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AGGREGATED DATA
AGGREGATION RESULTS
VOLTAGE DIPS 2006÷2008

Frequently asked questions

Terms and Definitions
NOMINAL VOLTAGE (Un)
Voltage by which a supply network is designated or identified and to which certain operating characteristics are referred

MEDIUM VOLTAGE (MV)
Voltage whose nominal r.m.s. value is 1 kV < Un < 36 kV

DECLARED SUPPLY VOLTAGE (Uc)
Supply voltage Uc agreed by the network operator and the network user Generally declared supply voltage Uc is the nominal voltage Un but it may be different.

FLICKER
Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time. Voltage fluctuation cause changes of the luminance of lamps which can create the visual phenomenon called flicker. Above a certain threshold, flicker becomes annoying. Intensity of flicker annoyance is evaluated by the following quantities:

short term severity (Pst) measured over a period of ten minutes;
long term severity (Plt) calculated from a sequence of twelve Pst-values over a two hours interval, according to the following expression:

HARMONIC VOLTAGE
Sinusoidal voltage with a frequency equal to an integer multiple of the fundamental frequency of the supply voltage Harmonic voltages can be evaluated:

individually by their relative amplitude (Uh) which is the harmonic voltage related to the fundamental voltage U1, where h is the order of the harmonic;
globally for example by the total harmonic distortion factor THD, calculated using the following expression:

RAPID VOLTAGE CHANGE
Single rapid variation of the r.m.s. value of a voltage between two consecutive levels which are sustained for definite but unspecified durations.

SUPPLY INTERRUPTION
Condition in which the voltage at the supply terminals is lower than 5 % of the reference voltage. A supply interruption can be classified as:

short interruption (up to and including 3 min);
long interruption (longer than 3 min).

VOLTAGE DIP
Temporary reduction of the r.m.s. voltage at a point in the electrical supply system below a specified start threshold. The dip start threshold is equal to 90 % of the reference voltage.

VOLTAGE DIP DURATION
Time between the instant at which the r.m.s. voltage at a particular point of an electricity supply system falls below the start threshold and the instant at which it rises to the end threshold.

VOLTAGE DIP RESIDUAL VOLTAGE
Minimum value of r.m.s. voltage recorded during a voltage dip For the purpose of this standard, the residual voltage is expressed as a percentage of the reference voltage.

VOLTAGE SWELL
Temporary increase of the r.m.s. voltage at a point in the electrical supply system above a specified start threshold. The swell start threshold is equal to the 110 % of the reference voltage.

VOLTAGE SWELL DURATION
Time between the instant at which the r.m.s. voltage at a particular point of an electricity supply system exceeds the start threshold and the instant at which it falls below the end threshold.

VOLTAGE UNBALANCE
Condition in a polyphase system in which the r.m.s. values of the line-to-line voltages, or the phase angles between consecutive line voltages, are not all equal.
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