Harmonics capacitors

When harmonic filter circuits are connected to a power system with saturated reactors (Figure 27.2(e)) resonance may occur between the reactor and the filter capacitors to give rise to overvoltages. 

Capacitors themselves do not generate harmonics but they do magnify their amplitudes, when connected in... 

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. 

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. 

Summarizing the above, the harmonic quantities when present in a system on which are connected a few capacitor banks affect the capacitors as follows ... 

Harmonic output of a capacitor unit The rating of a shunt capacitor unit... 

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. 

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. 

It is common practice to leave the star-connected capacitor banks ungrounded when used in the system or use delta-connected banks to prevent the flow of third harmonic currents into the power system through the grounded neutral. 

A filter circuit is a combination of capacitor and series reactance, tuned to a particular harmonic frequency (series resonance), to offer it the least impedance at that frequency and hence, filter it out. Say. for the fifth harmonic, =... 

Yet another reason for harmonic disorder in a power circuit is the occurrence of series or parallel resonance or a combination of both. Such a situation may occur at certiiin frequencies generated by the harmonic generating sources in the system. The capacitors installed in the system magnify the same. 

In an HT system, either the star is not grounded or it is a delta-connected system and hence the third harmonic is mostly absent, while the content of the. second harmonic nuiy be too small to be of any significance. For this purpose, where harmonic analysis is not possible, or for a new installation where the content of harmonies is not known, it is common practice to use a series reactor of 6% of the reactive value of the capacitors installed. This will suppress most of the harmonics by making the circuit inductive, up to almost the fourth harmonic, as derived subsequently. Where, however, second harmonics are significant, the circuit may be tuned for just below the second harmonic. To arrive at a more accurate choice of filters, it is better to conduct a harmonic analysis of the system through a harmonic analyser and ascertain the actual harmonic quantities and their magnitudes present in the system, and provide a correct series or parallel filter-circuits for each harmonic. 

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. 

Figure 23.15(a) Compensation of harmonics by tuning each capacitor circuit at different harmonics... 

Figure 23.14 Compensation of harmonic currents by the use of reactors in the capacitor circuits...

But w hen the third and/or second harmonics are also present in the system, at a certain fault level it is possible that there may occur a parallel resonanee between the capacitor circuit and the inductance of the system (source), resulting in very heavy third or second harmonic resonant currents, which may cause failure of the series reactor as well as the capacitors. In such cases, a 6% reactor will not be relevant and a harmonic analysis will be mandatory to provide more exacting filter circuits. 

When a capacitor circuit is compensated through a series reaetor. either to suppress the system harmonics or to limit the switching inrush currents (Section 23.11) or both, it will require suitable adjustment in its voltage and capacitive ratings, fhe series reactor will dampen the switching currents but consume an inductively reactive power and offset an equivalent amount of capacitive kVAr. and require compensation. The following example w ill elucidate this. 

In TSCs the thyristors are used in anti-parallel to switch a capacitor bank ON or OFF but without any phase angle control. A TSC therefore does not by itself generate any harmonics, unlike a TCR. 

Note Capacitors and reactors may be A connected to eliminate triple harmonics... 

D Thyristor switched capacitors Filter circuit to absorb harmonic currents caused by TCR... 

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. 

The factors discussed in Section 23.5.2 give rise directly to the current drawn by the capacitor unit and indirectly add to its rating. The relevant Standards on this device recommend a continuous overload capacity of 30% to account for all such factors. A capacitor can have a tolerance of up to -t-15% in its capacitance value (Section 26.3.1(1)). All current-carrying components such as breakers, contactors, switches, fuses, cables and busbar systems associated with a capacitor unit or its banks, must therefore be rated for at least 1.3 x 1.15/,., i.e. 1.54. For circuits where higher amplitudes of harmonics are envisaged, for reasons of frequent load variations or more... 

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