But some questions need to be asked. Moreover, whether in balanced or unbalanced cases we think that the triplen harmonic currents circulate in the windings of the transformer and thus cause an additional heating. The circulation of these currents is responsible for distortion of the voltage at the Delta winding due to impedances of transformer windings. This is what we wanted to show by in general. If the neutral of the load is not earthed the Delta winding does not do anything as there is no zero sequence to trap.
|Published (Last):||6 April 2019|
|PDF File Size:||10.53 Mb|
|ePub File Size:||9.71 Mb|
|Price:||Free* [*Free Regsitration Required]|
A 3,degree Fahrenheit electric arc furnace prepares to receive a "charge" of scrap metal for conversion into steel Saturday at Severstal Columbus Photo by Carmen K. The 3-phase system is transformed into three single-phase systems that are much simpler to analyze. The method allows any unbalanced set of phase currents or voltages to be transformed into three balanced sets. The positive sequence set contains three sinusoids displaced from each other, with the normal A-B-C phase rotation e.
The sinusoids of the negative-sequence set are also displaced , but have opposite phase rotation A-C-B, e. The sinusoids of the zero sequence are in phase with each other e. In a perfect balanced 3-phase system, the harmonic phase sequence can be determined by multiplying the harmonic number h with the normal positive sequence phase rotation. Phase sequence for all other harmonic orders can be determined in the same fashion.
They deserve special consideration because the system response is often considerably different for triplens than for the rest of the harmonics.
Triplens become an important issue for grounded-wye systems with current flowing on the neutral. Two typical problems are overloading the neutral and telephone interference. One also hears occasionally of devices that misoperate because the line-to-neutral voltage is badly distorted by the triplen harmonic voltage drop in the neutral conductor.
For the system with perfectly balanced single-phase loads illustrated in Figure 1, an assumption is made that fundamental and third harmonic components are present. Summing the currents at node N, the fundamental current components in the neutral are found to be zero, but the third harmonic components are three times the phase currents because they naturally coincide in phase and time.
Figure 1 — High neutral currents in circuits serving single-phase nonlinear loads Triplen Harmonics in Transformers Transformer winding connections have a significant impact on the flow of triplen harmonic currents from single-phase nonlinear loads.
Two cases are shown in Figure 2. In the wye-delta transformer top , the triplen harmonic currents are shown entering the wye side. Since they are in phase, they add in the neutral. Figure 2 — Flow of third harmonic current in 3-phase transformers The delta winding provides ampere-turn balance so that they can flow, but they remain trapped in the delta and do not show up in the line currents on the delta side.
When the cur- rents are balanced, the triplen harmonic currents behave exactly as zero-sequence currents, which is precisely what they are. This type of transformer connection is the most common employed in utility distribution substations with the delta winding connected to the transmission feed. Using grounded-wye windings on both sides of the transformer bottom allows balanced triplens to flow from the low voltage system to the high voltage system unimpeded.
They will be present in equal proportion on both sides. Many loads in the United States are served in this fashion. Some important implications of this related to power quality analysis are: Implication 1 — Transformers, particularly the neutral connec- tions, are susceptible to overheating when serv- ing single phase loads on the wye side that have high third harmonic content. Implication 2 — Measuring the current on the delta side of a transformer will not show the triplens and, therefore, not give a true idea of the heating the transformer is being subjected to.
The flow of triplen harmonic currents can be interrupted by the appropriate isolation transformer connection. Implication 3 — Removing the neutral connection in one or both wye windings, blocks the flow of triplen harmonic current. There is no place for ampere-turn balance. Likewise, a delta winding blocks the flow from the line. Therefore, a wye-wye with only one neutral point grounded will still be able to conduct the triplen harmonics from that side.
These rules about triplen harmonic current flow in transformers apply only to balanced loading conditions. When the phases are not balanced, currents of normal triplen harmonic frequencies may very well show up where they are not expected. The normal mode for triplen harmonics is to be zero sequence. During imbalances, triplen harmonics may have positive or negative sequence components too.
Figure 3 — Example of voltage notching caused by a 3-phase converter One notable case of this is a 3-phase arc furnace. The furnace is nearly always fed by a delta-delta connected transformer to block the flow of the zero sequence currents, as shown in Figure 3. Thinking that third harmonics are synonymous with zero sequence, many engineers are surprised to find substantial third harmonic current present in large magnitudes in the line current. However, during scrap meltdown, the furnace will frequently operate in an unbalanced mode with only two electrodes carrying current.
Large third harmonic currents can then freely circulate in these two phases just as a single-phase circuit. However, they are not zero sequence currents. The third harmonic cur- rents are equal amounts of positive and negative sequence currents. But to the extent that the system is mostly balanced, triplens mostly behave in the manner described.
What is the Notching? Notching is a periodic voltage disturbance caused by the normal operation of power electronics devices when current is commutated from one phase to another. Since notching occurs continuously, it can be characterized through the harmonic spectrum of the affected voltage.
However, it is generally treated as a special case. The frequency components associated with notching can be quite high and may not be readily characterized with measurement equipment normally used for harmonic analysis. Figure 3 above shows an example of voltage notching from a 3-phase converter that produces continuous DC current. The notches occur when the current commutates from one phase to another. During this period, there is a momentary short circuit between two phases pulling the voltage as close to zero as permitted by system impedances.
Harmonics for Arc Furnace Facilities Webinar Arc furnace installations are notorious for having some of the most unconventional power quality problems in any application.
Reactive power compensation is extremely important for many reasons in this kind of facilities, but it can also be a more complex affair than installing traditional notch filters because of the uncharacteristic harmonics present due to the arc furnace itself. McGranaghan and Roger C. It helps you to shape up your technical skills in your everyday life as an electrical engineer.
Download White Paper Here
Current harmonics[ edit ] In a normal alternating current power system, the current varies sinusoidally at a specific frequency, usually 50 Hz or 60 Hz hertz. When a linear electrical load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage though usually not in phase with the voltage. Current harmonics are caused by non-linear loads. When a non-linear load, such as a rectifier is connected to the system, it draws a current that is not necessarily sinusoidal.
What happens to the triplen harmonic currents when a three phase load is used?
Harmonics Harmonics: Harmonics by definition are a steady state distortion of the fundamental frequency 60 Hz. Harmonic distortion of current occurs when sinusoidal voltage is applied to a non-linear load ex. The result is a distortion of the fundamental current waveform. This distortion occurs in integer multiples of the fundamental frequency 60 Hz. Voltage distortion, on the other hand, is generated indirectly as result of harmonic currents flowing through a distribution system. It is important to note that the vast majority of harmonic currents found in a distribution system are odd-order harmonics 3rd, 5th, 7th, etc.
Harmonics (electrical power)