Ring Main Unit for Solar and Wind Power Projects: What Should You Consider?

Solar and wind farms need reliable medium-voltage switching between generators, transformers, and the utility grid. A Ring Main Unit for Solar and Wind Power Projects helps operators isolate faults, protect transformers, and restore service without shutting down an entire collector circuit. When comparing a ring main unit for solar power projects, engineers should review voltage, current, short-circuit withstand, protection functions, environmental conditions, and communication needs. These checks also help reduce outage time and improve the safety of field crews.

Why Renewable-Energy Projects Need a Ring Main Unit

Solar and wind projects often contain many distributed generation units. A solar farm may have dozens of inverter-transformer stations, while a wind farm may connect each turbine transformer to a medium-voltage collector network. The collector system must carry power over long distances and remain available when one cable section or transformer develops a fault.

An RMU places switching and protection functions in a compact metal-enclosed assembly. Depending on the design, it may include:

• Load-break switches for normal energizing and de-energizing.
• Earthing switches for safe maintenance isolation.
• Vacuum circuit breakers for fault interruption.
• Fuses for transformer or feeder protection.
• Current transformers and voltage transformers for measurement.
• Protection relays for overcurrent, earth fault, and other conditions.
• Remote terminal units or communication modules for SCADA control.

IEC 62271-200 covers AC metal-enclosed switchgear and controlgear above 1 kV and up to 52 kV. IEC 62271-100 covers high-voltage circuit breakers, while IEC 62271-103 covers switches. These standards provide a useful starting point for checking the construction and test requirements of an RMU.

For renewable plants, the main value is selective isolation. If a cable section fails, operators can open the affected section, earth it, and keep healthy sections available when the network design and protection settings allow it. The actual result depends on the single-line diagram, relay coordination, utility rules, and operating procedure.

Key Technical Data to Check Before Buying

1. Rated voltage and insulation level

First, match the RMU to the project’s medium-voltage system. Common project classes include 12 kV, 17.5 kV, 24 kV, and 36 kV, but the correct choice depends on the grid voltage and equipment specification.

Do not check only the nominal voltage. Request the following data:

• Rated voltage, in kV.
• Rated power-frequency withstand voltage.
• Rated lightning impulse withstand voltage.
• Insulation level at the project altitude.
• Internal arc classification, if required by the owner.

At high altitude, air insulation performance decreases. The manufacturer may need to apply an altitude correction or provide equipment tested for the installation site. Include the project elevation in the technical inquiry instead of relying on a standard catalog value.

2. Continuous current rating

The rated current must cover the maximum continuous current in each feeder. A simple three-phase estimate is:

I = P ÷ (√3 × V × power factor)

For example, a 5 MW collector feeder at 33 kV and a power factor of 0.95 carries about 92 A under balanced conditions. The RMU should not be selected at that value alone. Engineers must also consider transformer inrush, future expansion, ambient temperature, cable loading, reactive power, and emergency operating conditions.

Typical RMU current ratings may be 200 A, 400 A, 630 A, or higher. The chosen rating must match the full project load flow and the utility’s protection requirements.

3. Short-circuit withstand and breaking capacity

Renewable plants are not automatically low-fault-current systems. The utility grid, transformers, cable impedance, and inverter controls all affect the available fault current. Ask for:

• Short-time withstand current, such as 16 kA, 20 kA, or 25 kA.
• Short-circuit duration, commonly 1 or 3 seconds depending on the design.
• Peak withstand current.
• Short-circuit breaking current of the circuit breaker.
• Making current of the switch or breaker.

These values must come from the project fault study. A 24 kV RMU with a 20 kA short-time rating is not suitable if the calculated fault level exceeds that rating. The equipment must also coordinate with upstream and downstream protection so that the smallest practical network section is disconnected.

4. Protection for solar and wind collector feeders

Protection depends on the connection point and network design. Common functions include:

• Overcurrent protection: Detects phase faults and overloads.
• Earth-fault protection: Detects current flowing through an unintended ground path.
• Directional overcurrent: Useful where fault current can flow from more than one direction.
• Under-voltage and over-voltage: Helps separate abnormal grid conditions.
• Under-frequency and over-frequency: Supports grid-code compliance.
• Breaker failure protection: Trips an upstream device if the local breaker does not clear a fault.
• Transformer protection: May include fuse protection, temperature monitoring, or a dedicated relay.

Inverter-based solar and wind generation can produce fault current differently from synchronous generators. The inverter manufacturer, utility, and protection engineer should confirm the fault-current contribution and relay settings. Do not copy settings from a conventional industrial feeder without checking the inverter model and grid-code requirements.

5. Switching sequence and earthing safety

A suitable RMU should make the operating sequence clear. Interlocks should help prevent unsafe actions, such as closing an earthing switch onto an energized feeder. The project specification should define:

• Visible or position-indicating switch status.
• Mechanical and electrical interlocking.
• Key interlock requirements.
• Cable test access.
• Voltage presence indication.
• Safe discharge and earthing arrangements.
• Lockout and tagout points.

Operators should follow the approved switching procedure and local electrical-safety rules. An RMU is not a replacement for a complete risk assessment, arc-flash study, or permit-to-work system.

Choosing Insulation Technology for a Ring Main Unit

Gas-insulated RMU

Gas-insulated units use a sealed tank to reduce the effect of dust, humidity, salt, and animals. This can be useful in coastal wind farms, desert solar plants, and compact substations. However, the buyer should identify the insulating gas, its pressure-monitoring method, service requirements, and end-of-life recovery process.

SF₆ has strong dielectric and arc-quenching performance, but it is a greenhouse gas with a high global warming impact. The European Commission’s revised F-gas Regulation, Regulation (EU) 2024/573, increases restrictions on certain fluorinated-gas equipment and supports lower-emission alternatives. Project owners should therefore review local law, utility specifications, and the required service life before selecting an SF₆-based design.

SF₆-free and solid-insulated RMU

SF₆-free designs may use clean air, vacuum interruption, or other insulation systems. Solid-insulated units can reduce gas-handling needs and may offer a compact sealed construction. The buyer should still verify:

• Type-test reports for the exact voltage and current class.
• Partial-discharge performance.
• Temperature-rise results.
• Mechanical endurance.
• Moisture and pollution performance.
• Recycling and maintenance instructions.

“SF₆-free” describes the insulation approach, not the complete performance of the equipment. Compare the full technical file, service plan, and life-cycle requirements rather than selecting a product from the label alone.

Environmental Conditions in Solar and Wind Farms

Renewable-energy equipment is often installed far from a city and exposed to demanding conditions. The RMU specification should include:

• Ambient temperature range.
• Maximum altitude.
• Humidity and condensation risk.
• Dust, sand, and pollution level.
• Salt spray for coastal projects.
• Wind loading and enclosure protection.
• Flood risk and foundation height.
• Indoor or outdoor installation.

For outdoor equipment, check the enclosure’s IP rating under IEC 60529. For corrosive locations, ask about stainless-steel hardware, anti-corrosion coating, drainage, heaters, and anti-condensation control. A solar site with high daytime temperatures may also require derating or forced ventilation if the RMU is installed inside a container.

Wind farms can create special transport and maintenance problems. RMUs installed inside turbine towers or compact substations must fit through access doors and allow cable termination work in restricted space. Confirm dimensions, lifting points, cable bending radius, and maintenance clearance before approving the layout.

Communication and Remote Operation Requirements

Large solar and wind sites commonly use SCADA to monitor feeder status, alarms, voltage, current, and power quality. A remote-ready RMU may include motorized switches, a circuit-breaker trip coil, local/remote selection, protection relays, and a communication gateway.

Ask the supplier to confirm supported protocols, such as IEC 61850, IEC 60870-5-104, Modbus TCP, or Modbus RTU. The correct protocol depends on the plant control system and grid operator. Also define:

• Control command security and permission levels.
• Time synchronization method.
• Event and fault-record storage.
• Remote indication of switch and earth positions.
• Loss-of-communication behavior.
• Cybersecurity responsibilities.

Remote control can reduce travel to unmanned substations, but it must not bypass local safety interlocks. The design should keep local emergency operation available and record every remote switching action.

How to Evaluate a Ring Main Unit Manufacturer

When comparing a Ring Main Unit Manufacturer, look beyond purchase price. A reliable evaluation includes design evidence, factory quality, field support, and spare-part availability.

Request these documents

• General arrangement drawing and single-line diagram.
• Technical data sheet with guaranteed values.
• Routine-test procedure and sample report.
• Type-test reports to the applicable IEC standards.
• Short-circuit and internal-arc test information.
• Protection-relay setting range and coordination support.
• Material and coating specifications.
• Installation, operation, and maintenance manual.
• Recommended spare-parts list and warranty terms.
• Factory acceptance test plan.

Check manufacturing and service capability

Ask how the manufacturer controls insulation assembly, torque values, wiring, sealing, and final testing. The factory acceptance test should confirm electrical operation, interlocks, control circuits, relay functions, communication points, and labeling.

Also review the service network. A remote renewable site may need local commissioning support, training, emergency response, and replacement parts. A lower initial price can create a higher life-cycle cost if a failed relay, motor mechanism, or cable termination stops a feeder for several days.

Juhonkia can be included in the supplier comparison when buyers are looking for a project-focused RMU option. Request the exact voltage class, insulation technology, protection scheme, compliance documents, and after-sales scope for the intended solar or wind application.

Installation and Commissioning Process

A controlled process reduces commissioning errors. The main steps are:

1. Confirm the approved design: Check the single-line diagram, cable schedule, protection study, and utility requirements.
2. Inspect delivery condition: Look for damage, moisture, missing accessories, and incorrect nameplate data.
3. Prepare the foundation: Verify level, anchor locations, cable trench dimensions, and equipment clearances.
4. Install and bond the enclosure: Connect the main earth bar to the site grounding system according to the approved design.
5. Terminate cables: Use the correct termination kit, bending radius, screen bonding, and phase identification.
6. Test mechanical operation: Operate each switch and earthing device locally and confirm interlocks.
7. Test electrical circuits: Check insulation, continuity, control voltage, trip circuits, indication, and alarm circuits using the approved test method.
8. Verify protection: Inject test current or use the approved relay-test procedure to confirm pickup, timing, trip logic, and coordination.
9. Test communication: Confirm every SCADA point, command, alarm, timestamp, and fail-safe response.
10. Complete energization approval: Record test results, close defects, and obtain authorization before putting the feeder into service.

Testing must be performed by qualified personnel with suitable instruments. The exact test voltage and method depend on the equipment design, cable type, manufacturer instructions, and local standards. Avoid applying a test that could damage electronic sensors or connected surge-protection devices.

Maintenance Plan for Solar and Wind RMUs

Maintenance intervals should follow the manufacturer’s manual, operating frequency, fault history, and site conditions. A practical plan may include:

• Visual inspection for corrosion, dust, water entry, and damaged labels.
• Inspection of cable compartments and termination temperature.
• Operation of switches and breakers according to the approved procedure.
• Verification of gas pressure or insulation-system alarms where applicable.
• Protection-relay inspection and secondary-injection testing.
• SCADA point-to-point checks after control-system changes.
• Thermal imaging under load when permitted by the safety procedure.
• Review of trip records, alarms, and partial-discharge data if monitoring is installed.

Do not open a sealed gas compartment or modify relay settings without authorization. Keep an asset record containing the serial number, test reports, cable termination details, fault events, and replacement parts used during service.

Common Selection Mistakes to Avoid

• Choosing by voltage only: Current, fault level, insulation level, and environmental conditions are equally important.
• Ignoring inverter behavior: Protection settings must reflect the actual inverter and grid connection.
• Leaving communication until the end: A relay may support a protocol, but the complete SCADA system still needs point mapping and testing.
• Using a standard enclosure at a harsh site: Salt, dust, heat, and condensation can shorten service life.
• Skipping future capacity: A later feeder or transformer addition may require more space and a higher bus rating.
• Accepting incomplete test evidence: Ask for reports that match the equipment’s exact rated class.
• Underestimating cable work: Incorrect termination, screen bonding, or bending radius can cause failures even when the RMU itself is correctly rated.

Advantages of Using the Right Ring Main Unit

A correctly specified RMU can provide several measurable project benefits:

• Fault isolation: Operators can separate a damaged feeder section instead of treating the whole collector network as unavailable.
• Compact installation: Integrated switching and protection can reduce the number of separate medium-voltage panels.
• Improved operating safety: Interlocks, voltage indicators, and earthing devices support a controlled maintenance process.
• Remote supervision: SCADA integration can reduce unnecessary site visits and improve event response.
• Environmental flexibility: Sealed or suitably protected designs can operate in dusty, humid, or coastal locations.
• Lower life-cycle risk: Standardized spare parts, test records, and service procedures make future maintenance more predictable.

The RMU does not create energy or remove every outage risk. Its value comes from matching the switching, protection, insulation, and communication design to the actual collector network.

FAQ About Ring Main Units for Solar and Wind Projects

What does RMU mean in a renewable-energy project?

RMU means Ring Main Unit. It is a medium-voltage switchgear assembly used to connect, protect, isolate, and sometimes control feeders, transformers, and grid connections in a ring or loop network.

Is an RMU required for every solar or wind farm?

No. The need depends on the collector-system design, voltage level, utility interconnection rules, transformer arrangement, and protection study. Some projects use RMUs, while others use metal-enclosed switchgear lineups or other feeder equipment.

Should I choose an SF₆-free RMU?

An SF₆-free RMU may reduce greenhouse-gas management requirements, but it must still meet the project’s voltage, current, fault, insulation, environmental, and service requirements. Compare test evidence and local regulatory requirements before making the decision.

What voltage is common for solar and wind collector systems?

Medium-voltage collector systems commonly use voltage classes such as 11 kV, 22 kV, 24 kV, or 33 kV. The correct RMU class must match the project’s actual system voltage and the utility’s approved design.

Can an RMU be controlled remotely?

Yes. Motorized switches, circuit breakers, protection relays, RTUs, and communication gateways can support remote monitoring and control. The project must define the protocol, control logic, cybersecurity measures, and local emergency operation.

What information should I send to a supplier for a quotation?

Provide the system voltage, rated current, fault level, frequency, transformer data, cable details, number of ways, protection functions, insulation technology, enclosure location, altitude, environmental conditions, communication protocol, utility standards, and required delivery documents.

Next Steps Before Ordering

Start with the approved single-line diagram and fault study. Then prepare an RMU specification that lists voltage, current, short-circuit rating, protection, insulation, enclosure, communication, testing, and service requirements. Compare at least two technically compliant suppliers and ask each one for matching type-test evidence, factory-test details, and a commissioning plan.

For a practical next step, read the manufacturer’s user guide and request a project-specific review from Juhonkia or another qualified supplier. Include the phrase how to choose a ring main unit manufacturer in your internal evaluation checklist so that technical compliance, service support, and long-term operating cost are reviewed together.

Reference Standards and Industry Resources