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PFC in presence of Harmonics

The installation of capacitors in presence of current harmonics (voltage harmonics are generally irrelevant)
implies a series of problems to carefully evaluated.
Capacitors amplify the existing harmonics
and undergo current overloads which may
exceed their tolerable limits and resonance
phenomena may occur which could be
extremely dangerous both for the capacitors
as well as for the entire electrical system
When the frequency of the harmonic
coincides with the resonance frequency of
the capacitors/network/transformer circuit,
Harmonic current
the impedance of the system is minimum and the current reaches maximum value even though voltage
could remain constant.
At worst capacitors may explode but, if they do not, they anyway undergo a continuos overload and
overheating generating a very rapid deterioration of their dielectric therefore reducing drastically their useful
life; it is common practice to consider dead a capacitor when 20-30 % of its initial capacitance is lost.
Thereafter avalanche effects may take place which may generate explosion of the capacitors.
In order to limit these amplification phenomena, a PF correction system in presence of heavy harmonics is
made with capacitors protected by series chokes (see dwg ) generally arranged into a single three phase
choke) which form a low pass filter being “detuned” at a frequency LOWER than the frequency of the
Harmonics circulating in the line.
Scope of the choke(s) is therefore to avoid
resonance and amplification phenomena
and the consequent harmonics overload on
The “detuning” frequency of the chokecapacitors determines the “protection
factor” of the capacitors and it is generally
calculated for 189 Hz as per diagram below
taking into account that industrial non linear
loads (see also page 2) generate
harmonics from the 5th harmonic up.
Capacitors protected by chokes need
additionally to be oversized in dielectric due to the over-voltage generated by chokes across capacitors and
which depends on chokes’ detuning frequency. For example, the over-voltage is 8-10 % for a 189 Hz
detuning frequency.
A “detuning” frequency of 189 Hz is generally considered because :
industrial converters (6 diode total controlled) generate only harmonics above the 5th
- the value considered implies an impedance for the 5th harmonic sufficiently low as to be protected even
in case of loss of capacitance by the capacitors (tuning frequency shifts upward.
- lower chokes detuning frequencies (example 134 Hz = higher protection factor) imply much larger
chokes, higher capacitors voltage, higher dissipation and much higher size and cost of the PF
compensation equipment therefore they are generally considered only in those cases where other
solution is not possible.
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Three-phase 6 pulse converters
They represent the great majority of the nonlinear loads in industrial application.
A typical three phase 6 pulse converter
generates a wave form as indicated in the
diagram which is the result of a 50 Hz
sinusoid (linear) and a square waveform.
The above waveform may be considered as
the summation of several odd harmonics
according to the FOURIER transformed
IΣeff = √ Ih1 +Ih3 + Ih5 + Ih7 + ….
Where however the 3rd harmonics is not present in 6 pulse converters.
In terms of THD such converter generates a max theoretical THD-I% = 28,43% therefore values of THD-I
greater than 30% (measurement at converter output only) are generally anomalous and may be the
symptom of something wrong in the converter(s).
The converters are normally in parallel to other linear loads therefore a measurement taken at overall mains
input is likely to show a THD-I resulting from the summation of linear + non linear loads therefore it should
normally be below a 30% and show odd harmonics from the 5th UP.
If instead the readings indicate THD values > 30% or even harmonics or harmonics below the 5th order , this
may be the sign of :
1. non linear loads that do not belong to the above category (example 3 pulse converters i.e. older
technology) or other power supply systems. In this case, a specific investigation must be carried in order
to design a suitable PFC system.
2. something is wrong in the converters (broken or non calibrated diodes/control system) or with the
electrical system (transformers, impedances, saturated magnetic components, etc)
In this case the harmonic content may heavily impair both the correct performance of the machines
generate higher losses (cables, transformers, machines, etc.), imply a higher electricity bill and imply a
much higher cost of the PFC equipment.
It is therefore wise in these cases to first eliminate the causes of the problem then to proceed with a correctly
designed PFC equipment avoiding all the extra cost of a useless redundant design.
Harmo (EN0110)
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