Harmonics Currents

Overloading of neutral conductors due to third harmonic currents flowing through the neutral. The size of the neutral must be increased by 150-200% of the normal size in such cases, depending upon the severity of the harmonics. 

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),... 

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. 

When the supply system itself contains harmonics and the voltage is already distorted now even the linear loads will respond to such voltage harmonics and draw harmonic currents against each harmonic present in the system, and generate the same order of current harmonics. 

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

It is pertinent to note that since a filter circuit will provide a low impedance path to a few harmonic currents in the circuit (in the vicinity of the harmonic, to which it has been tuned) it may also attract harmonic currents from neighbouring circuits which would otherwise circulate in those circuits. This may necessitate a slightly oversized filter circuit. This aspect must be borne in mind when designing a filter circuit for a larger distribution network having more than one load centre. 

Shenoy, G., Reducing harmonic currents and improving power factor in installations having predominant thyristor loads. Siemens Circuit. XVII, No. 1 (1982). 

Higher harmonic contents may magnify the harmonic currents and affect the loading capacity of the line. 

A filter circuit to absorb the harmonic currents generated by TCRs and in certain conditions, w hen TCR is OFF. ... 

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

To limit the inrush and harmonic currents through the capacitor banks. 

These are meant to provide high impedance to harmonic currents and block their entry or reduce their amplitudes and are therefore also known as blocking reactors. They may have any of the magnetic characteristics shown in Figure 27.2 and have a fixed reactance, rated continuously. 

Martin, Robert Harmonic Currents, Compliance Engineering—1999 Annual Resources Guide, Cannon Communications, EEC, 103-107. 

A capacitor bank will represent reducing impedance to currents of increasing frequency. Such reducing impedance, if matched with a similarly increasing inductance impedance of a transformer or a supply system, can cause a resonant condition. In plants where equipment produces harmonic current, a full survey of the installation is recommended prior to installation of the capacitors. 

It can be seen that it was again difficult to obtain results from specimens where no stable rest potential could be measured. The harmonic currents in all cases were low and for certain specimens were of the same order as the distortion resulting from the input sine wave. The Tafel slopes obtained were in general anomalously high and the corrosion rates varied over several orders of magnitude. 

Branford S, Cross NC, Hochhaus A et al. Rationale for the recommendations for harmonizing current methodology for detecting BCR-ABL transcripts in patients with chronic myeloid leukaemia. 

Observed lineshapes in IETS are further modified by the finite modulation voltages used. We can write the second harmonic current signal at a bias voltage Vq for a given non-linear I-V characteristic as (10) ... 

The frequency dependence of SHG at simple metal surface has been the focus of a recent theoretical study of Liebsch [100]. Time-dependent density functional theory was used in these calculations. The results suggest that the perpendicular surface contribution to the second harmonic current is found to be significantly larger than had been assumed previously. He also concludes that for 2 a> close to the threshold for electron emission, the self-consistently screened nonlinear electronic response becomes resonantly enhanced, analogous to local field enhancement in the linear response near the bulk plasma frequency. 

Arc furnaces are operated in conjunction with large capacitor banks and harmonic filters to improve the power factor and also to filter the harmonic frequency currents so they do not unduly affect other power users sharing the same power fines. It is not uncommon to see arc furnaces supplied from dedicated utility power fines to minimize their impact on other power users. The presence of large capacitance in an electrical system can result in voltage rise due to the leading reactive power demands of the capacitors, unless they are adequately canceled by the lagging reactive power required by the loads. This is why capacitor banks, whether for power factor correction or harmonic current filtration, are switched on when the furnace is brought on line and switched off when the arc furnace is off line. 

Arc furnaces are normally operated with capacitor banks or capacitor bank/fil-ter circuits, which can amplify some of the characteristic frequency harmonic currents generated by the furnace, leading to severe light flicker. For arc furnace... 

Figure 4.7c shows the fifth harmonic phasors. Note that the phase sequence of the fifth harmonic currents is clockwise and opposite to that of the fundamental. The fifth harmonics are negative sequence harmonics. 

Figure 4.7d shows the seventh harmonic current phasors. The seventh harmonics have the same phase sequence as the fundamental and are positive sequence harmonics. 

The expressions shown so far for harmonics have zero phase shifts with respect to the fundamental. It is not uncommon for the harmonics to have a phase-angle shift with respect to the fundamental. Figure 4.8 depicts a fifth harmonic current waveform with and without phase shift from the fundamental. Expressions for the fifth harmonics with a phase-shift angle of 0 degrees are ... 

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