SEL-421L vs 311C and 311L 5

[Sn00ze]

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

 

[davidbeach]

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.

 

[Mbrooke]

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.

 

[Sn00ze]

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?

 

[Mbrooke]

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.

 

[piterpol]

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.

 

[Mbrooke]

Also consider breaker failure.

 

[Sn00ze]

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.

 

[davidbeach]

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.

 

[HamburgerHelper]

Davidbeach,

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

 

[Sn00ze]

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/Transmission-and-Subtransmission/

 

[HamburgerHelper]

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.

 

[Mbrooke]

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.

 

[davidbeach]

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.

 

[bacon4life]

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.

 

[submonkey]

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

 

[davidbeach]

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.

 

[Sn00ze]

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

 

[submonkey]

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

 

[Mbrooke]

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.