About the Step-Up Transformer Sizing
Step-up transformer sizing determines the rating, voltage ratio, impedance, and vector group of the transformer that steps up a solar plant's LT inverter output voltage (typically 415 V) to the grid interconnection voltage (11 kV, 33 kV, or higher) for export.
How to calculate it
- 1Total transformer rating (kVA) = total plant inverter AC output (kW) ÷ power factor × 1.1 overload margin.
- 2Primary voltage: match inverter output (typically 415 V LT or 690 V HT for large central inverters).
- 3Secondary voltage: match grid interconnection voltage (11 kV for most rooftop; 33 kV for 1–50 MW; 110/132/220 kV for large utility scale).
- 4Vector group: Dyn11 is the standard for solar step-up transformers — provides isolation, suppresses 3rd harmonics, and allows earthing of the secondary neutral.
- 5Impedance: 5–6% is standard; higher impedance (6.25%) reduces fault level contribution.
- 6Cooling: ONAN for ≤ 2.5 MVA; ONAF or OFAF for larger transformers.
Worked example
A 1 MWp plant with 800 kW inverter output at 0.98 PF. Required transformer rating = 800 ÷ 0.98 × 1.1 = 898 kVA → select 1,000 kVA. Voltage: 415 V / 11 kV. Vector group: Dyn11. Impedance: 5.5%. Estimated cost: ₹8–12 L for a 1 MVA distribution transformer.
What you can calculate
- MVA sizing
- IS 2026
- Impedance %
- IEC 60076
Standards & references
Frequently asked questions
Why is Dyn11 the standard vector group for solar transformers?
Dyn11 (delta primary, star-earthed secondary) is preferred because: (1) the delta winding traps 3rd harmonic currents from the inverter and prevents them from propagating to the grid; (2) the star-earthed secondary provides a neutral earthing point for protection; (3) the 30° phase shift limits circulating currents when paralleling transformers. Some DISCOMs specify Dyn1 or Ynd11 — always verify with the DISCOM's grid connection requirements before ordering.
Can I use a distribution transformer for solar step-up?
Standard distribution transformers (Dyn11, 11 kV / 415 V) are widely used for solar step-up at ratings below 2.5 MVA. For plants above 2.5 MVA, dedicated solar-duty transformers with enhanced overload capability and larger core cross-sections are recommended — solar plants generate DC-biased currents that can cause transformer heating in standard units.
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