Capacitors harmonic effects

Referring to the data available from experiments, as shi)wn in Table 23.1, it hits been estimated that a Vp, of I. Hj should be sufficient to account for the harmonic effects. For this dielectric strength is designed a capacitor unit and selected a switching or protective device. 

This means that the capacitor will offer a low reactance to the higher harmonics and will tend to magnify the harmonic effect due to higher harmonic currents on account of this. In fact, harmonic currents have a greater heating effect loo compared to the fundamental component due to the skin effect (Section 28.7),... 

Based on the system studies carried out and Table 23.1, it has been assessed that in actual operation, effective current through a capacitor circuit may increase up to 1.3 limes its rated cunent, /,., i.e. = 1.3 /, to account for all the harmonic effects (V, - Jf. equation (23.4)). A capacitor unit is thus designed for at least M)9r continuous overload capacity (Section 25.6). Its switching and protective devices are selected along similar lines. 

It is desirable to contain the harmonic effects as far as practicable to protect the capacitors as well as inductive loads connected on the system and the communication network, if running in the vicinity. 

Suitable lor systems svhere fre( uenl switching is not likely and the system is I ree frotn harmonic generating sources. They are the sell-healing type and their output may become reduced with lime (bectiuse of railures of capacitor elements as a result of ssvitching inrush currents and. system harmonics). If the system conditions are not conducive, an inductor coil may be provided to limit the harmonic effects (Section 23.9). Some manufacturers, as standard practice, provide an inductor coil inside the shell to contain the inrush current and also dampen the harmonics. 

In this part the author provides all relevant aspects of a reactive control and carries out an exhaustive analysis of a system for the most appropriate control. Harmonic effects and inductive interferences as well as use of filter and blocking circuits are covered. Capacitor switching currents and surges and methods of dealing with these are also described. 

But as the harmonics do exist in the system, they do affect an inductive load. They may also disturb a communications network as a result of capacitive coupling, whose effects are magnified in the presence of capacitor units in the power system. It is therefore considered relevant to discuss this subject in more detail to make the harmonic study more informative. 

The rating of a capacitor unit will thus vary in a square proportion of the effective harmonic voltage and in a direct proportion to the harmonic frequency. This rise in kVAr, however, will not contribute to improvement of the system p.f. but only of the overloading of the capacitors themselves. 

Refer to Table 23.1, which shows the average cumulative effect of all the harmonics that may be present in a power system. If we can provide a series reactor of 6% of the total kVAr of the capacitor banks connected on the system, most of the harmonics present in the. system can be suppressed. With this reactance, the system would be tuned to below the fifth harmonic (at 204 Hz) for a 50 Hz system as derived below. 

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