Stage 1 – feasibility study and Network Options Assessment (NOA) based cost benefit assessment. This will conceptually develop a solution capable of use at 275kV and 400kV, identify likely whole life costs (information about the costs will be commercially sensitive and will not be made public), identify network constraints where a QB is currently identified as an economic solution under the NOA and develop conceptual alternative solutions for comparison.

If the economic assessment at the end of stage 1 confirms that the solution is theoretically economically viable, then work will progress to stage 2. The Technology Readiness Level assessment TRL 3-4 below is based on stage 1. This will be updated if, and before, the project progresses to stage 2.

Stage 2 – demonstration at either 275kV or 400kV. This will include the purchase of a small number of units and trial at a suitable substation on the England and Wales transmission network with the objective of establishing real life construction costs, operational performance and reliability.

If the economic assessment at the end of stage 1 remains valid and the performance of the solution is proven to be satisfactory, consideration will be given to stage 3.

Stage 3 – testing the practicalities and economics of relocating the units. This will include planning and, if economically justifiable, testing the practical reality of relocating the units from the one site (the trial site) to another (either the Deeside test facility or an operational site at which additional impedance will be beneficial)

Benefits

The measures of success that will be used to inform the decision to continue with the project at each of the stage gates are:

Stage 1:

• A concept design for the solution that can be used at 275kV or 400kV,

• A NOA based assessment of economic viability based on concept design costs.

Stage 2 (if it progresses):

• Schedule of tests to be undertake to test performance and reliability of the units

• Specifications, design drawings, risk assessments and method statements for the construction and commissioning of the units,

• Objective evidence of the performance of the devices to add the designed impedance,

• Confirmation of actual construction and commissioning costs,

Stage 3 (if it progresses):

• Specification, risk assessments and method statements for the decommissioning and dismantling of the devices,

• Testing schedule to confirm devices remain fully functional after relocation and installation at another site,