Solar PV

The step-up stage of a PV plant

The output of a PV plant follows the sunlight: low early and late in the day, zero at night, while the step-up transformer stays connected to the grid around the clock. For a plant with limited annual full-load hours, no-load loss takes a clearly larger share of site losses than in conventional industrial supply.

Project types also differ widely. Ground-mounted plants place a step-up unit in each array block, on open but dusty sites with wide temperature swings; rooftop and industrial-park projects are limited by roof loading and ground space, and the connection point often has to fit into a very small area. Either way the step-up units are numerous and widely spread, with limited staff to maintain them.

What PV projects have to cope with

The cost per kilowatt-hour of a PV project is sensitive to site losses, and once equipment is on a roof or between the arrays, replacing it is expensive.

• Zero output at night and in overcast weather while the equipment stays energised, so no-load loss counts all year
• Harmonic content on the inverter side, affecting power quality at the connection point
• Many array blocks and step-up units, installed within a concentrated schedule
• Dust and temperature swings on ground-mounted sites; limited space and roof loading on distributed ones
• Largely unattended plants, calling for fully sealed equipment with long maintenance-free intervals

The selection logic for the step-up stage is straightforward: look at no-load loss first, then at size and method of installation, and last at whether the unit can stay closed for twenty years.

Coreda's approach for solar PV

We configure the step-up stage separately for ground-mounted and distributed projects, around a combination of amorphous-alloy transformers and prefabricated substations.

Recommended configuration:

• Chinese-style box-type substation: suited to the MV step-up node of a PV plant, with independent MV and LV rooms that open for maintenance, an enclosure protected against corrosion, condensation and salt fog, and space for an automation terminal reporting to the control centre.
• ZGS11 compact pad-mounted substation (American style): small and compact, with a fully sealed tank and fully insulated design, suitable for both ring-main and terminal supply — a good fit for distributed connection points on tight sites.
• SBH25 amorphous-alloy oil-immersed transformer: wound amorphous-alloy core in a fully sealed corrugated tank (ONAN), no-load loss roughly 1/3 to 1/4 of a silicon-steel S13 unit of the same rating, good immunity to harmonics and noise about 5–10 dB(A) lower.

Ground-mounted plants take box-type substations delivered block by block, distributed projects use the compact pad-mounted transformer to shrink the connection point; both rely on fully sealed construction to reduce site maintenance.