substation engineering

WELCOME WELCOME

TOTO

PRESENTATIONPRESENTATION

ON ON

CONCEPT OF SUBCONCEPT OF SUB--STATION ENGINERINGSTATION ENGINERING

2

Contents of PresentationContents of Presentation

�� PURPOSEPURPOSE

�� CLASSIFICATIONSCLASSIFICATIONS

�� VOLTAGE CLASS & RATINGSVOLTAGE CLASS & RATINGS

�� PLANNING OF SUB STATION INSTALLATIONPLANNING OF SUB STATION INSTALLATION

�� SUBSUB--STATION ENGINEERINGSTATION ENGINEERING

�� SUBSTATION EQUIPMENTSSUBSTATION EQUIPMENTS

3

1.0 PURPOSE OF ESTABLISHING A SUBSTATION1.1 The substations are very much essential to

• Evacuate power from generating stations.

• Transmit to the load centers.

• Distribute to the utilities & ultimate consumers.

1.2. The Electrical power generation from Hydel, Thermal, Nuclear and

other generating stations has to be evacuated to load centers. • The generation voltage is limited to 15/18 KV due to the limitation of

the rotating machinery. • This bulk power has to be stepped up to higher voltages depending on

quantum of power generated and distance to the load centers. • Again the power has to be stepped down to different lower voltages for

transmission and distribution.

1.3 In between the power houses and ultimate consumers a number of Transformation and switching stations have to be created. These are generally known as sub-stations

4

2.0 CLASSIFICATIONS

Accordingly the substations are classified as

a) Generating substations called as step up substations

b) Grid substations

c) Switching stations

d) Secondary substations.

2.1. The generating substations are step up stations as the generation voltage

needs to be stepped up to the primary transmission voltage so that huge

blocks of power can be transmitted over long distances to load centers.

2.2 The grid substations are created at suitable load centers along the primary

transmission lines.

2.3 Switching stations are provided in between lengthy primary transmission lines • To avoid switching surges.• For easy segregation of faulty zones.• For providing effective protection to the system in the A.C. network.• The switching stations also required wherever the EHT line are to be tapped and line

to be extended to different load centers without any step down facility at the switching stations.

• The number of outgoing lines will be more than the incoming lines, depending on

the load points.

5

2.4. Secondary substations are located at actual load points along the

secondary transmission lines where the voltage is further stepped

down to sub transmission & primary distribution voltage.

2.5. Distribution substations are created where the sub-transmission

voltage and primary distribution voltage are stepped down to supply

voltage and feed the actual consumers through a network of

distribution and service lines.

3.0. VOLTAGE CLASS AND RATINGS.

Generally the following voltage class substations prevailing in India

• 6.6 KV, 11 KV, 22KV, 33 KV ---------- High Voltage

66KV, 110/132KV,

� 400 KV and above 220/230KV ---------- Extra high Voltage

3.1 Sub station rating is defined as the capacity of power transformers installed.

6

4.0 PLANNING OF SUBSTATION INSTALLATION

The process of planning sub-station installations consists in

• Establishing the boundary conditions.

• Defining the plant concept, type, & Planning principles.

4.1 The boundary conditions are governed by following environmental

circumstances & availability of the land in the required place.

• Local climatic factors

• Influence of environment

• The overall power system voltage level

• Short circuit rating

• Arrangement of neutral point

• The frequency of operation

• The required availability or reliability

• Safety requirements

• Specific operating conditions

7

4.2. Boundary conditionsThe following boundary conditions influence the design concept and measures to be considered for different parts of substation installations.

Boundary conditions Concept and measuresOutdoor / indoor

Conventional / GIS

Equipment utilization

Construction

Protection class of enclosures

Creepage, arcing distances

Corrosion protection

Earthquake immunity

Short circuit loadings

Protection concept

Lightning protection

Neutral point arrangement

Insulation coordination

Environment, climate conditions

Net work data / Net work form

8

Boundary conditions Concept and measures

Bus-bar concept

Multiple in-feed

Branch configuration

Standby facilities

Un-interruptable supplies

Fixed/draw out apparatus

Choice of equipment

Network layout

Scope for expansion

Equipment utilization

Instrument transformer design

Automatic/conventional control

Remote/local control

Construction/configuration

Network layout

Arcing fault immunity

Lightning protection

Earthing

Touch protection

Step protection

Fire protection

Ease of operation

Safety requirements

Availability and abundance of power

supply

Power balance

9

4.4. Type of sub4.4. Type of sub--stationsstations

4.4.1. The types of Sub Stations depends upon:

• The availability of the land in the required place.

• Environmental conditions.

4.4.2. Sub-Station types are:

• Out door

• In door

• Compressed Air insulated

• GIS

10

4.5 Sub-Station Engineering

•The Sub Station Engineering comprises:

� Sub-station site selection

� Switching scheme.

� Bus-Bar.

� Safety clearances.

� Phase to phase clearances.

� Phase to ground clearances.

� Sectional clearance.

� Ground clearance.

11

4.5 Sub-Station Engineering(Contd)�Yard levels.

�Single line diagram & Layout.

�Bus levels.

� First level ---- Equipment interconnection level.

� Second level ---- Bus levels.

� Third level ---- Cross Bus / Jack Bus level.

�Bay widths

�Lightning protection.

�Earth mat.

�Civil Engineering works.

�Electrical Installation works.

�Main electrical equipments.

�Auxiliary supplies

12

4.5.1. Sub station site selection

• The aspects are to be considered for site selection

� Fairly level ground

� Right of way around the sub station yard for incoming & out

going transmission & distribution lines

� Preferably of soil strata having low earth resistance values

� Easy approach & accessibility from main roads for Heavy

equipment transportation and routine O & M of sub station

� Economy / Cost

13

4.5.2. Switching schemes:• The factors considered for selection of switching schemes

� Reliability factor

� Availability of the space

� Economics (project cost)

� There can be several combinations in which the equipments, bus-

bars, structures etc. can be arranged to achieve a particular

switching scheme.

� The switching schemes can be made more flexible

by making minor modifications like providing sectionalisers using

bye-pass path etc.

�The various types of switching schemes along with its advantages

and disadvantages are:

14

Switching SchemesSwitching SchemesSwitching Scheme Advantages Disadvantages

Bus fault or breaker failure causes station outage

Maintenance is difficult

No station extension works without complete

shutdown

For use only where loads can be disconnected

or supplied from another substation.

Single busbar with

sectionaliser

Shut down on the part of

the Bus can be availed Aditional cost for the isolator

Higher flexibility as

compared to single bus

Maintenance of main bus will involve outage of

substation.

One breaker can be taken

for maintenance at a timeAdditional cost for the Transfer Bus & Breaker

High flexibility with two

busbars of equal merit

Expensive for additional bus and BC breaker and

associated equipments and also extra space is

required

Each busbar can be

isolated for maintenance One Breaker maintenance possible at a time.

Each branch can be

connected to either of the bus

with bus tie breaker

There will be a time delay for restoration of the

circuit in case of breaker outage

The two buses can be

individually operated in case

of island operations

Single main and transfer

bus

Single busbar Least cost

Double main busbar

16

LinesLines

Lines LinesLines

Main Bus

Transfer

Bus

Main Bus1

Main Bus2

Transformer Transformer Transformer Transformer

17

Switching Schemes (Switching Schemes (contdcontd))

Switching Scheme Advantages Disadvantages

High flexibility with 3 buses

and 2 tie breakers

One breaker is available at a

time for maintenance

No time delay for restoration

of the circuit in case of breaker

outage.

Greatest operational flexibility

High reliability

Breaker fault on the busbar

side disconnects only one

branch

Each main bus can be

isolated at any time

All switching operations

executed with circuit-breakers

Bus fault does not lead to

branch disconnections

Greatest operational flexibility

Each branch has two circuit

breakers

Connection possible to either

bus bar

Each breaker can be serviced

without completely disconnecting

the branch

High reliability

Flexibility for breaker

maintenance

Each breaker removable without

disconnecting load

Only one breaker needed per

branch

Each branch connected to

network by two breakers

All change-over switching done

with circuit-breakers & hence

flexible

Area required will be more

2 breaker system Most expensive method

Ring bus

Double main bus with

transfer bus

Greater outlay for protection and auto-reclosure,

as the middle breaker must respond independently

in the direction of both feeders

Three circuit-breakers with associated equipments

required for two branches

Auto-reclosure and protection fairly complicated

Breaker maintenance and any faults interrupt the

ring

1½ Breaker system

Expensive consequent to additional two buses

and two breakers with associated equipments and

additional space is required.

18

Line

Main Bus1

Transfer Bus

Transformer

Main Bus2

Line

Transformer

22

4.5.3.4.5.3. Bus Bus -- BarsBars• Selection of bus-bars

�Type of Bus Bar

� Sizes of Bus Bar

• Types of Bus –Bars

� Strung Bus / Flexible Bus

� Rigid Tubular Bus

• Strung Bus:

The various Types of conductors used for Strung Bus are

� All Aluminum conductor (AAC)

� All Aluminum alloy conductor (AAAC)

� Aluminum conductor with aluminum alloy reinforced (ACAR)

� Aluminum conductor with steel reinforced (ACSR

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• RIGID TUBULAR BUS.

� Rigid tubular conductors are also used in substations.

� Rigid tubular buses are more advantageous than the

flexible conductors.

• Sizes of Bus Bar

The factors to be considered for selection of the Bus-Bar sizes are:

�Normal current carrying capability

�Short circuit heating with stand capability

�Surface gradient

�Corona free performance

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• Selection Criteria for Bus Sizes

� Electrical & Mechanical Stresses:

� The bus-bars must be designed for:

• The operating current.

• To withstand short-circuit fault currents.

• The anticipated stresses on the bus-bars and their supports

in the event of a short circuit must therefore be calculated.

�Thermal stresses

� Bus bars including clamps and connectors are also stressed

thermally under short circuit conditions.

� The bus bar conductors/tubes are suitably sized / designed to

with stand the short circuit currents not only mechanically, but

also thermally.

Case Case –– Study for 1000 MVA 400/220 S/SStudy for 1000 MVA 400/220 S/S

•• REQUIREMENTS:REQUIREMENTS:

� Normal full load current for 1000 MVA (2 X 500) capacity.

400 KV -- 1445 amps220 KV -- 2625 amps.

�Short circuit heating withstanding capability: Minimum cross sectional aluminum area required to with stand one KA for one second is 15.29 .

For 40 KA for 1 sec --- 610.7 sq mmFor 31.5 KA for 1 sec --- 481 sq mm

� Maximum Permissible conductor surface gradient -21 KV/cm.

� Permissible radio interference level --- 40 to 50 db

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Rph Yph Bph

1.

Single Moose 830 Amps *34.6 KA

Aluminum area for

with standing 1 KA

/sec is 15.29 sq

mm

400 Kv system

ph–ph 7 mtrs

Ph–gr 8.0 mtrs

a) Single moose

15.82 20.81 14.2 47.84

220 KV system

Ph-ph 5.0 mtrs.

Ph-gr 5.5 mtrs

a) Single Moose

34.54 41.34 24.4 139.11

11.94 14.16 13.32 38.76

3.

For 1000 MVA

Transformer

2665 Amps

four moose is

required

For 31.5 KA S.C.w ith

standing capacity for 1

sec single moose is

required for 220 KV

The characteristics of the ACSR Moose conductor are as follows.

Conductor Surface

gradient at KV/CM

Radio

interfere

nce level

db

2.

For 1000 MVA

Transformer

1445 Amps

Tw in moose is

required

For 40 KA S.C.w ith

standing capacity for 1

sec tw in moose is

required for 400KV

b) Tw in moose w ith 450

mm conductor spacing

Sl

no

Voltage system in

KV

Normal

Current

carrying

capacity

at 85 C

Short circuit heating

withstand capacity

for 1 sec having

cross sectional area

of 529 mm

Considering the Example of Moose A.C.S.R. Characteristics.

27

REQUIREMENTSREQUIREMENTS

• Normal full load current for 1000 MVA ( 2 X 500 )capacity

400 KV -- 1445 amps

220 KV -- 2625 amps

• Short circuit heating withstanding capability

Minimum cross sectional aluminum area required to with stand one KA for one

second is 15.29 sq mm.

For 40 KA for 1 sec --- 610.7 sq mm

For 31.5 KA for 1 sec --- 481 sq mm

• Maximum Permissible conductor surface gradient --- 21 KV/cm

• Permissible radio interference level --- 40 to 50 db

By the above it is found

� Twin moose conductor is required for 400 KV.

� Quadruple moose conductor is required for 220 KV main bus, bus coupler bay.

� Twin moose conductor is required for 220 KV transfer bus, transformer & line bays.

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The characteristics of 100 mm and 75 mm IPS aluminum tube are as follows:

* 400 KV system: conductor height 8 mtrs, phase to phase spacing 7 mtrs.** 220 KV system: Conductor height 5.5 mtrs, phase to phase spacing 4.5 mtrs.By the above it is observed that

For 400 KV system 100 mm IPS tubes are required For 220 KV system 100mm IPS tubes are required

*400 KV **220 KV

1 100 mm 114.2 97.18 2825 2665 18.08 11.63

2 75 mm 88.9 77.93 1428 1775 21.89 13.98

Surface voltage

gradient KV rms/cmSl.

No.

Size of

IPS

Outer dia.

mm

Internal dia.

mm

Aluminium

area sq mm

Normal current

carrying capacity

at 850C

• Rigid conductor selection.Rigid conductors are selected based on the following criteria.

� Normal current carrying capacity

� Short circuit heating withstand capability

� Surface voltage gradient

� Fiber stress in tube &Vertical deflection

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4.5.3.4 Fiber stress in tube & Vertical deflection

• Aluminum tube should be capable to with stand the gravitational wind & short circuit forces.

• The vibrations in aluminum tube are caused due to study wind blowing across the bus at right angles to aluminum tube span.

• The fiber stress/bending stress of Aluminum tube depends upon the span of the Aluminum tube between two supports.

• The vertical deflection also depends upon the span of tube and type of supports [i.e. Whether two ends are pinned (simple supported) orfixed, or whether one end is fixed and other is pinned].

• The safe vertical deflection should be less than the half of the outer dia. of Aluminum tube.

30

4.5.3.5 The maximum allowable span lengths are as follows

Size of Aluminum

tube.

Two ends pinned or

simply supported.

Permissible Span

Both ends Fixed

permissible Span length

in mtrs100 mm 11 **12.5

75 mm 9 **12.5

** Maximum permissible to limit the fibre stress.

� The adequacy of span of Aluminium tubes has to be verified depending upon sub-station layout arrangement.

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4.5.3.5 The standard sizes of aluminum tubular bus and conductors generally used for different substations are as follows

SL.

No.

Voltage

referenceAl. Tube ACSR conductor

1 33 KV 50 mm Coyote / Drake

2 66 KV 63 mm Falcon / Twin Drake

3 110KV 75 mm Falcon / Twin Drake

4 220 KV100/75

mm

Single / Twin Falcon Twin

/ Quadruple Moose

5 400 KV 100 mm Quadruple MOOSE

32

4.5.4 Electrical safety clearances.

The electrical , safety clearances to be adopted in substation are governed by following parameters.

• Basic Impulse Insulation levels (BIL).

• Basic Switching Impulse level (BSL).

• IE Rules.

• Allowances in tolerance in dimensions of structural work.

• Safety margins for unforeseen errors.

Based on the above, certain minimum clearances are defined for

a given voltage class and the same are applied in substation

• The various clearances need to be defined.

� Phase-to-earth clearance.

� Phase-to-phase clearance.

� Sectional clearance.

� Ground clearance.

� Equipment to equipment spacing

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4.5.4.1. Phase to earth and phase to phase clearancesThe minimum phase to phase and phase to earth clearance for 400 KV, 220 KV and other voltage classes are based on the BIL & BSL values.

The above mentioned clearances do not include clearance between the live and ground parts of equipments including bus post insulators for which insulation is prescribed as per relevant standards and guaranteed by the manufacturers and confirmed by type tests.

Sl.

No.

Voltage

classBIL BSL

Phase to phase

clearance in mm

Phase to ground

clearance in mm

1. 400 KV 1425 KVp 1050 KVp 4000 3500

2. 220 KV 1050 KVp 460 KVp 2400 2100

3. 110 KV 550 KVp 230 KVp 1100 1100

4. 66 KV 325 KVp 140 KVp 630 630

5. 33 KV 170 KVp 90 KVp 320 320

35

4.5.4.2Section clearance

• Section clearance is the distance between two sections of

substation, which enables a person to work on one section

of a substation in a safe manner, while the other section is

charged.

• Section clearance is chosen in such a manner that phase to

earth clearance is maintained between the live point and the

approach of the working personnel with adequate margin.

• In case of 400 KV:

� The phase to earth clearance of 3.5 meters.

� The approach of man is considered as 2.5 meters.

� Margin of 0.5 meters for unforeseen reasons like errors in

erections, dimensions of tools and platforms etc.

� Thus the section clearance is taken as 6.5 meters.

36

The section clearance for all voltage classes shall be:

Voltage

class in

KV

Highest

system

voltage

in KV

Minimum safety

working

clearance followed as

per the design

Minimum safety working

clearance as per rule no 64 of I.E.Rule1956

400 420 6500 mm 6000 mm

220 245 5000 mm 4300 mm

110 123 4000 mm 3500 mm

66 72.5 3500 mm 3000 mm

33 36 3000 mm 2800 mm

37

4.5.4.3. Ground clearance

• The ground clearance is the distance between ground level and bottom of any

insulator in an out door substation.

• This ensures that any person working in the area cannot touch or damage the

insulators accidentally.

• This clearance is kept as 2.5 meters for all voltage levels.

• However in cases, where the vehicles and cranes are allowed inside a

substation, the ground clearance for the equipment falling on both sides of the

road are to be enhanced as the vehicles and cranes height is generally 3.5

meters.

• The minimum ground clearances between the live point & ground at the

substation for the different voltage classes as per rule no 64 of I.E.Rule 1956

are as follows

� 400 KV 8000 mm

� 220 KV 5500 mm

� 110 KV 4600 mm

� 66 KV 4000 mm

� 33 KV 3700 mm

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4.5.4.4. Bus levels

Generally in all the substations two / three level bus arrangements are necessary.

� The first level is the equipment interconnection.

� The second level is main buses 1 & 2, which may be Rigid / Strung Bus.

� The third level cross bus / Jack Bus , required in few large sub stations.

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4.5.4.5. First level : Equipment inter connection levelThe first level height is fixed based on the following considerations

• The bottom part of the insulator or top most part of the earth metal

position should have the minimum ground clearance i.e. the height of the

man standing on the ground with shoes on holding the tools and

extending the arms upwards, which is already prescribed as 2.5 meters

in all voltage class substations.

• The insulator height / length as per I.E.C / I.S.S i.e., phase to earth

clearances as prescribed for different voltage classes.

• Live metal part height of the various equipments.

• Maximum value of electrical field at a height of 1.8 meters i.e. height of

an average person level.

� The electrical field is the deciding factor not only for the height of the

bus level but also for conductor configuration and phase spacing.

� It is generally considered that 10 KV per meter as safe design value of

an electrical field for a period of 180 seconds.

� The effect of electrical field reduces with increase in bus level.

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• 400 KV System.� The calculated electrical field at a level of 1.8 meters from

ground level with a bus level of 7 meters height is 12.11 KV per meter.

� The calculated electrical field at a level of 1.8 meters from ground level with a bus level of 8 meters height is 9.4 KV per meter.

• 220 KV System.

� The calculated electrical field at a level of 1.8 meters from

ground level with a bus level of 5.5 meters height, is 9.4 KV per

meter..

• The minimum bus level height for 400 KV is calculated as

� As per IEC & ISS ---------- 2500 + 3500 = 6000 mm.

� As per live metal part Ht of eqpt --- less than 6000 mm

� As per electrical field ------------ 8000 mm

� As per I.E. Rule also -------------- 8000 mm

• For 400 KV minimum first bus level shall be 8000 mm

• For 220 KV minimum first bus level shall be 5500 mm.

43

4.5.4.6. Second level:

• Cross Bus levels are generally called second levels in a sub-

station switchyard.

•The height of this bus is decided / designed based on the

following.� Height of the equipment inter connection level i.e., first level.

� The extension of top level live metal part of Bus post insulator / isolator with an expansion clamp.

� The maximum sag of the conductor if it is a strung bus.

� Phase to phase clearance.

� Half of conductor / rigid bus diameter.

� Some minimum factor of safety ie some adequate marginparticularly to maintain

� minimum phase to phase clearance between main bus & cross bus.

44

4.5.4 Standard Bus levels (First levels) / Equipment inter connection level and second level ie cross Bus / main Bus levels for different voltage classes in a sub-station designed as per above principles are follows.

* Third level or Jack bus level in 400 KV stations

SlSlSlSl. . . .

No.No.No.No.Voltage classVoltage classVoltage classVoltage class

First level First level First level First level

mmmmmmmmSecond level mmSecond level mmSecond level mmSecond level mm

1. 400 KV 8000 15000/22000 *

2. 220 KV 5500 11000

3. 110 KV 4500 9000

4. 66 KV 4250 8500

5. 33 KV 4000 8000

45

4.5.4.5 Equipment to equipment spacing.

The equipment to equipment spacing is decided based upon following factors.

• Adequate clearances (phase to earth, phase to phase, section and

ground clearances).

• Convenience of erection and security.

• Adjacent equipments should not foul physically while installing

terminal clamps.

• Equipment foundations should not foul with each other and cable

trenches.

• Technical requirements.

� Location of surge arrestors with respect to protected equipments such as

transformer and reactors.

� Position of CVT, wave-trap and shunt reactor approaching from line side.

� Maintenance flexibility

46

4.5.5 Bay widthsThe bay widths are chosen in such a way that the minimum clearances are maintained

even when the isolator is kept under fully open condition with one end energised. The

different types of the isolators like horizontal center brake, horizontal double brake,

pantograph and vertical break has a great impact in deciding the Bay widths.

• Vertical brake isolator

The bay width can be reduced, but the bus height increases.

Hence this type of isolator is not generally used.

• Pantograph isolator

It requires fine adjustment of sag and too expensive. Bay widths & Lay out sizes can be

reduced considerably. These type of isolators will be used in critical lay outs where

space is criteria.

• Horizontal center break isolator

This type is most commonly used isolator due to it’s low cost, and it will never be placed

under the gantry as it will intrinsically demands higher clearances and the bay width has to be

increased beyond the rational value.

• Horizontal double break center rotating isolator

It is very rigid, good performance, less bay widths and lay out sizes can be reduced. But

costlier compared to H.C .B.

50

4.5.5.1 The factor considered for computing bay widths are

• Phase to Ground / Earth clearance

� The distance between nearest point of tower from extreme phases

� Minimum Phase to earth clearance: as per IE Rule

� Maximum horizontally protruding live metal part from the center of the equipment

� Tolerances for execution

� Length of the isolator blade

� The movement due to swing of conductor and insulator string in case of strung

bus only and not applicable in case of rigid bus.

• Phase to phase clearance� Length of the isolator blade

� Horizontally protruding live metal part of adjacent equipment

� Phase to phase clearance as per IE rule

� Tolerances for execution

� The movement due to swing of conductor and insulator string in case of

strung bus only and not applicable for rigid bus system.

• Considering tolerances for execution etc, the phase to phase and phase to

ground clearances for 400 KV is as follows

� Phase to ground ---- 6500 mm� Phase to phase ----- 7000 mm� Bay width will be 6.5 + 7.0 + 7.0 + 6.5 = 27 meters

51

4.5.5. The standard bay widths, ground & sectional clearances based on above analogy for rigid and strung buses for different voltage classes of substations are as follows

Voltage

class

KV

Bay

width

in mtrs.

Phase

to

phase

clearan

ce in

mtrs

Phase

to earth

clearan

ce in

mtrs.

Bay

width

in mtrs.

Phase

to

phase

clearan

ce in

mtrs

Phase

to earth

clearan

ce in

mtrs.

1. 33 4.5 1.25 1 5.5 1.5 1.25 3.8 2.5

3. 110 8.2 2.1 2 10.5 2.75 2.5 4.6 3.5

4. 220 14 3.65 3.35 17 5 3.5 5.5 4.3

5. 400 27 7 6.5 27 7 6.5 8 6.5

3

Sectio

nal

clearan

ce in

mtrs.

2. 66 7.6 2 1.8 8.6 2.3 2 4

Sl.

No.

Rigid bus Strung bus

Ground

clearanc

e in mtrs.

53

4.6 Yard leveling4.6 Yard leveling

• Complete switchyard shall be generally maintained at the same level

to have sufficient ground clearances and easy for execution,

operation & maintenance.

• But in some cases leveling of the complete switchyard is too

expensive.

• There will be huge cutting and filling if there are large undulations of

the site.

• In such cases 2 to 3 different levels can be maintained for different

voltage classes viz. 400 KV, 220 KV, control room etc.

• Generally the levels of the switchyard will be mainly decided by

balancing volume of earth cutting and volume of earth filling for

economical considerations.

54

4.7. Single line diagram & lay out design.• Draw a single line diagram also called key diagram, before the locations of

various equipments in the substation are decided.

• This diagram indicates the proposed bus bar arrangement and relative

positions of various equipments. There are numerous variations of bus bar

arrangement.

• The choice of a particular arrangement depends on various factors viz.

System voltage, position of the substation in the system, flexibility, expected

reliability of power supply and cost.

• The following technical consideration must be borne in mind while deciding

upon any one arrangement.

� Simplicity is the key note of a dependable system

� Maintenance should be easy with minimum interruption of supply

� Safety to the operating personnel

� Alternative arrangement should be available in the event of an outage on

any of the equipments or sections of sub station

� The layout should not hinder for expansion and/or augmentation at a later

date, to meet the future load growth

� The installation should be as economical as possible keeping in view of the

requirements and continuity of supply

60

4. 8. LAYOUT DESIGN

The first task which a substation designer has to undertake after finalizing the single line

diagram, bus switching scheme, bay widths, section & ground clearances, is to translate

the selected scheme into a layout so as to physically achieve the feeder switching

required for ease in erection and maintenance.

4.8.1. BROAD PARAMETERS

Following are the broad parameters, which change from one substation to another.

a) Nature of bus bars i.e. Rigid or flexible ( Strung Bus)

b) Orientation of bus bar

c) Location of equipments

d) Manner of inter-connections

e) Structural arrangement

f) Direct stroke lightning Protection

4.8.2. FACTORS INFLUENCING THE CHOICEThe factors need to be considered while choosing a type of layouta) Reliability

b) Ease of construction and provision for extension

c) Ease of operation and maintenance

d) Safety of operating personnel

e) Land requirement

f) Safety of Equipment and installation

g) Aesthetic look.

h) Economy

61

4.9. SAFETY MEASURES:4.9. SAFETY MEASURES:

4.9.1.Power Distribution.

• The distribution of power and current is checked and the currents occurring in

the various parts of the station under normal and short circuit conditions are

determined.

• The power flow to be balanced to an extent possible by properly locating th

incoming / out going lines & Transformer bays.

4.9.2 Safety MeasuresThe safety measures for the substation and its components are to be designed in

respect of

a) Insulation co-ordination.

b) Lightning Protection system.

c) Safe Clearance.

d) Thermal and mechanical stresses

62

4.9.3.INSULATION – CO-ORDINATIONInsulation coordination is the total of all measures taken to restrict flash over or break down of the insulation caused by over voltages at places with in an installation at which the resulting damage is as slight as possible. This is achieved by using lightning arresters to limit over voltages.

The equipments are also to be designed to withstand lightning and switching surges. The nominal lightning impulse withstand voltage and power frequency withstand voltage for various voltage classes are as follows.

Nominal

lightning

impulse

Nominal power

frequency

withstand voltage

Peak value RMS value

36 KV 170 KVP 90 KV

72.5 KV 325 KVP 140 KV

123 KV 550 KVP 230 KV

245 KV 1050 KVP 460 KV

420 KV 1425 KVP 1050 KVP*

Maximum

voltage of

equipments

* Nominal Switching impulse with stand voltage

63

4.9.4 LIGHTNING PROTECTION:

In H.V.& EHV substations, the protection from the lightning is done either by shield wire or

lightning mast (high lattice structure with a spike on top) and sometimes combinations of

both depending upon type of layout of substation.

• Shield wire

Shield wire lightning protection system will be generally used in smaller sub stations of:

� Lower voltage class, where number of bays are less, area of the substation is

small, & height of the main structures are of normal height.

� The major disadvantage of shield wire type lightning protection is, that it

causes short circuit in the substation or may even damage the costly

equipments in case of its failure (snapping ).

• Lightning masts (LM)

This type of protection will be generally used in large, extra high voltage sub stations

where number of bays are more. It has the advantages,

� It reduces the height of main structures, as peaks for shield wire are notrequired

� It removes the possibility of any back flashover with the near byequipments/structure, etc.during discharge of lightning strokes

� Provides facility for holding the lightning fixtures in the substation forillumination purposes

� Aesthetic look.

64

4.9.4.1 PROTECTION ZONE BY SHIELD WIRE.

• The height of equipment to be protected by shield wires depends upon the

height of the earth wire and distance between them.

• As per experiment it has been found that, the displacement of the electrode

from shield wire at a distance of “B”=2h where ‘h’ is the height of the shield

wire, all the discharges will strike the shield wire, and protects all the

equipments from lightning discharges in the zone.

• There fore for two shield wires at a distance of “S”= 4 x h between them

(“h” is the height of the shield wire), the point situated on ground surface

mid way will not be struck by lightning.

• Similarly for protection of any equipment of height “h0”, the distance

between shield wire “S”, shall not be more than 4 times effective height ( h-

h0 ), i.e., the difference of height between. shield wire and the object to be

protected

“S” = or < 4(h-h0)

ho = or < (h-S/4)

ho the height of equipments shall be = or < h – S/4

65

• 400 KV switch yard.

� 400 KV bay width = 27 mtrs.

� Hence Shield wire distance ‘S” is 27 mtrs, apart..

� Height of shield wire “h” = 23.5 mtrs.

� Maximum height of equipments which can be protected by these two shield

wires are (23.5 – 27/4) = 16.75 Mtrs

� The height of the main bus level in the 400 KV station is15.00 Mtrs and all

the equipments will be with this level only. Hence the shield wires provided

on the peaks of the bus structures will protect all the equipments in the

respective bays.

• 220 KV switchyard.

� Bay width ------ 17 Mtrs.

� Height of shield wires ------ 19 Mtrs

� Maximum height of equipments which can be protected by these two shield

wires are (19 – 17/4) = 14.75 Mtrs

� The height of the main bus level in the 200 KV station is13.50 Mtrs and all

the equipments will be with in this level only.

� Hence protects all the equipments in the bay.

69

4.9.4.2 SELECTION OF LM HEIGHT

The factors to be considered

• The height of the LM will be decided, depending upon the height of equipment

to be protected

• The protection zone or coverage area of LM increases with the increase of its

height

• Hence LM’s height depends upon the height of equipment to be protected

• The protection zone of same LM would be more if the equipment height to be

protected is less

• The numbers of lightning masts in substation can be reduced by increasing

the height of LM, but this will cause increase in cost of structure and civil

foundations.

• The detailed analysis and experience revealed that 30 mtr. LM height is

economical proposition & hence to be limited to this height.

70

4.9.4.3. LOCATION & NUMBER OF LIGHTNING MASTS

The exact number and locations of LMs will be calculated for complete protection of equipment in substation, by considering the following aspects.

• The protection zone of one LM is very limited.

• In case of two LMs, the protective zone is considerably more than

the sum of protective zones of two single LMs.

• A point of height ho situated midway between two lightning masts

of height h can be protected if the distance ‘a’ between LMs is not

more than a seven times of active height ( i.e. difference of height

between a LM height h and height of equipments to be protected

ho ) or a = or < 7(h-ho).

• In case of 3 LMs forming triangle or 4 LMs forming rectangle, of

height h can protect the object of height ho situated inside the

triangle or rectangle if diameter D of the circle passing through

the tips of LMs is not more than 8 times the active height i.e. D

< = 8 (h-ho).

72

4.10. EARTH MAT REQUIREMENT

The main objectives of earthing system in the substation are:

• To ensure that a person in the vicinity of substation is not exposed to

danger of electrical shock

• To provide easy path for fault currents into earth under fault

condition without affecting the continuity of service

• Hence intentional earthing system is created by laying earthing rod

of mild steel in the soil of substation area.

• All equipments/structures which are not meant to carry the currents

for normal operating system are connected with main earth mat

• The earthing system in a substation serves :

� Protects the life and property from over-voltage

� To limit step & touch potential to the working staff in substation

� Provides low impedance path to fault currents to ensure prompt and

consistent operation of protective device

� Stabilizes the circuit potentials with respect to ground and limit the

overall potential rise

� Keeps the maximum voltage gradients within safe limit during ground

fault condition inside and around substation

73

4.10.1 SELECTION OF EARTHING CONDUCTOR SIZE FOR MAIN EARTH MAT

The selection of earthing conductor is based on

• The thermal stability criteria

• Jointing method

� Welded with maximum temperature rise of 6200C

� Bolted with maximum temperature rise of 3100C

• Magnitude of short circuit fault current & its duration

74

4.10.2. FORMATION OF SUBSTATION EARTHING:

• The main earth mat shall be laid horizontally at a regular spacing in both X & Y direction based upon soil resistivity value and substation layout arrangement.

• The main earth mat shall be designed to limit the following;

� Touch Potential – The potential difference between two points, one on the ground where a man may stand and any other point which can be simultaneously touched by either hand.

� Step Potential – The potential difference between any two points on ground surface which can be simultaneously touched by feet.

�Maximum ground mat resistance shall be less than 1.0 ohm for substations of 220kV class and below, and shall be 0.5 ohms for 400kV and above voltage class.

� The earth rods shall be capable of with standing short circuit current for specified period.

� For I KA SC current for 1 second the minimum cross sectional area of M.S. Rod / Flat shall be 12.16 sq mm with welded joints.

75

• The crushed rock (Gravel) of 15 mm to 20 mm size shall be

used as a surface layer of 150 mm in the substation for the

following reasons:

� To provide high resistivity for working personnel

� To minimize hazards from reptiles

� To discourage growth of weed

� To maintain the resistivity of soil at lower value by retaining

moisture in the under laying soil

� To prevent substation surface muddy and water logged.

• The main earth mat shall be laid at a depth of 600 mm from

ground.

• The earth mat shall be connected to the following in substation

i. Lightning down conductor, peak of lightning mast

ii. Earth point of S A, CVT

iii. Neutral point of power Transformer and Reactor

iv. Equipment framework and other non-current carrying parts.

v. Metallic frames not associated with equipments

vi. Cable racks, cable trays and cable armour

i.

76

5.0. INSULATORS.• Types

� Disc type� Post type.

a) Disc type

These are required for stringing the ACSR conductor for main bus and

jack bus/cross bus. The individual units are rated for 11 KV and string of

these units will be used for deferent voltage classes. The number of units

per string depends on following parameters.

i) System voltage.

ii) Insulation level

iii) Power frequency withstand level

iv) Tensile strength

v) Purpose – tension string or suspension string.

77

Typical details of string used for various system voltages

Tension

string

Suspen

sion

string

400 160 25 -

400 120 - 23

220 120 16 -

220 90 - 14

110 90 8 8

66 90 5 5

33 70 2 2

System

voltage

in KV

Tension

strength

in KN

No. of units per

string

78

b) Post type insulators

These are used for supporting ACSR conductor or Rigid

Aluminum tube for connecting main bus to equipment or

forming main bus. These are also used as supporting insulators

for isolators.

There are two types

i. Pedestal post or stacking type

ii. Solid core type

The solid core type are preferred

The design considerations are,

i) The phase to earth clearance which determines the height

ii) Insulation level

iii) Power frequency withstand level

iv) Mechanical strength i.e., mainly cantilever strength

v) Minimum creepage dimensions

79

Typical parameters for various voltage levels

System voltage in

KV

Height of stack

(mm)

Nol of units per

stack

Minimum

creepage

dimension in mm

at 25 mm/KV

Cantilever

strength KN

400 3650 3 10500 8

220 2300 2 6125 6

110 1220 1 3075 4.5

66 770 1 1815 4.5

33 380 1 900 4

80

6.0 Steel structures:

i) Towers and Beams:Required for stringing the ACSR conductor for main busand cross bus/jack bus

ii) Lightning masts:Required for providing protection against lightning andinstalling the luminaries fitting for illumination ofswitchyard.

iii) Support structuresThese are required for supporting the equipments andpost insulators to maintain the live point heights andother clearances as per the statuary clearancerequirements.

81

6.1. The steel structures can be classified broadly into two groupsi) Lattice type

Formed by mild steel angle sections/plate sections etc by fastening the various sections by bolts, nuts or by welding.

ii) Tubular typeFormed by using mild steel pipes. These are preferable for support structures for lightning arrestors, post insulators, & instrument transformers. etc.

6.2 Protection against corrosionThe steel structures are generally made of mild steel, which are galvanised / painted to protect against corrosion. Galvanising by applying zinc coating is preferable as the protection achieved is superior to painting & maintenance free. The coastal areas where due to saline weather conditions – corrosion phenomenon occurs very fast and hence only galvanising is recommended.

6.3 Design considerationsa)Towers

i) Wind loadii) Reactions of loads on the beams to which conductor is (along with

insulator) strungiii) Vertical loads on the beams like under strung isolatorsiv) Tension of conductor (if strung directly ) and ground wire.v) Short circuit forcesvi) Type of foundation – Stub type or anchor bolt type

82

b) Beams

i) Tension of conductor

ii) Wind load / weight of conductor and insulator string

iii) Vertical loads due to under strung isolators /post insulator etc.

iv) Short circuit forces

v) Configuration of conductors.

c) Lightning masts (25.0 to 30.0 meters height)

i) Wind load

ii) Weight of luminaries fittings.

d) Support structure

i) Weight of equipment/post insulator

ii) Wind load on conductor

iii) Load due to aluminum pipe and cantilever strength of post insulator

83

7.0. ILLUMINATIONThe indoor & out door areas of sub station are to be properly illuminated. The minimum lux levels to be maintained in the different areas are follows.

Sl No Location in sub station Minimum lux levels to be

provided

1 Control Room 350

2 L.T.Room. 150

3 CableGallery 150

4 Battery Room 100

5 Entrance Lobby 150

6 Corridor Landing 150

7 Conference Room &

Display Room

300

8 Rest Room 250

Main Equipment -- 50

Balance area. -- 30

10 Street / Road 30

9 Out Door Switch Yard

84

7.1. The aspects to be considered are

• The illumination design to be done by using the software

program to achieve specified levels of illumination most

economically

• The energy conservation methods are to followed by using CFL

fittings etc. where ever feasible with out compromising on

required illumination levels

85

8.0 Classification of the works to be executed in a sub-station

a) Civil engineering works

b) Electrical works8.1. Civil engineering works:

The Civil works comprise of

1. Buildings

i. Residential

ii. Non Residential – Office, control room, repair bay etc.

2. Design & construction of foundations for structures and equipment

structures and transformer plinth.

3. Cable trenches

4. Fencing around switchyard

5. Water supply

6. Drainage & Sewerage

7. Roads & paths

86

8.2. Electrical works comprise of:a) Choice of:

i. Switching schemes.ii. Bus bars.iii. Preparation of key diagram / single line diagram.iv. Preparation of Lay outs

b) Design & layout of earthing grids and protection against directlightning strokes.

c) Auxiliariesi. D.C. supply

• Battery set.• Battery Charger• D.C. Panel

ii. A.C. supply• Auxiliary Distribution Transformer.• Diesel Generator set.• A.C.Panel.

iii. Control cable & power cable schedule.iv. Switchyard lightingv. Fire fighting equipment.

Major SubMajor Sub--station Equipmentsstation Equipments

a)a) Power Transformers.Power Transformers.

b)b) Circuit breakers.Circuit breakers.

c)c) Instrument Transformers:Instrument Transformers:i.i. Current Transformers.Current Transformers.

ii.ii. Voltage transformers.Voltage transformers.

iii.iii. Capacitor voltage transformersCapacitor voltage transformers

d)d) Isolators / Disconnects.Isolators / Disconnects.

e)e) Lightning Arrestors.Lightning Arrestors.

f)f) Control & Relay panels.Control & Relay panels.

g)g) Shunt Capacitor Banks.Shunt Capacitor Banks.

h)h) Reactors.Reactors.

Technical parametersTechnical parameters

a)a)The principle points to be considered for The principle points to be considered for

selecting subselecting sub--station equipments arestation equipments are

•• Standards.Standards.

•• Principle parametersPrinciple parameters

•• Ratings & their choice.Ratings & their choice.

•• Technical requirements.Technical requirements.

•• Tests.Tests.

StandardsStandards -- Power TransformersPower Transformers

Sl.No Standards Title

1. IS – 10028 (Part 2 & 3)

Code of practice for selection,

installation & maintenance of

transformers (P1:1993), (P3:1991)

2. IS – 2026

3. IEC –76 (Part1 to Part 5)

4. IS-3347 (Part 1 to Part 8)

Dimensions for porcelain

transformer bushings for use in

lightly polluted atmospheres.

5. IS-3639(1991)Fittings and Accessories for power

transformers

6. IS – 6600 (1991)Guide for loading of oil immersed

transformers

7. IEC-354 (1991)Loading guide for oil immersed

power transformers

8. IEC-214 (1989) On-load tap – chargers

9. NEMA – TR – 1Transformers, Regulators and

reactors10.

Power transformers

CBIP Manual on Transformers

Principle Parameters Principle Parameters --Power TransformersPower TransformersSl. No. Item1. Type of power

transformer/installation

2. Type of mounting

3. Suitable for rated system

frequency

Rated voltage

Voltage ratio HV/IV/LV

5. No. of phases

6. No. of windings

7. Type of cooling

Maximum rating MVA 220 KV

winding HV

110/66 KV winding IV 100

11 KV tertiary winding LV

MVA rating corresponding to

cooling system.

a) ONAN cooling 60

b) ONAF cooling

c) OFAF cooling

11.

Connection symbol (vector

group)

4.

Examples of Technical requirements3 phase, auto winding

interconnecting transformer

suitable for outdoor installation.

3 phase core type winding

transformer suitable for out

door installation-----On wheel mounted on rails----

50 Hz 50Hz

245 KV class

220/110/11 KV

245 KV class

220/66/11 KV

Three Three

Auto Tr. With tertiary Three winding with tertiary

OFAF OFAF

8.

100

100

100

9.

80

100

80

100

10.

Winding connection HV Star

IV Auto

LV Delta

HV Star

IV Star

LV Delta

YNaod 11 YNynod 11

60



12.

System earthing

Percentage impedance voltage

on normal tap and at rated MVA

tolerance as per IS-2026.

a) HV-IV

b) HV-LV

c) IV-LV

Anticipated continuous loading of

windings

a) HV and IV

b) Tertiary

Tap changing gear :

a) Type

b) Provided on

c) Tap range

d) Step voltage

e) No. of steps

17.

Max. flux density in any part of

core and yoke at rated MVA,

frequency and normal voltage

(tesla)

18.

Current density of HV/IV/LV

winding

Insulation levels for windings:

a) 1.2/50 micro-second

wave shape impulse withstand

(KVP)

HV IV LV

950 325 170

HV IV LV

395 140 70

Type of winding insulation:

a) HV winding

b) IV winding

c) LV winding






door installation

10

-----Effectively solidly earthed-----

13.

10

The tertiary winding is for stabilising purpose without loading.

The impedance shall be designed in confirmity with BIS, to

with stand the short circuit currents for a specified period.14.

------ Not to exceed 110 % of its rated capacity --------

15.

On load, suitable for bi-directional power flow

--------Neutral end of HV winding -------

+5% to –15% +5% to –15%

1.25% of 220 KV 1.25% of 220 KV

16 16

16.

Over voltage operating capability

& duration

i) 115% of rated voltage continuously

ii) 125% of rated voltage for 60 seconds

iii) 140% of rated voltage for 5 seconds

----------------1.60-----------------

-----------not exceeding 3 Amps per sq. mm----------

19.

HV

950

b) power

frequency voltage

withstand (KV rms)

HV

395

20.

Graded

Graded

Full

Graded

Graded

Full

-------------------- Un loaded teritiary ----------------



21.

System short circuit level &

duration for which the

transformer shall be capable to

withstand thermal and dynamic

stress (KA rms/sec)

Permissible temperatures rise

over ambient temp. of 500C

i) Of top oil measured by

thermometer

ii) Of winding measured by

resistance method

Minimum clearance in air (mm).

a) H.V.

i) HVPhase to Phase

b) I.V.

i) Phase to Phase

ii) Phase to ground

c) L.V.

i) Phase to Phase

ii) Phase to ground

Bushings

a) HV winding Line end

c) HV/IV winding neutral end

(for solid grounding)

-------------40 KA for 3 seconds ------------






door installation

22.

Noise level at rated voltage and

frequencyLess than 83 db

---- as per table 01 of latest NEMA std. TR-1------

550C

500C

23.

500C

550C

ii) Phase to ground 1820 1820

20002000

350 350

700

1270 660

1430

320 320

25.

245 KV class OIP condenser 245 KV class OIP condenser

b) IV winding line end

24.

145 KV class OIP condenser

bushing

72.5 KV class OIP condenser

bushing

--------36 KV porcelain bushing-------

d) LV winding --------36 KV porcelain bushing-------



26.

Insulating medium

Terminal current rating

HV

IV

LV

HV/IV Winding neutral

28.

Max. Radio interference voltage

level at 1 MHz and 1.1 times

max. rms phase to ground

voltage for HV winding

Cooling equipments.

a) Number of Banks

c) No. of fans

Insulation level of bushings:

a) Lightning impulse withstand

(KVP)b) 1 minute power frequency

withstand voltage (KV rms)

c) Creepage

distance (mm)

a) Bushing current transformers

for tertiary provided in each

phase

i) Current Ratio (A/A)

ii) Accuracy class

iii) VA Burden






door installationOIL OIL

27.

800 Amp 800 Amp

800 Amp 1250 Amp

2000 Amp 2000 Amp

800 Amp 1250 Amp

------------ 5000 Micro volts--------------

29.

Two nos. of 50% Bank Two nos.of 50% Bank

One 100% pump & one 100%

standby pump in each bank

One 100% pump & one 100%

standby pump in each bank

Adequate number of fans 18”/24” sweep with one stand by fan

in each group

25 mm per KV of highest

system voltage

25 mm per KV of highest

system voltage

HV IV LV

1050 325 170

HV IV LV

1050 550 170

HV IV LV

460 230 70

b) No. of pumps

30.

31.

--------- To be provided for LV bushings-------

1000/1 1000/1

�-----------5P – 20------------�

�------------15--------------�

HV IV LV

460 140 70

Standard RatingStandard Rating --Power TransformersPower Transformers

Sl.

No.

KV CLASS RATING

1 33/11 KV 5 MVA

8 MVA

12.50MVA

16/20 / 31.5 MVA

a. 110/11 KV 10 MVA

16/ 20 / 31.5 MVA

b. 110/33-11KV 10 MVA

16/20 / 31.5 MVA

a. 220/110/11 KV 100 / 150 MVA

b. 220/66/11 KV 100 / 150 MVA

5 a. 400 / 220 /33 KV 315 MVA

b. 400 / 220 / 33 KV

3 Units of single phase

transformers.166 MVA

2 66/11KV

3

4

500 MVA

TestsTests --Power TransformersPower Transformers

TESTS:TESTS:

a) a) Type tests.Type tests.

b) Routine / Acceptance Testsb) Routine / Acceptance Tests

Type TestsType Tests

i.i.Temperature rise testTemperature rise test

ii. ii. Vacuum test on transformer tankVacuum test on transformer tank

iii.iii.Relief device testRelief device test

iv.iv.Short circuit testShort circuit test

v v Impulse test on principle tap. Impulse test on principle tap.

vi. vi. IPIP--55 test for OLTC cabinet and cooler control cabinet. 55 test for OLTC cabinet and cooler control cabinet.

TestsTests --Power TransformersPower TransformersRoutine tests:Routine tests:

i.i.Operation and dielectric test of OLTCOperation and dielectric test of OLTC

ii.ii.Magnetic circuit testMagnetic circuit test

iii iii OC & SC testsOC & SC tests

iv. iv. Oil leakage test on transformer tank after complete assembly.Oil leakage test on transformer tank after complete assembly.

v. v. Measurement of zero sequence and reactance Measurement of zero sequence and reactance

vi vi Measurement of acoustic noise levelMeasurement of acoustic noise level

vii vii Measurement of power consumption by fans and oil pumpsMeasurement of power consumption by fans and oil pumps

viii. viii. Measurement of harmonic level on noMeasurement of harmonic level on no--load currentload current

ix.ix.Measurement of capacitance and tanMeasurement of capacitance and tan--delta to determine capacitancedelta to determine capacitance

between winding and earth before and after series of between winding and earth before and after series of didi--electric tests.electric tests.

x. x. Insulation resistance testInsulation resistance test

xi. xi. Ratio and polarity test Ratio and polarity test

xii.xii.DiDi--electric and PPM test on oilelectric and PPM test on oil

xiii xiii TanTan--delta test on bushingdelta test on bushing

xiv. xiv. Measurement of copper and iron losses.Measurement of copper and iron losses.

xvxv Leakage tests for radiators / cooler tanksLeakage tests for radiators / cooler tanks

xvi xvi Weld test Weld test

97

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