Selectivity MV/LV in a consumer’s substation

In general the transformer in a consumer’s substation is protected by MV fuses, suitably rated to match the transformer, in accordance with the principles laid down in IEC 60787 and IEC 60420, by following the advice of the fuse manufacturer.
The basic requirement is that a MV fuse will not operate for LV faults occurring downstream of the transformer LV circuit-breaker, so that the tripping characteristic curve of the latter must be to the left of that of the MV fuse pre-arcing curve.
This requirement generally fixes the maximum settings for the LV circuit-breaker protection:

• Maximum short-circuit current-level setting of the magnetic tripping element
• Maximum time-delay allowable for the short-circuit current tripping element

(see Fig. H57)

Fig. H57: Example

Example:

• Short-circuit level at MV terminals of transformer: 250 MVA
• Transformer MV/LV: 1,250 kVA 20/0.4 kV
• MV fuses: 63 A
• Cabling, transformer - LV circuit-breaker: 10 metres single-core cables
• LV circuit-breaker: Compact NSX 2000 set at 1,800 A (Ir)

What is the maximum short-circuit trip current setting and its maximum time delay allowable?
The curves of Figure H58 show that discrimination is assured if the short-time delay tripping unit of the CB is set at:

• A level ≤ 6 Ir = 10.8 kA
• A time-delay setting of step 1 or 2

Fig. H58: Curves of MV fuses and LV circuit-breaker