over head power line

8/19/2019 over head power line

1/139


2/139

OVERHEAD POWER LINES

MODULE 5 – CONDUCTORS, NETWORK & ENVIRONMENTAL

CONSTRAINTS - CONTENTS

Proprietary Document

Overhead Power Lines

Module 5 Conductors, Network & Environmental Constraints - Contents

Page 2 of 3

Section Description

No Figure or Table

Page

No.

Table 5.7 ACSR Conductors Inductive Reactance, Ω/km(equivalent spacings given) (IEE Proceedings, Vol. 133, Pt.C,

No.7, November 1986)

23

5.5 Design Spans, Clearances and Loadings 24

5.5.1 Design Spans 24

5.5.1.1 Basic Span 24

Fig. 5.7 Overall Tower Height 26

26

Table 5.8 Relative Supply and Installation Costs for

Overhead Line

27

5.5.1.2 Wind Span 27

Fig.5.8 (a) and (b) Cost v span, Wind and Weight Span 28

Fig.5.8 (c) Illustrating Equivalent Span 29

5.5.1.3 Weight Span 29

5.5.1.4 Equivalent Span 30

5.5.1.5 Creep 30

Table 5.9a Creep coefficients for ACSR conductors

(Equation 5.4)

31

Table 5.9b Creep coefficients for AAAC conductors

(Equation 5.5)

31

Table 5.9c Creep coefficients for AAC conductors

(Equation 5.5)

31

Table 5.9d Creep coefficients for ACAR conductors

(Equation 5.5)

31

Table 5.10 Typical Creep Conditions 32

Fig.5.9 Creep Assessment Procedure 33

Table 5.11 The period for which creep compensation is

required

34

5.5.1.6 Catenary Equations for Sloping Spans 35

5.5.2 Conductor and Earth Wire Spacing and Clearances 355.5.2.1 Earth Wires 35

Fig.5.10 Overhead Line Earth Wire Lightning Screen 36

Fig.5.11a Earth Wire Lightning Screen Protection (Vertical

Formation)

37

Fig.5.11b Earth Wire Lightning Screen Protection

(Horizontal Formation)

38

5.5.2.2 Earthing Counterpoise 38

Fig.5.12 Wave Propagation Along Electric Lines 39


3/139


MODULE 5 – CONDUCTORS, NETWORK & ENVIRONMENTAL

CONSTRAINTS - CONTENTS



Module 5 Conductors, Network & Environmental Constraints - Contents

Page 3 of 3

Section Description

No Figure or Table

Page

No.

Fig.5.13 Cable and Transformer Characteristic Impedances 40

Fig. 5.14 MV Overhead Feeder with Downstream Reclosers

and Sectionalisers

42

5.6 Load Flow Constraints in Transmission and Distribution Networks 43

5.6.1 Security Standards 43

5.6.2 Load Flow Constraints 44

Fig 5.15 Simple Network 44

Fig.5.16 Simple Network (Economic Generation Level) 45

Fig. 5.17 Minimum acceptable Generation Level at C 46

Fig.5.18 Additional Circuit added between A and C 475.6.3 Overhead Line Short-term Overload Ratings 47

Fig.5.19(a) An Isolated 132kV Network 49

Fig.5.19(b) Real Power Flows in the neighbourhood of 8019

- Base Case Study

49

Fig. 5.19 (c) Showing Real and Reactive Power Flows 50

Fig. 5.19(d) Post Fault Real and Reactive Power Flows 50

5.7 Summary 52


4/139


MODULE 5 – CONDUCTORS, NETWORK AND ENVIRONMENTAL

CONSTRAINTS - PROPRIETARY DOCUMENT



Module 5 –Conductors, Network & Environmental Constraints

Page 1 of 52

5.1 Introduction

Overhead lines are, in essence air-insulated cables suspended from insulated supports

with a power transfer capacity approximately proportional to the square of the linevoltage.

As was stated in Module 1 overhead line are mire economic than underground cables and

their use is avoided as much as possible at the higher transmission voltages.

Here are some approximate cost comparisons which should be treated in more depth

since they do not take into account the additional costs of either solution imposed by

the wayleave/consent/clearance procedures, addressed in module 3, and which are an

unfortunate part of transmission/distribution development.

For the transmission of equivalent power

At 11 kV a cable feeder costs about 5 times that of a transmission line



5.2 Environmental Conditions

It is necessary to match the mechanical and electrical characteristics of the overhead

line conductor to the environmental conditions that pertain. These environmental

conditions must, therefore, be obtained and appropriately analysed. The parameters

required were addressed in Module 4, Section 4.2.1

Condition Comments

Temperature The maximum, minimum and average ambient temperature

influences conductor current rating and sag. For temperature

conditions in many countries typically 20OC with 55O rise. For

tropical conditions 35OC or 40OC with 40OC or 35OC temperature

rise.

Maximum conductor operating temperature should not exceed

75OC to prevent annealing of aluminium

Wind

Velocity

Required for structure and conductor design. Electrical

conductor ratings may be based on cross-wind speeds of 0.5 m/s

or longitudinal wind speeds of 1 m/s

Solar

Radiation

Required for conductor ratings but also for fittings such as

composite insulators which may be affected by exposure to high

thermal and ultraviolet (UV) radiation. Typical values of 850

W/mm2 and 1200 W/m2 may be assumed for temperature and

tropical conditions respectively


5/139


6/139







Page 3 of 52

4. The conductor thermal capacity must be adequate.

5. The conductor diameter or bundle size must meet the recognised international

standards for radio interference and corona discharge.6. The conductor must be suitable for the environmental conditions and conform to

constructional methods understood in the country involved (such as IEC, BS, etc)

5.3.2 Types of Conductor

For 36 kV transmission and above both aluminium conductor steel reinforced (ACSR to

IEC 209) and all aluminium alloy conductor (AAAC to IEC 208) may also be appropriate.

Aluminium conductor alloy reinforced (ACAR) and all alloy conductors steel reinforced

(AACSR to IEC 210) are less common than AAAC and all such conductors may be more

expensive than ACSR. Historically ACSR has been widely used because of its mechanical

strength, the widespread manufacturing capacity and cost effectiveness. For all butlocal distribution, copper-based overhead lines are more costly because of the copper

conductor material costs. Copper has a very high corrosion resistance and is able to

withstand desert conditions under sand blasting. All aluminium conductors (AAC to IEC

207) are also employed at local distribution levels.

From a materials point of view the choice between ACSR and AAAC is not so obvious

and 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 lower tower heights. With regard to long-term creep or relaxation, ACSR with its

steel core is considerably less likely to be affected. Jointing does not imposeinsurmountable difficulties for either ACSR or AAAC types of conductor as long as

normal conductor cleaning and general preparation are observed. AAAC is slightly

easier to joint than ACSR. The characteristics of different conductor materials are

given in Table 5.1

Figure 5.1 illustrates typical strandings of ACSR. The conductor, with an outer layer of

segmented strands, has a smooth surface and a slightly reduced diameter for the same

electrical area


7/139


8/139







Page 5 of 52

6 AL./1St

7 AL./1St

8 AL./1St8 AL./1St

6 AL./1St

4 AL./3St

12 AL./7St

26 AL./19St

18 AL./1St

18 AL./19St

30 AL./7St

30 AL./19St

54 AL./19St

54 AL./19St

42 AL./19St

76 AL./7St

42 Seg/30 AL./7St

nAL= no. of aluminium strandsnSt = no. of steel strands

24 AL./7St

26 AL./7St

Fig. 5.1 Conductor Arrangements for Different ACSR Combinations


9/139







Page 6 of 52

There is no standard nomenclature for overhead line conductors. Code names are based

on animals (Refer to Table 5.2).

Aluminium-based conductors are referred to by their nominal aluminium area. Thus

ACSR with 54 Al strands surrounding seven steel strands, all strands of diameter d =3.18 mm. is designated 54/7/3.18; aluminium area = 428.9 mm2, steel area = 55.6 mm2,

and described as having a nominal aluminium area of 400 mm2.

In France the total area of 485 mm2 is quoted and in Germany the aluminium and steel

area 429/56 are quoted. In North America, the area is quoted in circular mils (1000

circular mils = 0.507 mm2)

Codename

Stranding Alu area(mm2)

Steelarea

(mm2)

Diameter(mm)

Mass(kg/km)

BreakingLoad

(kN)

Resistance(Ω/km)

Horse 12/7/2.79 73.4 42.8 13.95 538 61.2 0.3936

Lynx 30/7/2.79 183.4 42.8 19.53 942 79.8 0.1441

Zebra 54/7/3.18 428.9 55.6 28.62 1621 131.9 0.0674

Dove 26/3.72 +

7/2.89 282 45.9 23.56 1137 99.88 0.1024

Table 5.2 Typical Properties of some ACSR Conductors

5.3.3 Aerial Bundled Conductor

Research onto excessive power failures occurring on open wire distribution systems

under storm conditions has lead to the development of the Aerial Bundled Conductor

(ABC). Their employment on distribution networks is now extremely popular in various

parts of the world as a means of improving consumer supply reliability.

The cost of ABC quite naturally exceeds that of the bare, air-insulated traditional

approach. For short lines up to 24 kV, employment of ABC is about 160% of the cost of

bare wires in terms of system losses (revenue). The limiting factor is volt drop, this

being determined by the line reactance. For longer lines and higher currents therunning cost differential diminishes. However, the capital costs per unit length for

ABC are approximately twice that for bare-wire. It could be argued that the ABC end

product is a more pleasing overhead line.

Delegates Notes


10/139







Page 7 of 52

There are two distinct ABC systems in use. One system uses a self-supporting bundle

of insulated conductors where all conductors are laid helically and where tension is

taken on all conductors which are of hard-drawn aluminium. An alternative system is

where all conductors are insulated and the hard-drawn aluminium phase conductors are

laid up around an aluminium alloy neutral which ahs a greater tensile strength and acts

as a catenary wire to support the whole bundle. The insulation material is either

polyvinylchloride (PVC), linear polyethylene (PE) or cross-linked polyethylene (XLPE).

A comparison of the advantages and disadvantages between ABC and conventional open-

wire distribution construction is given in Table 5.3

With ABC construction core identification and the need to distinguish between neutral

and phase conductors is essential and in practice such overhead line emergency work is

often carried out in poor light. One, two or three prominent ribs are introduced along

the length of the core insulation to identify the phases, and multiple low profile ribs

along the neutral conductors may be identified by touch irrespective of the position of

the neutral in the bundle.

Delegates Notes


11/139







Page 8 of 52

Property Pros (ABC vs open-

wire

Cons (ABC vs open-

wire

1. Ultimate tensile strength Higher for neutral

catenary wire support

Simple fittings

Less stock/stores

holdings

Possible

support/fittings failure

prior to bundle failure

2. Current ratings - Lower – however note

that the design of

distribution voltage

level is usually based onvoltage drop rather

than current carrying

capacity

3. Voltage regulation Lower AC reactance

(typically – 25%)

Higher DC resistance

(typically +15%)

4. Earth loop impedance - Line lengths will be less

(typically –15%)

because of higher DC

resistance. This is an

important issue in PME

systems

5. Short-circuit ratings - Thermal limits on both

conductor and

insulation means more

attention to speed of

protection is required

6. Costs

(a) Fittings

(b) Conductor

(c) Poles and stays

(d) Labour

- refurbishment- new works

- under eaves

- refurbishment

- new works

(e) Maintenance

Same

-

-10%

-36%-25%

-17%

-22%

lower costs

Same

1.6 to 2 times

Table 5.3 Comparison between Aerial Bundled Conductor and Open-wire


12/139







Page 9 of 52

5.3.4 Conductor Breaking Strengths

The declaration of breaking strength in conductors does not have a unique value. The

value depends upon the method of calculation employed as stipulated in the appropriateNational or IEC standards to which the material is supplied. Differences quoted in

breaking strengths for a given conductor configuration are not due to the material

itself but to the calculation methodologies.

The IEC and BS values listed in Table 5.4 are fairly close for the two types of

conductor displayed.

Calculation Standard ACSE conductor 50/8

Breaking strength

(kg)

ACSE conductor

380/50

Breaking strength (kg)

BS 215, Pt 2, 1970 1714 12330

ASTM B232-74 (Class A) 1779 12398

ASTM B232-74 (Class B) 1724 12059

ASTM B232-74 (Class C) 1669 11720

NF C34 120 1968 (R) 14700

NF C34 120 1968 (N) 12105

DIN 48 204 (declared) 1742 12552

DIN 48 204 (theoretical area) 1716 12314

DIN 48 204 (calculated area) 12306

CSA C49 1-75 1752 12579

IEC 209 (now IEC 1089) 1720 12305

Table 5.4 Calculated Conductor Breaking Strengths According to some Different

Standards

Such anomalies have presented a dilemma to manufacturers of conductors and fittings

as to how to decide whether test results were applicable to the conductor or to thefittings. The calculation of conductor behaviour under changing loading conditions

(wind, ice) and temperature is related to breaking strengths, and the design of fittings

(tension clamps, repair sleeves etc) must also be related to these values. Hence it is

necessary in any overhead line specification to state clearly which standard calculated

breaking strengths are to be based on in order to avoid inevitable disputes at a later

date. If in doubt use the IEC standards.


13/139







Page 10 of 52

5.3.5 Bi-metal Connectors

Special copper/aluminium joints are required when terminating an aluminium conductor

on to a copper terminal in order to prevent corrosion. A termination of this type usuallycomprises an aluminium sleeve compressed onto a copper stalk with an insulating disc

separating the two surfaces which are exposed to the atmosphere (refer to figure 5.2).

The two dissimilar materials are generally welded together by friction welding as this

process ensures a better corrosion resistance at the interface. An additional

protection is afforded by the use of an anticorrosion varnish. When using such fittings

it is always recommended that the aluminium component is above the copper one. Even

slight traces of copper on the aluminium have a drastic effect on the aluminium

material.

Copper Pin

AluminiumFerrule

This area around theAluminium /copper weldIs protected by anAnti corrosive varnish toPrevent galvanic action

Fig.5.2 Bi-metal Connector5.3.6 Corrosion

Overhead lines are built in widely different climatic conditions all around the world.

Over the years a great deal of experience has been acquired on their performance.

Aluminium conductors have a good corrosion behaviour due to an oxide layer which

forms on the outside. This provides a protective layer which remains undisturbed

throughout the life-cycle protecting the conductor from any further corrosion.

However ACSR conductor is known to suffer from “bulge corrosion”. This is an increase

in conductor diameter resulting from bimetallic corrosion.


14/139







Page 11 of 52

Early problems associated with deterioration of the steel cores of ACSR conductors

have been resolved over the years by the use of high temperature greases. These

greases prevent the onset of galvanic corrosion between the galvanised steel core and

the outer aluminium wires. They have a high drop point which allows continuous

operation of the conductor at 75OC and full service life protection. AAAC will obviously

offer superior corrosion resistance than un-greased ACSR. Conductors that are not

fully greased are not recommended for corrosive areas. The resistance properties of

ACSR also depend upon the number of layers of aluminium surrounding the steel core.

The conclusions of research carried out in the late 1960’s showed that: -

Pure aluminium had the best corrosion resistance under the majority of

environmental conditions

Smooth body conductors were the most corrosion resistant, especially if the

inner layers were greased.

Small diameter wires were most susceptible to corrosion damage and to failure.

Thus for a given conductor area it is preferable to have a fewer larger diameter

strands

The overall corrosion performance of aluminium alloy conductors depends upon

the type of alloy used.

For very aggressive environments the following order of preference is suggested: -

Aluminium conductor fully greased

Aluminium conductor with alumoweld core fully greased

ACSR fully greased

Aluminium conductor with alumoweld un-greased

ACSR with greased core.

Delegates Notes


15/139







Page 12 of 52

5.4 Calculated Electrical Ratings

5.4.1 Heat Balance Equation

The conductor thermal current rating in wind, ignoring any voltage regulation

considerations, is given by the following simplified heat balance equation as valid for

stranded conductors:

Heat generated (I2R conductor losses) = heat lost by convection(watts/km)

+heat lost by radiation (watts/km)

-heat gained by solar radiation (watts/km)

= HC + HR - HS

( )[ ) ( ) θ θ θ α xx3871202 448 0.

dVtR I =++= ( ) ( ) 273-273xxx 44 ++++ ttdsE c θ π

- (watts/km)xx dSSα ……….(5.1)

where I = current rating, amps

R20 = resistance of conductor at 20OC

α = temperature coefficient of resistance per OC (for ACSR at 20OC, α =

0.00403)

t = ambient temperature in OC

θ = temperature rise, OC (t1 = initial temperature and t2 = final temperature)

αS = 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. Average value of 0.8, say, may be taken for

initial design purposes.

S = intensity of solar radiation, watts/m2

D = conductor diameter, mm

V = wind velocity normal to conductor, m/s

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

s = Stefan-Boltzmann’s constant = 5.7 x 10-8 watts/m2

π = pi, constant (22/7) = 3.141592654…..

For design purposes 0.5 or 0.6 m/s wind speeds are usually taken. Higher wind speeds

would of course lead to higher ratings. In practice, the heat balance is a highly complex

process bit the above equation is adequate for calculation purposes. Further research

is on going using the following: -


16/139







Page 13 of 52

Deterministic models with values based on experience without attempting to correlate

wind speed with air temperature or solar radiation.

Probabilistic models based on availability of statistical data since practicalmeasurements have shown that in almost all cases the conductor temperature is lower

than that predicted by other methods.

5.4.2 Power Carrying Capacity

Approximate economic power transfer capacity trends for different line voltages

based on power transfer being proportional to the square of the line voltage are given

in figures 5.3a and 5.3b for transmission voltage up to 500 kV. In practice, the capacity

will be limited over long distances by the conductor natural impedance (voltage

regulation) as well as by conductor thermal capacity. Note also that transient stability

limits on longish inter-ties between large networks may also limit the power transfercapability to a value lower than the thermal limit.

Depending upon the required electrical load transfer, the number of overhead line

conductors of a particular type (used per phase) will vary. Conductor configurations are

given in figure 5.4

Therefore, under the following specific tropical conditions (40OC ambient temperature,

0.894 m/s wind speed, 100 mW/cm2 solar radiation and 35OC temperature rise), the

calculated ratings for typical ACSR twin conductors at 230 kV would be: -

Type (Nominal) 230 kV

2 x 200 mm2

2 x 300 mm2

2 x 400 mm2

2 x 500 mm2

2 x 600 mm2

1052 A (419 MVA)

1296 A (516 MVA)

1558 A (620 MVA)

1742 A (694 MVA)

1890 A (753 MVA)

Delegates Notes


17/139


MODULE 5 – CONDUCTORS, NETWORK AND ENVIRONMENTAL CONSTRAINTS




Page 14 of 52

Voltage/ Power 11kV 33kV 66kV 132kV 220kV 275kV

5MVA (a) 100- DOG 25 - GOPHER

(b) 100 - GOAT 25 - GOPHER10MVA (a) 300-GOAT 50 -RABBIT 25 - GOPHER

(b) INADEQUATE 50 -RABBIT 25 - GOPHER

25MVA (a) 200 -PANTHER 75 - RACOON

(b) 200 -PANTHER 75 - RACOON

50 MVA 200 -PANTHER

100 MVA Notes 200 -PANTHER

200 MVA 1. Numbers refer to nominal aluminium area e.g. 100m2 2 x 150-WOLF

200 MVA 2. For voltages up to and including 66 kV, conductor size is governed 250-BEAR

300 MVA by thermal rating and/or voltage drop – surface gradients are 2 x 175 LYNX 400 - ZEBRA

400 MVA normally acceptable 2 x 250-BEAR 2 x 175 LYNX

500 MVA 3. (a) is thermal rating b) is rating for 10% voltage drop - a power factor of 0.9

over a distance of 10 kM. Other

2 x 400 ZEBRA 2 x 250-BEAR

(2 x BATANG)

600 MVA (ratings for other assumptions 2 x 350–ANTELO

or BISON

700 MVA 4. For voltages 132kV and above conductor size is also governed by surface gradient and electrical stability of systems. Ratings

lines are affected by equipment in substations

800 MVA 5. Typically minimum conductor sizes would be:

1000 MVA 132 kV 1 x 14.2 mm

275 kV 2 x 19.3 mm

1200 MVA 400 kV 4 x 18 mm

1800 MVA6. This Table is prepared for tropical conditions. For lower temperature conditions ratings would be 20-30% higher

2000 MVA7. This is really a very complex subject so readers should use this chart only as a guide

Fig. 5.3 (a) Approximate Conductor Sizes (ACSR) for Power Transfer Capab


18/139







Page 15 of 52

100

200

300

400

500

600

L i n e

V o l t a g e

k V

Line Voltages from 66 kV to 600 kV

100,000 200,000 300,000Power Transfers MW-Kilometers

Line Voltages from 11 kV to 66 kV

L i n e

V o l t a g e

k V

100 200 300 400 500 600

Power Transfers MW-Kilometers

10

20

30

40

50

60

70

Fig 5.3 (b) Economic Power Transfer Capacities


19/139







Page 16 of 52

A typical set of power transfer curves for the 2 x 400 mm2 conductor case are given in

figure 5.5. The optimum rating for a particular line length is given by the intersection

of the regulation curves for, say, 0.9 power factor (pf) with either the thermal limit or

the voltage regulation lines, whichever does not infringe the voltage or current limitspecified for the line. It should be noted that adequate technical performance is

usually judged upon the load flow under single circuit outage conditions (n-1). On the

other hand economic loadings do not consider contingencies and are based on normal

operating conditions.

Calculated ratings for typical ACSR conductors at lower voltage levels of 11, 33 and 66

kV overhead lines using different conductors over different distances are given in

Table 5.5.

Single

Flat twin or Vertical twin

orTriple

Quad

Multiple bundle

Fig. 5.4 Typical Conductor Configurations


20/139







Page 17 of 52

Power transfer (MW)

V o l t a g e

a t r e c e i v i n g e n d ( % ) Voltage regulation limitThermal

Limit

230 kV Transmission line (2 x 400 mm2)Line length 45 kM Sending end busbar voltage 105%Max. current rating of line 1812 ampsLine parameters on 100 MVA base: resistance 0.000077 p.u./km

Reactance 0.000592 p.u./kmSusceptance 0.001830 p.u./km

Power Factor Key

(a) 1.00(b) 0.98(c) 0.95(d) 0.90(e) 0.85

Fig.5.5 Power Transfer Curves

Line

Voltage

(kV)

Conductor

Equivalent

Configuration

(mm)

ACSR

Conductor

code

AAC

Conductor

code

MWcapacity

Based upon

5%

Regulation

Sparrow Iris 0.95 0.49 0.33 0.25

11 1400 Raven Poppy 1.4 0.7 0.47 0.35

Linnet Tulip 3.00 1.5 1.00 0.75

16 (km) 32 (km) 48 (km) 64 (km)

Quail Aster 5.00 2.50 1.70 1.2533 1500 Penguin Oxlip 6.70 3.35 2.20 1.70

Linnet Tulip 8.35 4.18 2.80 2.10

Hen Cosmos 11.50 5.75 3.80 2.90

32 (km) 64 (km) 96 (km) 128 (km)

Quail Aster 12.50 6.25 4.18 3.14

66 3000 Linnet Tulip 16.00 8.00 5.32 3.99

Hen Cosmos 18.40 9.18 6.12 4.59

Table 5.5 Typical Load Carrying Capacity of Overhead Distribution Lines


21/139







Page 18 of 52

5.4.3 Corona Discharge

High voltage gradients surrounding conductors (above about 18 kV / cm) will lead to

breakdown of the air in the vicinity of the conductor surface known as coronadischarge. The effect is more pronounced at high altitudes. Generally the breakdown

strength of air is approximately 31 kV peak/cm or 22 kV rms/cm. This is a useful guide

for the selection of conductor diameter or conductor bundle arrangement equivalent

diameter. Corona discharge and radio frequency interference generated cause

problems with the reception of radio communication equipment and adversely affect

the performance of power line carrier signals.

At higher voltage levels, and certainly at voltages of 400 kV and above, interferences

due to corona effect can be the dominant factor in determining the physical size of the

conductor rather than the thermal rating characteristic. Increasing the conductor

diameter may be necessary in order to reduce the practical size, strength and handlingcapability for conductors. The bundling of conductors as described in section 5.4.2

assists in the effective increase in overall conductor diameter and hence leads to lower

stress levels.

The surface voltage gradient may be determined from Gauss’s theorem showing that an

increase in radius or equivalent radius leads to a reduction in surface voltage gradient.

xxx2

r

QgV

0ε π

= where Vg = voltage surface gradient (volts/cm)

Q = surface charge per unit length (coulomb/m)r = equivalent radius of smooth conductor (cm)

ε = permittivity of free space 910xx36

1

π = F/m

In practical terms this may also be expressed as follows: -

kV/cm2

log2

ed

Dd

UV

p

g = …………………………………………………(5.2)

where Vg = voltage surface gradient (kV/cm)Up = phase voltage (kV)

d = diameter of single conductor (cm)

D = distance between phases for single phase line orequivalent spacing for three phase lines (cm)

For the three phase line configuration 3 ry xxD br yb DDD = where Dry, D yb and Dbr are

the spacings between the different phases r, y, and b


22/139







Page 19 of 52

5.4.3.1 Worked Example for Corona Discharge

Consider a 132 kV single circuit Zebra ACSR line with conductor diameter of 28.62 mm

and spacings as shown on figure 5.66.261.862 22 =+=ryD 7.231.872 22 =+=ybD 3.746312 22 =+= .D br

3ry xxD br yb DDD = = 5.53m = 553 cm

Earth Wire

3 m

3 m

r

b

y

2.8 m

1.8 m

1.8 m

Fig. 5.6 Corona Discharge Calculation Example – 132 kV Zebra Conductor

Spacing

kV/cm2

log2

ed

Dd

UV

p

g = = ( ) ( )[ ]553/2.86x2log2.86/23132/

e

=( )386.71logx1.43

76.2

e

=

5.96x1.4376.2 =

= 8.94 kV/cm which is within the 18 kV/cm criteria


23/139







Page 20 of 52

5.4.3.2 Radio Frequency Interference

Radio frequency interference (RFI) noise is measured in decibels above 1 microvolt per

metre (dB > 1µV/m) from comparative equations of the form: -

0

mean0mean0n

n

d

d)EERFI-RFI 1010 10log40log3.8( +++=

0

+( )21010 1 1

20log30logf

nf

D

D2

00

+

++

where RFI = calculated radio noise (dB > 1µV/m)

Emean = calculated mean voltage gradient (kV/cm)

d = conductor diameter (cm)

n = number of sub-conductors in bundle

d = distance between phase and measuring antenna (m)

f = frequency (Hz)

The suffix “0” refers to the same quantities obtained from measurements. Acceptable

noise levels depend upon the quality of service required and is described in terms of an

acceptable signal-to-noise or signal plus noise-to-noise ratio. Some reception

classifications are given in Table 4.6

Signal-to-noise ration (dB) Subjective impression of reception quality

32 Entirely satisfactory

27

22

16

6

0

Very good, background not intrusive

Fairly satisfactory, background evident

Background very evident, speech easily understood

Difficulty in understanding speech

Noise swamps speech

Table 4.6 Effect of Various Levels of Signal-to-noise Ratio

Thus if a signal has a field strength of, say, 60 dB > 1µV/m and a fairly satisfactory

reception is required then the noise from the adjacent overhead line should not exceed

60 – 22 = 38 dB > 1µV/m. Audible noise is presently not considered to be a controlling

factor at voltage levels below 500 kV. However, further research is on-going.


24/139







Page 21 of 52

5.4.4 Worked Example for an Overhead Line Calculation

A simple hand calculation can be used as a check against the computer solutions to

calculate the conductor and tower size normally employed.Consider the need to transfer 40 MVA over a distance of 70 km on a Lynx ACSR under

the following tropical conditions: -

Maximum operating temperature 75OC

Maximum ambient air temperature 40OC (temperature rise = 35OC)

Lynx conductor max. resistance 0.1441 Ω/km

Lynx conductor diameter 19.53 mm

Emissivity 0.6

Solar radiation coefficient 0.8

Solar radiation intensity 1000 w/m

2

Wind velocity 1 mph = 0.447 m/s

Effective wind velocity = actual wind velocity x p/760 x 293/(273 + t)

= 0.447 x 760/760 x 293/313 = 0.418 m/s = 41.8 cm/s

(assuming normal atmospheric pressure)

Load Current at 132 kV for 40 MVA power transfer MVA17510x132x3

10x40

3

6

==

The conductor thermal rating capability is first determined, ignoring any voltage dropconsiderations, by comparing the 175 A load current requirement and the rating of the

conductor derived from the heat balance equation detailed in section 5.4.1 (Equation

No. 5.1)

( ) ( )0.448-4122 xx10x-12.8 dVttR I =

4

1

4

2 -xxxx TTdsEcπ + -

(watts/km)xx dSSα

( ) ( )0.448-4-52 1.953x41.8x10x40-7513.810x0.1441x =I

[ ] [ ]44 40273 -75273x1.953x10x5.7x0.6x -12 +++π

1.953x10x1000x0.8 -4− ( ) 0.156-318-348x10x20.990.347 44-12+= 0.2970.156-0.1060.347 =+

3

2

10x0.1441

0.297 =I 310x206.107=I

The conductor type is therefore more than adequate on thermal considerations for the

load required.


25/139







Page 22 of 52

A check is then made for any corona discharge limitations. Assume a conductor

configuration as shown in figure 5.7 and calculate for only one circuit r1, y1 and b1

Earth Wire

3.6 m

r13.5 m

4.5 m

4.5 m

r2

4.6 m

3.6 m

b1 b2

y2y1

Fig. 5.7 Calculation Example – 132 kV Lynx Conductor Spacing

m4.61142 22 =+= 5.D ry m4.61142 22 =+= 5.Dyb

Dbr = 9m3

ry xxD br yb DDD = = m5.769x4.61x4.613 ==D

2log

2

ed

Dd

UV

p

g = =

( ) ( )[ ]576/1.953x2log1.9533/23132/

e ( )589.86logx0.97776.2

e

= =6.38x0.977

76.2 =

= 12.22 kV/cm which is within the 18 kV/cm criteria and Lynx conductor is therefore

acceptable from both a corona and current carrying capacity aspect.

5.4.5 Available kVA km Neglecting CapacitanceIf capacitive reactance is ignored the voltage drop, Vd, for a line length, l , is calculatedfrom the usual formula: -

Vd = I (R cos Φ + X sin Φ)

If the load at the receiving end is given in kVA, then for a three phase system the load

currentU

I x3

kVA = where U is the line voltage in kV

The main practical problem is now to obtain accurate values for the line reactance.

Some typical reactance values are given in Table 5.7


26/139


MODULE 5 – CONDUCTORS, NETWORK AND ENVIRONMENTAL CONSTRAINTS - PR




Page 23 of 52

Equivalent Al area

(mm2)

25 30 75 100 125 150 175

Stranding 6/1/2.36 6/1/3/35 6/1/4.1 6/2.72+7/1.57 30/7/2.36 30/7/2.59 30/7/2.79

Current (temperate) A 157 242 311 371 429 482 528

Current (tropical) A 130 198 253 299 343 384 419

R Ω/km (20OC) 1.093 0.5426 0.3622 0.2733 0.2203 0.1826 0.1576

R Ω/km (75OC) 1.317 0.6539 0.4365 0.3294 0.2655 0.2203 0.1899

0.3m spacing (415V) 0.298 0.276 0.263 0.253 0.239 0.233 0.229

1.4m spacing (11kV) 0.395 0.373 0.360 0.350 0.336 0.330 0.326

1.5m spacing (33kV) 0.399 0.377 0.364 0.355 0.340 0.335 0.330 3.0m spacing (66kV) 0.442 0.420 0.408 0.398 0.384 0.378 0.373

3.6m spacing (110kV) 0.454 0.432 0.419 0.410 0.395 0.390 0.385

4.9m spacing (132kV) 0.473 0.451 0.439 0.429 0.415 0.409 0.402

Table 5.7 ACSR Conductors Inductive Reactance, Ω/km (equivalent spacings given) (IEE Proceedings,


27/139







Page 24 of 52

The value of (R cos Φ + X sin Φ) is approximately constant for overhead lineconfigurations with conductor sizes above 150 to 200 mm2. Therefore very large

conductors are necessary to improve any voltage drop problems if such conductor sizes

prove to be inadequate. In such circumstances consideration has to be given to

raising the transmission voltage level.

It is useful to introduce the concept of kVA km for a given voltage drop for a variety

of overhead line configurations and different conductors. For a 10% voltage drop the.

x3

kVAx3 0.1

U

U = x l x (R cos Φ + X sin Φ) x 10-3

kVA x l =

( )θ θ sincos

x1002

+R

U with the length, l in km…………………………….(5.3)

Tables may thus be prepared based on this equation for different conductors at

different power factors giving the available kVA km for a given % voltage drop.

5.5 Design Spans, Clearances and Loadings

5.5.1 Design Spans

The general parabolic sag/tension equation is addressed in module 4, section 4.2.3. In

order to design suitable tower dimensions for an overhead line it is necessary to

calculate the sags and tensions. The maximum conductor tension (occurring at minimum

temperature) is evaluated in order to ensure a sufficient mechanical strength marginfor a 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) or the everyday stress (EDS). The conductor has to be

designed such that the maximum anticipated loads do not exceed 50% of the breaking

load at –6OC (MWT condition) and about 20% about at an everyday temperature of 16OC

(EDS condition)

Delegates Notes


28/139







Page 25 of 52

5.5.1.1 Basic Span

The optimum spacing of towers and their height becomes an economic (financial)

exercise. With short spans and low towers the total number of towers and associatedfittings will be large to cover a certain route length but less steel for each tower will

be needed. If long spans are used then the conductor sag between tower points

becomes greater and fewer, stronger, higher towers and fittings, but with

correspondingly more steel, are necessary to ensure correct clearances. The extent of

labour associated with a variable number of towers for a given route length will also be

important. Refer to figure 5.7: the overall height of the tower is: -

H = C + SO + 3 SA + SB + SC + SE

Where C = statutory clearance to ground

SA = length of insulator suspension set

SB, SC and SE = vertical distances between cross-arms and conductor above or to earthwire

SO = sag of conductor (proportional to the square of the span)

Given the mechanical loading condition and phase to earth wire conductor types an

evaluation of the basic span may be made as follows. Assume as arbitrary length in a

flat area over say, 100 km. Inevitably there will be some angle/section towers whose

positions will be fixed beforehand (Refer to Module 4). From experience let this

number be NO. If L is the basic span and l the span length then the number of

suspension towers will be the next integer from O NL

-x100

l l

+

1. Conductors and earth wire – costs for supply and installation

2. Insulators – selection depending upon mechanical loading and pollution levels such

that SA, may be defined

3. SR, SC, SE – a function of the still air clearance coordinated with insulation level

4. Tower weight (W) – lengthy designs may be omitted at this stage by using a

formula such as that by P.J.Ryle: -

Approximate weight of tower 1MHK W xx1=

Approximate base width 1MK x2


29/139


30/139







Page 27 of 52

The summation of the costs involved will then give an indication of the approximate

total cost. By varying the span length l (with its influence on SO and associated

quantities), cost versus span may be evaluated and plotted. Such curves as illustrated in

figure 5.8 are in practice normally very flat at the bottom. Experience shows that aspan selected slightly greater than the minimum derived from such an initial analysis

gives an overall optimum choice. From a recent international survey the supply costs of

overhead lines may be broken down as in Table 5.8.

Description Up to 150 kV 150 – 300 kV > 300 kV

ConductorsEarth wires

Insulators

Towers

Foundations

31.64.1

8.8

36

19.5

31.53.5

9.3

36.0

19.7

34.13.9

6.9

36.4

18.7

Table 5.8. Relative Supply and Installation Costs for Overhead Lines

The breakdown given in Table 5.8 is only an approximation. It gives average values for

many lines and practices encountered by Balfour Beatty throughout the world and

caries according to line voltage, conductor configuration and the design of thesupporting structure. In addition, an allowance has to be made for the routing survey,

land clearance, erection and similar items. Basic spans might be approximately 365 m at

230 kV and 330 m at 132 kV. The minimum allowable ground clearance between phase

conductors and earth is derived from specified conductor clearances for the country

involved, in still air at maximum conductor temperature. Survey figures for the

proportion of tower costs compared to the overall line costs ranged from 8% to 53%

with ACSR, but from 25% to 45% with AAAC.

5.5.1.2 Wind Span

The wind span is half the sum of the adjacent span lengths as shown in figure 5.8b. At230 kV this might be 400m under normal conditions and 300m under broken wire

conditions. Correspondingly, at 132kV typical values are 365m and 274m respectively

Delegates Notes


31/139







Page 28 of 52

Cost

SpanBasicspan

Cost/span plot to determine most economic basic span.The basic span is the horizontal distance between centres

of adjacent supports on Level Ground

L1 L2 L3

Wind Span= L1 + L2

2

Weight Span

The wind span is half the sum of adjacent horizontal span lengthssupported on any one tower.The weight span is the equivalent length of conductor supportedat any one tower at minimum temperature

Cold sag templatecurve

(a)

(b)Wind and Weight Span

Cost v Span Plot

Fig.5.8 (a) and (b) Cost v span, Wind and Weight Span


32/139







Page 29 of 52

(c) Equivalent Span

TensionTower

Tension

Tower

L1 L2 L3 L4

SuspensionTowerSuspension

TowerSuspension

Tower

The equivalent span is used for determination of sag in spans for which the tension in anysection length is that which would apply to a single span equal to the equivalent span.

3 3 3 3 31 2 3 n4

1 2 3 n4

L +L +L +L +.......LEQUIVALENT SPAN

L +L +L +L +......L=

Fig.5.8 (c) Illustrating Equivalent Span

5.5.1.3 Weight Span

The weight span is the distance between the lowest points on adjacent sag curves on

either side of the tower as shown in figure 5.8b. It represents the equivalent length or

weight of conductor supported at any one tower at any time. For design purposes, it is

the value under worst loading conditions (minimum temperature in still air) which gives

the greatest value. A tower at the top of a hill may be heavily loaded and it is usual to

assume a weight span which can reach up to twice the value of the basic span. In fairly

level terrain a value of 1.6 to 1.8 will be adopted.

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 0.7 is usually considered acceptable. Thisratio is easily computed with the use of a “cold” template. When plotting tower

positions, the engineer must be aware of the maximum weight span and of such ratios.

Typical weight span values at 230 kV and 132kV are included overleaf.


33/139







Page 30 of 52

230 kV 132 kV

Suspension towers

750m Normal conditions

Suspension towers

680m Normal conditions

565m Broken wire conditions 510m Broken wire conditions

Tension Towers Tension Towers

750m Normal conditions 680m Normal conditions

750m Broken wire conditions 680m Broken wire conditions

5.5.1.4 Equivalent Span

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 of the

section. The mathematical treatment to obtain the equivalent span is based on parabolic

theory and there is no similar concept using full catenary equations. For sagging the

overhead line conductors the tension appropriate to the equivalent span and the

erection temperature as shown in figure 5.8c is used. Erection tensions are calculated

from final tension tensions making al allowance for creep . This is equated to a

temperature shift which is applied to final tensions.

5.5.1.5 Creep

Creep is a phenomenon that affects most materials subjected to stress. It manifests

itself by an inelastic stretch (or permanent elongation) of the material in the direction

of the stress. Certain materials such as aluminium are more susceptible than others.

For example steel suffers only a limited amount of creep. The increase in conductor

length resulting from inelastic stretch produces increased sags which must be taken

into account in the overhead line design and installation process so as not in infringe

clearances.

Some mathematical models have been evolved to help the engineer assess the effect of

creep and those used in the UK are included here as equations 5.4 and 5.5:

mm/kmδσ

µθβ teσK ε ϕ = ……………………………………………….….. (5.4)

for all types of conductors, and.

mm/km tθK σε µβ ϕ = …………………………………….………………(5.5)

for AAC, AAAC and ACAR where

ε = permanent inelastic elongation (creep )

K = constant σ = average stress in conductor

β, Φ, µ, δ = creep indices obtained by test

e = natural logarithm base = 2.7182818, t = time in hours, θ = temperature in OC


34/139







Page 31 of 52

Since the total inelastic strain can be considered as the result of geometric settlement

of the strands and of the metallurgical creep thereafter, the derivation of the

constants and of the indices (β, Ф, µ, δ) is of prime importance. In the UK it has been

decided that tests should be carried out in such a way that the geometric settlementwould be taken into account in the constants and indices and that the formulae

(equations 5.4 and 5.5) would give the total creep. Typical values for the constants

involved in these equations are given in Tables 5.9a through 5.9d.

Conductor

Stranding

Al Steel

Al/steel

Area

ratio

Process K Ф β µ δ

HR 1.154 7 7.71

EP 1.6

0.0175

0.0171

2.155

1.418

0.342

0.377

0.2127

0.1876

48 7 11.4 HR 3.0 0.01000 1.887 0.165 0.0116

30 7 4.28 EP 2.2 0.0107 1.375 0.183 0.0365

26 7 6.16 HR 1.9 0.0235 1.830 0.229 0.0365

24 7 7.74 HR 1.6 0.0235 1.882 0.186 0.00771

18 1 18.0 EP 1.2 0.0230 1.502 0.332 0.1331

12 7 1.71 HR 0.66 0.0115 1.884 0.273 0.1474

Note: Industrial processing of aluminium rod: HR = hot rolled; EP extruded or Properzi

Table 5.9a Creep coefficients for ACSR conductors (Equation 5.4)

Process K Φ β µ

Hot rolled 0.15 1.4 1.3 0.16

Extruded Not available Not available Not available Not available

Table 5.9b Creep coefficients for AAAC conductors (Equation 5.5)

Process K Φ Β µNumber of make up wires

7 19 37 61

Hot Rolled 0.27 0.28 0.26 0.25 1.4 1.3 0.16Extruded or

Properzi

0.18 0.18 0.16 0.15 1.4 1.3 0.16

Table 5.9c Creep coefficients for AAC conductors (Equation 5.5)


35/139







Page 32 of 52

Process K Φ β µ

Extruded or

Properzi 0.04 + (0.24m/m + 1) 1.4 1.3 0.16

Table 5.9d Creep coefficients for ACAR conductors (Equation 5.5)

When applying the technique of creep evaluation the designer must forecast reasonable

conductor history. Typical conditions might be shown in Table 5.10 where tm and tIV represent the periods for which compensation should be made.

Stage Stress Temperature Time

1. Running out Average ambient Time for running

out as decided by

design office

2. Pretension

(If provided

Average ambient As decided by

design office

3. Stress at given

Temperature

Mean yearly

temperature + 5OCtm

4. Stress at given

TemperaturetIV

5 Maximum stress tIV

Table 5.10 Typical Creep Conditions

Delegates Notes


36/139







Page 33 of 52

Virgin conductor on drum

Calculate creep strain

Is there apretension period?

Change the tension to thepretension value and

ignore creepduring this change

Calculate creep strainduring pretension period

Change tension to stringingtension and ignore creep

during this change

Yes

Calculate creep strainsince loading

Answer

Allowance to be madefor this creep only

No

Fig.5.9 Creep Assessment Procedure

Figure 5.9 illustrates an acceptable procedure for creep assessment. As an illustration

of the steps to be followed consider the following example: -

1. The EDS is to be 20% of the UTS of the conductor at 20OC

2. The maximum stress occurs when the conductor is subjected to a wind of

50kg/m2, no ice

3. The maximum operating temperature is 70OC

4. Accept a span length of 400m (In practice, three values should be taken: a

maximum and a minimum span both deduced from the profile, and a basic span.

The span which gives the highest value of creep strain is selected as a basis for

creep compensation)

5. Creep strain is to be calculated for a period of 30 years

6. Conductor is manufactured from aluminium rod obtained by the Properzi method

Some decisions based on experience are the necessary regarding the duration of the

maximum and minimum stresses, and values may then be inserted in a tabular format as

shown in Table 5.11


37/139







Page 34 of 52

Stage Stress Temperature Time

1. 20% UTS 20OC 1 hour

2. Nil

(no pretension)

Not applicable

(no pretension)

Not applicable

(no pretension)

3. Calculate by program 25OC 257544 hours(a)

4. Calculate by program 70OC 2628 hours(a)

5 Calculate by program 0OC 2628 hours(a)

Table 5.11 – Note (a) is the period for which compensation is required

If we consider the general change-of-state sag/tension equation the influence of creep

strain and temperature are both linear (Refer to section 5.5.2.5)

( ) ( ) ( ) 6-1212122

1

2

2

22

10x--

1

1 -

124

x

εεθθαTTEA

TT

LW

++−=

⎟⎟ ⎠

⎞⎜⎜⎝

⎛

It is possible, therefore to express creep strain, ε, by an equivalent temperaturechange, i.e.

610xe

α θ∆ε = ………………………………………………….(5.6)

where α is the coefficient of thermal expansion per OCThis is a widely employed concept when creep compensation is carried out with the help

of sag and tension charts. For example with Zebra conductor it has been assessed that

creep strain at the end of 10 years (t = 87,600 hours) is

ε = 616 mm/km giving ∆θe = 32OC approximately, then:

Maximum design temperature of conductor = 50OC, say

Equivalent temperature corresponding to creep, ∆θe = 32OC Temperature for evaluating sag time, t, and corresponding to the maximum design

temperature of the conductor when no pretension or over-tension regime are

applicable, θ + ∆θe = 82OC

This will clearly lead to a penalty in the height of all towers. An alternative would be to

reduce the sag at sagging time resulting in a temporary over-tension in the conductor.

However, this results in an over-design penalty on the angle towers. By applying several

combinations of temperature correction or pretension the designer is able to aim for

the least onerous solution.


38/139







Page 35 of 52

5.5.1.6 Catenary Equations for Sloping Spans

Basic catenary equations are useful. These can be found in most higher mathematics

textbooks.5.5.2 Conductor and Earth Wire Spacing and Clearances

5.5.2.1 Earth Wires

Where there is a risk of 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 path. Note that today’s earth wires on new

circuits are more often than not constructed with an optical fiber in the center of the

bundle. This is known as optical fibre ground wire (OPGW) and is used for end-to-end

data communications for protection relaying, Supervisory Control and Data Acquisition

(SCADA), other utility data transmission requirements and voice etc as a widebandalternative to power line carrier (PLC).

The degree of shielding of the overhead line phase conductors from lightning strikes is

determined by the shielding angle afforded by the earth wire(s) running over the line.

A single earth wire is considered to proffer a 30O shielding angle as illustrated in

figure 5.10a. Where lines are erected in areas of high lightning activity (refer also

module 6), or with supporting structures with wide horizontal spacing configurations

e.g. low height 400 kV towers in some networks, two earth wires are often provided to

permit a lower shielding angle and, therefore, better protection. Figure 5.10b shows a

0O angle protection employing two earth wires.

Delegates Notes


39/139







Page 36 of 52

Phase -conductorCross -arm

EARTH WIRE

30O

(a) TYPICAL 132kV DOUBLE CIRCUIT TOWER

WITH 30O SHIELD ANGLE

(b) TYPICAL 230kV DOUBLE CIRCUIT TOWER

WITH 0O SHIELD ANGLE

EARTH WIRES

0O

Fig.5.10 Overhead Line Earth Wire Lightning Screen


40/139







Page 37 of 52

The vertical spacing between the earth and phase conductors must be such as to

ensure sufficient clearance to prevent mid-span flashovers under transient conditions.

The sagging should be arranged so as to ensure that the vertical mid-span clearance

between the phase and earth conductors is about 20% greater than the supports.Galvanised stranded steel presents a low cost earth wire material (refer to comment on

PGW above). Where severe pollution exists or where relay protection schemes demand

a low impedance path, ACSR or other materials are employed.

In the UK the early 132 kV lines were designed with a 45O angle of protection and gave

satisfactory cover. When this angle was applied to the 275 kV and 400 kV lines it was

found advantageous to reduce the angle to 30O

The calculation of lightning behaviour of overhead lines is very complex. The

electromagnetic model is, however, a convenient way of visualizing the process. Assume

a cloud at height H above the ground with a stepped leader originating from point “O”as shown in figure 5.11a. If the distance OE to the earth wire is less than the distanceOC, the strike is more likely to hit the earth wire than the phase conductors.

H

D B

Ground

h

H

α

A

Overheadline

O

E

C

G

Cloud

Fig.5.11a Earth Wire Lightning Screen Protection (Vertical Formation)


41/139







Page 38 of 52

If a line EB is drawn at a tangent to the circle center, O, then by construction:

H

hH

OE

OA

EB

DB −=== sin α

If it is assumed that H = 2h then 30and0.52

2 sin O==

−= αα

H

hH. This angle of

protection is often adopted on the basis that the cloud height is likely to be twice the

height of the earth wire (top of the tower). If, however , H = 1.5h then

20and0.3331

1.5 sin O≈=

−= αα

H.5

hH

Ground

1.5hh

E E

2h

Cloud

Fig.5.11b Earth Wire Lightning Screen Protection (Horizontal Formation)

When considering conductors arranged in horizontal formation (e.g. low 400 kV towers

mid-cross-arm in DEWA) it is customary to assume a cloud level at 1.5h to 2h. Figure5.11b shows a single-circuit line with all three phases on horizontal formation.

5.5.2.2 Earthing Counterpoise

A lightning strike on the earth wire will be dissipated into the ground after passing

through the transmission structure and foundations. Wave propagation along electrical

lines obeys classical wave propagation theory. Wave reflections will occur at points of

discontinuity such as points of changing impedance. Consider the arrangement in figure

5.12. The voltage and current along the line at any time, t, are the vector sums of the

forward and reflected waves.


42/139







Page 39 of 52

Electrical line

Positive directionfor all voltage, v andCurrent, i , vectors

ZL = characteristicimpedance of the linef = forward

r = reflected

Forwardwave

Reflectedwave

v f v r

i f i r

Fig.5.12 Wave Propagation Along Electric Lines

The basic equations are as follows: -

_ _ _

r f

vvv +=……………………………………………………………(5.7) _ _ _

r f i i i += …………………………………………………………………………(5.8) _ _ _

r Lf Z i v = , _ _

r

_

LZ i vr −= ………………………………………………..(5.9)

therefore _ _ _ _

r f LZ vvi −= , _ _ _ _

2 f LZ vvi =+ ………………………………..(5.10)

For a line terminated on an impedance, ZT , the relationships between voltage v . and

current, i , at the receiving end is _ _ _

i v TZ=

Combining with the equations 5.7 through 5.10 allows resolution in terms of the incidentwave

x2 _ _

_ _ _

⎟ ⎠

⎞⎜⎝

⎛ +

=

TL

f T

ZZ

Z v

v …………………………………………(5.11)


43/139







Page 40 of 52

x

_ _

_ _ _

_

⎟ ⎠

⎞

⎜⎝

⎛

+

⎟ ⎠

⎞⎜⎝

⎛ −

=

TL

r LT

f

ZZ

ZZ v

v ………………….……………………(5.12)

voltageousinstantane _

=v

voltageforward _

f =v

voltagereflected _

r =v

For a line with open circuit at the receiving end ( _

TZ = infinity) then _ _

r f vv = and _ _

2 f vv = This illustrates possible voltage doubling effects.

Consider the example shown in figure 5.13

At the interface, A, between the overhead line and the cable there is an impedancemismatch. The incident wave incident

_

v will be transmitted through the cable dTransmitte _

v andreflected Reflected

_

v in accordance with the equations 5.11 and 5.12

x2

_

OHL

_

incident

_ _

dTransmitte

_

C

C

ZZ

Z

+

= v

v x-

_

OHL

_

reflected

_

OHL

_ _

reflected

_

C

C

ZZ

ZZ

+

⎟ ⎠

⎞⎜⎝

⎛

=

v

v

Fig.5.13 Cable and Transformer Characteristic Impedances


44/139







Page 41 of 52

The transmitted voltage dTransmitte _

v is fully reflected at the interface, B, between the

cable and the effectively open circuit transformer impedance. This process continues

between points A and B in the circuit with multiple reflections and wave distortion. TheBasic Insulation Level (BIL) of all the equipments (cables, terminations, transformer

bushings etc.,) has to be specified to match the maximum anticipated voltages. For the

above example, consider a 132 kV overhead line, cable and transformer with

incident

_

v = 830 kVZC = 10 Ω

ZOHL = 220 Ω

The surge voltage entering the cable is given by substituting in equation 5.11

x2 _ _ OHL

incident

_ _

dTransmitte

_

⎟ ⎠

⎞⎜⎝

⎛ +=C

vv

ZZZC

10220

830x10x2

+= = kV72≈

If the equipment BIL is specified as 650 kV (A standard IEC value for 145 kV rated

equipment) then the maximum voltage magnification allowed is 650/72 = 9 times. This

value will assist in the determination of protection equipment (Refer to Module 6).

Structures having a high impedance (or surge impedance) will cause the development of

extremely high potentials during the lightning strike conditions. This may in turn begreater than the phase-to-neutral insulation of the line and cause a back flashover to

the phase conductor. In order to minimize this effect the tower footing impedance is

specified to a low value. Typically this is less than 10 Ω

This is achieved by connecting the tower footing to bare counterpoise conductors laid

in the ground. A test is required to measure the tower to ground impedance by driving

an additional test rod at an appropriate distance from the tower footing and measuring

the resistance across the tower to this electrode.

The counterpoise conductors may radially project from the base of the tower. If

necessary, however, a continuous counterpoise is directly buried and connected to eachtower along the line length. Earth rods may also be used at the tower base to try to

reduce the footing impedance. National and international regulations require touch

potentials to be kept within defined limits. Since all transmission lines have slight

leakage from phase conductors to earth it is essential to ensure proper tower earthing.


45/139







Page 42 of 52

Supporting steelwork (cross-arms) on wooden pole lines may not be connected to earth

thus saving the cost of earth conductors and electrodes. The pole itself acts as the

insulator. This poses a problem when low current earth faults continue to pass through

the high resistance of the pole to ground, the protection relaying being insufficientlysensitive to cause a circuit trip. This can result in a complete burning of the pole and a

collapse of the circuit (Refer also to Appendix 4).

Auto reclose on the sending end breaker and downstream reclosers are usually provided

in distribution overhead line circuits to cover for transient faults such as lightning and

foreign objects. This is illustrated in figure 5.14

11kV OR 13.8kV

Primary Busbar

Sending end circuit breakerfitted with auto-reclose

Downstreamrecloser

Sectionaliser

Fig. 5.14 MV Overhead Feeder with Downstream Reclosers and Sectionalisers

Delegates Notes


46/139







Page 43 of 52

5.6 Load Flow Constraints in Transmission and Distribution Networks

5.6.1 Security Standards

Most utilities that generate and transmit electricity in bulk and hence are responsiblefor supply to a large number of consumers design their networks to fixed or

deterministic standards of security. The criteria for what is acceptable as a

consequence following any specific fault occurrence is most clearly dictated in these

standards. Operational standards of security for networks already constructed and

thus performing the day to day function of supplying electricity, stem from these

design standards and are essentially deterministic. I.e. they are not permitted to be

varied in accordance with the level of probability. There is, however, a tendency,

worldwide to vary the standards for economic gain in accordance with the fault

probability, this being largely weather dependent.

Unless very strong economic reasons exist, networks are thus operated so that at all

times a “loss” of any single generating unit or “Transmission Element” does not cause

(1) A Loss of supply to consumers

(2) An unacceptable change in system frequency or voltage

(3) Damage to other generating or transmission plant

Note that System Stability must be maintained since one or more of the above

consequences could otherwise result.

This is known as the single contingency and means

(1) Loss of Generation attributable to the failure of any single component (this is

normally one unit but could be more during abnormal station running)

(2) Loss of a double circuit overhead line

(3) Loss of a single circuit overhead line

(4) Loss of a single-circuit underground cable

(5) Loss of a single transformer

(6) Loss of a section of busbars

Where the loss is due to a short-circuit it is normally assumed that a single set of main

protection will also fail to operate.

Main transmission networks are designed to fulfil these standards at the anticipated

peak demand. One would imagine that this is the most stressful condition. In fact this

is seldom the case, because at times other than peak demand, circuits and generators

are our of service for a variety of reasons.


47/139







Page 44 of 52

When a circuit is out of service for a planned outage, the network must be operated so

that it can withstand the various forced outages above (often referred to as “credible”

faults)

The operation of power networks to the above standards of security can cost vast sums

of money in non-economic generation dispatch. This is one price that is paid for

security in the operational phase. SCADA/EMS systems play a very significant role in

this exercise

5.6.2 Load Flow Constraints

The current carrying capacity of any circuit is limited by the lowest rated element in

the series path.

Normally the substation terminal equipment such as cables, sealing ends, circuit

breakers, disconnectors and current transformers are arranged to have a continuousrating at least equal to the main component of the circuit. Care is required however, in

the application of seasonal or short-term circuit ratings. The terminal equipment often

has quite different time related thermal characteristics.

The power flows in interconnected networks depend upon the respective levels of

generation, position and levels of demand and the network characteristics. (Refer to

figure 5.15)

A

20 MW

120 MW

B

C

80MW

20MW

120MW100 MW

80 MW

(1)

120MW

100 MW

A

20 MW

120 MW

B

C

60MW

40MW

80 MW

(2) 20MW

A

20 MW

100 MW

B

C

52MW

28MW

120MW120 MW

80 MW

(3) 28MW

Fig 5.15 Simple Network


48/139







Page 45 of 52

Power flows in interconnected networks must be adjusted at all times to ensure that

circuits are within their continuous ratings. Furthermore, credible 'forced outages'

either of generating units or transmission circuits must also lead to unacceptable or

uncontrollable overload conditions (see Section 5.6.1 - Security Standards). Totalsystem demand and hence the demand taken at each transformer substation varies

enormously. Furthermore, the generation to feed this demand varies not only in total

output terms (i.e. matching demand) but also in its relative location. It is often

necessary therefore, to adjust generation output levels uneconomically because of

load-flow constraints.

Figure 5.16 continues with the very simple example introduced in Figure 5.15. The

Power Station at Junction 'A' (or node) has an available capacity of 220 MW and is

cheaper to run than the Power Station at node 'B' which has an available capacity of

100 MW. Economics dictate that the Power Station at node 'A' supplies all thenetwork demand of 220 MW. Each of the three feeders A-B, B-C and A-C has a rating

of 120 MW.

Figure 5.16.1 represents the 'Steady Status or 'Basic' load-flow at this ideal economic

generation dispatch. Circuits A-B and A-C are heavily loaded at 100 MW each but

within their continuous rating.

A

20 MW

220 MW

B

C

100MW

100MW

80 MW

(1) 20MW

80 MW

120 MW

A

20 MW

220 MW

B

C

80MW

120MW

(2)

120 MW

A

20 MW

220 MW

B

C

200MW

(3)

120 MW

80MW

A

20 MW

220 MW

B

C

200MW

(4)

120 MW

120MW

Fig.5.16 Simple Network (Economic Generation Level)


49/139







Page 46 of 52

However, if the security standards (refer to Section 5.6.1) are applicable to this

network dictate that it must be able to withstand a loss of any one of the three

feeders, then the ideal economic dispatch scenario is unacceptable. Loss of either A-B

or A-C feeder results in an overload condition (see Figures 5.16.3 and 5.16.4).

The minimum level of generation at point 'C' is 80 MW as shown in Figure 5.17. If an

additional circuit A-C exists with the same rating, then the economic dispatch scenario

is permissible (refer to Figure 5.18). This demonstrates the interrelation between

System Design and Operation Economics.

A

20 MW

140 MW

B

C

70MW

50MW

80 MW

(1)10MW

80 MW

120 MW

A

20 MW

140 MW

B

C

80MW

40MW(2)

120 MW

A

20 MW

140 MW

B

C

120MW(3)

120 MW

80MW

A

20 MW

140 MW

B

C

120MW

(4)

120 MW

40MW

80MW

80MW

80MW

80 MW

80 MW

80MW

Fig. 5.17 Minimum acceptable Generation Level at C

Delegates Notes


50/139







Page 47 of 52

A

20 MW

220 MW

B

C

60MW

80MW

80 MW

(1)20MW

120 MW60MW

A

20 MW

220 MW

B

C

115MW

80 MW

(2)80MW

120 MW85MW

Fig.5.18 Additional Circuit added between A and C

For the network in Figure 5.18 a planned outage on one of the feeders A-C changes the

dispatch from economic to non-economic. The simple example also demonstrates how

transmission outages can affect the cost of production.

It should be noted that outage cases for items of generation must also be considered

when constraints of load flow are being examined.

The solution of the power flows in the outage cases of Figures 5.15 - 5.18, if real power

is the only consideration, is possible without any complex analogue or mathematical

(digital) modelling. In normal networks this is not of course true and such modelling

techniques are required in order to ensure security of the network at optimum cost.

The rating of circuits is in any event current dependent and hence the load-flow

constraints depend upon examination of both real and reactive power flows.

5.6.3 Overhead Line Short-term Overload Ratings

As stated earlier the current carrying capacity is dependent upon the maximum

operating temperature of the conductors. The rise in conductor temperature is due tothe heating effect of the current flowing through them (I2R). The cooling of the

conductors is affected by the air surrounding the conductors, which also supplies the

insulation between the conductors. The larger the cross-sectional area of the

conductor the lower its resistance (R) to the flow of current and at the same time the

lower is the heating effect. It is clear that the size of conductors strung on overhead

lines is restricted by the strength of the supporting structure, the most common being

the wooden pole.


51/139







Page 48 of 52

Since the power transfer is proportional to the product of line voltage and current

√3 VL IL it is important to maintain system voltage levels at nominal or above, the limit

for power transfer of the overhead line being governed by the conductor carryingability as described.

The heating effect after a sudden change in current is carried, takes its time to

affect the rise in conductor temperature. Overhead lines thus have some time

dependent overload capacity, typically a line carrying 50% or less than its full load will

be able to carry about 30% more than its continuous rating following a sudden change

for approximately 5 minutes. The flow must then be reduced to a level less tha

over head power line

Documents