Electricity generated at power plants and transmitted through high-voltage networks must eventually be transformed and distributed at lower voltages suitable for local networks and end users.

A secondary substation is a substation used within a distribution network to transform and distribute electrical power, typically closer to consumers than primary substations.

A typical secondary substation may contain:

• Medium-voltage switchgear
• RMU or other MV switching equipment
• Distribution transformer
• Low-voltage switchboard
• Protection equipment
• Metering equipment
• Earthing system
• Control and communication equipment

The exact voltage levels and configuration vary by utility, country, network architecture, and application.

What Is a Secondary Substation?

A secondary substation is part of the electrical distribution system between higher-voltage network infrastructure and low-voltage consumers.

Its primary functions are to:

1. Receive electrical power from an MV distribution network.
2. Switch and protect the incoming and outgoing circuits.
3. Transform the voltage to a lower distribution voltage.
4. Distribute electricity to downstream loads.

A simplified power path is:

Medium-Voltage Distribution Network
                 │
                 ▼
          MV Switchgear / RMU
                 │
                 ▼
        Distribution Transformer
                 │
                 ▼
          LV Distribution Board
                 │
          ┌──────┼──────┐
          ▼      ▼      ▼
       Building Industry Commercial
         Loads    Loads    Loads

The exact arrangement depends on the distribution network.

How Does a Secondary Substation Work?

The secondary substation receives electricity at medium voltage.

The MV switchgear controls and protects the incoming supply.

The transformer then reduces the voltage to the level required by the low-voltage distribution network.

For example, a project may use an MV network feeding a transformer that supplies an LV system. The actual voltage values must be selected according to local network requirements.

The process can be summarized as:

MV Grid → Switching & Protection → Voltage Transformation → LV Distribution

This makes the secondary substation an important interface between the distribution network and local consumers.

Main Components of a Secondary Substation

A secondary substation is more than just a transformer.

1. MV Switchgear

MV switchgear provides switching, protection, isolation, and distribution functions.

It may include:

• Circuit breakers
• Load-break switches
• Disconnectors
• Earthing switches
• Busbars
• Protection relays
• Current transformers
• Voltage transformers

The configuration depends on the network and protection philosophy.

2. RMU

An RMU (Ring Main Unit) is commonly used in distribution networks where compact switching equipment is required.

An RMU can provide:

• Ring network switching
• Transformer protection
• Isolation
• Earthing
• Fault interruption, depending on configuration

RMUs are especially useful where equipment footprint is important.

3. Distribution Transformer

The transformer reduces the MV supply to a lower voltage.

Common transformer technologies include:

• Oil-immersed transformers
• Dry-type transformers

The appropriate type depends on:

• Installation environment
• Fire-safety requirements
• Load
• Space
• Maintenance strategy
• Local regulations

4. LV Switchboard

The LV switchboard distributes the transformer’s output to downstream circuits.

It may include:

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

MV Switchgear in a Secondary Substation

MV switchgear is one of the most important components in a secondary substation.

Its role includes:

Switching

Operators can connect or disconnect feeders under appropriate operating conditions.

Protection

Protection devices can identify abnormal electrical conditions and initiate circuit interruption where required.

Isolation

Sections of the electrical system can be isolated for maintenance.

Earthing

Earthing equipment can support safe maintenance procedures according to the applicable system design and operating procedures.

RMU for Secondary Substations

RMUs are widely associated with compact MV distribution systems.

A simplified ring arrangement can look like:

      Feeder A
         │
         ▼
   ┌─────────────┐
   │     RMU     │
   │             │
   │ Transformer │
   │   Feeder    │
   └─────────────┘
         │
         ▼
      Feeder B

The ring configuration can allow distribution networks to be operated from different directions, depending on the network design.

One important benefit of an RMU is its relatively compact arrangement.

This makes it suitable for:

• Urban substations
• Commercial buildings
• Industrial sites
• Residential distribution
• Renewable-energy facilities

Distribution Transformer

The transformer is responsible for voltage conversion.

Its rating should be selected based on the expected load and system requirements.

Important parameters include:

• Rated power
• Primary voltage
• Secondary voltage
• Frequency
• Impedance
• Cooling method
• Insulation system
• Temperature rise
• Installation conditions

Oil-Immersed vs Dry-Type Transformer

The best choice depends on the application and applicable regulations.

LV Distribution Equipment

After voltage transformation, the LV switchboard distributes power to end-use circuits.

Typical loads can include:

• Buildings
• Lighting
• HVAC
• Pumps
• Motors
• Manufacturing equipment
• EV charging infrastructure
• Commercial loads

Protection coordination between MV and LV systems is important.

The protection system should be designed so that faults can be cleared appropriately while maintaining service to unaffected circuits where possible.

Protection and Control

Protection is fundamental to secondary substation design.

Depending on the system, protection may address:

• Short circuits
• Overcurrent
• Earth faults
• Transformer faults
• Overvoltage
• Undervoltage
• Temperature conditions

Protection settings should be developed according to the system study and coordination requirements rather than using generic values.

Metering

Secondary substations may also include metering for:

• Energy consumption
• Voltage
• Current
• Power
• Power factor

The required accuracy and metering architecture depend on whether the equipment is used for operational monitoring, billing, or another purpose.

Secondary Substation Types

Secondary substations can be designed in different physical configurations.

Indoor Secondary Substation

Equipment is installed inside a dedicated electrical room or building.

Advantages can include:

• Protection from weather
• Controlled environment
• Integration with buildings

However, ventilation, fire protection, access, and room dimensions must be considered.

Outdoor Secondary Substation

Equipment is installed outdoors.

The design must account for:

• Rain
• Dust
• Temperature
• UV exposure
• Corrosion
• Pollution

Enclosures and equipment ratings should be appropriate for the installation environment.

Compact Secondary Substation

A compact substation integrates major components into a space-efficient arrangement.

A package can include:

MV Switchgear + Transformer + LV Switchboard

This approach can reduce installation complexity and is useful where available space is limited.

Prefabricated Secondary Substation

Prefabricated or packaged substations can be assembled and tested before delivery.

Potential benefits include:

• Factory-controlled assembly
• Faster site installation
• Integrated equipment
• Reduced on-site work

The actual project benefits depend on logistics and site conditions.

Applications of Secondary Substations

Secondary substations are used throughout electrical distribution networks.

Commercial Buildings

They can supply:

• Offices
• Shopping centers
• Hotels
• Hospitals
• Large commercial complexes

Industrial Facilities

Industrial plants can require multiple substations to distribute power to:

• Production equipment
• Motors
• Pumps
• HVAC
• Process systems

Residential Areas

Distribution substations can supply residential neighborhoods through LV distribution networks.

Renewable Energy

Secondary substations can be used within renewable-energy electrical systems where the voltage transformation and distribution architecture requires them.

Applications include:

• Solar projects
• Wind projects
• BESS facilities

Data Centers

Data centers have complex power distribution systems and can use MV substations to supply transformers and downstream distribution equipment.

Smart Secondary Substations

Modern distribution systems are increasingly incorporating digital monitoring and automation.

A smart secondary substation can integrate:

• Digital protection
• Remote monitoring
• Intelligent electronic devices
• Temperature sensors
• Power meters
• Communication gateways
• SCADA interfaces

A simplified architecture is:

MV Equipment → Sensors/IEDs → Communication → SCADA/Control Platform

Potential benefits include:

• Improved visibility
• Remote status monitoring
• Event recording
• Faster fault analysis
• Condition monitoring
• Better operational data

Smart functionality should be selected according to actual operational requirements.

SF6-Free Secondary Substations

Environmental requirements are influencing the design of modern distribution equipment.

A secondary substation does not inherently require SF6.

Depending on the MV switchgear design, alternatives can include:

• Vacuum circuit breakers
• Air insulation
• Solid insulation
• Alternative gas technologies

For projects seeking SF6-free switchgear, buyers should verify the complete switchgear assembly rather than assuming that one component’s technology defines the entire product.

Important questions include:

• Is SF6 used anywhere in the switchgear?
• What interruption technology is used?
• What insulation system is used?
• What are the equipment ratings?
• What standards are satisfied?
• What test evidence is available?

How to Select Secondary Substation Equipment

The selection should start with the electrical system requirements.

1. Determine the MV Voltage

Identify:

• Nominal voltage
• Maximum system voltage
• Frequency
• Insulation requirements

The switchgear voltage class should match the network requirements.

2. Determine Transformer Capacity

Calculate the expected load and consider:

• Present demand
• Load growth
• Diversity
• Motor loads
• Renewable generation
• Future expansion

Avoid selecting transformer capacity solely from the current connected load.

3. Determine Fault Level

The MV and LV equipment must have suitable short-circuit withstand and interruption capabilities for the system.

4. Select MV Switchgear

Consider:

• Circuit breaker or switch-fuse arrangement
• RMU configuration
• Busbar arrangement
• Protection
• Metering
• Cable connections
• Earthing

5. Consider Physical Space

Check:

• Equipment dimensions
• Maintenance clearances
• Cable bending radius
• Door access
• Ventilation
• Transformer clearances

6. Define Environmental Conditions

Consider:

• Indoor or outdoor installation
• Ambient temperature
• Altitude
• Humidity
• Pollution
• Dust
• Corrosion

7. Define Automation Requirements

Determine whether the substation needs:

• Local monitoring
• Remote monitoring
• SCADA
• Remote switching
• Digital metering
• Condition monitoring

Secondary Substation Buyer Checklist

Before ordering equipment, verify:

Electrical

• MV voltage
• LV voltage
• Transformer capacity
• Rated current
• Short-circuit level
• Frequency

Switchgear

• RMU or circuit-breaker switchgear
• Busbar arrangement
• Protection
• Earthing
• Cable entry

Transformer

• Transformer type
• Rating
• Impedance
• Cooling
• Installation environment

Automation

• Metering
• Protection relay
• Communication
• SCADA
• Remote control

Mechanical

• Indoor/outdoor
• Dimensions
• Ventilation
• Maintenance access
• Enclosure requirements

Compliance

• Applicable IEC standards
• Local standards
• Utility specifications
• Project requirements

Frequently Asked Questions

What is a secondary substation?

A secondary substation is a distribution substation that receives electrical power from a higher-voltage distribution network, transforms it to a lower voltage, and distributes it to downstream loads.

What equipment is used in a secondary substation?

Typical equipment includes MV switchgear or an RMU, distribution transformer, LV switchboard, protection devices, metering equipment, earthing equipment, and potentially digital monitoring and communication systems.

What is the difference between a primary and secondary substation?

A primary substation generally performs higher-level voltage transformation and distribution within the network, while a secondary substation is typically closer to end users and provides distribution at lower voltage levels. Exact definitions and voltage boundaries vary by utility and country.

Is an RMU used in a secondary substation?

Yes. RMUs are commonly used in MV distribution applications and can be installed in secondary substations where ring-network switching and compact equipment are required.

What voltage is used in a secondary substation?

There is no single universal voltage. The MV input and LV output depend on the local distribution network and project requirements.

What transformer is used in a secondary substation?

Both oil-immersed and dry-type distribution transformers can be used. Selection depends on load, installation environment, fire-safety requirements, maintenance strategy, and applicable standards.

Can a secondary substation be compact?

Yes. Compact secondary substations can integrate MV switchgear, a transformer, and LV distribution equipment into a space-efficient package.

Can secondary substations use SF6-free switchgear?

Yes. Depending on the switchgear technology and voltage class, a secondary substation can use SF6-free MV switchgear based on technologies such as vacuum interruption, air insulation, or solid insulation.

What is a smart secondary substation?

A smart secondary substation incorporates digital monitoring, protection, communication, and potentially remote-control functions into the conventional electrical distribution system.

Does a secondary substation need an RMU?

Not necessarily. An RMU is one possible MV switching solution. The appropriate equipment depends on the network topology, protection requirements, voltage, fault level, and project specification.

How do you size a secondary substation?

Sizing normally involves determining the expected load, transformer capacity, MV and LV voltage, current, fault level, protection requirements, environmental conditions, and future expansion requirements.

Where are secondary substations used?

They can be used in residential areas, commercial buildings, industrial facilities, infrastructure projects, renewable-energy installations, data centers, and other distribution networks.

Conclusion

A secondary substation is a key part of the electrical distribution network, connecting medium-voltage distribution systems with lower-voltage loads.

A typical installation combines:

MV Switchgear/RMU → Distribution Transformer → LV Switchboard → End Users

The design should be based on the electrical characteristics of the network rather than on a standard equipment package alone.

For modern projects, buyers should evaluate:

• MV switchgear
• RMU configuration
• Transformer capacity
• Protection coordination
• Short-circuit performance
• Physical layout
• Environmental conditions
• Digital monitoring
• Applicable IEC and local standards
• SF6-free technology where required

For space-constrained projects, a compact secondary substation can combine several functions into an efficient package. For modern distribution networks, smart monitoring and automation can provide additional operational visibility.

Ultimately, the best secondary substation is one whose electrical design, equipment ratings, protection system, physical configuration, and monitoring capabilities match the actual requirements of the distribution network and site.