L6-7 Power Systems

Section 10 Overhead Lines and Underground Cables Overhead lines Overhead lines supports (towers and poles) Overhead lines conductors Fittings Representation of lines Underground cables Insulation, thermosetting and thermoplastic for cables

1

Overhead Lines Overhead lines are usually suspended from insulators. The insulators are supported by towers or poles. The span between two towers depends upon the allowable sag

in the line. Example: the span for steel towers with very-high voltage lines

is normally 370-460 m. There are two main types of towers as follows:

Those for straight runs, which are designed to withstand the stress due to the weight of line alone.

Those for changes in route also known as deviation towers. They need to be able to withstand resultant forces due to line changing directions.

2

Overhead Lines Overhead lines are, in essence, air-insulated cables suspended

from insulated supports with a power transfer capacity approximately proportional to the square of the line voltage.

The overhead lines components under IEC standards are as follows: conductors fittings vibration dampers supports.

3

Overhead Lines Overhead lines versus underground cables Cables are more expensive than overhead lines, the higher

ratios being found at higher voltages. For the transmission of equivalent power at 11 kV, a cable feeder would cost some 5 times the cost of a transmission line, at 132 kV 8 times and at 400 kV, 23 times.

Overhead lines are a lot more forgiving when it comes to overloading than cables.

Catastrophic failures due to short circuits are rare. Line damages can be discovered and repaired more easily than

underground cables. Over head lines can be altered easily in situ to accommodate

change.

4

Overhead Lines Key elements: Transmission voltage evolution Voltage classification according to EN 60071 and General Use International and European standards Technical comparison of AC and DC transmission

5

Transmission Voltage Evolution

1910 1920 1930 1940 1950 1960 1970 1980 1990 20000

200

400

600

800

1000

1200

Year of inauguration

Tran

smis

sion

vol

tage

(kV

)

6 Development of three-phase AC network transmission voltages

Voltage Classification According to EN 60071 and General Use Below 1 kV: Low Voltage (LV) Between 1 kV and 45 kV : Medium Voltage (MV) Between 45 kV and 300 kV : High Voltage (HV) Between 300 kV and 75 kV : Extra-High Voltage (EHV) Above 800 kV : Ultra-High Voltage (UHV)

7

International and European Standards for Overhead Lines

Standard Description IEC 60383 Insulators for overhead lines with a nominal voltage above 1000 V

IEC 60471 Dimensions of clevis and tongue couplings on string insulator units

IEC 60720 Characteristics of line post insulators

IEC 60797 Residual strength of string insulator units of glass or ceramic material for overhead lines after mechanical damage to the dielectric

IEC 60826 Design criteria for overhead transmission lines

IEC/TR 60828 Loading and strength of overhead transmission lines

IEC/TR 61774 Meteorological data for assessing climatic loads for overhead lines

EN 12465 Wood poles for overhead lines durability requirements

EN 12479 Wood poles for overhead lines signs, methods of measurement and densities

EN 12510 Wood poles for overhead lines strength grading criteria

EN 12511 Wood poles for overhead lines determination of characteristic values

EN 12509 Timber poles for overhead lines determination of modulus of elasticity, binding strength, density and moisture content 8

International and European Standards for Overhead Lines Standard Description

EN 14229 Wood poles for overhead lines requirements

EN 12843 Precast concrete masts and poles

EN 50341 Overhead electrical lines exceeding AC 45 k V

EN 50423 Overhead lines AC 1 to 45 k V

Eurocode 1 EN 1991 Basis of design and actions on structures; 1991-1-3 covers snow loads; 1991-2-4 covers wind loads

Eurocode 2 EN 1992 Design of concrete structures 1992-3 covers concrete foundations

Eurocode 3 EN 1993 Design of steel structures

Eurocode 7 EN 1997 Geotechnical design

Eurocode 8 EN 1998 Design provision for earthquake resistance of structures

9

Technical Comparison of AC and DC Transmission Advantages of DC line towards AC lines DC lines present lower power losses. DC lines present lower switching overvoltage and therefore

lower clearance to tower. DC lines introduce no problem of stability. DC lines have the ability to interconnect systems of different

frequency through back-to-back converter stations. DC lines present lower levels of short circuit.

10

Technical Comparison of AC and DC Transmission Disadvantages of DC line towards AC lines DC lines cannot directly supply loads. DC lines cannot be economically taped to supply consumers. The converter stations need a high ratio of reactive power to

supply commutation equipments which reaches to 60% of active power.

11

Support Types and their Applications Definition: Towers or poles are parts of support and consist of

tower or pole body, earth wire peaks and cross arms. Transmission voltage, the number of circuits, the height of

supports and other aspects determine the support design and materials.

Self-supporting structures: Poles made of reinforced concrete, solid wall steel and wood as well as steel towers and portal structures.

12

Support Types and their Applications Key elements: Pole structures Tower structures Tasks of supports in an overhead line Support design and application

13

Pole Structures Pole structures are especially used for economic household

distribution at voltage levels of 380/415 V and 20/11kV where planning permission allows such arrangements in place of buried cables.

Pole structures are also used at the lower transmission voltage levels, typically at up to 145 kV but also with multipole and guyed (stayed) arrangements at voltages up to 330kV.

Low-voltage designs are based on matching the calculated equivalent pole head load to the particular type and diameter of wood, steel or concrete to be employed.

At higher voltages specific designs are used in order to select optimum size and relative cost.

14

Typical Pole Structures

15

Low voltage Wood pole

Low voltage thin wall steel pole

Single circuit Single circuit Trient (no earth wire)

Typical Pole Structures

16

Single circuit twin earth wire

Combined 3 wires 11 kV and 380 V

Double circuit twin earth wire

Tower Structures In order to standardise, towers are categorized typically to fulfill the

following duties: Suspension towers: straight line and deviation angles up to about 2 10 angle or section tower: angles of deviation up to 2 or at section

positions also for heavy weight spans or with unequal effective negative weight spans

30 angle: deviation angles up to 30 60 angle: deviation angles up to 60 90 angle: deviation angles up to 90 Terminal tower: terminal tower loading taking full line tension on one

side of tower and none or slack span on other typically at substation entry

17

Typical Tower Outlines

18

Single circuitSingle earth wire

Double circuitSingle earth wire

Typical Tower Outlines

19

Single circuitdouble earth wire

Double circuitdouble earth wire

Tasks of Supports in an Overhead Line 1. Suspension supports 2. Angle suspension supports 3. Angle supports 4. Strain and angle strain supports 5. Dead-end supports 6. Special supports

20

Tasks of Supports in an Overhead Line 1. Suspension supports:

Carry the conductors in a straight line.

2. Angle suspension supports: Serve as suspension supports for conductors where the line

changes the direction at line angle deflection. Important at voltages higher than 110 kV. The angle change is 0-20 degrees.

3. Angle supports: Carry the resulting conductor tensile forces where the line

changes direction at line angle deflections.

21

Tasks of Supports in an Overhead Line 4. Strain and angle strain supports:

Carry the conductor tensile force in line direction or in resultant direction.

Serve as rigid points in the line. Secure the line against cascading failures.

5. Dead-end supports: Carry the total conductor tensile forces in line direction on one

side. Are additionally loaded by conductors leading to the substation

portals. Act often under large angle to the horizon over a short distance.

22

Tasks of Supports in an Overhead Line 6. Special supports:

Supports that are used for several functions of individual supports. For example, a branch support which assumes the task of angle or angle-strain support concerning the circuit passing through.

23

Support Design and Application 1. Selection of support design 2. Self supporting lattice steel towers 3. Self supporting steel poles 4. Steel reinforced concrete poles 5. Wood poles 6. Guyed supports 7. Cross-armless supports

24

Support Design and Application 1. Selection of support design factors:

Optimum utilization of right-of-way Environmental impact Prospected life time Location Terrain and its access Number of circuits to be installed on the same structure Mechanical and climetical load Required height of structure Land use for the line and its vicinity Keraunic level and arrangement of earth wires Required methods of construction and maintenance Investment

25

Support Design and Application 2. Self supporting lattice steel towers

The most traditional overhead lines. Suitable for local conditions demanding narrow tower locations

and right of way Above 150 kV All kinds of conductor configuration Easy to be upgraded Easy to exchange cross arms Increasing of tower heights is possible Easy to repair Unnecessary towers can be scraped and recycled They are hot-dip galvanised for corrosion protection

26

Self Supporting Lattice Steel Towers

Self supporting lattice steel tower

27

Support Design and Application 3. Self supporting steel poles:

Used in congested urban areas Able to accommodate insulating crossarms Low visual impact Suspension poles made of H-beam sections can be adopted for

medium voltage lines. H-beams: low torsional stiffness. Seamless tubular steel poles: Section by section differing diameter

and canonical shape. Adjustable to bending momentum. Canonical steel poles: six, eight or more sides. Shape and cross

section can be adjusted for loads. Used f or high voltage. 110 and 400 kV have been used.

28

Self Supporting Steel Poles

Self supporting steel pole

29

FILIP BRNADICFILIP BRNADIC 4 September 2013 2:01 PM

Support Design and Application 4. Steel reinforced concrete poles:

Steel reinforced concrete poles are used as spun concrete poles for low and medium voltage installations, 110 and 132 kV double circuit lines

Long service life with out any efforts for maintenance Low visual impact for residential area High weight is a disadvantage. Hard to be transported and erected

30

Support Design and Application 5. Wood poles:

Wood poles are the simplest and cheapest support structures. They have been made from larch, spruce, cedar, pine and fir trees, selected specially for height and straightness.

They are very flexible and they can bend and give under sudden severe loads.

Under severe forces, such as strong storm winds perpendicular to the line, the force on the pole may be dissipated in the ground. Usually, they can be straightened.

Have been used for 100 kV and 220kV in North America and Australia.

31

Support Design and Application

Guyed V-tower

6. Guyed supports: Types: H, V and Y Frequently used for long

single-circuit lines Suitable for agricultural

areas.

32

Support Design and Application

Crossarm-less support

Mast

Spacer cable Cross ropeEarth wire

7. Cross-armless supports: Used to make compact lines First used for 735 kV line in Canada.

33

Design Spans In order to design suitable tower dimensions for an overhead

line it is necessary to calculate the conductor sags and tensions.

The maximum conductor tension (which will occur at minimum temperature) is evaluated in order to ensure a sufficient mechanical strength margin for the particular conductor.

The sag is calculated in order to fix the tower height. The ruling condition for the conductor has to be determined

based on either the maximum working tension (MWT), the everyday stress (EDS) or, potentially, the maximum erection tension (MET).

34

Design Spans Key elements: Basic span Wind span Weight span Equivalent span

35

Basic Span

36

3O O A B C EH C S S S S S

SE

SA

SA

SA

SC

SB

SO

C

H

C : statutory clearance to ground SA : length of insulator suspension set SB, SC, SE : vertical distances between crossarms and conductor above or to earth wire SO : sag of conductor (proportional to square of span)

Wind Span The wind span is half the sum of the adjacent span lengths. At 230 kV this might be 400 m under normal conditions and 300

m under broken wire conditions. At 132 kV typical values are 365 m and 274 m respectively.

37

Weight Span The weight span is the distance between the lowest points on

adjacent sag curves on either side of the tower. The ratio of weight span to wind span is also important since

insulators on lightly loaded towers may be deflected excessively thus encroaching electrical clearances.

A ratio of weight span to wind span of approximately 1.52 is often considered acceptable.

38

230 kV 132 kV

Suspension towers 750 m Normal conditions 680 m Normal conditions

565 m Broken wire Conditions 510 m Broken wire Conditions

Tension towers 750 m Normal conditions 680 m Normal conditions

750 m Broken wire Conditions 680 m Broken wire Conditions

Typical weight span values at 230 kV and 132 kV

Weight Span

39

L1 L2 L3

Cold sag temperature curve

Weight span

Equivalent Span

L1 L2 L3

40

The equivalent span is defined as a fictitious single span in which tension variations due to load or temperature changes are nearly the same as in the actual spans in a section.

Equivalent span

3 3 31 2 3

1 2 3

L L LL L L L Equivalent span :

Loads on Over-head Lines 1. Wind loading 2. Conductor loadings:

Conductor tensions Short circuit loadings Ice loading Seismic loads Combined loads Broken wire conditions

41

Loads on Over-head Lines Wind loading: It is normal practice to consider wind loads on structures due to a 3-second gust that occurs over a 50-year period. The wind load is related to the wind speed in accordance with the code of practice applicable to the country where the work is being carried out. In the EU the relevant information is set out in the NNAs of EN50341 and 50423. It describes procedures for calculating wind loads on both structures and conductors, with considerable variation in detail between individual countries.

42

Conductor Tensions

43

2

8W g LT NS

The tension, T, in the conductor for a given sag, S, is given by the formula:

W : weight of conductor per unit length (kg/m) L : span of the conductor (m) G : gravitational constant (1 kgf or 9.81 N) S : sag (m)

Short Circuit Loading Under short circuit conditions lateral mechanical attraction or repulsion forces will occur between the different phase conductors. The effect of conductor movement during short circuits is erratic and difficult to calculate. Such movement is taken into account in the overall design by allowing adequate clearances between the phase conductors. The conductor short circuit forces are usually ignored in the structural design of overhead line towers or substation gantries because of the very short durations of the faults.

44

Loads on Over-head Lines Ice loading

The build-up of ice on conductors will increase effective conductor weight, diameter and wind loading. Local experience must be used in the application of ice loads to structural design. As an example, EN 50341-3-9 calls for a uniform ice load on all spans of 5 kN/m3 to be considered for UK designs, or 9 kN/m3 in case of wind and ice.

Seismic loads The acceleration due to a seismic event is categorized as a

fraction of the gravitational constant, g. This may be given for both horizontal and vertical effects over a frequency spectrum.

45

Loads on Over-head Lines Combined loads

The simultaneous application of individual worst case loads is unlikely to occur in practice and the simple arithmetic addition of all such load cases would lead to an uneconomic and over-engineered solution. The individual loads are therefore factored to arrive at a sensible compromise.

wind load plus ice load is often taken as full ice loading plus wind load at, say, 50% basic wind speed. Similarly, wind load plus seismic load is normally taken as full earthquake load plus 50% wind load.

46

Broken Wire Condition The towers themselves are usually designed such that no

failure or permanent distortion occurs when loaded with forces equivalent to 2 times the maximum simultaneous vertical, transverse or longitudinal working loadings for suspension towers and 2.5 times for tension towers.

Under broken wire conditions the towers must be capable of withstanding typically 1.25 times the maximum simultaneous resulting working loadings.

47

Overhead Line Conductor and Technical Specifications Key elements: Environmental conditions Environmental effects Connector selection Calculated electrical rating Conductor and earth wire spacing and clearances

48

Environmental Conditions Temperature Wind velocity Solar radiation Rainfall Humidity Altitude Ice and snow Atmospheric pollution Soil characteristic Lightning Seismic factor General loadings

49

Environmental Effects Corrosion Corona

50

Corrosion Aluminum conductors have good corrosion behavior

essentially resulting from the formation of an undisturbed protective surface oxide layer which prevents further corrosion attack.

ACSR is known to suffer from bi-metallic corrosion which is noticeable as an increase in conductor diameter due to corrosion products in the steel core known as bulge corrosion.

Early problems associated with deterioration of the steel cores used in ACSR conductors have been resolved over the years by the use of high temperature greases.

AAAC will obviously offer superior corrosion resistance than ungreased ACSR.

51

Corrosion For very aggressive environments the following order of

preference is suggested: Aluminum conductor fully greased. Aluminum conductor with alumoweld core fully greased. ACSR fully greased. Aluminum alloy conductor fully greased. Aluminum conductor with alumoweld core ungreased. ACSR with greased core.

52

Corona High voltage gradients surrounding conductors (above about

18 kV/cm) will lead to a breakdown of the air in the vicinity of the conductor surface known as corona discharge.

This effect is more pronounced at high altitudes. Generally, the breakdown strength of air is approximately 31 kV peak/cm or 22kV rms/cm.

At higher voltage levels, and certainly at voltages of 400 kV and above, interferences due to the corona effect can be the dominant factor in determining the physical size of the conductor rather than the conductor thermal rating characteristic.

53

Corona The corona effect is more severe around small conductors and

at sharp points and corners. Corona absorbs energy from the line. Corona losses can be reduced by taking the following actions:

Separating conductors with spacers placed periodically along the line.

Bundling of high voltage conductors.

54

Corona Practical formula to calculate voltage surface gradient (Vg)

55

[ / ]/ 2 log 2 /p

ge

UV kV cmd D d

Up : phase voltage (kV) d : diameter of single conductor (cm) D : distance between phases for single phase line or equivalent spacing for three phase lines (cm)

Overhead line Conductor and Technical Specifications Connector selection: The selection of the most appropriate conductor

size at a particular voltage level must take into account both technical and economic criteria as listed below:

The maximum power transfer capability must be in accordance with system requirements.

The conductor cross-sectional area should be such as to minimize the initial capital cost and the capitalized cost of the losses.

The conductor should conform to standard sizes already used elsewhere on the network in order to minimize spares holdings and introduce a level of standardization.

The conductor thermal capacity must be adequate. The conductor diameter or bundle size must meet recognized

international standards for radio interference and corona discharge. The conductor must be suitable for the environmental conditions and

conform to constructional methods understood in the country involved (such as IEC61089).

56

Connector Selection Key elements: ACSR, AAAC, ACAR and AACSR Aerial bundled conductor (ABC) and BLX Conductor breaking strengths Bi-metal connectors

57

ACSR, AAAC, ACAR and AACSR For 36 kV transmission and above both aluminum conductor steel

reinforced (ACSR) and all aluminum alloy conductor (AAAC) are used. Aluminum conductor alloy reinforced (ACAR) and all aluminum alloy

conductors steel reinforced (AACSR) are less common than AAAC and all such conductors may be more expensive than ACSR.

ACSR has been widely used because of its mechanical strength, the widespread manufacturing capacity and cost effectiveness.

Copper (BS 7884 applies) has a very high corrosion resistance and is able to withstand desert conditions under sand blasting but it is expensive.

At larger conductor sizes, the AAAC option becomes more attractive. AAAC can achieve significant strength/weight ratios and for some constructions gives smaller sag and/or lower tower heights.

AAAC is slightly easier to joint than ACSR.

58

ACSR

59

Aluminum-based conductors have been referred to by their nominal aluminums area. Thus, ACSR with 54 Al strands surrounding seven steel strands, all strands of diameter d = 3.18 mm, was designated 54/7/3.18; aluminum area of 428.9 mm2, steel area of 55.6 mm2 and described as having a nominal aluminum area of 400 mm2.

54/7/3.18 ASCR

Aerial Bundled Conductor (ABC) and BLX At low voltage levels aerial bundled conductor (ABC) is now

becoming rapidly more popular because of improved reliability and the low installation and maintenance costs compared to conventional open wire pole distribution.

For the 1014 kV distribution levels the use of ABC is more problematic due to the requirement of employing underground cable joining techniques at high level (especially difficult in maintenance situations).

For medium voltage distribution lines, (10 kV to 25 kV) BLX conductors are now preferred in many countries.

60

Aerial Bundled Conductor (ABC) and BLX There are two distinct ABC systems in use: 1. One system uses a self-supporting bundle of insulated

conductors where all conductors are laid up helically and where tension is taken on all conductors which are of hard-drawn aluminum.

2. An alternative system is where all conductors are insulated and the hard-drawn aluminum phase conductors are laid up around an aluminum alloy neutral which has greater tensile strength and acts as a catenary wire to support the whole bundle. The insulation material may be polyvinylchloride (PVC), linear polyethylene (PE) or cross-linked polyethylene (XLPE).

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Conductor Breaking Strengths for ACSR

62

Calculation standard ACSR conductor 50/8 breaking strength (kN) ACSR conductor 380/50 Breaking strength (kN)

BS215, Pt. 2. 1970 16.81 120.96

ASTM B2 32-74(Class A) 17.45 121.62

ASTM B232-74 (Class B) 16.91 119.74

ASTM B232-74 (Class C) 16.67 114.97

NFC 34 120 1968 (R) - 144.21

NFC 34 120 1968 (N) - 118.75

DIN 48 204 (declared) 17.09 123.14

DIN 48204 (theoretical area) 16.83 120.80

DIN 48204 (calculated area) - 120.72

CSA C49 175 17.19 123.40

IEC 61089 16.87 120.71

EN 50182 16.81 121.30

Bi-metal Connectors

63

Where an aluminum conductor is terminated on a copper terminal of an isolator a special copper/aluminum joint is necessary to prevent the formation of a corrosion cell. A termination of this type usually comprises of an aluminum sleeve compressed onto a copper stalk with an insulating disc separating the two surfaces which are exposed to the atmosphere

Copper pin

Anti corrosion varnish

Aluminum Ferrule

Bi-metal connectors

Calculated Electrical Rating 1. Heat balance equation 2. Power carrying capacity

64

Heat Balance Equation

65

2 20 1 C R SI R t H H H

0.448387CH V d

4 4237 237R CH E s d t t S SH S d

I : current rating, amps R20 : resistance of conductor at 20C : temperature coefficient of resistance per C (for ACSR at 20C, =0.00403) t : ambient temperature, C : temperature rise, C (t1 , initial temperature and, t2 , final temperature)

Heat Balance Equation S : solar absorption coefficient depends upon outward condition of the

conductor and varies between 0.6 for new bright and shiny conductor to 0.9 for black conditions or old conductor.

S : intensity of solar radiation, watts/m2

d : conductor diameter, mm V : wind velocity normal to conductor, m/s. For design purposes 0.5 or 0.6

m/s wind speeds are often taken.

EC : emissivity of conductor differs with conductor surface brightness. Typical values are 0.3 for new bright and 0.9 for black aluminium, ACSR or AAAC conductor. Average value 0.6, say.

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Power Carrying Capacity

67

100

200

300

400

500

600

100000 200000 300000

Line

vol

tage

(kV

)

Power transfer (MW.km)

Line voltages 66 kV-600 kV

10

70

20

30

40

50

60

100 200 300 400 500 600

Power transfer (MW.km)Li

ne v

olta

ge (k

V)

Line voltages 11 kV-66 kV

Economic power transfer capacity trends for different line voltages based on power transfer are proportional to the square of the line voltage.

Economical power transfer

Power Carrying Capacity

68

Line voltage

(kV)

Conductor equipment

configuration spacing (mm)

ASCR conductor

code

AAC conductor

code MW capacity

11 1400

Sparrow Raven Linnet

Iris Poppy Tulip

8 (km) 0.95 1.4 3.00

160 (km) 0.490 0.7 1.5

24 (km) 0.33 0.47 1.00

32 (km) 0.25 0.35 0.75

33 1500

Quail Penguin Linnet Hen

Aster Oxlip Tulip Cosmos

16 (km) 5.00 6.70 8.35 11.50

32 (km) 2.50 3.35 4.18 5.75

48 (km) 1.70 2.20 2.80 3.80

64 (km) 1.25 1.70 2.10 2.90

66 3000

Quail Linnet Hen

Aster Tulip Cosmos

32 (km) 12.50 16.00 18.40

64 (km) 6.25 8.00 9.18

96 (km) 4.18 5.32 6.12

128 (km) 3.14 3.99 4.59

Earth Wire Spacing and Clearances Electrical clearances Earth wires Distribution voltage level clearances Transmission voltage level clearances

69

Earth Wire Spacing and Clearances According to EN 50314-1, there are five types if requirements to be considered for determining the minimum of electrical clearance distance: 1. Prevention of disruptive discharges between phase conductors

and earth during fast and slow front over voltages. This clearance can be considered either internal clearance between earthed tower components or external clearance between conductor and an obstacle (Del).

2. Prevention of disruptive discharges between phase conductors during fast and slow front over voltages. The minimum clearance is an internal nature in this case (Dpp).

70

Earth Wire Spacing and Clearances 3. Prevention of disruptive discharges between a live conductor

and objects with earth positional at power frequency (Dpf_pe). 4. Prevention of disruptive discharges between a live conductors

at power frequency (Dpf_pp). 5. Setting of minimum air clearance distances to obstacles or

cross objects in order to prevent discharges occur inside the overhead line.

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Electrical Clearances Clearances Del and Dpp corresponding to fast-front withstand voltages, of conductor-to-obstacle and conductor-to-conductor air gaps in altitudes up to 1000m above sea level, EN 50341-1.

72

Lightning surge withstand voltage (kV)

Del_ff (m) Dpp_ff (m)

400 0.77 .85

600 1.14 1.26

800 1.50 1.68

1000 1.88 2.08

1200 2.23 2.50

1400 2.61 2.92

1600 2.98 3.33

1800 3.35 3.75

2000 3.72 4.17

2050 3.82 4.27

2100 3.91 4.38

2150 4.00 4.48

Electrical Clearances Clearance Del and Dpp corresponding to slow withstand voltages, of conductor-to-obstacle and conductor-to-conductor air gaps in altitudes up to 1000m above sea level, EN 50341-1.

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Lightning surge withstand voltage (kV)

Del_sf (m)

Dpp_sf (m)

400 0.88 1.02

600 1.44 1.67

800 2.07 2.45

1000 2.84 3.41

1200 3.71 4.57

1400 4.77 5.97

1600 6.02 7.66

1800 7.50 9.70

Electrical Clearances Minimum clearances in air dependent on the highest voltage of equipment, EN 50341-1

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Highest voltage of equipment (kV)

Dpf-pe (m) Dpf-pp (m)

52 0.11 0.17

72.5 0.15 0.23

82.5 0.16 0.26

100 0.19 0.30

123 0.23 0.37

145 0.27 0.42

170 0.31 0.49

245 0.43 0.69

300 0.51 0.83

420 0.70 1.17

525 0.86 1.47

765 1.28 2.30

Electrical Clearances Empirical data for minimum clearance distances Del and Dpp, EN 50341-1.

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Highest voltage of equipment (kV) Del (m) Dpp (m)

52 0.60 0.70

72.5 0.70 0.80

82.5 0.75 0.85

100 0.90 1.05

123 1.00 1.15

145 1.20 1.40

170 1.30 1.50

245 1.70 2.00

300 2.10 2.40

420 2.80 3.20

525 3.50 4.00

765 4.90 5.60

A Typical Overhead Line Clearances (based on maximum conductor temperature or the load EN 50341)

Clearance (m) from line with highest system voltage of: Clearance consideration 52 kV 145 kV 245 kV

To ground in unobstructed countryside 5.6 6.2 6.7

To rockface or steep slope 3.1 3.2 3.7 To trees which cannot be

climbed 0.6 1.2 1.7

To trees which can be climbed 2.1 2.7 3.2

To buildings with fire-resistant roofs and

roofs with slope 15 to horizontal

3.1 3.2 3.7

To buildings with fire-resistant roofs and roofs with slope

Earth Wires Where there is a risk of a direct lightning strike to the phase

conductors, transmission lines are provided with overhead earth (or ground) wires to shield them and also to provide a low impedance earth return.

In the UK the original 132 kV overhead lines were designed with a 45 angle of protection and gave satisfactory cover.

This angle was applied 400 kV overhead with the angle of 30 in order to reduce the number of strikes in UK.

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Distribution and Transmission Voltage Level Clearances Distribution voltage level clearance: for open wire

construction at distribution voltage levels (380 V24 kV) the earth or neutral wire is normally placed at the bottom (nearest the ground) of the conductor set so as to minimize the danger caused by poles, ladders, etc. touching the wires from underneath.

Transmission voltage level clearance: There are no universally agreed clearances as they depend upon insulation level, pollution, span, type of overhead line construction, etc.

78

Overhead Line Fitting Definition: overhead line fittings serve for the mechanical

attachment, for electric connection and the protection of conductors and insulators.

Fittings for conductors serve to terminate, suspend or join the conductors and are directly connected to the conductors. Fittings for conductor include suspension and dead-end clamps, connectors, branch-off clamps, vibration protection fittings and bundle spacers.

Fittings for insulator sets and other attachments serve to connect the tension or suspension components with the attachment points at the supports such as: Yoke plates, index-yoke plate corona protection fittings and grading rings.

79

Fittings for Conductors 1. Conductor attachment at suspension insulator sets 2. Conductor attachments at dead-end terminations 3. Turn buckles 4. Connectors 5. Spacers for bundle conductors 6. Vibration dampers for single conductors 7. Sag adjusters 8. Miscellaneous fittings

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Conductor Attachment and Suspension Insulator Sets Articulated suspension clam: the most frequently used type of

suspension clamp. Armor rod suspension (ARS) Wedge-type dead-end clamp Armor grip suspension Suspended dead end arrangement: to prevent static bending

stress over long spans such as river crossing. Saddle-type clamp: which its suspension body can reach up to

two meters. Release suspension clams: are adopted occasionally to protect

not sufficiently proof suspension towers. Sliding suspension clamps: enable the conductor to slide

through the clamp above a stipulated difference in conductor tensile forces between adjacent spans.

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Articulated Suspension Clam

82

Suspension strap

Clamp cover

Clamp body

Armor Rod Suspension (ARS)

Suspension strap

Clamp cover

Clamp body Armor rods

83

Armor Grip Suspension (AGS)

84

Armor rodsRubber elastic lining

Suspension strap

Clamp body

Suspended Dead End Arrangement

85

Dead end clampTriangular yoke plate Turn bcuket

Saddle-type Clamp

86 Clamp body

Armor rods Clamp cover

Suspension insulator set using saddle-type clamp cover

Conductor Attachment and Suspension Insulator Sets Conductors can be terminated by dead-end clamps or by performed dead-ends. Wedge-type clamps: simple to install and can correct line sags,

adopted for high and medium voltage lines. Compression dead-end clamps: suitable for all standard types

of conductors in market. They are designed to achieve ultimate terminating forces up to rated tensile strength (RTS) of conductors, adopted for high and medium voltage lines.

Performed dead-end rods: suitable for terminating metal-reinforced cables with optical fibers (OPGW) and other aerial cables. They can be installed bare hand.

87

Wedge-type Clamps

Clamp body

Clamp wedge

Terminal strap

88

Compression Dead-end Clamps Clamp body with

sleeve for steel core

Lug for jumper loop

Outer sleeve

89

Turn Buckles and Connectors Turn buckles: are arranged in dead-end insulator sets to

compensate tolerances in lengths of elements in parallel or sub-conductors in bundles.

Connectors: including tension proof and non-tension proof are fitting jointing one or more phase conductors or earth-wires to each other or producing a branch-off. Performed splices: are used for medium-voltage. Protective patch rods or repair patch rods are frequently

adopted to restore the electric and mechanical function after damage or strand failures at conductors, earth wires or aerial cables.

90

Spacers for Bundle Conductors Spacers keep the sub-conductors within a span and in jumper loops at designed spacing to avoid damage caused by clashing, twisting or entwining. Rigid spacers: keep the sub-conductors at a constant distance

at the location of installation. Flexible spacers: permit small relative displacements of the

sub-conductors at the location of installation. Spacer dampers: reduce the vibration level by energy

dissipation in rubber-elastic elements. Phase spacers: are adopted in spans of lines where

conductor galloping occurs frequently due to topographical or climate peculiarities.

91

Rigid Spacers

Bar

Clamping jaw

92

Phase Spacer

Clamping arrangement

Central pivot

Suspensionbracket Composite insulators

Spacer sleeves

Grading rings

93

Phase spacer for quadruple bundles

Vibration Dampers and Spacer Dampers for Bundle Conductors Vibration dampers: offer effective protection against vortex-

induced vibrations. The most popular is Stockbridge-type. Spacer dampers for bundle conductors: suppress vortex-

induced conductor vibrations

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Fittings for Insulator Sets Fittings for insulator set compromise: 1. Fittings to attach the insulator sets to the support. 2. Fittings to join the insulator of one string , e. g. ball and socket

or clevis and tongue connections, yoke plates for multi-string insulator sets.

3. Fittings for grading of electrical fields and arcing protection, e. g. coordinating spark gaps, arcing horns and rings, as well as grading fittings.

4. Ball and socket connections are used for connecting long rod and cap-and-pin insulators, clevis and tongue connections for long rod and composite insulators. The dimensions of ball and socket connections are standardized by IEC 60120, for clevis and tongue connections by IEC 60471.

5. Arcing protection fittings should safe-guard insulators sensitive to abrupt temperature changes which may occur due to power arcs.

95

Sag Adjusters and Miscellaneous Fittings Sag adjusters: these consist of pivoted clamping plates with

adjustment holes to allow the sag of the conductor to be regulated after the initial erection in steps of, say, 10 mm over a 300 mm range.

Miscellaneous fittings: These include:

Tower or pole anti-climbing guards Climbing steps Danger plates Tower or pole number plates Phase plates Line circuit identifications Aircraft warning spheres

96

Representation of Lines The way in which lines and cables are represented with an

equivalent circuit depends on the following: Length Type Accuracy needed

There are three broad categories of length as follows: Short (up to 80 km or 50 miles). Medium (up to 240 km or 150 miles). Long (above 240 km or 150 miles).

97

Representation of Lines The actual line or cable is a distributed constant circuit and it

has: Resistance Inductance Capacitance Leakage resistance

These are all distributed evenly along its length. Except the long lines, the total resistance, inductance,

capacitance and leakage resistance of the line are concentrated to give a lumped constant circuit.

The various distances quoted are used as a rough guide only.

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Voltage Drop Along a Line The various loads (customers) connected across the line draw

current causing the voltage to drop. The farther one gets away from the substation, the higher the

voltage drop. If the voltage at the substation is set at nominal value, the

customers at the end of the line will not have enough voltage under heavy load.

If the voltage is set so that the end customers have nominal value, the voltage near the substation is too high for light loading conditions.

A compromise must be reached or maybe other methods can be employed for voltage regulation under various loading conditions.

99

Substation

1st customer 2nd customer last customer

Other laterals

Distribution Circuit

100

Substation

1st customer2nd customer last customer

V

VBase

Voltage Drop Along a Line with Heavy Load

101

Voltage Drop Along a Line with Light Load

102

Substation

1st customer 2nd customer last customer

V

VBase

Reactance The magnetic flux produced by the AC current flowing in a

conductor produces a series inductive reactance due to: Self inductance (which causes skin effect) along the conductor. Mutual inductance between conductors.

The reactance does not dissipate real power, but results in a voltage drop along the line and also draws reactive volt-amperes.

Any reactive power on the line must be supplied by the generator over and above to the reactive power the load will consume.

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Reactance Basic formula, geometric mean radius and geometric mean

distance Three phase formula Positive and negative sequence reactance Zero sequence reactance

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Basic Formula, Geometric Mean Radius and Geometric Mean Distance

105

0.2 0.25 log / [ / ]eL d r mH km L : inductance d : separation between conductor axes (mm) r : radius of conductor (mm)

d

r

Three-phase Formula for Transmission Lines Inductance (L)

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0.2 log / [ / ]eL GMD GMR K mH km

K : correction factor GMD : geometric mean distance between the A, B and C phases (mm) GMR : geometric mean radius of conductors (mm)

3 AB BC CAGMD d d d

rdAB

dAC dBC

A B

C

Three-phase Formula for Transmission Lines Inductance (L)

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All aluminum or all copper conductor ACSR

Number of strands GMR Number of Al strands

GMR

7 0.726 r 6 0.768 r

19 0.758 r 12 0.859 r

37 0.768 r 26 0.809 r

61 0.772 r 30 0.826 r

91 0.774 r 54 0.810 r

127 0.776 r

169 0.779 r

Solid 0.779 r

GMR values as function of conductor radius, r

Positive and Negative Sequence Reactance The positive and negative sequence inductive reactances (X1 , X2) of a three phase overhead line are equal and for a frequency, f Hz, become:

108

31 0.4 10 log /eX f GMD GMR K

Zero Sequence Reactance Zero sequence reactance (X0) is complicated to calculate. The value depends upon the position and materials used for the earth wires and the log of the square root of the ground resistivity.

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Earth wire Overhead line X0/X1 - Single circuit 3.5

- Double circuit 5.5

Galvanized steel Single circuit 3.5

Galvanized steel Double circuit 5.0

Non-magnetic Single circuit 2.0

Non-magnetic Double circuit 3.0

Typical values for the ratio of zero-to-positive sequence reactance for double and single circuit overhead lines

Capacitance Conductors separated by a given distance have capacitance.

The capacitance of a transmission line depends upon: Conductor size Spacing Height above the ground Voltage

Transmission line capacitance must be charged initially before transfer of real power can occur.

The expression for line-to-neutral capacitance, C, is:

110

1 [ / ]

18log /eC F kmGMD r

Resistance The series resistance of a conductor depends on:

Resistivity of the conductor material Length Temperature Skin effect

Conductor resistance varies with temperature in a linear fashion.

Losses occur because of the resistance of the lines, and these losses are kept minimum by using high voltages for transmission of bulk power.

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Power Factor Correction The term correction or compensation is used to describe the

intentional insertion of reactive power devices, either capacitive or inductive, to achieve one or more desired effects in an electric power system.

These effects include improved voltage profiles, enhanced stability and increased transmission capacity.

The correcting devices are either in series or in shunt (parallel) with the load(s) at one or more points in the power circuit.

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Why do we bother? The motivations behind power factor correction equipment are: Increase the efficiency of the source i.e. the AC mains in two

distinct ways as follows: Lower the source losses (harmonics are associated with undesired

components and losses). Increase the power that can be taken out from a given source.

Reduce the line harmonics and associated Electromagnetic Interference) (EMI) induced in the source (AC mains) by the load.

Market expectations for modern equipment. The aim is to shape the line current so that it will be in phase

with the input sinusoidal voltage.

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Shunt Capacitors for Voltage Regulation Capacitors have been effectively used to correct the power

factor since most of the loads are inductive (lagging). This is achieved by supplying Var. The capacitors are installed as close to the loads as possible. There are two ways for the capacitances to become available

for such function as follows: Fixed value: to cater for both light and heavy load conditions. Variable or switched capacitor banks in the case that the

load variation is so large and one capacitor value will not be good enough to serve all cases.

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Shunt Capacitors for Voltage Regulation Switched capacitor banks are expensive mainly because they

need sensing hardware to operate and appropriate control systems.

All capacitor banks require protection with a fuse or circuit breaker.

They can be connected either as delta or star. The underground start connection is preferred because if a

capacitor is shorted in one leg, the other two will limit the fault current.

Shunt capacitors are usually installed in radial feeders.

115

Shunt Capacitors for Voltage Regulation

116

Without shunt capacitor

Applying shunt capacitor

Distance from the source (generator)

Term

inal

vol

tage

Capacitor Banks

117

Circuit breaker

CT for protective relays

PT PT PT

Protevtive relay

Fuses

Outdoor Regulator Bank

118

Disconnect Switches for the Outdoor Regulator Bank for Maintenance

119

Shunt Reactors Shunt reactors are usually installed to remedy utility company power-generation and transmission issues including: Overvoltages that occur during low load periods at utility

substations served by long lines as a result of the inherent capacitance of the line.

Leading power factors at generating plants resulting in lower transient and steady-state stability limits.

Open-circuit line charging kVA requirements in extra-high-voltage systems that exceed the available generation capabilities.

120

Shunt Reactors

121

Without shunt reactor

Applying shunt reactor

Distance from the source (generator)

No-

load

term

inal

vo

ltage

Skin Effect The skin effect phenomena refer to the fact that as the

frequency of the AC current increases, the flow of current shifts towards the surface of the conductor.

Since the AC current reverses direction in a periodic way, each reversal results in the magnetic flux produced by the current also reversing.

The magnetic flux density is highest in the centre of the conductor, causing the inductive reactance of the conductor to also be highest in the centre.

The higher inductive reactance in the centre forces the current towards the surface.

The skin effect at a fixed frequency is proportional to the diameter of the conductor.

The skin effect causes the AC resistance to be 10 to 20 per cent of the DC one.

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Representation of Transmission and Distribution Systems Modern electricity supply systems are three-phase systems. The design of distribution networks is such so that normal

operation is reasonably close to balanced three-phase systems, i.e. one phase is usually sufficient to study and understand the systems full operation.

Equal loading on all three phases is ensured by connecting as far as possible equal domestic loads to each phase of the low voltage distribution feeders.

Industrial loads are most of the time three-phase ones.

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Reasons for Interconnection There are a number of reasons for interconnection as follows: The base load is satisfied by high efficiency generation plants

that feed into the system and not into a particular load regardless of geographical location.

To meet sudden increases, spinning reserve is available from generators running at normal speed and ready to supply power instantaneously.

Interconnection allows for alternative paths to exist between generators and bulk supply point. This offers security of supply should a given path fails.

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Transmission It implies the bulk transfer of power from generators to load

centers which is then distributed. The transmission network operates at high voltage. Generators usually produce voltages at around 11kV -25kV. Transformers increase this to the main transmission voltage

levels. Large amounts of power is transmitted at voltage levels of 400

kV and 275 kV in the UK. Slightly different in other countries. This network in many cases is called supergrid.

125

MLine, cable or busbar (three-phase)

Rotating machine-general

Synchronous machine

Two winding transformer

Three winding transformer

Current transformer

BreakerThree-phase star connected with the star point solidly connected to earth or ground electrode

Three-phase star connected

Load

Symbols of Power System Components

126

Large high-efficiency station

Substation (grid supply point)

Generator transformers

275 kV or 400 kV (345.5 0r 765 kV in U.S.A.) (busbars sectionalized)

Load

Substation

Loads

Load

Load

To the restof

system

Embedded generation(local generator)

Typical Power System

127

Large generators ( gas, coal, nuclear, hydro)

Tie line to other systems

CHP generators or small local independents

Small embedded generators, e.g. wind,

landfill gas

132 kV(230/115 kV)

400/275 kV(765 kV, 500/345 kV)

Distribution

Loads

Constituent Networks of a Supply System

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Transmission Line Repair The time to repair a failed transmission line typically varies from

2 to 10 days, depending upon: Type of line

Overhead Underground cable in conduit Pipe-type of cable

The time required to replace a failed bulk-power system transformer is typically 30 days.

As a result of the previous points, emergency ratings for a transmission line or transformer may include, 2-hours emergency, 2-to-10 days emergency, and in some cases 30 days emergency ratings.

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Overhead Transmission Line Ratings The ratings of an overhead transmission line is based on the

maximum temperature of the conductors. Conductor temperature affects the following:

The conductor sag between towers The loss of conductor tensile strength due to annealing.

If the temperature is too high the following may happen: Proscribed conductor-to-ground clearance may not be met. The elastic limit of the conductor may be exceeded such that it

cannot shrink to its original length when cooled.

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Overhead Transmission Line Ratings Conductor temperature depends upon:

Current magnitude and its time duration. Ambient temperature. Wind velocity. Solar radiation. Conductor surface conditions.

There are standards that allow use of standards assumptions on ambient temperature, wind velocity, etc. and these are calculated conservatively (IEEE Std. 738-85, 1985).

131

Overhead Transmission Line Ratings It is common practise to have summer and winter ratings,

based on seasonal ambient temperature differences. In locations that higher winds prevail, such as coastal areas,

larger than normal line ratings are used. Emergency line ratings vary between 110% to 120% of

normal ratings. In many cases real time monitoring of actual conductor

temperatures along a transmission line is used for on-line dynamic transmission line ratings.

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Stability Definitions System stability is the ability of all synchronous generators in

operation to stay in synchronism with each other while moving from one operating condition to another.

Steady-state stability refers to small changes in operating conditions, such as normal load changes.

Transient stability refers to larger, abrupt changes, such as the loss of the largest generator or a short circuit followed by circuit breakers opening, where synchronism or loss of synchronism occurs within a few seconds.

Dynamic stability refers to longer time periods, from minutes up to a half-hour following a large, abrupt change, where steam generators (boilers), automatic generation control and system operator actions affect stability.

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Planning Process and Stability Steady state stability is also evaluated during the planning

process with the use of power-flow programs and by the systems ability to meet equipment loading criteria and transmission voltage criteria under steady state conditions.

Transient stability is evaluated via stability programs by simulating the systems transient response for various types of disturbances, including short circuits and other abrupt network changes.

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Typical Disturbances and Stability Studies Scenario: All transmission lines in service, and a permanent

three-phase fault (short-circuit) occurs on: Any transmission line Both transmission lines on any double circuit tower At any bus The fault is successfully cleared by primary relay

Any one transmission line out of service, a permanent three-phase fault occurs on any other transmission line. The fault is successfully cleared by primary relaying.

With all transmission lines in service, a permanent three-phase fault occurs on any transmission line; back up relaying clears the fault after a time delay due to a circuit breaker failure.

135

Value-based Transmission Planning The method assigns a monetary value to various levels of

reliability in order to balance reliability and cost. For each outage, the amount of money of unserved energy is

determined and its value is calculated based on the type of customers and other information.

If the cost of the transmission project required to eliminate the outage exceeds the value of service, then that project is given lower priority.

This means that reliability is quantified, and benefit-to-cost ratios are used to compare and prioritise planning options.

136

Cables Cables are use for low voltage, medium voltage and high

voltage power systems. Besides there are many applications of using control,

communication cable and fiber optic cables. International Electrotechnical Commission (IEC) standards for

application and testing of cables are IEC 60055, 60096, 60141, 60183, 60227, 60228, 60229, 60230, 60287, 60331, 60332, 60364,60502, 60702, 60724, 60754, 60811, 60840, 60853, 60859, 60885, 61034, 61042, 61084, 61443 and 62067.

137

Classification of Cables

138

Voltage level Usage Voltage range Insulation Low voltage (LV) Telephone 50 V PVC or PE

Control 600/1000 V PVC

Solid dielectric 600/1000 V PVC, XLPE, EPR

MI or MIND 600/1000 V Paper

Fire resistant 600 & 1000 V Mineral, Silicone Rubber

Fire retardant 600/1000 V LSF, LSOH

Medium voltage (MV) Solid dielectric 3 kV to 7.2 kV PVC, PE, XLPE, EPR MI or MIND 3 kV to 7.2 kV Paper

XLPE : Cross-linked polyethylene PVC : Polyvinylchloride PE : Polyethylene EPR : Ethylene propylene rubber LSF : Low smoke and fume LSOH : Low smoke zero halogen MI : Mass impregnated MIND : Mass impregnated non-draining PPL : Polypropylene paper laminate

Classification of Cables

139

Voltage level Usage Voltage range Insulation High voltage (HV) Solid dielectric 10 kV to 150 kV XLPE, EPR

MIND 10 kV to 36 kV Paper

Oil- filled, gas pressure 80 kV to 150 kV Paper

Gas insulated ducts 10 kV to 150 kV SF6

Very high voltage (VHV) Solid dielectric

150 kV to 300 kV XLPE

Oil-filled 150 kV to 300 kV Paper, PPL

Gas insulated ducts 150 kV to 300 kV SF6

Extra high voltage (EHV) Solid dielectric Above 300 kV XLPE

Oil-filled Above 300 kV Paper, PPL

Gas insulated ducts Above 300 kV SF6

Insulation Synthetic insulation for wire and cable can be classified as

follows: Thermosetting Thermoplastic

The mixture of materials within each of these categories vary so much so that unlimited number of insulations are available to the power engineer.

Most of insulation is composed of compounds made of synthetic rubber polymers (thermosetting) and from synthetic materials (thermoplastics).

These synthetic materials are combined to prove specific physical and electrical properties.

140

Thermosetting Insulation These materials are characterised by their ability to:

be stretched compressed Deformed

within reasonable limits under mechanical strain and then return to their original shape when the applied stress is removed.

141

Thermoplastic Insulation Thermoplastic insulation materials have excellent electrical

characteristics. They are relatively inexpensive. They are popular since they allow thinner insulation thickness to

be used to obtain good electrical properties, especially at higher voltage levels.

142

Cable Armouring In order to protect cables from mechanical damages such as

pick or spade blows, ground subsidence or excessive vibrations cable armouring is employed.

For three-core cables this consists of one or two layers of galvanized steel tapes, galvanized steel wire braid or galvanized steel wires helically wound over the cable.

For single core cables aluminium is used instead of steel wire in order to avoid losses.

143

Underground Cables It is quite difficult and in most cases impossible to obtain right

of way in urban areas for overhead lines. Therefore, use of underground cables is usually confined to the

short lengths required in heavily populated areas. The cost of underground cable is higher than the overhead

cable. Technologies such as cryogenic cable and other types of cable

using forced cooling techniques are used. In this case the cable uses three separate cables, each having a hollow centre for cooling purposes.

The conductor is made of stranded aluminium. The conductors are wrapped with an insulating material that contains liquid nitrogen.

144

Cryogenic Underground Cable

145

Outer covering

Insulation containing liquid

nitrogen

Stranded aluminium conductors

Hollow core

High-pressure Oil-filled Pipe-type High-voltage Cable

146

Steel pipe (filled with insulting oil)

Skid wires

Paper or oil insulation

Screen

Metallic tapes

Conductor (stranded copper)

Submarine Cables Submarine cables require additional tensile strength to permit

laying on or under the sea or river bed under high tension conditions.

Paper, PVC or XLPE insulation is used together with additional protection measures against water ingress and mechanical damage and with special sheath compositions to repel worm attack.

Such cables are manufactured in the longest possible lengths in order to minimize the number of underwater cable joints.

When preparing the design for submarine cables an accurate knowledge of the prevailing currents and tidal variations is essential to assist in deciding the best cable route and most favourable times for the cable laying work.

147

Summary Key points Competence required

Overhead line towers Comparing towers and poles applications and structures

Overhead line fittings Ability to choose suitable fittings

Tower structure Differentiation between tower structures according to their applications

Design span Calculating clearances and spans according to standards

Transmission line model Describing transmission line model, overvoltage, voltage drop, reasons for connecting capacitors

and inductors to transmission lines

Cables Comparing different types of cables and ability to

use standards to choose cables for specific applications

148