132 kV underground cables: magnetic
field
see also:
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.
This table gives some actual field values for the same conditions.
| |
|
|
|
magnetic field in µT at distance from centreline |
|
0 m |
5 m |
10 m |
20 m |
|
132 kV |
separate cores |
0.3 m spacing
1 m depth |
typical |
9.62 |
1.31 |
0.36 |
0.09 |
|
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 µT = 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 are probably 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.
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