Any capacitance in an AC
network can produce a risk of resonance with the inductive parts of the
network. Although electrical networks are designed not to have any resonances
at fundamental frequencies, when the multiple frequency effects of harmonic
distortions are considered, there is always the possible risk of system
resonance.
Effects of harmonics on
capacitors and capacitor banks are as follows:
- Resonance imposes considerably higher voltages and currents in capacitors.
- The capacitor bank acts as a sink for higher harmonic currents, which increases the heating and dielectric stresses.
- The losses in a capacitor are proportional to the reactive output (kVAR), which, in turn, is proportional to the frequency. These losses are increased, and the overall capacitor life is shortened with increasing harmonics.
🔺 To avoid or minimize such
problems, capacitor banks can be tuned to reject certain harmonics by adding
reactance.
In most industrial
harmonics power systems, the primary objective for installing capacitors is to
meet the utility power factor requirements as expressed in its tariff rates. Additional
benefits are better voltage regulation and lower losses.
Commonly used locations are
shown in Figure 2 below.
 |
| Figure 2 – Typical SLD for an industrial system |
Any capacitor bank can be a
source of parallel resonance with the system inductance.
Avoiding resonance problems
The best approach to avoid
resonance problems is to install large capacitor banks at the main bus.
This
solution offers the following advantages:
- More available reactive power to the system as a whole
- Easier control of harmonic voltages and currents
- Lower capital costs, as large banks are more economical in terms of purchase cost
- Reactors can be added to shift the resonant frequency away from the characteristic harmonic frequency of the plant
Capacitors can also be combined with reactors to develop harmonic filters at the troublesome resonance harmonic frequencies.The resonant frequency at the capacitor bus can be calculated by:
Where:
fr is resonant frequency
fs is system frequency, 50
Hz
kVAsc is three-phase system
fault level in kVA
kVAc is three-phase
capacitor-bank rating in kVA
