iiee_electric power cables

8/22/2019 IIEE_Electric Power Cables

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IIEE Cebu Chapter Seminar on Electric Power Cables for Low Voltage and Medium Voltage up to 69KV

Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573) Page No.1

I. ELECTRIC POWER CABLES FOR LOW VOLTAGE AND MEDIUM

VOLTAGE SYSTEM UP TO 69KV

A. Classification of Cables.

Cables are classified according to their insulation. The insulating materialof a cable is the most important single component, the purpose of which is toprevent the flow of electr icit y fro m the energized co nductors to ground or to an

adjacent conductor. The insulation must be able to withstand the electricalstresses produced by the alternating voltage and any superimposed transient

voltage stresses on the conductor without dielectric failure and causing short-circuit.

The selection of insulation involves a number of factors, some of which

area:a.)Stability and length of life.b.)

Dielectric properties.c.)Resistance to ionization and corona.

d.)Resistance to high temperature.e.)Resistance to moisture.f.) Mechanical strength.g.)Flexibility.Common insulation materials are Polyvinyl Chloride (PVC), Natural

Polyethylene (PE), Cross-linked Polyethylene (XLPE) and Ethylene PropyleneRubber (EPR). Each of these materials has unique characteristics which render it

suitable for particular application. Those cables are commonly used for ordinaryindustrial users and a limited number of cables such as the above mentioned

cable materials can be used to fulfill practically all industrial applications. Thevoltage rating or class of a cable is based on the phase to phase voltage of the

system though the cable is single, two or 3-phase. For example a 15KV ratedcable (or a higher value) must be specified or a system that operates at 7,200 V,

7,620V or 7,968 volts to ground or a grounded wye system of 12,500V, 13,200Vor 13,800V. Another example is that a cable for operation at 14.4KV to ground

must be rated at 25KV or higher since 14.4KV x 1.732 is 24.94KV.

Underground power cables have 3 voltage classifications as follows:

a. Low voltage - limited to 2KV.b. Medium voltage above 2KV to 46KVc. High voltage above 46KV

The low voltage cables are unshielded.


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Common among all classes of cables is the conductor. Commonly usedconductor materials are copper and aluminum.

Historically copper has been used for conductors of insulated cables due

pr imarily to its desirable electr ica l and mechanica l properties. The use of

aluminum is based mainly on its favorable conductivity to weight ratio (highestamong the electrical conductor materials) Aluminum has a specific gravity of 2.7and 61.0% electrical conductivity while copper has a specific gravity of 8.90

and 100% electrical conductivity. The use of aluminum has a distinct advantagein weight and may result in a lower initial cost. However due to the lower

electrical conductivity of aluminum, a large size or greater number of cables isrequired to supply a given load. This could easily result in larger size or greater

number of conduits or supporting racks which would increase the installationcost.

The principal difficulty encountered with aluminum conductors is that of

making and maintaining satisfactory terminal connection. Aluminum exhibits thefollowing three characteristics which gives rise to this difficulty:

a) The first characteristic is that the surfaces of aluminum exposed to airimmediately form an oxide coating which has a high resistance. Thiscoating is what gives aluminum wire its excellent anti-corrosion

property. This oxide coating insulates the strand of a cable fro m oneanother and tends to insulate the conductor from the connector on the

end of it. Even if the oxide coating is scraped off, it immediately re-forms before connection can be made. This high resistance at the

connection point results in excessive temperature rise. One method ofovercoming this problem has been to dip the aluminum conductor in a

special compound before clamping the terminal to it. This compound isgrease containing small particles of zinc. When the terminal is

clamped to the conductor, the zinc particle penetrate the oxide coatingto make a good electrical contact also the grease excludes air which

prevent the re-format ion of the oxide.

b) The second problem with aluminum is that it is soft and exhibits acold flow characteristic. For example, a normal terminal or

connector can be properly applied to the end of an aluminumconductor. After a period of time, the contact pressure will have

decreased and the resistance of the connection will have increased.This is the result of the cold flow characteristics or tendency for

aluminum to be squeezed out of the connection. This problem has beensatisfactory solved by the use of spring-loaded connectors and special

long barrel types of connector which clamped to a considerable longerportion of the conductor, thus minimizing the cold flow characterist ic.


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c) The third problem with aluminum is that it is damaged by galvanicaction when connected to other kind of metal and moisture is present.

Such conditions occur when copper connectors are used withaluminum cable. Connector manufacturers have found that tin-plated

copper alloy connectors eliminate the galvanic action.

PEC specifies aluminum alloy series AA-8000 electrical grade, forXHHW, THW, THHW, THWN and THHN insulation. This has purity of 99.00 +

with an electrical conductivity of 61% of that of copper. Type 1350 Aluminumalloy, medium-hard drawn, is typical for medium voltage power cables. The

alloy has a purity of 99.5% and the electrical conductivity is about 61%. Fullhard drawn aluminum alloy is most often used in overhead lines due to its higher

breaking strength.

A-1. Low Voltage Power Cables (600V Rating)

Low voltage PVC insulated power cables are generally rated at 600V,regardless of the voltage used whether 120V, 208V, 240V, 277V, 480V or 600V.

The selection of 600V power cables is oriented more towards physicalrather than electrical services requirements. Resistance to force, such as crush,

impact and abrasion become a predominant consideration, although goodelectrical properties for wet location are also needed. Cables are classified by the

insulations operating temperature and insulation thickness. A list of the morecommonly used cables is provided below.

A-1.1 Thermoplastic Types

Cables with thermoplastic insulating material is a synthetic compound

composed of plasticizers stabilizers, fillers and Polyvinyl Chloride Resin(PVC). The thermoplastic material is one that will soften repeat edly when

heated and hardened by cooling, that is, they can be molded and remoldedany number of times. The extrusion process for these materials requires

that they be heated sufficiently to cause them to flow, but no significantreaction takes p lace so that they will soften when reheated. The insulation

is mechanically tough oil, moisture and heat resistant and flame retardantcables. Under this cable type are the following as listed in PEC or NEC:

TW The maximum operating temperature is 60C in wet or dry

location. This type has no jacket.THW The maximum operating temperature is 75C in wet or dry

location. This type has also no jacket.THWN The maximum operating temperature is 75C for use in wet or

dry location. This type has a nylon jacket.THHN The maximum operating temperature is 90C for use in dry

location. This type has a nylon jacket.


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A-1.2 Thermosetting Types

A Thermosetting material is one that requires heat to vulcanize or cross-link it. The vulcanization cause a permanent chemical reaction so that the

material will have very little tendency to soften if heated again. Cross-

linked Polyethylene (XLPE) and Ethylene Propylene Rubber (EPR) are inthis type.

A-1.2.1 Cross-Linked Polyethylene (XLPE)

The 600V compounds of XLPE are usually filled with carbon black ormineral fillers to further improve the relatively good toughness of

conventional or natural polyethylene. The combination of crosslinkingthrough vulcanization plus fillers produces superior mechanical

properties. Vulcanizat ion el iminates the main drawback of a low meltingpoint of 105C for conventional or natural polyethylene. Also, usage of

natural polyethylene has greatly been limited to circuits where overloadand short circuit conditions are not critical.

The 600V construction consists of copper or aluminum conductor withsingle extrusion of insulation in the specified thickness. The insulation is

abrasion, moisture and heat resistant black XLPE. The naturalpolyethylene insulat ion for power cables had been replaced by the XLPE

material.The insulation type has a strong effect on cable rating. From a thermal

point of view, a good insulat ion material should have low thermalresistivity and should result in low dielectric losses.

In this classification are the following:

Type XHHW for 75C maximum operating temperature in wet and 90C

in dry locations only.

Type XHHW-2 for 90C maximum operating temperature in wet and drylocations.

A-1.2.2 Ethylene Propylene (EPR)

Rubber-like insulation such as ethylene propylene (EPR) and styrene

butadiene rubber (SBR) compounds require outer jacket for mechanicalprotectio n such as PVC and neoprene. Recent advancement in EPR

insulation has improved physical properties that do not require any otherjacket for mechanical protection.


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In this classification are the following:

a) Type RHW for 75C maximum operating temperature in wet or drylocation.

b) Type RHW-2 for 90C maximum operating temperature in wet or drylocation.c) Type RHH for 90C maximum operating temperature in dry locationsonly.

All the preceding cables are suitable for installation in conduit, duct or

other raceway and when specifically approved for the purpose may be installedin cable tray (1/0 AWG and larger) or direct buried, provided NEC or PEC

requirements are satisfied. The common conductor material used are copper oraluminum.

A-1.3 Current Carrying Capacity

The current carrying capacity or ampacity of a cable is defined asthe maximum current it can carry continuously without the temperature at

any point in its insulation exceeding the limit prescribe for it according toits thermal class. The current capacity of all the cable types shall be

referred to the data in latest edition of PEC or NEC

A-1.4 Insulation Resistance of Cables

An important aspect of a power cable is its insulation resistance

which is the resistance to flow of direct current through an insulatingmaterial (dielectric). There are two possible paths for current to flow

when measuring insulation resistance:

a) Through the body of the insulation (Volume insulation resistance)b) Over the surface of the insulation system (surface resistivity)Volume insulation resistance of a cable is the direct current resistance

offered by the cable insulation to an impressed D.C. voltage to produce a radialflow of leakage current through the insulation material.

On surface resistivity, there is a current flowing over the surface of the

insulation when voltage is applied on the conductor. This current adds to thecurrent flowing through the volume insulation resistance which reduces apparent

volume insulation resistance unless measures are taken to eliminate that currentwhen measurements are being made. This measure could be a guard circuit

which will eliminate the surface leakage current from the volume resistivitymeasurements.


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The volume insulation resistance at 60F is given by the followingformula:

1000k log10 D/dR60 = Mega ohms

L

Where:

R60 = insulation resistance at 60F

K = Insulation Resistance constant, mega ohms 1000ft

D = outside Diameter of the cable insulation, Inches.

d = conductor Diameter, Inches.

L = length of cable, Feet.

The equation above is based on K values at 60F

The following data for the values of K for common types of low voltagecable insulation materials are taken from ICEA (Insulated Cable Engineers

Association of USA).

THW (75C PVC) -------------------- 2,000TW (60C PVC) ---------------------- 500

XHHW (600V XLPE) --------------- 10,000RHW ----------------------------------- 4,000

EPR (600V) --------------------------- 10,000

Example Calculation:

Assuming a type THW 500MCM copper power cable which has aninsulation resistance constant of K= 2,000 and a length of 500ft. Find the

insulation resistance.

1000K log10 D/d

RI =

L

2, 000, 000 log10

=500

25.03

20.65


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= 4000 log10(1.21156)

There: RI = 333.38 Megohms or say 333.0 Megohms

Please note that the cable insulation resistance will decrease as the cable length

increases as there will be more parallel paths of leakage currents. Also theleakage current is inversely proportional to the insulation resistance. Thus a high

value of insulation is desired.

In order to measure the insulation resistance of a cable, the insulation must beeither enclosed in a grounded metallic shield or immersed in water.

For temperatures other than 60F, a temperature correction factor will have to beapplied by multiplying the insulation resistance in megohms as calculated as per

equation stated previously by the temperature convection factor. The temperaturecorrection factor Tc can be found in the following equation:

Tc = 100.4343 a (t-60)

Values ofa for some insulation materials are as follows:

Natural Polyethylene (Ther moplast ic) 0.0Silicone Rubber ---------- 0.03

XLPE and EPR (LV Thermosetting) 0.0

For low voltage cables (600V) insulation resistance measurement, megohmetervoltage must be 500V to 1000V.

A-2. MEDIUM AND HIGH VOLTAGE POWER CABLES

As listed in PEC & NEC, medium voltage cables are designated type MVand have solid extruded d ielectric insulation rated 2000V to 35,000 volts. Single

conductor and multiconductor cable are available with minimal voltage rating of5KV, 8KV, 15KV, 25KV and 35KV. Also available are solid dielectric 46KV,

69KV and 138KV transmission cables but these are not listed by PEC & NEC.The succeeding discussions will be centered on cross-linked polyethylene

(XLPE) and Ethylene Propylene Rubber (EPR) cables.

Medium voltage and high voltage power cables in addition to beinginsulated are shielded to confine and evenly distributes the electric field within

the insulation. It is accomplished by means of conductor and insulation shields.Shielding of power cables will be discussed thoroughly in the succeeding parts of

this paper. The use of shielded cables is dictated by the following conditions:

a) Personnel safety. The advantage is obtained only if the shield is grounded.If not grounded, the hazard of shock may be increased.

b) Single conductors in wet locations.


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c) Direct earth buried.d) Where the cable surface may collect unusual amounts of conducting

material such as salt, soot, conductive pulling compounds, etc.

A-2.1 Cable Components and its Functions

Commonly used types of cables are the XLPE and EPR.

The components and its functions of a medium and high voltage cable are as

follows (Refer to fig.1):

Fig. 1

Typical Shielded Power Cable Design


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A-2.1.1 The Central Conductor .

The purpose of which is to conduct power to serve the load.The metals of choice are either copper or aluminum as discussed in

the early part of the paper. The central conductor may be composed

of a single element (solid) or composed of multiple elements(stranded)

A-2.1.2 The Conductor Shield.

A semi-conducting layer placed over the conductor toprovide a smooth conducting cylinder around the conductor .

Typical of todays cables, this layer is a semi conducting plasticpolymer wit h carbon filler, extruded direct ly over the conductor.

This layer represents a very smooth surface which, because ofdirect contact with the conductor is elevated to the applied voltage

on the conductor.

A-2.1.3 The Insulation.

A high dielectric material to isolate the conductor, the twobasic types used today is cross- linked Po lyethylene (XLPE) or

Ethylene (EPR). Both types have maximum operating temperatureof 90C and maximum short-circuit temperature of 250C. XLPE

and EPR are classified as thermosetting materials which do notsoften to any greater degree below their decomposition temperature

and therefore are not capable of being remoulded. For XLPE cable,the process of cross-linking or vulcanization consists of forming

chemical bonds between the long chain molecules of plainpolyethylene to give a ladder ef fect which restricts sl ippage

between molecules and produces good thermal stability. Crosslinking or vulcanization also means that the different long

molecules of plain or natural polyethylene are linked together. SeeFigs. 2A & 2B.

FIG. 2A P.E. FIG. 2B XLPE

Simplified Description of Cross-Linked Networks


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The process of cross-linking can be achieved by high energy

radiation or by chemical methods. Chemical cross-linking is thetraditional method but radiation cross-linking is increasing in

popular ity for wires and small cables where insulat ion thicknesses

are not excessive. The most common method of cross-linking is bythe addition of an agent such as peroxide to the plain polyethylenematerial which can be activated by heat. Details of the cross-

linking process are not included in this discussion.Because of an aging effect known as TREEING (see fig.3)

on the basis of its visual appearance, caused by moisture in thepresence of an electr ic fie ld, a modified version of XLPE,

designated Tree retardant (TRXLPE) has replaced the use of XLPEfor medium voltage application. TRXLPE is a very low loss

dielectric that is reasonably flexible and has a maximum operatingtemperature limit of 90C or 105C depending on type.

FIG. 3

Treeing in M.V. & H.V. Power Cable

Insulation

Conductor

Conductor

Shield

Insulation Shield

Water Trees


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A-2.4 The Insulation Shield.

This consists of the following components as follows:

A-2.4.1 A semi conducting layer to provide a smooth cylinder aroundthe outside surface of the insulation. Typical shield compound isa polymer with carbon filler that is extruded directly over the

insulation. This layer for medium voltage applications is not fullybonded to the insulat ion (str ippable) to allow relat ive ly easy

removal for the installation of cable accessories such as cabletermination. Transmission cables have this layer bonded to the

insulation, which requires shaving tools to remove.

A-2.4.2 The Metallic Shield layer, which may be composed of wires,tapes, or corrugated tubes. This shield is connected to ground

which keeps the insulation shield at ground potential and providesa return path for fault current medium voltage cable can utilize

the metallic shield as the neutral return conductor if sizedaccordingly. Typical Shield sizing criteria:

a.) Equal in capacity to the central conductor for single-phase

application.

b.) One-third the capacity for 3-phase applicat ions.

c.) Fault duty for 3-phase feeders and transmission application.

A-2.5 Overall Jacket.

This is a plastic layer applied over the metallic shield forphysica l protect ion. This po lymer layer maybe extruded as a loose

tube or directly over the metallic shield (encapsulated). Althoughboth provide physical protect ion, the encapsulated jacket removes

the space present in a loose tube design which may allowlongitudinal water migration. The typical compound use for jacket

is linear low density polyethylene (LLDPE) because of itsruggedness and relatively low water vapor transmission rate.

Jackets can be specified insulating (most common) or semiconducting (when jointly buried and randomly laid with

communication cables).

A-3. Percent Insulation Levels of Power Cables

Referring to the discussion in the initial pages of this paper re-voltagerating of the cable insulation, its selection is made on the basis of the phase,


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phase voltage of the system in which the cable is to be applied whether thesystem is grounded or ungrounded, and the time in which a ground fault on the

system is cleared by protective equipment. It is possible to operate cables onungrounded system for long period of time with one grounded due to fault. This

result in line-to-line voltage stress across the insulation of two ungrounded

conductors. Therefore such cable must have a greater insulation thickness thancable on grounded system where it is impossible to impose full line-to-linepotential on the other two unfau lted phase of an extended period of t ime.

The following are the cable insulation level:

A-3.1 100% level.

Cables in this category may be applied where the system is provided with

relay protection such that ground fault will be cleared as rapidly as possible, butin any case within one minute. While these cables are applicable to the great

majority of cable systems that are on grounded system, they may also be used onother system for which the application of cables is acceptable provided the above

clearing requirement are met in completely de-energizing the faulted section.

A-3.2 133% level.

This insulation level corresponds to that formerly designated forungrounded systems. Cables in this category may be applied in situations where

the clearing time requirements of the 100% category cannot be met and yet thereis adequate assurance that the faulted section will be de-energized in a time not

exceeding one hour. Also they may be used when additional insulation strengthover the 100% level category is desirable.

A-3.3 173% level.

Cables in this category should be applied on systems where the time

required to de-energize a section is indefinite. Their use is recommended also forresonant grounded system. Cable manufacturers will have to be consulted for

insulation thickness of their manufactured cables.

The percent insulation level does not necessarily mean the thickness ratioover the 100% thickness. For example 133% insulation does not necessarily have

33% more thickness over 100% level thickness. PEC shows the thickness to beless then 133%.

Ratings of low voltage cables as well as the medium voltage cable

previously discussed in that they are also based on phase-to-phase operation. Thepractical po int here is that a cable that operates at says 480 volts fro m phase-to-

ground on a grounded wye system requires an insulation thickness applicable to480V x 1,732 volts phase-to-phase. This of course, means that a 1,000 volts level


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of insulation thickness should be selected. There are no categories for lowvoltage cables that address the 100, 133 and 173 percent levels. One of the main

reasons for the thickness of insulation walls for these low voltage cables in theapplicable standards is that mechanical requirements of these cables dictate the

insulation thickness. As a practical matter all these cables are over-insulated for

the actual voltages involved.

A-4. Current-Carrying Capacity

The current carrying capacity of a cable is defined as the maximum

current it can carry continuously without the temperature at any point in itsinsulation exceeding the limit prescribed for according to its thermal class.

The PEC or NEC will be referred to in checking the amperage capacity for

different installation conditions.

A-5. Insulation Resistance

The same procedure as discussed in the low voltage cables will be used incalculating the insulation resistance.

For the common medium voltage cables such as XLPE and EPR, theinsulation resistance constant is 20,000 Megohms-1000ft.

B. 3 PRINCIPAL FACTORS IN THE SELECTION OF CABLE SIZE.

The determination of conductor size is principally based on three

considerations:

1.0 Current-carrying Capacity or ampacity as adapted by PEC and NEC.The term ampacity was suggested by W.A. Del Mar of Phelps Dodge

Wire and Cable Co. USA in 1951 to replace the term current-carryingcapacity.

1.0 Short-circuit current.2.0 Voltage drop calculationsThe first consideration in the ampacity of cable which is a ffected by many

things. Basically, the final consideration is the permissible operating temperatureof the insulation. The higher the operating temperature of the insulation, the

higher the current-carrying capacity of the cable. The temperature at which apart icular cable will operate is affected by the ability of the surround ing mater ial

to conduct the heat away. Therefore, the current capacity is materially affectedby the ambient temperature as well as by the installat ion conditions,


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For example, assuming 40C ambient temperature a 3-conductor 4/0AWG, 15KV cable in an overhead rack in open air will carry 325 Amps. The

same cable installed in a magnetic conduit encased in concrete will only carry289 Amps. Running a non-metallic cable through a magnetic conduit will

increase the apparent resistance of the cable and will result in a lower ampacity

due to additional resistance losses. Similarly, when cables are run closelytogether, the presence of other cables, in effect, increased the ambienttemperature due to mutual heating which decreases the ability of the cables to

dissipate its heat. PEC and NEC have correction factors for the ampacity ofcables (with aluminum or copper conductor) for different installation conditions

The second consideration in the selection of conductor size is that of the

short-circuit currents which the cables must carry. The construction of the cableis such that its mechanical strength is high and it can handle short-circuit

currents without any mechanical difficulty. From a thermal view point, however,there is a limit to the amount of short-circuit current which can be carried.

During normal operation the magnitude of current at a given cable may

carry is limited by the continuous temperature rating of the insulation. It isrecognized, however that under fault conditions there will be an abrupt elevation

in conductor temperature which will subject the insulation to a more severethermal stress for a short period of time.

It is very important to check the thermal stress limit (in term of current

and time for various conductor sizes) so as to have protection equipment that willprevent severe permanent damage to cable insulat ion during an interval of fault

current flow.

Under short-circuit conditions the u ltimate conductor temperature dependson the following:

a. The magnitude of fault current.b. Cross-sectional area of the conductor.c. The duration of fault current flow.d. The conductor temperature before the short-circuit occurs.

On the basis that all the energy produced during fault current flow is effective inraising the conductor temperature (since the time period is very short, this is a

valid assumption for engineering purposes) the conductor heating is governed bythe following equation.


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For copper

2

I T2 + 234

t = 0.0297 log10

A T1 + 234

For Aluminum

2

I T2 + 228

t = 0.0125 log10A T1 + 228

I = Short-circuit current or Amps.

A = Conductor Area in Circular Mils.

t = Time of short-circuit in secs.

T1 = Initial conductor temperature in degrees Celsius.

T2 = Final conductor temperature in degree Celsius.

It is important to note that the abnormal temperature persists much longer than the

duration of fault current flow. For example, a flow of 30,000 Amps in 500 MCM cablewill elevate the copper temperature from 75C to 200C in approximately 1 second. With

the current then reduced to zero as much as 3000 secs or 0.8333 hrs could be required forthe copper to return to normal operating temperature. The cooling time will vary with the

cable geometry (wall thickness, diameter, etc). This thermal lag in cooling is of specialimportance in cases where circuits are protected by automatic reclosers and where

immediate manual reclosing is practiced.

In the two above equations for sizing of cables based on short-circuit current,generally, the initially conductor temperature T1 is not accurately known since it depends

upon the loading of the cable and ambient conditions. To be conservative it is usually

assumed to be equal to the maximum continuous operating temperature of the insulation.The duration of the short-circuit is usually assumed to be 1 second.

Hereunder is the data for the maximum continuous temperature rating andmaximum short-circuit temperature rating of cables (low voltage and medium voltage).


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Temperature Limits of Cables

Type of InsulationMaximum Continuous

Temperature Rating

Maximum Short-Circuit

Temperature Rating

MV XLPE 90C 250C

MV EPR 90C 250C

TW 60C 150C

THW 75C 150C

THHN/THWN 90C 150C

RHW 75C 200C

RHW-2 90C 200C

XHHW 75C 250C

XHHW-2 90C 250C

Polyethylene (natural orconventional) 75C 150C

The third consideration is to check the voltage drop in the cable.

PEC requires maximum total steady state voltage drop on both feeders and branch

circuits to the farthest outlet not to exceed 5% and will provide reasonable efficiency ofoperation. PEC defined feeder as all circuit conductors between the service equipment the

source of a separately derived system, or other power supply source and the final branchcircuit over current device.

Conductors for feeders must be sized to prevent a voltage drop exceeding 3

percent at the farthest outlet of power, heating and lighting loads or combination of suchloads. Below is a drawing for branch circuits and feeders.

FIG.4

Feeders and Branch Circuits

Feeders

Services Equipmentor source of

separately derived

system

Final Branch circuit

over current protection

Panel

Board

Panel

Board

Feeders


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On alternating current circuits where the cable is installed as singleconductor in free air, or as single conductor in individual ducts or buried directly

in the ground, the voltage drop depends upon the spacing, arrangement, etc. of theconductors. There are some published, simplified voltage drops, tables, curves

and charts but the variations in cable installation works are so numerous that is

impractical to prepare a simplified voltage drop tables, curves and graphs. Enggcalculation will have to be undertaken.

The approximate cable voltage drop formula is as follows:

V = IR cos + IX sin

Where:V = Voltage drop in the circuit, line to neutral.

I = Current flowing in the conductor.

R = Line resistance for one conductor, in ohms.

X = Line reactance for one conductor, in ohms.

= Angle whose cosine is the load power factor.

Cos = Load power factor, in decimals.

Sin = Load power factor, in decimals.

The voltage drop V obtained for the formula is the voltage drop in one conductor,

one way commonly called the line-to-neutral voltage drop. The reason for using the line-to-neutral voltage is to permit the line-to-line voltage to be completed by multiplying by

the ff. constants:

Voltage System Multiply By:

Single - phase 2

3 - phase 1.732

In using the voltage drop formula, the line current is generally the maximum or

assumed load current carrying capacity of the conductor.The resistance R is the AC resistance of the particular conductor used and the typeof raceway in which it is installed as obtained for the manufacturer. It depends on the ff:

a.) Size of the conductorb.)Type of conductor ( copper or aluminum)c.) The temperature of the conductor


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d.)Whether the conductor is installed in magnetic (steel) or non-magnetic(aluminum or non-metallic raceway).

The resistance opposes the flow of current and causes heating of the conductor.

The resistance X is obtained for the manufacturer. It depends on the ff:

a.)The size and material of the conductorb.)Whether the raceway is magnetic or non-magnetic.

c.) Spacing of the conductor of the circuit. The spacing is fixed for multi-conductor cables but may vary with single-conductor cables so that an averagevalue is required. Reactance occurs because the alternating current flowing in

the conductor causes a magnetic field to build up and collapse around eachconductor in synchronism with the alternating current. This magnetic field as

it builds up and fall rapidly, cuts across the conductors of the circuit, causing avoltage to be induced in each in the same way that a current flowing in the

transformer induces a voltage in the secondary of the transformer.

The following tables for impedances for different installation conditions are in

the following. Tables of IEEE STD. 141-1993


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C. SHIELDING OF POWER CABLES AND GROUNDING OF CABLE

SHIELD

Shielding of an electric power cables is the practice confining the

dielectric field of the cable to the insulation of the conductor or

conductors. It is accomplished by mean conductor and insulation shields.

C-1. Function of Shielding

A conductor shield is employed to prevent excessive voltage stress on

voids due to irregularities of the conductor surface between conductor andinsulation. To be effective, the shield must adhere to or remain in intimate

contact with the insulation under all conditions. In cables rated over 2000V, aconductor shield is required by industry standards such a PEC or NEC.

An insulation shield has a number of functions:

a) To confine the entire dielectric field to the inside of the insulationmaterial. This will result in a symmetrical radial voltage stress withinthe insulation.

b) To protect the cables from induced or direct over-voltages such as inconnecting to overhead lines. Shields do this by making the surge

impedance uniform along the length of the cable and by helping toreduce surge potentials.

c) To limit radio interference.d) To reduce the hazard of shock. This advantage is obtained only if the

shield is grounded. If not grounded, the hazard of shock may beincreased.

C-2.Use of Shielding

The use of shielding involves consideration of installation and operating

condition. Definite rules cannot be established on practical bas is for all cases butthe following features should be considered as a working basis for use of

shielding.

Where there is no metallic covering or shield, the dielectric (or electric)field will be partly in the insulation and partly in whatever lies between the

insulation and ground. The external field, if sufficiently intense in air, willgenerate surface discharge and convert atmospheric oxygen into ozone which

may be destructive to the insulation and protective jackets. If the surface of thecable is separated from ground by a thin layer of air and the air gap is subjected

to a voltage stress which exceeds the dielectric strength of air, a discharge willoccur, causing ozone formation.


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The ground may be either a metallic conduit or a damp non-metallicconduit or a metallic binding tape or ring on an aerial cable, a loose metallic

sheath etc. Likewise damage to non-shielded cable may result when the surfaceof the cable is moist, or covered with sooth, soapy grease or other conducting

film so that the charging current is carried by the film to some spot where it can

discharge to ground. The resultant intensity of discharge may be sufficient tocause burning of the insulation or jacket.

Where non-shielded, non-metallic jacketed cables are used in undergroundducts containing several circuits which must be worked on independently, the

external field if sufficiently intense can cause shocks to those who handle orcontact energized cable. In cases of this kind, it may be advisable to use shielded

cable. Shielding used to reduce hazards shock should have a resistance lowenough to operate protective equipment in case of fault. In some cases, the

efficiency or protective equipment may require proper size ground wires assupplement to shielding. The same considerations apply to exposed installations

where cables may be handled by performed who may not be acquainted with thehazards involved.

C-3. Shield Material

Two distinct types of materials are employed in constructing cable

shields: The non-metallic and metallic shields.

C-3.1. Non-metallic shields may consist of either semi-conducting compoundsor material that have a high dielectric constant and are known as stress

control material. Both serve the same function of stress reduction.

C-3.2. The conductor shielding materials were originally made of semi-conducting tapes that were helically wrapped over the conductor. This is

done, especially on large conductors, in order to hold the strand togetherfirmly during the application of the extruded semi-conducting material

that is now required for medium voltage cables. Experience with cablethat only had semi conducting tape was not satisfactory, so the industry

changed their requirements to call for an extruded layer over theconductor. Present day extruded layers are not only clean (from

undesirable impurities) but are very smooth and round. This has greatlyreduced the formation of Water Trees (Refer to Fig.3) that could

originate from irregular surfaces. By extruding the two layers (conductorshield and insulation) at same time, the conductor shield and insulation

are cured at the same time. This provides the inseparable bond thatminimizes the chances of the formation of a void at the critical area

between the conducto r shield and insulat ion surface adjacent to theconductor.

For compatibility reasons, the extruded shielding layer is usuallymade from same or a similar polymer as the insulation material. Special


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carbon black is used to make the layer over the conductor semiconducting material to provide the necessary conductivity. Industry

standards (ICEA and NEMA) require that the conductor semi-conductingmaterial have a maximum resistivity of 1,000 meter-ohms. Those

standards also require that this material pass a longtime stability test for

resistivity at the emergency (over load) operating temperature level toinsure that the layer remains conductive and hence provides a long cablelife.

A water-impervious material can be incorporated as part of the

conductor shield to prevent radial moisture transmission. This layerconsists of a thin layer of aluminum or lead sandwiched between semi-

conducting materials.

C-4. Insulation Shielding

The insulation shield for a medium voltage cable is made up of twocomponents:

C-4.1. A semi conducting or stress relief layer.

C-4.2. A metallic layer of tape or tapes, drain wires, concentric neutralwires, braid, sheath or metal tube. This metallic shield must be

non-magnetic.

The two components mentioned above must function as a unit for acable to achieve a long service life:

The semi conducting or stress layer used with extruded cables

(example: XLPE and EPR) is a polymer material. This is an extruded layerand is called this extruded insulation shield or screen. Its properties and

compatibility requirement are similar to the conductor shield previouslydescribed except that standard requires that the volume resistivity of this

external layer be limited to 500meters-ohms.

The non-metallic layer is directly over the insulation and thevoltage stress at that interface is lower than at the conductor shield

interface. This layer is not fully bonded (strippable) to the insulation forvoltage up to 35KV. Above 35KV, this layer is fully bonded to the

insulation, which requires shaving tools to remove.

The metallic portion of the insulation shield or screen is necessary toprovide a low resistance path for charging current to flow to the ground. It is

important to realize that the extruded semi-conducting shield material will notsurvive a sustained current flow of more that a few milliamperes. Those

materials are capable of handling the small amounts of charging current butcannot tolerate unbalanced or fault current.


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The metallic component of the insulation shield system must be able to

accommodate these higher current. On the other hand, an excessive amount ofmetal in the shield of a single-conductor cable is costly due to following:

a)

The additional metal over the amount that is actually required increasethe initial cost of the cable.b) The greater the metal component of the insulation shield, the higher

the shield losses that result from the flow of current in the centralconductor. The higher the shield losses which increase the heat in the

cable thus the capacity of the cable will decrease. These conditionswill be discussed thoroughly on the section entitled Grounding of

Cable Shields.

Shielding of low voltage cables is generally required where inductiveinterference can be a problem.

In numerous communication, instrumentation and control application,

small electrical signals may be transmitted on the cable conductor and amplifiedat the receiving end. Unwanted signals (noise) due to inductive interference can

be as large as the desired signal. Thus can resu lt in false signals or audible noisethat can affect voice communications.

D. GROUNDING OF CABLE SHIELD OR SHEATH

This discussion provides an overview of the reasons why cable shields or

sheaths are grounded and the methods of grounding of cable shields or sheaths.The terms shield and sheath are being used interchangeable since they have the

same function, problem and solutions for the purpose of this chapter. The two aredefined as follows:

a) Sheath refers to a water impervious, tubular metallic component of a cablethat is applied over the insulation. Examples are a lead sheath and acorrugated copper or aluminum sheath. A semi-conducting layer may be

used under the metal to form a very smooth surface.

b) Shield refer to the conducting component of a cable that must grounded toconfine the dielectric field to the insulation. Shields are generally

composed of a metallic portion and a conducting (or semi-conducting)extruded layer. The metallic portion can be either tape, wires or a tube.

Generally, sheaths are used on paper insulated M.V. and H.V. cables

while shields are commonly used in M.V. and H.V. XLPE and EPR. Since we areconcentrating on the commonly used XLPE and EPR cables, the term shields will

be used in this discussion.


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As discussed earlier in the section on shielding, for personal safety themetallic shield must be grounded due to induced voltage which will be d iscussed

later. However, grounding of the cable shields must take into consideration theshield losses which may effect reduction of cable ampacity.

There are two methods of grounding the cable shields namely: single-

point and multi-po int ground ing systems. PEC & NEC have ampacit y tablesbased on single-point grounding which will not effect cir culat ing current, thus,there will be no additional heating on the cable. Besides, as of this time, studies

are still on-going in USA & EUROPE on the ampacity of cables based on amulti-point grounding of shields due to conflicting methods by engineers and

researchers. PEC & NEC advised that if the shields are grounded at more thanone point, ampacities shall be adjusted to take into consideration the heating due

to shield currents. No correction factors are given by PEC & NEC.The cable systems that should be considered for single-point grounding

are systems with cables of 1,000 MCM (500mm) and larger and with ant icipatedloads of over 500 amperes.

A cable may be considered a transformer. When alternating current flowsin the central conductor of a cable, that current produces electromagnetic flux in

the metallic shield, if present, or in any parallel conductor. This becomes one-turn transformer when the metallic shield is grounded, two or more times since a

circuit is formed and current flows.A single-conductor shielded power cable will be considered first. See

Fig.5 below:

D-1. Single-Point Grounding

FIG. 5

Single-Point Grounding of Metallic Shield

IC

Metallic Shield


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If the metallic shield is only grounded one time and a circuit is not

completed, the magnetic flux due to the flow of current in the centralconductor produces a voltage in the shield. The amount of voltage induced

in the shield is proportional to the current in the central conductor and

increases as the distance from the ground connection increases. See Fig.6.

Fig. 6

Single-Point Grounding, Induced Shield Voltage

Actually there are eddy current induced in the shield. It is a known

fact that whenever an alternating magnetic flux penetrates a piece ofconducting material, eddy currents will be produced therein. Thesecurrents circulate in the shield. However, eddy currents are not of

significant amount as the metallic shield are non-magnetic such as copperor aluminum which has a much higher magnetic reluctance than the

ferromagnetic materials such as iron, nickel, cobalt,etc. Generally, eddycurrents can be neglected.

In the connection diagram of Single Point Grounding of the metallicshield (See Fig.5), there is no close circuit and therefore no, induced

circulating current in the shield. This set-up avoids the considerableheating of the metallic shield due to the circulating current flowing along

the metallic shield and returning through the ground (See Fig. 7 for multi-point grounding). The losses in the shield due to the circulat ing current

could effect a reduction of cable ampacity.In view of the voltage to the ground at the free end of the shield,

part icular care must be taken to insulate and provide surge protection atthe free end of the shield to avoid danger from the induced voltages.

Voltage

Distance


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D-2. Multi-Point Grounding

Fig.7Multi-Point Grounding of Metallic Shield

If the metallic shield is grounded two or more times or otherwisecompletes a circuit, the magnetic flux produces a current flow in the field. The

amount of current in the shield is inversely proportional to the resistance of theshield, that is, the current in the shield increases as the amount of metal in the

shield increases. The voltage to ground of the shield stays at zero. See Fig.8below.

Fig.8

One other important concept regarding multi-point grounds is that the

distance between the grounds has no effect on the magnitude of the circulatingcurrent in the shield. If the grounds are one foot apart or 1000 feet apart, the

current is the same depending on the current in the central conductor and theresistance in the shield. This is the same condition as in the current transformer

operation. In the case of multiple cables, the cable spacing in arrangement is alsoa factor.

Voltage

0

I circulating

Vshi = 0Vshi = 0

IC

Metallic Shield

Distance


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D-3. Metallic Shield Losses

A very important factor that affects much the ampacity of the cable is themetallic shield losses. As briefly discussed earlier, when current flows in a

conductor, there is a magnetic field associated with that current flow. If the

current varies in magnitude with time, such as with 60Hz a lternating current, themagnetic field expands and contracts with the current magnitude. In the eventthat a second conductor is within the magnetic field of the current-carrying

conductor, a voltage that varies with the field will be introduced in thatconductor.

If that conductor is part of a circuit, the induced voltage will result in

current flow. This situation occurs during operation of metallic shieldedconductors. Current flow in the phase conductors induces a voltage in the

metallic shields of all the phase cables within the magnetic field. If the shieldshave two or more points that are grounded or otherwise complete a circuit,

current will flow in the metallic shield conductor.

The current in the metallic shields generates losses. The magnitude of thelosses depends on the shield resistance and the current magnitude. This loss

appears as heat. These losses not only represent a economic loss, but they have anegative effect on ampacity and voltage drop. This has the effect of reducing the

permissible phase conducto r current . In other words, shield loss reduces theallowable phase conductor ampacity.

In multi-phase circuits, the voltage induced in any shield is the result of

the vectorial addition and subtraction of all flux linking the shield. Since the netcurrent in a balanced 3-phase circuit is equal to zero when the shield wires are

equidistant from all 3 phases, the net voltage is zero. This is actually not thecase, so in actuality there is some net flux that will induce a shield voltage

flow or current flow.

D-4. Effect of Spacing Between Phases of a Single Circuit

In a 3-phase of shielded, single-conductor cables, as the spacing between

conductors increases, the cancellation of flux from other phases is reduced. Theshield on each cable approaches the total flux linkage created by the phase

conductor of that cable. Refer to Fig.9.

Fig.9

CBAS S


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As the spacing, S, increases, the effect of Phase B and C is reduced andthe metallic shield losses in A phase are almost entirely dependent on the A

phase magnet ic flux.

D-5. Methods of Minimizing Shield Losses

There are two general ways that the amount of shield losses can beminimized:

a.)Single-point grounding (open-circuit shield)b.)Reduce the quantity of metal in the shield.

The single-point grounding or open circuit shield will not result incirculating shield current but the voltage induced in the shield increases from

zero at the point of grounding to a maximum at the open end that is remote fromthe ground. The magnitude of the induced voltage is primarily dependent on the

amount of current in the central or phase conductor. It follows that there are twocurrent levels that must be considered as follows:

a.)Maximum normal currentb.)Maximum fault current in designing such a system. The amount

of voltage that can be tolerated depends on safety concerns and

jacket designs.

Another approach is the use of a shield having higher resistance thancopper. Since the shield c ircuit is basically a one-to-one transformer, an increase

of resistance of the shield gives a reduction in the amount of current that willflow in the metallic shield. Bronze and other copper alloys have been used for

the metallic shield as these have resistivities higher than copper. M.V. and H.V.cables are manufactured with shield material and its thickness in accordance with

industry standards.

As pointed out earlier, PEC and NEC have data on the ampacities ofconductors based on one point grounding only and nothing for multi-point

grounding.

Hence, discussions will be centered on induced voltages in the shield withthe shield grounded at one-point only to eliminate shield which can reduce cable

ampacity. There are other types of grounding schemes that are possible and arein service. Generally they make use of special transformers or impedances in the

ground leads that reduce the circulating current in the shields because of theadditional impedance in those leads. These were very necessary years ago when

the protective jackets of the cables did not have the high electrical resistance andstability that are available today.


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D-6. Calculations for the Shield Voltage Levels

When single-conductor shielded power cables are installed inseparate ducts or otherwise separated from each other a few inches,

current flowing in the central conductor will induce a voltage in the

metallic shield. Three cable arrangements will be assumed as follows:

Fig.10

Equilateral Triangle Configuration

Fig.11

Flat Configuration without Transposition

Fig.12

Right Angle or Rectangular Configuration without Transposition

CB

A

S

S

CBA

S S

CA

B

SS

S


37/48



1.0) For the equilateral triangle configuration (see Fig.10), the metallic shieldis grounded only at one end and the other points insulated from ground.

The magnitude of the induced voltage is given by:

Vshi = Ic Xm

where:

Vshi = Shield voltage in volts to neutral per foot of cable.Ic = Current in central conductor in amperes.

Xm = Mutual reactance between conductor and shield, Micro-ohm/ft.

Xm is calculated from the formula:

Xm = 52.93 log10 micro-ohms/ft

where:

S = cable spacing in inchesrm = mean radius of the shield. This is the distance from the center

of the conductor to the mid-point of the shield.

For the more commonly encountered cable arrangements such as a 3 -phasecircuit, other factors must be brought into the equations. Also, phases A

and C have same induced voltage while phase B has a different induced

voltage value. This assumes equal current in all the 3 phases and a phaserotation of A, B and C.

2.0) The flat configuration of cables without transposition (See Fig.11) iscommonly used for cables in a trench but this could be a duct bank

arrangement as well.

The induced shield voltage in A & C phases are calculated asfollows:

Vsh i = 3Y2

+ (Xm a)2

where:Vshi = shield induced voltage on A & C phases in micro-volts to

neutral per foot.Ic = current in each phase central

Y = Xm + a

where Xm = 52.93 log10 in micro-ohms /ft for 60 Hz operation.

Ic

2


38/48



a = Constant = 15.93 Micro-ohms/ft for 60 Hz operation.

S = Cable spacing in inches.

rm = mean radius of the shield in inches. This is the distance from thecenter of the central conductor to the mid-point of the shield.

The induced shield voltage in B phase is the same as for the equilateral

triangle configuration which is as follow:

Vshi = Ic (Xm) in microvolts/footwhere:

Ic = amperes in the central conductor

Xm = 52.93 log10 microvolts to neutral per foot

S = Cable spacing in inches.

rm = mean radius of the shield in inches. This is the distance fromthe center of the central conductor to the mid-point of theshield.

3.0) For the right-angle or rectangular configuration without transposition

(See Fig.12) is a probable configuration for large single-conductor cablesin a duct bank.

The induced shield voltages in A & C phases are determined as follows:

Vshi = 3Y

2

+ (Xm - )

2

where:

Vshi = shield voltage in A & C phase in Micro volts to neutral perfoot

Ic = current in each phase central conductor (balanced 3-phase)

Y = Xm +

where Xm = 52.93 log10 micro-ohms/foot for 60 Hz operation.

a = constant = 15.93 Microohms per foot for 60 Hz operation

And the variables S and rm are the same definitions as for thecables in flat configuration discussed previously. The induced shield

voltage in phase B is given as follows:Vshi = Ic Xm

With Xm as calculated in the above formula.

Ic

2

a

2

a

2


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4.0) Two currents, Flat configuration with the 3-phase conductors on the same verticalarrangements. See Fig.12.

Fig. 12

Flat configuration, two-circuits without transposition

The induced shield voltages in A & C phases are determined as follows:

Vshi = 3Y2

+ (Xm - )2

where:

Y = Xm + a +

b = new constant = 36.99 micro-ohms per foot per 60Hz operations

All other designations have same values as in the previous cases.

5.0) Two circuits, flat configuration with two phase conductors on opposite verticalarrangement.

Fig. 14

Two circuits, flat configuration with two phase conductorson opposite vertical arrangement.

The induced shield voltage is given below:

Vshi = 3Y2

+ (Xm - )2

where:

Y = Xm + a -

All other designations have same values as in the previous cases.

Ic

2

b

2

b

2

Ic

2b

2

b

2

A B C

A B CS

SS

C B AS

SS

A B C


40/48



D-8. Importance of Jacket Stress Determination

Under normal conditions, the shield induced voltage will probably be ofno great concern but under high load current and particularly currents of fault

magnitude, it is conceivable this voltage may reach a value that will overstress the

cable jacket.As previously discussed, the single-point grounding will produce aninduced shield voltage at the free end which is proportional to the current in the

central conductor and the distance from the ground point of the shield to he freeend of the shield.

The jacket average voltage stress may be computed as follows:

Sj =

where: Sj = average stress of cable jacket in volts per Mil caused by the inducedvoltage Vshi.

t = Jacket Thickness in Mils.

The maximum stress on the jacket can be determined by the followingequations.

Sjmax =

where:

d = Shield outside diameter in inches.D = Jacket outer diameter in inches

Sjmax = maximum voltage stress in volts/Mil

Vshi = Ic [52.93 log10 ] microvolts/ft for 60 Hz operation.

Sample Problem:

3 Single-Conductor Shielded Power Cables are installed on equilateraltriangle configuration on 3- inch cable spacing.

Cable data are as follows:

Thickness of jacket on the shield, T = 50 Mils

Thickness of shield material, t = 5 MilsOutside Diameter of shield = 1.0 inch

Length of Cable = 600 ftFault Current, Ic = 30,000 amps line-to-ground fault

Find the shield induced voltage and jacket voltage stress under fault

conditions.

Vshi

t

0.000868 Vshid log10D

d


41/48



Solution:

Shield Outside Diameter = 1.0

Shield thickness, t = 5 mils = 0.005 inchJacket Thickness, T = 50 mils = 0.05 inch

S = Cable Spacing = 3.0 inches

ThenVshi = Ic (52.93 log10 )) microvolts/ft

For equilateral triangle configuration for cables.

rm = mean radius of shield

=

= = 0.4975 inch

For 600 ft,

Xm = 52.93log10 x 600 x10-6

= 52.93 (0.780328)

= 0.02478 ohms

Then, Vsh i = Ic Xm

Ic = 30,000 Amps

Vshi = 30,000 (0.02478)= 743.40 Volts

3.0

0.4975

CA

B

S = 3.0

Srm

Shield Outside Diameter t

21 0.005

2


42/48



Then average stress, Sj = = = 14.808 Volts/Mil

Then, maximum jacket voltage stress for a 50 Mil jacket thickness will be:

Sjma x =

0.000868 (743.40) u

1.0 log10 1.0 + 2 (0.05)1.0

0.64529 u

1.0 (0.0413926)

= 15.589 volts/mil of jacket thickness

The cable manufacturer shall be consulted for their maximum voltagestress on the jacket. A safe value for the shield induced voltage is about a

maximum of 120V under normal operating conditions.

E. PURPOSE AND TYPE OF CABLE TERMINATION

Discussions on this subject will address the design, application and

preparation of cables that are to be terminated. The application of this materialwill cover medium & high voltage cable system.

E-1. Purpose of Termination

Because medium & high voltage power cables are shielded, special

method are required to connect them to devices or other cables. This method iscalled termination which is a way o preparing the end of a cable to provide

adequate electrical & mechanical properties. These essential requirements includethe ff:

E-1.1) Electrically connect the M.V. & high voltage cable conductor toelectrical equipment bus, or non-insulated conductor.

0.000868Vshi

d log10 D u

d

Vsh i

T

743.40

50

=

=


43/48



E-1.2) Physically protect and support the end of the cable conductor,insulation shielding system and overall jacket, shield or armor of

the cable.E-1.3) Effectively control electrical stresses to provide both internal and

external dielectric strength to meet desired insulation levels for the

cable system.The current carrying requirements are the controlling factors in the

selection of the proper type and size of the connector or lug to be used. Variations

in these components are related to the base material used for the conductor withinthe cable, the type of termination used, and the requirements of the electrical

system.

The physical protection offered by the termination will vary considerably,depending on the requirements of the cable system, the environment, and the type

of termination used. The termination must provide an insulating cover at he cableend to protect the cable components (conductor, insulation and shielding system)

from damage by any contaminants that may be present including gases, moistureand weathering shielded medium or high voltage cables are subject to unusual

stresses where the shield system is ended just short of the point of termination.This can be elaborated further as follows:

Wherever a medium or high voltage shield power cable is cut, the end ofthe cable must be terminated so as to withstand the electrical stress concentration

that is developed when the geometry of the cable has changed. The electricalstress or voltage stress is described as lines of equal length and spacing between

the conductor shield and the insulation shield. As long as the cable maintains thesame physical dimensions, the electrical stress will remain consistent. When the

cable is cut, the shield ends abruptly and the insulation changes from that in the

cable to air. The concentration of electrical stress is now in the form as shown inFig. 1 with the stress concentrating at the conductor and insulation shield.

FIG. 1Electrical Stress Field, Cut End

Insulation

Conductor

Shield (Metallic &

Semiconducting)

Electrical

Stress Field

Electrostatic

Flux Lineson radial formation


44/48



In order to reduce the electrical stress at the end of the cable, the insulationshield is removed a sufficient distance to provide the adequate leakage or creep

age distance between the conductor and shield. The said distance is dependent onthe voltage involved as well as the anticipated environmental conditions. At the

point where the shield is stopped, the dielectric filed is no longer confined to the

cable insulation but rather distributes itself between the conductor and the ground.Longitudinal electrical stress will be introduced over the surface of the cableinsulation. The voltage distribution insulation with the shield removed is shown in

Fig. 3. As shown in Fig. 2, it is apparent that a high concentration of longitudinaland radial electrical stresses will occur where the shield ends.

Fig. 2

Electrical Stress Field, Shield Removed

In most case, this local breakdown in the insulation known as partial

discharge which can cause erosion of the insulation and ultimately completebreakdown in the cables.

Fig.3 shows the voltage distribution in the insulation with cable shield

removed.

Longitudinal Stress

Radial Stress

End of Shield

Conductor

Semiconducting and

Metallic Shield

Insulation


45/48



Fig. 3

Voltage Distribution in the Insulation with Cable Shield Removed.

The high electrical stresses can be controlled and reduced to a value within

the safe working limits of the materials used for termination. The most commonmethod of reducing these stresses is to gradually increase the total thickness of theinsulation at the termination by adding, over the insulation, a pre molded rubber

cone or insulating tapes to form a cone. This form is commonly called a stresscone. This function can also be accomplished by using a high dielectric constant

material, as compared to that of the cable insulation either in tape form or premolded tube applied over the insulation in this area. See Fig.4.

Fig. 4Stress cone using High Dielectric Constant Material

Conductor

Insulation

Shield Pre-molded stress control tube


46/48



Some of the newer terminations do not require a stress cone. The pre-molded tube is the usual type being presently used. The cold-shrink 3M scotch

brand and the heat shrinkable Raychem brand are of pre-molded tube type on themarket. This method results in a low stress profile and is referred to as

capacitance stress control. The stress cones are becoming less popular than the

pre-molded tube of high dielectric constant material because it is easy to apply.Fig. 4 shows a basic cross section of the pre-molded type as applied over ashielded power cable. This type is used for indoor installation. For outdoor

installation such as in weather exposed areas, additional creepage distance fromthe conductor terminal lug to the grounded shield of the cable will be gained by

using a non-melting insulation skirts or rain hoods between the stress controlassembly and conductor lug. The insulation is usually a track-resistant material

like silicon rubber.

Heat-shrink pre-molded stress control must be slipped over the cable priorto installing the conductor lug. As the name implies, heat is applied to shrink the

pre-molded stress control tube assembly.

Cold-shrink pre-molded stress control tube has a removable liner made ofpolypropylene (for 3M brand) that is pulled out and the tube collapse over the

underlying surface. Please note that the tube overlaps the end of the shield at anappropriate distance. Fig. 5A and 5B below shows a typical 3M scotch brand

cold-shrink outdoor termination kit with rain hoods or skirts. Indoor type has norain hoods or skirts (see Fig. 6A and Fig. 6B). Please note the pre-molded tube

overlapping the semi-conducting insulation shield.

Fig. 5A3M Brand Outdoor Type Cold-Shrink Termination


47/48



Fig. 5B

3M Brand Cold-Shrink Termination with Removal Liner

Fig. 6A

3M Brand Indoor Type, Cold-Shrink Termination Kit

Fig. 6B

3M Brand Indoor Type, Cold-Shrink Termination Kitwith Removable Liner


48/48

IIEE Cebu Chapter Seminar on Electric Power Cables for Low Voltage and Medium Voltage up to 69KVFig.7 (below) shows an actual installation of outdoor cold-shrink type

termination.

Fig.7

Outdoor Cold Shrink Type 11kV polymeric insulated cables(XLPE & EPR)

Manufacturers instructions must be followed so that proper installation of

the stress control tube will be effected.

iiee_electric power cables

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