Solar power plants connect to the medium-voltage grid through several switching points. Ring main units (RMUs) play a central role in that chain. In this guide, we explain how RMUs work in photovoltaic (PV) plants. We also cover typical configurations and selection factors.

1. Where RMUs Fit in a Solar Plant

A typical utility-scale PV plant has several voltage levels. PV strings feed DC combiner boxes. Inverters then convert DC power to AC. A medium-voltage collection network then steps up to the grid. For example, this collection network usually runs in rings or strings. An RMU protects and switches each substation. The RMU lets operators isolate a faulty feeder. It also lets them connect inverters in groups. Meanwhile, the rest of the plant stays online.

2. Typical Ring Main Unit Configurations in PV Plants

• Ring-in, ring-out units with a transformer protection way using a fuse switch combination or vacuum circuit breaker
• Single-incoming configurations for string substations that connect one inverter group
• Units with cable entries sized for MV cables from inverter stations, with earthing switches for safe isolation
• Gas-insulated or solid-insulated units where space is tight, such as inside containerized substations

For compact projects, the GSRM6 Gas Insulated Ring Main Unit is a popular choice because of its small footprint.

3. Key Selection Factors for Solar Projects

1. Voltage class: match the RMU rating (typically 12 kV, 24 kV or 40.5 kV) to the plant collection voltage
2. Fault levels: confirm the short-circuit making and breaking capacity at the connection point
3. Protection: choose fuse-switch combinations for transformer protection or vacuum circuit breakers where reclosing or more sophisticated protection is needed
4. Environment: outdoor and containerized sites need high enclosure ratings and sealed units in dusty or coastal areas
5. Scalability: choose modular configurations so additional inverter groups can be added later

4. Standards and Compliance

RMUs in PV plants must comply with the same switchgear standards as other MV installations. IEC 62271-200 is the leading standard for metal-enclosed switchgear. Verify that the manufacturer supplies type test reports. Also check that the units suit your earthing system.

In practice, many EPC contractors standardize on one RMU model across a plant. This simplifies spares, training and documentation. It also speeds up installation, because crews learn one familiar unit. Ask the supplier for a detailed single-line diagram with recommended RMU ratings before you finalize the plant layout. Some plants also use outdoor RMUs on concrete pads near inverter stations. This reduces cable lengths and system losses.

5. Frequently Asked Questions

Does every solar substation need an RMU?

Almost always, if the substation has MV switching. For example, RMUs provide the isolation and protection required for safe maintenance. They also handle faults quickly and safely.

Can RMUs handle bidirectional power flow?

Yes. RMUs switch circuits regardless of flow direction. This is exactly why they suit PV plants with distributed generation.

Are gas-insulated RMUs preferred in solar plants?

Often, because they are compact and sealed. However, air-insulated and solid-insulated units are equally valid. The final choice depends on space and environmental requirements.

What voltage classes are used in solar RMUs?

Most PV collection networks run at 12 kV or 24 kV. Larger plants use 33 kV and 40.5 kV for higher-power clusters. The ring main unit must match the plant’s collection voltage and fault level. In addition, size the cable entries for the MV cables between inverter stations and the substation.

6. Conclusion

RMUs are the workhorses of solar plant MV collection networks. Choose configurations based on voltage, fault level, environment and scalability. Work with a supplier experienced in renewable projects. For outdoor RMU applications, take a look at the Outdoor ring main unit.

Electric-GS supplies ring main units, photovoltaic grid-connected cabinets and distribution equipment for solar projects worldwide. OEM and ODM support is available. For a quotation or technical consultation, contact info@electricgs.com or WhatsApp +86 135 8897 8050.

Related Products

GSRM6 Gas Insulated Ring Main Unit

HXGN15-12 Ring Main Unit Medium Voltage Switchgear