Electromechanical relay failing to trip
Electromechanical relay failing to trip
(OP)
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.
I'm not a protection expert, I know a bit, but any pointers would be greatly appreciated. Thanks.






RE: Electromechanical relay failing to trip
Please provide more information:
What is a CT: type, burden ( 60VA?), ratio(600/5A?)?
What do you mean cable 25VA to burden 15VA?
Regards.
Slava
RE: Electromechanical relay failing to trip
CT - current transformer with ratio of 600/5 amps. It is connected via a long cable to the relay in the control room. The burden of the cable is 25VA and the burden of the relay itself is 15VA. I've been told that the current transformer is rated 60VA. So the total burden on the CT is 40VA. We know the relay picked up, but never sent a trip signal. Do you think the burden on the transformer was too much, or would the large DC component of the fault current have caused the transformer to saturate?
RE: Electromechanical relay failing to trip
Burden of CT, 60VA is OK. what is type 5P10/20/30 or ANSI type.
What is a lengh of cable?. 25VA that mean 1Ohm, for my pinion is so much.
Problem is, I don't know this relay, what is requirements to
CT. Please check what is n'> requirements for this protection.
It's new installation, it's first fault on this line?
Regards.
Slava
RE: Electromechanical relay failing to trip
This is an old installation. The relay has probably been in place for 20 or 30 years, but has received regular testing and maintenance. I will check the settings of the relay - its a distance relay, not overcurrent.
RE: Electromechanical relay failing to trip
60VA CT, 600/5A, long cable, we put with this data digital relay with 0.3, 0.5VA current input, here is 15VA!!!!
From other side 20years, 110kV line w/o faults (also possible???).
Second option what was fault impedance, maybe via arc and not eq to pick-up.
Of course with CT was in saturation, this influence on dist protection.
In all cases, we don't have any recorders of fault and very dificalt say,what is a problem.
Check operation of relay from CT, not from terminals into cubicle.
RE: Electromechanical relay failing to trip
I think we will add a digital relay as main distance protection now. Should have been done years ago!
RE: Electromechanical relay failing to trip
Yes, it seems as fault via high resistance ( arc resistance, not tower). This type of fault with high level of DC component is also problem for digital protection.
3kA is not so high level of fault for 110kV (I assume you have 3-ph fault around 30kA in the system).
I'm also assume, that fault was in end of first zone of dist protection.
Regards.
Slava
RE: Electromechanical relay failing to trip
RE: Electromechanical relay failing to trip
RE: Electromechanical relay failing to trip
The easy way to test everything is to test it as a system; with the relay in service, do a trip test with secondary injection and allow it to trip the C/B.
RE: Electromechanical relay failing to trip
the relay and various components have been tested and are working normally. The finger of blame is being pointed at the CT - but I am wondering has anyone come across a similar failure to trip because of a large DC component to the fault current.
RE: Electromechanical relay failing to trip
Possibility n. 2. The fault was not of evolving type. The relay picked-up, on th enon instantaneous zone. During the timer the arc "gets longer", i.e. the fault resistance gets higher, larger than the setting. Old relays do not allow to have different fault resistance settings (only one setting for all zones), so when the fault resistance went out from the relay zone, it went out from all the zones. Hopefully some other back-up relay (in remote circuit breakers) tripped for that fault.
RE: Electromechanical relay failing to trip
So you are saying the fault appears initially in the zone 1 R-X circle, but then moves back out again as the fault evolves and the resistance grows.
RE: Electromechanical relay failing to trip
521AB , you are right, I know about only one case of 3-ph fault(one person closed ES on live line in 161kV). Protection usally started on 2ph-fauly or I think 90% faults are phase to earth(ground) fault.
Drchaos
What is a company this relay?
RE: Electromechanical relay failing to trip
For the moment I would more think about evolving fault, if the relay is a switched relay, and not a full scheme one, which is quite possible if the relay is old. Relay picked-up for a phase-ground fault (LN), but when it started to measure the distance, the fault was 2-phase-ground (LLN) and the measured distance was longer than the actual one, because it was measured with the single phase loop.
But this is just an assumption. Try to get the disturbance recorder. You can't say anything without it.
Remember that if you get the disturbance recorder (probably it will be in form of a file: comtrade standard), it is possible to play-out again the waveforms from the modern test equipments, so you can inject again the waveforms into the relay and verify the "missed trip".'
RE: Electromechanical relay failing to trip
RE: Electromechanical relay failing to trip
You are asking about CT check.
First of all, as Slavag wrote: do you have any CT requirement for the relay manufacturer?
The CT i 5P50. Hoe many VA? What about CT resistance?
To calculate the voltage across the CT during the fault, not considering CT saturation due to dc-component (this should available in the formulae for CT sizing given by the manufacturer), do like that:
E = Ishort * (Rct + Rwire + Rrelay)
Rct is the CT resistance (ohms)
Rwire = resistivity at 75 deg * length / area = 0.022E-6 * 400 / (10E-6)
Rrelay = relay burden / Irated
As I said, there should be some coefficients mutilplying Ishort, considering the dc-component (or the time constant of the fault). But let's assume 1 for the moment (it can also be 3 or 4..).
You should compare the calculated voltage with the so called Accuracy Limit Voltage (E2max).
E2max can be calculated from CT data (accuracy limit factor, 50, Rct ?? and the nominal buden), but you can assume E2max = 1.2 Vkneepointvoltage (according to British Standard).
If E > E2max (by using 1 as coefficients multiplying the symmetrical steady state fault current), for sure the CT has to be considered saturated.
If not we don't know, because we don't know the coefficients in the formula.
I hope this helps.
RE: Electromechanical relay failing to trip
521AB small correction, you need use in calculation 2 x R wire.
Drchaos please see link
thread238-194167: Understanding CT magnetization curve
calculate n'=50x(60+0.7x25 )/(0.7x25+15+0.019x800x25/10)=55
not bad.
In all case check CT and CT wiring and realy mnf requirements, but n'=55 I think is good data ( of course for digital relay, but I think same for EM relay too).
Not correct setting???? dificult say.
It's short or long line?
I back to 521AB very important Q, what clear this fault?
3ph fault with 3kA current level, seems very small.
What is a type of neutral grounded in your system: solid grounded, resistive, "ungrounded???"?
W/O DR data and event list of relay or system are dificult say, but try help.
RE: Electromechanical relay failing to trip
Were are you : IEC or ANSI land?
Regards.
Slava
RE: Electromechanical relay failing to trip
Anyway, if you take 2 times you are on the safe side!
I am in IEC land...
RE: Electromechanical relay failing to trip
Cable: 400 m 10 squared mm
Relay burden: 15 VA
Ishort three-phase = 3000 A
Let’s go.
E2max = 775 V (almost knee point voltage)
Because:
What is the voltage across the CT when it delivers accuracy limit current (50 * In = 50 * 5 = 250 A) on the nominal burden of 60 VA?
Go back inthe resistive world:
CT is a current generator of 250 A, in series with Rct, in series with Rburden.
Rburden = 60 VA / In^2 ? Rburden = 60 / 25 = 2.4 ohm.
E2max = 250 A * (0,7 ohm + 2.4 ohm) = 775 V.
What is the voltage across the current transformer whe it delivers the fault current of 3kA (25 A secondary)?
The same as before: The CT is a current generator (25 A) in series with Rct, Rcable, Rburdenrelay.
Rcable = rho * LENGTHcable / AREAcable = 0,022E-6 * 400 m / 10E-6 = 8,8 ohm
Rburdenrelay = 15 VA / Irated^2 = 15 VA / 25 A^2 = 0,6 ohm
E2 = 25 * (0,7 + 8,8 + 0,6) = 25 * 10.1 = 250 V
250 V is much less then 775 V.
If we consider a multiplication factor of 3 for the fault current (the factor that should take in consideration dc current and its time constant --> read CT saturation), we go to 750 V, very close to E2max.
3 is a factor that can be used for numerical relays, I wonder if mechanical relays do not require more, maybe 5 or 6? Then the CT IS saturated.
Slavag, do you follow me?
RE: Electromechanical relay failing to trip
I'm on 99% sure it's start as LN or LLN fault and finished as 3ph fault.
RE: Electromechanical relay failing to trip
RE: Electromechanical relay failing to trip
I only read new.
Regards.
slava
RE: Electromechanical relay failing to trip
In all case in two way we have same result.
Nice work with you.
BTW for this case I prefer use 5 or 6 factor.
RE: Electromechanical relay failing to trip
1. What is a reason for pick-up?
2. What is accurace of CT.
More or les we try closed noe all option, you checked options of HW faults ( relay, CB, etc), according to drchaos are OK, will try next options.
Regards.
Slava.
OR THIS CASE IS DEAD CASE (I don't know if it's right expression)
RE: Electromechanical relay failing to trip
But you are right.
This means that the voltage is much less:
E2 = 25 * (0,7 + 0,88 + 0,6) = 25 * 2 = 50 V
Even considering a factor of 6 we go to 300 V, when we have "750 available".
CT cannot be blaimed. I think we can say that.
DanDel, good observation (relay started), but what does start mean? Is it just a very large starting zone, which doesn't trip. Relay can start even if CT saturates, if the zone is very large, but then the relay will underreach a lot (measure higher impedance then tha actual one) because current is much less.
Anyway I think you can't blaim the current transformer, and it seems they knew what they did 25 years ago, when they put a 10 squared mm cable there!
If CT is ok. can be too low resistive reach setting? Can you try just to inject the steady state values of the fault quantities in the relay, to see if it trips?
RE: Electromechanical relay failing to trip
If there were never any flag, then I would say to check out the relay, CTs, etc. The flag indicates that the relay and CTs work. The fact that the flag is there with no C/B trip indicates that something in the trip circuit or C/B is not working correctly.
It's a simple test to trip the C/B from the relay (the problem is scheduling the outage). This will check out the complete trip circuit out from the relay.
RE: Electromechanical relay failing to trip
BTW, Are you have BFP in this substation?
I back to Q: what was clean this fault?
DanDel, we are base on: system tested from end to end.(that mean from CT/VT up to CB), not only relay with test plug. If not...
Drchaos, please provide small test ( if possible), inject 5A
from CT terminals and meas. voltages on them.
RE: Electromechanical relay failing to trip
meas also what current you see on relay in injection time ,and sorry forgot about it, please provide test of this cable insulation ( 20-30 years of cable).
RE: Electromechanical relay failing to trip
The line was being fed from one end only, and the fault was definitely 3 phase, as I've seen the disturbance recorder (not at that substation, maybe next one along.) A digital relay at the next substation also picked up and finally cleared the fault (in its zone 2 or 3 or overcurrent).
When I say the relay picked up, this relay has an overcurrent starter element which picked up, but the distance element did not send a trip signal. The trip circuit has been tested and is good.
The CT kneepoint voltage is 609V. The techies used one of the Doble kits or whatever they are called to test it.
The fault current started out around 3kA (AC) and 3kA DC, but actually increased to 5kA AC, as there was a simultaneous fault elsewhere on the system - when this cleared, the fault current obviously increased. Where we are(in western europe), the fault levels are not particularly high as the system is quite weak in this area.
RE: Electromechanical relay failing to trip
I'm hope that we cancelled all option of HW problem (trip circuit, current circuit, etc.).
This EM relay with O/C starter and I'm assume with single meas.system.
I don't work with EM relays many years, but as I remember this type of relay have some "problems"
1. "Long or slow time" of operation in the 1-st zone (ten's miliseconds).
2. Not realibility in case , as 521AB say, fault evolves.
I will ask my friends, that have very good excpirence with
EM distance relay, maybe them send me some tip.
Regards.
Slava
RE: Electromechanical relay failing to trip
the measured knee point voltage of 609 V indicates that probably our calculations on the CTs are correct. We got E2max = 750V. Considering E2max = 1.2 Vknee we are almost there. This is good.
Even with a fault current of 5 kA, I don't think we can blame the CT.
Do you have the possibility to show us the disturbance recorder? Do you have it on paper or on file?
There are not so many possibilities now.
You have the disturbance recorder. You can read the pre-fault voltages and currents. Get them and inject them in the relay.
Then you can read the fault voltages and fault currents (steadu state, I would get the 3kA level for the beginning). read them and inject them into the relay.
Will the relay behave as during the fault? (Only start indication)?
Hopefully yes. Then we know it is probably a setting problem. You mentioned MHO characteristic. MHO characteristic are not sensitive for fault resistance, close to the reactive reach (reactive border).
If the relay trips correctly, with that injection, you should try to inject also the transient dc quantity. If the disturbance recorder is in form of COMTRADE file, DOBLE is able to play-out it again.
Will the relay now behave as when in service (just trip indication), probably the dc component created some problems (to the main CT itself or to the relay itself).
CONSIDER that the disturbance recorder DOES NOT show WHAT THE RELAY has measured (that's why digital relays with disturbance recorder included ARE important).
Try with the first test (steady state quantities). Hopefully this test gives some answers. Then let us know.
AND PLEASE: can you show us the disturbance file?
RE: Electromechanical relay failing to trip
I haven't got the disturbance recorder graphics unfortunately - it was looking at another line in the same substation - so those currents are estimates only. Do you think the relay maybe saw something else? Maybe bigger currents?
I really need to get my hands on the manufacturers CT curves, and I will probably try and model the setup in EMTP or something.
I suppose if the relay was working, it just didn't see the fault, and so that must have been due to the CT output.
I have a final question - when a CT saturates, will that increase the fault impedance seen by the relay?
RE: Electromechanical relay failing to trip
Numerical relays may manage this situation by recognizing the waveform, and can try to reconstruct the waveform that had been generated if the CT had not saturated, but electromechanical relays just get what they get….
Yoe mentioned a radial feeder, and said that a remote numerical relay tripped. If the feeder is radial also for the remote relay, the fault current measured by that relay must not be far away from the fault current measured by YOUR mechanical relay. Do you follow me?
For the voltages it is a little bit more complicated, but let’s try to get the fault current level..
And one suggestion: don’t promise any result. No disturbance recorder available, we can in practice just speculate.
Anyway, let’s see if we can get something from the remote relay.
RE: Electromechanical relay failing to trip
521AB, I'm agree with all.
Drchaos, please check with your team what is a type of
distance characteristic in this relay?
It's more and more seems as high resistive fault.
RE: Electromechanical relay failing to trip
I disscaused with my several friends and all of them saied
same:
All distance protection (digital, numerical, static and EM)
have same problem:
Not enough sensetive to high resistive fault ( high DC component add problems).
For increase sensetive we add SDEF function.
It's all time game between sensetivity and stability of distance protection.
Only before two weeks (just now I recieved this information)
one of numerical protection send trip according to second zone, but fault was in first zone and near to SS. Same problem : ARC and high resistive fault.
More from this are possible that your relay see this fault as overload ( are possible also in the digital relay).
And I would ask again what is atype of grounding in this system???
But as saied 521AB, w/o fault data, all of our disscasion is only speculate!!!!!!
Regards.
Slava
RE: Electromechanical relay failing to trip
1. During the fault the voltage collapsed near zero. Most impedance relays do not operate when the voltage is zero to guard against miss operation due to PT fuse failure.
2. Because the fault was 3 phase the relay might have see it as a power swing, therefore blocking all distance elements form operating.
Bare in mind that I am not at all familiar with that particular relay so take from this what you may.
RE: Electromechanical relay failing to trip
Slavag, the fault was on 110kV system, which I think is solidly earthed at trafo neutrals.
We are blaming CT saturation at the moment. I will try to model the situation using digsilent or emtp soon.
RE: Electromechanical relay failing to trip
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.
RE: Electromechanical relay failing to trip
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....
RE: Electromechanical relay failing to trip
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 : )
RE: Electromechanical relay failing to trip
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
RE: Electromechanical relay failing to trip
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).
RE: Electromechanical relay failing to trip
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
RE: Electromechanical relay failing to trip
http://www.eng-tips.com/viewthread.cfm?qid=161609
RE: Electromechanical relay failing to trip
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?
RE: Electromechanical relay failing to trip
Otherwise, it's time to enjoy the weekend and not be worrying about relays!