Transformers for photovoltaic plants: why 0.8 kV and what else to specify

Utility-scale and large rooftop PV plants use central or string inverters with an AC output of 800 V (sometimes 690 V). Connecting them to the medium-voltage grid through a standard 0.42 kV transformer is possible only with a step-up stage in between – expensive and lossy. The right tool is a distribution transformer with a 0.8 kV low-voltage winding, which both PNACHO series offer from 630 kVA upwards.

Why the LV winding must match the inverter

Inverters are current-limited devices. At 800 V a 1 000 kW inverter delivers about 720 A; at 400 V it would need 1 440 A – twice the cable cross-section, twice the busbar cost and four times the I²R losses in every metre of LV cable. Manufacturers therefore design large inverters for 800 V and the transformer has to follow.

How to size the transformer for a PV plant

• Rated power = AC output of the inverters, not the DC peak power of the modules. A 1.2 MWp field with 1 000 kW of inverters needs a 1 000 kVA transformer.
• Power factor: grid codes require the plant to supply reactive power (cos φ 0.9 – 0.95 inductive or capacitive). Size the transformer for the apparent power at the required cos φ, e.g. 1 000 kW / 0.95 = 1 050 kVA → 1 250 kVA unit, or accept short overloads.
• Daily cycle: a PV transformer runs at full load for a few hours around noon and idles at night. Low no-load losses matter more here than anywhere else – overnight the transformer is a pure cost.

Vector group and grid operator requirements

Most European distribution operators accept Dyn11 for generation plants; some networks (parts of Poland, Germany and Austria) specify Dyn5. The vector group has no influence on the transformer's efficiency – it is purely a phase-shift convention that must match the existing network when transformers are paralleled or protection is coordinated. PNACHO transformers are supplied as Dyn11 as standard and as Dyn5 on request at the same price.

Harmonics and inrush

Modern inverters comply with IEC 61000 limits and produce low harmonic distortion, so a standard transformer does not need de-rating. Two points still deserve attention:

1. Inrush at morning start-up: if the MV switch closes with the transformer unloaded, inrush can reach 8–12 × rated current for a few cycles. Set the MV protection with an appropriate inrush restraint, or energise from the MV side before the inverters wake up.
2. Electrostatic screen: a screen between HV and LV windings reduces the transfer of high-frequency noise from the inverters to the MV network. It is a standard option on PNACHO PV versions.

Which unit to choose

For open-field plants the hermetically sealed PNACHO-O 1000 kVA 21/0.8 kV is the usual choice – outdoor rated, low no-load losses, no maintenance. For plants on industrial roofs where the transformer sits inside the building, the cast-resin PNACHO-D 1000 kVA 21/0.8 kV avoids any oil in the building. All PV variants are listed under Products; filter by 0.8 kV low voltage.