Connection types

We provide two main ways to connect to our network at our existing substations: a high voltage ‘bay’ connection or a lower voltage connection through a ‘tertiary’. If your project size justifies it, we can also ‘T’ into our overhead lines and build a new substation to connect you into. 

About Tertiary Connections 

Tertiary connection


Who is a tertiary connection suitable for? 

A tertiary connection is typically suitable for smaller projects looking to connect directly to National Grid’s network. It is called a tertiary as it uses the third winding on our existing high voltage transformers. 

  • Each connection provides up to 57MW (60MVA) demand or generation capability

  • The connection is to the ~13kV tertiary winding of a transformer 

  • Our standard design includes a transformer from ~13kV to 33kV on our site. Using 33kV mean we use smaller equipment, reducing the cost and complexity of your connection 

  • It’s possible to connect directly at ~13kV if your project is located very close to our substation

National Grid build the infrastructure assets and typically build the connection assets, although these can be built by you through a user self-build agreement. 

Please note: Compared to a bay connection, tertiary connections can have higher downtime. Please enquire through our Research Assistant for this or any other options and we will set up a pre-application discussion to explain all the options available to you. 

About Bay Connections 

Four Mills tertiary connection


Who is a bay connection suitable for? 

A bay connection is for larger projects up to 1800MW per bay (at 400kV). It’s called a bay as we provide a portion of our substation for you to put your assets into and provide a busbar (normally both main and reserve) for you to ‘clamp’ onto. We provide the connection point and you then build a transformer and all the assets up to the busbars. 

  • We provide bays at transmission voltages of 400kV, 275kV and 132kV 

  • The connection is at these voltages, providing up to 1800MW capacity at 400kV 

  • Compared to a tertiary, these connections typically have less down time

  • It takes approximately 36 months to connect to an AIS bay. It takes approximately 45 months to connect to a GIS bay. Both of these are from application to available for commercial load

We have two types of equipment in general: AIS and GIS (Air/ Gas Insulated Switchgear). Please use the Research Assistant to find out which type exists at the substation you are interested in connecting to. A GIS bay differs to an AIS one as the circuit breaker and isolator are part of the bay, therefore there is a different ownership regime at these substations. Please enquire through the Research Assistant for this or any other options and we will setup a pre-application discussion to explain all the options available to you.  

About T Connections 

A 'T' connection offers another way of connecting to our network. Our overhead lines are temporarily diverted whilst modifications are made to enable this connection, and a new, dedicated substation is built. It is generally not feasible to offer this connection near to our existing substations or for smaller projects, due to cost and planning reasons.

Due to the significant costs associated with connecting such projects, there are a couple of key considerations for you to think about: 

  • If your connection is not geographically close to our existing substations and under 150MW we would advise you to contact the DNO in the first instance as the costs to connect to their network (at a lower voltage) is likely to be more economic and efficient.  

  • If a transmission connection is preferable, we would recommend that we carry out a Feasibility Study. This would include: 

    • System Study analysis on both the circuit and substation design  

    • Potential works required to connect your project at that particular location 

    • Review consenting and planning considerations for the work we’d be carrying out 

    • High level programme, including diversion, overhead line and substation build 

    • Budgetary estimate for the works, including spend profile and liabilities 

  • Once you have reviewed the results of your Feasibility Study and you choose to apply for a connection, we would recommend that you elect for a variable application as we would anticipate being able to re-use most of this work.  

Please enquire through our Research Assistant for this or any other options and we will setup a pre-application discussion to explain all the options available to you.