tabela de correntes máximas
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Correntes máximas permitidas, amperes, capacidade, eletricidadeTRANSCRIPT
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".
Distance from the wall > 2 cm
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
NUMBER OF CABLES OR TRE-FOIL GROUPS
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
Note: When the separation between cables is equal to or greater than "2d", no correction is required.
• 67 •
THREE PHASE OR SINGLE CORE TRE-FOILCABLES LAID ON A WALL WITH A SEPARATIONBETWEEN CABLES EQUAL TO A DIAMETER "d".
Distance from the wall > 2 cm
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
NUMBER OF CABLES OR TRE-FOIL GROUPS
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
NUMBER OF CABLES OR TRE-FOIL GROUPS
2 3 6
1 0,93 0,90 0,87
Note: When the separation between cables is equal to or greater than "2d", no correction is required.
THREE PHASE OR SINGLE CORE TRE-FOILCABLES IN CONTACT LAID OVER STRUCTURESOR ON A WALL
1
CORRECTION FACTORS
NUMBER OF CABLES OR TRE-FOIL GROUPS
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
NUMBER OF CABLES OR TRE-FOIL GROUPS
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.