Electromechanical relay failing to trip

[drchaos]

Hi, I've been asked to look into why an R1Z23B electromechanical distance relay failed to react to a 3phase fault on a 110kV line, leading to a fairly big blackout in the affected county. The relay is 600/5 A, 60VA, and connected via a 25VA cable to a burden of 15VA. The fault had a significant DC component of about 3kA, and the AC component was also about 3kA. My question is: would that DC component have saturated the CT, or would the problem lie with the extra burden of the cable? The relay was tested and was working fine.
I'm not a protection expert, I know a bit, but any pointers would be greatly appreciated. Thanks.

 

[521AB]

I would like to comment Bluezee,

VT (or PT) fuse failure detection (and then blocking) is a separate element. The distance relay is designed to trip, if fault voltages go to zero, after a normal healthy state (healthy pre-fault).
The healthy pre-fault state is important, because if the pre-fault voltages are not given, and the relay immediately sees only fault voltages (and currents), this is called Switch Onto Fault (line is not energized, no pre-fault voltages, and then the line is energized on th epermanent fault). Requirements for SOTF trip are different than "normal instantaneous zone1 and delayed zone2": instantaneous trip for all faults on the protected line, without communication scheme. This means that SOTF is an additional (and complementary) function to distance protection relay.

FUSE failure function (maybe external function or built-in function in multi functional terminals) has the task to detect that VTs have been lost (fuse failure detection) and then block the distance protection by energizing the block imput of the relay.

ANYWAY SLAVAG, it is worth, if you have fuse failure relay, to verify that it did not block, eventually, your distance relay.

Regarding power swing, again: distance protection has nothing to do with power swing. It has to be detected by a special function (power swing detector) and if detected, it blocks the distance protection.

AGAIN: SLAVAG, check if you have the power swing relay and check if it eventually operated.

 

[521AB]

Doctor Chaos...
are you sure you are going in the correct direction?
What do you model with EMPT?
You can have the model of your network, fine, but which type of fault and which fault resistance will you ask EMTP to simulate?

Do you have the source impedance (positive sequence, as you talk about 3-phase fault) behind the relay? Can you give it to us?
Do you have the line data (positive sequence is enough, 3-phase fault)?
Let's see what is the fault current level at 3 km from the relay point, zero fault resistance, steady state (don't need EMTP for that) and let's see if the CT with the data we have obtained saturates (E2 > E2max).
Or let's see if the CT saturates with a 3-phase fault at relay point (0% of the line). if does not saturate, I think you can't blame the CT....

 

[drchaos]

Slavag, these are all good questions!
Not sure if i have the answers though.

I think I would like to put together a simple model with a few lines, short-cct MVA, model of a CT with saturation, and relay burden. THis will take time : )

 

[slavag]

Hi Freinds.
Drchaos, you don't need (from my point of view ) any simulation in any SW. I disscased with my freinde ( for me he is guru in distance protection), he also have good expirence in western europa, his opinion it's high resistive fault and we don't have back-up sensetive directional e/f protection ( BTW I recommend use this function in your new digital relay)on this line.

Bluezee, your reasons, I think more suitble for the digital relay.
I'm not sure if this relay have any power swing detector.

521AB,your:
ANYWAY SLAVAG, it is worth, if you have fuse failure relay, to verify that it did not block, eventually, your distance relay.
AGAIN: SLAVAG, check if you have the power swing relay and check if it eventually operated.
connected to my case, if yes:
This is one good digital relay, customer have event list and DR, no problems, no Q's.
Regards.
Slava

 

[521AB]

SLAVAG, I wrote SLAVAG but the message was for Drchais. Sorry

Now this is for you SLAVAG, we are discussing (don't know if true or not), a 3-phase fault. Earth fault protection, or sensitive earth-fault protection cannot do anything for this.

If the problem is the fault resistance, quadrilateral characteristic are quite good in detecting fault resistance even for 3-phase or 2-phase faults. MHO characteristic are not so good in that. To see more fault resistance, you need to increase the X setting (to make the circle bigger) and you risk to "overreach" (trip for faults putside your line).

 

[slavag]

Good day 521AB.
I don't believe in full 3-pf fault. Of course SDEF it's only help in case of ground fault.

"If the problem is the fault resistance, quadrilateral characteristic are quite good in detecting fault resistance even for 3-phase or 2-phase faults. MHO characteristic are not so good in that. To see more fault resistance, you need to increase the X setting (to make the circle bigger) and you risk to "overreach" (trip for faults putside your line)".
This is a point and I think it's a reason for no-trip.
Possible several option for this old EM relay: oval, MHO off set, hibryd, etc.
Regards.
Slava

 

[521AB]

Regarding 3-phase faults, look at that:

http://www.eng-tips.com/viewthread.cfm?qid=161609

 

[slavag]

Thanks a lot 521AB.
In additional to your two cases I would like add human mistake ( I wrote here about it), close ES on live line or
close CB on fault ( somebody forgot earhting on trafo or generator).In these cases you have really symetrical fault, according to theory. In other cases 3-ph fault is evaluation of other types of faults, as you and other wrote. In topic above you discussed about it and I don't want open this discussion again.
BTW, on this was build first types of power swing detectors.
Regards.
Slava
P.S.
Drchaos, what news?

 

[drchaos]

No news, except that my boss who is a protection expert, has ruled out a high impedance earth fault as the problem.
Otherwise, it's time to enjoy the weekend and not be worrying about relays!