Secondary substation equipment includes the electrical components used to receive medium-voltage power, transform it to a lower voltage, and distribute that power to local loads. Typical equipment can include medium-voltage switchgear or an RMU, a distribution transformer, low-voltage switchboards, protection devices, metering equipment, earthing systems, and control or monitoring equipment.

The exact equipment package depends on the network voltage, transformer capacity, fault level, installation environment, protection requirements, and local standards.

For distributors and EPC contractors, understanding how these components work together is essential when building a complete substation specification.

What Is a Secondary Substation?

A secondary substation is generally part of a distribution network and is located closer to the electricity loads than a primary transmission or distribution substation.

Its basic function is to take electrical power at a medium-voltage level and distribute it at a lower voltage suitable for local users.

A simplified arrangement is:

MV network → MV switchgear/RMU → transformer → LV switchboard → loads

Depending on the network, the substation may serve:

• Commercial buildings
• Industrial facilities
• Residential developments
• Infrastructure projects
• Renewable energy installations
• Small distribution networks

The physical configuration can range from a compact kiosk or packaged substation to a dedicated indoor electrical room.

What Equipment Is Used in a Secondary Substation?

A typical secondary substation may include several major equipment groups.

Not every secondary substation requires all of these components.

The equipment should be selected as a coordinated system.

Medium-Voltage Switchgear

Medium-voltage switchgear is one of the most important parts of a secondary substation.

It provides functions such as:

• Switching
• Isolation
• Fault protection
• Earthing
• Feeder control

Depending on the project, the switchgear may use:

• Vacuum circuit breakers
• Load-break switches
• Disconnectors
• Earthing switches
• Protection relays

For larger or more complex substations, metal-enclosed switchgear can provide greater flexibility for feeder arrangements and protection.

For compact distribution substations, an RMU may be more appropriate.

Ring Main Unit

A Ring Main Unit (RMU) is a compact medium-voltage switchgear assembly frequently used in distribution substations.

A typical RMU arrangement can provide:

• Incoming feeder connection
• Outgoing feeder connection
• Transformer feeder
• Load switching
• Earthing
• Transformer protection

A common configuration is:

Ring In + Ring Out + Transformer Feeder

The exact configuration depends on the distribution network.

RMUs are particularly useful where a compact substation footprint is important.

Modern RMUs can also incorporate:

• Vacuum circuit breakers
• Motorized operation
• Protection relays
• Fault indicators
• Remote monitoring
• Communication interfaces

This makes them suitable for distribution automation applications where the equipment supports the required functions.

Distribution Transformer

The transformer is responsible for reducing medium-voltage electricity to the lower voltage required by local loads.

For example, a project may receive power at medium voltage and transform it to a low-voltage distribution level.

Transformer selection should consider:

• Rated power
• Primary voltage
• Secondary voltage
• Frequency
• Impedance
• Cooling method
• Insulation system
• Installation environment

Two broad transformer approaches commonly encountered in distribution applications are:

Oil-Immersed Transformers

These use insulating liquid for insulation and cooling.

They can be suitable for many outdoor and utility distribution applications.

Dry-Type Transformers

Dry-type transformers use solid insulation rather than liquid insulation.

They can be attractive for certain indoor installations where fire and environmental considerations influence equipment selection.

The correct choice depends on the project design and applicable requirements.

Low-Voltage Distribution Equipment

After voltage transformation, power is normally distributed through low-voltage equipment.

A secondary substation can therefore include an LV switchboard or distribution board.

Typical components include:

• Main circuit breaker
• Outgoing circuit breakers
• Busbars
• Metering
• Protection devices
• Surge protection
• Monitoring equipment

The LV switchboard must be coordinated with the transformer and downstream loads.

Its current rating and short-circuit withstand capability should reflect the electrical system design.

Protection and Control Equipment

Protection systems help detect abnormal electrical conditions and isolate affected equipment.

Depending on the substation design, protection may include:

• Overcurrent protection
• Earth-fault protection
• Transformer protection
• Short-circuit protection
• Overvoltage protection
• Undervoltage protection

A vacuum circuit breaker combined with a numerical protection relay can provide a flexible protection arrangement for suitable medium-voltage feeders.

Smaller transformer substations may instead use an RMU with fuse protection.

The appropriate protection scheme should be established through the project’s electrical design and coordination study.

Metering and Monitoring Equipment

Modern secondary substations can include equipment for monitoring electrical conditions.

Depending on the project, measurements may include:

• Voltage
• Current
• Active power
• Reactive power
• Frequency
• Energy
• Power factor

Monitoring can be local or connected to a remote supervisory system.

For automated distribution networks, communication equipment can transmit information from the substation to a control center.

This can help operators understand network conditions without relying entirely on local inspection.

Secondary Substation Equipment for Renewable Energy

Secondary substations are also relevant to renewable-energy projects.

Solar Power

Solar farms use transformers and medium-voltage distribution equipment to collect electricity from multiple generating units.

A typical arrangement can include:

PV generation → inverter → transformer → MV switchgear/RMU → collector network

The equipment selection depends on the project’s collector voltage, transformer rating, fault level, protection requirements, and environmental conditions.

Wind Power

Wind farms may use transformer and MV switchgear equipment at individual turbine locations or within collection substations.

Equipment can include:

• MV switchgear
• RMUs
• Transformers
• Protection relays
• Monitoring equipment

Remote monitoring can be especially useful when electrical equipment is distributed across a large site.

Battery Energy Storage

Battery energy storage projects use power conversion systems and transformers to connect battery systems to the electrical network.

The secondary substation equipment may include:

• MV switchgear
• Transformer
• LV auxiliary systems
• Protection
• Metering
• Communication

The complete protection scheme needs to account for the characteristics of inverter-based equipment.

Indoor vs Outdoor Secondary Substations

The installation environment influences equipment selection.

Indoor Secondary Substation

Indoor substations can provide a controlled environment for electrical equipment.

Potential advantages include reduced exposure to:

• Rain
• Dust
• Direct sunlight
• Temperature fluctuations

However, the building needs appropriate ventilation, access, fire protection, and electrical clearances.

Outdoor Secondary Substation

Outdoor substations expose equipment to environmental conditions.

The design may need to consider:

• Rain
• Humidity
• Dust
• Pollution
• Solar radiation
• Corrosive environments
• Ambient temperature
• Altitude

Outdoor switchgear and transformers therefore require appropriate enclosure and environmental specifications.

How to Select Secondary Substation Equipment

1. Start With the Electrical System

Define:

• MV system voltage
• LV system voltage
• Frequency
• Maximum load
• Transformer capacity
• Available fault current

These parameters establish the foundation of the equipment specification.

2. Select the MV Switching Arrangement

Determine whether the project needs:

• RMU
• Load-break switch
• Vacuum circuit breaker switchgear
• Metal-enclosed switchgear

Consider the number of feeders and required protection functions.

3. Size the Transformer

The transformer should be selected according to:

• Maximum demand
• Load profile
• Future expansion
• Required redundancy
• Efficiency requirements

Avoid selecting transformer capacity based solely on the current connected load if significant future expansion is expected.

4. Define Protection Requirements

Coordinate the protection between:

MV network → MV switchgear → transformer → LV switchboard → final loads

Poor coordination can cause unnecessary interruptions or inadequate fault isolation.

5. Consider Automation

If remote operation is required, specify:

• Motorized switching
• Protection relays
• Sensors
• RTU
• Communication interface
• SCADA integration

Automation requirements should be defined before equipment procurement.

6. Check Installation Conditions

Confirm whether the equipment will be:

• Indoor
• Outdoor
• Underground
• In a coastal area
• In a polluted environment
• At high altitude

These conditions can affect insulation, enclosure construction, cooling, and material selection.

7. Review Standards and Testing

Ask suppliers to identify the applicable standards for each major equipment group.

Request appropriate documentation such as:

• Technical datasheets
• Type-test documentation
• Routine-test documentation
• General arrangement drawings
• Wiring diagrams
• Single-line diagrams
• Installation manuals

What Should Distributors Ask a Substation Equipment Manufacturer?

For distributors, purchasing a complete package is not simply about comparing prices.

Useful questions include:

Can the equipment be customized?

Check whether the supplier can adapt:

• RMU configuration
• Switchgear arrangement
• Transformer rating
• Protection system
• Communication interface
• Enclosure

Can different products be supplied as one package?

A supplier that can coordinate MV switchgear, RMU, transformers, protection, and monitoring equipment may simplify project procurement.

Can the manufacturer provide project documentation?

This can include:

• Drawings
• Datasheets
• Test reports
• Wiring diagrams
• Installation documents
• Operation manuals

Can the equipment support future automation?

Even if remote control is not required immediately, buyers may want to consider whether the equipment can accommodate future monitoring or automation.

Frequently Asked Questions

What is secondary substation equipment?

Secondary substation equipment includes the electrical components used to receive medium-voltage power, transform it to a lower voltage, protect the electrical system, and distribute electricity to local loads.

What are the main components of a secondary substation?

The main components can include MV switchgear or an RMU, distribution transformer, LV switchboard, protection equipment, metering, earthing, surge protection, and monitoring or communication equipment.

Is an RMU secondary substation equipment?

Yes. An RMU is a type of compact medium-voltage switchgear commonly used in distribution substations.

What switchgear is used in a secondary substation?

Depending on the project, secondary substations can use RMUs, load-break switches, vacuum circuit breaker switchgear, or other metal-enclosed MV switchgear.

What is the role of a transformer in a secondary substation?

The transformer reduces the incoming medium-voltage supply to a lower voltage suitable for local distribution and connected loads.

Can secondary substation equipment be automated?

Yes. Suitable equipment can incorporate motorized switching, protection relays, sensors, remote terminal units, communication systems, and SCADA interfaces.

What is the difference between a primary and secondary substation?

The distinction depends on the network architecture and terminology used by the utility or project. In general, primary substations handle higher-level network transformation or distribution, while secondary substations are closer to end-use loads and commonly perform local voltage transformation and distribution.

Can secondary substations be used for solar farms?

Yes. Secondary-substation equipment can be used in appropriate solar applications for transformer connections, medium-voltage collection, protection, and distribution.

How do I choose a secondary substation equipment supplier?

Evaluate the supplier’s switchgear technology, RMU and transformer options, electrical ratings, protection systems, testing documentation, customization capability, automation options, delivery capability, and technical support.

Conclusion

Secondary substation equipment forms the connection between medium-voltage distribution networks and local electrical loads.

A typical system can combine an RMU or other MV switchgear, distribution transformer, LV switchboard, protection equipment, metering, earthing, and monitoring systems.

For distributors and EPC contractors, the most important point is to treat these components as one coordinated electrical system.

Voltage and current ratings, transformer capacity, fault levels, protection coordination, installation conditions, automation requirements, and applicable standards should all be defined before selecting equipment.

As distribution networks become more connected, secondary substations can also incorporate automated RMUs, vacuum circuit breakers, digital protection, remote monitoring, and communication systems, creating more flexible and observable distribution infrastructure.