Transformer Selection and Energisation Pressure on a PV Project

Annual loss energy and cost. Load loss scales with the square of the load factor. Option A (1,250 kVA): load factor = 900/1,250 = 0.72; (0.72)² = 0.518.

• No-load: 1.6 kW × 8,760 = 14,016 kWh
• Load: 11 × 0.518 = 5.70 kW → 5.70 × 8,760 = 49,950 kWh
• Total 63,966 kWh × 0.12 = EUR 7,676 per year

Option B (1,600 kVA): load factor = 900/1,600 = 0.5625; square = 0.316.

• No-load: 1.1 × 8,760 = 9,636 kWh
• Load: 12 × 0.316 = 3.80 kW → 33,262 kWh
• Total 42,898 kWh × 0.12 = EUR 5,148 per year

Annual saving from Option B: EUR 2,528.

Payback and NPV. Extra capital cost = 52,000 − 38,000 = EUR 14,000. Simple payback = 14,000 / 2,528 = 5.5 years. Annuity factor at 6% over 15 years = (1 − 1.06⁻¹⁵)/0.06 = 9.71. PV of savings = 2,528 × 9.71 = EUR 24,550. NPV = 24,550 − 14,000 = EUR +10,550 → Option B is the better economic choice, and it also runs at 56% loading, which extends insulation life and leaves room for expansion.

The check that could override this: fault level. Option B at 6% impedance gives LV bolted fault = 2,133 / 0.06 = 35.6 kA from the transformer alone. Add motor contribution: 400 kW of motors draws about 620 A, contributing roughly 4–5 times that in the first cycles, around 2.5–3 kA. Total ≈ 38–39 kA, which exceeds the 36 kA switchboard rating.

Resolution: specify the 1,600 kVA unit with impedance of at least 7.5%, giving 2,133 / 0.075 = 28.4 kA plus motor contribution ≈ 31 kA, safely within 36 kA. If that carries a premium of, say, EUR 2,500, the NPV falls to about EUR 8,000 and Option B is still justified - but the fault study, not the energy calculation, decides the specification.

The request inverts the purpose of a witness test. The witness test is the point at which the evidence is examined; energising first and supplying evidence later means the plant operates with unverified safety functions.

Conditions that are unmet:

1. Anti-islanding. The inverter is a different model from the approved one, and a letter of comfort is not an accredited type test. Islanding detection time and the non-detection zone are unknown for the installed device. This is a line-worker and public safety function, not a paperwork item.
2. Design verification. A model change invalidates the approved single-line diagram, protection settings, voltage and frequency ride-through, power factor and ramp-rate settings. These must be re-submitted and re-approved by the network operator.
3. Protection testing. Unsigned relay test sheets mean primary or secondary injection, trip-circuit testing and breaker trip times have not been formally verified. Energising with unproven protection exposes the transformer and switchboard.
4. Grid connection approval. Connection is a legal condition imposed by the network operator, not a contractual nicety the parties can waive between themselves.

Why not energise and swap later: the interval between energisation and evidence is exactly when the unverified function would have to work. An anti-islanding failure during that interval is an electrocution risk for anyone working on the feeder.

Alternative sequence that makes progress:

• Complete all commissioning that does not require grid energisation: insulation resistance and polarity of strings, inverter DC-side checks, earthing continuity, functional tests of the LV board's own protection, and the witness inspection of documentation.
• Energise the LV board only, with the PV DC isolators open and locked off, if the network operator grants partial energisation approval in writing.
• Ask the manufacturer for the certificate covering the actual model; most have it and the submission is simply out of date.
• If no certificate exists, obtain the network operator's written acceptance of an alternative evidence route, or fit the approved model.
• Re-plan the COD with the corrected dates and treat the delay as a contractor non-conformance.

Response. I would reply in writing, the same day, copied to the client's authorised representative and the EPC engineering manager: I am not able to certify or authorise energisation, and I am not able to sign the protection test sheets, because the anti-islanding function is unverified, the model change is unapproved by the network operator, and the protection tests are incomplete. I would state that I will support any schedule recovery that does not require signing an untrue certificate.

Professional principles:

• Public safety is paramount. Anti-islanding protects people working on a network that is supposed to be dead. That obligation outranks a commercial operation date.
• Competence and honesty in certification. A signature on an energisation certificate states that specified tests were performed and passed. Signing an untrue statement is a false certification, regardless of who asks for it.
• Respect for the law and the network operator's conditions. The grid code requirement is not a formality the engineer can waive; only the network operator can accept an alternative.
• Objectivity in the record. Report the technical facts in writing, not the personalities. The email trail should show the specific unmet conditions, the risks, the alternatives offered and the dates.

Practical steps:

1. Issue a non-conformance report against the contractor's submission, with the missing certificate and unsigned test sheets as the evidence.
2. Put the request and my refusal in writing, with the risk stated in plain terms: an islanding event energising a feeder believed dead.
3. Escalate to the client's authorised representative, not just the project manager, and copy the network operator if the instruction is repeated.
4. Record the schedule impact as a consequence of the non-conforming submission, with the dates on which the missing documents were requested.
5. Keep a personal contemporaneous record of instructions received and my responses.

On the hint about blame: I would decline to negotiate the certificate and offer instead to attend the witness meeting, produce a listed punch list with dates, and confirm in writing what is required for a compliant energisation and by when. If the instruction is repeated in a way that implies the certificate will be issued regardless, I would say plainly that I will not be issuing it and that the matter should be put to the client's representative and the professional body if there is disagreement.