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]

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

 

[submonkey]

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

 

[Sn00ze]

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

 

[cranky108]

"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.

 

[jghrist]

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.

 

[marks1080]

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

 

[Sn00ze]

@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.

 

[Mbrooke]

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.

 

[marks1080]

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.

 

[Mbrooke]

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.

 

[marks1080]

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.

 

[Sn00ze]

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.

 

[Mbrooke]

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