SEL-421L vs 311C and 311L
SEL-421L vs 311C and 311L
(OP)
Hello everyone,
I am wondering if anyone has had any experience and thus feedback on these relays. They seem to be similar.
My design involved a single 240kV breaker that ties to another party's sub. probably inbetween there will be a circuit switching Station of some sort.
I am also wondering if there is a preference between 138kV and 240kV when picking the relays.
Any input is appreciated. thanks
I am wondering if anyone has had any experience and thus feedback on these relays. They seem to be similar.
My design involved a single 240kV breaker that ties to another party's sub. probably inbetween there will be a circuit switching Station of some sort.
I am also wondering if there is a preference between 138kV and 240kV when picking the relays.
Any input is appreciated. thanks






RE: SEL-421L vs 311C and 311L
An entity that rarely installs new line terminal protection may come to a different conclusion.
RE: SEL-421L vs 311C and 311L
SEL 311C and SEL311L works great for single breakers and are great in basic explications where nothing profound is required.
RE: SEL-421L vs 311C and 311L
So, we are assuming there will be no series line compensation and it will be a single breaker station. going to a switching station. In which case over reaching might be a consideration for the relay. The next stations are fairly close (11 kms) so i am thinking Line differential relays would be a better fit. For the size of the sub a 411, though really nice as you stated davidbeach, the cost might not be justifiable. I think i am leaning towards 311L, and 90L as redundant.
Is there any special considerations that you would recommend considering for 138kV lines vs 240kv lines?
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
Not sure but perhaps you may configure the relay for reclosing purposes as well.
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
Mbrooke, I am glad you brought up BF, as it seems 311L does not carry that feature or 59 overvoltage, out-of-step and under frequency. The the susbtation will be the collecting point for a windfarm collector system. And, correct me if i am wrong, but i think overvoltage and frequency would be necessary. So 311L is not looking good for this.
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
Do you use a SEL-411L for primary and backup?
RE: SEL-421L vs 311C and 311L
I thought it was weird that they didn't but they are not listed in their manual or datasheets.
What's people's take on Distance vs Differential?
@piterpole,
yeah seems like most EIDs do autoreclosing. so that's one thing off the list!
Edit: @Davidbeach, I stand corrected David (and i should have known better than to doubt you) but I have the SEL rep confirm that the SEL-311L does all those features much as you stated.
https://selinc.com/products/comparisons/Transmissi...
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
On a true two terminal line differential is an easy choice, but may be unnecessary; POTT and DUTT may do just as well in many cases. Throw in tapped loads on the line and there's a number of ways to get the differential to misoperate.
We always set stepped distance protection as though there is no communications. On rare occasions we wind up with lines that can't remain in service without transfer trip, or are so short that we can't set an underreaching zone 1; but almost always have the full complement of stepped elements. Then add the comm assisted tripping on top of that.
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
I think it is worth noting that the SEL-311x relays do not support
quadrilateral phase distance elements. On a short line, having
quad distance elements can make a big difference to the resistive
coverage - particularly near the reach boundaries.
Thanks,
Alan
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
So phase quadrilateral, is that mainly for over reaching? how necessary would you deem it?
the datasheet does say that the 311L does Quad ground
RE: SEL-421L vs 311C and 311L
The importance of quad phase elements depends a bit on your
application - the worse combination I've seen is traditional
stepped distance on short feeders with no comms for intertripping.
I have twice had an SEL-311C covering a remote 33 kV bus in
zone 2 fail to operate due to lack of resistive coverage,
combined with infeed over arc resistance. An upstream relay
with quad elements eventually tripped - resulting in a
substantial outage. In this case the zone 2 reach covered
the bus with a 20% margin, and line impedance data was
measured and correct. A margin of more than 20% could not
be provided due to grading with other feeders. Interestingly,
the relay was replaced with a Schneider P545 (quad/quad) and
a subsequent fault in the same position resulted in correct
tripping.
It can be argued that the case above was an application
problem (which it was), but why use a mho when something
so much better is available? Mho characteristics only make
sense for electromechanical relays where the alternative
is hard to build.
I specify quad/quad as a mandatory requirement for distance
protection.
Thanks,
Alan
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
There was no bus differential - the utility which originally built
the sub chose to rely on remote protection. After a number of faults
(and replacement of breakers with ones containing BBP CTs) a project
was eventually justified to install bus protection.
Thanks,
Alan
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
?? This statement leads me to believe there might be some other problem.
But I could be wrong. There could be something I don't know, which is not that unusual.
Bus faults should not be that common.
Using an over reaching element for bus protection was not that uncommon at one time. But now it is looked upon as wrong.
It was a cost tradeoff because of the cost of electromechanical relays.
Now many of us would install two bus protection systems on the same bus.
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L
Typically 21 protection becomes difficult to implement when you have a multi terminal line with weak in-feeds from one of the terminals. This will cause apparent impedance to be an issue when doing your settings. You may find that coordination is impossible.
For 87, the biggest issue is the number of tapped stations that will pull current that will not be calculated within the differential scheme. This can be adjusted for, but there is a limit. Generally if you have more than 4 or 5 tapped stations 87 could get tough to set.
So what's the way forward? If you have a simple primary configuration just go with 87. Make sure you are aware of all the communication issues that exist for 87 line protections. It's not necessarily straight forward. If you're stuck in a place where neither 21 or 87 are coordinating properly you can look at some hybrid logic (like how we supervise the 87 with a zone 2 21). The 311L has distance capability. I've never actually used the 411L, so I can't comment.
In terms of breaker failure, I like to use a separate IED, but I don't believe that's absolutely necessary. Probably comes down to available I/O (both analog and digital) capacity of the IED.
Mark
RE: SEL-421L vs 311C and 311L
@marks1080, though i am not the one making the final call, i would like to know how you'd go about doing this study? you mean with SC values and clearing times? We will be installing a C60 for the main 240kV breaker which will implement the 96 feature, among others.
RE: SEL-421L vs 311C and 311L
The only time I could argue a separate BF relay is in an ultra high reliability applications holding a double breaker double bus station. While I have never heard of a line protection relay malfunctioning and initiating inadvertent BFs, in theory its far less likely two separate relays would do that. Of course that is assuming everything is wired correctly.
RE: SEL-421L vs 311C and 311L
Yes a SC study would be the best way to figure these things out.
Mbrooke: The decision over separate breaker fail relays is sometimes a preference decision, but not always. Personally, I prefer a seperate relay, but i respect the logic of others to combine. A couple of rational arguments to use a separate relay would be:
1. There isn't enough logic available in a single line relay to handle all the line logic, breaker fail logic and re-close logic, along with any other site specific logic that may be required.
2. If the line protection is out of service, the breaker can remain in service with its own breaker fail protection available.
3. Allows for easier standardization - ie: using the same relay for HV and LV applications.
4. It's a lot cleaner (in my opinion) and easier to operate (in my opinion).
5. I/O limitations of a single IED, especially when you have auxiliary systems (or "Special Protections" - think load shedding or load flow systems) that want to trip breakers.
Whenever I'm a part of this discussion and people start arguing over it I just leave. I think at the end of the day there is a lot of value in consistency of design. So to me it would be more important to make a decision one way or the other and stick to it.
RE: SEL-421L vs 311C and 311L
2. The backup or secondary relay can also be configured for BF protection.
3. I don't see how the same BF relay results in standardization of the protection relay.
4. In my experience the opposite.
5. Other than very special or demanding applications SEL lets you order 3 I/O boards which will do fine. One extra terminal can be dedicated for BF out of all the others.
RE: SEL-421L vs 311C and 311L
I agree with your points, but my opinions are different, and self admittedly of little value lol. Again, consistency is more important in my world.
RE: SEL-421L vs 311C and 311L
RE: SEL-421L vs 311C and 311L