Transformer Differential Protection 1

[rashid_UK]

Hi all,

I have been reading technical literature on transformer differential and am having difficulty visualizing why some of the values are what they are, I would be grateful for any comments from members or pointers to good reference material to aid my understanding.

My understanding is that the protection scheme compares the load current to differential current flow and will find the ratio.
A typical differential scheme will have a number of sections:

First section - allows for differential current due to tap change + CT transformation error + magnetizing current + Relay error
Second section - slope of X% which starts at say 0.5 * Iprim to say 2 * Iprim

Q: If we decrease the slope end point from 2*Iprim we will make protection somewhat more stable for out of zone faults because we increase the restraint area.​

Q: What is the reasoning behind the number choosing a value around 1 - 2? (I understand it can be set much higher if required but would like to know why these values are typical, is it because the transformer could see 2 times FLC in am=n extreme case?)​

Q: The slope of the line, should this be the same % allowed as for the % of primary current allowed in the first section (Tap+CT+Relay error) because the differential error current will increase somewhat proportionally as the load increases?​

Slope of second line - Setting this to 100% allow out of zone faults near transformer to pass through without issue and allow for CT saturation. There is a Highset Stage that can be set which will clip the restrained area at a X A (80 - 100% of max primary theoretical fault current) I see quoted in some application guides.

​

Q: Is it not better to leave the slope continuous without a High-Set stage as defining a value here will greatly increase the possibility of a trip for out of zone faults?

Thank you

 

[Mbrooke]

Not sure if this helps, but its a good resource imo:

http://digitalcommons.mtu.edu/cgi/viewcontent.cgi?article=1576&context=etds

http://www.ijareeie.com/upload/2013/september/5A_A GENERAL.pdf

 

[DiscoP]

The relay manuals are normally pretty good references.

For the second section, normally you work out what the expected differential current at the extreme tap positions at allowable full load current, and see that there is adequate biasing and adjust if necessary.

The slope end point is normally the maximum allowable load current - if the transformer is designed to be able to supply twice nominal load for a period of time, 2 is the setting that should be applied.

Note that the biasing method and characteristic of the relay need to be understood for all these settings - as well as the transformer ratings.

With some relays a second slope setting of 100% will mean the relay will not operate, with others a 100% setting may lead to a false operation due to CT saturation. This is why the manual is normally the best reference to use.

The high-set is normally used to allow a trip for a large fault with in-rush current present.

In-rush harmonics can restrain the biased diff protection for a number of cycles, which may not be acceptable.

 

[prc]

Here is a good tutorial on the subject:

http://www.iea.lth.se/publications/Theses/LTH-IEA-1055.pdf

- Transformer differential Protection –PhD thesis by Zoran Gajic