C17 Spring DraftGuideforPlanningandDesigningTransitionFacilities

8/10/2019 C17 Spring DraftGuideforPlanningandDesigningTransitionFacilities

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Last saved by DJohnson March 23, 2010 IEEE P1793/D1

Copyright 2009 IEEE. All rights reserved.

This is an unapproved IEEE Standards Draft, subject to change. 7

Guide for Planning and Designing TransitionFacilities between Overhead and Underground

Transmission Lines

1. Overview

The overhead to underground transition facilities of a hybrid overhead/underground transmission line mustbe carefully planned and designed. The purpose of this guide is to provide general recommendation of thefactors that need to b e considered in the planning and d esign of the transition facility. This guide assumes

that the decision to install an underground section has already been determined and this guide is intended to

provide guidance in the selection and in stallation of th e appropriate transition facility. Although this guideis primarily intended for use on transmission class underground cable circuits (69 kV and higher), some ofthe information presented herein can be applied to distribution class electric power cable systems as well.

1.1 Introduction

Underground cable s ections ar e s ometimes n ecessary in o verhead t ransmission l ines. A n und ergroundsegment may be needed to avoid environmentally sensitive areas, to cross obstacles such as rivers or majorhighways, to cross airport runway safety zones, or to permit other land uses that would not be feasible withoverhead l ines. W hen a n und erground s egment i s a dded to a n o verhead tr ansmission line, a transitionfacility is required. The transition facility must provide a means to terminate the overhead transmission line,terminate the underground cable, and accommodate any ancillary systems associated with the underground

cable. Underground cables have electrical and operating characteristics, which are different from those ofoverhead lines, and which can affect the design of transition facilities. Underground transitions facilities areneeded for short underground sections (dips), which might be measured in the hundreds to thousands offeet. Transition facilities are also requires for longer underground segments, which are measured in miles.The length of the underground segment can affect the transition facility design. Overhead to undergroundtransition facilities have p lanning, s iting, d esign, and construction c onsiderations that must be evaluated

beginning in the initial stages of a transmission line project.

1.2 The Scope of the Guide

This gui de p resents factors to be considered in the p lanning a nd d esign o f tr ansition f acilities b etweenoverhead and underground transmission lines. These include the system implications of a hybrid installationas they relate to the transition facility.

While this document focuses on transmission lines only, some of the considerations listed in this guide arecommon to both transmission and distribution installations.

1.3 The Purpose of the Guide

The purpose of this guide is to list and describe typical factors that should be considered in the planning anddesigning of transition f acilities between overhead and underground transmission lin es. Some of t hese


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factors r elate to th e in stallation, o peration a nd m aintenance o f th e tr ansmission s ystem. This gui de isintended to be comprehensive, but may not be all inclusive.

2.0 Planning

Transition facilities for overhead to u nderground transmission lines can be as s imple as an overhead line

dead-end pole, w ith f acilities f or a ttaching a tr ansmission c able, o r they can b e as ex tensive as a l argetransition s tation. Because o f t he el ectrical ch aracteristics of underground transmission cable, anunderground system may often trigger the need for installation of additional equipment in order to deal withvoltage control, thermal rating issues, switching requirements, and o ther factors. Often, these factors maytrigger t he ne ed for a tr ansition s tation. Planning i s n eeded t o car efully co nsider an d establish variouselectrical system and cable system requirements. Such factors include the following:

Type of u nderground c able system to b e installed: extruded dielectric, high-pressure fluid-filledpipe-type (HPFF), high-pressure gas-filled pipe-type (HPGF) or self-contained fluid-filled (SCFF)

Requiring multiple cables to match an overhead line for either capacity or reliability

Switching requirements/switching equipment between the overhead and underground lines

Need for reactive compensation (shunt reactors) for the underground cable

Future capacity upgrades (for example, overhead line re-conductoring)

For pipe-type cables, the need for a p ressurizing plant with its power supply, alarm, control, andmonitoring requirements, and possibly circulation equipment and forced cooling equipment

For self-contained liquid-filled cables, the need for pressurizing equipment.

Ability to repair underground cable failures while leaving the overhead line and/or other cables inservice.

For self-contained and extruded cables, t he facilities r equired for b onding and grounding of thecable s ystem. Pipe t ype ca bles w ould also have b onding r equirements as w ell as cat hodicprotection requirements.

Once the necessary components have been identified, the layout of the transition site or structure can bedetermined to provide an efficient design that incorporates site selection, community/environmental impact,safety, construction, maintenance and physical constraint considerations. This document will discuss factorsto consider for either type of transition.

2.1 Site Selection

When planning for an underground to overhead transmission line transition, be it a single transition pole or

a more complex transition station, one must consider several issues relating to the siting of the structure orstation. Some of the issues to be considered are:

Community, Environmental and Permitting Considerations

Physical Site Considerations

Economic Considerations


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2.1.1 Community/Environmental Impact Considerations

A primary consideration when siting an underground to overhead transmission facility is to be aware of, andto the extent possible, avoid activity in or near the following types of environmentally sensitive areas.

Wetland ar eas Wetlands p rovide h abitat f or m any p lant a nd w ildlife s pecies in a ddition to

providing a method for replenishing the earths reserve of fresh water. There are threecharacteristics that define a wetland, 1) s tanding water, 2) aquatic vegetation an d 3) h ydric (orhydraulic) soils. If any of these three characteristics ex ist, t hen t here i s a h igh p otential f or awetland. T hese types of areas are federally protected, and may b e protected o n state and locallevels. Various permits are required if a wetland is to be disturbed.

Archeological sites Construction activity in and around known archeological sites is regulated bya State H istorical P reservation O fficer ( SHPO). If the SHPO kno ws o f o r s uspects t hat a narcheological site exists at or near the site o f th e p lanned f acility, further in vestigation m ay b erequired. An archeologist may need to b e en gaged t o d eal w ith t he p rocess o f w orking i n o r

around these sites.

Land Contamination sites Pollutants that have contaminated land generally come from industrialprocesses an d s torage f acilities. D uring s ite s election, each site should be screened for thepotential o f h aving c ontaminated s oils. L ikely areas i nclude ex isting o r ab andoned i ndustrial

developments or l and adjacent to t hese developments. A lso, land adjacent to commercial marineactivities and railroads has a high potential to c ontain contaminated soils. If a transition facility isto be sited in an area with contaminated soils, permits may be required, and special constructiontechniques m ay b e n ecessary f or w orker p rotection, cab le s ystem protection, and long termmaintenance.

Public Lands Siting of high voltage transition structures or s tations on p ublic lands require theapplication of al l the necessary permits and certificates including acquiring a r ight-of-way permitto cross the lands. There exist federal and state environmental compliance regulations that must bemet prior to c onstruction. I t may b e necessary to prepare specific environmental studies whichmust accompany the application to obtain regulatory approval from the appropriate jurisdiction. Insome cases, s pecial l egislative approval at t he local, s tate, o r f ederal level may b e r equired t o

obtain permission to construct on public land.

Community Impacts There are a number of community issues to be considered in siting a transitionfacility. These include:

Noise: Noise sources a t transition facilities can include shunt reactors, p ressurizing and c oolingequipment ( for pipe type cables), switching noise, and co rona discharge related noise. Standardnoise evaluating techniques f or s ubstations can b e u sed t o ev aluate r eactor an d p ressurizingplant/cooling plant n oise. S witching no ise t ends t o b e ve ry i nfrequent, a nd i s ge nerally no t a

significant siting issue. Corona is a luminous discharge that emanates from energized high voltageelectrical accessories and conductors due to ionization of the surrounding air. Corona is caused bya voltage gradient that exceeds the breakdown strength of air. Corona discharge generates audiblenoise (AN), radio interference (RI) and to a lesser extent television interference (TVI). Selection of

appropriate hardware f or t he o perating voltage w ill g enerally r educe o r el iminate co ronadischarges. It may be necessary to evaluate the anticipated audible noise levels and the potential tointerfere with communication signals d uring the design of hi gh voltage transition structures and

transition stations.

Aesthetics: A transition facility may have the appearance of a single transmission pole or tower, orit may look like a substation. In siting a transition facility, viewscapes may need to be evaluated totry to assess a nd/or reduce v isual impact. Screening, s uch as p lantings o r architectural f eatures

(walls, fences, etc) may be required. It may not be possible to f ully screen a tr ansition facility,since the overhead line and d ead-end tower must have appropriate electrical clearances from any


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water i ssues (wetlands, m arshes, f lood p lain, e tc.) should be a voided for b oth e nvironmental a ndconstruction r easons. Areas d esignated as h igh seismic zones would r equire s pecialized en gineeringand construction techniques, and should be avoided to the extent possible.

Land Use

Depending on the type o f transition facility (individual transition structures or a transition station) theland use requirements will be different. In the c ase of individual monopole riser, the transition willtypically be done within the same overall width as the balance of the overhead right of way.

A transition station equipped with multiple cables p er phase, shunt reactors, switching eq uipment, acontrol h ouse, an d p ossibly p ressurizing eq uipment ( for p ipe t ype cab les), w ill represent a moresignificant land use. A transition station might occupy up to 4 or more acres of land.

Land u se is sues o ften f all u nder lo cal z oning r egulations. The p roponent of a transition station sitemight have to demonstrate consistency with zoning requirements, or obtain variances.

Access

Regardless o f whether a m onopole o r a t ransition s tation is r equired, u nderground l ines need to beconstructed in areas that have access f or t rucks, t railers, an d o ther eq uipment u sed t o s upport t heduct/pipe, manhole, and cable installation. Unrestricted access to the transition site is essential both forinitial construction, and for ongoing operation and maintenance. F or monopole transition structures,

the access should be such to allow a line truck to drive up to the pole to facilitate the disconnection ofjumpers, and to allow for cable installation equipment to get to the pole for the event of a cable failure.If the transition site is larger, the access should be s uch to a llow large equipment to be tr ansported tothe site. T his requires construction of access roads at suitable grades for the passage of constructionvehicles. Within a f enced transition station, sufficient room should be created to allow cable reels andreel handling equipment to get to the cable pulling location, both originally, and for f uture repairs. Ifshunt reactors are r equired, then the transition station needs to be located where very large trucks can

access t he s ite t o deliver and replace the reactors in the event o f a failure. This requires e ither theconstruction of a road or location of the transition station close to a road.

Tree growth in the vicinity of the transition station needs to be controlled, both to p revent b ranchesfrom contacting the overhead line, and to prevent roots from interfering with the underground lines.

Interactions with adjacent facilities (telecommunication, railroads, pipe lines, highways,)

Whenever an el ectric u nderground l ine i s i nstalled, ef fect o n o ther ad jacent f acilities n eed t o b econsidered. The most o bvious ef fect i s t he p otential in duced c urrent o n p arallel c ommunicationfacilities (telecommunication and railroad communications) due to the magnetic field generated by theelectric l ine. I f t he el ectric l ine cr osses p erpendicular, there is essentially no influence tocommunication circuits. If a line runs p arallel to a c ommunication c ircuit for a substantial d istance,discussion should be made with the owner of t he f acility t o d etermine t he ad equacy o f t he cab leshielding on th e cable. T ypically, th e communication c ircuit is adequately shielded so no impact is

expected.

Another concern is the ef fect o n o ther m etallic p ipe l ines. T hese t ypes of p ipes ty pically a recathodically p rotected. HPFF an d H PGF s ystems ar e al so cathodically protected and an evaluation

should be made to determine the type of cathodic protection systems being used and determine if anystray currents are expected that could jeopardize either cathodic protections system. Also, paralleling asteel pipe and the effect the generated magnetic field would have on the cathodic protection should beconsidered.

Future Extensions


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Another factor to consider in designing a tr ansition facility is whether there is a future possibility ofhaving to underground more of the overhead l ine a nd e xtend t he und erground p ortion. If th ispossibility exists, a manhole o r other splicing area s hould be c onsidered. This is b est located n ear thebase of the transition structure. The purpose of this splicing area is to provide a convenient location toextend the underground line. I f a manhole or splice pit is not originally installed and the undergroundline has to be extended in the future, it would be necessary to install a manhole or splice pit around the

cable pipe, duct bank, or buried cable section. Great care is needed to prevent damaging the cable pipe,conduit and cable during this operation. Once the manhole or splicing chamber is in p lace, the cablewould then be cut at this location and new cable installed. The other option would be to r emove thecable back to t he n ext m anhole p rior t o an y ex cavation. This is ty pically m ore c ostly, a nd w ouldtypically require a more extended cable outage during the cutover.

Distribution Underbuild

Distribution underbuild (consisting of overhead distribution lines underbuilt on the same structures asoverhead tr ansmission lin es) a dd a n a dditional le vel o f complexity to transition stations, both from

clearance issues on the o verhead line s ide, and for the amount of additional equipment required at t hetransition facility. In g eneral, transitions with distribution underbuild should be a voided to the extentpossible. Given that the d istribution transition will likely be substantially smaller than the transmissionfacility, it may be possible to transition the distribution line a short distance away from the transmission

line dead-end, possibly reducing electrical clearance issues and simplifying the transition station.

Monopole Transition Structure Height Limitations

While there is no height restriction for extruded dielectric or HPGF cable systems, it is important tokeep the mounting height of the terminations as low as possible, to facilitate an easier installation. For

HPFF and SCFF, because of hydraulic p ressure lim itations, it is im portant to k eep th e te rminationmounting heights as short as possible. In addition, if the area is not fenced it is important to make surethe NESC electrical clearances are achieved.

Monopole Transition Structure Placement Limitation

Many of the same issues f or p lacement o f o verhead l ine s tructures ( avoid p lacement n ear r oad

intersections or driveways, p lace aw ay f rom t raffic, et c.), ap ply t o t he p lacement o f a m onopoletransition structures.

2.1.3 Economics

A thorough evaluation of the total project cost should be considered in determining the best-valuedinstallation for either an overhead or underground transmission line. A large cost associated with a hybridoverhead/underground line is the cost of the transition facility.

Overhead to underground transition facilities can vary widely in their size and complexity, thus ultimatelyinfluencing the cost of the underground project. The transition facility can vary from a single monopole,with no fenced in area, to a mini-substation that is from 2-4 acres in size, complete with: terminal structures,cable terminations, station class arresters, circuit breakers, disconnect switches, shunt reactors,

interconnecting bus work, CCVTs, CTs, PTs, an overhead line terminal, relays, and control equipment,station batteries, battery chargers, back-up power supplies and a control house for the variouscommunication devices.

A single monopole could accommodate multiple circuits to transition from overhead to underground.However, depending on the utilities operating procedures, it may not allow a portion of the circuit to remainin-service while another portion of the circuit is taken out of service to be repaired or for otherconsiderations. If the utility desires this type of flexibility, a larger, more substation facility is required.


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In addition to the components within the transition facility, the facility will also require: foundations for allthe equipment, a station ground grid, fencing, lighting, landscaping, etc. The size and complexity (and cost)of the transition station is dependent upon:

Voltage rating

Physical location

Number of cables or circuits coming into the transition station

Number of overhead lines leaving the transition station

How will the circuit(s) be used if one line is out of service

How much remote control is desired

Is switching required

Major cost items to consider for a transition facility, but not limited to, are the following

Cost of total installation versus benefits of the installation.

Cost of the right-of-way

Digging co sts i n contaminated land an d wetland areas should include the cost of removing andtransporting the contaminant to an acceptable land site.

Cost f or landscaping and pos sible irrigation r equirements, r epaving costs of roads, parking lots,and related items.

Cost of yearly maintenance.

Cost of system interruption in case of failure and cost of time for restoration.

Review alternate solutions that might cost more initially, but would offer cost savings in total owning costs.

2.2 System Impacts

Since an u nderground cab le system has s ignificantly d ifferent el ectrical ch aracteristics, s pecial

consideration needs to be taken when planning to connect an underground line to an overhead line. Some ofthese considerations are as follows.

Reliability vs Availability

Cable Rating

Electrical Characteristics

Protection and Control

Effect on Switching Devices


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2.2.1 Reliability vs Availability

Reliability is defined as the number of times a particular transmission line is lost over a year period of time,while Availability is d efined as the duration of time th at a tr ansmission line is out of service over a yearperiod of time due to a fault or other problem on the transmission line. While an underground line typicallyhas a higher r eliability than an o verhead line ( fewer outages), it typically has a lower availability (longeroutage duration). It takes a significant amount of time, to place an underground line back into service after

an electrical failure. Typical repair times for underground transmission lines can be extensive, ranging fromweeks to months depending on the fault location, the availability of personnel and spare materials. Withextruded dielectric or SCFF cable systems, s ome of this outage time can b e reduced if a fourth cable isinstalled a s a n in -place energized spare. T his increases the co st and complexity of t he i nstallation. Thetransition facility must be set up to a llow c onnection f rom th e f ourth c able to a ny o f th e o ther c ablepositions.

Since the power transfer capability of an underground cable is usually lower than that of an overhead line,the cable alternative may need two circuits, or two cables per phase, to provide the required power transfer.Multiple cables per phase will increase the s ize and complexity of the transition facility. Electrical issues,particularly cab le charging effects, will b e increased with each additional c able installed. On the positive

side, in addition to providing higher ratings, multiple cables per phase have the possibility of increasing theoverall availability of the cable system. If one line fails, the remaining line can typically carry 60 percent or

more of the total design power transfer for the time it takes to repair t he f ailed line. T here are severalconsiderations, however:

If the line trips e lectrically, the operator will need to d etermine if the trip was due to a fault in theoverhead line section, in which case the entire line can g enerally be returned to s ervice relativelyquickly. Relaying as described in Section 2.2.4.1, can help with this determination. If the fault isin one of the underground cables, the entire line must remain out of service until the failed cable isidentified, disconnected, and grounded. This can take anywhere from very little time to a day orlonger, depending on the level of fault detection equipment and switching equipment present at the

transition s tation. If the transitions ar e in r emote areas, separation of the faulted cable from thesystem and restoration times for the un-faulted cable can be longer still.

Once r epairs ar e u nderway on t he f aulted cab le, r epair cr ews n eed t o co ntend w ith i nduced

voltages from the energized cable. This may extend repair times.

System planners and/or system operators should evaluate whether power flows would exceed thethermal rating of a single cable of a two cable set.

In t he cas e o f o verhead transmission lines that m ay b e cap able o f car rying s everal t housandamperes, multiple cables may be r equired to pr ovide full power t ransfer along with the r equiredreliability. T his c an require a r ight-of-way 6 0 f eet or w ider t o a llow w orking r oom f or t heindividual lines and to reduce mutual heating between cables, which can de-rate the undergroundlines. In addition, transition stations will be large, or multiple transition structures will be required.

2.2.2 Cable Rating

While an o verhead l ine co nductor i s d esigned and rated f or t he ex pected em ergency co ndition, anunderground line conductor is typically selected on the normal continuous expected load. T he reason for

this is that an underground cable has the capability of operating at a higher temperature for short durationsof time without damaging the cable. T his is called the thermal capacitance of the cable. It is important tospecify the normal maximum continuous load for t he line a nd the maximum short term emergency ratingand duration in order to pr operly s ize t he cab le. Cable rating issues c an a ffect tr ansition f acility d esignprimarily i f m ultiple cab les ar e r equired f or each o verhead l ine, an d if cable spacing issues (to reducemultiple heating) affect structure spacing. Riser conduits a t transition stations may require evaluation, andpossible venting, so that cable risers dont limit the overall circuit rating.


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2.2.3 Electrical Characteristics

The electrical characteristics of cables are significantly different than similar capacity overhead lines. Abrief discussion of some of these differences is included below.

2.2.3.1 Surge Impedance

Overhead lines of longer lengths may have limits on capacity for one of three reasons:

Thermal constraints (usually related to sag).

Voltage s tability lim its w here the voltage drops significantly over t he l ength o f l onger l ines,making power transfer from one end to the other difficult. T his is most often an issue with linesthat are 80-320km.

Surge impedance loading limits where phase shift between one end of a l ine and the other is to ogreat for the s ystems at either en d to r emain s ynchronized. This is g enerally an issue for lineslonger than 320km.

Surge Impedance:C

LZSURGE

= [Ohms]

Surge Impedance Loading Limit (SIL): ]MVA[Z

VSIL

SURGE

2

LinetoLine =

Cables are always thermally limited because cables generally have ten t imes the surge impedance loadinglimit of a comparable overhead line, and cable circuit lengths are generally short, mostly because of cost butalso because the charging current limits the allowable line length.

As a general characteristic, overhead lines have m uch h igher s eries i nductance an d m uch l ower s hunt

capacitance than underground cables. As a result, the positive sequence surge impedance of a cable is muchlower than that of an overhead line. W hen inserting a s ection of cable into an overhead system, the usermust consider that the cable particularly in a n etworked power system may carry greater load than theoverhead line because the cable has lower positive sequence impedance. This phenomenon is sometimes

referred to as load hogging.

Loadflow analysis may determine that an underground cable will experience unacceptable load hoggingbased o n t he r elative i mpedance o f t he cable to other parallel o verhead l ines. I n t his cas e, it m ight b enecessary to incorporate additional equipment at the transition station to better balance cable loading to the

rest of the transmission system. This equipment could include series reactors or phase shifting transformers.Should either of these elements be required, the transition station will have to become larger toaccommodate this equipment and the associated accessories

2.2.3.2 Cable Capacitance and Reactive Compensation

Since overhead lines are l argely inductive while underground cables are essentially distributed capacitors,the overhead lines generally consume reactive VARS while cables generate reactive vars. Thesecharacteristics can i mpact t he voltage o n a t ransmission s ystem an d ultimately af fect p ower flow. Highvoltage cab le s ections i n an o verhead ci rcuit m ay r equire s hunt reactive compensation to mitigate t hevoltage e ffects o n th e tr ansmission s ystem d ue to th e capacitive V ARS from the cab le. Reactivecompensation can take the form of air insulated reactor coils at lower voltages, or oil immersed reactor coils

at higher voltages. If r eactive compensation is needed, the overhead to underground transition cannot b e


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done on a monopole, and a transition station will be required. Reactors, the electrical connections to them,and the maintenance space around them can add significantly to the transition station size.

In an overhead line that may have capacitor banks at the terminal stations, , inserting a cable section maymean that the capacitor banks are no longer necessary, or must be switched out during light loading periods.

Capacitive effects from underground cables become more pronounced as line length increases. Capacitiveeffects also are much more p ronounced at h igher t ransmission o perating v oltages. I n ad dition t o t hepossibility that shunt reactors may be required at transition facilities, cable charging current affects the typeof switches that can be installed at tr ansition stations. Switches carry an i nterrupting ration for capacitivecurrent, which may be significantly lower than o ther interrupting ratings. If a transition facility will haveswitching capability, it is important for the u ser to e valuate all the switching requirements (load pickup or

load dropping, making or b reaking p arallels, i nterrupting c harging c urrent, an d p erhaps ev en f aultinterrupting), and ensure that adequate switch capability is specified.

2.2.3.3 Short Circuit Currents

Short ci rcuit cap abilities o f cab les m ust b e car efully co nsidered i n t he cab le design; particularly f orextruded d ielectric an d s elf-contained f luid-filled c ables. T he pipe of a pi pe-type cab le g enerally h assignificant fault current carrying capacity, although pipe grounding must be configured carefully to allowthe fault current to reach ground. The fault current capability of a transmission cable is normally evaluated

for the case o f a single-line-to-ground fault, where the fault current passes through one o f the cable p haseconductors and may return through the cable metallic shield o r sheath. Many utilities conservatively usecable shield/sheath designs that allow for 1 5-30 cycles in the e vent that primary protection fails to operate

and secondary protection must clear the fault. A diabatic conditions are usually assumed when sizing theshield/sheath cross sectional area.

A cable in a hybrid circuit may increase the fault current levels further out on the line because of the lowerzero sequence impedance for cable than overhead lines. Consequentially, distance relays may have to b e

adjusted to consider a section of cable inserted into an overhead circuit.

The tr ansition s tructures w ill ty pically r equire p rovisions f or attaching link boxes, polarization cells orisolator s urge p rotectors (IS P) (for cat hodically p rotected cab le s ystems), cab le s hield an d ar resterdownleads, and other bonding and grounding facilities. The transition site may also be where an anode bed

would b e in stalled f or c athodically p rotected s ystems, which c an a ffect th e transition site footprint. Inaddition, adequate shield connections to g round a t the transition f acility, and adequate b uried gr oundingconductors to establish low ground resistance and low step/touch potential issues, should be evaluated.

2.2.3.5 Overvoltage and Insulation Coordination

As w ith a ny tr ansmission s ystem, th e effects o f o vervoltage on the t ransmission s ystem n eed t o b eaddressed. Overvoltage on any transmission s ystem c an b e c aused b y l ightning, s witching a nd s ysteminstability. Insulation Coordination S tudies should be p erformed to d etermine the correct insulation andprotection level for h ybrid o verhead an d u nderground t ransmission l ines. T he m ost ef fective w ay o f

limiting overvoltages on the underground line is to install properly sized arresters at each end of the cable.Transition facilities should have accommodations for surge arresters close to each cable termination. Thiscan be accomplished with the installation of suitable brackets on monopole transitions, or be incorporated inthe termination support structure in transition stations.

2.2.4 Effect on protection and control

An underground segment in an overhead transmission line can cause a n umber of difficulties that must beaddressed when designing a t ransmission l ine relay protection and control system. Depending on systemrequirements, the transition station could be as simple as a single pole used to connect the overhead line tothe u nderground cab le o r as co mplicated as a substation with circuit breakers, shunt reactors, protectiverelaying, AC and DC power supplies and communication facilities. Each transition station design provides

its own relaying concerns.


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Some of the main points to consider concerning protection and control of hybrid transmission lines includethe following:

Variations i n the o verall l ine i mpedance cau sed b y h aving o ne o r more parallel cab les o ut o fservice for maintenance while the remaining cable(s) and overhead portions remain in service.

Impedance mismatch between the overhead and underground sections.

Shunt reactors, if located at transition station, must have protection installed locally, requiring all

normal system redundancy (e.g. dual AC and DC systems).

Fault detection and/or location equipment at th e transition station may be needed to determine ifthe fault is in the cable section to enable or prohibit automatic reclosing.

System studies may determine that it is desirable to remove one of the parallel cables from serviceduring light load periods to reducing line charging. Switching devices (circuit breakers or circuitswitchers) would be required for this capability.

If there is automatic reclosing installed on the line, the trapped charge on the cable portions must

be discharged to a safe level prior to re-energization.

2.2.4.1 Relaying concerns

There are various protective relay types used for the protection of transmission lines. These can range fromsimple o vercurrent r elays to d istance o r d ifferential r elays w ith c ommunication b etween s ubstations.Overcurrent relays are generally used on lower voltage systems not requiring high speed fault clearing. Onhigh voltage transmission systems, high speed clearing is desirable, and requires a protection system with acommunication channel(s). O ne co mmon f orm o f co mmunication ch annel i s p ower l ine car rier. W ithpower l ine car rier, a h igh f requency s ignal is coupled onto the tr ansmission lin e a nd tr ansmitted ( or

received) at the substation. P ower line carrier is sensitive to the characteristic impedance of the line, andany changes in the characteristic impedance along the line. T he overhead and underground portions of acombination o verhead/underground l ine h ave d ifferent ch aracteristic i mpedances, which would cause apower line carrier signal to have wave reflections and signal loss. Thus, power line carrier communicationis n ot r ecommended f or a hybrid transmission line. O ther f orms o f c ommunication f or lin e r elayingpurposes must be considered, such as audio tone, microwave or fiber optics.

System studies may indicate that under light load periods, it is desirable to remove a parallel cable on acombination overhead/underground line from service to reduce line charging. When the cable section isremoved from service, the overall line impedance between substations will change. The impedance changeneeds t o be accounted f or i f distance t ype relays ar e being u sed t o protect the line. M onitoring of th e

number of cables in service at the tr ansition stations, with communication to the remote (substation) ends,can allow for the appropriate relay settings to be employed.

The preferred relaying scheme f or an u nderground l ine i s a cu rrent d ifferential s cheme, which r elies o nmonitoring the incoming and outgoing current on the line.

When a t ransmission l ine i s a ll und erground c able f rom s ubstation t o substation, shunt reactors, whenrequired, are installed at those substations and protected similar to a transformer. If system studies indicatethat shunt reactors need to be installed a t a tr ansition s tation, p rotective r elaying w ill a lso n eed to b einstalled, along with all the a ncillary e quipment n eeded to s upport a p rotective system. T he an cillary

equipment may include relay and control enclosure, AC & DC systems with appropriate backup, SCADARTU as well as either local circuit breaker(s) or remote tripping at the substation.

System protection issues may be addressed with equipment located at the remote substation ends of a hybridoverhead/underground system, or it may be necessary to install protective equipment at the actual transition


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facility. In that case, it is often necessary to have a control house, with associated climate control, AC andDC power supplies, and communication equipment located at the transition station.

2.2.4.2 Restoration/Reclosing

Overhead t ransmission lin es u sually e mploy a utomatic r eclosing to r estore a lin e after a fault. Withcombination ov erhead/underground l ines, o ne ne eds t o e nsure t hat t he f ault is not in the undergroundportion of the line prior to permitting reclosing. R elaying and in strument transformers can b e installed atthe transition station to detect if the fault is in the underground portion of the line and send a signal to theremote substations to permit or p rohibit reclosing. H owever, due to the possibility for trapped charge toremain on the cable, especially for extra high voltage cable systems, special in terrupting devices may b e

needed to discharge the cable to a safe level prior to permitting a reclose to occur. There are different issuesassociated w ith r estoration s witching f or f luid/paper in sulated c ables ( HPFF, H PGF, and SC FF) a ndextruded dielectric cables. Regardless of the type of cable system, it is strongly recommended that adequateprotective equipment be installed to prevent reclosing into a cable fault. T ransition facilities may have tocontain C Ts, P Ts, co mmunication eq uipment, an d other protective equipment to a llow d ifferentiationbetween overhead and underground faults.

It is recommended that the cable manufacturer be contacted and operating guidelines be set up in advance tohave these on hand to cover different reclosing situations and outage time.

Pipe Type Cables:

If the fault is in the p ipe cable section and the initial fault arc d id no t burn a ho le through the p ipe wallthickness initially, there is a much greater risk that this could occur on a reclosure. Pipe-type cable systemsoperate at a n ominal 2 00 p sig i nternal p ressure an d an i nstantaneous l eak will occur if th e p ipe is

compromised. In a HPFF system, a leak could require a costly environmental cleanup. If the initial fault arcdid burn through the pipe, a r eclosure could ignite a fire. In a HPGF system, there is no fluid leak but theloss of nitrogen could allow ground water to e nter through the hole in the pipe, resulting in cable damageand p ossibly e xtending t he outage duration. Reclosing m ay al so d amage ad ditional p hases, ex tendingrestoration time.

If an outage of a hybrid overhead/pipe type cable system is caused by a f ault on the overhead section and

the circuit outage time is one hour or less, no special precautions or procedures are needed for the HPFF orHPGF system when re-energizing the circuit. For prolonged circuit outages, it may be necessary to vent theterminators (potheads) on H PFF s ystems an d al so r e-establish th e o riginal n ominal 2 00 p sig s ystempressures on both HPFF and HPGF systems prior to re-energizing the circuit. This could take several days.

A discussion on the repair of a faulted or damaged pipe type cable system is provided in a later section.

Extruded Cables:

If the fault is in the underground section and the initial fault arc did not melt the cable to the conduit in thecase of a duct system installation or damage the adjacent cables if the cables are touching (ie: cable installeddirect buried in a t refoil arrangement), there is a much greater risk that this could occur on a reclosure. Ifthe cab le i s melted to the inside of the c onduit, it m ay b e difficult or im possible to r emove the faultedsection of cable and replace it with a spare cable. If the cable is installed with the cables touching, reclosingmay also damage additional phases, extending restoration time. If the initial cable fault was in a splice in a

manhole or splicing vault, a reclosure has the potential to damage adjacent cables, causing more damage,and e xtending t he outage. A d iscussion o n t he repair o f a f aulted o r d amaged extruded cable system isprovided in a later section

Installed Spare Cable for Extruded Dielectric Cable Systems

In some cases for extruded dielectric cable systems or self-contained fluid-filled cable systems, a spare

cable is installed and terminated to reduce the chance of prolonged system outages. A spare cable isinstalled for one or more of the following reasons.


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1. The circuit is of particular importance for system reliability2. The circuit is a radial feed where there is limited ability to serve the load by other means.3.

Replacement of a failed cable will require an unusually long period of time4.

The economic consequences of circuit down time are high

Close attention must be given to placement of the spare cable terminations to minimize connection time toany of the three cable system phases. The spare cable is usually energized from one end to minimizeswitchover time and to gain assurance that the spare cable is in good operating condition. In some cases thespare cable is connected in parallel with one of the three normal service cables; however, this is uncommonbecause it creates an unbalance in the phase impedances and may result in circulating currents. A sparecable will be in a different relative position to the normal cable geometry. This can affect cancellation forcross bonded shield systems, making the cross bonding less effective, This should be evaluated for cross

bonded systems.Because of cable charging issues, the installation of a spare cable is generally not recommended for extrahigh voltage cable systems or very long underground segments.

Manhole/Takeup Loop for Extruded Dielectric Cable Systems

For direct buried transmission cables, slack cable is sometimes installed for all three cable phases in the

immediate vicinity of the cable termination structure. This allows for the re-termination of the cables ifthere is a termination failure or damage to the above ground portion of the cable. Re-termination isaccomplished by excavating the slack loop and drawing the slack out until there is enough undamaged cable

available at the structure. The transition facility layout must be done in such a manner that there are noobstructions in the area where the cable relocation would occur.

Another approach to cable termination replacement is to install a splice pit or pull-through manholes withinseveral hundred feet of the cable terminations. In this case, the repair plan is to cut the cable of the failedtermination in the pull-through vault, pull in a short replacement cable, install a splice in the pull-through

vault, and terminate the newly installed length of cable. This approach is sometimes used for systemsinstalled in conduit, or for direct buried cables when there is no room for the slack cable lengths close to thecable terminations support structures.

2.2.4.3 Faults

A fault on an underground line typically has long repair times. The repair times are about one to two weeksfor extruded cables up to and including 138kV, two weeks to a m onth or m ore for higher voltages, and amonth or more for a HPFF or HPGF system. Repair times are dependent on the following.

Ability to find the fault

Physical constraints at the fault location

Extent of repairs required

Availability of spare parts

Availability of experienced repair personnel

In d eveloping a t ransition s tation o r m onopole s tructure, th e a bility to r epair f aulted c ables should beconsidered. Clearances to p arallel transmission cables on the s ame or ad jacent s tructures and clearance to

the o verhead l ines s hould b e ev aluated for t erminal cab le pulls and f or installation o f terminations. Thedesigner should consider that during installation of the porcelain or polymer insulator of a termination, theheight of the insulator plus an allowance for the crane boom and lifting cables must be added to the finishedheight of the termination,


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If adequate clearance is n ot provided for cable r epairs, i t may be n ecessary to switch out additional lines,perhaps at a t ime w hen t hey ar e n eeded m ost b ecause o f f ailure o f t he f irst l ine. T his co uld r esult i ntransmission system restrictions, loss of load, or perhaps delay cable repairs to a lower load time of year.

2.2.5 Effect on switching devices

An important design consideration for t ransitioning between overhead and underground is the decision ofwhether a s witching device is needed to disconnect the underground circuit from the overhead l ine. T hefollowing factors should be considered.

Voltage

Length of line/charging current

Space availability

Operational Issues

In-service switching of multiple cables per phase

Structure design

Switch design

All this issues must be addressed to be able to design a suitable disconnecting means.

One of the main concerns in selecting the disconnect means is the ability of the method selected to deal withthe charging current of the underground line.

A common method used for disconnecting a short underground circuit from the overhead line is the removal

of the jumper. T he main advantage is that the transition can be accommodated on a s ingle pole s tructurewith a compact design. Disadvantages include the need for a line outage, and that it requires a construction

crew to go to the site and physically remove the jumper.

Where the underground line is longer and thereby has a higher charging current, a disconnect switch may beneeded. It is im portant to c onfirm th at the cable charging c urrent does n ot exceed t he i nterruptingcapability o f t he d isconnecting d evice. Cable charging current is almost p urely cap acitive, s o t hecapacitive switching r ating o f t he d evice m ust b e ex amined. T he m ain d isadvantage o f d isconnectswitches is the need for additional space on a structure or an additional structure and land.

If a d isconnect s witch i s u nable t o h andle the c harging c urrent, t hen more s ophisticated d isconnectingmeans, such as circuit breakers or circuit switchers, would need to be used.

If the switches at transition facilities will be used to make or break parallels (for multiple cable per p hase

installations), o r to d rop o r p ick u p lo ad, th e c apability o f th e switching d evice m ust b e examined f orsuitability for these applications, as well as for charging current capability.

3. Design

Before the design or selection of a transition facility can begin, a comprehensive design criterion should be

developed. This design criterion should consist of the following:

Maintenance requirements


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Operation requirements.

Cable System Type (Extruded Dielectric, HPFF, HPGF, SCFF)

Reliability Requirements (installed fourth cable)

Cable System Requirements (voltage, multiple circuits, multiple cable per phase, etc)

Once this criterion has been established, the decision of whether a single structure transition or a transitionstation can be made. If switching is required to isolate the o verhead from the underground, a transitionstation is recommended. Single shaft structures are not currently being used for installations over 230kV.

3.1 Single Pole Structure Layout/Design

The following is a list of items to consider in the design of a single shaft transition structure.

3.1.1 Structure Layout

Design o f t he o verhead t ransmission tr ansition s tructure should provide t he proper el ectrical clearances

defined in the latest version of the National Electric Safety Code (NESC). These clearances should addressthe el ectrical cl earance from the c onductors, j umper loops, equipment, an d other en ergized parts t o thesurfaces of the supporting structures and to t he certain spaces that shall be designated for c limbing and /orworking on the structures. D esignated c limbing a nd w orking s paces a re i ntended t o p rovide f or s afelyperforming maintenance on energized overhead conductors using hot-line tools. The need to p rovidinghot-line climbing and working areas may be reviewed on the transition structure type and the associated

clearances m ay b e w aived where l ine co nfiguration a nd installed e quipment o n th e s tructure w ould n otpermit or where the structure will not be climbed or maintained with the line energized.

If a monopole structure will be u sed f or a t ransition b etween a s ingle o verhead l ine an d t wo s ets o funderground cables per phase, the user may want the ability to operate the line with one set of cables in

service while repairs are being performed on the second set of cables. Typically, the overhead line would beattached to the t ower w ith t he p hases ar ranged v ertically. E ach cab le ci rcuit w ould b e t erminated o n

opposite sides of the monopole structure with removable connections from the overhead line to each cabletermination. In d esigning the monopole structure, careful evaluation of electrical clearances is r equired todetermine whether the cable r epairs can b e performed safely. As a p ractical matter, this type of design isonly achievable a t lower transmission v oltages, and e ven a t the lo wer transmission v oltages, a transition

station may be more suitable.

If a monopole structure will be used in conjunction with an installed spare fourth cable, the structure mustbe arranged so t hat the termination f rom the fourth p hase can b e electrically connected to any of the threeoverhead line p ositions. This is typically accomplished by terminating the active phases on t he sides of

the m onopole, an d t erminating t he s pare cab le o n t he back o f t he p ole ( opposite t he o verhead l ineattachment) an d near the h eight o f th e lo west active te rmination. V ertical bus or open wire on s tandoffinsulators is installed up the pole from the spare termination, allowing installation of jumpers to any of thethree o verhead p hases. Note: A cable s heath b onding s ystem u tilizing single point bonding or m ultiplesingle point bonding with an energized spare cable will el iminate the need to modify the SVL groundingsystem if the need arises to use the spare cable as a power conductor occurs. This configuration will result

in a small de-rating of the cab le ampacity when compared to cross-bonding system without an energizedspare cable. If cross bonding is required with an energized spare, the spare cable will not be as balanced aswith the active cables and will negatively affect cable ampacity ratings. Cross bonding will create the needto reconfigure link b oxes in order to place the spare phase in service and remove the faulted cable. Anengineering evaluation should be completed no matter which sheath bonding method is chosen to determinethe impact of an energized spare cable with regards to the voltage rise at the open end or floating end and to

evaluate the spare phase capacitance contribution to the system.


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3.1.2 Structural Loadings

Structure should be designed for all local wind, ice, seismic, and code loads. Structure loads should includeall typical attachments for a m onopole ty pe s tructure w ith c onductors, s tatic w ires, a nd in sulators. Inaddition, the structure should be designed for the additional weight and transverse loads due to:

Cables

Shrouds/cable guards if installed (to protect cables at groundline)

Terminations

Surge Arresters

Link Boxes

Donut-type Current Transformers

Special d esign an d de tailing s hould be de veloped for a s tructure i n which t he cables pass through the

structure base plate or structure foundation, such as accessibility to the cable for construction and futuremaintenance, cable clamping, installation requirements, etc.

Support arms should be designed for a ll local wind, ice, seismic, and c ode loads on a rm and equipment.Design consideration should be given to possible personnel on arms and of possible weight of cable beingsuspended from arms during installation.

The at tachment p late f or t he t erminations s hould h ave an o pen s ide. V ertical s teel m embers may berequired below support arms to support cables.

3.1.3 Types of Structures

As with overhead line structures, various types of structures can and have been used for the transition pole:

self-supporting steel, guyed direct-embedded steel, guyed wood, guyed laminated, etc. T he most commonis self-supporting steel. A self-supporting steel structure eliminates the potential clearance issue that wouldexist if th e s tructure h ad to b e g uyed. S ince s teel typically lasts a lo ng time, it eliminates th e potentialproblem of having to replace a wood pole af ter a f ew years due to normal deterioration. In a true overheadto underground transition facility, the structure will almost always serve as a dead-end for the overhead line.

Occasionally, this structure may be a tangent structure, for example if the underground line is a tap to a newload.

Structure m aterial ch oices n eed t o b e car efully ev aluated. W ood poles for transmission-class cab leinstallations should generally be discouraged for transition poles due to potential maintenance issues (pole

replacement due to decay). Concrete and steel poles are considered more suitable for transition poles due totheir anticipated longevity. If steel poles are utilized, consideration must be given to joining multiple piecepoles. Both slip f it and bolted flange joint are c ommonly used, but with cables being installed on terminalpoles extra attention must be given to ensure there is no movement of the joint. Slip fit poles should have a

method for locking the joint to prevent movement (a through bolt is usually sufficient). In addition, with aflanged joint, special attention needs to be made for the flange, when running the cable on the outside of the

pole.

3.1.4 Structure Grounding

Grounding of the transition facility should follow the utilitys appropriate grounding standard.


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Typical structure grounds may consist of a single rod, or may involve multiple rods, grillages, foundations,counterpoise, potential control rings, etc. Selection of a grounding method is a function of:

(1) Transmission line structure

The resistivity of the transition structure is dependent on the material used. As discussed in Section 3.1.3,the most common structure is s elf s upporting s teel. T his s tructure h as a lo wer r esistivity th an a w oodstructure.

Wood pol es may r equire s upplemental grounding that would b e c onsistent a nd i n c ompliance w ith t heNational Electrical Safety Code, ANSI/IEEE Standard C2. A separate ground conductor should be installedon the wood poles to provide bonding to surge arresters, link boxes and terminations.

(2) Soil resistivity conditions

Soil resistivity should be measured at the location of the transmission structure. The resistivity of the soilvaries with depth, type and concentration of materials, moisture content and temperature. If the existing soilconditions ha ve a high resistivity, s upplemental grounding may b e required to provide an adequate faultpath. H igher r esistivity w ill c reate a h igher g round p otential d uring a bnormal c onditions, r esulting in

possible insulator or equipment flashover.

(3) Required ground resistance

The s tructure gr ounding s hould p rovide a r esistance t hat i s sufficiently low enough to keep groundpotentials to acceptable levels during abnormal c onditions. T he u tilities s tandard g rounding p rocedureshould be consulted to determine the appropriate level of resistance desired for each location.

(4) Need to control earth surface potentials.

During abnormal conditions like l ightning strokes and p hase-to-ground faults, very high magnitude, shortduration currents will flow within the grounding network. Part, or all, o f this current will flow through thestructure ground. This will cause a momentary voltage to appear on the structure ground that is a function

of the c urrent magnitude a nd t he s tructure gr ound r esistance. T his vo ltage often referred to as G round

Potential Rise (GPR) is of special interest with respect to earth surface potentials.

Earth Surface Potentials GPR effects include the presence of earth surface potentials between the structureground and remote earth. This creates the possibility of exposure to step and touch potentials for persons in

the vicinity of the structure at this specific instant. If the transition structure is located in an area accessibleto th e g eneral p ublic, a dditional g rounding s tudies m ay b e r equired t o as sess t he ef fectiveness o f t hestructure ground in controlling earth surface potential.

It may be necessary to install a supplemental grounding system at the transmission structure (interconnected

ground rods, ground grids or mats, etc.) to reduce the tower to ground resistance, and to keep step and touchpotentials to acceptable limits.

Supplemental Grounding Direct-embedded s teel structures and s tructures with d rilled-pier foundationswhere the f oundations s teel r einforcing b ars are electrically b onded to th e structure g enerally c onstitute

effective and acceptable ground. A separate ground conductor provided for bonding of cable accessoriessuch as arresters, link b oxes and t erminations is recommended o n the structure and this separate groundconductor should be bonded to the structure ground and the parallel ground continuity conductor. Should aseparate ground conductor not be installed on steel structures and bonding is performed directly to the metalpole, it is recommended that bonds be installed across the interface of multiple piece steel poles.

Additional Precautions


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Gas Pipelines If there are adjacent buried pipelines near the transition pole, additional precautions may bewarranted. The pipeline owner may have requirements for minimal separation distance between structuregrounds and the pipeline. In general, bonding between the structure and the pipeline is usually prohibited.

Coordination of cable shield grounding with structure grounding Chris G.

STRUCTURE SHIELDING -

3.1.5 Extruded Dielectric Installations

There are two ways that are commonly utilized to install underground cable on a steel pole.

Install the cable inside the pole

Install the cable on the outside of the pole

3.1.5.1 Cable Installed Inside the Pole:

Installation of the cable inside of the pole provides complete protection from external damage, but is a more

costly installation than installing the cable on the outside of the pole because of the following:

Requires a larger diameter pole and foundation to allow internal access.

Requires special foundation design.

More difficult to install cable.

3.1.5.2 Cable Installed Outside the Pole:

Installation of the cable outside of the pole is less costly than installing cable inside of the pole because ofthe following:

Ease of installation and restoration.

Smaller diameter pole and foundation.

Lower pole and foundation costs.

The cable should be supported o n t he o utside o f t he p ole b y n on m agnetic cab le cl amps. Since lowersections of the cable may be susceptible to possible external damage, provisions should be made to protectthe cab le at t he l ower s egment o f t he p ole (approximately 8 10 f rom gr ound l evel). T his c an b eaccomplished by non magnetic conduits or a metallic shroud covering all three phases. Plastic conduits arenot recommended since they could be susceptible to UV rays and deteriorate over time.

In case of multiple risers, the individual conduits are usually supported by pipe straps and by pipe supports.

The risers should be p laced on the side of the supporting structure away from vehicular traffic, wheneverpractical. Caution should be exercised to insure that only non-magnetic materials are used for riser conduitsand hardware where single phases are installed in each riser. Riser cables should be supported at the upperend of the conduit by a cab le grip. I n addition, a lower cable support may be installed at the duct mouth.An ad ditional i ntermediate cab le s upport m ay b e r equired i f el evation differences exceed grip holding

capacities or cable design parameters. T he design of the cable terminations may also dictate the type andamount of support that is required for a particular installation.

If a cable shroud is used to protect all three phases, the shroud should not extend to the base of the structure,where it will block off air entering the bottom of the shroud. The top and bottom of the cable shroud should


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be designed to allow air to flow through the shroud, in a ch imney effect. It is recommended that the shroudend several inches above the base of the structure. A wire mesh or other similar material shall be installedbetween the top of the foundation and bottom of the shroud, to prevent nesting in this area.

3.1.6 Pipe-type installations

In general, only one end of a pipe-type line can be installed using a transition pole. The other end, whetherHPFF or HPGF, is generally installed in fenced-in transition stations because of the accessory requirements

described in Section 3.5.3.

Several p ipe-type cab le l ines h ave t ransition p oles at o ne end, r esembling ex truded-dielectric tr ansitionpoles in appearance and simplicity. Two basic designs are employed:

The transition from carbon-steel line pipe c ontaining the three phases to stainless-steel r iser pipesfor the three individual phases, can be made below ground using a spreader head (steel sleeve thatmakes the transition in pipes) or a below-ground trifurcating joint. T he s tainless-steel p ipes arethen run up the transition pole to th e individual terminations and the completed installation veryclosely resembles that used for extruded-dielectric cables.

The carbon-steel line pipe is b rought above ground through a 90 degree sweep, the spreader-head is placed above ground, and the stainless steel riser pipes are routed to the terminations.

3.2 Transition Site Design

3.2.1 Site Layout

The layout of the site is predicated on the amount of equipment that is needed, such as disconnect switches,reactors, breakers, control house, etc. The design of this station is similar to a switching station. The utilityshould us their normal design specifications and standards.

3.2.2 Structure Design

When designing the transition structure the following items should be considered.

Structure s hould b e d esigned f or a ll lo cal w ind, ic e, s eismic, a nd c ode l oads on arm andequipment.

Design consideration should be given to p ossible p ersonnel on arms, forces associated with thecable pulling and of possible weight of cable being suspended from arms during installation.

The at tachment p late f or t he te rminations s hould h ave a n o pen s ide to eliminate potentialcirculating currents in th e steel and standoff insulators should be installed to isolate cable sheathvoltages or pipe voltages from the grounded structure.

Vertical steel members may be required below support arms to support cables.

If metal conduits are required to protect the cable up the pole or for pulling purposes at the sweep

at the base of t he p ole, t hese co nduits s hould b e n on-magnetic c onduit. Mat erials s uch asaluminum and stainless steel are suitable non-magnetic conduits. Steel conduit should not be usedunless all the cables are installed together in the conduit. I f an individual cable is installed in asteel conduit, circulating c urrents w ill r esult, u ltimately r esulting in a dditional h eating a nd areduction in the current carrying capacity of the circuit.


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If a spare (fourth) phase cable is installed, the transition structure must be designed to accomodatethe fourth cable, including the associated termination and arrester. Additionally, the transition mustaccommodate buswork or other means to allow the spare phase to be connected to any of the otherthree phases. On a monopole transition, the spare p hase i s t ypically t erminated at t he s ameelevation as t he lowest normal phase, but on t he back side of the p ole f rom t he overhead line.Conductors, on suitable standoff insulators, are installed vertically on the pole, to the height of the

highest termination, allowing the spare phase to be connected to any of the other phase conductorsthrough the use of suitable jumpers. For a transition station, the spare phase is terminated in alocation s uch t hat any of the three normal p hase locations can be reached by the installation ofbuswork and jumpers.

If two cables per p hase are required for e ither rating or r eliability purposes, consideration shouldbe given as to whether the unfaulted cable can remain in service while the faulted cable is repaired.If the unfaulted cable is to remain in service, electrical clearance for a terminal cable pull and fortermination i nstallation s hould b e ev aluated. I n m ost cases, t he n ecessary cl earance will not beachievable on a monopole, and a transition station w ould b e r equired. E ven w ith a tr ansitionstation, electrical clearances to o ther cab le t erminations, o verhead l ines, a nd s tation b usworkshould be evaluated.

3.2.3 Interrupting Devices

Depending on the operational practices of a u tility, interrupting devices or d isconnecting devices may beneeded. These would be as follows.

Disconnect Switches Disconnect switches are used to provide a visible disconnect or in the eventof multiple cables per phase allow the utility to take a set of cables out of service but still operatethe other cables.

Circuit S witchers If s witching i s n ecessary at an o verhead t o und erground t ransition, and adisconnect switch does not have the required interrupting capability to switch out a cable underload, and/or to break parallel load between cables, and/or to break cable charging current, a CircuitSwitcher may provide the necessary interrupting capability. A circuit switcher typically isconstructed like a n air in sulated switch, but it is equipped with interrupting devices to break the

current and extinguish the resulting arc. Circuit switchers have short-circuit interrupting capabilityalso. Circuit Switcher manufacturers should be consulted for in terrupting capability and availablevoltage ratings. Circuit Switchers typically r equire a s eparate mounting structure, a nd they mayalso require an AC or DC power supply and communication equipment.

Circuit Breakers Depending on the amount of fault current, a circuit breaker may be needed toisolate the underground lines. Pre-insertion Resistors may be needed.

Grounding S witches A transmission c able is e ssentially a distributed cap acitor. W hen a

transmission c able is d e-energized, a t rapped ch arge r emains on t he cab le. W hen operatingpersonnel apply safety grounds to a cab le after i t has been d e-energized, significant sparking canoccur. W ith l ong c ables o r hi gher vo ltage c ables, t his s parking c an a pproach unne rving, andpotentially unsafe, levels. In these cases, a grounding switch may be necessary. A grounding switch

is a n ormally open switch connected between the line and s tation ground. Once a cab le has beende-energized, the grounding switch is closed to safely d ischarge t he cab le. I n m any cas es, agrounding switch is installed only on one end of a c able system. The operating procedure would

require closing the grounding switches first, then applying portable grounds to the other cable end.If a cable system terminates into Gas Insulated Switchgear (GIS), a g rounding switch should beincorporated into the first gas zone beyond the cab le t o al low for cable g rounding. Groundingswitches are sometimes i ncorporated i nto p rotective r elaying s chemes. A fter a m ixedoverhead/underground s ystem is interrupted d ue t o a n o verhead l ine f ault, a gr ounding s witchmight be closed to remove trapped charge o n the cable, and then opened pr ior to reclosing th e

overhead line.


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3.2.4 Grounding

Since a transition station is similar to a s ubstation, the g rounding o f the equipment within the tr ansitionstations would follow the utilities standard grounding procedure for substations.

Supplemental Grounding A separate ground conductor provided for bonding of cable accessories such asarresters, link boxes and terminations is recommended on the s tructure and this separate ground conductorshould be bonded to the station ground and the parallel ground continuity conductor.

3.3 Other Design Considerations

In addition to the design of the transition structures, the following items should also be considered.

3.3.1 Arrester and Termination Material

The following safety issue should be considered when cable terminations and surge protection devices areinstalled on transition structures outside and in substation environments.

Cable terminations ( potheads) an d s urge p rotection ( arresters) ar e av ailable w ith b oth p orcelain an dcomposite ho usings f or e xtruded-dielectric cables, w hereas p ipe-type cab les u se o nly p orcelain.Termination and arresters with porcelain housings have been known to f ail violently and this failure willspray the ground adjacent to the structure with porcelain fragments. These fragments (much like shrapnel)may d amage ad jacent accessories; p roperty a nd potentially s eriously in jure in dividuals in the im mediatevicinity of the structure.

Terminations an d ar resters f or extruded-dielectric cab les ar e al so av ailable w ith co mposite h ousings.Composite h ousing may f ail b ut th e f ailure i s l ess v iolent w hen co mpared t o p orcelain an d s hould b econsidered for transition structures in and outside of substations for the safety of the general public.

Some utilities have installed protective covers around porcelain terminators, especially when installed on

transition structures. T hese covers are installed to p rotect the te rminator from being hit from a projectileand to p rotect th e p ublic by c ontaining t he p orcelain f ragments in the e vent th e te rminator explodes.Depending on the spacing between the terminator and the arrester, these protective covers are d esigned to

encapsulate each terminator or the combination terminator and arrester. These covers are commonly madeout of fiberglass.

Placement of Arresters and number of arresters on a structure? Jay Williams.

3.3.2 Cable Support

Extruded cables installed when transitioning from overhead to underground, generally outdoors and in air,frequently require a s ystem o f cl amps o r cl eats t o s upport t he cab les. T he cab le s upport s ystem i sprincipally a r igid s ystem s uch th at lo ngitudinal o r la teral m ovement is not permitted. It is extremely

critical to restrain cable lo ngitudinal a nd la teral m ovement where the cab le en ters t he t ermination o n

transition structures.

For extruded cables, clamps or cleats are typically manufactured of plastic or non-magnetic metals such asaluminum alloys or to a le sser e xtent non-magnetic g rade s tainless s teel. W hen m etallic cl amps ar e

employed, as a rule, liners are used such as elastomeric materials. These liners are used to prevent damageor deformation to the cable insulation, designed to accommodate limited thermal expansion and provide therequired restraint. Cable clamps or cleats may also be designed to provide thermal expansion by using boltswith spring washers.


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Clamps or cleats typically installed on substation riser structures or riser poles and/or structures are spacedevery 3 t o 6 f eet. T he spacing i s a f unction of the cable d iameter, bending stiffness, weight and type ofmetallic s hield o r s heathing s ystem. T he p rimary sources for s upply o f cl amps o r cl eats ar e the cab lemanufacturers. Cable manufactures t ypically h ave a co mplete l ine o f cl amps o r cl eats f or t heir cab leconstructions, which are designed to accommodate a range of diameters, and weights. Cable manufacturerswill also provide recommendations for spacing of the clamps or cleats on their product.

The transitioning structure should also be designed to accommodate the cable clamp or cleat selected for theproject. B ased on the clamp design a steel structure or riser pole may require fangs or pre-drilled holes tofasten the clamps to the structure. T he fang locations or the pre-drilled holes will be spaced based on thefrequency of units necessary to restrain the cables.

Note that the s tainless-steel riser p ipes for p ipe-type cables must b e very securely at tached to a transitionpole to prevent thermal movement and vibration that could create a leak in the termination seals.

3.3.3 Ladder Clips

Some utilities still want the ability to climb any structure, so may require ladder clips to be installed.

3.3.4 Fiber-optic boxes

In most installations, the utilities have gone to installing OPGW on their overhead lines. Because of this, atransition o f th e fiber-optic cable also n eeds to b e made. T his i s done through a fiber-optic s plice boxmounted on the structure.

Temperature Monitoring boxes On cable installations where temperature monitoring or dynamic rating ofcables i s b eing p erformed w ith f iber o ptics, a separate fiber-optic s plice box m ay be r equired bycommunication p ersonnel. It may be necessary to plan for a lo cation of this splice box on the t ransitionstructure.

3.3.5 Link Boxes

The most common method of grounding the cable in a transition station or transition structure is through a

link box mounted to the structure.

3.3.6 Pipe-type Considerations

The most common and practical arrangement is to use a trifurcating joint inside a manhole located as

close as possible to the transition structure.

Three very short cable lengths are pulled, one into each o f three non-magnetic stainless s teel riser pipes.The terminators (potheads) are then in stalled at the to p end first. The temporary n ight cap bolted to the

trifurcating r educer at t he l ower en d i nside t he m anhole is then removed and the 3 1 /C cab le s plicescompleted.

The manhole is not an absolute necessity. The trifurcating joint can be direct buried but the necessity for

maintaining dry conditions and low humidity during cable splicing operations is usually accomplished

best using a permanent manhole.

Other arrangements such as an above ground spreader head or pull through trifurcator (no cable splice) arealso possible and sometimes used depending on the circuit configuration and transition structure design.

3.3.7 Current Transformer Design

In many of the installations the underground segment of cable is only a portion of the overall circuit, withthe protection of the cable, from a breaker or other device, is often some distance away and at the end of anoverhead section. In some installation, the utilities want to have additional data on the performance and


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functionality of the underground portion of the circuit only. In addition if the cable seems to have apotential problem then a quick determination and trip of the circuit may potentially save the whole circuit.

One method of detection is to install CTs on the cable and measure and track the readings. The CTsshould be mounted as close to the termination/transition as possible and can often be designed directly in

the termination support. The output of the CT is them routed to a central control box or termination pointand then to a central operating system to determine overall actions. Often on larger systems a dual CTsystem can be installed to further provide protection to the system. The CTs should be installed on allphases of the circuit.

3.3.6 Additional Mechanical Protection

When designing the structure, the area in which the structure is to be installed must be considered. Oftenadditional protection for the c able and the structure must be considered when completing the design. Thestructures n eed th is p rotection f rom p reventing o r d eterring in dividuals f rom th e o utside g etting to thestructure and damaging the s tructure or harming themselves. Many installations will be fenced off, b ut in

other areas, more stringent barriers such as concrete posts, o r steel structures are required, such as i n thecase when the structure is close to a r oad or in a p arking lot where it could be h it by a vehicle. Also notethat proper signage, color and location of markings needs to b e taken into consideration when completingthe final design.

Additional guards on the cable, such as a cable shield or shroud may be considered in areas where the cablemay be exposed. Wildlife guards and deterrents (Guard Owls) should also be considered in the final design.

3.3.6 Overhead Interface:

When designing the interface between the overhead line and the underground cable terminations,consideration should be given to the ampacity of the jumpers, the air gaps between energized conductorsand surfaces of poles and ar ms, insulation s trength of jumper s truts an d t erminations, working clearancebetween circuits for structures with more than one circuit.

The ampacity of the jumpers should be equal to or greater than the ampacity of the of the underground cablecircuit. The ampacity of the jumpers is calculated using IEEE Standard 738 with ambient air temperature,

wind speed, conductor resistance and the utilitys maximum allowable conductor temperature. Theallowable conductor temperature should take into account the maximum temperature of the terminal lug ofthe cable terminations as specified by the manufacturer. At higher operating voltages, 3 45kV and above,bundled conductors are specified to reduce audible noise generated by the conductor surface gradient. Theconductor size and bundle diameter is typically the same as the overhead line.

The air gap between energized conductors or h ardware and the surface of p oles and arms is the greater ofthe clearances specified by the National Electric Code and the air gap equivalent to the insulators used onthe structure. Also consider clearance to climb p oles an d ar ms an d to work w ith t he ci rcuit o r ci rcuitsenergized. If more than one ci rcuit is on the s ame s tructure, design t he s pacing to allow for work to be

performed on one circuit with the other in service.

Phase orientation should be designed to match phases o n the overhead and und erground circuits on bothends of the underground circuit. When using an installed cable as a s pare cable, o rientation and design of

the structure should be such that the spare cable can be easily connected to replace a failed cable.

3.4 Structure Installation

The te rminal s tructure a nd its f oundation s hould b e d esigned to act together as a s tructural uni t. T heinstallation of the structure will vary slightly depending on the type of foundation it is setting on. Utmostcare should be taken during the installation of the structure so that it is done in a safe and accurate manner.The i nstaller n eeds to h ave t he p roper eq uipment o f ad equate s ize so as to not damage the individualstructural components, the protective coating of the s tructure, a nd p lace it in th e c orrect o rientation


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according to the manufacturers and owners design. The use of slings may be necessary to properly lift andhandle the structure. The structure needs to be erected plum and the use of cambering in the design shouldbe discouraged. A ll bol ts should be properly torqued and installed in acco rdance with the manufacturersdesign. The structure should be grounded soon after it is set to prevent lightning damage. I f a reinforcedconcrete foundation is used to support the structure, the structure should not be set on the foundation until atleast 7 days after the concrete has been placed and the concrete strength requirements for t hat period have

been met.

3.5 Cable Installation

3.5.1 Pipe-type

For m ost p ipe type in stallations, cable r eels are typically p ositioned a t the t ermination s tructure a nd the

cables are pulled into the above ground risers toward an adjacent splicing vault or pit. However, differentmethods are to be considered when s ingle p ole s tructures ar e utilized, b ecause o f the increasingly l argevertical height of each riser and large separations between the cable terminations. One installation approachis to p osition the r eel or r eels at the nearest splicing location or s preaderhead, and make single conductorpulls toward each termination using a heavy duty bull lin e. T he tr ansition la yout a nd d esign may alsoshorten the length of the installation due to cable and pulling limitations.

3.5.2 Installation of Extruded Cable

Installation of the cable onto a termination structure is often governed by the type of structure and right-of-way constraints. Consideration must be given to location of a crane, manlift, payout reel, winch, and cables.

Since the location is a transition station, safe distance from energized conductors must be maintained, or ifpossible, a scheduled outage can be taken on the circuit. Whatever installation method is used, it is advisedthat t he cab le m anufacturer h as r eviewed t he p rocedure an d i s in agreement. It is important to alwaysprotect the

Documents

C17 Spring DraftGuideforPlanningandDesigningTransitionFacilities