flat,trofoil & triplex

8/10/2019 Flat,Trofoil & Triplex

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Cross-bonding

A system is cross-bonded if the arrangements are such that the circuit provides electrically continuous sheath runs fromearthed termination to earthed termination but with the sheaths so sectionalized and cross-connected in order to eliminatethe sheath circulating currents. In such case, avoltage will be induced between screen and earth, but no significant current will flow. The maximum induced voltage will

appear at the link boxes for cross-bonding.This method permits a cable current-carrying capacity as high as with single-point bonding but longer route lengths thanthe latter. It requires screen separation and additional link boxes.

XLPE cable system configurations

Trefoil and flat formation

Trefoil or flat formation of three-phase XLPE cable?

The three cables in a 3-phase circuit can be placed in di ffe ren t fo rmat ions . Typical formations include t refoil( tr iangular) and f l a t fo rmat ions . The choice depends on several factors like screen bonding method, conductor area andavailable space for installation.

Bonding of the metallic screens

The electric power losses in a cable circuit are dependent on the currents flowing in the metallic sheaths of the cables.Therefore, by reducing or eliminating the metallic sheath currents through different methods of bonding, it is possible toincresase the load current carrying capacity (ampacity) of the cable circuit.

The usua l bond ing method s a re descr ibed be low: Both-ends bonding

A system is both ends bonded if the arrangements are such that the cable sheaths provide path for circulating currents atnormal conditions. This will cause losses in the screen, which reduce the cable current carrying capacity. These losses aresmaller for cables in trefoil formation than in flat formation with separation.

Single-point bonding

A system is single point bonded if the arrangements are such that the cable sheaths provide no path for the flow ofcirculating currents or external fault currents. In such case, a voltage will be induced between screens of adjacent phasesof the cable circuit and between screen and earth, but no current will flow. This induced voltage is proportional to the cablelength and current.

Single-point bonding can only be used for limited route lengths, but in general the accepted screen voltage potential limitsthe length.


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Cross-bonding

A system is cross-bonded if the arrangements are such that the circuit provides electrically continuous sheath runs fromearthed termination to earthed termination but with the sheaths so sectionalized and cross-connected in order to eliminatethe sheath circulating currents. In such case, avoltage will be induced between screen and earth, but no significant current will flow. The maximum induced voltage will

appear at the link boxes for cross-bonding.This method permits a cable current-carrying capacity as high as with single-point bonding but longer route lengths thanthe latter. It requires screen separation and additional link boxes.

o Static field limits

132 kV

132 kV overhead lines are usually carried on lattice steel pylons, but smaller than used for 275 kV and 400 kV

lines. Sometimes they are carried on wood poles.

Magnetic field

The maximum field is produced by the largest design of line the L7 when the ground clearance is the

minimum allowed 7.0 m and the loads are the highest allowed 1.4 kA in each circuit. The field also

depends on the phasing. 132 kV lines usually have Untransposed (U) phasing.
http://www.emfs.info/static-fields/static-limits/http://www.emfs.info/static-fields/static-limits/


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Typical fields are lower than the maximum field because the clearance is usually higher and the loads are

usually lower. The three curves shown here are for typical loads, the normal U phasing, and three different line

designs: L7 (the highest), a smaller pylon design, the L132, and a wood-pole design (the lowest field).

This table gives some actual field values for the same conditions.

magnetic field in T at distance from

centreline

maximum

under line

10 m 25 m 50 m 100 m

132

kV

largest

lines

L7

twin

bundles

0.305 m

lynx

maximum

clearance

7 m

phasing U

load

1.4/1.4 kA

30.445 20.532 5.553 1.528 0.392

typical clearance 1.848 1.359 0.468 0.138 0.036


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centreline

maximum

under line

10 m 25 m 50 m 100 m

10 m

phasing U

load

0.13/0.13

smaller

lines

L132

single

conductors

0.4 sq in

maximum

clearance

7 m

phasing U

load

1.2/1.2 kA

24.585 17.217 4.587 1.247 0.318

typical

clearance

10 m

phasing U

load

0.13/0.13

kA

1.731 1.317 0.451 0.132 0.034

smallest

wood-

pole

trident maximum

clearance

7 m

single

12.347 3.633 0.738 0.192 0.048


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centreline

maximum

under line

10 m 25 m 50 m 100 m

design 150 m span

single

conductors

lynx

circuit

load 0.7

kA

typical

clearance

10 m

single

circuit

load 0.1

kA

1.764 0.385 0.099 0.027 0.007

Note:

1. All fields calculated at 1 m above ground level.

2. All fields are given to the same resolution for simplicity of presentation (1 nT = 0.001 T) but are not accurate

to better than a few percent.

3. Calculations ignore zero-sequence current . This means values at larger distances are probably

underestimates, but this is unlikely to amount to more than a few percent and less closer to the line.

4. The maximum field under the line is the largest field, which is not necessarily on the route centreline; it is

often under one of the conductor bundles.

5. Sometimes, a 132 kV circuit could be carried on a line designed for 275 kV or 400 kV. Then the magnetic

fields could be larger than shown here.
http://www.emfs.info/sources/overhead/factors/balance-within/http://www.emfs.info/sources/overhead/factors/balance-within/http://www.emfs.info/sources/overhead/factors/balance-within/http://www.emfs.info/sources/overhead/factors/balance-within/


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Electric field

The maximum field is produced by the largest design of line the L7 when the ground clearance is the

minimum allowed 7.0 m. The field also depends on the phasing. 132 kV lines usually have Untransposed (U)

phasing.

Typical fields are lower than the maximum field because the clearance is usually higher. The three curves

shown here are for the normal U phasing, and three different line designs: L7 (the highest), a smaller pylon

design, the L132, and a wood-pole design (the lowest field).



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electric field in V m -1 at distance from

centreline

maximum

under line

10 m 25 m 50 m 100 m

132

kV

largest

lines

L7

twin

bundles

0.305 m

lynx

maximum

clearance

7 m

phasing

U

3615 913 182 81 23

typical

clearance

10 m

phasing

U

2372 890 103 72 23

smaller

lines

L132

single

conductors

0.4 sq in

maximum

clearance

7 m

phasing

U

2628 697 154 66 19

typical

clearance

10 m

phasing

U

1780 689 86 59 18

smallest

wood-pole

trident maximumclearance

7 m1174 588 73 11 2


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electric field in V m -1 at distance from

centreline

maximum

under line

10 m 25 m 50 m 100 m

design 150 m span

single

conductors

lynx

single

circuit

typical

clearance

10 m

single

circuit

583 458 89 15 3

Note:

1. All fields calculated at 1 m above ground level.

2. All electric fields are calculated for the nominal voltage. In practice, voltages (and hence fields) may rise by

a few percent.

3. All electric fields calculated here are unperturbed values.

4. All fields are given to the same resolution for simplicity of presentation (1 V/m) but are not accurate to better

than a few percent.

5. Calculations ignore zero-sequence voltages. This means values at larger distances are probably

underestimates, but this is unlikely to amount to more than a few percent and less closer to the line.

6. The maximum field under the line is the largest field, which is not necessarily on the route centreline; it is

often under one of the conductor bundles.

7. Sometimes, a 132 kV circuit could be carried on a line designed for 275 kV or 400 kV. Then the electric

fields could be larger than shown here.

Underground cables


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Two main types of 132 kV underground cable are used.

separate cores: the three conductors for the three phases are laid separately but close together in the

ground, typically 1 m deep

single cable: the three cores are twisted round each other in a single outer sheath.

With a single cable, because the cores are so close together and twisted, the fields they produce directly are

very small. Instead, the field comes from any net current in the sheath. This cannot be predicted accurately.

The following graph shows typical fields for these two types of cable (separate cores produce higher fields

close to the cable but lower fields away from it).

Underground cables do not produce any external electric fields.



centreline

0 m 5 m 10 m 20 m

132 separate cores (flat 0.3 m typical 9.62 1.31 0.36 0.09


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kV formation) spacing

1 m depth

single cable 1 m depth typical 5.01 1.78 0.94 0.47

Notes

1. All fields calculated at 1 m above ground level

2. All fields are given to the same resolution for simplicity of presentation (0.01 T = 10 nT) but are not accurate

to better than a few percent.

3. Calculations for separate cores ignore zero-sequence current . This means values at larger distances areprobably underestimates, but this is unlikely to amount to more than a few percent.

4. Cable designs are not standardised to the same extent as overhead lines and the examples given here are

representative.

5. In practice, there are often several cables nearby, and the fields interact with each other.
http://www.emfs.info/sources/overhead/factors/balance-within/http://www.emfs.info/sources/overhead/factors/balance-within/http://www.emfs.info/sources/overhead/factors/balance-within/http://www.emfs.info/sources/overhead/factors/balance-within/

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