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Anonymous Poster #1

Protection of Transmission Lines by Over Current Relay

05/30/2014 5:19 AM

Hi,

In protection of transmission lines by overcurrent relays, in current discrimination and time discrimination:

In current discrimination:

- relays are always set to pick up at a current equal to three phase symmetrical fault current at 80% of the line section to be protected. Why ? What happens with remaining 20%? Is it protected in the next section?

In time discrimination:

- definite time current relays are used. Here, the problem of uncatlled tripping of relays due to overcurrent can be solved by setting the in-built instantaneous units to reach upto 80% of the section to be protected. What about next 20%? Is it protected in the next section?

I thought this 80% is for "reach" of a distance relay only. How to understand this?

Regards,
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PWSlack
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#1

Re: Protection of Transmission Lines by Over current relay

05/30/2014 5:38 AM

<...relays are always set to pick up at a current equal to three phase symmetrical fault current at 80% of the line section to be protected. Why ?...>

To give a margin. If the setting were >100% the line would fail before the device operated!

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Anonymous Poster #1
#2
In reply to #1

Re: Protection of Transmission Lines by Over current relay

05/30/2014 5:50 AM

I think this 80% has to do with length of line and not the value of current.
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PWSlack
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#3
In reply to #2

Re: Protection of Transmission Lines by Over current relay

05/30/2014 5:52 AM

Then why ask the question (rhetorical question - NNTR)?

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Anonymous Poster #1
#4
In reply to #3

Re: Protection of Transmission Lines by Over current relay

05/30/2014 5:56 AM

i am concerned about the protection of remaining 20% of the line....does that form the blind spot?

Regs
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PeterT
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#5
In reply to #4

Re: Protection of Transmission Lines by Over current relay

05/30/2014 8:40 AM

The other end of the line's relays are set similarly, to cover 80% of the line from that end. They overlap in the middle 60%. The main idea is that the fault current is not exact, and you don't want the relay in one station (if set at 100%) to trip for a fault that is inside the other station (due to inexact value of current). Faults just barely inside the opposite end station should be cleared by the protection at that station, not the opposite end (near station) of the line. That way if there are radial loads tapped off the line along its length, they will still remain energized from this near end, even if a fault occurs within the far end station.

For more info, see GE's "The Art & Science of Protective Relaying," Chapter 13 (free download):

http://www.gedigitalenergy.com/multilin/notes/artsci/art13.pdf

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Anonymous Poster #1
#6
In reply to #5

Re: Protection of Transmission Lines by Over current relay

05/31/2014 1:48 AM

thanks Peter T, I guess this 80% section protection is concerned with overcurrent relays and time delay relays. Is similar the case for distance relays?

Regs
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PeterT
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#7
In reply to #6

Re: Protection of Transmission Lines by Over current relay

06/02/2014 8:10 AM

Yes. See subsequent chapter(s) in "The Art & Science" document, on distance relaying and pilot wire relaying.

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