Grid Reliability Standards
Grid Reliability Standards (GRS) are a set of standards against which the reliability performance of the existing grid (or future developments to it) can be assessed. The Authority is required to determine and review the GRS if requested. The relevant Code provisions are set out in clauses 12.55 to 12.62.
The capability of the grid to meet the GRS is a key aspect of a number of part 12 processes to do with grid investment. The Electricity Commission developed the GRS over 2004 and early 2005.
The GRS is a schedule to Part 12 and consists of:
- an economic (probabilistic) standard for the whole grid,and the associated assessment of the costs and benefits of investment for reliability; and
- a 'safety net' minimum reliability standard of N-1 for contingencies on the core grid. N-1 means that the system is planned such that, with all transmission facilities in service, the system is in a secure state, and for any one credible contingency event, the system moves to a satisfactory state. However, if more than one contingency event was to occur, loads may have to be shed to return to a satisfactory state.
When Transpower reasonably expects that the existing connection or interconnection assets are unlikely to meet the GRS at the relevant grid exit point over the next 5 years, it must enter a process for upgrade of assets or amendment to the transmission agreement or service levels in the interconnection rules.
The Electricity Commission determined that transmission planning should be carried out using a probabilistic framework. Accordingly, reliability levels on the grid should be determined by economic considerations, taking into account the cost to consumers of losses of supply, while meeting technical and safety requirements.
On 1 April 2005, the Electricity Commission made a recommendation to the Minister of Energy for the inclusion of the Grid Reliability Standards (GRS) as a schedule to section III of part F. This recommendation was formally notified in the Gazette on 7 April 2005.
The Commission's recommendation was agreed to by the Minister and the GRS and the associated rule amendments were formally notified in the Gazette on 14 April 2005. The GRS came into force on 13 May 2005.
Peer Review of the GRS
In 2008 an external expert view of the application of the GRS and overseas grid planning trends was commissioned. The report was published in October 2008.
Core Grid Determination
In developing the GRS, the Electricity Commission signalled its commitment to pursuing an economic approach to grid reliability, strongly linking the GRS with the application of the regulatory investment test.. However, concerns about the uncertainties and implementation issues associated with moving to such an approach at this time were acknowledged with the development of a two-limb grid reliability standard. The two-limb approach consists of an economic standard for the whole grid, underpinned by a “safety net” of an N-1 standard for contingencies on the core grid. The two-limbed grid reliability standard (GRS) gives rise to the need to determine the core grid for the purpose of giving effect to the second ('safety net') limb of the GRS. The cord grid is some subset of the entire national grid on which a minimum level of reliability must be maintained. The Code requires a determination of the most appropriate core grid determination (Rule 5A of Section III, Part F).
Once the core grid is determined, the capability of the grid to meet the GRS can be fully assessed. The core grid determination also provides a basis for Transpower to prepare grid upgrade plans, and for other parties to appraise opportunities for transmission investment and transmission alternatives.
The core grid definition, including a listing of the transmission lines included in the core grid, is included in the Part F Rules (Schedule F3A). It was intended that the definition be reviewed within 5 years of initial schedule coming into effect.
The core grid recommendation to the Minister on 12 December 2005 was approved, and a Gazette Notice was published 22 December 2005. Due to an administrative error subsequently discovered in the original recommendation, a further recommendation to the Minister on 16 January 2006 amending the Gazette Notice to correct the error, and extending the date on which those rules come into force to 16 February 2006.
external-peer-review.pdf | pdf | 242 KB | Last Changed: 31/10/2010 5:23pm
This page is related to: Transmission.