Skip Rate Exclusions
NESO excludes orders from the skip rate calculations to reflect operational reasons why orders might not have been accepted. This page sets out what each exclusion stage removes. For how the calculation works before any exclusion, see Skip Rates.
NESO states its skip rates at Stage 0, before any exclusion, and then at Stages 1 to 5, each applying a further set of exclusions on top of the last. A Stage 6, published for PSA only, also removes units providing Mandatory Frequency Response. NESO's headline skip rate is PSA at Stage 5.
An exclusion removes an order from the Feasible Merit Stack. If the order was accepted, its acceptance can also leave the Acceptance Stack, which reduces the Imbalance Requirement. The table below shows what leaves each stack at each stage.
| Stage | Feasible Merit Stack | Accepted Stack (Imbalance Requirement) |
|---|---|---|
| 1 | Wind offers | Accepted wind offer volume on any bid-offer pair, including Unwinds |
| 2 | Orders which would exacerbate an Active Constraint or push a boundary beyond its limit | Acceptances NESO deems taken to manage an Active Constraint (PSA only. Under All BM they are forced back into the stack.) |
| 2 | Unaccepted volume from Inaccessible Very Long Notice Units and Ramping Units | - |
| 2 | Orders from units with an invalid physical or dynamic parameter (see note below) | - |
| 3 | - | Acceptances system tagged in Elexon's data, not already caught by Stage 2 (PSA only: under All BM they are forced back into the stack.) |
| 4 | Unaccepted orders which would lead to Unwinds | - |
| 5 | Unaccepted volume from Inaccessible Long Notice Units | - |
| 5 | Volume from 0 to SEL (offers) or 0 to SIL (bids) on Long Notice Units that were off and then accepted | The same accepted volume, from 0 to SEL (offers) or 0 to SIL (bids) |
| 5 | Bids that would take a running Long Notice Unit offline: it must stay between SEL and MEL, or SIL and MIL | - |
| 5 | Pumped Storage / Hydro units whose Dispatched Level would need to pass through 0 | - |
| 6 | Units providing Mandatory Frequency Response (MFR), PSA only. NESO does not publish which stack. |
Under All BM, acceptances excluded at Stages 2 and 3 for system reasons are forced into the In Merit stack rather than removed. How that, and the shrinking Imbalance Requirement, moves a unit's skip rate at stages where the unit itself is not excluded is explained in Skip Rate Exclusion and Displacement.
The definitions of inaccessible units come from the Stage 2 and Stage 5 steps of NESO's Skip Rate Methodology and Implementation Guide (v2.0), which apply only to units not accepted in the period.
"Invalid physical/dynamic parameter" appears in NESO's exclusion data and in its list of exclusion reasons by stage, but neither NESO document defines it.