DOC protection for over head line

[keyhan2001]

I have a question about protection of high voltage (over 200kv) line.

Based on Siemens siprotec recommendation, high voltage line use two distance relay as sub1 and sub2 protection (main function). Also as common protection over voltage and DEF (directional earth fault) are used.
My question “DOC (directional over current) function is necessary for completion protection system or not?”
I know all famous brand distance relays including DEF and DOC. But I don’t know , if just DEF is enough why they were included DOC function.
On the other hand, we use two distance relay as main and backup. In this case, is there any need to additional function as DOC function for high voltage line?
Or is there any fault that may not clear by distance relays and need DOC function to clear it?

Another question is about 132kv line,
Also based on siprotec recommendation , they use under voltage as protection function,. But there is nothing about over voltage.
I think over voltage is also important for protection. And if this is not recommendation it means no fault leads to overvoltage.
Please give me some reference about that.

 

[bacon4life]

Over voltage relays are typically only used on equipment that are permanently damaged by moderate over voltages (>110%) such as generators, transformers or film capacitors. For generators & transformers, volts/hertz is better than over voltage because it will also protect an islanded generator & GSU from saturation at off nominal frequency. An overhead transmission line can withstand a very high voltage without flashing over, and even if it does flash over, there is a low probability of permanent damage.

 

[tem1234]

When the distance relay are directed foward, there's a little part they see backward.

DOC will just see foward.

If you don't want the distance protection to operate on a fault backward of the protection, you combine it with a DOC with a logical function "AND".

 

[keyhan2001]

bacon4life : thanks for your answer. But if over voltage is not important function for 132KV line, why under voltage is recommended for this line? also why over voltage function is recommended for 230Kv line?
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tem1234 :
Distance function have 4(or 5) zone that 3 of them are forward and one of them covers backward. It is not related to DOC function( Directional Over Current). My question is about necessity of DOC function when we use 2 different distance relay.

 

[slavag]

Im not see any added value in DOC.
Distance protection is best solution "( ok, 87L too ).

Undervoltage protection for line is some exotic functionality. maybe used is undervoltage load shedding.

 

[bacon4life]

Do you have link to the recommendations you are referring to? Numeric relays often have many more types of elements built in that is actually prudent to use. On very long 230 kV lines, there might be enough Ferranti voltage rise to consider using over voltage elements. Under voltage elements are sometimes used to block distance elements during low voltage.