The products > Harmonics > Influence of harmonics on capacitors

INFLUENCE OF HARMONICS ON CAPACITORS

Note: since the inductance of the motor is much higher than that of the source, it
becomes negligible in a parallel assembly.

• Scc (kVA): Short-circuit power of source
• Q (kvar): Capacitor bank power
• P (kW): Non-interfering load power

a) Decrease in capacitor reactance

The reactance of capacitor 
 

is inversely proportional to the frequency, its curve is reciprocal and its ability to block harmonic currents decreases considerably when the frequency increases.

b) Parallel resonance or anti-resonance between the capacitors and the source

• The reactance of the source XLT is proportional to the frequency.
• The reactance of the capacitors XC is inversely proportional to the frequency.
At the frequency Fr.p., there is parallel resonance or anti-resonance (since the two reactances are equal but opposite) and amplification (F.A.) of the harmonic currents in the capacitors and in the source  (transformer) where:

It is important to note that:

• the higher the short-circuit power of the source (Scc) is, the further the
resonance frequency moves away from the dangerous harmonic
requencies.
• the higher the power (P) of the non-interfering loads is, the more the
amplification factor of the harmonic currents is reduced.

Main harmonic current:

The main harmonic currents present in electrical installations are produced by semi-conductor based systems, i.e.:

harmonic 5 (250 Hz) - 15 = 20% I1
harmonic 7 (350 Hz) - 17 = 14% I1
harmonic 11 (550 Hz) - I11 = 9% I1
harmonic 13 (650 Hz) - I13 = 8% I1

*I1 Current of semi-conductor system at 50 Hz.

Harmonics summary

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