Harmonics and 3rd order harmonic
Harmonics are the result of nonlinear loads that convert AC line voltage to DC. Harmonics flow into the electrical system because of nonlinear electronic switching devices, such as variable frequency drives (VFDs), computer power supplies and energy-efficient lighting.
Today, the most prevalent and growing harmonic sources are:
Such as Swith-mode power supply, LED lights, Arcing furnance will cause 3rd order harmonic usually.
3rd harmonic and high current in Neutral current
In three phase four wire system,sinusoidal currents with linear loads sum to return on the neutral conductor. The 120° phase shift between the sinusoidal load currents causes their vector sum to be quite small. In fact it will be zero if the linear loads are perfectly balanced.
The instantaneous sum of the currents in the three phases taken at any moment will also be zero if the linear loads are perfectly balanced. If they are not, then there will be a small residual neutral current.
With linear loads, the neutral conductor can be the same size as the phase conductors because the neutral current will not be larger than the highest phase current. Unfortunately, this is definitely not true for non-linear phase-to-neutral loads.
120VAC non-linear loads like the SMPS used in computers and in monitors draw current in two distinct pulses per cycle. Because each pulse is narrow (less than 60 degrees), the currents in the second and third phases are zero when the current pulse is occurring in the first phase. Hence no cancellation can occur in the neutral conductor and each pulse of current on a phase becomes a pulse of current on the neutral.
Even if the phase currents of the SMPS loads are perfectly balanced in RMS amperes, the RMS value of the neutral current can be as much as √3 times the RMS value of the phase current because there are 3 times as many pulses of current in the neutral than in any one phase. If the phase current pulses do overlap because they exceed 60 degrees in width, then there will be some cancellation so that the neutral current will be less than √3 times the phase current. Overlapped or not, because there are 3 times as many pulses in the neutral than in a phase, the predominant component of the neutral current will be the 3rd harmonic (180Hz for a 60Hz system). The linear current completes only 2 cycles in the same time period that the non-linear neutral current completes 6 cycles or 3 times the fundamental.
3rd Harmonics currents add to nuetrals (3times 3rd Harmonic currents in Phases). Triple-N harmonic currents add arithmetically in the neutral conductor rather then summing to zero as do balanced fundamental and other harmonic currents. The result is neutral currents that are often significantly higher, typically up to 170%, than the phase currents.
3rd harmonics is dangerous
Now let us examine why the third harmonics are dangerous. Power system harmonics increase the current in the system rapidly. This is particularly correct for the third harmonic. The third harmonic causes a sharp increase in the current in a neutral conductor. Harmonics cause malfunctioning of electronic parts, resulting in transformers heating etc.
Digitalisation, computerisation and automation has the flipside to it, the power supplies (SMPS) and the addition of control gears of lighting add to the serious issue of 3rd Harmonics. As these harmonics are high amplitude and Magnitude they add to the woes of problem of poor Power quality.
The problem of 3rd Harmonic is as follows:
1. Overload Neutral conductors
2. Overload distribution Transformers
3. High neutral to ground Voltage.
4. Poor Power Quality and distortion of voltage supplying the load.
5. Current imbalance in transformer, excess loading of transformer in individual Phases.
Solution of 3rd harmonic
YTPQC-AHF based on 3-level topology, is an Active Harmonic Filter (AHF) system designed to eliminate harmonic oscillations and reduce costs consequently. AHF is a versatile solution, easily tailored to deliver power factor improvement, voltage variation control, flicker mitigation and load balancing functionality, and highly improved power quality in networks while reducing harmonic pollution.
YTPQC-AHF Active Filter can filter 3rd harmonics perfectly. Welcome to choose our Active Filter.
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