Reduced Neutral

[george t. everett]

I just received the new rules & regulations from the local power company, they say I can not reduce the size of the neutral on commerical services, 3-phase & single phase. My question is can a neutral be overloaded on a single phase service & does non linear loads affect single phase services? Thanks for any help.

 

[renosteinke]

Re: Reduced Neutral

Harmonics are a concern only with three-phase services.

 

[george t. everett]

Re: Reduced Neutral

If there are no harmonics on a single phase service, then there is no reason the neutral can't be downsized. I understand it only carries the unbalanced current. I don't understand there reasoning.

 

[bennie]

Re: Reduced Neutral

A neutral conductor serves other purposes besides unbalanced load.

 

[charlie]

Re: Reduced Neutral

This is interesting. It seems the local electric utility has decided to set the rules for wiring on the load side of the service point . . . kinda outa their jurisdiction, I'd say. I can't help but think that you have misread the new edict; I can't imagine an electric utility requiring something they have no authority over . . . HMMM LOL

 

[bphgravity]

Re: Reduced Neutral

It has been estimated that in 1990, 15% of utility loads contained harmonic currents. By 2000, this figure increased to about 50%. Harmonic conditions are a great concern to utilities because of transformer saturation and core losses. The power companies have great lobying power over NEC and electrical insatallations. Equipment must be manufactured to reduce power factor problems, and services should also be designed to assist power quality issues.

 

[charlie b]

Re: Reduced Neutral

Originally posted by renosteinke: Harmonics are a concern only with three-phase services.

I?m no expert on power quality, and can?t comment on the degree to which harmonic currents can influence single phase systems. But I do know that single phase systems can and do experience harmonic currents. Electronic ballasts, or switching power supplies, or any other non-linear loads will give you current waveforms that are not "pure sine waves." Once you have a current that (1) is not a pure sine wave, and that (2) repeats in the same pattern cycle after cycle, then you have harmonics.

THE MATHEMATICAL CONCEPT (without any real math):
Any repetitive waveform can be mathematically described by a sum of pure sine waves. You take some constant value times the cosine of ?1 x 60? units of time, and add some other constant value times the cosine of ?2 x 60? units of time, and add some other constant value times the cosine of ?3 x 60? units of time, and add some other constant value times the cosine of ?4 x 60? units of time, and add some other constant value times the cosine of ?5 x 60? units of time, and keep going until the amount you add each time becomes to small to matter. The ?1 x 60, 2 x 60, 3 x 60,? etc. are the first (or fundamental), second, third, fourth, etc. harmonics. The ?some other constant value? numbers are the amounts of current at each harmonic level. It turns out that for all the even harmonics, the constant value of ?zero? is multiplied by the cosine terms, meaning that there are no even harmonics in a power system.

 

[don_resqcapt19]

Re: Reduced Neutral

Charlie,
While harmonics may cause other problems in single phase systems, they do not cause excessive neutral current. They do not add in the single phase grounded conductor like they do in the 3 phase grounded conductor.
Don

 

[bphgravity]

Re: Reduced Neutral

Don't you get higher R x I squared heating where harmonic currents are supplied due to skin effect? Higher frequency currents flow more toward the skin, in effect reducing cross-sectional density. On a single phase multiwire branch circuit, the odd numbered harmonics will not cancel out on the neutral. They will be additive resulting in higher neutral load.

 

[don_resqcapt19]

Re: Reduced Neutral

Bryan,
Yes, the odd order harmonics do cancel in the grounded conductor of a single phase system. The odd order harmonics on one hot are exactly oposite to those on the other hot and they cancel in the same manner as normal grounded conductor currents. IN three phase, because the hots are 120? out of phase with each other results in these odd order harmonic currents adding.
Don

 

[dereckbc]

Re: Reduced Neutral

Don, this is true only if the the load is 240. But not true if it is 120.

 

[george t. everett]

Re: Reduced Neutral

Charlie # 59

The power company has control to the main disconnect. After that is the load side.

 

[don_resqcapt19]

Re: Reduced Neutral

Dereck,
There is no grounded conductor current to cancel in a 240 volt single phase load. When you use 2 hots and a grounded conductor from a 120/240 volt 3 wire single phase system, the harmonic currents do not add in the grounded conductor. While it is not technically correct to say that the two 120 volt legs of the single phase system are 180? out of phase, they do have opposite positive and negative peaks, therefore the harmonic currents in one leg will be exactly opposite from those generated in the other leg and they will cancel each other when they get to a common grounded conductor. About a year ago I spoke with the chief electrical engineer of a manufacturer of harmonic neutral current mitigation equipment and he agreed that the harmonic currents do not add in the single phase system. This information was originally published in the "EDI" (electrical design and installation) magazine about 13 years ago. This magazine was edited by Joe McPartland, and was, in my opinion, the best ever electrical trade magazine.
don

 

[dereckbc]

Re: Reduced Neutral

Don I agree with you on the 240 circuits. Maybee I stated it wrong. I am talking about 120 single phase circuit. For example your PC. There is not an identical PC on the other leg to get the cancellation effect.

The only way I know how to do this is use a balanced system for 120 loads. Didn't mean to insult you.

 

[bphgravity]

Re: Reduced Neutral

I had the great honor of meeting Mr. Joe McPartland. I am currently a consulting editor with Brian McPartland on a Tom Henry publication. Tom has expressed to me on a few occasions that Joe may be the most knowledgable electrician that he has ever known or met!

Back to harmonics on single phase loads, what about motor centers with capacitor banks or speed drives fed from a power source high in harmonic currents? Since there is an overall decrease in reactance from the raised frequencies, the capactiors in effect will have less impedance. Also, the peak voltages are affected along with RMS values, however it is the peak-to-peak value that can result in insulation breakdown. These problems affect the OCD and cause nusiance tripping. There is no OCD for the neutrals resulting in higher temperatures, not from overcurrent, but harmonic effects.

 

[roger]

Re: Reduced Neutral

Bryan, I don't understand your post.

Are you talking about phase to neutral sources feeding these MCC's or VSD's?


Roger

[ April 03, 2003, 06:10 AM: Message edited by: roger ]

 

[bphgravity]

Re: Reduced Neutral

I believe what Don has posted is acurate, but I have never heard of harmonics cancelling out in any wiring configuration. I understand the concept of of currents cancelling each other out on alternating sytem neutrals because of the reversing current when it is being shared. I have been made to understand that whether or not the harmonics cancel each other out, the effects they have on the load and the circuit in general reduce the current carrying capability of the nuetral conductor. This is not necessarily because of excessive overcurrent, but I squared X R losses and reduced efficency and phase shift in loads that are specifically affected by harmonic conditions. I have also been made to understand that the respective circuit may not be the source of the harmonic currents, but the system has been effected and therefore washing harmonic conditions over all circuits. I attended a very good power quality seminar class that clearly stated the residential market in the next coming years is going to change the way code and electricians install grounded systems due to the increase of harmonic conditions of the utility and within the home. I am also aware that there are several copanies that are working on "magnetic flux compensators" and "current injection devices" for both 3-phase, 4-wire and single phase, 3-wire systems. I certainly agree that the greater hazard of harmonic conditions are on the 3-phase, 4-wire systems, but I don't think it is safe to say that single phase nuetrals are not effected in some way that can lead to hazards. It is sort of like circuit containg both XL and XC, one may cancel the other out mathematically, but in reality they are both still influencing the circuit.

[ April 03, 2003, 07:17 AM: Message edited by: bphgravity ]

 

[don_resqcapt19]

Re: Reduced Neutral

I am also aware that there are several companies that are working on "magnetic flux compensators" and "current injection devices" for both 3-phase, 4-wire and single phase, 3-wire systems.

Why do we continue to treat the symptoms of this problem and not the problem itself? Why should the electrical industry be forced to solve the problems created by the electronics industry? The electronic equipment manufactures should be forced to redesign their equipment so that the harmonic currents are not transferred to the electrical system. This would be a case of "user pays". That is the creator of the problem pays to solve the problem instead of passing it on so we all have to pay to solve it.
Don

 

[Ed MacLaren]

Re: Reduced Neutral

Why should the electrical industry be forced to solve the problems created by the electronics industry? The electronic equipment manufacturers should be forced to redesign their equipment so that the harmonic currents are not transferred to the electrical system.

I agree 100%.

It seems to me that any electronic equipment manufacturer that would build in the necessary filtering, and could advertise a low harmonic distortion product, should have an advantage in the marketplace.

This might depend on the consumer being sufficiently educated about the subject to buy the more expensive equipment.

Ed