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65 EHV - SUB-ST A TION v SUB-ST A TION PLANNING CRITERIA v The maximum fault level on any new Sub-Station Bus should not exceed 80% of the rated rupturing capacity of the Circuit Breaker. The 20% margin is intended to take care of the increase in short circuit level as the system grows. The rate of breaking current and making currnt including fault clearing time capability of Switch-gear at different voltage levels may be taken as :- Fault cleaning Voltage Operating Breaking Acking Time level Time current current 150ms 33kV 60-80ms 25KA 62.5KA 120ms 132kV 50ms 25/31.5KA 70KA 100 ms 220kV 50ms 31.5/40KA 100KA 100ms 400kV 40ms 40KA 100KA 765kV 40KA v The capacity of any single sub-station at different voltage levels shall not normally exceed. 765 KV.- 2500 MVA. 400 KV.- 1000 MVA. 220 KV.- 320 MVA. 132 KV.- 150 MVA. v Size and Number of inter-connecting Transformer (ICTs.) shall be planned in such a way that outage of any single unit would not over load the remaining ICT (s) or the underlying system. v A stuck breaker condition shall not cause disruption of more-than four feeders for 220 KV. system and Two Feeders for 400 KV. system and one Feeder for 765 KV. system.

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65

(ii) The specific clearance between power and telcom lines, earth wires and earth-structures areto be adhered to. The minimum clearance between lines of various voltages to be maintained areas follows :

L.T. lines (400/230 V) 1.22 Mtr 220 KV lines 4.58 Mtr

11 KV lines 1.83 Mtr 400 KV line 5.49 Mtr

33/66 KV lines 2.44 Mtr 800 KV lines 7.94 Mtr

132 KV lines 3.05 Mtr

(iii) Guardians are to be provided at crossings of telecom lines with power lines upto 33 KV.

(iv) Maximum value of induced electromagnetic volt - 250 volts (faults duration equal to or less than200 ms.)

(v) Maximum value of induced noise (noise interference) -200 microvolts. (to be taken congnizanceif noise is persistent)

EHV - SUB-STATION

v SUB-STATION PLANNING CRITERIA

v The maximum fault level on any new Sub-Station Bus should not exceed 80% of the ratedrupturing capacity of the Circuit Breaker. The 20% margin is intended to take care of the increasein short circuit level as the system grows. The rate of breaking current and making currnt includingfault clearing time capability of Switch-gear at different voltage levels may be taken as :-Fault cleaning Voltage Operating Breaking AckingTime level Time current current

150ms 33kV 60-80ms 25KA 62.5KA120ms 132kV 50ms 25/31.5KA 70KA

100 ms 220kV 50ms 31.5/40KA 100KA

100ms 400kV 40ms 40KA 100KA

765kV 40KA

v The capacity of any single sub-station at different voltage levels shall not normally exceed.

765 KV.- 2500 MVA.

400 KV.- 1000 MVA.

220 KV.- 320 MVA.

132 KV.- 150 MVA.

v Size and Number of inter-connecting Transformer (ICTs.) shall be planned in such a way that

outage of any single unit would not over load the remaining ICT (s) or the underlying system.

v A stuck breaker condition shall not cause disruption of more-than four feeders for 220 KV.

system and Two Feeders for 400 KV. system and one Feeder for 765 KV. system.

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66

Sl No. Description of Technical Parameter Unit System

1. Nominal system voltage kVrms 400kV 220kV 132kV 33kV

2. Maximum system voltage kVrms 420kV 245kV 145kV 36kV

3. Power frequency with stand voltage kVrms 630kV 460kV 275kV 70kV

520 kV

4. Switching surge withstand voltage kVp

(for 250/2500ms)

1. Line to earth 1050kVp Not Not Not

2. Accross isolating gap 900kVp+345kVrms applicable applicable applicable

5. Lightinging impluse withstand voltage kvp for

1.2/50(ms)

1. Line to earth 1425 kvp 1050kvp 650kvp 170kvp.

2. Across isolating gap 1425kvp+ 1200kvp 750kvp 195kvp.

240kvms

6. One minute power frequency withstand

value

Dry kVrms 520 460 275 70

Wet kVrms 610 530 315 80

7. System frequency : Hz 50

8. Variation in frequency % +2.5

9. Corona extiniction voltage 320kV 156kV 84kV

10. Radion interference voltage 1000 mV at 1000 mV 1000 mV at

266kv at 167 kv 93kv

11. System neutral rating Solidly earthed

12. Continous current rating 1600A or 2000A 1600A 800A 600A

13. Symmetincal short circuit fault current kA 40 40 31.5 25

14. Duration of short circuit fault current Second 1 1 1 3

15. Dynamic short circuit current rating kAp 100 100 79 62.5kA

16. Conductor spacing for AIS layouts meters

Phase to ground meters 6.5 4.5 3 1.5

Phase to phase meters 7.0 4.5 3 1.5

17. Design ambient tempertures oC 50

18. Pollution level as per IEC-815 and 71 III

19. Creepage distance mm 10500 6125 3625 900

20. Maximum fault clearing time ms <100 <100ms <150ms

21. Bay width meter 27 16.4-18 10.4.12.0 5.5

22. Height of bus equipment interconnection meter 8 5.5 5 4

from ground

23. Height of strung busbar meter >15 10 8 5.5

EHV SUB-STATIONSYSTEM REQUIREMENT

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1. NomenclatureReliability :

The reliability of a power system means supply ofuninterrupted power at the specified voltage and frequency.

The reliability of a substation depends on the reliabilityof associated equipment such as busbars, circuit breakers,transformers, isolators and controlling devices.Failure Rate :

It is the average number of failures per year.Outage time :

It is the time taken to repair the failed component orrestore supply from an alternative source by switching.Switching time

It is the time taken from the initiation of outage torestoration of service by switching action.Switching scheme

It is the type of arrangement of bus bars andequipments considering cost, flexibility of operation andreliability of the system.Phase to ground clearance

The phase to ground clearance in a substation are, (a)distance between the conductor and the structures. (b)distance between the live parts of the equipment and structuresand (c) distance between the live conductor and ground.Phase to phase clearance

The phase to phase clearances in a substation are (a)distance between the live conductors (b) distance betweenthe live conductors and apparatus and (c) distance betweenthe live terminals in equipment like, circuit breakers, isolatorsetc.Ground clearanceIt is defined as the minimum clearance from any point, where aperson may be required to stand, to the nearest part (which isnot at earth potential) of an insulator supporting the liveconductor.Sectional Clearance

It is defined as the minimum clearance from any point,where a person may be required to stand, to the nearestunscreened live conductor. The basis for fixing the sectionalclearance is to take the height of a man with stretched handsplus the phase to ground clearance.Safety Clearance

This comprises of ground clearance and sectionalclearance.Electro static Field in Substations

An energised conductor or metallic part of theequipment produces electrostatic field. The magnitude of theelectrostatic field varies at different points in an EHV sub-station (above 400 KV), depending on the geometry ofenergised conductor/metallic part and the nearby earthedobject or ground.

STUDY ON SUB-STATIONS2. General

Substations or switching stations are integral part ofthe transmission system, and function as a connection orswitching point for transmission lines, sub-transmissionfeeders, generating circuits and step-up and step -downtransformers Substations of voltages 66 KV to 40KV aretermed as EHV sub-stations. Above 500KV, they come underthe terminology of UHV system.

The design considerations and procedures are almostthe same for the sub-stations in the EHV range except thatcertain factors become predominant at different voltage levels.Switching surges are very important at 345 KV and above,whereas it can be safely neglected upto 220 KV level.3. Design Criteria and Studies.

The following studies are to be performed to establishthe design criteria for a substation.1. Load flow studies

The purpose of a substation is to provide a path forreliable delivery of power to system loads. Load flow studiesestablish the current carrying requirements of the newsubstation or switching station, when all lines are in andwhen selected lines are out for maintenance. After studyinga number of load flow cases, the continues and emergencyratings required for various equipment can be determined.2. Short circuit studies.

In addition to the continuous current ratings, thesubstation equipment must have short time ratings,. Thesemust be adequate to permit the equipment to sustain, withoutdamage, the severe thermal and mechanical stresses of shortcircuit currents. In order to provide adequate interruptingcapability in the breakers, strength in post insulators andappropriate setting for protective relays, which sense thefault, the maximum and minimum short circuit currents whichwill flow for various types and locations of short circuits andfor different system configuration must be established.3. Transient Stability Studies.

Under normal conditions, the mechanical input to agenerator will be equal to the electrical output plus generatorlosses. So long as this is continues, the system generatorsrotate at 50 Hz. If this balance is destroyed by upsetting eitherthe mechanical or the electrical flow, the generator speeddeviates from 50Hz. and begins to oscillate about a newequilibrium point.

The most common disturbance is a short circuit. Whena short circuit occurs close to the generator, the terminalvoltage drops and the machine accelerates. When the fault iscleared, the unit will try to revert to its original state by feedingthe excess energy into the power system. If the electrical tiesare strong, the machine will quickly decelerate and becamestable. If the ties are weak, the machine will become unstable.

The factors which affect the stability arei. Severity of the fault.ii. Speed with which the fault is cleared.iii. Ties between the machine and the system after the

fault is cleared.

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68

The aspects of transient stability that are important in (iv) Ability to limit short circuit levelssubstation design are, (a) the type and speed of the line and Any arrangement which incorporates means ofbus protection relaying, (b) the interrupting time of the breaker providing a substation into two separate sections eitherand (c) the bus configuration after the fault has been cleared. completely or through reactor coupling, is suitable for limiting

The last point has a considerable bearing on the bus short circuit levels. By careful use of circuit breakers in ringarrangement. If a fault is cleared in the primary relaying time, system, a similar facility can be provided.only one line will be lost. If the fault is cleared in breaker failure (v) Maintenance facilitiesrelaying time, owing to a stuck breaker, more than one line

During the operation of the substation, maintenancemay be lost which will weaken the tie to the system.

will have to be carried out, either planned or emergency. The4. Transient over-voltage studies. performance of the substation during maintenance is also

Transient overvoltage may be due to lightning stroke dependent on the protection arrangements.or circuit switching . The most reliable means to establish (vi) Ease of extensionswitching over voltage is through the use of a Transient

The substation arrangement shall be such thatNetwork Analyser (TNA) study.

extension of bays for new feeders are possible. As the system4. Substation Arrangement expands, there may be need to convert a single bus

The substation arrangement depends on physical and arrangement to double bus system, or to expand a meshelectrical aspects and is influenced by the following factors. station to a double bus station. There shall be space and

expansion facilities.(i) System Security.(vii) Site considerationsThe ideal sub-station is one were each circuit is

controlled by a separate breaker with facilities for replacement The availability of site plays an important role inof bus-bar or breaker in the event of a fault or during planning the substations. When the areas is limited, a stationmaintenance. System security may be specified, based on with less flexibility may have to be constructed. The substationwhether complete reliance on the integrity of the substation which are simple in diagram and use least number of breakersor a percentage of outage due to periodic faults or maintenance occupy the least site.is permissible. (vii) Economy

Double bus-bar system with double breaker A better switching arrangement on technicalarrangement comes to near ideal, but the cost of such a requirements can be constructed, if the economics aresubstation is prohibitively high. reasonable.(ii) Operational flexibility 5. Substation Layout and Switching arrangement.

For the efficient loading of the generators it is A number of factors are to be considered whilenecessary to control the MVA and MVAR loading under all finalizing the layout and switching arrangements of an EHVconditions of circuit connections. The grouping of load circuits substation. It must be reliable, safe and must provide a highrequires to be capable of being arranged to give the best level of service continuity.control under normal and emergency conditions.

Normally used substation schemes are detailed below.(iii) Simplicity of protection arrangements

1. Single Bus arrangementIf more than one circuit is to be controlled from one

This arrangement is a simple scheme adopted in lesscircuit breaker or greater number of circuit breakers are to be

important substations. A breaker or bus failure can causetripped during fault conditions, the protection arrangements

total outage. By providing a bus sectionalism scheme, thisare complex. The most advantageous arrangement is single

can be overcome to some extent. Even though the protectivebus-bar with no sectionalising. Ring bus arrangement where

relaying is simple, single bus scheme is inflexible (Fig. 1)each circuit breaker can be in two zones of protection, causesfor complex protection scheme.

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E.Sw

Feeder

Fig. 1. Single Bus Arrangement

PT

LATrans

S i naliserect o

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Ttrans

BI

CT

CB

LALI

LA

BI

.SwE

PT

Feeder Feeder FeederTRANSFER BUS

MAIN BUS

2. Main and Transfer Bus breaker relaying must be so arranged to protect thetransmission line or transformer, if the protective relaysA transfer bus is added to the single bus scheme.also are not transferred. As the relaying selectivity isAn extra bus-tie breaker is provided to tie the main andpoor this scheme is considered as unsatisfactory. Failuretransfer buses together.of the main bus can cause for total outage of the

When a circuit breaker is in maintenance, the bus-substation (Fig-2)

tie breaker can be used for energising the circuit . Bus-tie

69

Fig-2 Main and Transfer Bus Arrangement

The circuit may operate all from one bus, of half of3. Double Bus, Single Breakerthe circuit connected in each bus. For a bus fault, only halfThis is superior to the single bus and main and transferthe no. of circuits will be lost. In some cases the tie breaker isbus schemes. There are two main buses and each circuit canpermanently closed and both the buses stand connected. Abe connected to either of the buses by bus isolators. A bus-tiebus protection scheme will be necessary for opening the tiebreaker connects the two main buses when closed allowsbreaker in the event of a bus fault.the transfer of a circuit from one bus to the other without a

Possibility of operator error is more as two busbreak in supply (Fig.3)isolators are involved for every circuit.

BUS 2

CB

BI

CT

LI

PT

LA

E.Sw

BusCoupler

Feeder

Trans

Fig. 3. Double Bus Arrangement

BUS 1

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Feeder

ESW

Fig. 4. Double Bus, Double Breaker Arrangement

Trans

LA

PT

CT

BI

BI

BUS 1

CB

BUS 2

4. Double bus, Double breaker Arrangement breakers for every circuit. The use of two circuit breakers percircuit makes the arrangement very expensive, but this providesThis scheme involves two main buses and two circuita very high order of reliability (Fig.4)

5. Breaker - and -a- half-scheme.

In this scheme two main buses are there and threebreakers are connected in series between them. Two circuitsare connected between the three breakers. Hence this is

1/2called 1 breaker scheme (Fig-5)

Normally all the 3 breakers are in closed position, andboth the buses are energised. When a line trip involves, twobreakers open. No additional feeder or source is lost whenone circuit is tripped. Any bus or any breaker can be takenout of service for maintenance without loss of service.

70

When a source and a line are connected in oppositedirections in a 3 breaker series, even when both the busesfails, it is possible to operate and provide some service.

It is more expensive then other schemes, except thedouble bus-double breaker scheme. Protective relaying and

1/2 automatic reclosing schemes are complex in 1 breaker busarrangement. But this arrangement is superior in flexibility,reliability and safety.

Fig. 5. Breaker and A Half Scheme

Trans

Trans

LA

PT

ESW

CT

BI

BUS 1

BUS 2

Line

CB

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6. Ring Bus arrangement During a breaker maintenance, the ring is broken,but the service is fully maintained. The circuits are generallyIn this scheme (Fig-7) the breakers are arranged in aarranged such that sources and loads are alternated.ring with circuits connected between the breakers. There

are some number of breakers as the number of circuits. For a Where five or six circuits are to be provided, ringcircuit fault, two breakers are tripped. In the event of a breaker bus arrangement is ideal. This scheme is economical andfailure during a line fault, an additional breaker trips as backup provide good reliability, safety and flexibility. Protectiveprotection. In that case an additional feeder will also be out of relaying and automatic reclosing schemes are complex inservice. the case of ring bus arrangement.

71

7. Other Layout Designs bus with bypass arrangement. These arrangements aremostly used in gas filled substations where more flexibility isIn additional to the above mentioned common busensured.arrangements, some other layouts are also employed. They

are (i) Double bus arrangement with transfer bus (ii) Triple Simple schematics are as given below. All equipmentbus arrangement (iii) Double bus with bypass and (iv) Triple are not shown.

Fig. 6. Ring Bus

Feeder

Feeder

Trans

TransLA

LA

PTESW

ESW

BI

LI

CT

CB

Fig. 7. Double Bus with Transfer Bus

BUS 2

Coupler

CB

CT

Feeder

TRANSFER BUS

BUS 1

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7. Switchyard Structures and (vii) Impact load, if any, during operation of equipment.

Structures are required to support and install buses, The substation gantry structures shall be designed

electrical equipment and to terminate transmission line to terminate the overhead line download span. Which may

conductors. The structures may be of steel, wood, RCC or enter +

PSC. They need foundations according to the soil conditions

of the side. Generally, fabricated steel structures are used in

the substations due to various advantages. The design of the

structures is affected by the phase clearance, ground

clearance, types of insulators, length and weight of buses

and other equipment.

Steel beams and girders shall be designed to prevent

failure by bending, flange buckling vertical and horizontal shear

and web crippling. The depth of the lattice box girders shall be

about 1/10 to 1/15 of the span and square in section. Maximum

beam defluxion shall not normally exceed 1/250 of the span

length. All bolts and nuts for structures shall be not less than

16 mm diameter, except in light loaded section, where they

may be 12 mm dia.

The design load on columns and girders shall include

(i) Conductor tension (ii) Earth wire tension (iii) Wt. of insulators

and hardwares (iv) Fraction load (about 350 kg) (v) Weight of

man & tools to works on them (about 200 kg) (vi) Wind load

72

30 degrees horizontally and +15 degrees vertically.

The yard structures may be hot dip galvanized or

painted. Galvanized structures require less maintenance. But

in some highly polluted locations, painted structures provided

more corrosion resistance.

Normally adopted phase spacings

11 KV 1.3 m

33 KV 1.5 m

66 KV 2.0 to 2.2 m

110 KV 2.4 to 3 m

220 KV 4.5 m

400 KV 7.0 m

8. Bus Design

The present day trend is to use rigid bus rather than

strain bus due to various reasons. Rigid bus can be constructed

at a lower profile and are aesthetically pleasing. Increased capacity

for the bus can be provided and corona level is lower.

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73

8.1 Rigid Bus

Aluminium bus materials used for rigid bus may be of

different shapes. They may be round tubings, square tubings,

channels, angles or integral web designs.

Round tubing is used in all voltages, whereas

square tubing is used only at lower voltages. Channel

bus is the same as square tubings except that they

provide more capacity. Angle bus is used only at

distribution voltages. Integral web bus is structurally

strong and is used for high current and long spans-

generally at lower voltage.

8.2 Capacity

The rigid bus must be able to carry the excepted

maximum load current without exceeding the temperature

limit. The capacity of the bus shall also be checked for

maximum temperature under short circuit conditions using

the equation.

I = K. A. 1 / t x104

Where I = Symmetrical rms current in amps.

A = Cross sectional area in inches

t = Time in seconds

K = Coefficient for alloy bus at maximum temp.

specified.

Maximum Value of K for various

Temperature aluminimum Alloys

2000C 5.50 to 5.71

2500C 6.28 to 6.52

3000C 6.94 to 7.18

The general practice is to limit the temperature of rigid

aluminium bus to 1000C for emergency ratings and 2500C for

short circuit duty.

8.3 Vibration

A span of rigid bus has a natural frequency expressedas follows :

f =K2

24L2 (Ei

M)12

Where f = Natural frequency of span in Hertz

L = Span length in feet

E = Modulus of elasticity PSI

i = Moment of inertia of cross sectional area (in 4)

M = Mass per unit length (Mass = Wt./32.2)K = Constant (1.0 for pinned ends and 1.5056 for fixed ends)Assuming there will be spans with pinned ends

where K = I

f = 2.153 x 103 r

L2

Where r = OD2 + ID2

4

r = radius of gyration (inches)

OD =Outside dia. of tubing (inches)

ID = Inside dia. of tubing (inches)

L =Span length (feet)

As vibration may be induced in the bus by the action

of 50 Hz. current, a natural frequency of 50 or 100 may be

avoided.

Another force which creates vibration in the bus isthe wind flowing across the tubing. The maximum aeolianfrequency f in Hertz will be

f = 3.26V

OD

Where V = maximum wind speed (mph)OD =Conductor out side dia. in inches

8.4 Short Circuit ForceShort circuit force produced between two parallel

conductors, in the event of a line to line fault can be expressedas follows.

f = 43.2.I

sc2

107 ( )D

Where f =short circuit force (Ib/ft.)Isc = Symmetrical rms short circuit current (amps)

D = Conductor spacing centre to centre (in)

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74

For a 3 phase fault, the maximum instantaneous

force will be

F = 37.4.I

sc2

107 ( )D

8.5 Bus support systemThe bus support system must be capable of taking the

following weights.i. Weight of the tubingii. Weight of damping materialsiii. Wind on the tubingiv. Short circuit force calculatedThe resultant load establishes minimum strength of

tubing material, span length and expected deflection, the busdeflection shall not generally exceed 1/200 of the span lengthwithout ice loading.8.6 Corona

For HV and EHV substations, the diameter of the busshould be checked for corona discharge. Bus tubing can beconsidered satisfactory, if the voltage gradient at the surfacedoes not exceed 2 KV/cm. The voltage gradient can bedetermined by

g = r. In D / r

E

Where g = voltage gradient (KV/cm)

E =Line to neutral voltage (KV)r =Bus outside radius (inches)D = Bus spacing (feet)

8.7 Strain Bus

Strain Bus is widely in use in most of the stations due to

the ease of construction. Even in stations where rigid buses

are predominant, some spans will be invariably of strain bus

construction . The design is followed based on simple sag-

tension calculations.

The down drops from the strain bus appear as a

concentrated load and depending on the length and weight ofthe dropper, tension on the bus will vary considerable.

Where bundled conductors are used in strain bus, thetypes of spacers used may have an influence on the resultingtension. If rigid spacers are used, then during short circuit,the two conductors will attempt to draw together and can

cause for increase tension in the strain bus.

8.8 Substation Bus Accessories

(i) Tubular bus conductorSystem Nominal Diameter

Voltage KV External (mm) Internal (mm)72.5 42 35145 60 52

60 49.2589 7889 74

101.6 90.1245 101.6 85.4

114.3 102.3114.3 97.2114.3 102.3114.3 97.2

420 127 114.5127 109.0

For rigid bus arrangement 7000mm spacing betweenphases are given for 400 KV and 4500mm for 220KV.(ii) ACSR Conductor for strain bus.

Suitable ACSR conductors having the desired capacityshall be used for bus stringing. According to requirement,quadruple Moose, Twin Kundah and Single Kundah ACSRConductors are used for strain bus.

Some Commonly used conductor forBus Stringing

SystemVoltage (KV) Bus Conductor

Lynx ACSR, Kundah ACSR72.5 19/3.53 AAC145 Panther ACSR, Kundah ACSR

19/4.22 AAC245 Kundah ACSR, Moose ACSR

19/5.36 AAC, 37/5.23 AAC400 Moose ACSR

a. Conductor tensionThe following conductor tensions are generally taken

for designing the switchyard structures for bus arrangement.

Details 400KV yard 220 KV yard 132/110 KV yard(Kgs/conductor) (Kgs/conductor) (Kgs/conductor

Line termination 2000 1000 1000

Main Bus/Sub Bus 1000 900 800

Interconnections1000 900 800

between yards

Earth wire 800 600 600

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