IIEE_Electric Power Cables

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

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. insulation. The insulating material of a cable is the most important single component, the purpose of which is to preve pr eve nt the th e flo w o f elec el ec tr icit y fro m the th e en erg er g ized iz ed co nduc nd uc tors to rs to gro und or to an adjacent conductor. The insulation must be able to withstand the electrical stresses produced by the alternating voltage and any superimposed transient voltage stresses on the conductor without dielectric failure and causing shortcircuit. The selection of insulation involves a number of factors, some of which area: a. ) b.) b. ) c. ) d. ) e. ) f.) g. )

Stability and length of life. Dielectric properties. Resistance to ionization and corona. Resistance to high temperature. Resistance to moisture. moisture. Mechanical strength. Flexibility.

Common insulation materials are Polyvinyl Chloride (PVC), Natural Polyethylene (PE), Cross-linked Polyethylene (XLPE) and Ethylene Propylene Rubber (EPR). Each of these materials has unique characteristics which render it suitable for particular application. Those cables are commonly used for ordinary industrial users and a limited number of cables such as the above mentioned cable materials can be used to fulfill practically all industrial applications. The voltage 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 rated cable (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, 13,200V or 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 46KV c. High voltage – above 46KV The low voltage cables are unshielded.

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

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Common among all classes of cables is the conductor. Commonly used conductor materials are copper and aluminum. Historically copper has been used for conductors of insulated cables due pr imar ima r ily il y to its desi de sira ra ble bl e elec el ectr tr ica l and an d mech me chan an ica l pro pr o pert pe rties ies . T he us e o f aluminum is based mainly on its favorable conductivity to weight ratio (highest among the electrical conductor materials) Aluminum has a s pecific gravity of 2.7 and 61.0% electrical electrical conductivity conductivity while copper has a specific specific gravity gravity of 8.90 and 100% electrical conductivity. The use of aluminum has a distinct advantage in 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 is required to supply a given load. This could easily result in larger size or greater number of conduits or supporting racks which would increase the installation cost. The principal difficulty encountered with aluminum conductors is that of making and maintaining satisfactory terminal connection. Aluminum exhibits the following three characteristics which gives rise to this difficulty: a) The first characteristic is that the surfaces of aluminum exposed to air immediately form an oxide coating which has a high resistance. This coating is what gives aluminum wire its excellent anti-corrosion prop pr op ert y. Th is o xid e co at ing ins u lat es the th e st rand ra nd o f a ca ble fro m o ne another and tends to insulate the conductor from the connector on the end of it. Even if the oxide coating is scraped off, it immediately reforms before connection can be made. This high resistance at the connection point results in excessive temperature rise. One method of overcoming this problem has been to dip the aluminum conductor in a special compound before clamping the terminal to it. This compound is grease containing small particles of zinc. When the terminal is clamped to the conductor, the zinc particle penetrate the oxide coating to make a good electrical contact also the grease excludes air which preve pr eve nt the th e re -fo r mat ion io n o f t he ox ide . b) The second problem with aluminum is that it is soft and exhibits a “cold flow” characteristic. For example, a normal terminal or connector can be properly applied to the end of an aluminum conductor. 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 o f the connection. This problem has been satisfactory solved by the use of spring-loaded connectors and special long barrel types of connector which clamped to a considerable considerable longer po rtion rt ion o f t he co nduc nd uc to r, thus th us min imiz im izing ing the th e co ld flo w char ch arac acte te rist ri st ic.


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c) The third problem with aluminum is that it is damaged by galvanic action when connected to other kind of metal and moisture is present. Such conditions occur when copper connectors are used with aluminum cable. Connector manufacturers have found that tin-plated copper alloy connectors eliminate eliminate the galvanic action. PEC specifies aluminum alloy series AA-8000 electrical grade, for XHHW, THW, THHW, THWN and THHN insulation. This has purity of 99.00 + with an electrical conductivity of 61% of that of copper. Type 1350 Aluminum alloy, 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%. Full hard drawn aluminum alloy is most often used in overhead lines due to its higher bre ak ing st reng re ngth th.. 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 physical rather than electrical services requirements. Resistance to force, such as crush, impact and abrasion become a predominant consideration, although good electrical electrical properties for wet location location are also needed. Ca bles are classified by the insulation’s operating temperature and insulation thickness. A list of the more commonly 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 remolded any number of times. The extrusion process for these materials requires that they be heated sufficiently to cause them to flow, but no significant reaction reaction takes p lace so that they will so ften when reheated. The insulation is mechanically tough oil, moisture and heat resistant and flame retardant cables. Under this cable type are the following as listed in PEC or NEC: TW – The maximum operating temperature is 60˚C 60˚C in wet or dry location. This type has no jacket. THW –The maximum operating temperature is 75˚C 75˚C in wet or dry location. This type has also no jacket. THWN – The maximum operating temperature is 75˚C 75 ˚C f or o r use in wet or dry location. This type has a nylon jacket. THHN – The maximum operating temperature is 90˚C 90˚C for use in dry d ry 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 crosslink it. The vulcanization cause a permanent chemical reaction so that the material will have very little tendency to soften if heated again. Crosslinked Polyethylene (XLPE) and Ethylene Propylene Rubber (EPR) are in this type. A-1.2.1 Cross-Linked Polyethylene (XLPE)

The 600V compounds of XLPE are usually filled with carbon black or mineral fillers to further improve the relatively good toughness of conventional or natural polyethylene. The combination of crosslinking through vulcanization plus fillers produces superior mechanical pro pr o pert pe rties ies . Vu lca lc a niz at ion io n el imina im ina te s the th e ma in draw dr aw bac k o f a low melt me lting ing po int of 105 10 5˚C for conventional or natural polyethylene. Also, usage of natural polyethylene has greatly been limited to circuits where overload and short circuit conditions are not critical. The 600V construction consists of copper or aluminum conductor with single extrusion of insulation in the specified thickness. The insulation is abrasion, moisture and heat resistant black XLPE. The natural po lyet lye t hyle hy lene ne insu in su lat ion for fo r po we werr cabl ca bles es had ha d been be en repl re plac acee d by the th e XLPE XL PE material. The insulation type has a strong effect on cable rating. From a thermal po int of vie w, a go o d ins u lat ion mat er ial ia l sho sh o uld ha ve low lo w ther th er mal ma l resistivity and should result in low dielectric losses. In this classification are the following: Type XHHW for 75°C maximum operating temperature in wet and 90°C in dry locations only. Type XHHW-2 for 90°C maximum operating temperature in wet and dry locations. A-1.2.2 Ethylene Propylene (EPR)

Rubber-like insulation such as ethylene propylene (EPR) and styrene buta bu ta d iene ie ne ru bber bb er (SBR (S BR)) co mpo unds un ds requ re qu ire ou te r jac ket ke t for fo r mech me chan an ica ic a l pr otec ot ectio tio n such su ch as PV PVC C and an d neo pr ene. en e. Re ce nt adva ad vanc ncee ment me nt in EP R insulation has improved physical properties that do not require any other jack ja cket et for mec hanic ha nic al prot pr ot ectio ec tio n.


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In this classification are the following: a) Type RHW for 75C maximum operating temperature in wet or dry location. b) Type RHW-2 for 90°C maximum operating temperature in wet or dry location. c) Type RHH for 90°C maximum operating temperature in dry locations only. All the preceding cables are suitable for installation in conduit, duct or other raceway and when specifically approved for the purpose may be installed in cable tray (1/0 AWG and larger) or direct –buried, provided NEC or PEC requirements are satisfied. The common conductor material used are copper or aluminum.

A-1.3 Current Carrying Capacity

The current carrying capacity or ampacity of a cable is defined as the maximum current it can carry continuously without the temperature at any point in its insulation exceeding the limit prescribe for it according to its 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 insulating material (dielectric). There are two possible paths for current to flow when measuring insulation resistance: a) Through the body of t he 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 radial flow 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 the current flowing t hrough the volume insulation resistance which reduces apparent volume insulation resistance unless measures are taken to eliminate that current when measurements are being made. This measure could be a “guard” circuit which will eliminate the surface leakage current from the volume resistivity measurements. Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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The volume insulation resistance at 60˚F is given by the following formula: 1000k log10 D /d R 60 =

Mega ohms L

Where: R 60 = insulation resistance at 60˚F 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 60˚F The following data for the values of K for common types of low voltage cable insulation materials are taken from ICEA (Insulated Cable Engineers Association of USA). THW (75˚C PVC) -------------------- 2,000 TW (60˚C PVC) ---------------------- 500 XHHW (600V XLPE) --------------- 10,000 RHW ----------------------------------- 4,000 EPR (600V) --------------------------- 10,000 Example Calculation: Assuming a type THW 500MCM copper power cable which has an insulation resistance constant of K= 2,000 and a length of 500ft. Find the insulation resistance.

1000K log10 D /d R I

= L 2, 000, 000 log10 =

25.03 20.65

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

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= There: R I

=

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4000 log10 (1.21156) 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 the leakage 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 be either enclosed in a grounded metallic shield or immersed in water. For temperatures other than 60˚F, a temperature correction factor will have to be applied by multiplying the insulation resistance in megohms as calculated as per equation stated previously by the te mperature convection factor. The te mperature correction factor Tc can be found in the following equation: Tc =

100.4343 a (t-60)

Values of a for some insulation materials are as follows: Natural P olyethylene (Ther mop last ic) – 0. 0 Silicone Rubber ---------- 0.03 XLPE and EPR (LV Thermosetting) – 0.0 For low voltage cables (600V) insulation resistance measurement, megohmeter voltage must be 500V to 1000V. A-2. MEDIUM AND HIGH VOLTAGE POWER CABLES

As listed in PEC & NEC, medium voltage cables are designated type MV and have solid extruded d ielectric insulation rated 2000V to 35,000 volts. Single conductor and multiconductor cable are available with minimal voltage rating of 5KV, 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 being insulated are shielded to confine and evenly distributes the electric field within the insulation. It is accomplished by means of conductor and insulation shields. Shielding o f power cables will be discussed thoroughly in the succeeding parts o f 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. Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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c) Direct earth buried. d) Where the cable surface may collect unusual amounts of conducting material such as s alt, 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

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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 co mposed 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 to pr ov ide a smo oth co nducting cy lin der ar ou nd the co ndu ctor. Typical of today’s cables, this layer is a semi conducting plastic po lymer wit h carbon filler, extruded dire ct ly over the co nduc tor. This layer represents a very smooth surface which, because of direct 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 two basic types used to day is cross- lin ked Po lyethylene (XLP E) or Ethylene (EPR). Both types have maximum operating temperature of 90˚C and maximum short-circuit temperature of 250˚C. XLPE and EPR are classified as thermosetting materials which do not soften 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 plain po lyethylene to give a “la dd er” ef fec t wh ich restric ts slipp age between molec ules and prod uces go od ther mal stability. Cross linking or vulcanization also means that the different long molecules of plain or natural polyethylene are linked together. See Figs. 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 the traditional method but radiation cross-linking is increasing in po pular ity for wir es and small ca ble s wh ere ins ulat ion thicknesses are not excessive. The most common method of cross-linking is by the addition of an agent such as peroxide to the plain polyethylene material which can be activated by heat. Details of the crosslinking 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 the pr esence of an elec tr ic fie ld, a mod ified version of XLP E, designated Tree retardant (TRXLPE) has replaced the use of XLPE for medium voltage application. TRXLPE is a very low loss dielectric that is reasonably flexible and has a maximum operating temperature limit of 90˚C or 105˚C depending on type.

Insulation Shield

Insulation Water Trees Conductor Shield Conductor

FIG. 3 Treeing in M.V. & H.V. Power Cable


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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 around the outside surface of the insulation. Typical shield compound is a polymer with carbon filler that is extruded directly over the insulation. This layer for medium voltage applications is not fully bonded to the insulat ion (str ippab le) to allo w relat ive ly ea sy removal for the installation of cable accessories such as cable termination. 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 compose d of wires, tapes, or corrugated tubes. This shield is connected to ground which keeps the insulation shield at ground potential and provides a return path for fault current medium voltage cable can utilize the metallic shield as the neutral return conductor if sized accordingly. Typical Shield sizing criteria: a.) Equal in capacity to the central conductor for single-phase application. b.) One-t hird the capacity for 3-phase applic ations. 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 for ph ysica l prot ect ion. This po lymer layer maybe extruded as a loo se tube or directly over the metallic shield (encapsulated). Although bo th provide ph ysical prot ect ion, the encapsu lated jac ket re moves the space present in a loose tube design which may allow longitudinal water migration. The typical compound use for jacket is linear low density polyethylene (LLDPE) because of its ruggedness and relatively low water vapor transmission rate. Jackets can be specified insulating (most common) or semi conducting (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-voltage rating of the cable insulation, its selection is made on the basis of the phase, Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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pha se vo lta ge of the system in wh ich the ca ble is to be applied whether the system 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 on ungrounded 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 than cable on grounded system where it is impossible to impose full line-to-line pote ntial o n the ot her two unfau lted phas e of an exte nded 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, but in any case within one minute. While these cables are applicable to the great majority of cable systems t hat are on grounded system, they may also be used on other system for which the application of cables is acceptable provided the abov e clearing requirement are met in completely de-energizing the faulted section. A-3.2 133% level.

This insulation level corresponds to that formerly designated for ungrounded systems. Cables in this category may be applied in situations where the clearing time requirements of the 100% category cannot be met and yet there is 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 strength over 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 for resonant 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 ratio over the 100% thickness. For example 133% insulation does not necessarily have 33% more thickness over 100% level thickness. PEC shows the thickness to be less then 133%. Ratings of low voltage cables as well as the medium voltage cable previou sly discu ssed in that they are also bas ed on phase-to-pha se operatio n. The practica l po int here is that a ca ble that op erate s at says 480 vo lts fro m phase-t oground on a grounded wye system requires an insulation thickness applicable to 480V x 1,732 volts phase-to-phase. This of course, means that a 1,000 volts level Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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of insulation thickness should be selected. There are no categories for low voltage 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 the applicable 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 its insulation 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 in calculating the insulation resistance. For the common medium voltage cables such as XLPE and EPR, the insulation 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-carrying capacity. 1. 0 Short-circuit current. 2. 0 Voltage drop calculations

The first consideration in the ampacity of cable which is a ffected by many things. Basically, the final consideration is the permissible operating temperature of the insulation. The higher the operating temperature of the insulation, the higher the current-carrying capacity of the cable. The temperature at which a particu lar cable will operate is affected by the ability of the surround ing mat er ial to conduct the heat away. Therefore, the current capacity is materially affected by the ambient temperatu re as well as by the installat ion co nditions,


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For example, assuming 40˚C ambient temperature a 3-conductor 4/0 AWG, 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 carry 289 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 closely together, the presence of other cables, in effect, increased the ambient temperature due to mutual heating which decreases the ability of the cables to dissipate its heat. PEC and NEC have correction factors for the ampacity of cables (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 cable is 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 is recognized, however that under fault conditions there will be an abrupt elevation in conductor temperature which will subject the insulation to a more severe thermal 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 will prevent severe permanent da mag e to cable insulat ion during an int erva l of fault current flow. Under short-circuit conditions the u ltimate conductor temperature depends on the following: a. The magnitude of fault current. b. Cross-sectional area o f the conductor. c. The duration of fault current flow. d. The conductor temperature before the s hort-circuit occurs. On the basis that all the energy produced during fault current flow is effective in raising the conductor temperature (since the time period is very short, this is a valid assumption for engineering purposes) the conductor heating is gove rned by the following equation.


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For copper 2

I

T 2 + 234

t = 0.0297 log 10 A

T 1 + 234

For Aluminum 2

I

T2 + 228

t = 0.0125 log 10 A

T 1 + 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 cable will elevate the copper temperature from 75˚C to 200˚C in approximately 1 second. With the current then reduced to zero as much as 3000 secs or 0.8333 hrs could be required for the 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 special importance 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 T 1 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 and maximum short-circuit temperature rating of cables (low voltage and medium voltage).


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Temperature Limits of Cables

Maximum Continuous Temperature Rating

Maximum Short-Circuit Temperature Rating

MV XLPE

90 C

250 C

MV EPR

90 C

250 C

TW

60 C

150 C

THW

75 C

150 C

THHN/THWN

90 C

150 C

RHW

75 C

200 C

RHW-2

90 C

200 C

XHHW

75 C

250 C

XHHW-2 Polyethylene (natural or conventional)

90 C

250 C

75 C

150 C

Type of Insulation

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

˚

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 of operation. 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 branch circuit 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 such loads. Below is a drawing for branch circuits and feeders. FIG.4 Panel Board Services Equipment or source of separately derived system

Feeders Feeders

Final Branch circuit over current protection Panel Board

Feeders and Branch Circuits Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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On alternating – current circuits where the cable is installed as single conductor 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 the conductors. 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. Eng’g calculation will have to be undertaken. The approximate cable voltage dro p formula is as follows: V = IR cosθ + IX sinθ 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. θ = Angle whose cosine is the load power factor. Cosθ = Load power factor, in decimals. Sinθ = 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 lineto-neutral voltage is to permit the line-to-line voltage to be completed by multiplying by the ff. constants:

Voltage System Single - phase 3 - phase

Multiply By: 2 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 type of raceway in which it is installed as obtained for the manufacturer. It depends on the ff: a.) Size of the conductor b.) 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 conductor b.) Whether the raceway is magnetic or non-magnetic. c.) Spacing of the conductor of the circuit. The spacing is fixed for multiconductor cables but may vary with single-conductor cables so that an average value is required. Reactance occurs because the alternating current flowing in the conductor causes a magnetic field to build up and collapse around each conductor in synchronism with the alternating current. This magnetic field as it builds up and fall rapidly, cuts across the conductors of the circuit, causing a voltage 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 condu ctor 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 and insulation. To be effective, the shield must adhere to or remain in intimate contact with the insulation under all conditions. In cables rated over 2000V, a conductor 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 insulation material. This will result in a symmetrical radial voltage stress within the insulation. b) To protect the cables from induced or direct over-voltages such as in connecting to overhead lines. Shields do this by making the surge impedance uniform along the length of the cable and by helping to reduce 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 be increased. 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 but the 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, will generate surface discharge and convert atmospheric oxygen into ozone which may be destructive to the insulation and protective jackets. If the surface of the cable 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 will occur, causing ozo ne formation.


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The ground may be either a metallic conduit or a damp non-metallic conduit 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 surface of 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 to cause burning of the insulation or jacket. Where non-shielded, non-metallic jacketed ca bles are used in underground ducts containing several circuits which must be worked on independently, the external field if sufficiently intense can cause shocks to those who handle or contact energized cable. In cases o f this kind, it may be advisable to use shielded cable. Shielding used to reduce hazards shock should have a resistance low enough to operate protective equipment in case of fault. In some cases, the efficiency or protective equipment may require proper size ground wires as supplement to shielding. The same considerations apply to exposed installations where cables may be handled by performed who may not be acquainted with the hazards 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 compounds or 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 semiconducting tapes that were helically wrapped over the conductor. This is done, especially on large conductors, in order to hold the strand together firmly during the application of the extruded semi-conducting material that is now required for medium voltage cables. Experience with cable that only had semi conducting tape was not satisfactory, so the industry changed their requirements to call for an extruded layer over the conductor. Present day extruded layers are not only clean (from undesirable impurities) but are very smooth and round. This has greatly reduced the formation of “Water Trees” (Refer to Fig.3) that could originate from irregular surfaces. By extruding the two layers (conductor shield and insulation) at same time, the conductor shield and insulation are cured at the same time. This provides the inseparable bond that minimizes the chances of the formation of a void at the critical area between the co nducto r shield and ins ulat ion surfac e ad jacent to the conductor. For compatibility reasons, the extruded shielding layer is usually made from same or a similar polymer as the insulation material. Special Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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carbon black is used to make the layer over the conductor semi conducting material to provide the necessary conductivity. Industry standards (ICEA and NEMA) require that the conductor semi-conducting material 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 to insure that the layer remains conductive and hence provides a long cable life. A water-impervious material can be incorporated as part of the conductor shield to prevent radial moisture transmission. This layer consists of a thin layer of aluminum or lead sandwiched between semiconducting materials. C-4. Insulation Shielding

The insulation shield for a medium voltage cable is made up of two components: C-4.1. A semi conducting or stress relief layer. C-4.2. A metallic layer of tape or tapes, drain wires, concentric neutral wires, braid, sheath or metal tube. This metallic shield must be non-magnetic. The two components mentioned above must function as a unit for a cable 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 layer and is called this extruded insulation shield or screen. Its properties and compatibility requirement are similar to the conductor shield previously described 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 the voltage stress at that interface is lower than at the conductor shield interface. This layer is not fully bonded (strippable) to the insulation for voltage up to 35KV. Above 35KV, this layer is fully bonded to the insulation, which req uires shaving tools t o remove. The metallic portion of the insulation shield or screen is necessary to provide a low resistance path for charging cu rrent to flow to the grou nd. It is important to realize that the extruded semi-conducting shield material will not survive a sustained current flow of more that a few milliamperes. Those materials are capable of handling the small amounts of charging current but cannot tolerate unbalanced or fault current. Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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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 of metal in the shield of a single-conductor cable is costly due to following: a) The additional metal over the amount that is actually required increase the 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 central conductor. The higher the shield losses which increase the heat in the cable thus the capacity of the cable will decrease. These conditions will be discussed thoroughly on the section entitled Grounding of Cable Shields. Shielding of low voltage cables is generally required where inductive interference can be a problem. In numerous communication, instrumentation and control application, small electrical signals may be transmitted on t he cable conductor and amplified at the receiving end. Unwanted signals (noise) due to inductive interference can be as lar ge as the de sired signal. Thus ca n resu lt in false signals or audible no ise that 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 o f 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 are defined as follows: a) Sheath refers to a water impervious, tubular metallic component of a cable that is applied over the insulation. Examples are a lead sheath and a corrugated 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 co mponent o f a cable that must gro unded to confine 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, s heaths 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 are concentrating on the commonly used XLPE and EPR cables, t he term shields will be used in th is discu ssion.


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As discussed earlier in the section on shielding, for personal safety the metallic shield must be grounded due to induced voltage which will be d iscussed later. However, grounding of the cable shields must take into consideration the shield losses which may effect reduction of cable ampacity. There are two methods of grounding the cable shields namely: single po int and multi-po int ground ing systems. PE C & NEC ha ve ampacit y ta ble s based on sing le-point grounding wh ich will not effec t cir cu lat 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 a multi-point grounding of shields due to conflicting methods by engineers and researchers. PEC & NEC advised that if the shields are grounded at more than one 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 (500mm²) and larger and with ant icipat ed loads of over 500 amperes. A cable may be considered a transformer. When alternating current flows in the central conductor of a cable, that current produces electromagnetic flux in the metallic shield, if present, or in any parallel conductor. This becomes oneturn 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

Metallic Shield

IC

FIG. 5 Single-Point Grounding of 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 central conductor 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 gro und connection increases. See Fig.6.

Voltage

Distance

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 of conducting material, eddy currents will be produced therein. These currents circulate in the shield. However, eddy currents are not of significant amount as the metallic shield are non-magnetic such as copper or aluminum which has a much higher magnetic reluctance than the ferromagnetic materials such as iron, nickel, cobalt,etc. Generally, eddy currents can be neglected. In the connection diagram of Single Point Grounding of the metallic shield (See Fig.5), there is no close circuit and therefore no, induced circulating current in the shield. This set-up avoids the considerable heating of the metallic shield due to the circulating current flowing along the metallic shield and returning through the ground (See Fig. 7 for multi po int grou nding). The losses in th e sh ield du e to the cir cu lat 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 mus t be taken to ins ulate and provide surge protection at the free end of the shield to avoid danger from the induced voltages.


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D-2. Multi-Point Grounding

Metallic Shield IC

Vshi = 0

Vshi = 0

I circulating

Fig.7 Multi-Point Grounding of Metallic Shield

If the metallic shield is grounded two or more times or otherwise completes 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 the shield, 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.8 below.

Voltage 0

Distance

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 circulating current 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 the resistance 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 also a factor. Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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D-3. Metallic Shield Losses

A very important factor that affects much the ampacity of the cable is the metallic 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 a s with 60Hz a lternating current, the magnetic field expands and contracts with the current magnitude. In the event that a second conductor is within the magnetic field of the current-carrying conductor, a voltage that varies with the field will be introduced in that conductor. If that conductor is part of a circuit, the induced voltage will result in current flow. This situation occurs during operation of metallic shielded conductors. Current flow in the phase conductors induces a voltage in the metallic shields of all the phase cables within the magnetic field. If the shields have 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 the losses depends on the shield resistance and the current magnitude. This loss appears as heat. These losses not o nly represent a economic loss, but they have a negative effect on ampacity and voltage drop. This has the effect of reducing the permissible phas e co nducto r current . In ot her words, shield loss reduce s the allowable 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 net current 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 the case, 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. The shield on each cable approaches the total flux linkage created by the phase conductor of that cable. Refer to Fig.9.

A

B

S

S

C

Fig.9 Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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As the spacing, S, increases, the effect of Phase B and C is reduced and the metallic shield losses in A phase are almost entirely dependent on the A pha se mag net ic flu x. D-5. Methods of Minimizing Shield Losses

There are two general ways that the amount of shield losses can be minimized: 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 in circulating shield current but the voltage induced in the shield increases from zero at the point of grounding to a maximum at t he open end that is remote from the 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 two current levels that must be considered as follows: a.) Maximum normal current b.) Maximum fault current in designing such a system. The amount of voltage that can be tolerated depends on safety concerns and jacket desig ns. Another approach is the use of a shield having higher resistance than copper. Since the shield c ircuit is basically a o ne-to-one transformer, an increase of resistance of the shield gives a reduction in the amount of current that will flow 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 of conductors based on one point grounding only and nothing for multi-point grounding. Hence, discussions w ill be centered on induced voltages in the shield with the 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 are in service. Generally they make use of special transformers or impedances in the ground leads that reduce the circulating current in the shields because of the additional impedance in those leads. These were very necessary years ago when the protective jackets of the cables did not have t he high electrical resistance and stability that are available today.


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D-6. Calculations for the Shield Voltage Levels

When single-conductor shielded power cables are installed in separate 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:

B

S A

S

S C

Fig.10 Equilateral Triangle Configuration

A

B

S

C

S

Fig.11 Flat Configuration without Transposition

A

S B

C

S Fig.12 Right Angle or Rectangular Configuration without Transposition


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

The magnitude of the induced voltage is given by: V shi = I c X m where: V shi Ic Xm ohm/ft.

= Shield voltage in volts to neutral per foot of cable. = Current in central conductor in amperes. = Mutual reactance between conductor and shield, Micro-

X m is calculated from the formula:

X m = 52.93 log10

micro-ohms/ft

where: S = cable spacing in inches r m = 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 -phase circuit, 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 phase rotation of A, B and C. 2.0) The flat configuration of cables without transposition (See Fig.11) is commonly 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 as follows: Ic 2 2 V sh i = 3Y + (Xm – a)

2

where: V shi = shield induced voltage on A & C phases in micro-volts to neutral per foot. I c = current in each phase central Y = Xm + a where Xm = 52.93 log 10

in micro-ohms /ft for 60 Hz operation.


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a = Constant = 15.93 Micro-ohms/ft for 60 Hz operation. S = Cable spacing in inches. r m = mean radius of the shield in inches. This is the distance from the center 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/foot where: Ic = amperes in the central conductor Xm = 52.93 log10

microvolts to neutral per foot

S = Cable spacing in inches. r m = mean radius of the shield in inches. This is the distance from the center of the central conductor to the mid-point of the shield. 3.0) For the right-angle or “rectangular” configuration without transposition (See Fig.12) is a probable configuration for large single-conductor cables in a duct bank.

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

V shi =

Ic 2

2

a

3Y + (Xm - 2 )

2

where: V shi = shield voltage in A & C phase in Micro volts to neutral per foot I c = current in each p hase central conductor (balanced 3-phase) Y = Xm +

a 2

where Xm = 52.93 log 10

micro-ohms/foot for 60 Hz operation.

a = constant = 15.93 Microohms per foot for 60 Hz operation And the variables S and r m are the same definitions as for the cables 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. Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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

S

A

B

C

A

B

C

S

S

Fig. 12 Flat configuration, two-circuits without transposition The induced shield voltages in A & C phases are determined as follows:

V shi =

Ic 2

b

2

3Y + (Xm - 2 )

2

where: Y = X m + a + b 2 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 vertical arrangement.

S

A

B

C

C

B

A

S

S

Fig. 14 Two circuits, flat configuration with two phase conductors on opposite vertical arrangement. The induced shield voltage is given below:

V shi = where:

Ic 2

2

b

3Y + (Xm - 2 )

2

Y = X m + a - b 2 All other designations have same values as in the previous cases.


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D-8. Importance of Jacket Stress Determination

Under normal conditions, the shield induced voltage will probably be of no 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 an induced 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 free end of the shield. The jacket average voltage stress may be computed as follows: S j = Vshi t where: S j = average stress of cable jacket in volts per Mil caused by the induced voltage Vshi. t = Jacket Thickness in Mils. The maximum stress on the jacket can be determined by the following equations. 0.000868 Vshi S jmax = d log10 D d

where: d = Shield outside diameter in inches. D = Jacket outer diameter in inches S jmax = maximum voltage stress in volts/Mil Vshi = Ic [52.93 log 10

] microvolts/ft for 60 Hz operation.

Sample Problem:

3 Single-Conductor Shielded Power Cables are installed on equilateral triangle 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 Mils Outside Diameter of shield = 1.0 inch Length of Cable = 600 ft Fault Current, I c = 30,000 amps line-to-ground fault Find the shield induced voltage and jacket voltage stress under fault conditions.


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Solution:

B

A

C

S = 3.0” Shield Outside Diameter = 1.0” Shield thickne ss, t = 5 mils = 0.005 inch Jacket Thickness, T = 50 mils = 0.05 inch S = Cable Spacing = 3.0 inches Then V shi = I c (52.93 log 10

S )) microvolts/ft r m

For equilateral triangle configuration for cables. r m = mean radius of shield = =

Shield Outside Diameter – t 2 1 – 0.005 = 0.4975 inch 2

For 600 ft, X m = 52.93log10

3.0 0.4975

x 600 x10

-6

= 52.93 (0.780328) = 0.02478 ohms Then, Vsh i = I c X m I c = 30,000 Amps V shi = 30,000 (0.02478) = 743.40 Volts


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Then average stress, S j =

V sh i T

=

743.40 50

= 14.808 Volts/Mil

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

S jma x =

=

=

0.000868Vshi d log10 D u d

0.000868 (743.40) 1.0 log10 1.0 + 2 (0.05) 1.0 0.64529 1.0 (0.0413926)

u

u

= 15.589 volts/mil of jacket thickness The cable manufacturer shall be consulted for their maximum voltage stress 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 material will 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 is called termination which is a way o preparing the end of a cable to provide adequate electrical & mechanical properties. These essential requirements include the ff: E-1.1) Electrically connect the M.V. & high voltage cable conductor to electrical equipment bus, or non-insulated conductor.


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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 within the 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 cable end to protect the cable components (conductor, insulation and shielding system) from damage by any contaminants that may be present including gases, moisture and 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 of the cable must be terminated so as to withstand the electrical stress concentration that is developed when the geometry of the cable has changed. The electrical stress 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 the same 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 in Fig. 1 with the stress concentrating at the conductor and insulation shield.

Insulation

Shield (Metallic & Semiconducting)

Electrical Stress Field

Conductor

FIG. 1 Electrical Stress Field, Cut End Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

Electrostatic Flux Lines on radial formation

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In order to reduce the electrical stress at the end of the cable, the insulation shield is removed a sufficient distance to provide the adequate leakage or creep age distance between the conductor and shield. The said distance is dependent on the 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 cable insulation. 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 longitudinal and radial electrical stresses will occur where t he shield ends. Longitudinal Stress

Conductor

Radial Stress Insulation

End of Shield

Semiconducting and Metallic Shield 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 complete breakdown in the cables. Fig.3 shows the voltage distribution in the insulation with cable shield removed.


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Fig. 3 Voltage Distribution in the Insulation with Cable Shield Re moved.

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 common method of reducing these stresses is to gradually increase the total thickness of the insulation 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 stress cone. 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 pre molded tube applied over the insulation in this area. See Fig.4. Shield

Pre-molded stress control tube Conductor

Insulation Fig. 4 Stress cone using High Dielectric Constant Material Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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Some of the newer terminations do not require a stress cone. The premolded 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 the market. 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 a shielded power cable. This type is used for indoor installation. For outdoor installation such as in weather exposed areas, additional creepage distance from the 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 control assembly 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 prior to 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 of polypropylene (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 an appropriate 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 no rain hoods or skirts (see Fig. 6A and Fig. 6B). Please note the pre-molded tube overlapping the semi-conducting insulation shield.

Fig. 5A 3M Brand Outdoor Type Cold-Shrink Termination


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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 Kit with Removable Liner Seminar Resource Speaker : Engr. Oscar P. Pasilan, P.E.E. No. 0573)

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