the results is the user's responsibility. The IEC (International Electrotechnical Commission) and Neher-McGrath cable ampacity program is undergoing continuous development to make it as comprehensive and as easy to use as possible. Additional analysis capabilities will be made available as they are developed. Any comments, suggestions or errors encountered in either the results or the documentation should be immediately brought to EDSA’s attention. It is recommended that users of the program experiment with the sample job files that are included before creating their own job files. It is also recommended that users consult the relevant papers on which the program is based: IEC Standards 287, Neher-McGrath IEEE paper 57-660 and Underground Transmission Systems Reference Book, Electric Power Research Institute, 1992 Edition. This program is intended to be a very easy to use tool. However, it is expected that the user of the program have good knowledge of the cable construction and ampacity calculations.

What’s New in This Release of the Cable Ampacity Program 1) The cable ampacity program was enhanced to facilitate the simulation of touching cables. When three cables are in touching arrangement either directly buried or inside a duct/ductbank, the user need not enter the coordinates of each cable. Instead, it is sufficient to enter just one cable whose coordinates is the same as the center of the three cables. Also, the user should select “Cables Touching” option in the program. This option, when selected, instructs the cable ampacity program to consider three cables even though the user has just given the coordinates of one cable. 2) Computation of the cable sheath losses when a cable has no skid/concentric neutral wires, no armor, or no reinforcement tape was modified to correctly reflect the modeling of this type cables. 3) As part of the program V&V, the sample case for the pipe type cable from the Neher-McGrath paper was simulated. The computed ampacity for this sample case, using the EDSA’s cable program, is 904 amperes versus 905 amperes which is reported in the Neher-McGrath paper. The discrepancy between the two result is less than 0.1 %

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Overview Cable ampacity assessment and temperature rise calculations is an important but time consuming task for cable manufacturers, designers and operators. This is due to the fact the computations often includes numerous mathematical calculations and extensive table look up and data processing. EDSA has developed an efficient computer program in order to facilitate such calculations. The EDSA cable ampacity program utilizes the techniques and formulae suggested in the IEC (International Electrotechnical Commission) standard publication No. 287-1982 to compute the temperature rise and ampacity of power cables. This program also offers an alternative computational method to handle nonunity load factor based on the Neher-McGrath technique. Several enhancements to both Neher McGrath and IEC 287 standard have been implemented. These include:

9 Simulation of soil drying out in the neighborhood of energized cables 9 Nonisothermal earth surface 9 Cables without metallic sheath but with copper concentric neutral that can be single or both ends bonded and grounded

9 Steel armoured submarine cables with or without concentric neutral or metallic sheath 9 Cables on riser poles 9 Single phase circuits consisting of one single core cable with concentric neutral wires or sheath serving as the return conductor.

9 Ductbanks and backfills of any size 9 PPP (Paper-polypropylene-paper) laminated cables. Upcoming Features in the Next Program Release The program update will include the following enhancements:

9 Transient analysis: In the next release, the EDSA users can assess the short term transient response

of power cables given load variation pattern. The computation methodology is based on the principles recommended in the IEC Publication 853-2. The program adopts lump parameters and image techniques to calculate the transient temperature rise of cables among a set of equally or unequally loaded, similar or dissimilar cables. The temperature profiles, ampacities, and time information are computed using the equivalent thermal circuit of the cable. Each cable in a study can be subjected to different time dependent load curve

9 The steady-state computation will support mix of ampacity and temperature calculation where a group

cables will have assigned ampacities while the program will compute ampacities for another group of cables without exceeding their maximum temperature

9 The program will have full graphical display of result including temperature distribution along cable cross section

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Background The EDSA’s advanced power cable ampacity program supports all AC as well as DC voltages. Cables can be directly buried, be in ducts, be in steel pipes, as well as in air. The permissible current rating of an a.c. cable can be derived from the expression for the temperature rise above ambient temperature:

[

] [

]

[

]

Δθ = I 2 R +0.5Wd T1 + I 2 R(1+λ)+ Wd nT2 + I 2 R(1+λ1 +λ2 )+ Wd n(T3 +T4 ) Where:

I is the current flowing in one conductor (Amps).

Δθ is the conductor temperature rise above the ambient temperature (K). Note: The ambient temperature is the temperature of the surrounding medium under normal conditions in a situation in which cables are installed, or are to be installed, including the effect of any local source of heat, but not the increase of temperature in the immediate neighborhood of the cables due to heat arising there from. R is the alternating current resistance per unit length of the conductor at maximum operating temperature (ohm/m).

Wd is the dielectric loss per unit length for the insulation surrounding the conductor (W/m).

T1 is the thermal resistance per unit length between one conductor and the sheath (K.m/W). T2 is the thermal resistance per unit length of the bedding between the sheath and armor (k.m/W). T3 is the thermal resistance per unit length of the external serving of the cable (K.m/W).

T4 is the thermal resistance per unit length between cable surface and the surrounding medium (K.m/W). n is the number of load carrying conductors in the cable (conductors of equal size and carrying the same load).

λ1

is the ratio of losses in the metal sheath to total losses in all conductors in that cable.

λ2

is the ratio of losses in the armoring to total losses in all conductors in that cable.

The permissible current rating is obtained from the above formula as follows:

I=

Δθ −Wd[0.5T1 +n(T2 +T3 +T4 )](m − 1)ΔQ RT1 +nR (1+λ1)T2 +nR (1+λ1 +λ2 )(T3 +mT4 )

ΔQ is temperature difference between critical isotherm (50 ºC) and the ambient (critical isotherm is one at which drying out occurs), m is the ratio of the thermal resistivities of the dry and moist soil zones. The nonisothermal surface is modeled by introducing an imaginary additional layer of soil d meters thick at the earth surface, where:

d=

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a is the convection coefficient and ρ 0 is the thermal resistivity of the moist soil. The program computes the convection coefficient.

General Data General site configuration data as outlined below should be specified. The following items require special attention:

9 Ambient temperature and soil resistivity values should correspond to the installation situation and not to the test condition of the manufacturer. Ambient temperature is the soil ambient at the depth of the cable if the cable is buried. If the cable is installed in air, ambient temperature means air ambient.

9 Soil thermal resistivity is normally in the range of 0.8 to 1.3 C-W/m. Values as low as 0.4 and as high as 4 C-W/m have been recorded in field. Thermal resistivity of the soil is one of the most important parameters affecting cable ampacity. The higher the value of the resistivity, the lower the ampacity. Thermal resistivity increases with the decrease in moisture content in the soil. Thermal resistivity of dry sand can be as high as 5 C-W/m, whereas, thermal resistivity of dry crushed limestone usually cannot be higher than 1.5 C-W/m. Another factor affecting the value of the soil thermal resistivity is its compaction. The higher the soil compaction, the lower is its thermal resistivity. If the soil thermal resistivity is unknown, the more conservative value of 1.3 can be used as a starting point.

9 The heat source/sink data if any. 9 For a nonisothermal surface, the user should enter the air ambient temperature. This temperature should be greater than the soil ambient. If the cables are located at a depth greater than 1.5 m, the nonisothermal condition does not apply.

9 For simulation of soil drying out, the user should enter dry soil (and backfill if present) thermal resistivity. The dry thermal resistivity is larger than the moist values.

Cable in Air For cables in air, the following possible configurations are supported. Installation

Z

E

GC

Configuration

a) Cables in free air, installed on non-continuous brackets, ladder supports, or cleats:

Single cable

0.21

3.94

0.60

Two cables, touching, horizontal

0.29

2.35

0.50

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Three cables in trefoil

0.96

1.25

0.20

Three cables, touching, horizontal

0.62

1.95

0.25

Two cables, touching, vertical

1.42

0.86

0.25

Two cables, spaced D'e , vertical

0.75

2.80

0.30

Three cables, touching, vertical

1.61

0.42

0.20

Three cables, spaced D'e , vertical

1.31

2.00

0.20

Single cable

1.69

0.63

0.25

Three cables in trefoil

0.94

0.79

0.20

b) Cables directly clipped to a vertical wall:

Solar Radiation When cables are in air, the following information is required in addition to the cable arrangement as shown above: a) Shaded or un-shaded cable. For shaded cable the next two items do not apply. b) Intensity of solar radiation (W/m2). The radiation should represent the long-term average value. c) The cable surface absorption coefficient. The following default values are provided, however, the user can change the value if required:

9 Compounded jute/fibrous materials 9 Polychloroprene Version 5.10.00

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9 Polyvinylchloride 9 Polyethylene 9 Lead or armour

= 0.6 = 0.4 = 0.6

Heat Source/Sink The heat source/sink in proximity of cables can be simulated with the following four possibilities:

9 External source specified as constant temperature source inside backfill. 9 External source specified as constant heat flux source inside backfill. 9 External source specified as constant temperature source not in backfill. 9 External source specified as constant heat flux source not in backfill. Note that the unit of heat source for the heat flux option is defined as W/m2. Temperature is given in degrees C for the case of constant temperature.

Installation

9 Cable geometrical coordinates should be defined with origin of the coordinate system such that the Y values for buried cables are always a positive number (Y=0.0 at the ground level). The X values may be either positive or negative. The choice of the origin of the X-axis in normally decided by the ease of entering cable coordinates. For cables installed in air, the Y location has no significance and can be set to 0.

9 Circuit number identifies the three phases of the cable circuit. The user should not define more than

one cable specified for the same circuit number; all of the three cables of the same circuit should be of the same cable type.

9 Selection of a reference (for dissimilar or unequally loaded cables) cable. The program finds the ampacity of the reference cable at its maximum operating temperature and the ampacities of the remaining cables will be the highest possible without exceeding their thermal ratings.

Possible Cable Installation Conditions The following is a list of possible cable installations for self contained cables:

9 Cables are in air or cables are in duct and duct is in air. 9 Cables are directly buried. 9 Cables are in thermal backfill. Version 5.10.00

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9 Cables are in duct or in duct bank underground. For pipe type cable:

9 Cables are in pipe and pipe is directly buried. 9 Cables are in pipe and pipe is in thermal backfill. 9 Cables are in pipe and pipe is in air. Note that Pipe type cables are treated as three-conductor cables.

Backfill Backfill data pertain to thermal backfills or to ductbanks. The program can only handle two different materials surrounding the cable. Only rectangular backfills/ductbanks can be simulated. Backfill or ductbank is defined by its dimensions and thermal resistivity. The thermal resistivity of the backfill is usually lower than that of the native soil. Concrete thermal resistivity is usually in the range of 0.5 to 0.8 ºC-W/m.

Conductor Material Conductor material can be copper, aluminum, or user defined. If conductor material is your defined, then, the user should provide the conductor resistivity at 20 ºC (in Ohm-m) as well as thermal coefficient of resistance in 1/(ºC).

Conductor Construction The conductor construction can be any of the following available choices:

9 round, stranded 9 round, compact or compressed 9 type m, round segmental type m, 4 segment hollow core 9 hollow core 9 type m, six segment hollow core 9 sector shaped 9 oval 9 solid Version 5.10.00

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Advanced Power Cable Ampacity Program Note: sector shaped cables are 3-phase cables.

Dried And Impregnated

9 cable dried and impregnated 9 cable not dried and impregnated or not applicable Depending on the selection the program will establish the proper coefficient for calculation of skin and proximity factors.

Conductor Losses The a.c. resistance of the conductor at its maximum temperature is computed from: Racm = Rdcm ( 1 + Ys + Yp )

Where:

Rdcm is DC resistance at maximum temperature; Racm is AC resistance at maximum temperature; Ys is skin effect factor; Yp is proximity effect factor. Yp and Ys are functions of Kp and Ks as well as arguments of Bessel function. Ks and Kp are assumed to be the same for copper and aluminum defined as follow: Round, stranded dried and impregnated Round, stranded not dried and impregnated Round, compact dried and impregnated Round, compact not dried and impregnated Round, segmental (values apply to conductors having four segments ) Hollow, helical stranded dried and impregnated Sector shaped dried and impregnated Sector shaped not dried and impregnated decided based on

*

d ' −d ⎛ d ' +2d i ⎞ ⎟ K S = c' 1 ×⎜⎜ c' d c +d1 ⎝ d c +d i ⎟⎠

Kp=0.8 Ks=1.0 Ks=1.0 Kp=1.0 Ks=1.0 Kp=0.8 Ks=1.0 Kp=1.0 Ks=0.435 Kp=0.37 Ks=* Kp=0.8 Ks=1 Kp=0.8 Ks=1 Kp=1.0

2

which is function of inside and outside diameter of conductor

Cable Insulation The insulation material can be selected from the following available choices. When a user has a different insulation material than those listed below, then, thermal resistivity of the insulation should be provided:

9 user supplies RHI (insulation thermal resistivity in ºC-m/W) 9 solid type or mass impregnated, non draining cable, 9 LPOF (low pressure oil filled) self contained cable, 9 HPOF (high pressure oil filled) self contained cable, 9 HPOF (high pressure oil filled) pipe type cable, 9 external gas pressure cable, Version 5.10.00

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9 internal gas pressure, preimpregnated cable, 9 internal gas pressure mass impregnated cable, 9 butyl rubber, 9 EPR, 9 PVC, 9 polyethylene, 9 cross linked polyethylene (XLPE) (unfilled), 9 cross linked polyethylene (XLPE) (filled), 9 paper-polypropylene-paper-laminate (ppp or ppl),

RHI=6.5 RHI=6.0 RHI=5.0 RHI=5.0 RHI=6.0 RHI=3.5 RHI=3.5 RHI=3.5 RHI=6.5

The program selects dielectric constant and loss factor coefficients according to the following table (see the 1988 revision of IEC 287) Dielectric constant Loss factor Cables insulated with impregnated paper solid type, 0.01 fully-impregnated, pre-impregnated or mass-impregnated non-draining 4.0 Self-contained, oil filled, low pressure 3.3 0.004 Self-contained, oil filled, high pressure 3.5 0.0045 Oil-pressure pipe-type 3.7 0.0045 External gas-pressure 3.5 0.004 Internal gas-pressure 3.4 0.0045 Butyl rubber 4.0 0.05 EPR up to 36 kV 3.0 0.020 above 36 kV 3.0 0.005 P.V.C. 8.0 0.1 2.3 0.001 PE XLPE 2.5 0.004 up to 36 kV (unfilled) above 36 kV (unfilled) 2.5 0.001 above 36 kV (filled) 3.0 0.005 Paper-polypropylene-paper-laminate (ppp or ppl) 3.5 0.00095 The dielectric loss factor is only taken into account for cables operating at equal or greater phase to ground voltage than the following: Cable Type Insulated with impregnated paper solid type oil-filled and gas pressure Butyl rubber EPR P.V.C. PE XLPE unfilled XLPE filled Paper-polypropylene-paper-laminate (ppp or ppl)

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Skid/Concentric Neutral Material Skid wires are used in pipe-type cables. Concentric neutral wires can be either bonded and grounded at one end or at both ends. This version of the program cannot handle both metallic sheath and concentric wires (but concentric wires and armour wires can be entered). If the cable has both metallic sheath and concentric neutral wires, then the user should enter sheath with electrical resistivity chosen such that the final resistance of the sheath per unit length correspond to the parallel connection of the sheath and concentric wires. For corrugated sheath, the user should enter the average sheath diameter in the dimension dialog.

Bonding Arrangement Sheath means metallic sheath/metallic shield/concentric wires. The bonding arrangement is a very important factor in the computation of ampacity. When sheaths are bonded and grounded at both ends, large circulating currents may result considerably decreasing cable ampacity. For cross-bonded and single point bonded systems, only eddy current losses are present (if continuous cylindrical sheath is present) which are much smaller than the circulating currents. For single point bonded systems, standing voltages can develop at the open end. If a cross-bonded system has sections of unequal lengths, circulating currents may occur and are computed by the program. The program will consider unequal spacing between phases in two-point bonded systems if this information is available. The lengths of the sections with unequal spacing can also be entered. The program will compute an average inductance of the cables and this will affect the magnitude of circulating current.

Loss Factor Constant This constant is used to relate daily load factor (DLF) to the conductor loss factor. A value of 0.3 for ALOS is suggested in the Neher-McGrath paper. The loss factor is computed using the following formula: LOSS FACTOR = ALOS*DLF+(1 - ALOS)*DLF2 If the user wishes to specify a particular loss factor, it is sufficient to specify ALOS=1 and DLF equal to the required value of the loss factor.

Jacket/Pipe Coating Material Jacket normally applies to the outer covering of the cable for self-contained cables or it can represent pipe coating for pipe type cables.

Armor/Reinforcement Material Armor here can represents reinforcing wires or tapes which can be either magnetic or nonmagnetic. Armor serves as cable protection and should be distinguished from concentric wires that can serve as neutral conductors for distribution cables or reinforcing conductors for transmission cables. Armor wires are assumed always to be bonded and grounded at both ends whereas concentric wires can be either single or two point bonded.

Armor Bedding, Serving Material

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Advanced Power Cable Ampacity Program If the cable has an armor, then the insulating material below the armor is called armor bedding and over the armor it is called armor serving. If armor wires are embedded in an insulating material, then the user has a choice of either a) entering both armor bedding and armor serving with thicknesses equal to one half of the thickness of the material in which the wires are embedded or b) representing armor bedding only. If the cable has an armor, then the insulating material below the armor is called armor bedding and the material over the armor is called armor serving.

Armor Permeability User can provide values for the longitudinal and transverse relative permeability (AME and AMT) and angular time delay (GAMMA) for steel wire armor of single conductor cables. For three conductor cables with steel tape armor user can provide just AME. If program selects the following values: AME = 400, AMT = 10 when wires are in contact and AMT = 1 when wires are separated, GAMMA = 45 deg.

Insulation Shielding

9 belted cable or no insulation shielding 9 copper tape 9 aluminum tape This information is not used in the computation of ampacity. The program will add properly the thickness of insulation shielding to that of the insulation itself.

Sheath/Reinforcing Material This can represents: a) reinforcing tape for self contained cables or b) metallic tape over insulation shielding for pipe type cables. The following choices are available:

9 user supplies reinforcing tape resistivity, RHT (ohm-m) and temperature coefficient of resistance, ALFAT ( 1/deg c)

9 copper 9 brass/bronze 9 zinc 9 stainless steel 9 steel Version 5.10.00

RHT=1.7241E-8,

ALFAT=0.00393

RHT=3.5E-8,

ALFAT=0.003

RHT=6.11E-8,

ALFAT=0.004

RHT=70.E-8,

ALFAT=0.000

RHT=13.8E-8,

ALFAT=0.0045

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Advanced Power Cable Ampacity Program Pipe type cables usually have metallic tape over insulation shielding made of copper, bronze or stainless steel.

Cable Transposition Transposition of cable reduces the circulation current for single conductor cables which are bonded at both ends. The choices are:

9 cables are not transposed 9 cables are regularly transposed Pipe Material and Configuration a) Multiplier for In-Pipe Effect This is a multiplier in the computation of skin and proximity effects for pipe type cables.

9 user supplies PIPFAC (multiplier for 'in pipe effects') 9 stainless steel pipe, PIPFAC=1.0 9 steel pipe, PIPFAC=1.7 9 iron pipe, PIPFAC=1.36 Two possible configurations namely triangular and cradled are considered in the program.

Material and Construction of the Ductbank or Duct

9 user supplies duct material thermal resistivity, RHD (c-m/w) 9 metallic conduit RHD=0.0 9 fibre duct in air RHD=4.8 9 fibre duct in concrete RHD=4.8 9 asbestos duct in air RHD=2.0 9 asbestos duct in concrete RHD=2.0 9 PVC duct in air RHD=7.0 9 PVC duct in concrete RHD=7.0 Version 5.10.00

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9 polyethylene duct in air 9 polyethylene duct in concrete 9 earthenware duct 9 high pressure gas filled pipe type 9 high pressure oil filled pipe type

RHD=3.5 RHD=3.5 RHD=1.2 RHD=0.0 RHD=0.0

The duct/ductbank material along with its dimensions are used for the computation of appropriate thermal resistance. The type of construction is one of the 12 listed above. Construction information together with the value of RHD is used to compute external thermal resistance of the duct.

Cable Dimensions The following is the list of cable components for which the dimensions will be required depending on cable construction:

9 Cable Conductor Data 9 Cable Insulation Data 9 Sheath and nonmagnetic reinforcing tape or metallic binder data 9 Cable jacket, armor bedding and armor serving data 9 Skid wires/concentric neutral wires or wire armor or magnetic reinforcement tape 9 Cable outer diameter, pipe/duct diameters Conductor

9 Number of conductors in the cable 9 Conductor cross sectional area 9 Diameter or geometric mean diameter of the conductor 9 Diameter over the conductor shield 9 Inside diameter of hollow core cables 9 Diameter of a round conductor having the same cross section and compaction as the shaped one 9 Radius of circle circumscribing three sector shaped conductors 9 Distance between conductor centre and cable centre Insulation

9 phase to phase voltage, kV 9 diameter over the insulation 9 diameter over the insulation shield 9 insulation thickness between conductors Version 5.10.00

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9 insulation thickness between conductors and sheath 9 thickness of conductor insulation including insulation shielding tapes plus half of any nonmetallic tapes

9 thickness of metallic shield 9 outside diameter of circle circumscribing three insulated sector shaped conductors in a shielded cable

Sheath and Nonmagnetic Reinforcing Tape or Metallic Binder

9 diameter over sheath 9 sheath thickness 9 diameter over reinforcing tape or metallic binder 9 thickness of reinforcing tape or metallic binder 9 width of reinforcing tape or metallic binder for Neher-McGrath. Enter 0.0 for IEC method. 9 number of reinforcing tapes 9 Neher-McGrath: length of lay of tapes, m. IEC: 0.0 9 IEC: enter 10.0 for tapes with long lay, enter 0.0 if tapes are wound at approximately 54 degrees to the cable axis, enter -1.0 for tapes with short lay. Neher-McGrath: 0.0.

Cable Jacket, Armor Bedding and Armor Serving

9 diameter over cable jacket 9 thickness of cable jacket 9 thickness of armor bedding 9 thickness of armor serving Skid Wires, Concentric Neutral Wires, Wire Armor or Reinforcement Tapes

9 diameter over skid wire/concentric neutral assembly 9 diameter over wire armor 9 mean diameter over tape armor 9 diameter of skid wire/concentric neutral wires 9 diameter of armor wires 9 cross sectional area of tape armor 9 length of lay of skid wire/concentric neutral wires 9 length of lay of armor wires 9 number of skid wires or concentric neutral wires Version 5.10.00

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9 number of armor wires Cable Outer Diameter, Pipe/Duct Diameters

9 overall cable diameter 9 inside diameter of duct or cable pipe 9 outside diameter of duct or cable pipe 9 Axial spacing between conductors of the same circuit Last Item is applicable only to 3-core cables with round conductors. Conductor Dimensions The present version of the program can handle only either single or three-conductor cables. Pipe type cables are treated as three-conductor cables, therefore, the number of conductors in this case is 3. Sector shaped conductors are replaced in the calculations by equivalent circular ones and therefore the user should provide appropriate dimension as asked. Oval shaped conductors are replaced by equivalent circular conductors with diameter d =

Dmajor xDmin or

Insulation Dimensions In the computation of thermal resistance of the insulation for sector shaped and circular three-conductor cables, insulation thickness between conductors as well as insulation thickness between conductor and the sheath should be provided. Sheath Dimensions For corrugated sheath, the thickness of the sheath should be the average thickness and the diameter over the sheath should be equal to the arithmetic average between the internal and external diameter. The lay of tape is the distance along the tape length between two points where the tape makes one full turn around the cable. Skid Wire/Concentric Neutral/Armor Dimensions Length of lay of the wire is the distance between two points measured along the wire length where the wire makes a full turn around the cable. The following relation holds between the lay factor and the length of lay x =

Length of lay Diameter under the wire

Where: X Lay factor

4 1.27

6 1.13

8 1.07

9 1.06

10 1.05

12 1.03

Outer Dimensions/Overall Cable Diameter

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Running Cable Ampacity Program To start the cable ampacity program, select the program icon from EDSA technical main menu as shown below or alternatively, select “Analysis->Cable Ampacity IEC/NM”:

Figure 1: Main Menu of Cable Ampacity program

Loading Sample Cable/Project Library There are a number of cable types and projects that are prepared for the users to assist them in running and operating the program. The program maintains a set of cable type/projects library and the user can create a new set of cable/project library. Here we will load the sample cable/project library. Select “Open” by selecting its icon as shown in the above figure. The provided sample cable/project library can be found in the “\edsa2005\sample\CableAmpacity” directory. Select the sample library named “sample-jobfiles-4cable-ampacity” as shown below:

Figure 2: Opening a cable/project library Then, the program will load the entire cables/projects library in the sample file into memory.

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Advanced Power Cable Ampacity Program As can be seen below, the program window will have three parts. The upper left hand side will list the available cable types library. The lower left hand side, shows the projects library and right hand side will show either cable construction details (when a cable type is selected, note the highlighted row in the upper left hand side below) or cables locations in a highlighted project. A project is the same as a study where the user seeks cables ampacity/temperature for a given cable installation conditions.

Figure 3: Main Cable Menu after a cable/project library is loaded

Provided Sample Projects Several sample cable types and sample projects have been prepared for the users convenience. The following is a short description of the provided sample projects: Case

Project Name Sample for Directly Buried Cables

1 Sample for Cables in Duct Bank 2 3

Sample paper

from

Neher-McGrath

Sample for Dissimilar Cables 4 Sample for Cables on Riser Pole 5

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Description In this study, 3 single core XLPE, Concentric Neutral Submarine Cables are directly buried and the ampacity of the cables are computed, cables have non unit daily load factors Six 1000 KCMIL, XLPE, with PVC duct cables are arranged in two circuits and placed in a duct-bank. Ampacity of the cables are computed, cables have non unit daily load factors The 138 kV pipe type cable used in the Neher McGrath paper is used in this study which is discussed extensively in this guide. In this study we installation that uses two different cable types (dissimilar), one cable is Paper Insulated single Core Cable and the other is a Pipe-type 3 core, 230kV cable. This example uses the same cable type as the one used in the second study, i.e., the XLPE, Concentric Neutral Submarine Cable. However in this example cables are riser pole and the cable temperature calculation is request and not the ampacity

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Step by Step Instructions for Adding a New Cable Type The user can examine/delete an existing project, make a copy of an existing project, or create a new project. The same options are also available for a cable, i.e., examine/delete an existing cable, make a copy of an existing cable, or create a new cable type. To edit a cable type already in the library point with mouse to row where the desired cable is and with one left mouse click highlight it as shown above, then, either use the “Edit..” icon or double left mouse click will open the cable type dialogs as shown below:

Figure 4: Cable Type Dialogs Several tabs are shown above each corresponding to different cable construction layers, e.g. Insulation, Sheath, Armor, etc. To edit any of the cable layers, simply select the corresponding tab. Below the dialog for the “Conductor” tab is shown (For example, in the figure below, cable conductor material can be either copper, aluminum or can be defined by the user):

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Figure 5: Cable Conductor Dialog

Using the Drop-Down Menu Materials of each cable layers can be define by the user. Of course, there are many default materials defined for each cable layer/section that the user can select. To choose an item from a dropdown menu, use the mouse to point to the dropdown arrow as shown below (here selection of cable insulation material is shown as an example) and left mouse click once on the “arrow”:

Figure 6: Selecting an Item from the Dropdown Menus

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Advanced Power Cable Ampacity Program The list of available materials (in this case insulation) will be presented. To select, point with the mouse on the desired row and left mouse click once on the highlighted item as shown below:

More details for each cable layers and their materials will be shown in the later sections. Now, let’s see how a new cable type can be added/created.

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Adding/Creating a New Cable Type Using the Wizard To add/create a new cable type, simply, press the “New..” icon in the upper left hand side of the main dialog as shown below:

A Wizard has been implemented to assist the user in the process of creating a new cable type. The Wizard will take the user through a step-by-step instructions/dialogs until all of the cable data are completed. The first screen will prompt the user for general cable data as shown below:

Figure 7: Cable General Data Dialog

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Advanced Power Cable Ampacity Program We will be entering the same cable data for this example as the 138 kV pipe type cable that was used in the Neher McGrath Paper. In the below dialog, a 3-phase 138 kV, steel pipe type cable along with the pipe inside and outside diameters is specified.

Upon completion of the above information, select “Next” to continue to the next step. The “Conductor” data dialog is shown below:

Figure 8: Conductor Data Dialog

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Advanced Power Cable Ampacity Program The cable construction per Neher McGrath paper is a round 4-segmental cable. Also, the conductor does not have any screen/shield. Next, conductor dimension data dialog will be presented as shown below:

Figure 9: Conductor Dimension Data Dialog The cable cross-section area, diameter over conductor shield (if any) should be entered in the above data dialog. The data based on the paper is shown in figure below.

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Advanced Power Cable Ampacity Program The cable insulation data dialog will be shown next as seen in the figure below.

Figure 10: Cable Insulation Data Dialog The insulation material and loss factors are all user defined for this example. The insulation thermal resistivity based on the paper is 5.5 ºC-m/W. This cable has no insulation shield/screen.

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The insulation dimension data dialog will be the next data dialog that should be completed.

Figure 11: Cable Insulation Dimension Data Dialog

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Advanced Power Cable Ampacity Program The diameter over insulation and over insulation shield is the same in this case since we have no insulation shield.

In the next screen, the cable sheath and bonding and transposition data should be provided.

Figure 12: Cable Sheath Data Dialog

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Select “Next” to proceed to the next data dialog, i.e., the cable reinforcing data dialog as shown below:

Figure 13: Reinforcing Tape Data Dialog

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Advanced Power Cable Ampacity Program The reinforcing tape made of brass/bronze is selected based on the data supplied in the Neher McGrath paper.

Figure 14: Reinforcing Tape Dimension Data Dialog

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Advanced Power Cable Ampacity Program The reinforcing tape dimension data is entered next that is shown in the figure below:

The cable concentric neutral or skid wire can be defined next. For the cable at hand, the cable does not have any concentric neutral wires but it has a brass/bronze skid wire.

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Figure 15: Concentric Neutral/Skid Wire Data Dialog

The cable Jacket or in case of a pipe type cable, pipe coating material can be specified in the data dialog shown in the figure below:

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Figure 16: Jacket/Pipe Coating Data Dialog

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Advanced Power Cable Ampacity Program For this cable a user defined pipe coating material is selected where thermal resistivity of the pipe coating is provided, i.e., RHJ = 1.0 ºC-m/W

Figure 17: Armor/Serving/Bedding Data Dialog This cable has no armor bedding, armor, or armor serving as specified and shown in the above figure. The last data dialog, is the data regarding the overall cable diameter, pipe material (in case of pipe type cable), and cable loss factor.

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Figure 18: Overall Cable Dimension Data Dialog

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After completing the overall cable dimension dialog, this cable that is just created will be added to the cable/project library. As shown in the figure below, the newly added cable type is now can be seen in the list of available cable types.

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How to Add/Create a Project/Study To add a new study/project select “New..” from the lower left hand side of the program’s main screen as shown below:

When creating a new project/study, the program requires a number cable installation conditions in addition to the cables types and their location. In the next screen shown below, there are two tabs one marked as “General” where general installation data can be entered and the other tab named “Cable Installation” where cable type and location can be specified. First the general project data should be completed. The general data requirement is show the dialog illustrated in the figure below:

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In the above there are several groups of data which is required. These are:

9 Study/Project Title: The user is recommended to assign an identification record to each study for easy reference in the future.

9 Soil resistivity and ambient temperature 9 Computation option: program can compute ampacity of cables given their maximum operating temperature or alternaively, program can compute temperature if cable ampacities are given.

9 Solution method can be either IEC or Neher McGrath. If all of the cables have unity daily load factor, then, IEC is the recommended method, otherwise, Neher McGrath should be selected

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9 Cables can be in air, buried directly, being in backfill or ductbank 9 Heat source/sink data in any adjacent to the cables 9 Duct dimension and material 9 Non-iso thermal earth surface simulation 9 And finally if the program should include soil drying out condition

Figure 19: Project/Study General Data Dialog

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Advanced Power Cable Ampacity Program The general installation data entered based on the Neher McGrath paper is shown in the below screen:

After completing the “General” data, the cable locations and type needs to be identified. Use the mouse to select the “Cable Installation” tab as shown above. The data dialog appears in the figure shown below.

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To add a cable type to the study, use the mouse to point to “Add Cable..” icon and then click once the left mouse button.

Figure 20: Adding a Cable to a Study/Project

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Advanced Power Cable Ampacity Program In the data dialog shown below, first we should specify which cable is to be used in the study. Press “Select Cable..” icon as shown below to see a list of available cables in the library.

List of cables in the library are shown in the figure below. Simply, use the mouse to highlight the desired cable and then press “OK” button to accept the selection. In the case at hand we will choose the cable we created in the previous section “138 kV Pipe Type Cable from NM paper”.

Figure 21: Selecting a Cable from the Cable Type Library

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Advanced Power Cable Ampacity Program Now the select cable identification is shown in the upper portion of the data dialog below.

The cable in this study based on the Neher McGrath paper is directly buried 3 feet below the ground surface with the maximum conductor temperature of 70 ºC. Daily load factor of 85% or 0.85 p.u. is also specified. Press “OK” to accept the data entries. It can be seen from the figure below, it is possible to add more cable type (s) to this study by just repeating the same process (choosing “Add Cable..” icon) .

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Since there is no additional cable in this study, press “OK” to complete study/project data .

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The study is just created is now can be seen in the list of existing studies as shown in the lower left hand side of the figure shown below:

How to Run a Simulation Now that we have completed all the necessary data for the cable installation conditions, the ampacity computation can be performed. To start the ampacity calculation, first use the mouse to point to the desired study in the list of existing studied/projects as shown in the lower left hand side of the above screen. For example, in the above screen in the list of studies, the study named “This is a new project to use newly created pipe type cable” is highlighted. Now that the study is selected choose the “Run>Project Calculation” option from the menu bar shown at the top portion of the above figure. Once this is selected the program will start the ampacity computation and will show the result in the report browser window that is shown below:

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The report browser screen show below can be used to view the result and also save a copy of the text result for later printing or inclusion in a documents. Select the “DONE” icon once the examination of result is completed.

Figure 22: Report Browser Window Showing the Result of Amapcity Calculation

Full result report is shown in the following papes:

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EDSA Cable Ampacity Program v5.00.00 ==================================== Project No. : Project Name: Date Title : Time Drawing No. : Company Revision No.: Engineer Unit System : U.S. Standard Jobfile Name: sample-jobfiles-4-cable-ampacity

: July 29, 2005 : 12:42:38 PM : :

Project ======= Title

:

This is a new project to use newly created pipe type cable

Solution Method

:

Neher-McGrath

Ambient temperature Native soil resistivity

: :

25.00 deg. C 0.800000 C-M/W

Cables in Project ================= [ 1] 138 kV Pipe Type Cable from NM paper, Circuit No.

Type Index

Horz Vert Temp DLF Outer ConfigPos Pos DIA uration (ft) (ft) (deg.C) (ft) --------- --------- --------- --------- ---------- ---------- --------- --------1 1 0.000 3.000 70.000 0.850 0.2220 Cradled

Cables Types ============ TYPE[ 1] 138 kV Pipe Type Cable from NM paper, Voltage

Phase Type Conductor Conductor Dried Bonding Cross Sec. Cnstrctn Area (V) in² ------------ ------------ ----------- ------------ ------------ ------------ ---------------- -----------138.000 3 Pipe-type copper 4 seg. Y 3c.cmn or pipe 1.566 ALOS

Cond. Cond. Sh Axial Insulation Insul. Insul.Sh TANDELTA DIA DIA Spacing DIA DIA (in) (in) (in) (in) (in) ------------ ------------ ------------ ------------ ------------ ------------ ------------ -----------1.000 1.63189 1.63189 0.00000 user def. 2.64212 2.64212 0.00500 EPSILN

Reinf.Tape

Reinf.Tape Reinf.Tape Reinf.Tape Reinf.Tape Reinf.Tape Concentric DIA Thickness Width Length/Lay # Tapes (in) (in) (in) (in) ------------ -------------- ------------ ------------ ------------ ------------ ------------ -----------3.50000 brass/bronze 2.66102 0.00315 0.87519 0.98425 1 brass Concentric Concentric Concentric Concentric DIA Wire DIA Length/Lay # Wires (in) (in) (in) ------------ ------------ ------------ -----------2.66400 0.14134 1.50000 1 Pipe Overall Pipe Outer Pipe Inner Pipe RHJ DIA DIA DIA Coating (ft) (ft) (ft) -------------- ------------ ------------ ----------- -------------0.8022 0.7188 0.6771 user def. 1.00000*E-08

Cable Ampacity Results ====================== This is a new project to use newly created pipe type cable Solution converged after 4 Cable No

X Location Ft

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1

0.00

3.00

70.

904.

Program Validation and Verification The cable ampacity program has been used numerous to assess its validity. In this section we will compare the result of this program against the result for a pipe type cable in the Neher McGrath paper. The cable is a 138 kV - 2000 KCMIL High-Pressure Oil-Filled Pipe-type Cable in 8.625 inch outside diameter pipe. The Cable shielding consists of an intercalated 7/8 × 0.003 inch bronze tape with 1 inch lay and a single 0.1 × 0.2 inch D shaped brass skid wire having 1.5 inch lay. The cables will lie in cradled configuration. The pipe is located 36 inches (3 feet) below the earth surface in soil having thermal resistivity of 0.85 K.m/W. An ambient temperature of 25 ºC is assumed. This problem is chosen from page 25 of the Neher McGrath paper and cable cress section as well as dimensions are shown below:

Figure 23: 138 kV, 2000 MCM high pressure oil-filled, 3-conductor, pipe type cable.

9 2000 KCMIL copper conductor - diameter (DC) = 1.632” = 41.453 mm. 9 Insulation thickness (t) - 0.505” = 12.83 mm. 9 Outside diameter of sheath (DS) - 2.661” = 67.589 mm. 9 Inside diameter of pipe (DP) - 8.125” = 206.375 mm. 9 Outside diameter of pipe (DPO) - 8.625” = 219.075 mm. The EDSA cable ampacity program for the above case produces versy close result as those reported in the IEEE paper as summarized below:

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Advanced Power Cable Ampacity Program Table 1: Comparing Results of Neher McGrath and EDSA’s Cable Ampacity Program Reference Cable Ampacity (amp) Error (%)

Neher McGrath IEEE Paper 905 -

EDSA’s Cable Ampacity Program 904 0.1

References – Bibliography

9 AIEE Trans., "Symposium on Temperature Rise of Cables," Vol. 72, Part II, p. 530-62, 1953. 9 AIEE Committee Report, "A-C Resistance of Pipe-Cable Systems with Segmental Conductors," AIEE Trans. Vol. 71, Part III, p. 393-414, 1952.

9 AIEE Publication S-135-1, Power Cable Ampacity Tables, ICES Publication, pp. 46-426, 1962. 9 Arnold, A. H. M., "Proximity Effect in Solid and Hollow Round Conductors," Journal IEE, Vol. 88, Part II*, p. 349-59, Aug., 1941.

9 Arnold,

A.H.M., "Eddy-Current Losses in Multi-Core Paper-Insulated Lead-Covered Cables, Armored and Unarmored, Carrying Balanced 3-Phase Current," Journal IEE, Vol. 88, Part I, p. 52-63, Feb., 1941.

9 Beasley, W.A., "Hot Circuits Can Be Expensive," IEEE Trans. Ind. Appl. Vol. 1A-19, July/August 1983.

9 Bosone, L., "Contribution to the Study of Losses and of Self-Induction of Single-Conductor Armored Cables," L'Elettrotecnica, p.2, 1931.

9 Buller, F.H. and Neher, J.H., "The Thermal Resistance between Cables and a Surrounding Pipe or Duct Wall," AIEE Trans, Vol. 69, Part I, p. 342-9, 1950.

9 Buller, F.H. and Woodrow, C.A., "Load Factor and Equivalent Hours Compared," Electrical World, Vol. 92, No. 2, p. 59-60, 1928.

9 Buller, F.H., Neher, J.H. and Wollaston, F.O., "Oil Flow and Pressure Calculations for SelfContained Oil-Filled Cable Systems," AIEE Trans. Vol. 75, Part I, 1959.

9 Buller, F.H., "Thermal Transients on Buried Cables," AIEE Trans. Vol. 70, Part I, p. 45-55, 1951. 9 Buller, F.H., " Artificial Cooling of Power Cable," AIEE Trans. Vol. 71, Part III, p. 634-41, 1952. 9 Burrell, R.W.; Morris, M., "A-C Resistance of Conventional Strand Power Cables in Non-Metallic Duct and in Iron Conduit," AIEE Trans. Vol. 74, 1955, Part III, p. 1014-23.

9 Greebler, P.; Barnett, G.F., "Heat Transfer Study of Power Cable Ducts and Duct Assemblies," AIEE Trans. Vol. 69, Part I, 1950, p. 351-69.

9 Heiman, R.H., "Surface Heat Transmission," ASME Trans. Vol. 51, Part I, p. 287-302, 1929. 9 IEEE Trans. on Ind. Appl., "Neher-McGrath Calculations for Insulated Power Cables," Vol. 1A-21, No. 5, September/October 1985.

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9 IPCA Publication, “Ampacity Tables for Solid Dielectric Power Cables Including Effect of Shield Losses,” pp. 53-426 and NEMA Publication WC 50-1976.

9 Meyerhoff, L. and Eager, Jr., G.S., "A-C Resistance of Segmental Cables in Steel Pipe," AIEE Trans Vol. 68, Part II, p. 816-34, 1949.

9 Meyerhoff, L., “Pipe Losses in Non-Magnetic Pipe,” AIEE Trans. Vol. 72, Part III, p. 1260-75, 1953.

9 National Electric Code, National Fire Protection Association Tables 310-20 through 310-30, 1988. 9 Neher, J. H., "The Temperature Rise of Buried Cables and Pipes," AIEE Trans. Vol. 68, Part I, p. 9-21, 1949.

9 Neher, J.H., "The Temperature Rise of Cables in a Duct Bank," AIEE Trans. Vol. 68, Part I, p. 540-9, 1949.

9 Neher, J.H., "The Determination of Temperature Transients in Cable Systems by Means of an Analogous Computer," AIEE Trans. Vol. 70, Part II, p. 1361-71, 1951.

9 Neher, J.H., "A Simplified Mathematical Procedure for Determining the Transient Temperature Rise of Cable System," AIEE Trans. Vol. 72, Part III, p. 712-8, 1953.

9 Neher, J.H. and McGrath, M.H., "The Calculation of the Temperature Rise and Load Capability of Cable Systems," AIEE Trans. on Power Appl. Sys., Pt. III, Vol. 76, pp. 752-772, October 1957.

9 "NEMA Report of Determination of Maximum Permissible Current Carrying Capacity of Code Insulated Wires and Cables for Building Purposes," June 27, 1938.

9 Rosch, S.J., "The Current Carrying Capacity of Rubber Insulated Conductors," AIEE Trans. Vol. 57, p. 155-67, April 1938.

9 Schurig, O.R. and Frick, G.W., "Heating and Current-Carrying Capacity of Bare Conductors for Outdoor Service," G.E. Review, Vol. 33, p.141, 1930.

9 Schurig, O.R., Kuehni, H.P. and Buller, P.H., "Losses in Armored Single-Conductor, LeadCovered A-C Cables," AIEE Trans. Vol. 93, No. 1, p. 417, 1929.

9 Simmons, D.M., "Calculation of the Electrical Problems of Underground Cables," The Electric Journal, May-November, 1932.

9 Wedmore, E.B., "The Heating of Cables Exposed to the Sun in Racks," Journal IEE, Vol. 75, p. 737-48, 1934.

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APPENDIX I: IEC & Neher-Mcgrath Cable Ampacity Calculations Methodology Traditional cable sizing methods address the issue of maximum allowable ampacity strictly from the point of view of the cable and system load characteristics. In other words, they are generally limited to factors such as insulation rating and voltage drop. In reality, cable ampacity is a much more complex concept that hinges on many other factors. The allowable conductor temperature for the type of insulation being used is the main factor that defines the ampacity of a cable. A complete approach to the problem requires the integration of all aspects of cable system design such as:

9 Load characteristics 9 Cable type 9 Conductor material & size 9 Insulation thickness and properties 9 Shield connections 9 Environment 9 Installation conditions

Figure 24: Basic Thermal Circuit Of course, other system conditions and load-flow characteristics may limit the rating of the cable to values lower than its ampacity. The basic calculation procedure for studying the thermal behavior of an element corresponds to the thermal equivalent of Ohm's law, which is shown in above Figure. Here we can see that Heat (Watts) is equivalent to electrical current, and Thermal Resistance (thermal ohm-foot) corresponds to electrical resistance (ohms). When heat circulates through the circuit's thermal resistance, a temperature drop ( °C ) is established. Thermal resistance values depend on material properties, thermal resistivity, and geometric characteristics.

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Advanced Power Cable Ampacity Program Thermal resistivity is defined in units of °C − cm / watt for both, the metric and imperial systems. In Europe, however, is defined as °C − m / watt . To illustrate the significance of this unit, consider the following: a material that possesses a resistivity of 1°C − cm / watt , will experience a 1°C temperature 2

rise when a heat flow equivalent to 1 watt / cm flows through a 1 cm section of the material.

dT = 1oC

1 Watt

1 Foot (1Meter)

1 Thermal Ohm-Foot (1 Thermal Ohm-Meter)

Figure 25: Definition of Thermal-Ohm Units In the United States, the ampacity calculations are based on a unit cable length of 1 foot, thus defining a Thermal-Ohm-Foot (TOF) ( °C − foot / watt ) as the thermal resistance that causes a 1°C -temperature increase, when 1 watt of heat per foot of conductor is generated. The equivalent metric unit is called the Thermal Ohm Meter (TOM), and is expressed as ( °C − m / watt ). The above Figure illustrates the concept of Thermal-Ohm-Foot/Meter.

Tcond

Rins/2

Ccond

Conductor Ohmic Losses

Rins/2

Tshield

Cins

Dielectric Losses

Rshield-fluid

Cshield

Rfluid

Cfluid

Shield Losses

Rfluid-pipe

Tpipe

Cpipe

Rpipe-earth

Rearth

Cearth

Pipe Losses

Tearth

Figure 26: Mathematical Model of a Cable Thermal Circuit Figure above, illustrates a mathematical model that describes the thermal circuit of a self-cooled buried transmission cable. This model is typical of a pipe-type cable system, which is one of the most common types of cable used in transmission applications. While conventional models may include conductor and dielectric losses and maybe even shield effect, they fail to take into account crucial thermal elements such as pipe fluid, and earth losses. Each of the components are summarized as follows:

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Advanced Power Cable Ampacity Program Temperatures:

Conductor Cable/earth interface Ambient

Losses:

Ohmic I 2 ∗ R Dielectric (insulation) Shield/sheath (eddy & circulating currents) Pipe (edddy-current & hysteresis )

Thermal Resistance:

Electrical insulation Dielectric Pipe covering Earth Mutual heating effect of nearby heat sources (steam mains) Mutual heating effect of other nearby cable systems Shield - Fluid Fluid - Pipe Duct material Air space Concrete envelope

Thermal Capacitance:

Conductors Insulation Shield Fluid Pipe Earth

(

)

In the particular case of cables in free air, one must consider the effect of solar radiation, which increases the temperature rise, and the effect of the wind, which decreases it. As seen from the above list of elements, the true ampacity of a cable depends on many factors, which are often ignored by traditional methods. This leads to poor/dangerous design specifications. Virtually every steady-state ampacity calculation in the United States is performed according to the Neher-McGrath method. This procedure quantifies with extreme accuracy the added heating effect imposed by each and every one of the elements previously listed. A non-US practice is to employ the IEC 287 method, which is the metric equivalent to the Neher-McGrath procedure. IEC 287, however, has a better treatment of sheath losses for single conductor cables. Both methods are applicable to both single and three-conductor cable in various installations, such as cable in air, cable in duct, and cable buried in earth.

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APPENDIX II: Some Useful Diagrams and Figures

Cable type: Pipe / Location: In Air

Air

Conduit/Pipe

Air

Pipe Type Cable

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Advanced Power Cable Ampacity Program Cable type: Pipe / Location: Buried

Soil Soil

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Advanced Power Cable Ampacity Program Cable type: Duct / Location: Air

Air

Air Duct Bank Ground Surface Duct Bank

Ground Surface

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Advanced Power Cable Ampacity Program Cable type: Duct / Location: Buried

Ground Surface

Soil

Duct Bank

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Soil

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Advanced Power Cable Ampacity Program Cable Construction Type

Cradle Configuration

OIL Single conductor Oil-filled cable 3-Conductor Cables in Duct

Triplex Configuration Cables in flat configuration

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Advanced Power Cable Ampacity Program Magnetic Armor or Reinforcement Type

Magnetic pipe/conduit 3-Conductor Cables.

Non-Magnetic Armor 3-Conductor Cables.

Magnetic pipe/conduit 1-Conductor Close Triangular Cables.

Non-Magnetic Armor 1-Conductor Close Triangular Cables.

Magnetic pipe/conduit 1-Conductor Cradled Cables.

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Non-Magnetic Armor 1-Conductor Cradled Cables.


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Advanced Power Cable Ampacity Program Thermal Circuit Type

Two Core Belted Circular Conductor Three Core Belted Circular Conductor

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Advanced Power Cable Ampacity Program Conductor Shield/Screen and Jacket

Jacket

Insulation

Conductor

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ti

tC


DJ

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Advanced Power Cable Ampacity Program Insulation

Jacket

Armor, Binder for RI (tai)

Belt for RI (tbi)

Core (tci)

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Conductor Material Conductor Shield Insulation Type Insulation Shield Sheath Type Sheath Reinforcing Material Concentric Wires Armour Bedding Armour Type External Cover

Figure 27: Cable Topology / General Parameters Self Contained Cables The figure above is a complete cable topology, which outlines the general range of the information required for a typical cable ampacity calculation.

Figure 28: Typical Pipe Cable Cross-Section

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Advanced Power Cable Ampacity Program Configuration of Cable in the Duct/Conduit:

Figure 29: Configuration of Cable in the Duct/Conduit

Figure 30: Ductbank Gb Factor

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APPENDIX III: Tables of Material Properties Different types of insulating material are listed below. The user can select specific inductive capacitance of insulation (εr) based on the type of cable. Table 2: Specific Inductive Capacitance of Insulation

ε

Materials

Insulation ( r)

Polyethylene Paper Insulation (Solid type) Paper Insulation (Other type) Rubber and Rubber-like compounds Varnished Cambric

2.3 3.7 (IPCEA Value) 3.3 - 4.2 5.0 (IPCEA Value) 5.0 (IPCEA Value)

The thermal resistivities of different materials are listed in table below. The user has the option to select the material thermal resistivities depending on the type of cable, its protective coverings, and the materials for duct installations. Table 3: Thermal Resistivity of Various Materials Material Paper Insulation (Solid type) Varnished Cambric Paper Insulation (other types) Rubber and Rubber-like Jute and Textile Protective Covering Fiber Duct Polyethylene Transite duct Somastic (Jacket) Concrete

ρ in Ccm/watt 700 (IPCEA Value) 600 (IPCEA Value) 500 - 550 500 (IPCEA Value) 500 480 450 200 100 85

For cables in ducts or pipes, the constants A' and B' from below table are used for calculation of thermal resistance (RSD) of the air space between the cable surface and the duct internal surface. The user needs to supply the values of the constants from table below depending on the installation of the cable. Table 4: Pipe Constants Condition In Metallic Conduit In Fiber Duct in Air In Fiber Duct in Concrete In Transite Duct in Air In Transite Duct in Concrete Gas - Filled Pipe Cable at 200 psi Oil - Filled Pipe Cable

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A’ 0.190 0.330 0.270 0.260 0.220 0.680 2.450

B’


Dec 2005


Table 5: Conductor Material Material

Resistivity ( ρ )ohm.m @ 20 ºC.

Copper

1.7241 x 10

Aluminum

2.8264 x 10

Lead or Lead Alloy

21.4 x 10

Steel

13.8 x 10

−8

3.5 x 10

Stainless Steel

70.0 x 10

Aluminum

2.84 x 10

Brass

6.24 x 10

B

−3

4.03 x 10

−8

4.0 x 10

−8

4.5 x 10 3.0 x 10

−8

B

−3

3.93 x 10

−8

−8

Bronze

TemperatureCoefficient (δ20) per K at 20 ºC

−3 −3 −3

Negligible

−8

−3

4.03 x 10

−8

4.5 x 10

−3

Table 6: Dielectric Loss Uo (kV)

Types of cables

B

Cables insulated with impregnated paper Cables with other kinds of insulation: Butyl rubber EPR PVC PE XLPE

B

(line to ground)

30 15 15 6 110 45

Table 7: Resistivities of Materials Material

Thermal resistivity (K.m/W)

Paper insulation in solid type cables Paper insulation in oil-filled cables Paper insulation in cables with external gas pressure Paper insulation in cables with internal gas pressure: a. Pre impregnated b. Mass impregnated PE XLPE Polyvinyl chloride: up to and including 3 kV cables greater than 3 kV cables EPR: up to and including 3 kV cables greater than 3 kV cables Butyl rubber Rubber Protective coverings: Compounded jute and fibrous materials Rubber sandwich protection Polychloroprene PVC: up to and including 35 kV cables greater than 35 kV cables PVC/bitumen on corrugated aluminum sheaths PE

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6.0 5.0 5.5 6.5 6.0 3.5 3.5 5.0 6.0 3.5 5.0 5.0 5.0 6.0 6.0 5.5 5.0 6.0 6.0 3.5


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Materials for duct installations: Concrete Fibre Asbestos Earthenware PVC PE

1.0 4.8 2.0 1.2 7.0 3.5

Table 8: Absorption Coefficients of Solar Radiation Materials

Absorption Coefficients

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Bitumen/Jute serving Polychloroprene Polyvinylchloride (PVC) Polyethylene (PE) Lead of Armor

0.8 0.8 0.6 0.4 0.6

For the cable diameters in the range of 25 mm to 100 mm, the constants UU, V and Y in following tbale are used for calculation of thermal resistance of the air space between the cable surface and the duct internal surface. Table 9: Constants For Ducts Or Pipes Installation condition

UU

V

Y2 B

B

B

B

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ In metallic conduit In fibre duct in air In fibre duct in concrete In asbestos cement: duct in air duct in concrete Gas pressure cable in pipe Oil pressure pipe type cable Plastic ducts Earthenware ducts

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5.2 5.2 5.2

1.4 0.83 0.91

5.2 1.2 5.2 1.1 0.95 0.46 0.26 0.0 Under consideration by IEC 1.87 0.28

0.011 0.006 0.010 0.006 0.011 0.0021 0.0026 0.0036


Dec 2005

Cable Ampacity

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