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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 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 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 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 contains typical layouts for various types of bus-bar systems.
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One of the primary requirements of a good substation layout is that it should be as economical as possible, but it should ensure the desired degree of flexibility and
reliability, ease of operation and maintenance, expansion and meets all safety requirements of the operation and maintenance personnel. Besides, the layout" 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 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 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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Substation types
introduces lhe di(fll!rent types 01 sub·sla/ions
Generation station
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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, step-up stations.
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Grid station
(.)
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.
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Distribution station Distribution Stations are located at the load points where the power is stepped down to • 11 kV - 110 kV levels.
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Bulk Industrial supply stations
•
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 have their own generotion focilities besides the. SEB supply and these s1a1ions oct as step-up stations as well.
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Sur' stc POSHI\;,;n
can also be classified as Step-up stotions, Primary grid Stations, Secondary Sub-secondary stations and Distributions stations depending upon their in the power system hierarchy. :1S
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 Substations (GIS) are extremely costly and requires extra maintenance and hence are preferred only when it is absolutely necessary.
Salient features of major equipment
Major eqc.. ,Omenl In a $vbslalion.
T
r.... substation layout is influenced to a great
r .,
·~xtent
by the dimension of the
eCjUlpment and their accessories within the substwlon.
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Circuit Breakers
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Circuit Breaker is a mechanical device capable of making, carrying and breaking
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currents undN normal circuit conditions and making, carrying for a specified time and breaking j
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under short circuit conditions. Circuit Breakers of the types indicated
below are used in India.
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36 kV
Minimum oil/ Vacuum / Sulfur hexa fluoride (SF6)
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72.5 kV
Minimum oil/ Sulphur hexa fluoride (SF 6)'
145 kV and above
Sulphur hexa fluoride (SF,,).
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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
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requirement. When a CB interrupts a transformer or a reactor circuit, switching over
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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
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currents take place causing restrikes which can set in oscillations of a few hundred Hz.
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CBs with self. generating pressure and comparatively slow contad movement, such as.,
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bulk·oil, minimum- oil, SF" puffer type might restrike. However, modern SF 6 puffer type breakers are designed, restrike-free.
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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 CTs on either side of the CB. This type of compared with a live tonk type
ca and
ca is less expensive when
two free standing (generally oil filled) CTs
combination. These are not popular in Indio. ~
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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
PlCnensive than dead tank CBs.
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
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Qlsconnect. It can also carry currents under normal circuit
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!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$«
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equipment for maintenonce.
Although a
variety of disconnect switches are available, the fadar which hos the maximum
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influence on the station layout is whether the disconnect switch is of the verticol breok
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type or horizontal break type. Horizontal break type normally occupies more space \
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than the vertical break type.
Between the horizontal center break and horizontal
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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.
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The disconnect switch serves as adamonaf protection for personnel, with breoker 11\
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or!'ln, during maintenance or repair work on the feeder and also enobles the breaker ;... ,,;e isolated from the bus for inspection and maintenance.
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Earth
~itch
is a mechanical switching device for earthing different ports of a circuit,
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which is capable of withstanding short-circuit currents, for a specified time but not
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required to carry normal rated currents of the circuit.
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Instrument Transformers
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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.
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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
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are seporateJy mounted. The location of the
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breaker depends on the protection scheme and the layout ofsubstotion as. well. So
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:
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• They
capable of withstanding high short circuit currents, due to their short and
ngid: mary conductar and hence more reliable, • They r.:Jve
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reactance and therefare hove better transient performance.
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
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handled and
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transported carefully, ,.".. -,' .~
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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 are used. For protection, the CT should faithfully reproduce the changes in the current 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
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low reactance and their performance will be spec"
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terms of the following charaderis:;cs. 1. Turns Ratio, which will be numerically the same as the roled
transformation ratio. 2. Minimum Knee-Point Voltage (Vk), specified in accordance with the formula; Vk K
= K I, ( R.:, + RJ
-+ poromete~ specified by the purchaser, which depends on the system foult level
and the characteristics of the refoy, intended 10 be used
-+ rated secondary current of Ihe CT R.:, -+ resistance of the secondary corrected 1o 7O'"C ~ -+ impedance of the secondary circuit as pacified by the purchaser
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3. Maximum Exciting Current, at the rated knee-point voltage or at any specified fraction of the rated knee-point voltage.
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
secondary currenl. The standord composile errors '"
P -+ Y
rohO hOles the
~rcent
oct"ur
are 5. 10 and 15
Protection
-+ 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
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lhe highest primory currenl up la which the transformer will comply w.th the specified
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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
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type
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(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
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their lower cost and can be used as a coupling capacitor, as well. for the Power line
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Carrier Communication (PlCq equipment. Each CVT will be earthed through an
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earth electrode.
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For ground fault relaying, on additional core is required in the VTs, which can oe
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connected in open delta. The VTs are connected on the feeder side of the circuit
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breaker and on the bus bars for synchronization.
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The standard accuracy classes for ClTs will be
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for m~csurement, 0.2, 0.5, 1.0 and 3.0
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for protection, 3P and 6P
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.ormer
Transformer is the largest piece of equipment in a substation ond it is, therefore,
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dimensions and reliability, it is generally not possible to accommodate two
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wider than the bay width.. In order to reduce the risk of fire, large transformers are
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provided with stone metol filled sooking pits with voids of capacity adequote to contain
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important from the point of view of station layout.
For instance, due to its large
transformers in adjacent boys. One of the problems could oe, the radiators being
the total quantity of oil. Besides, separation walls are provided in-between the
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transformers and between transformers and roads within the substation.
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One of the important factors governing the layout of the substation is whether the V
transformer is a three-phose unit or a bank of three single-phose transformers. The ~.
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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 phose transformer, which is kept in the service boy and used in case of a fault or
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01 one d the single-phose
o~rmonen!iy
installed in the switchyord ready to replace the uni:,
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Allernatively, the spore
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be 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
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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
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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 ....
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equlprr;
the earth is provided to facilitate single·pole auto reclosing. Since these :00
contain oil, all fire-safety precautions that are token for transformers
should be followed.
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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.
Surge Arrestors/Lightning Arrestors
Besides direct strokes, the substation equipment has also to be protected against travelling waves due to surge strokes on the lines entering the substation. equiprlent most commonly used for this purpose is the surge arrestor
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The
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. :~;bstal;on
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The fixing up of insulation level for equipment within a
requires a detailed insulation co-ordination s1udy with surge arrestor as the for protecting the equipment from power frequen-:
,-/er-voltoge exceeding
the or- estor rating. Besides protecting the transformers, the surge arrestors also
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protee to the equipment located W"',in their protection zone
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arresters con be provided, depending up on, the isocerounic level, anticipoted
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Additional surge
overvohoges and the protection requirements.
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Insulators
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Adequate insulation should be provided in a substation for reliability of supply ond However, the station design should be so evolved that the_
safety of personnel.
quantity of insulators required is the minimum and commensurate with the expected security of supply.
An importont consideration in determining the insulation in a
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 line washed periodically and this aspect has to be kept ,in mind while deciding the loyout of the substation. -.:,..~iying suitable type
Another method, which hos proved to be successful, is
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
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ADOPTED AS PER IS/IEC
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Pollution Level
Min. Norrinal Creepage Distance (mm/kV)
Type of Pollution
Light
16
Non-Industrial, Agricultural, Mountainous areas beyond 20 Km from sea
Medium
20
Industrial Area without polluting smoke and chemical effl uents and not too dose to sea
Heavy
25
Industrial Area with polluting smoke & chemical efffuents close to sea and exposed to strong winds from sea
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Very Heavy
31
The highest line-to-Iine voltage of the system .'
Industrial Area subjected to conductive dust polluhon, smoke very close to sea, exposed to sea and very strong winds from sea, desert areas etc. IS
used to determine the creepage
distance
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The following types of insulators are normally used: a)
Bus Support Insulators (i)
b) 'c, _
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Solid core type
Strain Insulators (i)
Disc insulators
(ii)
long Rod Porcelain insulators
(iii)
Polymer insulators
Structures 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 structures also ploys on important port and the choice is usually between using a few 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 structures. When, galvanizing is not effective; particularly in a substation located In coastal or industrial areas, paInting becomes essential.
Q Power Line Carrier Communication (PLCC)
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The carner equipment required for communication, relaying and tele metering is
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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 on the line side of the wave trap and are normally base mounted. The wave traps for voltage levels up to 145 kV can be mounted on the gantry structure on which the line 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 •
Inter-circuit coupling
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
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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
dela11s the switching sCMmes
election of a bus bar scheme for a porticular sub station is on important step in
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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
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•
Availability of land
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Cost
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The relative importance of these factors varies from case to case and depending on
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the voltage level, number of circuits, desired level of security, etc.
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Types of schemes
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The various bus-switching schemes that are in pradice are,
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Single bus
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Sectional Single bus
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Main and Transfer bus
•
One and Half breaker
•
Mesh scheme
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Double Main Double Main and Transfer bus
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
•
Double bus and double breaker
•
i
Criteria for seledion
__
lhe following criteria are usually followed when selecting a switching scheme for a
sub-station. • 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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Economy
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• The acceptable level of reliability has not been defined by any standard and therefore jt
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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 Tronder 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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own circuit breaker.
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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
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the reliability of supply to the feeders using this type of layout is considerably
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unimportant. But this is a simplest arrangement where each circuit is provided with its
~1e
that if the incoming circuit breaker is to be shut down for mainteno that feeder has also to be shut down. If the bus is supplied by more
t~1i
load on
one feeder,
increased.
Sectionalised single bus bar scheme
:; "
,
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
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section circuit breakers acts as backup to the circuit breakers of the main circuits. ThE'
...
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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reliability. The sectional ising breaker may also be used at medium sized substations
) .) receiving supply from more thon <;me source to synchronise or segregate the supplies,
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as per. the opemting requirements.
The interlocking arrangement is simple for both the types of arrangements .
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Main and Transfer bus bar scheme
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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
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bus idle. Each circuit is also connected to the transfer bus bar through on isolator.. In
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the transfer
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
0
breaker. Since the arrangement to the transfer bus is through the isolators coreful
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interlocking is necessary with bus transfer breolcer so that only one circuit transferred
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at a time.
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• •
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 ..y 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
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energised through the bus bar coupler circuit breaker as in the main and t,ansfer bus
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scheme, and the controlling circuit breaker of the circuit token out for mainter.ance.
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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
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operation. In case maintenance of the circuit Ix-eakers can be arranged by taking the
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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
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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
~
()
0
.. s~erne is complicated whereas; the protectian scheme is simple. ------~------.-..
. One and a half breakers scheme
a I
In one ant..
0
feeders ond hence the name. This scheme is more flexible than any other scheme
"
•
.t)
;l
holf breaker scheme, three circuit breakers are used for connecting two
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 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
e·
breoken in circuit.
• 0
• •
This scheme ovoids the necessity of bus coupler circuit.
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
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
~
a ;)
Mesh scheme contains a ring with circuit breakers as many as the number of feeders, 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 preferred to mesh scheme.
IS
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
generoting station is of utmost importance for the overC'; network, since loss of
0
~Iiability
0
of a power
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 in a substation, designers cater to this eventuality. In schemes like breaker and half a
•
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
.. t2
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
~
Q }, I
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
0
breaker and disconnector operations are required. The only drawback with active
0
redundancy is the requirement of interrupting bath feeds during isolation of a feeder.
0
Operational Flexibility
0
•
0
.
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,
:J
more complicated will be the protection arrangement. This is porticula.)
.~
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
'::l
....
;0
when
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.
;--,.
Breaker and Half scheme no such transfer is necessary. Further, multiplicity of bus
"',;
While in
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.
Maintenance - effects on System Security
)
,
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
,~,"
...;
)
1)
should be suitably designed to provide security in operation and avoid catastrophic 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.
0
electrically interlocked such that they cannot be aperated unless the associated
•
associated earth switches are in the dosed position.
()
USYOUT
0
Earth switches are
disconnedars are opened. Circuit breaker cannot be operated locally unless its
,
• •
aescribed hereunder.
~
CLEARANCES
Z)
The position of equipment in an EHV switchyard is greatly influenced 'by the air
r)
designing a switchyard layout, various aspeCts are considered which are
clearances to be adopted.
Two types of air dearances are calculated for the
purpose, which are phase to ground clearance and phose to phase clearance.
)
Sedional clearance in
'\
reasons during the maintenance of equipment.
-
.I
Q
swilchyard is derived from these, which is used for safety
The clearances are calculated considering the insulation levels adopted for a system.
Highest System Voltage
kV
400kV
220 kV
132 kV
110 kV
66 kV
420
245
145
123
72.5
,
.
LIghtning impulse with standvoltoge SWitching surge withstond voltage 1 min. Power freq. Withstand voltoge
kV"
1425
kVp
1050
kV,....
630
6501 550
10501 950
5501 450
325
140
460
Phase to ground Clearance -'
-
The phase to ground cleorances for a substation is calculated considering various
0
overvoltoges and the highest of the values is adopted.
...... • >
() " 'J J}
0
3
electrode configurations and their eJedrical response to the above mentioned
This he . ·.!r does not apply to the length of post insulator where the phose to ground . orance can be adopted based on tests conduded on them and margin for inaccuracy in erection & variations in equipment geometry is provided.
Phase to Phase Clearance
()
D
It is well known that lightning surge stresses between phases will not be normally
•a
higher than phase to ground lightning. surge stress.
voltages above
~
variation in equipment geometry.
•
Sectional clearance is obtained by rounding off the sum of PIE clearance and
Considering this asped the
phase to phase clearance is calculated hosed on switching surge stresses for system ~45
kY. A design margin is provided for the inaccuracy in erection,
\I
9
clearance to the ground from the lowest port of insulator.
:3
-a
Minimum Cearances Based on CSIP Manual on Substation Equipment, Illumination & layout, Dec. 1996. inm
400 leV
220kV
132kV
110 kV
66 kV
Phose - Phose Phose - Earth
4.2 3.4
2.1/1.9 2.1/1.9
1.1/0.9 1.1/0.9
0.63 0.63
Sedion
6.5
5/4.5
1.3/1.1 1.3/1.1 4
4/3.5
3
Ground
8
5.5
4.6
4.6
4
Boy Width
27
18/17
12
10
f}
:)
') "~
"
.:i '"\ J
""\
./
')
Equipment Spacing
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
. I
• Distance between LA and protected equipment has been decided considering J
protection reach of LA.
~"\
.
.... ,...,;
Bus Bars
a
The bus bars of 400 kV Switchyard .·,.,1 consist of flexible and rigid conductors conductors.
0 i ;~
Sequence of installation of wave tra'ps lightning arresters and capacitive transformers:
{
vo~tage
•"
The sequence of installation of line traps, lightning arresters and capacitive voltage transformers is decided based on insulation co-ordination considerations.
Structure
;3
a e
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.
• ••
design to toke care any future increase in load during replacement.
a~·:;lees..
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.
3
Road Layout
.)
.,.,
.;
.)
)
)
The maximum wind loading will be taken os per IS ; 802. The structure
sholl be hot dipped golvonised.
·3
)
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
0
)
A slightly higher vertical load sholl be considered in
•
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
.......
../
s"ltKfIDn of a bus pos/ insula/or
election of a bus post insulator is based on both electrical and mechanical
S
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
;0
~)
frequency
voltage,
creepage
distance,
corona
and
radio
interference voltage.
~
For s~stem voltages up to 300 kV the Basic Insulation Level assumes importance in
0
the design, whereas for higher system voltages the bosic characteristics of the
i)
•
insulators are determined by Switching Surge Level and Creepage Distance. Mechanical design
(Ref: Electrical Enginetlf"s' Handbook by Knowlton)
D
~
•
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
i
~
Short circuit due to electro -mognetic force,
7# I
)
N
Fs
X
M x K x 2.05
\2
x Lx 10.8
= p.
"
V
Where,
.J
Fs
;J
.., ~.
-'
.
~
"!
~
=
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 buses K= 1.
=
M
Multiplying factor
Short Circuiting (AI ·IS1
Short-circuit current(l) expressed as Max. peak R.M.S., Asymmetrical R.M.S., Symmetrical
!A)-(B)·,q
M
Force on eonductor
1.00 2.66 8.00
j,/
or 8
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
J
) f
·5
for N can be 0.4 to 0.45 for three phose and phose to phose faults for most of the field conditions. Although strudure could be safely designed assuming even smaller values for N, a value of 0.5 is token generally.
{) it)
. I
& i
~
• '"
The fador N
M is called as Stress factor.
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, FE
""F,
»
=S x W
.... ,
S
't:# 1)
X
=
Horizontal earthquake force component in Kgf.
=
Seismic intensity. A fador of 0.25 is considered to be in the very strong.
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 2s 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.
=
Where,
L
_
length of the insulator
"J"
B
=
breadth of the insulator
0 ;D
Generally, 5% design margin is added to Ihe calculated wind force.
''-
10 .
..J
• •
I
p x Lx B x 1.2 x 1.92
.
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 projected area.
Tha wind pressure acting on a column is considered uniformly
disturbed load for bending moment calculation.
8
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
~
under short-circuit condition is most unlikely in actual serVice.
'.
• i
0
)
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
Bus Hars
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 .
.~~) J
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
0
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
'lJ
,')
•
Bus·8ft, "~tr,*,'
~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
) )
available as IPS (Iron Pipe Size) type and ExIra Heavy IPS type, depending on
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 1s rating. The area of cross section In mm 1 required to corry the short circuit current for the specified time is,
".
),
I" x
=
A
14 X10
0
4
9 ,~
;8
F"'.
......
X[Og [T.. + 258 To
]]0.5
+ 258
Where,
= = =
I" t To T",
=
0 ,9
"t x (2SAf
symmetrical short circuit current in A duration of fault in seconds initial temperature of the condudor before short circuit in °C final temperature of the condudor after short circuit in PC
Mechanical The mechanical characteristics thot has to be considered for seledion of a rigid bus , 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
= =
wind pressure in kg/m1
Where,
p D
diameter of the rigid bus bar
S~;:)rt
circuit force N x M x K x 2.05 x I" x 10 8
Snort cir.::uit force per meter length
:=
\Nnere.
Fs P, -c-
Electro-magnetic force in Kgf.
=
Peak value of maximum short-circuit current.
=
Center to center spacing between phases in m.
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
,
(}
0
Both wind load and short circuit force act in the horizontal direction whereas the force due to the weight of the bus bar acts vertically.
0
·0
8 g
The bending stress on the rigid bus bar is
=
MIl Be:lding moment in kgm
where,
•.
L
= = = =
Z
=
,-"\
The maximum allowable bending stress in aluminium alloy is 2.1098 x 10 7 kg/m2
M
~
."",
a ') .,..,.
~
\.
WL'/8 resultant force in kg/m length of the bus bar in m section modulus m 3
and the factor of safety specified by IE rules is 1.5.
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'
MI
==
moment of inertia m A
,
. 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)
J
conductor) exposed to a potential gradient near a grounded facility.
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 conditions menDoned above. For instance, German Grounding Standard DIN 57141
(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.
...J
I
Both tests and experience show that the chance of servere injury or death is 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
9
primary protective devices, or that of the back-up protection.
a
Effed of Reclosing:
~
.•
circumstances, a person might be subiected to the first shock, which would not
~
fast automatic redosure could result in a second shock, occurring after a relatively
•
Redosure ofter a ground fault is common in modern operating practice.
In such
permanently injure him, but would upset and disturb hiJ!l temporarily. Next, a single
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 distance of 1 m with his feet without contacting any other grounded obiect.
)
0
Touch Voltage: The potential difference between the ground potential rise (GPR) and
lne surfc::ce potential at the navinfl
!~!5
point where a person is standing. while at the some time
hands in contact with a grounded structure. convenhonol sub!Jolion, the wont ,ouch voltoge is usuolly found the potenhol dlHerence
IIOTE:
.1
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. -~.
I
'
.
... -'
Transferred Voltage: A special case of the touch voltage, where a voltage
IS
transferred into or out of the substation . I
f
t3
Calculations based on IEEE Guide for safety in AC substations - ANSI/IEEE
a
Std 80 - 1986
·3
Sizing the Conductor
D
The area of cross section for the conductor is given by the expression
•
0
=
~
./
te a, p, 10"
TCAP
• ')
~
~.
Where
r.,l A
.:,.
,.
RMS current in kA
T",
conductor cross section in mm 2 maximum allowable temperature in °C
0
To
)
= =
ambient allowable temperature in °C reference temperature in °C
T, 00
=
thermal coefficient of resistivity at 0 °C
a,
=:
thermal coefficient of resistivity ot reference temperature T,
p,
=
resistivity of the ground conductor at reference temperature Tr in ~
1I
ao
or ( 1 / ex,. ) - T
f
duration of current flow in s
:') ,')
=
thermal capacity fador in J/cm 3/ "'C
=
4.184· SH· SW
SH
=
specific heat capacity in Col/gram / "C
SW
=
specific weight in gram/cmJ
TCAP
:) :) ,~)
,.· , .~.
Material Constants
If ..
'-J
(}
'J
-
~j
J
ar
leo
Fusing
p,
Description
%
@20·C
@o·c
Temp.
@20"C
Standard Annealed Cu wire
100.0
0.00393
234
1083
1.7241
3.422
Commercial hard drown Cu Ware
97.0
0.00381
242
1084
1.7774
3.422
Cu clod steel care wire
40.0
0.00378
245
1084/
4.397
3.846
5.862
3.846
Cu clod steel core wire
40.0
0.00378
245
:0
.
9
0
• • • ~
J/cm 3
rc
1300
) !
leAP
Conductivity
0
1084/ 1300
Commercial EC AI wire
61.0
0.00403
228
657
2.862
2.556
AI allay wire 5005
53.5
0.00353
263
660
3.2226
2.598
AI alloy wire 6201
52.5
0.00347
268
660
3.2840
2.598
AI clod steel care wire
20.3
0.00360
258
660/
8.4805
2.670
20.1
3.931
72.0
4.032
.
1300
Zinc coated steel core wire
8.5
0.00320
293
419/ 1300
,
Stainless steel # 304
2.4
0.00130
749
1400
Step and Touch Voltage Criteria: The safety of a person depends on preventing the critical amount of shock energy from being absorbed before ~ the fault is cleared and the system de-energized. The maximum driving voltage of any accidental circuit should not exceed the limits defined below. For step voltage the limit is E".so
=
(1000 + 6C. p,)0.116 /.J t,
or
E step70
=
(1000 + 6C, p,}0.157 / .J t.
The actual step voltage, E" should be less than the maximum allowable step voltage,
ESItIp, to ensure safety. Similarly, the touch voltage limit is
E tovd>50
=
(1000 + 1.5C, P.)O. 116/" t,
E sfap50
=
{1000 + 1.5C, p,}0.1571" t,
=
1; for no protedive surface layer
Where,
C.
I
1
=
0.96
[
a 1+2 L
-:-:=:=K="::;::::;;:::--J' ; otherwise
n_1
..J
1+(2nhjO.08f
)
Simple c: 'ernative approaches, based on the equivalent hemisphere, such as
= f
1-0 [l-P/P. I 2h,+a)
.::,;
approximately; a= 0.106 m. which avoids infinite summation series, olso possible
the resistivity of the surface material in Om t. = duration of shock cu' 'ent in seconds The actual touch voltage, mesh valtagt:, or transferred voltage should be less than the =
p.
::) .~
maximum allowable touch voltage, Eloudv to ensure safety. However, ElINp50 &
0
~
are taken into consideration, os these would give lesser
limiting volues.
J
}
0
Grounding System Elements
0 Q
1. Ground electrode: A condudor imbedded in the earth and used for collecting ground current from or dissipating ground current into the earth.
0
•
0
.Z)
•
2. Grounding grid:
A system of horizontal ground electrodes that consists of a
number of interconnected, bare condudors buried in the earth, providing a common ground for eledric;al devices or metallic structures, usually in one specific location. NOTE:
Grids buried horizontally near the earth's surfac. or. alS? effective in controlling the svrfoce
potential gradients. A typical grid usually is supplemented by a number of ground rods and may be f, ., '
connected to ouxiIiory ground electrodes, to lower its resiAance with respect to remote earth .
3. Ground mat: A solid metallic plate or a system of closely spaced bore condudors that are connected to and often placed in shallow depths above a ground grid or elsewhere at the earth surface, in order to obtain an extra protective measure minimizing the danger of the exposure to high step or touch voltages in a critical operating area or places that are frequently used by people.
Grounded metal
gratings, placed on or above the soil surface or wire mesh placed directly under the crushed rock, are common forms of a ground mat. 4.
Grounding System: Comprises all interconnected grounding facilities
In
a
specific area.
Basic Aspects of Grid Design: Conceptual analysis of a grid system usually starts with inspedion of the station layout plan, showing all mojor equipment and strudures. In order to establish the basic ideas
and .concepts. the following points may serve as guidelines for starting a typICal grounding grid design:
L
A continuous conductor'loop should surround the perimeter to enclose as
much area as pradical.
This measure helps to ovoid high current
concentrotion and hence high grodients both in the grid area and near the projecting coble ends. Enclosing more area also reduces the resistance of the grounomg grid.
2.
Within the loop, conductors should be laid in paralleled lines and, where practical, along the structures or rows of equipment, to provide for short ground connections.
3.
A typical grid system for a substation may include 4/0 bare copper conductors buried 1.3-0.5 m below grade. spaced 3-7 m aport, in a grid pattern. At cross-connedions, the condudors would be securely bonded together. Ground rods may be at the grid comers and at each second junction point
o
•o o
• • 1)
e,
along the perimeter. Ground rods may also be installed at major equipment. In multi layer or very resistive soils, it might be useful to use longer
rod~.
(lengths exceeding 100 ft. have been used by some utilities.) 4.
This grid system would be extended over the entire substation switchyard and often beyond the fence line. Multiple ground leads or larger sized conductors would be used where high concentrations of current may occur, such as at a neutral-to-ground connection of generators, capacitor bonks, or transformers.
5.
The ratio of the sides of the mesh usually is fro", 1: 1 to 1:3, unless a precise (computer-aided) analysis warrants more extreme values.
Frequent cross
cannections hove (.; relatively small effed on lowering the resistance of 'grid. Their primary role is to assure adequate control of the sUrMee potel"'
The
cross-connections are also useful in securing multiple paths for Ine fault current, minimizing the voltage drop in the grid itself. and providing a certain measure of redundancy in the case of a condudor failure.
Design In Difficult Conditions:
In areas where the soil resistivity is rother high or the subslcrtion spoce is at
0
Q ' "
premium, it may not be possible to' obtain a low impedance grounding system by
D ;;
spreading the grid electrodes over a large area, as is done in more favorable
:)
the control of surface gradients difficult. Some of the solutions include:
:> ~
conditions. Such a si1uation is ty;:>icol of many GIS installations, occupying only
0
fraction of the land area normally used for conventional equipment. This often makes
J
., ,"
(1)
Connection(s) of remote ground grid(s) and adjacent grounding facilities; a
"
combined system utilizing separate installations in buildings, underground
'"",
vaults, etc. A predominant use of remote ground electrodes requires careful consideration of transferred potentials, surge arrester locations, and other CI
itical paints. A significant voltage drop may develop between the local and
remote grounding facilities. 12)
Use of deep-driven ground rads and drilled ground wells, in combination with
a chemical treatment af sails, ar use af bentonite days for backfilling.
(3J
Use af caunterpoise wire mats. In exposed areas, it is feasible ta combine
both an insulating material and fabricated mats made of wire mesh,
I :)
expanded metal, ar gratings; first ta equalize the gradient field near the
~J
surface and then ta reduce ca.nductance from the surface to the underlying metal strudures. A typical counterpoise mesh might consist of copper dad
J
steel wires of AWG No.6 size, arranged in a 0.6 • 0.6 m (24-24 m) grid paHern, installed 0.05.• 0.15 m (2-6 m) below the earth's surface and
.
overlaying the main grounding grid, which is installed in greater depth, usually between 0.3 - 0.5 m (12 • 18 mI. (4)
Where feasible, controlled use of other available means to lower the overall resistance of a graund system, such as connecting static wires and neutrals ta the ground (see 13.3)~ Typical is the use af metallic objects an the site that qualify for and can serve os auxiliary graund electrades, ar as ground ties to other systems. Cansequences of such
applica~ons,
of course, have to be
carefully evaluated.
•
Wherever prodicol, a nearby deposit of low resistivity material af sufficient volume can be used to install on extra (satellite) grid. This satellite grid, when sufficiently connected to the main grid, will lower the overall resistance and, thus, the ground potential rise of the grounding grid.
The nearby low
resistivity material may be a day deposit or it may be a part of some large structure, such as the concrete mass af hydroelectric dam.
Connections to Grid:
Candudors of adequate ampacity and mechanical strength should be used for the
cannections between:
[l)
All ground electrodes, such as graunding grids, rodbeds, ground wells, and,
where applicable, metal, water, or gas pipes, water well casings, etc.
:>
(2)
All fault current sources such as surge arresters, capacitor banks, or coupling capacitors, fransformers
)
and,
where appropriate,
secondory lighting, and power circuits.
machine
neutrals,
Design Criter,ia: There are two main design goals 10 be achieved by any sl?b:;tation ground system l?nder normal as well as fault conditions. These are •
to provide means ta dissipate elec"lc currents into the earth without exceeding any operating and equipment limits
•
to assure that a person near grounded facilities is not exposed to the danger of critical eledric shock.
Critical parameters, which hove impad on grid design, are: 1.
Moximum Grid Current(IG) The magnitude of the grid current is didated by system requirements.
2.
1::_, ·tt Duration (tf) and Shock Duration (t,): The fault duration and shock Jtion is normally assumed equal, unless the fault duration is the sum of
o
successive shocks, such as from redosures. The selection of t, should reflect
D
distribution and industrial substations. The choices t, and t, should result
tJ
the most pessimistic combination of fault current decrement factor and
t)
• •
fast dearing time for transmission substations and slow dearing times for
allowable body current. Typical values for tf and t, range from 0.25 - 1.0 s.
3.
Soil Resistivity (p): The grid resistance and the voltage gradients with
4.
a
Resistivity of Surface loyer (pJ: A thin surface layer of crushed rock helps in limiting the body current by adding resistary:e to the equivalent body resistance. Values from 1000 to 5000 12m have been used for p,.
"
Designing a Ground Grid:
The following step should be followed to design a ground grid:
(i)
Coiled field data
)
(ii)
Choose the condudor
)
(iii)
Assume Spacing
(iv)
Calculate lir:'liting ElOl>p and
(v)
Calculate Em and E" L.:,
(vi)
Check Em< E..,...;., Es< E.,ep, L.:>l,.q & Rs < R,eq.
(vii)
If yes increase the spacing and check until the conditions foil.
)
In
substation are diredly dependent on the soil resistivity
t)
9
~n
~.
Rc
(viii)' If no decrease the spacing and check until the conditions are passed.
Calculation of Maximum Step and Mesh Voltage:
Em
=
pK",KHl and
E,
=
pK.,KHl
._,.;
.
Mesh Voltage (E...):
,,,\
,·
:~e
.
spcclng fador Em for mesh voltage by simplified method is:
v
'\,-
[t In
:::;
2j[
::J K
~:'}
1,.
=
with ground rods ~nroughout
.~
K
I:}
•• ••
~
"•
•
• 0
...
.J
f'l
:J
16hd
BOd
h
J
--
+
4d
8'
K..
p(2n.l)
10
the grid corne .~. as well as both along the perimeter and
the grid area•
=
for grids with no ground rods or grids with only a few ground rods,' none located in the corners or on the perimeter .
+ h/ho
K_
=
..)1
h..
=
1 m (reference depth of grid)
D
=:
spacing between parallel conductors in m
h
=
depth of ground grid condutors in m
n
=
number of parallel condudors in one direction
d
=
diameter of the grid condudor in m
Corrective Factor:
0.656 + 0.172 n
K
For mesh voltage calculation,
I
n
=
~
Where x & yare condudors in each diredion.
For easy identification, K; for mesh voltage calculation is denoted as K!!l'
For step voltage calculation,
. 'J
n
~
For easy identification, K; for step voltage calculation is denoted as K;!.
=
max(x,y)
-
;j
:)
) ~
J
K,.
-In
for grids with ground rods along the perimeter, or for grids
..., }
'~
(O+2h)'1
·· ..ere
s}
0
°'1
-+
Step Voltage (EJ:
The spacing foetor ~ for step voltage by simplified method is
K
=
1
-
1
1
1
'I
+ - + -O-O:Sfto').,
i
2h
o
D+h
Moreover, for depths smaller than 0.25 m.
K.,
=
1 [
1
1
:----;
+ -
D+h
+ 0
w]
Where
w
"..,
+ 2
4
n-1
Or for n ~ 6
.:t
W
,..j.
= - -
+ In (n-1) - 0.423
2(n-1)
.....,
~
'" )
~
+
+
3
I
The use of a different equotion for 1(., depending on the grid depth h, reflects the fad that the step voltage decreases rapidly with increased depth.
~
•
In equotion for Em ond E,
L
4J 0
• • •• ,
0
)
=
L.+L,
for grids with no ground rods or only a few rods in e the center oway from the perimeter
=
1..+
for grids with ground rods predominantly around the perimeter.
1.15L,
Estimation of Minimum Buried Condudor Length .
K", K P IG ,,~
.
L >
116
+ 0.174 C, P
Refinement of Preliminary Design: If colculations based on the preliminary design indicate that dangerous potential
f)
differences can exis1 within the station, the following possible remedies should be
~
studied and applied where appropriate.
t>
{1)
Decrease in total grid resistance will decrease the maximum ground grid
;,
" potential rise and hence the maximum transferred potential. The most effective way to
"...
decrease ground grid resistance is by increasing the area occupied by the grid. Deep driven rods or wells may be used if the ovailable area is lirnited. Decrease in
-' .."
. .
-" "
.; 1't
stotion resistance mayor may not decrease appreciably the local gradients, depending on the method used .
(2)
Improvement of Gradient Control.
By employing closer spacing of
grid condudors. the condition of the continuous plote can be appraached more eiosely. D::'~gerous
potentiols within the station can thus the eliminated at a cosl. The
problem c" ''"Ie perimeter may be more difficult, especially at a smal! station where earth res::' ,ity is high.
-However, it is usually possible, by burying the grid
perimeter ground condudor outside the fence line, to ensure that the steeper gradients
l~'mediatety
outside this grid perimeter do no1 contribute to the more
dangerous ::::>uch contacts. Another effedive and economical wav to control perimeter ...,.,
gradients cnd step potentials is to bury two or more parallel conductors around the perimeter at successively greater depth as distance from the slalion is increased.
)
v
,)
.~
(3)
Diverting a greater part of the fault current to other paths. For
example, conneding overhead ground wires of transmission lines or by increasing the tower footing resistance near the substation.
Concerning the lotter, however. the
0
effed on fault gradients near tower footings should be weighed.
C)
(4)
Limiting of short-circuit currents flowing in the ground mat to
lower values. If feasible, this will decrease the total rise in ground mot voltage and
!)
all gradients in proportion. Other fadors, however, will usually moke this impractical.
0
Moreover, if accomplished at the expense of greater fault clearing time, the danger
a
may be increased rather than diminished.
n
•
~
I'
Barring of access to limited areas where it'may be impractical to
e:ote possibility of excessive potential differences during a fault.
By uSing one or more of the above methods where necessary, designs can be
~
completed for construdion purposes.
D
grounding facilities can usually be installed more cheaply if all go in as port of the
-"
general construction job, without the necessity of making addITions later.
...i
)
These should be reasonably liberal, as
Limitations of Simplified Equations for Em and E.:
"")
....:
..
Severol simplifying assumptions are mode in deriving the equations for Em and Es. These assumptions may result in inaccurate results, for some cases, in comparison with the results from more rigorous computer analysis or scale model tests.
The
inclusion of correction fadors into the equations for Em and E, practically eliminates the inaccuracy (within certain ranges for the various parameters) for most pradical grid designs.
)
\}
0 3
When using the equatlons for E.., and E•• the following limits are recommended for
")
square grids, or for rectangular grids having the some number of condudors in both diredions:
'-, ....__ 0'
n
0
S
25
0.25 m
~
h
-:;
d < 0.25 h
1)
D > 2.5m
2.5m
Although the equations for Em and E. have been tested for n greater than 25 and
0
found to be sufficiently accurate, the tests were not extensive enough to form solid
o.
conclusions.
Thus, caution should be exercised before exceeding the limits given
above.
0 ~
~
;tance:
Grid
) 1
G
e 0 tl)
Where,
• 0
h
=
grid buried depth in m
A
=
Area of grid in m'
L
=
totollength of condudor in m
0
• ~
Calculation of Maximum Step and Mesh Voltage:" Based On IS 3043 Area of Cross Section )
The areo of cross section required for the ground condudor is,
3
•
l...n
s
k
0
Where,
0
S
0 :J ...., ~
=
k
=
Cross section areo in mm'
=
rms value of fault current in A
=
duration of fault in s
=
fador dependent on material of the protective conductor
The factor k is
{) Q., (8 + 20)
~ ~.
K
=
~
~
+
B
.:-}
')
9;
\"'i~=re,
:> <)
Q
Volumetric heat capacity of the material in JrC mm J
E
reciprocal of temperature coefficient of resistivity of the cond~ctor at O"C in °C
(.J
J 1:1.
.)
electrical resistivity of 200c in Omm
=
initial temperature of conductor °C
=
initial temperature of conductor °C
Material Constants
()
\)
)
()
=
)
--
0
Material
B,oC
Copper'
234.5
3.45 x 10.3
17.241
X
10- 6
Aluminum
228
2.5 x 10.3
28.264
X
10-6
Lead
230
1.45 x 10-3
241
Steel
202
QCI
.; ..-
JrCmm 3
5201 Omm
",
3~8 x'10"
X
10.6
138 x 10"
0
•
D
Current Rating of Various Protective Earthing Materials
o
For bore conductor without any risk of fir. or 0I'tf other touching moleriol,
Material ~ __ .
•
.'• 0
·9
0 Q
Copper'
1 s current rating
205
3 s current rating
118
IniliollemperOlvre: 400c;
"I-n'.:::' ~ ..
~,.~,-
finalt.m~rolvre:
in A/mm2
Aluminum·
Steel
126
80
73
46
• • .~I.,
395"<: (Cu). 325"C (A1),500"C (Sleel)
A note on Fences: )
Fence grounding is of major importance because the most dangerous touch contacts ore involved. The outside of the fence is usually accessible to the public, In addition. the fence may occupy a position on the periphery of the ground-grid area where surface potential gradients are the highest.
Post utility practices have been quite
varied, but a few facts are dear.
Two different general philosophies of fence grounding have been followed: ground~grid
(l)
Inclusion of the fence within the
(2)
Placement of the fence outside the ground-grid area, either with or
area
without close electric coupling between fence and adjacent earth along its length, but with no electric coupling between fence and main station grid.
Inclusion of the fence within the ground-grid area increases the size of the area and
"
thereby reduces. often substantially, the ground-grid resistance. and hence the maximum ground-grid voltage. rises as well. While the fence now tokes port fully in ~his
rise. this is not of concern if intemal and perimeter gradients of the grid are kept
-
Vlithin acceptable limits.
...
Under the firs: philosophy, the perimeter condudor of the grid will normally either
1)
follow the fence line, or paraUel it at about O.5-1.5m outside.
,
I
'
In either case. the
oerimeter ground condudor and fence should be bonded eledrically at frequent
"""
<..,.'
;ntervals.
0 j
Placement of the ground condudor diredly on the fence line permits the latter to be
)
~
"
condudor a short distance outside the fence line will decrease the possible touch ;:)otentiol to. which a person outside the fence could be subject, Whether or not this
0
difference is importont will depend on the circumstonces.
()
0
•
0
0
l
'\
0
0 {) !
:)
{)
iD """\ 'IV'
I
0
ground condudor on adjacent property. On the other hand, plocement of the ground
I
•
iocated on the property line if desired, without obtaining on easement to place
D
D ~
Direct Stroke lightning Protection
,)
L
ightning conductors ore used to protect the equipment ond
I:::~IS
bars in the
Switchyord from lightning strokes. Each conductor consists of a lightning receiver, a ground grid and an interconnection. Typically the lightning conductor
can be lightning rods or ground wires.
0 .~
'J
Proper earthing of lightning conductors is essential to prevent breaking down of the
D
insulating medium, generally air, between the lightning conductor and the object to be protected due to very high voltages. The protective odion of the lightning conductors is
j .r Q
bosed on the fact that charges, stored on it's tip in the leader stage of lightning
)
discharge, produce the greatest field intensity along the path between the head of the leader canal and the tip of the lightning condudor to which the discharg3 is directed.
0 0
The space around a lightning conductor, in which the probability of a lightning stroke is
0
small, is called the "Protective Zone." The break down voltages of air gaps of tens of
0
meters have considerable probability scatter, hence lightning conductors provide
•
•
•
protection with a certain degree of probability.
0 (
()
0 () "
,J
() "-' -~
""" J "
::>
\
)
Ra:zevig Method Pr("\hcctive Zones of a Lightning Mast
Lightning Most
.)
. 'l J.J
,.. ..."
.
!
c·
_-_:: ">.,. ------..1
0- )
0
I
I
Q
~:ross
5cchorl n~ Ihe f'rnteeliy"
Inne 01 u t'u:I:;n*
I
_~ __L __ _
.0
I
I I
t;)
I
0
9 0
Construction of the protective zone 0; b sinqlc lightning rnosl. 1 -:;implilicd con:slruciion
0 4.-
!l
2 - prolective to ic conslructod I rom \
I
The protective zone of a single lightning mast up to 0 height 30m is shown in the figure
·3 ~
...
-j
r.
=
1.5 h
=
0.75 h
--" r) "~-~
l~ - ~J 0.8 h
if h. >
2/:l
",-"
~
,~l
,J' ~
:./
J
Eq
Where H - height of the lightning conductor r. - radius of protective zone at 0 height h. h. - height of the obiect to be protected
h
·;.
As the effectiveness of lightning masts of height greater than 30 becomes less, the volue obtained has to be multiplied by a co-efficient '-, ~
!
=
p
5.5 -.JH
Two Ughtning Masts
The protective zone for two lightning masts has considerably greater dimensions than sum of the protedive zones of two single mosts.
Ie
The internal port of the protective zone of two lightning
masl~ In
the plane passing
-+ , I
I
I, I I r I
k
,I
I
I
I
I
I I I I
----.,._--- I
I
I I
--------------- --'Il.:--.:-:~~----T '0
I
,..
I I
,
""
I I
.. 1--_, :- :.....
---
.
,r-,."
' ..
"
through ooth the lightning masts is bound by the are of 0 circle which can be constructed on three points: two of them are the tips of the lightning masts at a height equal to ho. The height of protedion at mid way between the lightning masts is a
ho
=
; h < 30m
h 7
!"'\
...J
a "'~)
J
..
=
h
; h > 30m
_
7p a
h . h,
=
Provided, the distance between the masts is less than seven times the active height. The external port of the protective zone is determined in the some way as for
.,
..,
..
single lightning masts. Generally, in large substations there are more than two
0
whereas the internal part is different. The objects of height h. falling inside the
lightning masts. Th., external part of protection is similar to that of two lightning masts
...,;
rectangle formed by four masts (or Ule triangle formed by three masts) will be protected
~
'.I
in the case of the diagonal of the rectangle formed by four masts (or the
\)
diameter of the circle passing through the tips of the masts in triongular formation)
0
is less than eight times the active height of the lightning most. The active height of
......
V
')
the most is the difference between the height of the most and the object protected. for 3 lightning masts
0 0
o
!
for 4 lightning masts
s
!
i~
i~
....1
i
<)
I
i
~
e
• •
o
0
'. &..
o
~
<=
8 (h - h.);
ifh <= 30m
<
8 ( h - h. )p;
if h > 30 m
<=
7 ( h - h. );
if h <= 30 m
<
7 ( h - h. )p;
if h > 30 m
S)
a
s
:)
0
0 [) ~
Protective Zones of Ground Wires The protective zone of a ground wire is shown in the figure. The cross section of protection zone on a perpendicular plane to the ground wire is constructed in the some way as for the lightning most with the only difference that breadth of the zone at the ground level for a wire less than 30m high is 1.2h.
) .)
b.
=
0.6h
r
h.
1 --
hx> 2/3 h
h
1.2h
[ h'J 1 --
.
hx > 2/3 h
O.Sh '
J
o :0 ()----
o )
- " , + ' •.
_ . _ - ......
8
• • • • •
~
f
"
I,
!
9
~'Ol. s(tC'tiOf'll
!).
:t'
I'lot.eh. . C" "' ••9....
ron_
h.
.
·--'r·----~
/
Ii
l
0
I
~
0 0 ;)
;J ;)
=
)
Furthermore, if two ground wires are placed at a distance S 4ft between them the point situated on the ground surface on the midway between" ground wires will not be struck by lightning. If the distance between the grind wires S < 4h then the point situoted midway between the ground wires ot a level of hO wiD be protected. S
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Angle Method
Protection angle instead of protedion zone can also be specified. It is the angle between the vertical line passing through a ground wire and the line joining a condudor and the ground wire and lying on the plane perpendicular to the conductor axis.
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lightning Conductor
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Object to be prolected
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Ground level
Protection angle
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Generally, the protection angle should not exceed 60" when placed between two ground wires and 45° when protected by one wire only.
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DESIGN
INPUTS~:
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AlSTO'M Pre-tendering Design Requirements The basic objective of pre-tendering design for the Switchyardl Sub-station is to provide a simple, reliable and economical configuration having maximum flexibility for operation and maintenance with minimum possible interruption in the event of equipment! component failure. Preliminary single line diagram and corresponding layout plan/ sectional drawing are bore minimum requirement for proposing a Switchyord/Sub-station ------ even for a budgetory quote.
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In case these drawings are not furnished by the customer alongwith the enquiry, the same are to be developed based on customer's requirement of switching schemes. In the absence of details of switching schemes from customer, the some has to be proposed to them' with advantage/disadvantage of different schemes. The bus switching schemes generally followed are:
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For 400 kV (Primary Transmission)
i) One and Half breaker scheme
ii} Double main with Transfer bus
i) Double main with Transfer bus ii) Double moin bus .~..
For 220 kV - (Primary/Secondary Transmission) For 132/66 kV (Secondary Transmission)
i) Double Main bus
ii) Single main with-Transfer bus
iii} Single bus
Apart from these schemes (i) Sectionalised Main with. Transfer bus, (ii} DoublE' .;n with by pass Isolators, (iii) Sectionalised Double Main bus, [IV) Mesh scheme etc. ar~;jO adopted.
• Once the SLD and Layout Plan & Sections are available, the bill of quantities can be prepared for major equipment as well as auxiliary equipments/materials. I
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Major eguipments are categorised as follows:
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1.
Power Transformers.
2.
Circuit Breakers (with pre-insertion resistor, if required) .
3.
Current Tronsformers.
4.
Voltage Transformers.
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5.
Capacitive Voltage Transformers.
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6.
Isolators.
7.
lightning Arrestors .
8.
Control & Relay Panel.
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9.
Power line Carrier Communication Equipments.
9
10.
EHV Coble terminating bushings (for U.G. Coble lines)
.
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-1l Auxiliary eguipments/materials are categorised as follows:
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Bus Post Insulators.
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Disc Insulators.
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Boy Morshalling Kiosks.
5.
Cf/VT/CVT Junction Box.
6.
Clamps & Connectors for equipments and busbars.
7.
Busbar materials.
8.
a) 1.1 kV Power & Control Cables and Cable Glands. b) H.T. Power Cables and Jointing Kit.
9.
Coble Trays & Support Angles.
10.
Sattery & Bc:ttery Chargers.
11.
AC Dish
12.
.. 6c Distribution Boord. a} Earthing Materials. b) lightning Protection System.
14.
Illumination for Switchyord & Control room.
15.
Fire Fighting System (portable/spray hydrant system)
16.
Structures.
17.
Neutral Grounding Resistors.
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18.
Diesel Generating Sets.
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19.
SCADA.
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20.
Toriff metering system.
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21.
Auxiliary Transformers.
22.
Air Conditioning & Ventilation.
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Insulator Hardwares with Sag compensation spring (if required).
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Major technical parameters considered for equipment are:
0)
Rated voltage.
b)
Design ambient temperature with permissible maximi.:m temp. rise.
c)
Breakdown insulation level.
d)
Creepage distance.
e)
Rated current carrying capacity. Rated short circuit current capacity with duration. Materiological data like altitude, wind speed, maximum & minimum temperature, se;<::-~ic level.
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of bushing/support insulator .
Major technical parameters considered for Switchyard layout are:
-
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Phose to phose clearance.
4)
Sectional clearance. Ground clearance.
5) .».Short circuit force on equipment / supports / gontry structure.
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I)~::".". Electro mechanical strength of string insulators.
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Cantilever stren:;)
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---~-- 6J-' --JiOximum sag for longest proposed span. \
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Main busbar height from ground (finished level}.
.Following parameters are to be obtained from customer in the absence of detailed specification and drawings for rough estimation:
l.
Soil data with soil bearing capacity and soil resistivity.
2.
Plot plan of the proposed area.
3.
location of the Control Room with resped to Switchyard.
4.
Distance of the Switchyard fence from the Power House, in case of Power Station.
5.
location of the Generating Transformer with respect to Switchyard.
6.
length of the Transmission lines connected to the Switchyard.
7.
Available space for Switchyord (fence area}.
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8: -.. Additional provision for spore bays.
Follo~!_ng
information are to be obtained for availability of adeguate site faciliti€...:.
1.
location of proposed site and nearest railway station.
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2.
Acces$ibility to site by rood.
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3.
Construction & drinking water (free/chargeable).
4.
Construction power (free/chargeable).
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Following information are to be obtained for pl0viding post commissioning services to customer.
J. Requirement of mandatory spores.
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2. Requirement of recommended spores. ... ;.
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:--,.=~:·'j~'-:R~Uii~ment of special tools and tackles for operation & maintenance. ~~~:'~1.~~~'~~.&:
~~:.",=. ~~'R~uirement of testing equipments.
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Following datos are required generally from customers for reasonable quotation:
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~_~1:':~:t'~~);"~: Breaker -
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: Single pole/gang operated, live/dead tonk, pneumatic/spring operated type, duty cycle requirement, creepage for interrupter (arc chamber), as well as support insulator, closing and opening time, 'indudive/capacitive charging curr~nt rating. live/dead tonk type, nominal capacitance in case of cvr, Transformers creepage of the bushing • Shed profile, creepage, cantilever strength.
d)Disc Insulators
Disc insulators string/long rod insulator, electro mechanical strength.
e)C&R Panels
1) Numerical/static/eledro-magnetic relay. 2) Additional requirement of Tariff metering with closs of accuracy. 3) Requirement of busbar protection. 4) Requirement of synchronising panel/trolley. S} Requirement of separate disturbance recorder with event logging. 6) 'Requirement of recorders like voltage, frequency etc. 7) Requirement of interfacing with SCADA.
8} Requirement of mimic panel. 9) Simplex/duplex type of panel.
f) Isolators
Single/double break, Conventional/Pantograph, single pole/gong operated, motor/manual operated, aluminium/copper blades, creepage and cantilever strength of suaport insulators.
g) Cables
PVC/XlPE, Copper/Aluminium, Flame retardant/ordinary
h}Busbar
Flexible/ Rigid bus, Copper/Aluminium material
i)Earthing material
MS, GI/Copper bars
j) Battery
lead acid/NICAD, ordinary/maintenance free acid.
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Armoured/U narmoured,
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k) PLCC
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lightning Protection
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Illumination
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i} Phase to phase/Phase to Earth/Phase to Phose inter circuit coupling (in case of double circuit line).
iii} Milli Henry requirement of line trap .
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Structures
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By lightning Mast/Shield Wire/Spikes on the Gantry Towers. Using lightning Mast/separate lighting Most or poles for light fixtures .
1} Conductor tension for line take off/line termir
gantries.
2} Short circuit forces.
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4} Gantry arrangement.
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5) Conductor span.
3) Wind pressure .
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6} Minimum & maximum temperature of the proposed areas.
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Since major equipments with standard rating are supplied by different manufacturers with marginal differer.ce in Ex-works costs, following items need to be near accurately estimated for a competitive quotation in on EHV Switchyard project of turnkey nature,
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Post Insulators.
.....
2)
HT & LT Power & Control Cables and Accessories .
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case of lead
ii) Programmable/non-programmable PlC terminal.
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3)
Structures
4)
Busbar materials.
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Clamps & Connectors
6)
Earthing material
7)
Illumination System
8)
Post Insulators
....-
9)
Disc insulators & hordware sets
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Lightning protection system
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Battery sizing for totol D.C. loads
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Air conditioning & ventilation (if in scope).
__ me customers are also interested in alternate offer with better economical design, in addition to the base offer as per layouts specified by them. Within the stipulations made in specification for eledrical clearances, altemate offer can be proposed, if economized on following aspects: 1) 2) 3) 4) S) 6) 7) 8)
Switchyard space•. Busbar materials. Insulators & Hardwares. Structures. Illumination. Civil Works involvement• Power & Control Cables. Earthing materials.
A sic.;ard desir;put sheet developed for overall system requirement herewith for necc:, .~ data required from customer for pre-tendering design .
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NDER INfORMATION REPORT II. COMMERCIAL INPUT CUSTOMER iA) CUSTOMER'S NAME, ADDRESS & CONTACT PERSON
,8) PRESENT LEvEL OF CONTACT
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NEW I WElL·KNOWN
2 !cUSTOMER REF
3 !DUE DATE
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4 ICOMPETITORS
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A) DOCUMENTS OBTAINED BY
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~-'ISOURCE 6~ FUNDING
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'~AST ~~PERI'ENC~ 9F THE Cl!STOMER wiTH, , I I
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A) AlSTOM 8) COMPETITIORS
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PREPARE SPECIFICATIONS
,EVALUATE OffERS· TECHNICALLY
,EVALUATE OFFERS· COMMERCIAllY
iPLACEMENT OF ORDEf<
IDETAILED ENGINEERING ON RECEIPT OF ORDER
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,DIVISIBLE/INDIVISIBLE
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SALES TAX 8. SURCHARGE
" ' SALES TAX 8. SURCHARGE (CONCESSIONAl
,rrfl'''l"r!NST ANY SPECifiC FORM AND APPLICABILITY) WPfkS CONTRACT TAX & SURCHARGE, IF ANY ", p,}' pCT~OI ;~{ nSERVICE TAX ON DESIGN 8. ENGG, If ANY ::1 ANYOT~ER TAXES, LIKE ENTRY TAXES, ETC
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2, PIE SID CONFERENCE'S DATE & VENUE -- -.. ,-- ---- -_ ........ -- "- ..
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YES I NO (If YES. WHAT IS THE PERCENTAGE?) YES I NO YES I NO
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10QfOl08.'. 2000 O~ 07
Page 2 017
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APPROVED/PREfERRED MAKES
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.f~eQU,4.l.lfICATION REO (PLEASE REFER NOTE NO. 21 YES YES 'YES YES :YES
'KTAANSFQRMERS
;~~h~\i ~WITCHGEAR
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!,ROTECTION & CONTROl PACKAGE lAi.STOM AS A CONTRATOR
NO NO NO NO NO
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/ / / / /
SUPPlY / ERECTION / CIVil WORKS
"!f9.~q ~~~URITY
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VAlUE • ... OF THE ...... PROJECT -. __ .
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Eg. BYA lACK-UP GUARANTlI, ITC. . "', 13. ALL INHOUSE EQUIP'MENTS, ,E_HV-CB-'~ScAriA. ">_" _,PANELS, RESPEtii,,! FORMATS~ •. _-, M.V~• SWITCHGEAR,'THE -_..... __.. _____ .. ,." __ ... __ ". • • ,.
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DIRECTLY TO THE RESPECTIVI UNITS. '. .' . . , ,
iN'~CASE OriUDG£TAiiY"OFF-Ea5;ITEM NOS: ~f 12.1','21 & 23-24 ARE NOT·MANDATORY.
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PaQ41 3017
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I Double main bus with transfer
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Other (please specify) SYltem rated fault current, kA ; period, s , I
Number of Bays
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Tie for 1 feeder Tie for 2 feeders
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Breaker and a half
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Single main bus with transfer
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Double main bus
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REMARKS
SYltem switching scheme (tick among the following, as applicable) Single main bus
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System Rated Voltage, kVrms
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PARAMETERS
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• TECHNICAL INPUT
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Spore / future (unequipped)
Meteorological Dato •
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Altitude
(less than 1000 m in case more than 1000 m indicate value)
b, Wind Pressure kg /m 2 c, Pollution/Creepage
.' Light / Medium / Heavy / Very heavy
Site· Plot plan (Reference. drawing /I if available)
lor Space limitations
Sile . conlour mop (Reference. drawing /; if available, j
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Soil be~rin{:) c~p'(J~ity or soil type $a~dy, ~Iock cott~n, (not required if civil works is ~xduded)
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Soil Resi5tivity, Ohm· m . '
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Location of site
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of transformer , ,.
Whether site is reasonably levelled, if nol indiucole quantum of filling required Ust of local erection controdors and civil contarciors enclosed
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Control Room
Struc;lurol
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