Page 8 - ELECTRONICON_Katalog_PFC-60Hz

INTRODUCTION

COMMON METHODS OF POWER FACTOR CORRECTION

SINGLE OR FIXED PFC, compensating for the reactive power constant power single-
of individual inductive loads at the point of connection so red- demand and long or group-fixed
ucing the load in the connecting cables (typical for single, duty cycle PFC
permanently operated loads with a constant power),

GROUP PFC - connecting one fixed capacitor to a group of variable power automatic
simultaneously operated inductive loads (e.g. group of mo- demand and/or (“bulk“)
tors, discharge lamps), variable duty cycle PFC

BULK PFC, typical for large electrical systems with fluctuating load where it is common to connect a number of capacitors to a
main power distribution station or substation. The capacitors are controlled by a microprocessor based relay which continuous-
ly monitors the reactive power demand on the supply. The relay connects or disconnects the capacitors to compensate for the
actual reactive power of the total load and to reduce the overall demand on the supply.

A typical power factor correction system would incorporate a number of capacitor sections determined by the characteristics and
the reactive power requirements of the installation under consideration. Sections of 12.5 kvar, 25 kvar, and 50 kvar are usually
employed. Larger stages (e.g. 100 kvar and above) are best achieved by cascading a number of smaller sections. This has the
beneficial effect of reducing fluctuations in the mains caused by the inrush currents to the capacitors and minimizes supply di-
sturbances. Where harmonic distortion is of concern, appropriate systems are supplied incorporating detuning reactors.

INFLUENCE OF HARMONICS, HARMONIC FILTERING

Developments in modern semiconductor technology have led to a significant increase in the number of thyristor- and inverter-fed
loads. Unfortunately these non-linear loads have undesirable effects on the incoming AC supply, drawing appreciable inductive
reactive power and a non-sinewave current. The supply system needs to be kept free of this harmonic distortion to prevent
equipment malfunction.
A typical inverter current is composed of a mixture of sinewave currents; a fundamental component at the supply frequency and
a number of harmonics whose frequencies are integer multiples of the line frequency (in three phase mains most of all the 5th,
7th, and 11th harmonic). The harmonics lead to a higher capacitor current, because the reactive resistance of a capacitor reduces
with rising frequency. The rising capacitor current can be accommodated by constructional improvements in the manufacture of
the capacitor. However a resonating circuit between the power factor correction capacitors, the inductance of the feeding trans-
former and/or the mains may occur. If the frequency of such a resonating circuit is close enough to a harmonic frequency, the
resulting circuit amplifies the oscillation and leads to immense over-currents and over-voltages.

Harmonic distortion of an AC supply can result Typical non-linear loads (generating harmonics)
in any or all of the following: • converters, rectifiers, inverters, choppers
• Premature failure of capacitors. • thyristor controls, three-phase controllers
• Nuisance tripping of circuit breakers • electronic valves
and other protective devices. • phase controls
• Failure or maloperation of computers, motor drives, • UPS units (inverter technology)
lighting circuits and other sensitive loads • discharge lamps with magnetic ballasts

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