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:
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
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