Mixed Technology Substations

Mixed technology substations (MTS) incorporate the use of air-insulated and gas insulated components and switchgear. In a substation, some or all of the switching bays consist of compact GIS modules with CBs, instrument transformers, disconnectors and grounding switches. The busbars, bay connections and often complete switching bays originate in conventional AIS technology.

Technology Types

The main focus when discussing MTS originates in compact GIS modules with bushings and with special requirements resulting of interaction with larger AIS substation parts. MTS can differ in their engineering design depending on the required configuration. Three combinations are possible:

  • AIS in compact and / or combined design
  • GIS in combined design
  • Hybrid IS in compact and / or combined design assembled together and using a common structure to minimise the installation time

Components & enablers

  • Encapsulation

  • Insulation gas (commonly SF6)

  • Bushings

  • Circuit-breakers

  • Combined disconnector and earthing switch for cables and busbars

  • Current and voltage transformer

  • Earthing switch

Advantages & field of application

The first MTS modules were developed in the 1990s. They are compact switchgear applications mainly used in the refurbishment and expansion of substations with air-insulated outdoor and indoor switchgear. The application of MTS allows the bay footprint to be reduced by up to 50%.

Depending on the national regulation, this can result in a lower cost of ownership for the technology compared to AIS.

Technology advancements in hybrid insulated substations (HIS) follow the same trends of GIS and AIS installations. MTS modules are commercially available for all voltage levels up to 420 kV.

Technology Readiness Level

The TRL depends on the gas mixture used in the GIS technology-based components.

SF6 free:

2020: TRL 5

2025: TRL 7

2030: TRL 9

For SF6 based MTS, TRL is 9.

Research & Development

Current fields of research: The research on MTS focuses on requirements resulting from the interface between different insulating media and equipment stresses (e.g. switching of loop currents).

Power system technologies’ manufacturers focuses their effort on the transportability of large assembled MTS modules. Key innovation is the rotating bushing concept, which takes less than 30 seconds per bushing from the in-service position to the transport position and back again at the installation site.

Innovation priority to increase overall TRL: N / A

Best practice performance

Maximum continuous current: 63 kA

Maximum voltage rating: 420 kV

Standard definition: IEC 62271-205

Expected lifetime: 50 years

Best practice application

Sicily, Italy


A 420 kV MTS was commissioned in 2015 to help ensure the reliability of a new high voltage 2 GW power link between Sicily and the Italian mainland.

With very limited space on the island substation and the need to perform maintenance on the modules while in service, conventional solutions were ruled out. The 420 kV MTS arrived on site factory assembled and tested, ready for rapid installation and energization.

Ensuring the reliability of 2000 MW power transmission capacity between Sicily and the Italian mainland while abiding to the limited available space.

Wilster, Germany


Construction of a substation for the link between two network levels in the region of Schleswig Holstein.

Two transformers connect the 380 kV level with a 110 kV network with a capacity of 300 MVA.

Onshore generated wind-electricity will be easily integrated and transmitted from the north to the south in the future.


[1] Cigre. Mixed Technology HV Switchgear and Substations: Optimised Service Strategies. [Link]

[2] EE publishers. Mixed technology switchgear saves space and time. [Link]

[3] Pandit A, Sinha A. High Voltage Mixed Technology (Hybrid) Switchgear - Steps Up to 400kV. [Link]

[4] ABB. ABB hybrid switchgear PASS 420 kV supports reliable power supply in strategic new Terna power line. [Link]

[5] Tennet. Umspannwerk Wilster: zwei Transformatoren erreichen einen der wichtigsten Netzknoten Deutschlands. [Link]