Output of MV / LV Transformers

One of the main causes of magnetic field exposure in MV / LV electrical substations is represented by the output of the LV transformer.

As shown in Fig 2, the output is equivalent to the three sections of the conductor which are spaced out the same distance as the terminals of the transformer (D) on the transformer side. They are closer together (d) on the other side, where they form the bundle of cables directed toward the LV distribution substation. The height of the cables is a parameter that can vary depending on the installation mode. The distances on the different axis (with reference to Fig 2) when magnetic induction is equal to 3 μT (quality target) have been calculated on the basis of the nominal power and therefore on the basis of the secondary LV currents. The results are shown in Tables 1, 2 and 3 respectively for x, y and z.

It is clear from the tables that the output LV is a substantial pollutant component and that in the case of major power supplies the distances affected can be significantly more than 10 metres.

When substations are located in the vicinity of civilian, commercial or industrial settings where the quality target must be satisfied, it is necessary to implement shielding systems for almost all power supply levels.


Table 1. Distance from the centre of the system’s coordinates along the X axis to obtain 3 μT.
Rated power (kVA) Rated secondary power h=0.5 (m) h=0.6 (m) h=0.7 (m) h=0.8 (m) h=0.9 (m) h=1.0 (m)
250 361 2.47 2.69 2.9 3.09 3.26 3.43
315 455 2.77 3.03 3.25 3.47 3.68 3.86
400 577 3.13 3.41 3.68 3.92 4.14 4.36
500 722 3.49 3.81 4.11 4.38 4.64 4.88
630 909 3.91 4.28 4.61 4.92 5.22 5.49
800 1155 4.41 4.82 5.2 5.55 5.88 6.19
1000 1443 4.93 5.39 5.81 6.21 6.58 6.93
1250 1804 5.5 6.03 6.5 6.94 7.35 7.75
1600 2309 6.23 6.81 7.35 7.86 8.32 8.77
2000 2887 6.96 7.61 8.22 8.78 9.31 9.81
2500 3608 7.78 8.51 9.19 9.82 10.41 10.97
Table 2. Distance from the centre of the system’s coordinates along the Y axis to obtain 3 μT.
Rated power (kVA) Rated secondary power h=0.5 (m) h=0.6 (m) h=0.7 (m) h=0.8 (m) h=0.9 (m) h=1.0 (m)
250 361 3.1 3.14 3.16 3.2 3.23 3.26
250 361 3.1 3.14 3.16 3.2 3.23 3.26
315 455 3.54 3.57 3.6 3.63 3.67 3.69
400 577 4.1 4.13 4.16 4.19 4.22 4.25
500 722 4.65 4.68 4.7 4.73 4.77 4.79
630 909 5.27 5.3 5.32 5.35 5.39 5.41
800 1155 6.05 6.08 6.11 6.14 6.16 6.2
1000 1443 6.87 6.9 6.93 6.96 6.99 7.02
1250 1804 7.86 7.88 7.9 7.94 7.96 7.99
1600 2309 9.05 9.07 9.09 9.12 9.14 9.18
2000 2887 10.37 10.39 10.42 10.45 10.47 10.5
2500 3608 11.94 11.96 11.98 12.01 12.04 12.07

Table 3. Distance from the centre of the system’s coordinates along the Z axis to obtain 3 μT.
Rated power (kVA) Rated secondary power h=0.5 (m) h=0.6 (m) h=0.7 (m) h=0.8 (m) h=0.9 (m) h=1.0 (m)
250 361 3.26 3.36 3.47 3.59 3.7 3.82
315 455 3.72 3.83 3.95 4.07 4.21 4.33
400 577 4.29 4.41 4.54 4.68 4.81 4.96
500 722 4.86 4.99 5.14 5.28 5.43 5.59
630 909 5.51 5.66 5.81 5.97 6.14 6.32
800 1155 6.32 6.48 6.65 6.82 7.01 7.2
1000 1443 7.18 7.34 7.52 7.71 7.92 8.13
1250 1804 8.17 8.35 8.54 8.75 8.96 9.19
1600 2309 9.39 9.59 9.8 10.02 10.26 10.5
2000 2887 10.74 10.94 11.17 11.4 11.65 11.92
2500 3608 12.33 12.53 12.76 13.02 13.28 13.56
Notes:

(1) The D parameter is an average value which is not linked to any particular make of transformers.

(2)The d parameter is calculated on the basis of the diameter of the output cables.