Curves

[rcdallas]

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[topgroove]

fault current is probebly one of the hardest concepts to wrap your head around. When you have a fault on a feeder the greatest current on that circuit will be at the feeder breaker inside the station. You could have a car pole accident at the end of the circuit and see a thousand amps at the first isolation device from the accident. Back at the station, the breaker may see ten thousand amps or more fault current. Engineers use all kinds of data like wire size, loading , and length of feeder ect.

They've got fancy formula's and algorythms to figure out the trip settings at the recloser. The bottom line is, you want the recloser to opperate first and not have the feeder breaker open inside the station. Since the recloser in the field see's way less actual current it has to opperate quicker and clear the fault before the feeder breaker opperates.

[thrasher]

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Most of the time I agree with everything you say from the technical side, but this time I gotta disagree. There is a difference between Maximum available fault current all up and down the line, as you said, with the highest possible being generated at the lowside of the power transformer in the station. However once you are dealing with an actual fault the current from That particular fault is the same all the way down the line.
An example: A Rural station has 20 miles of line on one feeder. Maximum fault current at the station is 6000 amps, but at the end of the line maximum fault current is only 400 amps. If a windstorm blows a piece of tin roof into the phase and neutral on the last span, the fault across the tin would be 400 amps and the station feeder breaker would see 400 amps + load on the line, it would not see 6000 amps. To see 6000 amps the tin would have to blow into the station buswork. Therefore when engineers "coordinate" devices down a line they are looking for the fault current past that device and then set the fastest devices at the end of the line with the devices progressivly becoming either slower or with a higher pickup the closer the device is to the source.
What curves are picked and devices used is a multi-day course for engineers and is also influenced by the individual company "philosophy" . Variations include (1) do you use instantenous settings (2) do you use phase and ground or only phase (3) what are your dead or open times between shots (4) do you set reclosers with a fast curve that will reclose thru a fuse or only slow curves that let the fuse blow first, etc,etc...

[T-Man]

Good topic and some good info. I might add the reclosers come in oil filled and electronic. The newer ones being electronic. Thrasher explains it pretty well. "A" settings are the fast seting meaning they trip first and reclose in a shorter time. The "D" settings trip at the same fault setting but take a tad longer to close. If a tree branch blows out of a tree and hits the line the recloser sees the fault and opens but the branch falls to the ground and is no longer there and the recloser closes and the fault is gone. The recloser resets.
But lets say the whole tree tips over from a wind storm and lays against the line, the recloser goes thru the first operation and closes only to see the fault still there, It opens again and takes a bit longer to close, when it does, the fault is still there and the recloser locks out. In an oil filed recoser this works thru hydralics and locking dogs, in an electronic unit it's all done on circuit boards that are more programable.

A typical feeder will have an area from the station to the recloser protected by the station breaker. After the recloser the system will be protected by the recloser to a set of line fuses, then there may be tap line fuses and fuses on the individule Transformers. All this protection needs to be coordinated so the fault will take the first protection between the fault and the source out. Otherwise a fault on a tap could take out a bigger area than needed and way harder to find the cause. This is where fault indicators come in handy another topic.
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If you look at your Circuit Breaker pannel in your home you'll have main breakers, then seperate circuits, and you might have protected extension cords for your computer, but the system is similar, just a smaller version.

[tramp67]

The different trip curves also depend on the type of fuses the utility uses, such as Q, K, T, etc, which all designate the speed at which the fuse blows. Proper coordination, which includes the selection of recloser curves and amperage settings, tries to clear temporary faults such as the tree branch, or insulator flashover, or lightning arrestor faults and flashes, without blowing a fuse. If a fault remains, the objective is to isolate the faulted section through the blowing of a fuse. If the fault is ahead of a fuse, then the recloser hopefully will lock out before the station breaker dumps the whole feeder. Many utilities also use sectionalizers in conjunction with reclosers. The sectionalizers count the operation of the recloser ahead of them, and open one operation ahead of the recloser lockout. An interesting fact about sectionalizers is that they only open under no load, they will operate during the part of the recloser cycle where the recloser is open. Good engineering will keep outages to as small an area as possible, replacing a blown fuse with something other than what the engineer had specified can do a lot to screw up the coordination and increase outage areas.

[rcdallas]

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[topgroove]

One thing ya'll brought up was sectionalizers. I have never seen one in person/I have a generic device book that only adds about a 4 sentence paragraph as to what it is. It's been awhile since I've looked at it, if I reckon it looks pretty close to a 3 phase kyle.

Can ya'll detail it a little more or point me to where I can dig a little more on em?

sectionalizers are easy. The thing to remember is they only opperate on loss of potential, they can't sence fault current.

You can program them from the controll box, but in a nutshell.... Lets say a tree comes down and pins the primary down, The station breaker or field recloser sences the fault and opens.


The sectionalizer see's loss of potential and opens.


The recloser or station breaker times out and auto closes


The sectionalizer see's potential again and closes into the fault

The breaker or recloser see's fault current again and opens

Now depending on how the sectionalizer is programed it may go more cycles but most times its programed to simply lockout on the second loss of potential within so many minutes.

The breaker or recloser times out once again and closes.

This time the sectionalizer is locked out so the fault is isolated and the circuit stays hot.


Many times its a branch or animal that blows clear and the circuit may restore normal before the sectionalizer locks out.


From the ground you can opperate the sectionalizer. You can place it in manual or auto,

or open and close it from the controll box. If the controll box fails you can even pump it closed with a switch stick.


I hope this makes sence.... I'm a terrible communicator sometimes

[rcdallas]

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[Booger1944]

This may be old news to you, but if not, you may find it interesting and helpful http://www.sandc.com/support/coordinaide.asp
From what I understand the maximum number of sectionalizers that will coordinate is 4. So say you have 10 in a circuit and you have a fault past the last one, then 6 of them will open. The good thing is that if the fault is transient than they will all close back in in sequence after they sense potential voltage. That is the self healing part of the "Smart Grid" we all hear so much about. Is anyone using the pulse closers that send a pulse down the load side of the recloser before they close?

[topgroove]

Damn..... what a great idea! We don't have any pulse reclosers on our system but I think its a fantastic idea. Instead of sending all that fault current down the line why not send a quick pulse of a few miliseconds before the recloser closes into a fault.

The only problem I see is that the seconalizers wont be able to isolate the fault and reduce the overall customer outage.