Solar Step-up Transformer: Inverter LV to MV Grid Connection

Definition

A step-up transformer raises the low-voltage (typically 400–800 V AC) output of utility-scale solar inverters to medium-voltage (10–35 kV) for the plant's collection grid. Critical component in utility-scale plant electrical infrastructure.

Key Takeaways

Step-up transformer raises inverter LV to MV.
Typical: 0.69/33 kV or 0.8/13.8 kV.
Critical for utility-scale plant collection grid.
Specified per IEC 60076 / IS 2026.
One per central inverter; or one per group of string inverters.

Where the Step-up Transformer Sits in the Plant

On a utility-scale ground-mount plant, each central inverter converts DC from the array into three-phase AC at a relatively low voltage — commonly 690 V or 800 V — because that’s the practical output level for inverter power electronics. Carrying that current any real distance at LV would mean thick, expensive cabling and significant I²R losses, so the plant’s step-up transformer sits immediately downstream of the inverter’s AC disconnect, often on the same skid, and raises the voltage to the 10–35 kV level used for internal collection feeders. From there, multiple transformer outputs are gathered on the plant’s medium-voltage ring and routed to the site substation, where a separate main power transformer steps up again for grid evacuation. This chain of voltage levels — inverter LV, step-up transformer MV, substation HV — is exactly what a plant’s three-line diagram is drawn to document, and it’s the section of the drawing set reviewers and utilities such as PGCIL scrutinize most closely during interconnection approval.

Sizing, Ratios, and Standards

Voltage ratio is set by matching the inverter’s AC output to the plant’s chosen collection-grid standard — 0.69/33 kV and 0.8/13.8 kV are the two ratios most commonly specified in Indian and international utility-scale designs. Transformer kVA rating is sized to the inverter’s continuous AC output plus a margin for ambient derating, and units are typically specified per IEC 60076 (or the equivalent IS 2026 in India) covering insulation class, temperature rise, and factory acceptance testing. On plants built around central inverters, it’s standard to dedicate one step-up transformer per inverter; plants using distributed string inverters more often group several units onto a shared transformer and switchgear station to keep the electrical balance-of-plant simpler to design and maintain.

Step-up transformer sizing rarely gets decided in isolation — it’s one line item in the broader electrical and civil scope of a utility-scale plant. Our guide to open access solar in India covers how plants using this same LV-to-MV-to-grid chain structure their power evacuation and commercial arrangements, and the PVsyst tracker yield study methodology walkthrough shows how the same MW-scale layouts that drive transformer sizing get modeled for yield before construction. For teams benchmarking an operating ground-mount build against this same inverter-to-transformer-to-substation architecture, Heaven Green Energy’s ground-mount solar park EPC work is a useful reference point.

Frequently Asked Questions

5 commonly searched questions about Step-up Transformer.

What does a step-up transformer do in a solar plant?

It raises the low-voltage AC output of utility-scale inverters — typically 400–800 V — to a medium voltage of 10–35 kV so the power can be carried on the plant's internal collection grid without excessive current and line losses.

What are typical step-up transformer voltage ratios?

Common ratios seen on utility-scale sites are 0.69/33 kV and 0.8/13.8 kV, matched to the inverter's AC output voltage and the plant's chosen MV collection-grid standard.

Is there one step-up transformer per inverter?

Usually yes for central inverters — one dedicated transformer per unit, often packaged on the same skid. Sites using string inverters more commonly group several inverters onto a shared step-up transformer at a combiner/switchgear station.

What standards govern step-up transformer design?

Utility-scale solar step-up transformers are typically specified per IEC 60076 (or the equivalent IS 2026 in India), covering rating, insulation, temperature rise, and testing requirements.

How does the step-up transformer relate to grid evacuation?

After the step-up transformer raises inverter output to medium voltage, that power feeds the plant's MV collection grid and ultimately the main power transformer at the substation before evacuation to the transmission network.