Electricity quality reflects two complementary measurements: continuity of supply and voltage quality.

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Continuity of supply

Continuity of supply is the grid’s ability to continuously supply power to all customer points of delivery. It may be affected by interruptions in supply or outages (for instance a weather event with lightning). Continuity of supply is estimated based on equivalent outage time and outage frequency.

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What is an exceptional event?

Electricity quality indicators are calculated either “excluding exceptional events” or “including exceptional events.”

In application of article 19 of the Specifications for the Public Transmission System, exceptional events (or major events) are events that are independent of RTE’s will or actions and beyond its control, such as:

• atmospheric phenomena that are exceptionally significant in terms of impact on networks;
• decisions by public authorities to take infrastructure out of service for reasons related to public safety or public order, when this decision is not due to behaviour or inaction on the part of the concession holder;
• the sudden, unplanned and simultaneous unavailability of several generation units connected to the public transmission system, when the power made unavailable exceeds what the safety rules mentioned in article 28 of the Specifications for the Public Transmission System allows;
• natural disasters as defined in law 82-600 of 13 July, 1982, as modified;
• damage caused by accidental and uncontrollable events, attributable to third parties, such as fires, explosions, or plane crashes;
• destruction due to acts of war, riots, pillage, sabotage, terror attacks, or criminal acts.

If operations are disrupted, RTE takes all appropriate measures to return to normal operating conditions as quickly as possible.

Equivalent outage time

Expressed in minutes + seconds, equivalent outage time is calculated as the ratio between energy not delivered during long power cuts (non-distributed energy, NDE) and the average power supplied during the year to all industrial customers and distributors.

NDE is calculated, for each long outage, as the product of the power withdrawn at the time of the outage and the time until supply is fully restored, from which is deducted, where applicable, the energy supplied through another network source. This energy is expressed in megawatt hours (MWh)

In 2020, equivalent outage time was 3 minutes and 4 seconds, excluding exceptional events. This was in line with the average for past years.

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Outage frequency

Outage frequency is the average number of outages per customer site per year. It is broken down into long and short outage frequency. Short outages last between 1 second and 3 minutes. Long outages last more than 3 minutes.

In 2020, outage frequency was the best on record: 0.341 outage/site, excluding exceptional events.

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Short outages and lightning density

Lightning density is a leading determinant of the brief outages observed during the year, and thus has an impact on electricity quality. In most cases, short outages are more frequent in the regions that see the most lightning. Conversely, regions where lightning is rare show a lower frequency of brief outages.

The rate of occurrence of lightning was relatively low in the first half of 2020, despite significant local storm activity. This explains why lightning density ended 2020 at a low 0.544 strike per km2 across France.

Voltage quality

Different factors can affect voltage quality. It is important to draw a distinction between factors that cause: momentary disruptions resulting from specific events (hazards for grid components, weather conditions, equipment damage, actions by third parties…), such as voltage dips, and phenomena continuously impacting the nominal characteristics of the voltage wave, related to the functioning of the facilities connected to the grid (imbalance, rapid fluctuations in voltage, harmonics…) and to variations in consumption and generation (frequency variations, slow voltage variations).

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Voltage management

RTE must keep voltage on the grid stable to ensure that the system operates safely, to prevent the risk of equipment deterioration, and to meet its contractual commitments.

Voltage levels notably depend on the electricity carried over the lines, which fluctuates with consumption: lower demand means higher voltage, and vice versa: voltage is lower when demand is greater.

To keep voltage within contractual ranges, RTE can use different levers: generating units, compensating equipment (particularly reactors, condensers and Static VAR Compensators), and grid topology. Since all electrical areas do not react in the same manner, it may be necessary to invest in compensating equipment.
In 2020, substation reactors were installed in Revigny (55), Aube (61), Pusy (70) et Cirolliers (91).