A solar plant does not need one substation. It needs a collection system that gathers inverter output at a low voltage, steps it up, and delivers it to the grid at the voltage the utility asks for. The compact substation is where the step happens. Getting it wrong shows up as curtailment, nuisance tripping, or a grid connection that fails its commissioning tests.
Where the Substation Sits in a Solar Plant
Inverter output feeds a step-up transformer, and the medium-voltage side collects several of those transformers into a ring or radial before it reaches the point of common coupling. Depending on plant size and layout, that means either several smaller units distributed across the array, or one larger unit near the grid connection. The choice is an economics question: cable losses and trenching on one side, equipment cost and spares on the other.
For smaller commercial and industrial rooftop or ground-mount systems, a single compact unit usually covers the whole plant. Details for that case are on the solar power applications page.
What Makes Solar Different
- The load is a source. Power flows the other way, so protection coordination, reverse power and islanding detection behave differently from a normal distribution point.
- Output is intermittent and harmonic-rich. Inverters inject harmonics, and the transformer has to live with them. Winding configuration and, where needed, K-factor rating are decided at the design stage.
- The duty cycle is unusual. Peak output runs for a few hours a day, and the transformer sits at low load the rest of the time. That changes which losses matter.
- Grid codes are local. Fault ride-through, voltage regulation and reactive power requirements come from the utility, and they are not the same everywhere.
Sizing the Step-Up Transformer
The transformer rating is set by the inverter AC output, not by the panel DC nameplate. Inverters are commonly sized slightly below the array peak, so the transformer ends up smaller than a naive DC-to-kVA conversion suggests. Ambient temperature is the other input that matters: a transformer in a sealed enclosure in a hot climate is working in conditions well above the outdoor figure, and that has to be reflected in the rating or in the cooling.
Getting the voltage right matters too. The secondary has to match what the inverter produces, and the primary has to match the collection voltage. Both are fixed by your design before the transformer is quoted.
Battery and Storage Integration
Where the plant includes storage, the substation has to handle power in both directions and the protection has to be coordinated for it. This changes the switchgear arrangement and sometimes the transformer specification. It is worth raising at the enquiry stage rather than as a variation later.
What to Send With the Enquiry
- The single-line diagram up to the point of common coupling.
- Inverter AC output, quantity and locations.
- Collection voltage and the grid connection voltage.
- The grid code or utility requirements that apply.
- Site conditions: altitude, ambient temperature range, and whether the enclosure is in full sun.
With that, a proposed arrangement and a price can be produced. Without the grid requirements in particular, any quotation is a guess.
Standards and Testing
As with any project, the standard and clause set are agreed against your specification before production. Solar projects frequently carry utility-specific requirements that sit on top of the equipment standard, and those are the ones that catch people out at commissioning. Test reports travel with the equipment; the sequence we follow is described on the testing page.
Related reading: our MV switchgear selection guide.
Frequently Asked Questions
How many compact substations does a solar plant need?
It depends on plant size and layout. Several smaller units distributed across the array reduce cable losses and trenching. One larger unit near the grid connection reduces equipment count and spares. The comparison is done on your layout, not in general.
Do inverters affect the transformer specification?
Yes. Inverter output carries harmonics, and the transformer winding configuration and insulation have to be chosen for it. Where harmonic content is significant, a K-factor rated design is specified rather than a standard one.
What voltage should the transformer secondary be?
It matches the inverter AC output. That is fixed by your inverter selection and is one of the first things confirmed on the drawing.
Does ambient temperature change the rating?
It can. A transformer in a sealed enclosure in a hot climate runs hotter than the outdoor ambient suggests. Altitude does the same through reduced cooling air density. Both are inputs to the rating, not afterthoughts.
Get a Drawing and a Price
Send the single-line diagram, the load list and the site conditions – altitude, ambient range, indoor or outdoor, enclosure rating. We come back with a proposed arrangement, the standard set we would work to, and a quotation. Email info@xsdfftransformer.com or call +8615867897761.