Transformer Differential Protection 3

[prc]

I have a query on Transformer differential protection. How is the harmonic restraining feature is set in the relay? I thought the relay should sleep( block operation) only when there is second harmonics in all three phases. But Iam told cross blocking is used nowadays so that relay will operate only when there is no second harmonics in all three phases. Is this not a dangerous situation? In case there is an internal winding failure in one phase and if the unit is energised then the relay will not operate as the healthy phases will draw current with second harmonics and relay will sleep. The transformer will be isolated by relays such as gas operated relay only after much time delay. What is the practice followed in various parts of the world for large power transformers?

 

[pwrtran]

Harmonic restraint - does not block 87 but to add the harmonic current to the restraint current
Harmonic blocking - set a threshold to block the 87 element
To detect internal turn-to-turn fault, may try negative sequence differnetial

 

[davidbeach]

Also, most transformer differential relays have an unrestrained differential that will operate on high differential currents, so an internal fault on energization should trip on the 87U, completely ignoring harmonics.

 

[prc]

The problem is many times the differential current drawn from line will not be high,but the inter turn fault current will be quite high melting copper and consequent break causing severe arcing.What is 87 U, negative sequence differential? Why not we programme such that relay shall operate in case any phase become with out second harmonics? Any problem anticipated?

 

[davidbeach]

Sometimes one phase will have very little 2nd harmonic. 87U is Unrestrained. 87Q is negative sequence differential, and is the best method for a relay to detect the currents associated with a turn-to-turn fault.

 

[stevenal]

How harmonic restraint/blocking works is very much dependent on the relay and manufacturer chosen as well as on the philosophy of the engineer setting it. One pretty good discussion of the subject can be found at

http://pattersonpowerengineers.com/papers/2000_GA_TECH_paper.pdf.

The paper is a bit dated now, but I think it is still relevant.
I think you'll find that tripping on inrush into an un-faulted transformer is a bit more common than failure to trip on a faulted one.

 

[slavag]

You can take in account, if you have block of 2nd harmonic, you are have deblocking too.
its depend of protective terminal type.
Cross blocking is very important function, especially in case of one phase autorecloseres in the grid.

You can use additional functionality in few newer protective terminals: waveform blocking.

 

[prc]

Thank you stevenal for pointing out that paper.My present case is exactly same. This paper is with my data bank and I glanced through this at least two times while seeking a solution to my present problem,but never read it! Thank you Davidbeach.

 

[tinfoil]

We are a SEL Shop, so I heartily recommend a read through ...
[link file:///C:/Temp/6261_ConsiderUsingHarmonic_KB_20061019.pdf]Link[/url]

 

[stevenal]

[link 6261_ConsiderUsingHarmonic_KB_20061019.pdf]Link[/url]

 

[stevenal]

Trying again

 

[tinfoil]

thanks, stevenel.
I guess the Link button on the forum has some issues...

 

[prc]

Thank you tinfoil for that excellent tutorial.

 

[inpran]

Just to add on to the previous replies, during a fault on one phase, the harmonic blocking settings(settings mostly in percentage - ratio of harmonics to the fundamental component)will take care of the case.

The general settings I have come across is 15% for second harmonics(I2/I1 - I2 is second harmonic current and I1 is the fundamental)and 25% for 5th harmonics.

During a turn to turn fault,since the rise in the magnitude of fundamental is high than the second harmonics, the possibility of harmonic blocking getting activated is remote.

However, in the unlikely event of such a case where the ratio has been exceeded, schemes mentioned by davidbeach will ensure the operation of differential protection.