Transformerless Inverter

ALMM and inverter compliance: MNRE’s current ALMM page publishes solar PV module and cell lists, not an inverter list. Verify the inverter’s exact model documents and applicable scheme or DISCOM requirements separately.

What is a transformerless inverter

A transformerless inverter, also called TL inverter, is a solar inverter that omits the galvanic isolation transformer found in older inverter designs. Instead, the inverter uses high-frequency PWM switching, an LCL output filter and dedicated protection circuits to interface the PV array directly with the AC grid.

The transformer removal eliminates 2 to 4 percent of conversion losses (transformer copper and core losses), raising peak inverter efficiency to 97-98.6 percent for modern designs; see transformerless vs transformer-based solar inverters for a side-by-side breakdown. The inverter also becomes 30 to 50 percent lighter and 20 to 40 percent cheaper than equivalent transformer-isolated designs.

The trade-off is the absence of galvanic isolation between PV DC and AC grid. This requires Type B RCCB for fault current detection, PV insulation monitoring (Riso test on startup) and a floating (ungrounded) PV array configuration instead of conventional earthing. Modern transformerless inverters from major brands meet all relevant safety standards.

Why transformerless matters

Higher efficiency. 2 to 4 percentage points better than equivalent transformer-based designs.

Lower cost. Smaller bill of materials, less copper, less iron.

Lighter weight. Easier installation, lower shipping cost.

Smaller footprint. Wall-mount installations easier.

Industry standard. Almost all modern Indian solar inverters are transformerless.

How transformerless works

A typical transformerless inverter has these stages:

1. PV input with isolation monitor (Riso test).
2. Boost converter to raise PV voltage to DC bus level.
3. H-bridge or multi-level inverter switching at 16-50 kHz PWM.
4. LCL filter smoothing chopped output to clean sine wave.
5. Direct AC connection to grid through contactor.

Compared to transformer-isolated designs, the omitted line-frequency transformer would have provided 50 Hz galvanic isolation. Without it, the inverter’s safety design must include ground fault detection (Type B RCCB), PV insulation monitoring, and common-mode current control - the same core process our how does a solar inverter work guide covers stage by stage.

Technical deep dive

Common transformerless topologies

Required safety circuits

Efficiency comparison

For the full context on why each percentage point of efficiency matters over a 25-year system life, see solar inverter efficiency explained.

Real-world applications

Indian residential rooftop solar.

C&I rooftop solar string inverters.

Utility-scale string inverters.

Central inverters with external step-up transformer.

Hybrid inverters for solar-plus-storage, where correctly sizing the battery bank alongside the transformerless front end determines usable backup.

Advantages

Highest efficiency among inverter topologies.

Lower cost than equivalent transformer-isolated.

Lighter and smaller footprint.

Mature technology with wide market availability.

Standardised safety design.

Limitations

No galvanic isolation requires special protection.

Type B RCCB more expensive than Type A.

PV array must be floating (not directly earthed).

Common-mode current filtering required.

Cannot directly support systems requiring isolation (e.g., medical).

Common transformerless problems

Riso test failure on startup due to moisture on PV cables.

Type B RCCB nuisance tripping from accumulated capacitive leakage.

Insulation monitor false positives during heavy rain.

Ground fault detection requiring inverter shutdown for resolution. See solar inverter ground fault error troubleshooting for the full diagnostic sequence.

Best practices

Use Type B RCCB rated for solar DC leakage.

Specify minimum Riso threshold per inverter datasheet.

Use PV cable with adequate insulation rating.

Maintain dry PV junction boxes.

Verify floating PV configuration during commissioning.

Indian market context

Major brands (Sungrow, Solis, Goodwe, Growatt, Solax, Deye, Tata Power Solar) ship transformerless residential and C&I inverters, and ready-stocked units are also available through Heaven Green Energy’s solar inverter catalogue.

Utility central inverters from Sineng, Huawei, Sungrow use transformerless multi-level topology.

PM Surya Ghar residential installations use transformerless string inverters universally.

The CEA connectivity regulations and inverter safety standards such as IEC 62109 accommodate transformerless designs.

Standards and certifications

Further reading

For how Transformerless Inverter plays out in real projects, these guides go deeper:

• Solar Inverter Efficiency: 95% vs 97% vs 98%
• Transformerless vs Transformer Solar Inverters

Sources

• IEC 62109-1, -2, Inverter safety.
• IEC 61727, Utility interface characteristics.
• CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013 (as amended 2019), Central Electricity Authority.
• SMA HERIC and H5 patent documentation.
• Teodorescu R., Liserre M., Rodriguez P., Grid Converters for Photovoltaic and Wind Power Systems, Wiley-IEEE.
• NREL Power Electronics for Photovoltaic Systems.
• Manufacturer documentation from Sungrow, Solis, Goodwe.
• ITRPV Roadmap, latest edition, VDMA.
• IEA PVPS Task 14 publications.