Every solar plant connecting to a medium-voltage (MV) grid — typically above 1 MW — requires a step-up transformer that converts the inverter's low-voltage AC output (typically 400 V or 630 V) to the utility's medium voltage (6 kV to 35 kV). Selecting the wrong transformer kVA, vector group or loss class creates either a bottleneck that limits plant output or an expensive overspecification. This guide covers the transformer selection process for solar plant engineers and project developers.
Why transformers for solar are different
Solar transformers have unique duty characteristics compared to industrial transformers:
- Variable loading: Solar transformers ramp from zero at dawn to full load at noon and back to zero at dusk — a daily cycle the transformer must handle for 25+ years
- Harmonic content: Modern string inverters produce very low harmonic distortion (Total Harmonic Distortion, THD ≤ 3% for Sungrow SG-CX series), but high DC voltage (1,000–1,500 Vdc) creates requirements for specific transformer insulation design
- No magnetising inrush on load: Unlike motor transformers, solar transformers don't see mechanical load inrush — but the inverter MPPT algorithm can cause rapid load changes that require transformer impedance consideration
- Overloading in DC coupling: For DC-coupled BESS configurations, the transformer may need to handle both PV and BESS discharge simultaneously — increasing peak power above the PV nameplate rating
kVA sizing methodology
The transformer kVA rating must cover the maximum AC output power of the connected inverters with margin for losses and overloading:
Transformer kVA = ΣInverter AC kVA × 1.05 to 1.10
The 1.05–1.10 oversize factor accounts for:
- Transformer copper losses (load losses) at rated current — typically 0.5–1.5% of rated power
- Overloading during irradiance spikes (cloud edge enhancement can briefly push inverter output above nominal)
- Future capacity expansion (10% headroom prevents transformer replacement if a second phase is added)
Example: a cluster of 10 × Sungrow SG150CX inverters (150 kW AC each) requires a minimum transformer of: 10 × 150 kVA × 1.08 = 1,620 kVA. Select the next standard size: 1,600 kVA or 2,000 kVA.
Vector group selection
The transformer vector group defines the phase relationship between primary (HV) and secondary (LV) windings and how the neutral is handled. For solar plants, the most common configurations are:
| Vector group | When used | Notes |
|---|---|---|
| Dyn11 | Most European solar applications | Delta HV / Star LV with neutral. Most common for string inverters. Provides zero-sequence current path on LV side (inverter ground fault detection). |
| YNyn0 | US / North American solar | Star-neutral / Star-neutral. Common for utility applications where both HV and LV neutrals are grounded. |
| Dyn1 | China utility solar (GB/T) | Phase shift 30° between HV and LV. Required by some Chinese grid operators for harmonic cancellation in double-transformer configurations. |
For most international projects connecting at 11 kV to 33 kV MV networks, Dyn11 is the standard choice. Always confirm the required vector group with the utility's interconnection study or grid connection standards document before ordering the transformer.
Efficiency classes and losses
Transformer losses consist of two components:
- No-load losses (core losses): Constant regardless of loading — the transformer uses this power 24 hours a day even when the plant is not generating. Specified in watts at rated frequency.
- Load losses (copper losses): Proportional to current squared. At 50% loading, load losses are 25% of the full-load value.
For solar applications, no-load losses deserve special attention because solar plants are at zero or low load for 10+ hours per day (night). A high-efficiency transformer with low no-load losses pays back its premium in energy savings over the plant's life.
| IEC efficiency class | No-load loss benchmark (1,000 kVA) | Use case |
|---|---|---|
| Standard (AAA) | ~1,000 W | Standard grid infrastructure |
| High efficiency (A0/A0k) | ~700 W | Good for solar — lower night losses |
| Amorphous core (premium) | ~250 W | Best for solar — pays back in 3–5 years vs. standard |
For a 1,000 kVA transformer with 1,000 W vs. 250 W no-load losses running 8,760 hours/year at $0.10/kWh energy cost: annual saving = 6,575 kWh × $0.10 = $658/year. At a 3–5 year payback on the amorphous core premium, this is almost always economically justified.
Transformer impedance and inverter compatibility
Transformer impedance (expressed as a percentage, typically 4–6%) determines the fault current delivered during a short circuit and affects the inverter's grid voltage stability under load changes. Higher impedance reduces fault current (safer switchgear) but increases voltage drop under load. For most solar plant transformer connections, 4–6% impedance is standard; consult the inverter manufacturer's specification for acceptable impedance range.