Page 51 - Energize September 2022

TECHNICAL

Capacitor banks Static VAR generators (SVG)
Fixed capacitor banks (FCBs) are used where there is a single The SVG overcomes the problem of a varying PF by continuously
load, and where there is little variation in PF. The capacitor’s size monitoring the voltage and current and continuously correcting the
is chosen to match the PF of the load. FCBs are most commonly PF. The SVG consists of a combination of capacitors and inductors
installed at the connection point of the load. controlled by SCR switches to provide a continuously variable
Automatic capacitor banks (ACBs) are used where the load and reactance. A typical example is shown in Figure 5.
the PF vary over a considerable range. An ACB consists of a bank of SVGs are available for industrial and commercial use from as low
capacitors which are switched in and out of circuit according to the as 2 kVA up to hundreds of kVA.
instantaneous PF of the combined loads. ACBs are most commonly
installed at the common point feeding the loads, such as the main
distribution board.
PFC-using capacitors may be applied either at a common point
or individually at each load as shown in Figure 4. The choice will
depend on the characteristics of the location. With centralised
PFC there is increased loss in the cables connecting the load to the
common point. Individual PFC requires a capacitor at each load,
which can be more expensive than a common PFC.
Shunt capacitance also acts as a shunt path for harmonics, and
the reactance will decrease with increasing harmonic numbers.

Figure 5: Static VAR generator basics (EEP)

Harmonic correction
To solve distortion PF problems, there are two options:
• Passive PF correction: Improves PF by filtering out harmonics using
passive filters. This is typically used in low power applications but
is not enough at high power.
• Active PF correction: Uses a switching converter to modulate
the distorted wave in order to shape it into a sinewave. The
only harmonics present in the new signal are at the switching
frequency, so they are easily filtered out. This is considered the
Figure 4: Common vs individual PFC.
best PFC method but adds complexity to the design.
Harmonic resonance
A serious concern arising from the use of capacitors in an electrical Passive filters
power system is the possibility of system resonance. This effect Passive filters consist of capacitors and inductors tuned to the
imposes voltages and currents which are higher than would be the harmonic frequency which needs to be reduced and are used where
case without resonance. Harmonic resonance in a power system there is single load, and the harmonic content is stable. The filter
may be classified as parallel or series resonance, and both types may be shunt or series form. This is typically used in low power
are present in a harmonic-rich environment. applications but is not enough at high power. Passive filters are used
Parallel resonance causes current multiplication, whereas series in situations where harmonic distortion is constant, are of fixed
resonance produces voltage magnification. Substantial damage design and are thus relatively cheap.
to capacitor banks would result if the amplitude of the offending The filter, connected in parallel with the supply, consists of a
frequency is large enough during resonant conditions. Also, there is combination of capacitive and inductive components, providing a
a high probability that other electrical devices on the system would low impedance path at the tuned frequency. Designed for a specific
be damaged too. site, passive filters may consist of a filter for a single harmonic
For this reason, harmonic analysis must be performed frequency or may consist of a bank of parallel filters covering several
before installation of a PF improvement capacitor bank, to frequencies. Figure 6 shows several possible configurations. Filters
ensure that resonance frequencies do not correspond with may also be of the damped wideband type used to cover a range of
prominent harmonics contained in the currents and voltages. harmonics.
Filtering efficiency is defined by the impedance ratio of the There is a move away from passive solutions to active ones
passive filter impedance and the network impedance, and it in low and medium-voltage applications. Typical loads, such as
is difficult to guarantee compliance with standards by using variable frequency drives, low-energy lighting systems, etc., which
passive filters incorporate PFC in their design, have a good PF which could lead

energize | September 2022 | 49