Protection for the supply circuit of a LV/LV transformer
From Electrical Installation Guide
The protective device on the supply circuit for a LV/LV transformer must avoid the possibility of incorrect operation due to the magnetizing inrush current surge, noted above.It is necessary to use therefore:
- Selective (i.e. slighly time-delayed) circuit-breakers of the type Compact NS STR (see Fig. N32) or
- Circuit-breakers having a very high magnetic-trip setting, of the types Compact NS or Multi 9 curve D (see Fig. N33)
Fig N32: Tripping characteristic of a Compact NS type STR (electronic)
Fig N33: Tripping characteristic of a Multi 9 curve D
Example
A 400 V 3-phase circuit is supplying a 125 kVA 400/230 V transformer (In = 180 A) for which the first inrush current peak can reach 12 In, i.e. 12 x 180 = 2,160 A.
This current peak corresponds to a rms value of 1,530 A.
A compact NS 250N circuit-breaker with Ir setting of 200 A and Im setting at 8 x Ir would therefore be a suitable protective device.
A particular case: Overload protection installed at the secondary side of the transformer (see Fig. N34)
Fig N34: Example
An advantage of overload protection located on the secondary side is that the short-circuit protection on the primary side can be set at a high value, or alternatively a circuit-breaker type MA (magnetic only) can be used. The primary side short-circuit protection setting must, however, be sufficiently sensitive to ensure its operation in the event of a short-circuit occuring on the secondary side of the transformer.
Note: The primary protection is sometimes provided by fuses, type aM. This practice has two disadvantages:
- The fuses must be largely oversized (at least 4 times the nominal full-load rated current of the transformer)
- In order to provide isolating facilities on the primary side, either a load-break switch or a contactor must be associated with the fuses.
