jackymechanical

Effects of harmonics in power system:

Considering the trend in electricity consumption and the increasing sources of harmonics in industrial, commercial, and residential installations over the next 25 years, in that order, it shall be necessary to clearly assess the effects that unfiltered harmonics from scattered nodes in the power system will have in different equipment, on operation, and on diverse processes.

Thermal Effects on Transformers:

Modern industrial and commercial networks are increasingly influenced by significant amounts of harmonic currents produced by a variety of nonlinear loads like variable speed drives, electric and induction furnaces, and fluorescent lighting. All of these currents are sourced through service transformers. A particular aspect of transformers is that, under saturation conditions, they become a source of harmonics. Delta-wye connected transformers trap zero currents that would otherwise overheat neutral conductors. The circulating currents in the delta increase the RMS value of the current and produce additional heat in power system harmonics. This is an important aspect to watch out for. Currents measured on the high-voltage side of a delta-connected transformer may not reflect the zero sequence currents, but their effect in producing heat losses is there.

Neutral Conductor Overloading:

In single-phase circuits, the return currents carrying a significant amount of harmonic components flow through transformer neutral connections increase the RMS current. Furthermore, zero-sequence currents (odd integer multiples of 3) add in phase in the neutral. Therefore, the operation of transformers in harmonic environments demands that neutral currents be evaluated in grounded-wye connected transformers to avoid the possibility of missing the grounding connection as a consequence of overloading. In balanced three-phase, four-wire systems, there is no current on the neutral, for which the neutral currents under these conditions should be attributed to the circulation of zero sequence harmonics, which are mostly produced by single-phase power supplies. To accurately analyze power system harmonic analysis Resonant Conditions

These conditions involve in the reactance of a capacitor bank that at some point in frequency equals the inductive reactance of the distribution system, which has an opposite polarity. These two elements combine to produce series or parallel resonance. In the case of series resonance, the total impedance at the resonance frequency is reduced exclusively to the resistive circuit component. If this component is small, large values of current at such frequency will be developed. In the case of parallel resonance, the total impedance at the resonant frequency is very large (theoretically tending to infinite). This condition may turn into a large overvoltage between the parallel-connected elements, even under the small harmonic currents. Therefore, parallel resonant conditions may represent a hazard for solid insulation in cables and transformer windings and for the capacitor bank, and their protective devices as well.