Power frequency overvoltages (POP)


Besides the phenomenon of transient overvoltages, which may affect any type of conductor, electric transmission lines can transmit another type of overvoltage, known as power frequency overvoltages. These are considered to
be any voltage increase above 10% of the effective nominal value for an indefinite period. POP overvoltages are caused by problems in the electricity distribution network or, more commonly, by bad connections or breakage
of the neutral conductor.

Most electric distribution systems in the world use a neutral conductor, usually grounded, which acts as a reference for the phase voltages. The return current through this conductor allows the effective voltage between each phase and neutral (line-to-neutral voltage) to remain constant. Therefore, if this conductor should break, the line-to-neutral voltage would become decompensated; the voltage received by any installation connected
between phase and neutral is floating and depends on the load imbalance in the three-phase network. An increase in effective voltage may lead to the premature aging of receivers, current consumption excess or even destruction, with the resulting fire risk.



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