Transformer procurement · 2 min read
Inverter-duty transformers for solar plants: what to specify
UD Fomra Group ·
An inverter-duty transformer connects solar inverters to the plant's medium-voltage collection system. It looks like a distribution transformer, but it is fed by power electronics and follows the sun's daily cycle. Those two facts change what the specification has to say.
What makes inverter duty different
Inverter output carries harmonic currents that add to winding and stray losses and to heating. The transformer should be designed for the harmonic spectrum the inverter manufacturer states, not only for the fundamental current.
Many plants connect two inverters to one transformer through two separate LV windings, so the transformer may have one MV winding and two LV windings with a vector group such as Dy11y11. Some specifications also ask for an electrostatic shield between windings. These choices come from the inverter and plant design.
Losses follow the plant's day
The transformer usually stays energised through the night while the plant produces nothing, so no-load loss is paid every hour of the year. Load loss matters during generating hours. Plants often evaluate both through loss capitalisation in the tender, which should be stated with the enquiry.
Indian sites also face high ambient temperatures and outdoor installation, frequently inside an inverter station or skid. State the ambient, altitude, enclosure and cooling assumptions explicitly.
What the specification should state
IS 2026 and IEC 60076 are the base transformer standards. IEEE C57.159 gives guidance specific to transformers in photovoltaic generation. The plant specification should translate these into clear values.
- Rating in kVA or MVA, MV voltage and the LV voltage of each winding
- Number of LV windings and the vector group
- Inverter make, model and harmonic spectrum
- Loss values or loss capitalisation basis, and efficiency requirements
- Cooling, ambient, altitude, enclosure and installation type
- Tests, inspection and documents required, and the delivery location
Sending the enquiry
UD Fomra Group's transformer range includes inverter-duty transformers from Uttam Bharat. Send the transformer schedule with the inverter datasheet so the offer reflects the actual connection.
If the inverter has not been finalised, say so and give the candidate models; the winding arrangement can be confirmed once it is fixed.