harmonics on a 120/240 system

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hde_electric

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If harmonics are such a big problem on a 3 phase system why use it then on new office buildins with a lot of non-linear loads and use 120/240 system instead? Will somebody explain please. Thank you.
 
Re: harmonics on a 120/240 system

Properly designed, 3 phase systems have no problems with harmonics. You get extra capacity within the 3 ph panelboard, less net transformer footprint, etc.
 
Re: harmonics on a 120/240 system

<big snip>Harmonics only mean trouble if the power system is not designed to handle them. High harmonic neutral currents are a problem only if the neutral is not properly sized. Current harmonics are not a problem to a transformer if it is derated appropriately. Even some voltage distortion below 8% THD at the point of utilization) is acceptable as long as sensitive equipment is not affected. However, it is always important to be aware of the presence of harmonics and to try to minimize them by purchasing low distortion electronic ballasts and reactors for PWM ASDs. This will not only keep the harmonics in check and improve the power factor in the facility, but will also save energy by reducing losses on power system components. In addition, any time there is a considerable increase of non-linear loads, it is important to check power system components to prevent problems.
<snip>
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For the whole article go here:
http://www.pge.com/002_biz_svc/002c1i_power_sys.shtml
 
Re: harmonics on a 120/240 system

Gotta go with Ron on this issue. If you use "K" rated transformers, and size the neutrals properly for non-linear loads. harmonics are not a problem. Ron is correct, it is a design, capacity, and real estate issue.
 
Re: harmonics on a 120/240 system

Ron's point is the bottom line.

Natural resources, fuel for generation, compacting equipment into given spaces, overall money savings to the customer etc...

Don Graniere has been bringing this to the table for quite sometime and I like the idea since Harmonics are not a problem in single phase systems, but we have to look at the points in the sentence above.

In reality the harmonic issue is greatly over exagerated.

Roger
 
Re: harmonics on a 120/240 system

Originally posted by hde_electric: But why are 120/240 systems not used in some offices?
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I can think of two reasons. First, let?s presume a total load of 100 KVA. If you serve it with 120/240, the current will be 416 amps, and you will need a 600 MCM feed (or smaller parallel conductors). If you serve it with 120/208, the current will be 120 amps, and you will only need an AWG #1. Secondly, a large single phase load would cause a balance problem for the utility system. They might be able to achieve an overall balance by feeding other nearby customers with the other single phases, but that?s not the ideal solution. Better to have each customer balance their loads by using a 3-phase service.

By the way, I suspect that this is not news to many. But I?ll add it, just in case it might be news to some. A ?k-rated transformer? does nothing to make harmonics go away. It?s just bigger and heavier, so that the harmonics won?t damage it. It costs the owner more, not only because of the extra copper, but also because it needs a bigger HVAC system to take away the heat. It?s like a tall guy who bumps his head every time he goes through a low doorway. If he buys and wears a hard hat, then the bumps will not hurt as much. But the hard hat won?t stop him from bumping his head. He would need to look for other solutions to the head-bumping problem.

So saying that a ?properly designed? 3-phase system has no problems with harmonics is an over-simplification. A more accurate statement is that a properly designed 3-phase system will not be damaged by any harmonic loads that happen to be connected to it. That notwithstanding, even with a properly designed 3-phase system, one load that is a significant source of harmonics can create problems (even to the point of causing damage) to another load on the same system.
 
Re: harmonics on a 120/240 system

A non-linear load that draws current at frequencies other than 60 hz will do so on a 240/120V or 208/120V system. Equipment that will have trouble due to poor power quality, being served by a common distribution panel with other high harmonic drawing loads, will have similar trouble on 240/120V or 208/120V systems.
"one load that is a significant source of harmonics can create problems (even to the point of causing damage) to another load on the same system" will happen on either set of voltages 240/120V or 208/120V.
Properly design the system to withstand (not eliminate) the harmonics and the resulting neutral currents for the best approach to 3 phase power distribution.
 
Re: harmonics on a 120/240 system

"K" Factor Transformer Rating:

<big snip>In the 1980's, power quality engineers began encountering a new phenomenon: non-linear loads, such as computers and peripherals, began to exceed linear loads on some distribution panels. This resulted in large harmonic currents being drawn, causing excessive transformer heating due to eddy-current losses, skin effect, and core flux density increases.

Standard transformers, not designed for nonlinear harmonic currents were overheating and failing even though RMS currents were well within transformer ratings.

In response to this problem, IEEE C57.110-1986 developed a method of quantifying harmonic currents. A "k" factor was the result, calculated from the individual harmonic components and the effective heating such a harmonic would cause in a transformer. Transformer manufacturers began designing transformers that could supply harmonic currents, rated with a "k" factor. Typical "K" factor applications include:
K-4: Electric discharge lighting, UPS with input filtering, Programmable logic controllers and solid state controls

K-13: Telecommunications equipment, UPS systems, multi-wire receptacle circuits in schools, health-care, and production areas

K-20: Main-frame computer loads, solid state motor drives, critical care areas of hospitals

"K" factor is a good way to assure that transformers will not overheat and fail. However, "K" factor is primarily concerned with thermal issues. Selection of a "K" factor transformer may result in power quality improvement, but this depends upon manufacturer and design. <big snip>
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For more on Transformer Ratings go here:
http://www.teal.com/resources/an7.htm

../Wayne C.

[ October 13, 2003, 08:15 PM: Message edited by: awwt ]
 
Re: harmonics on a 120/240 system

Charlie,
The way I see the single phase installation for harmonic loads is that the utility would provide 480/277Y and the local panels that serve the computer and other nonlinear loads would be fed from single phase 480-120/240 volt transformers. Ideally these single phase transformers would be installed in multiples of 3 to balance the load on the 480 volt system. The transformers would be installed in the area of the 120/240 volt single phase panels. Local panels and lower grounded conductor currents also will reduce the grounded to grounding conductor voltage that many of the IT people complain about.
Don
 
Re: harmonics on a 120/240 system

Does anyone else ever wonder why almost all of the information on this "problem" comes from organizations that have an economic interest in selling products to fix the "problem"???
Don

[ October 14, 2003, 08:49 AM: Message edited by: don_resqcapt19 ]
 
Re: harmonics on a 120/240 system

Don,
Does anyone else ever wonder why almost all of the information on this "problem" comes from organizations that have an economic interest in selling products to fix the "problem"???
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absolutely.

And on a further note, not many have actually had to repair or fix wiring damaged from additive harmonic heating.

Roger
 
Re: harmonics on a 120/240 system

i personally found the harmonics issue to be a "money maker", the reason being that if you make sure you have a full sized or double neutral, use low harmonic ballast, and have the load separated so that the distribution is driven to one area (not spread throughout the grid), then we've have had not problem.

i see a problem when neutrals aren't considered CCC, and when the proper precautions are not made. so...as far as i'm concern, i spec k-4 or k-13 transformers, and use 200% neutrals. after doing test in the field, i find that the neutral current is much lower than expected.

i could be wrong, though :)
 
Re: harmonics on a 120/240 system

Does anyone else ever wonder why almost all of the information on this "problem" comes from organizations that have an economic interest in selling products to fix the "problem"???
Click to expand...
I'm not sure if it is worth arguing your point with my anecdotal situations, but I have found tremendous information regarding this possible problem from the IEEE, trade magazines and questions/discussions with on site technicians.
I have had to search for information from sales folks.
Maybe an electrician is closer to the sales folks on a daily basis, and they get their ear first, I don't know. I have found many electricians interested in the topic due to possible cost increases in a project, so I think it goes both ways.
 
Re: harmonics on a 120/240 system

Ron,
Look at the authors for the trade magazine articles. Most of the trade articles are written by the manufacturers who sell the equipment. Very few are written by independent authors.
Don
 
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