Scotland's Electricity Grid Owners
The Scottish Generation, Transmission and Distribution system is owned and operated by two private sector companies, Scottish Power and Scottish & Southern Electric (Scottish Hydro Electric). Between them they own the majority of generation and the entire Transmission and Distribution-line, cable and transformer network.
This network basically consists of an extensive 132kV system, used to interconnect load centres that are supplied by 33 kW or 11kV systems, which often contain embedded generators-- normally relatively small hydro generators and wind farms. In northern Scotland the 132 kV system operates in parallel to their 275 kV system as an integral part of Scottish and Southern Energy's transmission system. In southern Scotland, Scottish Power's territory, the 132 kV system extends from Glasgow down the west coast to the Solway Firth, across to Chapel Cross power station, through the border to Galashiels and Berwick and back north to Edinburgh.
Characteristics of Centralised Generation and Transmission
The basic nature of a centralised generation and the connecting transmission & distribution system--is hierarchical. Where power flows from the generator at high voltage to the user at low voltage. This means that lines farthest from the generating source are generally of low voltage, limited capacity, with well-established fault current relay settings.
The resources that a centrally despatched system operator requires include:
The third point above is considered by this project as having even more relevance to a system that has increasing levels of wind penetration. Where levels of wind penetration reach above 10% the system operator should have a fourth requirement:
4. Sufficient capacity of plant (conventional) to cover for prolonged periods of low to no wind. (this project proposes the use of energy storage instead of conventional thermal backup).
Generators can be designed and defined as being flexible or inflexible, with flexible plants being able to provide frequency response or a load following capability. At present there is a significant capacity of inflexible generation connected to the system, in the form of CCGT and CHP plants, and all nuclear power stations. Wind generation schemes are unlikely to be able to operate as flexible plant due to the nature of their energy source and the characteristics of the generators; in fact wind generation will increase the burden of flexibility on the thermal plants.
Connection of New Renewable Generation
In general-at present, there are no major constraints on the Generation system that would inhibit the connection of significant generation capacity. However, the Transmission system is less forthcoming. Connection capacity is location specific with the rural 132kV system less capable of accepting large capacity connections than higher voltage circuits at major nodes.
In an interconnected transmission system major reinforcements are required when projected power flows exceed the fixed transmission capability. Power flows are directly related to the magnitude and location of connected generation and demand. If new generation is located in an exporting area (where generation already exceeds demands), power flows will increase and reinforcements could be precipitated. It is therefore logical to give careful consideration to the siting of new generation in an importing area to reduce or eliminate the need for such reinforcements. Selecting suitable locations for generation in this way will have the additional benefit of reducing system losses.
Renewable Energy Transmission Studies (RETS)
This was a joint venture among SP, S&SE and NGC to investigate and quantify the necessary transmission system development required to accommodate the renewable targets. This report delivered estimated costs for a three-stage development. The grid upgrade required accommodating 2, 4 and 6GW of renewable power. The remit of the study only considered developments around transmission system node points.
What reinforcements are planned and where?
Scottish Power has plans to reinforce this network by overlaying large sections of the existing system with 275 kV circuits.