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Effects Of Harmonics In Power System:

By Author: Omprakash M
Total Articles: 1

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.

Unexpected Fuse Operation:

The RMS voltage and current values may increase under harmonic analysis in power system. This can produce undesired operation of fuses in capacitor banks or in lateral feeding industrial facilities that operate large nonlinear loads. Capacitor banks can be further stressed under the operation of a fuse on one of the phases, which leaves the remaining units connected across the other phases. They are thus left subject to an unbalanced voltage condition that can produce overvoltages and detune passive harmonic filters if they are not provided with an unbalance detection feature. An important aspect to look after with the advent of cogeneration and microturbine schemes using power inverters with electronic switching technology and the presence of their harmonic current contribution and effect on the operation performance of islanding protective relays.

Abnormal Operation of Electronic Relays:

Variable frequency drive (VFD) operation leading to shut-down conditions is often experienced in applications involving oil fields in which solid material (sand) abruptly demands higher thrust power, mining works in which sudden increases in lifting power occur, and high inertia loads, among others. In all these cases, the protective relays trip as a response to overcurrents exceeding the established settings. Therefore, when the protective relays are triggered during the operation of a nonlinear load, the harmonic analysis in power system to be assessed. It might well be that an unpredicted overloading condition is the cause of the unexpected operation, but often increased harmonic levels following nonlinear load growth are the reason for the similar behavior.

Thermal Effects on Rotating Machines:

The transformers and rotating machines are also exposed to thermal effects from harmonics. Because the effective resistance of the conductor moves up as frequency increases, a current wave rich in harmonics may cause greater heating on winding conductors than a sine wave of the same RMS value. The overall effect can lead to a decreased transformer lifetime.
Pulsating Torques in Rotating Machines:
Additionally, magnetomotive forces (mmfs) induced by positive and negative sequence harmonics interact with the nominal frequency mmf force, creating torque components of different frequencies. This may create problems on the shaft of rotating machines subjected to influence of harmonic torsional duo, including
• Equipment fatigue
• Unexplained operation of mechanical fuses
• Increased vibration
• Bearing wear-out

Abnormal Operation of Solid-State Devices:

Under some circumstances of harmonic analysis in power system, harmonic distortion can lead to multiple zero crossings of the voltage waveform disturbing the operation of control systems that use the zero crossing as a timing or counter reference.

Considerations for Cables and Equipment and Operating in Harmonic Environments:

A steady increase in harmonic distortion in power systems, the specifications and selection criteria of electrical equipment in industrial installations should be revised. These are the criteria to be looked after for a better Harmonic Analysis Services.

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