About the Transformer Sizing
Transformer sizing determines the kVA rating of the step-up transformer that connects a solar plant to the HT grid. It is driven by the plant's AC output, a margin for future expansion and losses, and the standard rating steps and loss limits defined in IS 1180 for distribution transformers.
Formula
Transformer kVA ≥ Plant AC capacity (kW) ÷ PF × (1 + margin)
- Plant AC capacity
- Combined inverter AC output (kW)
- PF
- Power factor at the interconnection (≈1.0 for grid-tied inverters)
- margin
- Head-room for losses/expansion, typ. 10–25%
How to calculate it
- 1Sum the inverters' rated AC output to get the plant AC capacity.
- 2Divide by the power factor at the point of interconnection (near unity for modern inverters).
- 3Add a margin (typically 10–25%) for transformer losses and future expansion.
- 4Round up to the next standard kVA rating and verify no-load and load-loss limits and ONAN cooling per IS 1180.
Worked example
A 1 MW (1,000 kW) plant at unity power factor needs at least 1,000 kVA; adding a 20% margin gives 1,200 kVA, so you select the next standard 1,250 kVA ONAN transformer.
What you can calculate
- kVA sizing
- ONAN cooling
- IS 1180
- Loss calculation
Standards & references
Frequently asked questions
What transformer kVA do I need for a 1 MW solar plant?
Around 1,250 kVA. The plant delivers ~1,000 kW at near-unity power factor, and after a typical 20–25% margin for losses and future head-room you round up to the next standard rating — usually 1,250 kVA with ONAN cooling.
What does ONAN mean on a solar transformer?
ONAN = Oil Natural, Air Natural — the transformer is cooled by natural oil circulation and natural air convection, with no pumps or fans. It is the standard, maintenance-light cooling class for distribution transformers at solar plant HT connections under IS 1180.
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