Alstom Training Manual

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Introduction

o &tie" cwetYiew 01 substation engineering

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ubstations form on important port of the transmission and distribution networks

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of electric p;;,wer system. They control the supply of power on different circuits by

means of various equipment such 0$ transformers, compensating equipment,

:;) _{circuit breakers, etc. Various circuits are joined together through these components to}

bus bar systems at the substations. While the bus-bar systems follow certain definite

~ patterns, limiting the scaP'! for variation, there is practically no standardization

regarding the physical arrangement, called the layout of the various components

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_{relating to one another. For the some type of bus-bar system different layouts have}

been used in different countries and in fact in Indio there are variations in this regard

j _{not only among the various State Electricity Boards but also within a State Electricity}

Board. This manual gives the basic requirements ond for the sake of illustration

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} _{contains typical layouts for various types of bus-bar systems.}

One of the primary requirements of a good substation layout is that it should be as

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i economical as possible, but it should ensure the desired degree of flexibility and

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reliability, ease of operation and maintenance, expansion and meets all safety

requirements of the operation and maintenance personnel. Besides, the layout"

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should not lead to breakdowns in power supply due to faults within the substation, os

such faults are more serious. A brief discussion on the various components and

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auxiliary facilities required in substation and how they affect the layout is included.

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Many standards viz. IS,

as,

lEe, IEEE and the like guide the design of substations. It is

essential that the equipment used and the practices followed conform to the latest

standards, as required by the customer.

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This manual is aimed at understanding the basis of sub-station design. If deals with

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voltage levels between 33 kV and 400 kV and standard switching schemes. It also discusses, briefly about sele"~'on of major equipment.

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introduces lhe di(fll!rent types 01 sub·sla/ions

Generation station

Generation is done at 11 kV - 15 kV level. As power of very high capacities cannot be

.,;::nsmitted for long distances at these voltages it is stepped up using generator

transformers to 110 kV - 400 kV levels. Generation stations are. in simple terms,

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step-up stations.

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Grid station

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Grid Stations are used to interconnect different grids/regions/sectors. They are

generally 400 kV substations. They are stotions, switching power from one

generation/grid station to other. They can olso be called Switching Stations.

Distribution station

Distribution Stations are located at the load points where the power is stepped down to •

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11 kV -110 kV levels.

Bulk Industrial supply stations

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Bulk Industrial Supply Stations are distribution stations catering to one or 0 few

consumers. The supply voltage can range from 33 kV to 110 kV. Industriol users do

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have their own generotion focilities besides the. SEB supply and these s1a1ions oct as

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_{step-up stations as well.}

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Sur' stc :1S Sub-secondary stations can also be classified as Step-up stotions, Primary grid Stations, Secondary and Distributions stations depending upon their

POSHI\;,;n in the power system hierarchy.

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Generally the Substations are of outdoor type for 33 kV and above. EHV Stations can

be indoor depending upon the environmental conditions like, pollution, salinity etc.,

and space constraints. Indoor stations are Air - Insulated or SF6 gas - insulated

depending' upon the availability of space and financial constraints. Gas Insulated

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Substations (GIS) are extremely costly and requires extra maintenance and hence are preferred only when it is absolutely necessary.

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-Salient features of major equipment

Major eqc.. ,Omenl In a $vbslalion.

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r.... substation layout is influenced to a great ·~xtent by the dimension of the

eCjUlpment and their accessories within the substwlon.

Circuit Breakers

Circuit Breaker is a mechanical device capable of making, carrying and breaking currents undN normal circuit conditions and making, carrying for a specified time and

breaking .IS under short circuit conditions. Circuit Breakers of the types indicated

below are used in India.

36 kV Minimum oil/ Vacuum / Sulfur hexa fluoride (SF6)

72.5 kV Minimum oil/ Sulphur hexa fluoride (SF6)'

145 kV and above Sulphur hexa fluoride (SF,,).

245 kV and higher voltage outdoor circuit breakers, generally necessitate the

provision of approach roods for breaker maintenance.

400 kV CBs may hove pre-insertion resistors depending up on the system

requirement. When a CB interrupts a transformer or a reactor circuit, switching over

voltages can be' more than 1.5 p.u. or 2.5 p.u. respectively (maximum limit

recommended by IEC). resistors are required to prevent restrikes due to current

chopping. When lightly loaded tines are disconnected, interruption of capacitive currents take place causing restrikes which can set in oscillations of a few hundred Hz. CBs with self. generating pressure and comparatively slow contad movement, such as.,

bulk·oil, minimum- oil, SF" puffer type might restrike. However, modern SF6 puffer

type breakers are designed, restrike-free.

CBs can be live tank type or dead tonk type depending up on ihe substation design

and economy. Dead tank type CBs come by design with sets of current tronsformers

on the bushings. They are normally used in the l'h breaker or Ring bus scheme,

where, there are CT s on either side of the CB. This type of

ca

is less expensive when

compared with a live tonk type

ca

and two free standing (generally oil filled) CTs

combination. These are not popular in Indio.

Live tank CBs are used in other schemes where CTs are not required on either sides

of the

ca,

like double main scheme, double main transfer scheme etc. as they ore less

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Disconnect Switches and Earth Switches

Disconnect switches are mechanical devices which provide in their ope.. ' positions, isolating distances to meet the specified dearances. A disconnect switch can open and dose a circuit when either a negligible current has to be broken or mode or when ·';"ere is no significant change in voltage across the terminals of each pole of the

Qlsconnect. It can also carry currents under normal circuit !itions and the short

circuit currents for a specified time. Disconnect switches are used for transfer of load

from one bus to another cnd to i$« ,13 equipment for maintenonce. Although a

variety of disconnect switches are available, the fadar which hos the maximum influence on the station layout is whether the disconnect switch is of the verticol breok type or horizontal break type. Horizontal break type normally occupies more space

than the vertical break type. Between the horizontal center break and horizontal

double break types, the former requires large phase to phose clearance.

The location of disconnect switches in substations affects not only the substa,ian­ loyouts but maintenance of the disconnect contacts also. In some substations, the disconnects are mounted of high positions either vertically or horizontally. Although such substations occupy lesser area, the maintenance of those disconnect switches is more difficult and time consuming.

The disconnect switch serves as adamonaf protection for personnel, with breoker or!'ln, during maintenance or repair work on the feeder and also enobles the breaker ;... ,,;e isolated from the bus for inspection and maintenance.

Earth ~itch is a mechanical switching device for earthing different ports of a circuit,

which is capable of withstanding short-circuit currents, for a specified time but not

required to carry normal rated currents of the circuit.

Instrument Transformers

Instrument transformers are devices used to transform currents and voltages in the

primary system to values suitable for ins1ruments, meters, protective relays etc. They

isolo:e the primary system from the secondary.

Current Transformers (CTs) may either be of the bushing type or wound type. The

bushing type is accommodated within the transformer bushings and the wound types

are seporateJy mounted. The location of the

cr

with resped to associated circuit

for. Ihe wcund type CTs with dead tonk construction has been useo. Howeve,. current

transformers with live tonk construction also are being offered. It is ck:lImed thot These

transform"":; offer the following advantages:

• They .~ capable of withstanding high short circuit currents, due to their short and

ngid: mary conductar and hence more reliable,

• They r.:Jve "0W reactance and therefare hove better transient performance.

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These current transfarm€;: s do nat have their majar insulation over the high

currer' carrying primary. Therefore, the heat generated is easily dissipoted due to which "1e insulation has superior thermal stability and longer life. However, these

,,'"., have "mitations in withstanding seismic forces and have 10 handled and

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transported carefully, ,.".. -,'

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'..I ) Different classes of accuracy

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The two different uses of a CT are

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• Protection

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• Metering

These two requires conflicting properties of saturation, hence different types of cores

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are used. For protection, the CT should faithfully reproduce the changes in the current

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for higher magnitudes, whereas for metering, the CT should saturate at higher magnitudes in order to prevent any damage to the meters.

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Protection Classes· (110.

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, ; • PS

Closs PS CTs are Ot low reactance and their performance will be spec" . In

terms of the following charaderis:;cs.

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1. Turns Ratio, which will be numerically the same as the roled

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transformation ratio.

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2. Minimum Knee-Point Voltage (Vk), specified in accordance with the

' j formula; Vk

=

K I, ( R.:,

+

RJ

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K

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poromete~ specified by the purchaser, which depends on the system foult level and the characteristics of the refoy, intended 10 be used

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rated secondary current of Ihe CT

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resistance of the secondary corrected 1o 7O'"C

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impedance of the secondary circuit as pacified by the purchaser

3. Maximum Exciting Current, at the rated knee-point voltage or at any specified fraction of the rated knee-point voltage.

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In this way, a CT designated in terms of percent composIte error ond accuracy limit factor

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~ Composite errDI'". Ihe RMS value of Ihe difference oetweefl til" ,nSlontancous volues at Ihe prtmory current and lhe rated Iranstormohon rohO hOles the oct"ur secondary currenl. The standord composile errors '" ~rcent are 5. 10 and 15 P

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Protection

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Accuracy limit factor, Ihe ralio of the raled accuracy 1.01.1 pnmary :urreonllo lhe rated primClrf current, where raled occ:vracy Iim.1 primary current IS th. value of

lhe highest primory currenl up la which the transformer will comply w.th the specified limits of the compqsile error. The standard accuracy hmit foclors are 5. 1O. 15. 20

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Voltage Transformer (VTs) may be either Electro-magnetic type (IVT) or capacitor

type (CVT). IVTs are commonly used where high accuracy is required, like revenue metering. For other applications CIT is preferred particularly at high voltages due to their lower cost and can be used as a coupling capacitor, as well. for the Power line Carrier Communication (PlCq equipment. Each CVT will be earthed through an earth electrode.

For ground fault relaying, on additional core is required in the VTs, which can oe connected in open delta. The VTs are connected on the feeder side of the circuit breaker and on the bus bars for synchronization.

The standard accuracy classes for ClTs will be • for m~csurement, 0.2, 0.5, 1.0 and 3.0

• for protection, 3P and 6P

T. .ormer

Transformer is the largest piece of equipment in a substation ond it is, therefore, important from the point of view of station layout. For instance, due to its large dimensions and reliability, it is generally not possible to accommodate two transformers in adjacent boys. One of the problems could oe, the radiators being wider than the bay width.. In order to reduce the risk of fire, large transformers are provided with stone metol filled sooking pits with voids of capacity adequote to contain the total quantity of oil. Besides, separation walls are provided in-between the transformers and between transformers and roads within the substation.

One of the important factors governing the layout of the substation is whether the transformer is a three-phose unit or a bank of three single-phose transformers. The space required for single-phase banks is more than that with three-phase transformers. Besides, single-phose bonks are usually provided with one spare single­

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~olntenOr.ce 01 one d the single-phose uni~. Allernatively, the spore un:' [l~;:::, be

o~rmonen!iy installed in the switchyord ready to replace the uni:, wn::~ I~ ;)u! of

::;",'Vlce. Tni:;, however, requires on elaborate bus arrangement and isolalor SWitching.

Reactivi' Compensation Equipment

Reactive compensation may be switched or non-switched type as indicated by system

studies 01 Ine network. The non-switched type compensation usually comprises shunt

reactors p-:::rmonently connected to transmission line or to bus bars at the substation.

t-.lext to Ih· transformer, shunt reodor is the largest piece of equipment. These also

can be In the form of single-phase units or three· phose units. Often, neulral

grounding reador, which is connected between the neutral bushing of the line shunt

reactor the earth is provided to facilitate single·pole auto reclosing. Since these

equlprr; :00 contain oil, all fire-safety precautions that are token for transformers

should be followed.

Switched compensotion can be through switched reodors, switched capacitors or· thyristor controlled readors and thyristor switched capacitors known as Stotic VAr Compensators (SVC). These are selected according to the system requirements and conneded diredly to the system through their own dedicoted tronsformers. The shunt capacitor bonks ore composed of 200·400 kVAr copocitor units mounted on rocks in

series/parallel operated in.groups

to

provide the required reodive power (MVAr)

output at the system voltage. Mony.o.time only some of trese moy be required in the initial stage and may undergo alteration as the system develops.

Dired Stroke Lightning Protection

Any substation hos to be shielded from direct lightning strokes either by provision of overhead shield wire/earth wire or spikes (masts). The methodology followed for systems up to 145 kV is by suitable placement of earth wires/masts to provide coverage to the entire station equipment. Generally, 60° angle of shield for zones covered by 2 or more wires/masts and 45° for single wire/most is considered adequate. For installations of 245 kVand above, eledromognetic methods are used. The commonly used methods for determining shielded zones are the Mousa Method and Razevig Method.

Besides direct strokes, the substation equipment has also to be protected against

travelling waves due to surge strokes on the lines entering the substation. The

equiprlent most commonly used for this purpose is the surge arrestor 01 the line entry

of the __ ostalion. The most important and the costliest equipment in a sub_ .1110n is the trans: - -ner and the normal practice is to install surge arrestors as near the

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_{transL cner as possible. The fixing up of insulation level for equipment within a}

· \ :~;bstal;on requires a detailed insulation co-ordination s1udy with surge arrestor as the

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[ocal ~oint for protecting the equipment from power frequen-: ,-/er-voltoge exceeding

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the or- estor rating. Besides protecting the transformers, the surge arrestors also

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.J protee to the equipment located W"',in their protection zone Additional surge

arresters con be provided, depending up on, the isocerounic level, anticipoted

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overvohoges and the protection requirements.

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) Insulators

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Adequate insulation should safety of personnel. However, the station design should be provided in a substation for reliability of supply ond be so evolved that the_

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quantity of insulators required is the minimum and commensurate with the expected

security of supply. An importont consideration in determining the insulation in a

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substotion, porticularly if it is located near sea, a thermol power generating station or on industrial place, is the level of pollution, which can be combated using insulators of higher creepage distance. In case this does not suffice, the insulators need to be hot

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line washed periodically and this aspect has to be kept

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in mind while deciding the

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_{loyout of the substation. Another method, which hos proved to be successful, is}

-.:,..~iying suitable type of greases or compounds on 1he surface of the insulators ofter

cleaning, the frequency depending upon ~degree and the type of pollution.

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_{FOLLUTION LEVELS AND MINIMUM NOMINAL CREEPAGE DISTANCE TO BE}

ADOPTED AS PER IS/IEC

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Pollution Min. Norrinal Creepage Type of Pollution

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Level Distance (mm/kV)

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Light 16 Non-Industrial, Agricultural,

Mountainous areas beyond 20 Km from sea

~ Medium 20 Industrial Area without polluting

smoke and chemical effl uents and

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not too dose to sea

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Heavy

25

Industrial Area with polluting

smoke & chemical efffuents close

Very Heavy 31 Industrial Area subjected to conductive dust polluhon, smoke very close to sea, exposed to sea and very strong winds from sea, desert areas etc.

The highest line-to-Iine voltage of the system IS used to determine the creepage

.' , distance

The following types of insulators are normally used:

a) Bus Support Insulators

(i) Solid core type

b) Strain Insulators

(i) Disc insulators

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(ii) long Rod Porcelain insulators

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) (iii) Polymer insulators

Structures (3

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The cost of structures also is a major consideration while deciding the layout of a'

{) substation. For instance, in the case of flexible bus-bar arrangement, cost of

structures is much higher than in the case of rigid bus type. Similarly, the form of

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structures also ploys on important port and the choice is usually between using a few

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heOYy structures or more number of smaller structures.

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Hot-dip galvonized steel is the most commonly used material in Indio for substation

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structures. When, galvanizing is not effective; particularly in a substation located In

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coastal or industrial areas, paInting

becomes

essential.

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Power Line Carrier Communication (PLCC)

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The carner equipment required for communication, relaying and tele metering is

connected to line through high frequency coble, coupling capacitor and wove trap.

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The wave trap is installed at the line entrance. The coupling capacitors are installed

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on the line side of the wave trap and are normally base mounted. The wave traps for

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voltage levels up to 145 kV can be mounted on the gantry structure on which the line

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is terminated at the substation or mounted on top of the capacitor voltage

-transformer. Wave traps for voltage level:.; of 245 kV and above generally require

separate supporting insulator stock mounted on structures of appropriate height,

however, 245 kV wave traps can also be suspended from the line side gantry.

The differ-ent types of coupling used are

Incase of double circuit lines one phose on each circuit need be used lor communicotion. This type of coupling is called inter-circuit coupling.

• pr.~:e to Phose coupling

Incose of single circuit lines coupling con between any two pi-::Jses of

::) tne circuit depending up on the impedance of the phases

• Phose to Earth coupling

Any one phose only can be use~ for carrier communication where the

earth is used as the return path.

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Substation switching schemes

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dela11s the switching sCMmes

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election of a bus bar scheme for a porticular sub station is on important step in

design.

The

choice of the bus-switching scheme is ~overned by various factors,

which aim at a simple. ~elioble, safe and economic sub-station. Some 01 the

important fodors that dictate

the

choice of the bus-switching scheme are,

• System reliability and ovailat ,y

• Operational flexibility

• Limitation of short circuit level

• Simplicity of protection arrangements

• Ease of extension

• Availability of land

• Cost

The relative importance of these factors varies from case to case and depending on the voltage level, number of circuits, desired level of security, etc.

Types of schemes

The various bus-switching schemes that are in pradice are,

•

Single bus

•

Sectional Single bus

•

Main and Transfer bus

•

Double Main

•

Double Main and Transfer bus

•

One and Half breaker

•

Mesh scheme

Aport from these schemes, there are a few which are less frequently used

• Sectionolized Main and Transfer bus

• Double Main with bypass isolator

• Sedionalized Double Main and Transfer bus

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Criteria for seledion

lhe following criteria are usually followed when selecting a switching scheme for a sub-station.

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• It should be possible to take out any circuit breaker or any other equipment for 1J'0intenance without removing the corresponding circuit from service.

• The rr",in bus could be isolated for maintenance without loss of any circuit. • CB failure, Bus fault should couse minimum loss of circuits

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The acceptable level of reliability has not been defined by any standard and therefore

jt is extremely difficult to quantify it for a system. In such a situation, the prevailing practices and experience gained from system operation are token into consideration.

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For instance, in 400 kV systems, One and Half breaker scheme is preferred over other

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schemes os a major shut down cousing loss of 2 or more feeders is just improbable,

albeit being more expensive than others. Furthermore, for 220 kV systems, the Double

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Main T ronder scheme is preferred.

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Single bus bar scheme

This type of arrangement can be used only where interruption to service is relatively

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unimportant. But this is a simplest arrangement where each circuit is provided with its .:..;

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own circuit breaker .

The circuit breaker enables the feeder to be removed from service while it is carrying

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the loads when there is fault on the feeder. The disadvantage with this r-"'Ongement is

that if the incoming circuit breaker is to be shut down for mainteno ~1e load on

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that feeder has also to be shut down. If the bus is supplied by more t~1i one feeder,

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the reliability of supply to the feeders using this type of layout is considerably

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increased.

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_{Sectionalised single bus bar scheme}

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If adequate number of bus sections are made, the single sectionalised bus provides an

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'"" economical way of limiting circuit outage is case of fault on a bus sedion, as the

..,! section circuit breakers acts as backup to the circuit breakers of the main circuits. ThE'

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arrangement may be considered for intermediate switching stations or smo/i

generating stations where mil1imising of circuit outage is important for systern

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receiving supply from more thon <;me source to synchronise or segregate the supplies, as per. the opemting requirements.

The interlocking arrangement is simple for both the types of arrangements .

Main and

Transfer bus

bar

scheme

In this type of cnongement, the main ond transfer bus bors are coupled by means of

a normally open circuit

brealcer.

All

the incoming and outgoing circuits are connected

with the main bus bars through thei; controlling circuit br~kers keeping

the

transfer

bus idle. Each circuit is also connected to the transfer bus bar through on isolator.. In

case the circuit breaker of any circuit is shut down for maintenance, this circuit is

cannected to the transfer

bus

bar through its tronsfer bus isolator. Under such

circumstances,

Jhis

particular circuit will

be

controlled by the bus transfer circuit

breaker. Since the arrangement to the

transfer

bus is through the isolators coreful

interlocking

is

necessary

with

bus transfer breolcer so that only one circuit transferred

at a time.

Double bus

bar scheme

In this arrangement, each incoming and outgoing circuit has its own controllingdrcuit

breaker and,

bt·

means of

bus

selection isolators, can be conneded to either of the

buses. Each bus bar is designed to take the station total load and either bus bar

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be

token out for moin1enonce. Each circuit may, in addition, be provided with a

bye-pass

isol....,

enabling it to be connected directly to one of the bus bars. bye­

passing the controlling circuit breakers of the circuit. The circuit can in that case

be

energised through the bus bar coupler circuit breaker as in the main and t,ansfer bus

scheme, and the controlling circuit breaker of the circuit token out for mainter.ance.

Use of a bye-pass isolator with double bus necessarily requires adoption of a relatively

costly switchyard arrangement. It should, therefore, be resorted to only in case where

outage of the particular circuit will have undesirable repercussions on the system operation. In case maintenance of the circuit Ix-eakers can be arranged by taking the relevant circuit out (e.g. where two or more circuits or alternative routes are available),

the normal double bus arrangement without bye-pass should be favoured as simpler

and cheaper physical layouts can then be used. The two buses of a double bus bar

arrangement enn be sectionalised through circuit breokers or isolators as required from reliability considerations.

)

)

Double Main and Transfer scheme

In this scheme, ther!! are two bus bars, which can carry the total ~tation load, and one

spore bus bar, which can carry the load far anly one bay. As in single main and

)

transfer bus bar scheme, the transfer bus bar is "jle and the feeders are fed from .) _{either of the buses which, can}

_be

_{selected through isolators. There is a bus coupler to}

couple the bus bars and a bus transfer bay to couple the main buses ond the transfer

,)

bus.

Careful interlacking is required to transfer anly one bay at a time. Interlockin!3

~

_{..s~erne is complicated whereas; the protectian scheme is simple.}

()

---~---.-..

.

One and a half breakers scheme

0

"

a

I

•

In one ant.. ;l holf breaker scheme, three circuit breakers are used for connecting two

0

feeders ond hence the name. This scheme is more flexible than any other scheme

described previously and the continuity of supply is assured. Interconnection of grid feeders in each boy can be maintained even without energising the bus bars. The

.t)

feeders con

be

energised without energising the bus bars. If one of the breakers in

0

one boy requires· any maintenance, it can be attended to by keeping the other two

breoken in circuit. This scheme ovoids the necessity of bus coupler circuit.

•

e·

Interlocking scheme is simple with this arrangement. The only disadvantage is that it

is a costlier orrangement and the protection scheme is complicated. It is often

0

questionable whether the expense of such an arrangement is justified and it should be

•

used only where the importance of the continuity of service warrants it.

•

Mesh scheme

~

Mesh scheme contains a ring with circuit breakers as many as the number of feeders,

a

with associated isolators. Each feeder is connected between two circuit breakers. This

;)

provides a double feed to each circuit; opening one breaker for maintenance or

)

otherwise does not affect supply to any circuit. AI! sections of conductor in the station

ore covered by the Feeder differential protection and no separate bus protedion is

)

needed. Though it is cheaper than the double bus or main and transfer bus schemes,

)

it would be advisable to use mesh arrangement only at substations where a limited

:>

number of circuits are to be conneded. However, in Indio 1112 breaker scheme IS

preferred to mesh scheme.

)

)

layout consideraUons and clearance

deloils Ihtf swilching sdurm«S

O

verall system security and reliability of supply to consumers is dependent on the cumulative effect of the reliability of individual systems and components of

the power system. For instance, the reliability of the step-up switchyord of 0

generoting station is of utmost importance for the overC'; ~Iiability of a power

network, since loss of 0 generator or group of generators may result in not only

interruption of supply to loads but a;~') sequential tripping of other generotors ond

instability.

The

main planning philosophy of a grid is to ensure that available

generation is transmitted reliably even under conditions of outage of a transmission line.

a. Feeder Oearance

Feeder Fault· Ideally, only one circuit breaker has to operate to isolate a faulty feeder. However, certain schemes like Breaker and Half requires operation of two breakers to isolate a fault.

Bus Fault -Though the bus faults are rare in switchyards, these may lead to extensive loss of generation or circuit outage occur because all breakers connected to that particular bus have to be opened to isolate the faulty bus. The aim of the design is to

F ;It the loss of generation or circuit outage to the maximum extent possible.

In two bus bar schemes, continuity of supply is maintained even in case of a bus fault

becalJse each circuit is feed through two paths.

b. Failure of main equipment and bus bar components

The reliability of a switchyard is directly related to the total number of

equipment/components and failure rate of each of these. Ideally, when any

equipment or component fails out~Clge of feeders should be minimum. The effect of

failure af these is discussed below:

Equipment Failure - Though experience says that main equipment are quite reliable, substation design has to cater to failure of main equipment without disturbing the

continuity of supply, as for as possible. Albeit, stuck breaker condition is uncommon

.~---'

t2

-

,

~

Q

_}, I

0

0

0

0

•

0

..

~ ~

"

:J

.~ ... '::l

....

.

...,.

;--,. "',;

•

stuck breaker would result in loss of either one or two feeders only, depending on

which (bus side or tie) breaker is stuck.

Component Failure - Failure of bus bar components like clamps etc. is more

common than equipment failure. Component failure would result in conditions

identical to those in bus fault. It is, pertinent to recognise that for any failure of

components or faults in the feeder boys, there shall be no or minimum inte~ruption of

service.:

c. Redundancy in Design

..

The reliability of a feeder con be increased by providing redundant paths either active

. or standby dePending on whether these are permanently connected in service or are

switched on when required. Each feeder is fed from two paths and has definite

advantage during bus fault or stuck breaker because alternate poth is available. Even

during breaker maintenance, because of this active redundancy only less number of

breaker and disconnector operations are required. The only drawback with active

redundancy is the requirement of interrupting bath feeds during isolation of a feeder.

Operational Flexibility

Operational flexibility in a sub-station is th~ possibility of achieving the different

switching arrangements, which may be required, and the ease of changing from one

arrangement to another.

Simplicity of protedion Arrangements

More the number of circuit breo!:ers required to be tripped during fau! ,ditions,

more complicated will be the protection arrangement. This is porticula.) ;0 when

automatic operanon or redosing is used. Some schemes require operation of one breaker while others two. However, the situation is different when a breaker has to be taken out for maintenance. In some schemes like Double Main Transfer, the trip circuits have to be transferred to the bus coupler/bus transfer breakers. While in Breaker and Half scheme no such transfer is necessary. Further, multiplicity of bus bars and the provision of connecting a feeder to either of the two buses may complicate the bus differential protection but in Breaker and Half scheme bus differential protedion is simple.

)

, From maintenance, the best scheme is one in which each component can be taken

J

out for maintenance without any loss of feeder and with ease of changeover. Circuit

breaker manufacturers hOYe bas~ the design of EHV circuit breakers on modular

concept The maintenance period is dependent on mean annual duration of circuit breaker maintenance.

Ease of Extensions

Substation arrangement should be suitable far further extension without loss af

feeders. ."\

'" .'

"' _Interlocks

:J

,~," interlocking arrangement between circuit breakers, disconnectors and earth switches

...;

)

should be suitably designed to provide security in operation and avoid catastrophic

1)

_{consequences arising out af operators' mistakes.}

<.J

Disconnectors are interlocked electrically or mechanically, such that they cannot be

,~

operated unless the associated circuit breakers are opened. Earth switches are

0

electrically interlocked such that they cannot be aperated unless the associated

•

disconnedars are opened. Circuit breaker cannot be operated locally unless its

associated earth switches are in the dosed position.

0

()

_USYOUT

designing a switchyard layout, various aspeCts are considered which are

•

aescribed hereunder.

,

•

~ CLEARANCES

The position of equipment in an

EHV

switchyard is greatly influenced 'by the air

Z)

clearances to be adopted. Two types of air dearances are calculated for the

r)

purpose, which are phase to ground clearance and phose to phase clearance.

) _{Sedional clearance in} Q swilchyard is derived from these, which is used for safety

'\

.I reasons during the maintenance of equipment.

-The clearances are calculated considering the insulation levels adopted for a system.

400kV 220 kV 132 kV 110 kV 66 kV

Highest System kV

420

245

145

123

72.5

,

-LIghtning impulse with standvoltoge kV" 1425 10501 950 6501 550 5501 450 325 SWitching surge withstond voltage kVp 1050

1 min. Power freq. Withstand voltoge

kV,.... 630 460 140

Phase to ground Clearance

-'

The phase to ground cleorances for a substation is calculated considering various ...

-electrode configurations and their eJedrical response to the above mentioned

overvoltoges and the highest of the values is adopted.

• >

0

()

This he . ·.!r does not apply to the length of post insulator where the phose to .

"

'J _J} ground . orance can be adopted

based

on tests conduded on them and margin for

inaccuracy in erection & variations in equipment geometry is provided.

0

3

_{Phase to Phase Clearance}

()

It is well known that lightning surge stresses between phases will not be normally

D

•

higher than phase to ground lightning. surge stress. Considering this asped the

phase to phase clearance is calculated

hosed

on switching surge stresses for system

a

voltages above ~45 kY. A design margin is provided for the inaccuracy in erection,

variation in equipment geometry.

•

~

\ I

Sectional clearance is obtained by rounding off the sum of PIE clearance and

9

clearance to the ground from the lowest port

of

insulator.

:3

Minimum Cearances

Based on CSIP Manual on Substation Equipment, Illumination & layout, Dec. 1996.

-a

_inm 400 leV 220kV 132kV 110 kV 66 kV

f}

Phose - Phose 4.2 2.1/1.9 1.3/1.1 1.1/0.9 0.63

:)

Phose - Earth 3.4 2.1/1.9 1.3/1.1 1.1/0.9 0.63 Sedion 6.5 5/4.5 4 4/3.5 3

')

Ground 8 5.5 4.6 4.6 4 "~ Boy Width 27 18/17

12

10 " .:i '"\ J ""\ ./ _{Equipment Spacing}

')

. I J ~"\

.­

....

_,...,;

a

0

i _;~

{

"

•

_;3

a

•

e­

•

0

•

·3

!)

3

.)

)

•

)

.,.,

.;

.)

)

)

-The spacing for the placement of equipment, between them is decided by considering • Terminal clamps of adjacent equipment.

• Ease of maintenance/removal of equipment

• Equipment foundation & their coble trenches.

• Land availability

• Distance between LA and protected equipment has been decided considering

protection reach of LA.

Bus Bars

The bus bars of 400 kV Switchyard .·,.,1 consist of flexible and rigid conductors

conductors.

Sequence of installation of wave tra'ps lightning arresters and capacitive

vo~tage transformers:

The sequence of installation of line traps, lightning arresters and capacitive voltage transformers is decided based on insulation co-ordination considerations.

Structure

All switchyard structure will be designed for a factor of safety of 2 under normal

conditions and 1.5 under broken wire conditions & 1.1 under combined short circuit

& broken wire conditions. A slightly higher vertical load sholl be considered in design to toke care any future increase in load during replacement. The gantry

structures are designed to terminate the conductor at 30_, degree angular deviation,

hOWF.!'Ver considering design safety the allowable maximum angular deviation is 15

a~·:;lees.. The maximum wind loading will

be

taken

os

per IS ; 802. The structure

sholl be hot dipped golvonised.

Equipment Supports

Support design sholl be done by considering the most severe conditions of wind and

short circuit forces. Support structures are foreseen to be lattice type.

Road Layout

Proper road facilities sholl be provided so as to facilitate movement of the heavy equipment and machinery. Roods are provided throughout the periphery for security & patrolling and also across the switchyard as well as ease of maintenance.

...

../

Bus Post

Insulator

" ~ ~)

;0

~

0

i)

•

_D

•

~ .-", i ~

7#

I

)

" V

.J

;J

..,

~. -'

.

~ "! ~

s"ltKfIDn of a bus pos/ insula/or

S

election of a bus post insulator is based on both electrical and mechanical

requirements. This chapter deals with both el~ctrical and mechanical design.

Electrical design

The important parameter which are to be considered in post insulators designs, for

use in outdoor and indoor substations, are the basic insulation level (impulse withstand voltage), temporary over voltage, switching surge, dry and wet power frequency voltage, creepage distance, corona and radio interference voltage.

For s~stem voltages up to 300 kV the Basic Insulation Level assumes importance in

the design, whereas for higher system voltages the bosic characteristics of the insulators are determined by Switching Surge Level and Creepage Distance.

Mechanical design (Ref: Electrical Enginetlf"s' Handbook by Knowlton)

Post insulators for supporting bus bars and disconnecting switches have to be

designed to withstand abnormal operating loads, viz., electromagnetic force due to

short-circuit, seismic load ond wind load .

Short Circuit Force

Short circuit due to electro -mognetic force,

N X M x K x 2.05 \2 x Lx 10.8

Fs =

p. Where,

Fs = Electro-magnetic force in Kgf.

= Peak valve of maximum short-circuit current.

p.

₌

Center to center spacing between phases in meters.

l = Span between two supporting points in meters.

N = Correction fodor for actual field condition.

K ::: Correction fador for shope and arrangement of buses, for tubular

M

=

Multiplying factor

Short Short-circuit current(l) M Force on eonductor

Circuiting expressed as

(AI ·IS1 Max. peak 1.00

!A)-(B)·,q R.M.S., Asymmetrical 2.66 j,/ or

8

R.M.S., Symmetrical 8.00

IAI. (B).IC'-represent phase conductors

·1 H) - represent short-circuits between phose c:onduc:IOrs

,-~.

Generally multiplying fodor, M. is token to be 8, considering the worst condition of a

-'

~,

.

three phose symmetrical fault.

. ,

~'\ _{, ,}

The fadar N is generally used for calculating the steady short circuit force to which the

()

support insulators are to

be

designed for field conditions. Analysis show that the value

for N can be 0.4 to 0.45 for three phose and phose to phose faults for most of the

J )

f _{field conditions. Although strudure could be safely designed assuming even smaller}

·5

_{values for N, a value of 0.5 is token generally.}

{)

The fador N X M is called as Stress factor.

it)

I

_{Seismic Force}

..

The predominant frequency range of seismic vibration is considered to be in the range

of 3 Cps to 15 Cps, which is dose to the frequency spectrum for electrical switchgear

&

and the:r insulator iUPPOrts. The horizontal earthquake fprce component,

i

FE

=

S x W

» .... ,

•

~

""

_F,

₌

_{Horizontal earthquake force component in Kgf.}

'"

_S

₌

Seismic intensity. A fador of 0.25 is considered to be in the very

't:# strong. 1) W

₌

Weight of insulator in Kg.

D

Wind Force ~)

Force due to wind pressure is one of the important criteria to be considered in the

. C"\

. J mechanical design of support insulators .

--:;,

The wind pressure is calculated based on measured wind velocities, called Basic wind

..., _{speed in different regions. The wind pressure in kg/m}2_{s given by the relation (in IS}

802) is, I

"

--..

:.:" p

=

0.6 X V;l

)

f~

.~

Where,

,)

_v,

_{= Vt, x k, x k;}

basic wind speed, m/s

J

.,

k,

=

0

wind force in kg is 1·-.... _F.

=

_{p x Lx B x 1.2 x 1.92} ''-<II Where,

10

L

_

. length of the insulator

.

""

J B

=

breadth of the insulator

0

Generally, 5% design margin is added to Ihe calculated wind force.

;D

..J For bus c..:.rs, wind pressure is assumed acting on full projected area whereas, for support insulator, the effective projected area of the insulalor is assumed 50% of Ihe

•

I

projected area. Tha wind pressure acting on a column is considered uniformly

disturbed load for bending moment calculation.

•

8

The cantilever load at the support insulators is calculated considering lotal load either

due to

short-circuit force and wind force

or

short-circuit force and

earthquake force,

whichever is

higher.

This is due to the fact thallhe occurrence of earthquake and maximum wind pressure together with the Electro-magnetic force

•

8

'.

under short-circuit condition is most unlikely in actual serVice.

•

~

i

0

0

Bus Hars

-"': , "

.

~~) J

/

\)

.~

:)

"

'lJ

,') Bus·8ft, "~tr,*,'

•

.

"'"

)

) Sclce 'n of blls bars

B

US bars are either rigid or flexible type. In the rigid type, ,PIPes/tubes are used

for bus bars for making connections to the equipment wherever required. The

, bu's bars and the connections are supported on p e . ; insulators. Since thf

bu;; bars are rigid, the clearances remain constant ana as the bus bars and

conn~ctjons are not very high from ·'.d ground, their maintenance is easy. Due to

large diameter of the pipes, the corona loss is substantially reduced. It is also claimed that lhe system is more reliable with the rigid bus than thm with the flexible bus .

The flexible type of bus bars is on overhead system of conductors strung between supporting structures and flexible type insulators. The stringing tension may be limited to 5· 9 kN for installations up to 132 kV. For 220 kV and 400 kV installations limiting tension for a sub-conductor (of a bundle condudor) may be as high as 20kN. Design of structures for 245 kV and higher voltage substations can economized by suitably locating the spacers in the conductor bundles.

The materials in common use for flexible bus bars and connections are Aluminum

Conductor Steel ~einforced (ACSR) or Ali A1umioom Conductors (MC). For the rigid

bus bar, aluminum pipes of Grode 63401 WP conforming to IS: 5082 is commonly

U$~j Copper rigid bus bars can also be

used,.

however their use in Indio is not

encouraged due to reasons of economy and pilferage. In case of fong spans,

expansion joints should be provided to avoid strain on

the

supporting insulators due to

thermQI expansion or contraction of pipes. In adcition to this, at I~ast one end of bus

bar WIll be provided with expansion damps and circuit breakers and transformers will

alwoys be provided with expansion clamps

to

toke core of the vibrations during

operation .

The bus-bar sizes should meet the electrical and mechanical requirements of the

specific application for which these are chosen. Rigid Bus Bor

Rigid bus bars Can be mode of copper or aluminium. Aluminum bus bars are

the weight of the tube. Electncol and mechamcal characteristIcs nove to be token in to considerat:on while deciding on a rigid bus bar.

Electrical

The electrical parameters that have to be considered for deciding on a bus btlr are. Continuous current rating and

Shof' ';me current rating

Continuous current ratings .n indoor and outdoor conditions will be different due

convection of heat produced due to f1R effect. Short circuit current rating for 3s will be

1/\'3 times that of 1 s rating.

The area of cross section In mm1 _{required to corry the short circuit current for the}

specified time is, ". I"

x

"t

x

(2SAf

A

=

),

0

₁₄

_X

₁₀

4

X[Og

[T...

+

258 ]]0.5

9

To

+

258 Where,

,~ _{symmetrical short circuit current in A}

I"

=

t

₌

duration of fault in seconds

;8

_To

₌

_{initial temperature of the condudor before short circuit in °C}

T",

=

final temperature of the condudor after short circuit in PC

0

Mechanical

,9

The mechanical characteristics thot has to be considered for seledion of a rigid bus _,

F"'.

...

_{bar are} Bending Stress Vertical Deflection

"

_{Aeolian Vibration}

Bending stress

Three loads which causes the bending stress are, , \

Wind load Short circuit Force Dead load Wind load

Wind load on bus bar per meter length,

WI

=

pxD

Where,

p

₌

wind pressure in kg/m1

S~;:)rt circuit force

10 8

N x M x K x 2.05 x I" x

Snort cir.::uit force per meter length :=

\Nnere.

Fs

Electro-magnetic force in Kgf.

=

Peak value of maximum short-circuit current.

P,

₌

Center to center spacing between phases in m.

-c-L Span between two supporting points in m.

-r"~

.. J

N

₌

Correction factar for actual field condition, for calculating steady

"~ _{.,- force N=O.S}

K = Carrection factor for shape and arrangement of buses, for tubular

;~)

buses K= 1

..(" M = Multiplying factor, for 3 phase symmetrical faults M=8

,

(}

Both wind load and short circuit force act in the horizontal direction whereas the force

0

due to the weight of the bus bar acts vertically.

0

The bending stress on the rigid bus bar is

·0

=

MIl

8

where,

g

_M

=

Be:lding moment in kgm

=

WL'/8

•

~

_..

\.

=

_{resultant force in kg/m}

L

₌

length of the bus bar in m

Z

₌

section modulus m3

,-"\ _{The maximum allowable bending stress in aluminium alloy is 2.1098 x 10}7

kg/m2 ."",

and the factor of safety specified by IE rules is 1.5.

a

')

Vertical deflection

.,..,.

The vertical deflection is ~ 0.0054 X LA X W )

=

Ex MI Where, L = unsupportedlengthinm

W :: _{weight of the tubular bus bar in kg/m}

E

== Young's modulus in kg/m'

) A

MI == moment of inertia m

,

.

Tne verlical ceflecllon should be less than half the diameter of the tube or l/200.

Aeolian vibration

The natural frequency of vibration is

::: 5.61/ ~deflection

which should be more than 2.75 cycles per second.

-~ Flexible Bus Bar

,'\

,­

. for flexible bus bar, Sag tension and spacer spon calculations are performed .

...

)

-j } "'\

1)

0

0

·V

"'"

0

fl)

..,

"""

:0

"

~ ~

D

D

..;

..

} ~l

-Grounding

IEanhingl

Grounding is very essential for ensuring saltily for personnel ~'d equipment

t : '

rounding is done to provide means to carry electric currents into the earth under

I',J

normal and fault conditions without exceeding any .. aling and equipmen!

...

'

, _{limits or cdversely affecting continuity of service and to assu~e that a person near}

grounded facilities is not exposed to tl- danger of critical electrical shock,

-',

Grounding can be of one the following two types

...,.. Intentional

(

-0

This consists of ground electrodes buried to about 2.5 to 3 m below the earth

Q

_surface.

0

Accidental

,~

g

This is temporarily established by a person or a thing (good or poor

1) conductor) exposed to a potential gradient near a grounded facility.

J

CL:umstances that lead to a shock:

0

1 . Relatively high fault current to ground in relation to the area of ground system

and its resistance to remote earth.

2. Soil resistivity and distribution of ground currents such that high potential

gradients may occur at some points on the earth surface.

3. Presence of on individual at such a point times and positions that the body is

bridging iwo points of high potential difference.

4. Absence of sufficient contact resistance or other series

.

resistance, to limit

current through the body to a safe value, under the above circumstances.

5. Duration of the fault and body contact, and hence, of the flow of . current

through a human body for a sufficient time to couse harm at the given current intensity.

The relative infrequency of accidents of this type, os compared to accidents of other

kinds, is due largely to the lo,!", probability of coincidence of all the unfavorable

(1977 edition) recognizes this lOW' probability and allows reduction for grounding

calculations of a given fault current magnitude by a certain foetal. A 0.7 value is

-,

recommended for stalions of 110 kV closs ond above.

Importance of High-Speed Fault Clearing:

,

, j

Considering the significance of fault duration, high-speed clearing of ground faults is advantageous for two reasons: .

!~

,I0'I0-;

> - - '

1 . The probability of electric shock is greatly reduced by fast fault clearing time,

:J

_{in contrast to situations in which fault currents could persist for several minutes}

"\) or possible hours.

2. Both tests and experience show that the chance of servere injury or death is

...J

_I

greotly reduced

if

the duration of a c~rrent flow through the body is very brief;

0

the allowed current value moy therefore be based on the clearing time of

primary protective devices, or that of the back-up protection.

9

a

Effed of Reclosing:

Redosure ofter a ground fault is common in modern operating practice. In such

circumstances, a person might be subiected to the first shock, which would not

..

•

~

permanently injure him, but would upset and disturb hiJ!l temporarily. Next, a single

~ fast automatic redosure could result in a second shock, occurring after a relatively

•

short interval af time bek:9 the person has rlKOVered, thot might cause a ,,!.tfJaus

accident. With manual redosure, the possibility of exposure to a second > k is

reduced since the redosing time interval may be substantially greater.

Potential Difference during Shock Situations:

Ground Potential Rise (GPR): The maximum voltage that a station grounding grid may attain relative to a distance grounding point assumed to be at the potential of remote earth.

Step Voltage: The difference in surface potential experience by a person bridging 0

distance of 1 m with his feet without contacting any other grounded obiect.

TCAP -~. I

.

'­

...

-' I f

t3

a

·3

D

•

0

•

~ ') ~ ~.

r.,l

.:,.­ ,. 0

)

Touch Voltage: The potential difference between the ground potential rise (GPR) and

lne

surfc::ce potential at the point where a person is standing. while at the some time

navinfl !~!5 hands in contact with a grounded structure.

IIOTE: .1 convenhonol sub!Jolion, the wont ,ouch voltoge is usuolly found the potenhol dlHerence

oetwee" 'IOnd and the feel 01 a point of nlOIIimum ,each distance. However, ,n the ...~a of'o metol-to· .ne.ol c. :.:Jct from hand-to-hand or from hand-to-leel, which is of concern in the gos·insulo.ed \ubstohc" , both sIIuaIions should be inve!Jigoted for lhe possible worsl reach condition, ,ncludlng both 'lands.

Mesh Voltage: The maximum touch voltage to be found within a mesh of a ground

grid.

Transferred Voltage: A special case of the touch voltage, where a voltage IS

transferred into or out of the substation .

Calculations based on IEEE Guide for safety in AC substations - ANSI/IEEE

Std 80 - 1986

Sizing the Conductor

The area of cross section for the conductor is given by the expression

te

a,

p,

10"

=

./

Where A

=

T", To

₌

T,

=

00 =: a, p, = RMS current in kA

conductor cross section in mm2

maximum allowable temperature in °C

ambient allowable temperature in °C

reference temperature in °C

thermal coefficient of resistivity at 0 °C

thermal coefficient of resistivity ot reference temperature T,

resistivity of the ground conductor at reference temperature T r in

~

1

I

ao

or ( 1 / ex,. ) - Tf