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Controlling Harmonics Through Wave Shifting Using Transformers
A transformer can be used to phase shift the voltages between primary and secondary.
By using phase shifting to your advantage, you can actually cancel harmonics on the secondary at low impedance (Zigzag winding), and on the primary by phase shifting loads on different transformers. One can also improve power factor with all the saving that this implies.
Many precautions must be taken to succeed with this solution: installation, location, and impedance, fault protection, etc. But in return, almost all harmonics may be eliminated.
By taking this approach, 10 years from now, harmonics will still be canceling each other instead of adding up. Let us look at standard and special construction three-phase transformers and see where they fit in this approach.
Three Phase transformer
In the new electrical environment, we have to familiarize ourselves with the concept of phase shifting. We know that to cancel current on the neutral, at 60 Hz, we have to balance our three phases. This is due to the 120º between the phases; when one phase is in the positive the two other phases act to cancel it. This should be applied to our systems. If a system is 30º shifted to the primary line (Delta-Y Transformer), the 5th and 7th harmonic should largely be canceled on the primary line by another system that is phase shifted 0º to that line (Delta-Delta Transformer). Again this is due to the fact that when a system 5th harmonic is on the positive sequence of the sinusoid the other system 5th is on the negative side.
Lets look at three phase transformers:
Delta-Y Transformer (30° Secondary To Primary)
This is the most common transformer winding in a four wire secondary application. It has the capacity to stop the zero sequence harmonics (3, 9 triplens) which cause neutral conductor heating. It accomplishes this by capturing these harmonics on the delta side primary because the triplen harmonics have no rotation.
The downside of this is that the triplens will generate heat due to the increase of your RMS current value, thereby requiring derating of the transformer (See the CBEMA Formula). It is important to realize that the negative sequence harmonics will go through the transformer, and appear on the input side (primary).
Even with cancellation of the triplen on the primary side, close attention should be given to the impedance of the transformer. Ohms law applies here (E=IZ). If there is high impedance, the harmonic current will distort the voltage and this is when we start having problems on the secondary. To reduce most of the problems (ex.: voltage between neutral and ground) we must install these transformers as close as possible to the nonlinear loads, thus reducing the impedance that the line might offer.
By installing the transformer close to non-linear loads, you greatly reduce the voltage neutral to ground (due bonding on the secondary neutral to ground as required by code). High current on the neutral creates this voltage, by the addition of all zero sequence harmonics present on the phase. Transformers must be derated anywhere from 80 to 50% of their nominal value if they are not built to withstand the harmonic heating.
We can classify the K factor Delta-Y transformers as a power conditioner if it is equipped with TVSS on the secondary and a double extra-wide electrostatic shield. These "K-rated" transformers are designed primarily for their ability to withstand the heating effects of harmonics and should be built using copper windings. A copper winding transformer is smaller in size and mechanically stronger, it has a better resistance to mechanical and thermal shock. For the same wire electrical resistance, it takes 1.6 times the surface of aluminum compared with a copper winding.
Delta-Delta Transformers (0° secondary to primary)
Delta-Delta transformers are very well known and are frequently used in the industry for a three-wire system. They will trap zero-sequence harmonics in the primary, which is good news, but the impedance to these harmonics ought to be closely examined since it could distort the voltage harmonics (THD). They also have to be derated for non-linear load applications.
TT Connected Open Delta, Two Coil Transformers (30° second ary to primary)
This connection is very useful in the field when you lose one phase of a three-phase system; by reconnecting the two coils we could make a three-phase system.
Due to its low cost in construction, some manufacturers have used them as distribution transformers. (Only one manufacturer remains in North America). Phase unbalancing can lead to voltage unbalances between the phases.
Please note the recommendation of IEEE Std 1100-1992 9.17.6
Delta Zigzag Transformers (0° or -30° secondary to primary)
A Delta Zigzag has a Delta Primary and a Zigzag Secondary, it is used in a 4 wire system.
Because of the ability of a zigzag winding, it will be able to cancel the zero sequence harmonics on the secondary at low impedance. If the phases are not well balanced, the triplen harmonics will not be properly canceled on the Delta primary. Therefore most of the triplen harmonics will appear on the primary side.
Canceling the triplens on the secondary with a zigzag connection, means that the triplen harmonics will see very little impedance, and consequently, little voltage harmonic distortion (THD) (E=IZ) on the secondary system.
Again, they have to be installed close to the application, for the same reasons we stated earlier. The negative sequence that will be going on the primary line will be canceled, again by the harmonics coming from a system connected to a Delta Y transformers (30°) or a Delta V - 30°.
Remember this transformer looks like a Delta Y transformer and is connected to the line in the same way.
&nbs p;Delta Double Zigzag Transformer (0°, 60° or 0°, 30° between systems, prim. and sec.)
On the secondary of a Delta Zigzag we add a second secondary zigzag system at 60° to the first one.
This transformer has one delta primary and six-phase secondary (3 phases per system). The secondary system’s "A" first phase is 180° to secondary system’s "B" second phase and so on.
When one system is going in one direction (0°, 60°) the other is going in the opposite direction on the common neutral, this make the common neutral a good point of cancellation of all harmonics.
You also have, due to a special winding (0°, 30°), a cancellation of the triplen harmonics and of the 5th, 7th harmonics, on the secondary of the transformer. A cancellation on the primary also occurs, due to phase shifting of primary to secondary
Zero Sequence Harmonic Filter (Traps, Eliminator, etc..) Although they are called filters, they are just zigzag autotransformers connected in parallel to the line. One of their applications is to create a fourth wire in a three-wire system.
Installed near the application, they offer low impedance to triplen harmonics, canceling them near the application. This means that you do not see these triplen harmonics coming back on the neutral, and this greatly reduces the harmonic load to the transformer.
It is a great tool for retrofitting some installations that would otherwise be very expensive to replace with a new transformer and new winding. It will also reduce the voltage neutral to ground at the application, which is created from the effect of the line impedance an d the neutral current. This technique should only be applied by specialists with experience in the field (poor design of installation and product could add a lot of 5th & 7th harmonic making your problems worse).
Due to the change of impedance, fault current will change and this has to be addressed accordingly, as per electrical codes.
Remember, the following must be applied to achieve maximum effect:
1. Install near the applications
2. Segregate your loads (your non-linear loads on one system)
3. Balance the phases
4. Balance the systems (Phase shifting)
Some Installation Basics
A) The first thing that you need to know is the correct value of the loads. You must also determine the harmonic distortion of all equipment on the premises. The manufacturer usually gives that information on their nameplate.
Good instrumentation is essential to know the RMS value of all your loads; old instruments and inexpensive ones do not give this RMS value but the average values. This means that although you will read 10 amps on your neutral the RMS values may be 15 amps, and this can make a major difference in choosing or sizing your transformers or your protection.
If your values are too high add extra neutrals and derate your installation.
There is an easy way to do this if most of your loads are single phase switch mode power supplies:
Calculate all your RMS am ps value on your three phases and establish the average; then, take the peak RMS value of every phase and calculate the average. Multiply your amps by a crest factor of 1.414 (sine wave) and divide it by the average of the peak values (ref. As per Computer & Business Equipment Manufacturer Association (CBEMA).
|Phase A||Phase B||Phase C||Average|
|RMS Amps||310 A||346 A||337 A||331 A|
|PEAK Amps||705 A||793 A||729 A||742 A|
So: 331 A x 1.414 (crest factor) = .630
Therefore: Manufacturer KVA rating x .630 = KVA usable
B) Your loads should be balanced as much as possible.
C) Segregate your loads: linear or non-linear loads. Your non-linear loads should be well balanced on all the three phases.
D) All protection device should be installed as close as possible to the loads, however with TVSS you would like to install this at the service entrance and distribution panels.
E) Verify that there are no ground loops present in the system (two computers at two different sites (Ground potential difference) with a communication cable grounded at both ends, the difference between the two outlet voltages would circulate into that communication cable). Or two non-linear loads with one up stream of the ground wire with communication cable.
F) Try to phase shift your non-linear loads so that harmonics in your facility will cancel each other rather than adding up. This will be beneficial in the future when you add even more non-linear loads.
PHONE: (678) 546-6780 FAX: (678) 546-6782
5044 B U Bowman Drive #102
Buford, Georgia 30518
PHONE: (678) 546-6780
Last Updated: 07 Jun 2004
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