Assymetry in 3 phase supply

[mithko]

"Transformers handle negatvie sequence components the same way they handle positive sequence components"
1 .Does that mean that the negative sequence do not have bad effects on transformers such as increased heating?

"zero sequence components affect transformers in a way that depends on their confirguration (i.e. whether delta/wye, wye/wye, etc.) and the presence of a nuetral"
2. What does this mean and what are the exmaples?

3. Can anyone explain the sequence components in a simple and practical way?

 

[davidbeach]

Lots about sequence (symmetrical) components in many of the references in faq238-1287.

 

[electricpete]

1 .Does that mean that the negative sequence do not have bad effects on transformers such as increased heating?

Rotating machinery can experience dramatic localized heating from negative sequence currents which is far larger than for comparable positive sequence currents. This is due to the fact that the frequency of these components when seen by the rotor is dramatically different. (fundamental negative sequence component is close to twice supply frequency as seen by the rotor). This causes things like surface heating of rotor conductors/core from skin effect, and increased rotor core losses. Additionally, the negative sequence stator component induces a negative sequence rotor component and the 2 combine to produce a negative seqeuence (Reverse) torque, which must be overcome by the positive sequence to drive the load.

I don't see any reason that transformers would experience similar effects since in that case the negative sequence has the same frequency as the positive sequence.


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(2B)+(2B)' ?

 

[dpc]

These are very general questions - difficult to answer in the context of a forum like this. I'd refer to the references in the link that David Beach gave you.

After doing this, if you have more specific questions, feel free to post them.

Regarding the transformer and negative sequence currents, I agree with Electricpete - the transformer doesn't care.



David Castor

http://www.cvoes.com

 

[mithko]

thanks guys...what I meant was "does the presence of negative sequence currents in transformers mean that you have additional heating that you would otherwise not have in the case of a symetrical supply".

 

[davidbeach]

No, the transformer doesn't give a hoot about the rotational order of the phases.

 

[jghrist]

Sequence (symmetrical) components are mathematical constructs used to enable balanced circuit analysis on a system with unbalanced voltages or currents. They are not normally used to determine heating effects on transformers. For transformer heating, forget about sequence components and limit the highest winding load to the 1/3 of the 3Ø transformer rating.

 

[bacon4life]

As the other posters stated, the most common use of symmetrical components is for analysis of unbalanced systems with only fundamental components (50 or 60 Hz).

A less common way to use them is to describe a system that has harmonics instead of unbalanced currents or voltages. The harmonics 1, 4, 7 ... 3n+1 are positive sequence, 2, 5, 8 ... 3n+1 are negative sequence, and 3, 6, 9 ... 3n are zero sequence. If you are referring to harmonic sequence components, they do have higher heating effect that are solved by using k-factor transformers.

The Whys of Wyes does a great job explaining this other use of symmetrical components and also how the delta/wye configuration impacts zero sequence components.

http://www.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White Papers|GET-3388B|generic

This paper has a more in depth explanation of how to denote the different meanings of symmetrical component.

http://itee.uq.edu.au/~aupec/aupec05/AUPEC2005/Volume2/S192.pdf

 

[sibeen]

bacon4life, a great point, well made.

I think this is one of those examples where two EEs can be talking to each other, both using the same words, and yet meaning completely different things.

When electricpete stated "the negative sequence has the same frequency as the positive sequence", my immediate reaction was...WTF?

I was about to send off a hastily worded missive until I suddenly realised that Oh, they're talking about the 'other' sequence currents. I'm someone who works with harmonics on a regular basis and so for me the sequence currents has to do with the harmonic order and it's nice to be reminded (hit over the head with) that there's other ways of looking at things.

 

[mithko]

Thanks guys, form what you guys are saying I gather the following:
1. the reason why assymetry will cause increased heating in transformers is purely due to the presence of zero sequence current.(SINCE NEGATIVE SEQUENCE COMPONENTS DONT HAVE ANY EFFECT)
2. Tripple-N harmonics (3,9,15,21,...) can be thought of as zero sequence component.

Am I right or wrong?

 

[davidbeach]

Are you talking power system frequency sequence components or are you talking harmonics? It really makes a difference.

 

[bacon4life]

Not quite right yet. All currents going through the transformer cause loss; some types just cause additional losses compared to normal currents.

Power frequency negative sequence components have the same heating affects on transformers as power frequency positive sequence components.

Harmonic components will cause extra heating do to frequency dependent losses in the copper and steel. As the frequency goes up, the amount of extra heat generated goes up. For these additional losses, the frequency is important, not a particular sequence.

Zero sequence components passing through a grounded wye/grounded wye transformer would cause additional losses just due to frequency dependent losses.

Zero sequence harmonics cause additional heating as they are trapped by delta/wye transformers and the current circulates in the delta winding. This is in addition to any frequency dependent losses.

 

[jghrist]

Introducing harmonics into this is unecessarily confusing things. It is a different subject. The OP was about sequence currents not harmonic currents.

True, triplen harmonic currents have a higher frequency and higher frequency currents cause more losses. Not all zero-sequence currents are harmonics. An unbalanced fundamental frequency load can cause zer0-sequence currents.

Balanced 5th harmonic currents are negative-sequence. They also have a higher frequency. Does this mean that negative-sequence currents cause more losses than positive-sequence?

Balanced 7th harmonic currents are positive-sequence. They also have a higher frequency. Does this mean that positive-sequence currents cause more losses than positive-sequence?

 

[dpc]

It's really misleading to refer to harmonic currents as sequence currents. By definition, symmetrical component sequence voltages and currents are at the fundamental frequency.

It is true that certain order harmonic currents **behave like** certain sequence currents. But they are not the same thing at all.



David Castor

http://www.cvoes.com

 

[FPelec]

Also the presence of a 50 Hz, zero sequence voltage component will result in additional heating (expecially of transformer shields and tank) in case of 3 limbs grounded wye - grounded wye transformer without delta winding, due to zero sequence flux that exits from transformer core.