A rooftop PV installation at a port, feeding back into an existing 11 kV network. The grading problem runs the opposite way to a normal industrial study, and the original brief had it the wrong way round.
The Brief
- A new 2500 kVA transformer circuit serving two sheds, fed from a spare 11 kV breaker on an existing substation panel.
- Four devices in the chain: a numerical relay on the 11 kV breaker, a 4000 A incoming air circuit breaker, and two outgoing ACBs.
- Two coordination paths, from the 11 kV relay through the incomer to each outgoing breaker.
How It Was Done
- The critical correction was direction. The outgoing breakers are generation infeed, not load, so the fault current through them runs backwards relative to a conventional study. That inverts the grading problem.
- Fault levels were established to IEC 60909 at every point in the chain, sufficient to inform settings rather than as a standalone deliverable.
- Settings were checked against each relay and trip unit’s own achievable ranges, not against idealised curves, so every recommended value can actually be entered on site.
What Came Out Of It
- A settings schedule for all four devices with time-current curves proving the margins hold on both paths.
- Three limitations that would otherwise have been discovered at commissioning were written into the report body: the absence of an earth fault element in the LV trip units, a self-protection caveat on the incomer, and a trade-off on the incoming rating.
- Upstream coordination was folded in at the same revision when it became clear the study would be read as if it covered it.
Standards applied
IEC 60909 · IEC 60255
Published without naming the client. The technical detail is exactly as delivered.
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