transformer differential protection

http://www.patentstorm.us/patents/6356421/description.html

http://www.emo.org.tr/ekler/ff75f52998a477f_ek.pdf

second hormonics components comprises highest percentage(UPTO 80%)at the time of transformer charging,so that it can be easily and distinctively sense by protection relay.Also third hormonic current predominant in delta windings.

You see! the 2nd Harmonic and the 5th harmonic are called "Restraint Features" in a transformer differential relay. The 2nd Harmonic Restraint is used for restraining the relay operation during a switching inrush (which the relay might sense as a differential fault as this current appears only on one side of the trafo.). It has been studied and found out that the 2nd Harmonic Component in the trafo. inrush current is significant and hence can be detected easily. Not so with the other harmonics.

So also, the 5th Harmonic Restraint Feature is used during sudden load throw-off conditions, which might cause over-fluxing and hence differential currents. Also, it was found out that during such over fluxing conditions, the 5th Harmonic Component is predominant and hence detection becomes easy for restraining the relay operation.

Dear electricalexpert65.........

I have read many of your postings which all are were very useful & really indicates that you are an expert in this field!!!What I couldn't understand here is that we are not considering even harmonics in electrical systems since they just cancel out in one half cycle of the fundamental waveform as we can see that they repeat twice(2nd) or four times(4th) in one half cycle unlike odd where the total effect is zero.(This is mentioned in one of the topics).So how 2nd harmonic can be used to detect the inrush??

How that the inrush current is not reflected to the secondary of the transformer?

Thank you.

Please note that we are not considering only the EFFECTS of even harmonics because of half wave symmetry, as had been explained by me in another thread. But, nevertheless, all even harmonics will be PRESENT. And, as you must be aware, the magnitude of a particular harmonics is: (the magnitude of the fundamental / the order of the harmonics). So, naturally, among all harmonics, the 2nd Harmonics will have the highest magnitude. And, when the magnitude is high, the detection becomes easier. This detection is utilised to restrain the diffeential relay operation.

Coming to the second part of your question, the inrush current will not be reflected on to the secondary side of the transformer as the entire inrush current is spent only to excite the core.

One more question regarding this....

As you told "the 5th Harmonic Restraint Feature is used during sudden load throw-off conditions, which might cause over-fluxing and hence differential currents. Also, it was found out that during such over fluxing conditions, the 5th Harmonic Component is predominant and hence detection becomes easy for restraining the relay operation."

Why not the 3rd harmonic is used as restraint feature during overfluxing conditions as its magnitude is more than that of 5th ?

How its gets cancelled in delta winding?

Sorry for the delayed response. I was on tour.

As you must be aware, third and all triplen harmonics are zero sequence in nature. That is, the three line currents will add up arithmatically.

In the Delta side of the transformer, in Line 'R' when there is a third harmonic current, say, Ir3h, flowing into the 'U' winding, at the same instant, there will also be the return third harmonic current from 'B' Line, say, Ib3h, through the 'W' winding, which flows in the opposite direction in the 'R' line. As the magnitude and phase angle are same but the direction is different, they cancel out each other.

+ Ir3h at an angle 0 degree PLUS - Ib3h at an angle 0 degree becomes a net current of zero amperes at R line. So, they cancel out.

This does not happen for 5th harmonics as 5th is Negative Sequence. That is, it would be: + Ir5h at an angle + 120 degree PLUS - Ib5h at an angle - 120 degree in R Line. So, they cannot add arithmatically but only vectorially. They cannot cancel out each other.

Hope it is clear.

Thank you ,.........but now my question is why 3rd harmonics are zero sequence currents......how it can be proved?I have forgotten most of equations studied earlier in college & I remember that a sinusoidal waveform can be represented as the sum of its orders like in Fourier transform & any one phase has zero,negative,possitive sequence componenets......hope it can be clarified..!

If fundamental rotates at 50Hz. third harmonic rotates at 150Hz. (i.e.) by the time fundamental completes one revolution, third harmonic would have revolved three times. Let us take Y phase as reference vector. If fundamental Y is at 0 degrees, fundamental R will be at + 120 degrees and fundamental B will be at - 120 degrees.

For third harmonic you will have to multiply this by 3. So, third harmonic Y will be again at 0 degrees, third harmonic R will be at + 360 degrees and third harmonic B will be at - 360 degrees. You would agree that + 360 degrees & - 360 degrees is also 0 degrees. So, the third harmonic vectors of all the three phases will be at 0 degrees (i.e.) there is no phase displacement between the three vectors. Hence, they are zero sequence in nature and will be in phase.

Excellent........now it seems to be so simple!........thank you for the interest you are taking.....really an expert you are!

thanks a lot. but why 4th harmonics are not considerd in relay application? how 5th harmonic appear during overfluxing?

Just go through the all postings.......Again,WAReagle has given a link where it can be understood also!

I have explained this in an earlier posting too. Not only the 4th, but all even order harmonics will not have any effect on the system because of a phenomenon called Half Wave Symmetry. That is, in all even harmonics, the number of positive half cycles and the number of negative half cycles will be equal in one cycle of the fundamental. Whatever the effects of the even harmonics' positive half cycles will be cancelled out by the effects of the even harmonics' negative half cycles.

To clarify a few things, it is not that only 2nd harmonic or 5th harmonic will appear during transformer energisation or during over fluxing. All harmonics (including inter-harmonics) will appear. We trap only which is traceable and effective.

thanks once again. i have a question in my mind for a long time , can you tell me what type of core materials are generally used in metering CT, protection CT and Class PS( for differential) CTs. plz reply.

Sir,

I have some doubts on your statement , can you please explain these,

• you said that, the Y phase ,R phase, B phase means current or voltage.
• As a result , you said that they are in zero sequence and will be in phase, what is the advantage and disadvantage of this?