The System Operator–Transmission Owner Code (STC) defines the high-level relationship between the GB system operator and the three transmission owners.
Prior to April 2005 electricity was traded and transmitted in two distinct markets in Great Britain, one in Scotland and one in England and Wales. To facilitate the operation of a single GB market for the trading and transmission of electricity, the move to the British Electricity Trading and Transmission Arrangements (BETTA) introduced National Grid Electricity Transmission (NGET) as the single System Operator for GB on 1 September 2004. NGET is now the only interface with Users for connection to and Use of transmission services in GB.
The STC was introduced as part of BETTA and defines the interaction between Transmission Licensees. The STC defines arrangements for:
- transmission Owners to make transmission services available to the GB system operator for its use so it can discharge its obligations under its Licence and the user facing Codes (CUSC, BSC, Grid Code etc);
- the GB System Operator to issue directions to Transmission Owners about the required configuration of the GB transmission system; and
- the planning of transmission outages and co-ordination of investment planning for the development of the GB transmission system (including new connections to the GB system).