System for monitoring the electric network voltage waveform, comprising: - switching means connected to the voltage lines (A, B, C) of a three-phase system, wherein said means comprise two thyristors (T1, T2) connected to two of the voltage lines (A, B, C) and at least two capacitors (C1, C2) connected to said thyristors (T1, T2), said means being configured to open and close said thyristors (T1, T2) in response to a corresponding trigger signal (DT1, DT2); - means for measuring the voltages in said voltage lines and at the input of said capacitors (V A, V B, V C, V C1. V C2); - a thyristor trigger circuit configured to provide a trigger signal from a trigger order to either thyristor (DT1, DT2) when the voltage in the terminals of said thyristor (T1, T2) crosses zero; - control means (13, 23) configured to give said trigger order from the measured voltages (V A, V B, V C, V C1, V C2) or not; - a zero crossing detector (235, 535); wherein said control means (13, 23) comprise: - a PLL (231, 331) configured to obtain an image signal from the voltage signals of said voltage lines (A, B, C), from the phase angle (φ) of the three-phase system and from the temporal variation thereof; the system being characterized by: - an adaptive notch filter (232, 432) the constants of which are calculated from the derivative of said phase angle (φ), configured to provide filtered voltage signals between the voltage lines (V AC_f, V BC_f), wherein said filtered signals can represent frequencies different from the fundamental frequency; - means for filtering the voltages measurement at the input of the capacitors (V C1_f, V C2_f); and wherein said zero crossing detector (235, 535) is configured to provide said trigger order from said filtered voltages (V AC_f, V BC_f, V C1 _ f, V C2_f); and wherein said PLL (231, 331) comprises: - means for applying a Clarke transformation to the voltage signals (V A V B V C) to convert the balanced three-phase system into an orthogonal stationary quadrature two-phase system (X D X Q); - means for applying a Park transformation to the two-phase quadrature (X D X Q) to convert the orthogonal stationary two-phase system (X D X Q) into a rotating orthogonal system (X d X q); - means for applying a proportional-integral-derivative control to obtain the rotation speed of the system and the frequency of the original electrical system; - means for integrating said rotation speed and calculating the phase angle of the voltage from the rotation speed of the system; - means for obtaining an inverse Park transformation to convert the quadrature rotating reference frame into an orthogonal stationary two-phase system; - means for obtaining the inverse Park transformation to obtain an image of the original three-phase system (V A V B V C). - switching means connected to the voltage lines (A, B, C) of a three-phase system, wherein said means comprise two thyristors (T1, T2) connected to two of the voltage lines (A, B, C) and at least two capacitors (C1, C2) connected to said thyristors (T1, T2), said means being configured to open and close said thyristors (T1, T2) in response to a corresponding trigger signal (DT1, DT2); - means for measuring the voltages in said voltage lines and at the input of said capacitors (V A, V B, V C, V C1. V C2); - a thyristor trigger circuit configured to provide a trigger signal from a trigger order to either thyristor (DT1, DT2) when the voltage in the terminals of said thyristor (T1, T2) crosses zero; - control means (13, 23) configured to give said trigger order from the measured voltages (V A, V B, V C, V C1, V C2) or not; - a zero crossing detector (235, 535); wherein said control means (13, 23) comprise: - a PLL (231, 331) configured to obtain an image signal from the voltage signals of said voltage lines (A, B, C), from the phase angle (φ) of the three-phase system and from the temporal variation thereof; the system being characterized by: - an adaptive notch filter (232, 432) the constants of which are calculated from the derivative of said phase angle (φ), configured to provide filtered voltage signals between the voltage lines (V AC_f, V BC_f), wherein said filtered signals can represent frequencies different from the fundamental frequency; - means for filtering the voltages measurement at the input of the capacitors (V C1_f, V C2_f); and wherein said zero crossing detector (235, 535) is configured to provide said trigger order from said filtered voltages (V AC_f, V BC_f, V C1 _ f, V C2_f); and wherein said PLL (231, 331) comprises: - means for applying a Clarke transformation to the voltage signals (V A V B V C) to convert the balanced three-phase system into an orthogonal stationary quadrature two-phase system (X D X Q); - means for applying a Park transformation to the two-phase quadrature (X D X Q) to convert the orthogonal stationary two-phase system (X D X Q) into a rotating orthogonal system (X d X q); - means for applying a proportional-integral-derivative control to obtain the rotation speed of the system and the frequency of the original electrical system; - means for integrating said rotation speed and calculating the phase angle of the voltage from the rotation speed of the system; - means for obtaining an inverse Park transformation to convert the quadrature rotating reference frame into an orthogonal stationary two-phase system; - means for obtaining the inverse Park transformation to obtain an image of the original three-phase system (V A V B V C).