- short if between 1 second and 3 minutes;
- long if over than 3 minutes;
- transient if less than 1 second;
Voltage interruptionsThe interruptions are the complete loss of supply voltage or loss of load.
They are classified in:
Voltage holesVoltage dips are sharp drops of voltage lower than 90% of the nominal voltage, for a time between 10 ms and 1 minute. The Standard indicates generic limits in terms of the number of events a year, from a few tens to a thousand, depending on the type of supply. This phenomenon can be determined by the start-up of electric motors, the insertion of transformers and short-circuit faults on the network.
The reference standard IEC EN 61000-4-11: electromagnetic compatibility (EMC) - Part 4-11: Testing and measurement techniques - Voltage dips, short interruptions and voltage variations immunity tests.
Frequency variationThe European frequency of 50 Hz must be maintained for 95% of the supply year within a tolerance of ± 1%, while at any time, it must not exceed an increase of 4% or a decrease of 6%. As an average value, the standard assumes the one measured within an interval of 10s.
The frequency variation may be due to faults in the generation and transmission system, or to unexpected deactivation of generators.
These phenomena can induce variations in engine speed and possible malfunction on electronic equipment.
Unbalance of tensionsIt is when the voltages supplied are different and can be caused by the external power supply network or, which happens more often, from loads of the plant. More specifically, it occurs in three-phase power systems where a branch provides a higher power than single-phase equipment, while the system continues to supply power to three-phase loads.
The negative effect that could be had is the heating in the motors and the failure of the engine control devices.
The Standard IEC EN 50160 establishes, with regard to LV electrical systems, that "under normal operating conditions for each period of a week, 95% of the effective average values, calculated in 10 minutes, of the reverse sequence component of the supply voltage must be in the range between 0 and 2% of the direct sequence component".