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
Ultra-rapid circuit breaker
As installed power increases, electrical distribution has to shift from a LV design to a HV design. Indeed, a high short-circuit level can be a threat to the installation and make impossible the selection of low voltage equipments (Switchboard and bus bars, circuit breaker…)
These situations could be met in the following applications:
Bus bars coupling onboard merchant vessels, off shore platform, loop networks (in industry), where the current and energy are important because of the installed power (several transformers or generators in parallel) and HV design not easy.
Two solutions could be used:
