Low Inst. Recloser

[tctctraining]

Hydro defines low inst. setting on their breaker to protect temp. faults on the line.
If we add a new recloser , we can't trip faster than the breaker and trip before breaker unless a time delay is introduced on breaker low inst. setting or raise the low inst.
the issue is that there is another feeder off the TS which doesn't have any recloser and using time delay on low set will delay temp faults on that line and that will probably damage the line.
low set is set for line current value which is lamost 600A.
I woould be interested in discussing this with exerts out there and appreciate your feedbacks.

Thanks

 

[jghrist]

In what way would the delayed tripping damage the line? If the fault is not temporary, the trip after reclosing will be delayed anyway.

Unless you have sequence coordination (see Cooper Power website for literature), you will have a fast trip at the station for a fault beyond the line recloser if the initial reclosure is not successful.

 

[cranky108]

Sequence coordination dosen't always work well. Ask Cooper for there technical papers on that.

Depending on the recloser, the electronic reclosers can have an instantious trip, which can be applied at a value, say 125% of the next recloser max fault current level. This will allow fast tripping for a close in fault, and a time delay for faults where you need coordination.

Given some short lines, this may not always work. It also dosen't work on hydrolic reclosers.

Another idea would be to reconductor the small wire sections.

 

[tctctraining]

Whats the point of having low inst. trip setting?
Is this a common practice for most utilities?
it seems it's set at breaker long time delay pick up value!

 

[cranky108]

Actually having a fast curve or inst. trip is common during hot line work.
It also works well for fuse saving.

 

[tctctraining]

What's the common value to pick up low inst. trip?
I need to suggest to the hydro to
1) raise
2) eliminate
3)or use a time delay
for low inst. pick up, so I can coordinate it with downstream reclosers( by the way these reclosers could have infinite instantaneous pick up as a curve)
A) I am not sure to what value if could be raised to
B) Emlemitaing low inst. pick up is not a safe practice for hot line work( customer is not using fuse saving scheme)
C) Time delay will cause fault current flow on other feedres which don't have reclosers and also there won't be any protection for hot line work.

any feedback or reference would be appreciated.

 

[JMatt]

A recloser should certainly be disabled during any hotline work. I also recommend a temporary low-set instantaneous that can be armed/disarmed using a selector switch or pushbutton.

As for your other feeder where a time delay may allow the line to be damaged on temp. faults, this just isn't good engineering practice. Any delay in tripping should still trip a fault quicker than any point on the cable damage curve.

 

[dpc]

It's done to allow temporary faults to clear themselves without blowing a fuse. This reduces the downtime for the line and reduces restoration costs.

It's called "fuse saving", as mentioned above.

But sometimes this approach creates other coordination problems so compromise may be needed.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[cranky108]

I've seen this before where at the current fault levels the conductor would burn down before the recloser protecting the line would operate.

Sort of goes along with buying larger transformers without consitering what it will do to the fault current levels. It's common within utilities that are managed by accountants, and old linemen.
The solution is simple, bite your lower lip, and spend the money to fix the problem right (And the managers won't want to hear this).
I worked for a company like this, and I hated it.

But then again that's my thoughts on old small conductor.

 

[racobb]

Unless you can set the breaker inst. high enough, or set it with enough delay so that it will not "see" faults beyond the recloser, it will always trip for faults betond the recloser if the actual fault current is above the breaker inst. setting. Obviously needs to be viewed based on any other system protection requirements. Device coordination usually involves compromise and can be as much art as science.

Unless you have the breaker inst. set to trip for all operations, it will revert to time delay on subsequent trips anyway, so how is it protecting the smaller wire if that is actually the case? Sounds a bit unusual at best.

Does TS mean transfer switch? How many feeders are you serving from this breaker?

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[tctctraining]

TS ( Tranmission Station)
I have attached the TCC curve for this study.
Upstream breaker is set at 630A indiefinte. I added the time delay of 300ms

There are two downstream reclosers, I also put one at 100 msec. and the other one 200 msec. I relize that I need at least 200ms delay between curves.

There is no way to add communication. This option is not feasible.

If I use the time delay on low isnt. setting the portion of fuse saving will be very small for example for 200A fuse only from 630A to 1800 Amp for fuses between breaker and the first recloser

It will be 630A to 2000 for fuses between the first recloser and the second recloser

It will be 630A to 3000 for fuses between the second recloser and end of the line

So , still introducing time delay seems to be a good option.


My question and concern is why Hydro had chosen 630A , I think it's the line maximum loading.

If I use 300msec on breaker low inst. trip , any fault on the feeder which doesn't have reclosers or the portion of line between the breakr and the first recloser will be delayed fopr 300msc.

 

[cranky108]

I don't know there reasoning. But...
Consiter a 300a feeder, max load = less than 300a. Now if you want to work on a breaker, recloser, etc.. You put that load on the device next to it, therefore the phase setting should be something like 600 amps or greater.
Just a guess.