Trouble Comes in Threes
It seems that no matter how many times our experts address the topic of triplen, there's always another facet of this disruptive harmonic current to be discussed. This month, Editorial Director John DeDad weighs in on how configuring a transformer can reduce its effects. And with that out of the way, EC&M's PQ consultants Mike Lowenstein and Mark McGranaghan explore UPS configuration and the effects of voltage distortion on rms values.
Q. I've heard and read that a distribution system's responses to harmonics generated by 120V loads like PCs and older-style electronic ballasts are different than those for harmonics generated by phase-to-phase connected loads like variable speed drives. Does the configuration of the distribution or service transformer have any effect on this?
DeDad's answer: The problematic harmonics are the 3rd, 9th, 15th, etc., which are called triplen harmonics. Phase-to-neutral nonlinear loads having switch-mode power supplies generate them. These power supplies draw current only at the peak of the voltage sine wave.
Basically, triplen harmonics are problematic for grounded-wye systems with current flowing on the neutral conductor because these harmonic currents are additive on the neutral conductor and can cause that conductor to overheat, a potential fire hazard.
Transformers can also be affected, depending on their winding configuration. Fig. 1a above shows a typical, non k-rated, delta-wye transformer, the most common type used in utility distribution substations (delta winding connected to transmission) and in 480V-120/208V, 3-phase, 4-wire distribution transformers in office buildings. The triplen harmonic currents enter the secondary (wye) winding from the nonlinear load. Since these harmonic currents are in phase, they add on the neutral. The primary (delta) winding actually provides ampere-turn balance, which allows the triplen harmonic currents to flow. However, these currents remain trapped in that winding while not showing up in the line currents on the primary side of the transformer. The higher the content of triplen harmonic currents in the delta winding, the greater the amount of transformer overheating.
Using a grounded wye-wye transformer, as shown in Fig. 1b, will allow the triplen harmonic currents to flow through both the secondary and primary windings unimpeded. However, you must be careful here. If you remove the neutral connection on one or both windings, you will be blocking the flow of triplen harmonic currents. Because 3-legged core transformers operate or behave as if they have what may be called a “phantom” delta tertiary winding, a wye-wye connected transformer with only one neutral point grounded will still conduct the triplen harmonic currents from the grounded side.
Finally, the above discussion of triplen harmonic current flow really applies only to balanced loading conditions. When this isn't the case, you may find triplen harmonic currents flowing where you least expect them. And, they may have positive or negative sequence components as well.
Q. Why does the rms value of a sinusoid wave move up and down when subjected to different levels of voltage distortion? How can frequency affect the sinusoid's rms value when it's subjected to distortion?
Lowenstein's answer: The rms value of a voltage sine wave is governed by the mathematical definition of rms, according to the following equation: Vrms = =(V12 + V22 + V32 + V42…). As the value of each harmonic voltage increases or decreases, the rms value of the voltage increases or decreases. The rms value doesn't depend on which harmonic voltage component (frequency) is present, but only on the amplitude of each harmonic. To further complicate the situation, if high harmonic voltages are present, the value of the fundamental (V1) may decrease, resulting in a lower rms value.
The Table should help clarify the situation. (For convenience we'll deal with just the first three harmonics: 3rd, 5th, 7th.) Looking at the top portion of the Table, note that the levels of voltage distortion commonly found in electrical distribution systems don't have much effect on the rms voltage. This is because of the way the rms summation works.
In some situations, however, voltage distortion can be much higher than the values shown in the top portion of the Table. These situations can arise when a large number of computer loads are served by a “soft” electrical distribution system, with, perhaps, a downsized neutral wire. Then, high 3rd harmonic current results in high 3rd harmonic voltage distortion at the computers. A situation of this type is shown in the bottom portion of the Table. Note that even the rather high voltage distortion still doesn't have much effect on the rms voltage.
Since distortion voltages are relatively low, the effect of harmonic voltages on rms voltage values is, in general, small. On the other hand, since distortion current values can often reach more than 90% of the fundamental current, rms current values can be much larger than the fundamental current and can cause serious overloading of electrical distribution systems.
Q. I have vital electronic equipment that requires UPS protection. The equipment is currently protected by a building UPS. The bypass/backup to the building UPS is an unfiltered power supply. The engineering is such that I'm unable to put a voltage regulator on the bypass/backup power feed, nor am I able to put a UPS on this bypass line. What would happen if I were to connect my vital electronic equipment to a local rack-mounted UPS, which in turn, is plugged into a power grid that's protected by a building UPS? If the primary UPS-protected power feed has to be bypassed, I think I have a modicum of protection provided by a rack-mounted UPS. However, I'm worried I may be creating a potential explosive situation by daisy chaining the UPS units.
McGranaghan's answer: Manufacturers used to recommend not using a UPS to supply another UPS because of the problems that could occur when a ferroresonant-type UPS (also known as a hybrid UPS) is added to protect a local load on a bus also protected with a larger UPS that includes a ferroresonant type of output. Interaction between the two ferroresonant transformer circuits could cause a voltage regulation problem. For instance, some computer systems came with UPS protection (ferroresonant type) as part of the standard design, and the problem showed up when the customer installed them on a circuit with a UPS that protected the computer room.
However, this doesn't have to be a problem. As a rule of thumb, you should never have a problem if the upstream UPS is at least 10 times larger than the downstream UPS. Smaller ratios may be acceptable as well, depending on the UPS designs. For instance, it's much less likely to be a problem for standby UPS configurations that don't include ferroresonant transformers or for online UPS systems that have full rectifier/inverter systems.
It might be important for the upstream UPS to have a sine wave output. (This would surely be the case if it were a full double conversion system with rectifier and inverter.) If the upstream UPS has a square wave output, the downstream UPS might interpret this output as a problem with the supply and go into backup mode at the same time as the upstream UPS, and the batteries would effectively discharge together. In this case, the downstream UPS might not improve the reliability for the supply to the vital load, as you would expect.
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