Solar Compact Substation: What Changes When the Source Is PV
A solar compact substation looks like any other from the fence line: same enclosure, same foundation, same MV cable entering from below. The difference is the duty. Instead of taking power from the utility and handing it to a building, it takes power from inverters and pushes it the other way.
That reversal changes the LV board, the protection settings, the transformer specification and the metering. Price a PV project from a conventional quotation and the gap usually appears at drawing approval, which is late enough to cost money.
What makes it a solar unit rather than a box in a field
The enclosure does not make it solar. Three things do. Power flows from the LV side to the MV side during generation. The LV source is a set of power electronic converters rather than a utility transformer, so the current is not a clean sine wave. And the load profile is a daily cycle that starts and stops, rather than a continuous one.
A PV plant inside the range we build for, up to about 10 MVA at 33 kV, is usually laid out as several inverter stations feeding one step-up unit. Either way the transformer is sized on the inverter AC rating, not the array’s DC rating: a 1.2 MWp array on a 1.0 MW inverter is a DC/AC ratio of 1.2, and the transformer sees 1.0 MW.
Reverse power flow changes the LV board
In a conventional substation the LV breaker is an outgoing device. In a solar unit it is the source breaker, and the panel it sits in is built as an incomer. That sounds like a labelling exercise and is not. The incomer has to carry the full inverter current continuously, it has to be interlocked with the inverter’s own protection, and the busbar has to be rated for a fault fed from the inverters as well as from the grid.
Current sets that rating. I = S / (√3 × V), with S in VA and V the line-to-line LV voltage. A 2,500 kVA unit at 400 V carries 2,500,000 / (1.732 × 400) = 3,608 A, and the incomer, the busbar and the cable between inverter station and transformer are all sized from that figure rather than from a downstream load list.
Protection also has to work in both directions. An overcurrent relay set for a load fed from the MV side will not see a fault fed backwards from the inverters unless the settings allow for it.
Inverter harmonics: the K-factor question
Grid-tie inverters switch in the kilohertz range, so the harmonic spectrum they inject is dominated by low-order components, typically the 5th and 7th with smaller 11th and 13th. Those harmonics add eddy current loss in the windings and stray loss in the core, and the transformer runs hotter than its rating suggests.
K-factor is the usual way to express the requirement. Each harmonic is taken in per unit of the fundamental, squared, and weighted by the square of its order:
K = Σ (Iₕ / I₁)² × h²
With a fundamental of 1.0, a 5th harmonic at 0.35 pu and a 7th at 0.15 pu: (1.0)² × 1² = 1.00, (0.35)² × 5² = 3.06, and (0.15)² × 7² = 1.10. The sum is 5.16, so the next standard rating up, K-9, covers it. Where the inverter datasheet gives a current spectrum, run the same arithmetic rather than guessing.
For a PV step-up unit the harmonics are usually modest compared with a drive-fed industrial load, so a K-rated dry type design or an oil-immersed unit with a small derating is normally enough. State the spectrum in the enquiry: a transformer designed for a sinusoidal load and one designed for a harmonic-rich load are not the same order.
The daily thermal cycle matters more than the annual peak
Generation peaks at midday, which is also when ambient is highest and the enclosure is in full sun. The transformer therefore reaches its hottest oil or winding temperature at exactly the moment the enclosure has least margin, and that coincidence sets the design ambient rather than the annual average.
The overnight half of the cycle is the other extreme. A unit de-energised each evening and re-energised each morning breathes: the air inside contracts as it cools, drawing in moisture, and the cycle repeats every day for twenty years. That is why the enclosure needs ventilation sized for the daily swing, why an oil-immersed unit needs its breather maintained, and why the LV compartment carries anti-condensation heaters.
Metering, protection and the grid interface
| Item | Conventional substation | Solar compact substation |
|---|---|---|
| Power direction | MV to LV | LV to MV in daylight |
| LV device role | Outgoing feeder | Source incomer |
| Metering | Import register | Import and export, two registers |
| Protection | Overcurrent from MV side | Directional, plus inverter interlocks |
| Duty cycle | Continuous | Daily start and stop |
The grid interface items come from the network operator, not from the supplier: voltage ride-through, reactive capability, power factor range at the point of connection and the metering arrangement. Collect them before the design is frozen, because several change the protection scheme and the instrument transformer ratings.
Frequency also has to be settled early. A 50 Hz plant in a 60 Hz market, or the reverse, changes the transformer core and winding design, and no amount of enclosure commonality covers it. State the frequency and the utility standard, and the assembly is designed to comply with them as applicable to the order.
What to send with the enquiry
- Inverter AC rating in kVA, the number of units, and the DC/AC ratio
- LV voltage and MV voltage, plus system frequency
- Inverter current harmonic spectrum, or the manufacturer and model so it can be checked
- Expected maximum ambient, altitude and whether the enclosure stands in full sun
- Whether the transformer stays energised at night or is switched daily
- Metering and protection requirements from the network operator
- IP rating, corrosion class and the enclosure family the site expects
Standard delivery is 30 days and the warranty is 2 years. Sending the inverter datasheet with the enquiry shortens the review more than any other single document, because the harmonic and current figures both come from it. For the enclosure side of the same decision, see the compact substation range and the solar and PV compact substations built for this duty, or the solar and wind step-up transformers where only the transformer is in scope. The step-up transformer guide covers the grid-tie side, and the comparison of enclosure families covers the kiosk versus pad-mounted decision that comes before it.
Questions buyers ask
What is a solar compact substation?
A prefabricated enclosure containing the step-up transformer, MV switchgear and LV panel for a PV plant, wired and tested as one package. It differs from a conventional compact substation in duty: power flows from the inverters on the LV side out to the MV network, so the LV board is built as a source rather than a load.
Does a PV plant need a special transformer?
It needs a transformer specified for the actual duty, which means the inverter harmonic spectrum, the daily load cycle and the site ambient rather than a generic distribution specification. In many cases a standard distribution transformer with the right derating is enough; the requirement is that the spectrum is stated, not assumed.
Why does the LV breaker change in a solar unit?
Because it becomes the source breaker. It must carry the full inverter current continuously, coordinate with the inverter’s own protection, and interrupt fault current fed from the inverters as well as from the grid. The busbar and the cable from the inverter station are sized from the same current, I = S / (√3 × V).
Can the transformer be switched off every night?
It can, and many plants do it to save no-load loss. Daily switching means daily energisation inrush and a breathing cycle as the unit cools and reheats, so the breather, the ventilation and the anti-condensation heaters are chosen for that pattern. A unit kept energised at night trades that for continuous no-load loss.
Which standards apply?
The transformer is designed to comply with IEC 60076 as applicable to the order, the LV assembly with IEC 61439, and the enclosure with the prefabricated substation standard the project cites. National grid codes add their own requirements at the point of connection, and those are collected before the design is frozen.
Send us the specification
Rating, primary and secondary voltage, frequency, quantity and the application. We answer within one working day.
Email info@xsdfftransformer.com · WhatsApp +852 54162620