Tabela de correntes máximas

8

TABLE OF MAXIMUMINTENSITIES

PERMISSIBLE INPERMANENT

SERVICE

8.1 INSTALLATION CONDITIONS

Tables 6 to 9 show the maximum permanent intensities admissible for copper or

aluminium conductors, with single or three core , EPR or PRC (XLPE) insulators,

installed either overhead or underground, for medium-voltage cables (1.8/3 kV to

18/30 kV). Those values given with (*) have been determined with the calculation

system published in IEC – 287 (translated to the UNE – 21144 Standard).

Consequently, it should be borne in mind that, at these intensities, the temperature

that the conductor is subjected to is 90ºC.

It should also be taken into account that these intensity

values are below the following installation conditions

UNDERGROUND CABLES

Cable laid over tubes at a depth of 0.7 metres, the

thermal resistivity of the terrain being 2.5 k·m/W and

the temperature of the terrain of 25ºC.

OVERHEAD CABLES

A circuit of three single cores in tre-foil configuration, or a

single or three core cable arranged in such a way that between

them there is efficient air replacement, an ambient

temperature of 40ºC, and they are not exposed to sunlight. For

both cases, if the conditions do not correspond exactly to the

aforementioned reference models, the corresponding

correction factors should be applied. These factors are given

on the following pages, where the most common and usual

conditions are described.

With the exception of certain coefficients, such as those that

are applicable to temperatures that are lower than the typical

ones that are given, or thermal resistivities below the one

given as typical and which increase the maximum intensity

admissible in Tables 6-9, the remaining coefficients make the

• 62 •

intensity given as maximum decrease, at times significantly, especially when

various conditions, that clearly have to be linked, play a role.

For low-voltage cables, the application will adhere to the same correction values as

indicated; although, in addition, it is highly important to take into account potential

drops, it being common to have to increase the cross-section of the conductors for

medium and long paths in order to obtain an intensity that will flow through smaller

sections.

(*) NOTE: With respect to the intensities that appear for each section,

whether the installation is overhead or underground, it is of paramount

importance to remember that the value given is the maximum

permissible permanent one, and for that intensity the conductor will be at

a temperature of 90ºC if the insulators are thermoset (XLPE, EPR) or 70ºC if

they are thermoplastic (PVC, PE). Consequently, any reducing coefficient that is

applicable to the installation, e.g. cables exposed to sunlight (coefficient 0.9), will

reduce the original maximum capacity by 10%. For various reasons, there may often

be a number of coefficients in the installation which, once applied, will cause us to

select cables with a larger section than originally planned.

On the other hand, in an economic study we must consider that when the service

temperature of a cable is increased due to the Joule effect, we are using a large

quantity of energy in kW/h in calorific form. Clearly it is better to opt for cables with

a greater section; the increased investment will be recouped in a short time.

• 63 •

8.2 MAXIMUM INTENSITIES

PERMISSIBLE IN PERMANENT

SERVICE (UNE 20460-5-523)

RATED VOLTAGE CABLES: 0.6/1 kVTABLE 6

1,5

2,5

4

6

10

16

25

35

50

70

95

120

150

185

240

300

400

500

630

SECTION NUMBER OF COPPER CONDUCTORS NUMBER OF ALUMINIUM CONDUCTORS

mm2 ONE* TWO THREE ONE* TWO THREE

OVERHEAD INSTALLATION (Air temperature: 40ºC)MAXIMUM TEMPERATURE OF CONDUCTOR: 90ºC

CABLES INSULATED WITH RETICULATED POLYETHYLENE (XLPE)

21

29

38

48

68

91

116

144

175

224

271

314

363

415

490

-

-

-

-

24

33

45

57

79

105

123

154

188

244

296

348

404

464

552

-

-

-

-

20

26,5

36

46

65

87

110

137

167

214

259

301

353

391

468

-

-

-

-

-

22

29

38

53

70

88

109

133

170

207

239

277

316

372

-

-

-

-

-

25

35

45

61

83

94

117

145

187

230

269

312

359

429

-

-

-

-

-

20

27,5

36

50

66

84

104

127

162

197

228

264

301

355

-

-

-

-

If there are any special conditions in the choice of cross-section, correction factors should be applied.

• 64 •

* Considering 3 loaded conductors

MAXIMUM INTENSITIES

PERMISSIBLE IN PERMANENT

SERVICE (UNE 20460-5-523)

RATED VOLTAGE CABLES: 0.6/1 kV

1,5

2,5

4

6

10

16

25

35

50

70

95

120

150

185

240

300

400

500

630

SECTION

TABLE 7

NUMBER OF COPPER CONDUCTORS NUMBER OF ALUMINIUM CONDUCTORS

mm2 TWO THREE THREE

UNDERGROUND INSTALLATION(Temperature of ground: 25ºC) Thermal resistivity of the ground: 2,5 K.m/W

MAXIMUM TEMPERATURE OF CONDUCTOR: 90ºCCABLES INSULATED WITH RETICULATED POLYETHYLENE (XLPE)

24,5

32,5

42

53

70

91

116

140

166

204

241

275

311

348

402

455

-

-

-

21

27,5

35

44

58

75

96

117

138

170

202

230

260

291

336

380

-

-

-

-

24,5

32,5

40

53

70

89

107

126

156

185

211

239

267

309

349

-

-

-

-

21

27,5

34

45

58

74

90

107

132

157

178

201

226

261

295

-

-

-

If there are any special conditions in the choice of cross-section, correction factors should be applied.

• 65 •

* Circuits with single core cables according to UNE 20460-5-523 the values of columns “TWO” and “THREE” can betaken depending on the number of loaded single core cables of the circuit in question

THREE

8.3 OVERHEAD CABLES

CORRECTION FACTORS

OVERHEAD CABLES AT TEMPERATURES OTHER THAN 40ºC

Correction coefficient for an ambient temperature other than 40ºC.

Cables insulatedwith PVC

TEMPERATURE 15 20 25 30 35 40 45 50 55 60

Cables insulatedwith XLPE, EPR

1,35

1,22

1,29

1,18

1,22

1,14

1,15

1,10

1,,08

1,05

1,00

1,00

0,91

0,95

0,81

0,90

0,71

0,84

-.58

0,77

OVERHEAD CABLES IN DUCTS OR GALLERIES.

THREE PHASE CABLES OR SINGLECORES IN TREFOIL INSTALLEDOVERHEAD AND IN GROUPS

Three phase or Single core cables in tre-foil

aid over continuous trays (where air

circulation is restricted), with a separation

between cables equal to a diameter "d".

Distance from the wall > 2 cm

• 66 •

It is clear that under certain installation conditions (in ducts, galleries??, etc), the

heat dissipated by the cables cannot be diffused freely, giving rise to an increase in

air temperature. Many factors depend on the size of this increase, and it must be

determined in each case. For an approximate evaluation, it must be borne in mind

that the increase in temperature is in the region of 15ºC; thus, the intensity

permitted in the conditions for this rating must be reduced with the coefficients in

the table above.

1

2

3

6

NUMBER OF TRAYS1

CORRECTION FACTORS

NUMBER OF CABLES OR TRE-FOIL GROUPS

2 3 6

0,95

0,90

0,88

0,86

0,90

0,85

0,83

0,81

0,88

0,83

0,81

0,79

0,85

0,81

0,79

0,77

THREE PHASE OR SINGLE CORE TRE-F O I L C A B L E S L A I D OV E RPERFORATED TRAYS WITH ASEPARATION BETWEEN CABLESEQUAL TO A DIAMETER "d".


Note: When the separation between cables is equal to or greater than "2d", no correction is required.

1

2

3

6

NUMBER OF TRAYS1

CORRECTION FACTORS


2 3 6 9

1

1

1

1

0,98

0,95

0,94

0,93

0,96

0,93

0,92

0,90

0,93

0,90

0,89

0,87

0,92

0,89

0,88

0,86


• 67 •

THREE PHASE OR SINGLE CORE TRE-FOILCABLES LAID ON A WALL WITH A SEPARATIONBETWEEN CABLES EQUAL TO A DIAMETER "d".


THREE PHASE ORSINGLE CORE TRE-FOILCABLES IN CONTACT ANDWITH THE WALL, LAID ONCONTINUOUS ORPERFORATED TRAYS(WHERE AIRCIRCULATION ISRESTRICTED).

1

2

3

6

NUMBER OF TRAYS

CORRECTION FCTORS


2 3 6

0,84

0,80

0,78

0,76

0,80

0,76

0,74

0,72

0,75

0,71

0,70

0,68

• 68 •

1

CORRECTION FACTORS


2 3 6

1 0,93 0,90 0,87


THREE PHASE OR SINGLE CORE TRE-FOILCABLES IN CONTACT LAID OVER STRUCTURESOR ON A WALL

1

CORRECTION FACTORS


2 3 6

0,85 0,78 0,73 0,68

BUNCHING OF THREE PHASE ORSINGLE CORE TRE-FOIL CABLES, WITHA SEPARATION OF LESS THAN ONEDIAMETER AND GREATER THAN AQUARTER OF A DIAMETER, SUPPOSINGTHAT THEY ARE INSTALLED ON APERFORATED TRAY, I.E., ALLOWING AIRTO CIRCULATE FREELY BETWEEN THECABLES.

1

2

3

Over 3

NUMBER OF TRAYS

CORRECTION FACTORS


1 2 3 >3

1,00

0,89

0,80

0,75

0,93

0,83

0,76

0,70

0,87

0,79

0,72

0,66

0,83

0,75

0,69

0,64

CABLES EXPOSED TO DIRECT SUNLIGHT.

The correction coefficient to be applied to a cable exposed to direct sunlight is

highly variable. The recommended figure is 0.90.

• 69 •

8.4 UNDERGROUND CABLES

CORRECTION FACTORS

UNDERGROUND CABLES WITH AN AMBIENT TEMPERATURE OTHERTHAN 25ºC.

Correction coefficient for ambient temperature other than 25ºC.

CABLES INSULATED WITH PVC

TEMPERATURE 10 15 20 25 30 35 40 45 50

CABLES INSULATEDWITH XLPE, EPR

1,15

1,11

1,10

1,07

1,05

1,04

1

1

0,94

0,96

0,88

0,92

0,81

0,88

0,74

0,83

0,66

0,78

CABLES BURIED DIRECTLY OR IN CONDUCTS BURIED IN TERRAINWITH A THERMAL RESISTIVITY OTHER THAN 150ºC PER cm/W.

CORRECTIONFACTOR

THERMAL RESISTIVITY OF THE GROUND ºC cm/W 80 100 120 150 200 250

1,28

1,23

1,18

1,15

1,09

1,08

1

1

0,88

0,90

0,80

0,82

UNIPOLARES

TRIPOLARES

THREE PHASE OR SINGLE CORE TRE-FOIL CABLES BUNCHEDUNDERGROUND

WITH A SEPARATION OF 7 CM(THICKNESS OF A BRICK)

LAYOUT OF CABLES 2 3 4 5 6 8 10 12

IN CONTACT

0,85

0,80

0,75

0,70

0,68

0,64

0,64

0,60

0,60

0,56

0,56

0,53

0,53

0,50

0,50

0,47

CORRECTION FACTORS

• 70 •

CABLES BURIED IN TRENCHES AT DIFFERENT DEPTHS

For a THREE PHASE OR SINGLE CORE TRE-FOIL CABLES buried directly, the

permitted intensity will be corrected by applying a correction coefficient in the

basis of the depth of the installation. In the table, the coefficients to be applied

are given depending on the installation type, of 70 cm or of 100 cm. It is

recommended that only one single core or three core cable be installed per tube.

The ratio of the tube diameter to that of the cable will be greater than or equal to

2. Whenever it is necessary to install one triad of cables per tube, the ratio

between the diameter of the tube and the apparent diameter of the triad of cables

must be greater than or equal to 2.

CABLES BURIED IN TRENCHES INSIDE SHORT TUBES OR SIMILAR

By short we refer to tubular installations with a length no longer than 15 m

(crossovers for paths, roads, etc.) In this case, no correction coefficient need be

applied.

CABLES BURIED IN TRENCHES INSIDE LONG TUBES OR SIMILAR

The correction coefficient to be applied to these cables will

depend on the type of bunching employed, and will vary for

each cable, according to whether it is placed in the centre of

the tube or on the periphery. Each case needs to be

studied individually. It is recommended that one single

core or three core cable be installed per tube. The ratio of the tube diameter to

the cable must be greater than or equal to 2. When a triad of cables needs to be

installed in a tube, the ratio of the tube diameter to the cable must be greater

than or equal to 2. As a rule of thumb, it is recommended that a correcting

coefficient of 0.8 should be applied in the case of a line with three core cable or

with three single cores in tre-foil inside the same tube. In the case of a line of

three single core cables inside their respective tubes, a correcting coefficient of

0.9 should be used.

• 71 •

Depth of installation (m)

Correction Factor

0,4

1,03

0,5

1,02

0,6

1,01

0,7

1

0,80

0,90

0,90

0,98

1,00

0,97

1,20

0,95

8.5 SHORT CIRCUITS

• 72 •

GENERAL COMMENTS

Electric networks must be capable of withstanding, while sustaining no permanent

damage, not only operating currents, but also the intense currents that are

produced under conditions of faults (short circuits) in the network itself or in the

receptors connected to it. These are short duration currents (a few seconds, at

maximum), and disappear when the protection devices that must be installed to

this end come into action. But in spite of this, their thermal effect may be very

important due to the intensity under these conditions being many times higher

than that of normal working conditions, and due to the heat production per time

unit depending on the square of the intensity of the current. In three phase

networks short circuits may be of different types, depending on which conductors

come into accidental contact. At most points of an electric system, the most

unfavourable case is a solid three phase short circuit, as this is when the highest

currents appear.

Only at points that are very close to generators or conversions with neutral rigidly

earthed can the current of a single phase or two phase fault exceed a three phase

one. In any case, the intensity of the three phase one is the easiest to calculate,

as it corresponds to a symmetrical case in the network, while in non-symmetrical

cases more complex methods are required for the calculation (symmetrical

component method, Clarke's fault analysis method, etc.)

The calculation of the current in a short circuit in general is beyond the scope of

the present work; nevertheless, in the case of a tripolar short circuit, it can be

calculated using the suitable conductor section.

THREE PHASE SHORT CIRCUIT

The current in a short circuit is necessary for determining the thermal and

mechanical loads to which the installations, and thus the cables, are going to be

subjected. In order to determine the thermal loads, the load time and the

evolution of the current must be taken into account in the most complete way

possible; the shorter the predicted duration of the phenomenon the better. The

evolution of the current depends on the permanent short circuit current at the

point being considered, on the sub-transitory, transitory and synchronic reactance

of the short circuit in question, and on the instant when they are produced.

For t values in the order of 1.5 seconds, it is sufficient to consider the current of

the permanent short circuit. The permanent symmetrical current of a THREE

PHASE short circuit can be calculated, to an effective value, on the basis of the

following formula:

This formula is shown in Figure 1 for the normal application field of our cables.

The dynamic loads are proportional to the square of the current surge in the

short circuit (peak value): for the most severe cases, this value, which is

dependent on the aforementioned reactance and on in the instant of the short

circuit, is considered to be equal to 1.8 2 Icc. Dynamic loads subject cables and

terminals to high mechanical forces. In tripolar cables these forces are absorbed

by the effect of the wiring, sheathing or armour. Single Core cables must be fixed

firmly over the entire cable run.

• 73 •

Pcc =

U =

Permanent potential of the short circuit

Nominal VOLTAGE between phases

• 74 •

300

200

100

50

40

30

20

10

5

4

3

2

11 2 3 4 5 10 20 30 4050 100 200 300 400 500 1000

GRAPH 1

PERMANENT SHORT CIRCUIT POWER Pcc (MVA)

PE

RM

AN

EN

T S

HO

RT

CIR

CU

IT P

OW

ER

Icc

(kA

)

U=0,380 kV 1

3

6

10

15

20

2530

45

65

• 75 •

GRAPH 2SHORT-CIRCUIT INTENSITY ADMISSIBLE IN THE CONDUCTORS IN VULPREN AND

HERSATENE CABLES (COPPER CONDUCTORS)

1

2

3

4

55

10

20

30

40

50

100

200

300

630 mm2

500 “

400 “

300 “

240 “

185 “

150 “

120 “

95 “

70 “

50 “

35 “

25 “

16 “

0,1 0,2 0,3 0,4 0,5 1 2 3

TIME (SEC.)

INT

EN

SIT

Y (

kA)

MAXIMUM SERVICETEMPERATURE 90ºC

MAXIMUM SHORT-CIRCUITTEMPERATURE 250ºC

• 76 •

GRAPH 3SHORT-CIRCUIT INTENSITY ADMISSIBLE IN THE CONDUCTORS IN VULPREN AND

HERSATENE CABLES (ALUMINIUM CONDUCTORS)

1

2

3

4

55

10

20

30

40

50

100

200

300

630 mm2

500 “

400 “

300 “

240 “

185 “

150 “

120 “

95 “

70 “

50 “

35 “

25 “

0,1 0,2 0,3 0,4 0,5 1 2 3

TIME (SEC.)

INT

EN

SIT

Y (

kA)

MAXIMUM SERVICETEMPERATURE 90ºC

MAXIMUM SHORT-CIRCUITTEMPERATURE 250ºC

1.

2.

3.

4.

=Icc Sc

t

• 77 •

SHORT CIRCUIT CURRENT IN THE CONDUCTOR

Graphs 2 and 3 show the short circuit intensities permitted by VULPREN and

HERSATENE cables with copper or aluminium conductors, according to the time in

seconds of the duration of the short circuit, and of the nominal section of the

conductor. These intensities have been calculated supposing that:

The phenomenon has a limited duration.

The temperature prior to the short circuit is the maximum admissible

under a permanent regime for each type of insulation.

The temperature at the end of the short circuit is the maximum admissible

for the insulation for this regime.

All heat that is generated is accumulated in the conductor mass, thus

increasing its temperature, and consequently that which is transmitted to

the exterior is null (an adiabatic process). Under these conditions the

following formula can be applied:

C

ON

DU

CT

OR

INSULATION

C VALUES

EPR Y XLPE

Cu

Al

141,8

92,8

Icc =

S =

t =

C =

Short circuit current admissible, in A

Conductor section in mm2

Duration of the short circuit, in seconds.

Coefficient that depends on the nature of the conductor and the

temperatures at the onset and the conclusion of the short circuit.

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Tabela de correntes máximas