harmonics

[mickeyrench]

how is a circuit determined to have harmonics? and the only thing i know about eliminating them would be to use a larger size neutral. does harmonics have anything to do about having poor pf?

 

[bphgravity]

Harmonics are the by-product of certain load types. We now know what type of loads cause this condition, but harmonics show up as an altered waveform. That is, the nice 60-hz sine wave becomes greatly distorted in predictable ways depending on the physics of the harmonic. There are methods of reducing harmonics and there are methods to protect against the effects of harmonics.

I would suggest a google search, or a seach thru this forum for more conversations about specific instances of harmonics. There is a lot of good information out there...

 

[zog]

how is a circuit determined to have harmonics?

With a PQ meter that can look at the voltages and currents at different frequencies, harmonics are all at multiples of the system frequency, mostly odd numbered multipliers (3rd harmonic 180Hz, 5th - 300Hz)

and the only thing i know about eliminating them would be to use a larger size neutral.

That dosent eliminate them at all, just allows the system to handle them better, filters, isolation transformers, etc are ways to reduce harmonics,

does harmonics have anything to do about having poor pf?

Not really, no, they are not related, You get harmonics from loads that do switching at high frequencies, like VFD's for example.

 

[Besoeker]

I agree mostly with what bphgravity and zog have posted.
I couple of points I would add.
A lot of equipment produces harmonics these days. Almost any mains powered piece of equipment with electronics does - television, computers, printers, chargers, light dimmers, variable speed drives, CFLs.
It has been a growing problem for some years.
Overloaded neutrals and excessive currents in capacitors in conventional fluorescent lighting are obvious targets.
I think the problem will get worse.

I slightly disagree with zog on the matter of power factor. The input kVA for a harmonic generating load is generally greater than the kW. If you define PF as kW/kVA, then it is a less than unity power factor. For consumer electronics and cheap(er) VSDs, it can be very much less than unity.
It's often referred to as distortion (as opposed to displacement) power factor.
Most light dimmers adversely affect both.

 

[jim dungar]

how is a circuit determined to have harmonics? and the only thing i know about eliminating them would be to use a larger size neutral. does harmonics have anything to do about having poor pf?

Circuits containing harmonics typically have low PF, but low PF does not in itself cause harmonics.

Increasing the size of the neutral copes with harmonics it does not eliminate them.

The harmonic content of any single device is not necessarily detrimental to the system, it is the combination of all harmonics and circuit reactances that determine if a problem exists. For example: a single VFD that draws 50A and has 25%THDI (total harmonic current distortion) may have very little impact, if any, on the output of a fully loaded 500kVA (600A) transformer with 5.75%Z.

 

[nolabama]

what little i know of harmonics can be found here - pick up their catalog if you can it is very informative
http://www.hammondpowersolutions.com/products/product_literature/technical_articles/index.php

 

[robbietan]

With a PQ meter that can look at the voltages and currents at different frequencies, harmonics are all at multiples of the system frequency, mostly odd numbered multipliers (3rd harmonic 180Hz, 5th - 300Hz)

That dosent eliminate them at all, just allows the system to handle them better, filters, isolation transformers, etc are ways to reduce harmonics,

or get better VFDs, the ones that have these filters already installed in them

 

[zog]

I slightly disagree with zog on the matter of power factor. The input kVA for a harmonic generating load is generally greater than the kW. If you define PF as kW/kVA, then it is a less than unity power factor. For consumer electronics and cheap(er) VSDs, it can be very much less than unity.
It's often referred to as distortion (as opposed to displacement) power factor.
Most light dimmers adversely affect both.

Ah Ha, less than unity lagging or leading? Most plants have very poor Pf in the lagging side of unity and therefore adding those harmonic generating loads that are in the leading side of a unity Pf the overall Pf for the facility becomes closer to unity.

 

[Besoeker]

Ah Ha, less than unity lagging or leading?

Depends on whether they are leading edge, trailing edge, or something else.
Many, perhaps most dimmers are phase-controlled and leading edge (chop off the first part of the cycle) so produce lagging PF. Conversely, trailing edge produces leading PF. Both produce very high levels of harmonic currents - a delay angle of 90deg for example, results in a third harmonic content of about 53% of the fundamental.
In the category of "something else" there are those that rectify the supply and then control the resulting DC. These take non-sinusoidal current from the supply and adversely affect distortion PF rather than displacement or both.

 

[nc5p]

Harmonics are often caused by equipment that rectifies the AC into DC. VFDs and frequency converters are two examples. What happens is with a simple rectifier and filter capacitor, the capacitors charge up to a certain value and current only flows when the AC voltage is just above the capacitor voltage. In single phase particularly this can be severe. Europe lead the effort through regulation to require power factor correction on loads. This is done through (most commonly) the boost converter, which allows the load to draw current exactly as a resistive load would, over the entire waveform. The output of the rectifier bridge goes into an inductor with a switched IGBT (or MOSFET) momentarily shorting out the secondary at a very high frequency. That causes the inductor to "charge" its magnetic field. When the short is removed, the inductor's stored energy will boost the voltage on the output. A diode then conducts a higher voltage (380 is common) into the capacitor bank. It is possible to obtain power factors very close to 1.0 and more importantly the harmonics are reduced to very low values.

The Fluke 345 is one example of a power quality analyzer. It runs around $1500 but is worth it if you need to work with power conversion or VFD equipment.

 

[MadeInUSA2007]

Laymens terms

Laymens terms

In plain english, I know that computers and florescent lights produce harmonics, which can almost double the load on the neutral. I don't really care why but I run #10 neutrals on those 20 Amp circuits when I can. Afterall, I don't need to know the engineering behind my car engine in order to drive to the supply house.

 

[brian john]

In plain english, I know that computers and florescent lights produce harmonics, which can almost double the load on the neutral. I don't really care why but I run #10 neutrals on those 20 Amp circuits when I can. Afterall, I don't need to know the engineering behind my car engine in order to drive to the supply house.

So on lighting circuits you pull an oversized neutral?
Do you do this on single phase 2-wire circuits?
On multiwire branch circuits with PCs you pull a number 10 AWG, with how many PCs per phase?

 

[don_resqcapt19]

In plain english, I know that computers and florescent lights produce harmonics, which can almost double the load on the neutral. I don't really care why but I run #10 neutrals on those 20 Amp circuits when I can. Afterall, I don't need to know the engineering behind my car engine in order to drive to the supply house.

However the increased neutral current from harmonics only happens on 3 phase wye systems. This is not an issue on 120/240 volt single phase systems.

 

[cschmid]

yet harmonics are leading the way in poor power factors..so when the POCO,s start charging for the poor power quality usage of the customer..who is responsible for correcting the problem??so is running an over sized nuetral going to have any real effect on the power quality??So if you are worrying about power quality..would you choose a capacitor correction or another form of correction..

 

[ohmhead]

harmonics

Well to me the only thing the double neutral size does is it can carry the extra current produced from harmonics , it does not stop harmonics i think power 3 phase or single phase can have harmonics balanced load or not they come from electronic switching high speed switching of computers vfds inverters and equipment today . harmonics can also effect your electric meter on your service high harmonics can effect power factor this can effect a lower reading on your meter lower your bill but increase the power companys load to make power that is needed but not measured . comments ?

 

[cschmid]

I believe the POCO has to generate more power due to low efficiencies due to an increased load due to poor comsumption..Harmonics are sinusoidal distortions and cause an increase in power consumption..

 

[gar]

090106-1532 EST

When you have poor power factor that means there there is a higher RMS current in the supply lines, transformers, and generators, than if your power factor was 1.0 . This produces wasted energy in the supply system and physically larger components in that system. Both of these increase the power company costs with no energy benefit to the customer.

One past the metering point these increased losses also cost you some small amount. For residential and many small businesses there is no penalty for poor power factor. What the meter reads is energy used and not the RMS current. To what extent harmonics cause KWH meter errors I do not know. But for the amount of typical harmonic content on a residential load I would suspect that the errors are rather small. The basic calibration of the meter might be a bigger error.

.

 

[cschmid]

090106-1532 EST

When you have poor power factor that means there there is a higher RMS current in the supply lines, transformers, and generators, than if your power factor was 1.0 . This produces wasted energy in the supply system and physically larger components in that system. Both of these increase the power company costs with no energy benefit to the customer.

One past the metering point these increased losses also cost you some small amount. For residential and many small businesses there is no penalty for poor power factor. What the meter reads is energy used and not the RMS current. To what extent harmonics cause KWH meter errors I do not know. But for the amount of typical harmonic content on a residential load I would suspect that the errors are rather small. The basic calibration of the meter might be a bigger error.

.

I must agree with this..and in our area they are annually having the calibration of the meters checked by an extrernal source (an idependant company)..

 

[brian john]

Well to me the only thing the double neutral size does is it can carry the extra current produced from harmonics , it does not stop harmonics i think power 3 phase or single phase can have harmonics balanced load or not they come from electronic switching high speed switching of computers vfds inverters and equipment today . harmonics can also effect your electric meter on your service high harmonics can effect power factor this can effect a lower reading on your meter lower your bill but increase the power companys load to make power that is needed but not measured . comments ?

And where have you seen this.

And you use a oversized neutral for SINGLE PHASE?

 

[gar]

090108-1708 EST

Brian:

I do not know of an immeadiate application but if I use the transformer as a full-wave center tapped rectifier, then the neutral RMS current will be higher than either hot line current.

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