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SEL-421L vs 311C and 311L
5

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

RE: SEL-421L vs 311C and 311L

No 421L. There's the 421 and there's the 411L, but no 421L. On our system we've standardized on the 411L for all transmission line terminals. It can do anything we need for any terminal configuration. It may seem to be a bit of over-kill for some installations, but having only a single design to maintain and having only one setting criteria to maintain goes a long way to off-set the somewhat higher relay costs where we might be able to use a 311 or 421. "Optimizing" the design for each terminal will cost more than using a single design everywhere.

An entity that rarely installs new line terminal protection may come to a different conclusion.

RE: SEL-421L vs 311C and 311L

Well, it depends on many things. One of the biggest being if your line will have access to only one breaker or multiple breakers. With multiple breakers the SEL421 and SEL 411L would be a first choice, in addition to things like bay control, series line compensation, faster processing, and so forth.

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

(OP)
Davidbeach - correct! 421, Doing all this research is making me dyslexic with "L"s. haha

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

In such a case go for a 311L is all the required protection parameters are met. Bear in mind that even low end modern relays are capable doing far more than what the most complex electromagnetical relays could do.

RE: SEL-421L vs 311C and 311L

The SEL-311L relay has also 87L and 21P/21G single-pole tripping.
Not sure but perhaps you may configure the relay for reclosing purposes as well.

RE: SEL-421L vs 311C and 311L

Also consider breaker failure.

RE: SEL-421L vs 311C and 311L

(OP)
Again, thanks for the input guys I am compiling a list with my reasearch and with feedback from various sources.

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

The 311L can do all of those; breaker failure protection just takes a bit of logic programming. Voltage, frequency, and out of step are part of the basic feature set, at least in the -6 and -7 versions of the relay.

RE: SEL-421L vs 311C and 311L

Davidbeach,

Do you use a SEL-411L for primary and backup?

RE: SEL-421L vs 311C and 311L

(OP)
davidbeach,

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

You go with differential if you can. It requires no coordination and is very secure. You'll still be including distance or ground overcurrent elements in addition in case your you lose fiber communication. In our system we only go with line differential for short segments <1 mi due to the cost of fiber and the small impedance of a short line makes it difficult to confidentially set undereaching distance and ground instantaneous elements.

RE: SEL-421L vs 311C and 311L

Differential would be my choice provided you can acquire a data transmission route. But in any case still set the distance elements. IF you loose the communications between the line protection relays you need to have a backup method to clear the fault.

RE: SEL-421L vs 311C and 311L

A and B, no primary and backup. Matched relays provide a lower level of risk of misoperation given our misoperation history. We rarely have failures to trip, but we have a variety of ways of getting trips we don't need. If we get that from two identical relays, non-identical relays would simply increase the risk of misoperation. Other people will have had different operational histories and will come to other conclusions as to how to reduce their risk.

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

2
Be aware that the 311L can't talk to the 411L. We are trying to standardize on 411Ls for the reasons David mentioned, but it does mean ripping out some perfectly fine 311Ls when a new intermediate substation is added.

RE: SEL-421L vs 311C and 311L

Hi,

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

It is true that there's no phase quad on the 311s or the older 421s, but the phase mhos are polarized by a positive sequence memory voltage, resulting in a dynamic characteristic that provides considerably more resistive coverage than the static characteristic might imply.

RE: SEL-421L vs 311C and 311L

(OP)
Wow, an abundance of information coming from this thread. I am really glad i made it. Thank you guys so much.

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

Hi sn00ze,

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

submonkey, you hit the nail on the head. The relays have evolved, the engineering has not. Much of what we know today is the byproduct of what eletromechanical technology limited us to. Today we know of conditions which make MHO and impedance elements look highly impractical. Wave analysis and arc signature analysis can do what other things never could.

RE: SEL-421L vs 311C and 311L

So why didn't the bus diff at the remote station clear that bus fault?

RE: SEL-421L vs 311C and 311L

Hi David,

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

(OP)
Thanks for the explanation Submonkey. That's an interesting, preventable example, but really drives the point home. Thank you. :)

RE: SEL-421L vs 311C and 311L

"After a number of faults a project was eventually justified to install bus protection."

?? 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

Quote (OP)

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.
If the other end of the line is at another party's sub, and you are using a piloted distance scheme or differential, your choice of relay may be dictated by what the other party uses.

RE: SEL-421L vs 311C and 311L

You need to do a system study to see which option is best. Depending on system configuration 21 or 87 may simply not be settable. For ideal situations, the industry seems to be moving towards 87 for lines. We still supervise our 87 operation with a 21 (Z2) element, but one day I think our confidence will be high enough to stop doing that.

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

(OP)
@jghrist, Thanks for weighing in. it is true that what that our choice may be limited to the other party. They use distance on the next sub over. However, i do believe we will be the ones building and owning the switching station where we would be placing the other differential relay, from there on, the other client would take care of their side. That is my understanding, so far.

@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

Personally having a separate BF relay is not necessary in my experience because most modern SEL relays come with that ability either standard or can be built using programing logic. A separate relay simply adds more cost, complexity, error, and failure points.

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

Hi Snooze:

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

1. With an SEL 311C, 311L, 411L and 421 all of those functions can coexist in the same relay.

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

Mbrooke: *Me walking away, as this is where we get off topic. Could be a good topic for another thread though...

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

(OP)
the C60 is mainly for breaker managment/control. Yes, according to the SEL function comparison sheet, most of their relays can do BF through logic.

RE: SEL-421L vs 311C and 311L

No, they are still of value :) Don't put yourself down.

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