Service continuity and quality


EU28Continuity is the most important parameter for measuring the performance of the electricity service. All the segments of EU29the electricity system (generation, transmission and distribution) contribute to the final result: to ensure society the availability of electricity with outages below pre-established thresholds and with appropriate standards of technical quality.

Terna has always monitored service quality by means of various indexes and identifies specific targets as annual goals for improvement. The following pages show benchmarking trends of “service quality and practices of Terna's NTG plants”, as defined by AEEG Resolution 250/04 and Terna's Grid Code.

It should be noted that the variation of the indexes within the given time period does not indicate significant trends: each index undergoes indeed minimal changes in relation to the whole service measured. Moreover, among the causes of variation, are both external factors, such as weather conditions, and events (e.g. failures) attributable to the management of the NTG: an analysis of the latter shows no systematic trends.

Service continuity is a goal that since 2008 has also been the subject of incentives by the Electricity and Gas Authority (AEEG) through specific bonus/penalty schemes based on performance compared to predetermined targets. In 2011, the two indexes for which incentives were provided have performed better than the targets set by the AEEG:

  • Regulated Energy Not Supplied (R-ENS) amounted to 1,210 MWh, lower than the target of 1,369 MWh;
  • Number of Outages per Users (NOU) equaled 0.179, lower than the target of 0.209.

During 2011, the Company continued the campaign to measure the voltage quality data in its plants through the monitoring network that has been operating since 2006. The campaign also included cooperation with end HV customers and distributor companies. Devices installed on the grid provide important information on the quality of electricity supplies.


ASA (Average System Availability)

This measures the average availability of the electricity grid components for use in a given period.
This index can be expressed with regard to specific categories (for example, by voltage level), grid areas, or – as in this case – the entire National Transmission Grid. Service performance improves as the indicator level rises. The performance achieved in 2011 exceeded the target set.



Short Average Interruption Frequency Index + Medium Average Interruption Frequency Index (SAIFI+MAIFI)

This interruption frequency index is calculated as the ratio between the number of customers involved in short (less than 3 minutes) and long (more than 3 minutes) interruptions and the number of users of the National Transmission Grid. A lower indicator level indicates better service performance. The performance achieved in 2011 exceeded the target set.

Continuity indicator


Average Interruption Time (AIT)

The average interruption time of the electricity system (NTG) in one year. It is calculated as the ratio between the energy not supplied in a given period (ENS value) and the average power absorbed by the system in the period in question. The figure is rounded to the second decimal. A lower indicator level indicates better service performance. The performance achieved in 2011 exceeded the target set.

System continuity indicator


Energy Not Supplied (ENS)

Until 2007 the ENS indicator was used as an internal process indicator for the purpose of constantly improving Terna’s performance. This indicator regarded the energy not supplied to users directly connected to the NTG because of events that affected the latter and did not consider the shares that were due to significant incidents(2).

Regulated Energy Not Supplied (RENS)

Since 2008, with AEEG Resolution 341/07, the Authority regulated the quality of the service provided by Terna through a mechanism based on incentives and penalties, which, among other things, revised the ENS indicator. The new index also includes the energy not supplied to directly connected users caused by events on other connecting grids that are not part of the NTG and a share of the energy not supplied because of force majeure events or by significant incidents.

Service continuity indicator

(2) “Significant incident” means any interruption with net energy not supplied exceeding 250 MWh. The share that affects the ENSR index is a percentage that decreases as the energy not supplied increases during a single significant incident.