Loop feeder

[electrics]

Here is the definition of loop feeder. So what does the bold parts mean?
Thanx in advance
Loop Feeder. A loop feeder has its ends connected to a source (usually a single
source), but its main function is to supply two or more load points in between. Each load point
can be supplied from either direction; so it is possible to remove any section of the loop from
service without causing an outage at other load points. The loop can be operated normally closed
or normally open. Most loop systems are, however, operated normally open at some point by
means of a switch. The operation is very similar to that of two radial feeders

 

[iwire]

When the switch is open the loop is not a true loop.

 

[charlie b]

Draw a large circle. Put a symbol for a generator at the left end, and another at the right end. Draw a few symbols connected at several points along the circle, to represent loads. Finally, at the top and bottom of the circle, draw a pair of dots on the circle itself, close to each other. Let those represent switches that, both at top and again at bottom, allow the left half of the circle to be connected to the right half of the circle, or causes the two halves to be disconnected to each other.

If both switches are closed, you have a complete loop, and the two generators are supplying all the loads in parallel, and power can flow in any direction throughout the circle. That is the "closed loop" configuration. If both switches are open, then each generator is supplying its loads independently of the other. That is the "island" configuration. If one switch is open and the other is closed, then the generators are again in parallel, and are supplying all the loads together, but power can flow only in one direction. That is the "open loop" configuration.

 

[iwire]

Charlie I am curios why your description includes two switches when the bolded part of the OP is singular?

 

[charlie b]

A loop feeder can have any number of switches along the "circle." That provides flexibility for isolating portions of the system for maintenance or repair. If the loop feeder is intended to be operated as an "open loop," they will only have to open one of the many breakers along the path. The bolded portion of the quotation is merely saying that only one breaker needs to be open.

 

[don_resqcapt19]

Often there are not "real" switches in a loop feed. They use load break elbows at the transformers and the transformers are often suppied with double primary bushings and a "parking stand". For closed loop operation the load break elbows are connected to the bushings, for open loop, one set of load break elbows is placed on the parking stand.

 

[dicklaxt]

Here's a connection diagram for a lighting loop feed,as drawn when the switch is in the NO position,it is a ring loop feed,when in the NC position it is a standard feed.

I have used this a number of times for Living Quarter's receptacles on offshore platforms, but for the life of me I can't remember why on the receptacles and not the lights.

 

[dbuckley]

Loop feeders are widely used at MV in both distribution, and large facilities.

The "ends" of your loop may come from different substations, so you don't want all the switches closed as that parallels the substations. You have a switch open. If something fails, by operating switches you can generally bring the facility back on line in its entirety, unless its one of the transformers loop switching that knacked, in which case you lose thyat transformer.

 

[electrics]

"unless its one of the transformers loop switching that knacked, in which case you lose thyat transformer. "
This part is intangible to me,
1- what do u mean with losing ?
2- knack means here "broken or off"?
3-Why do we need one switch open while having closed since it will be feeding the loads better ?
Sorry I couldnt get it..

 

[charlie b]

If something fails, by operating switches you can generally bring the facility back on line in its entirety, unless its one of the transformers loop switching that knacked, in which case you lose thyat transformer.

1- what do u mean with losing ?

2- knack means here "broken or off"?
3-Why do we need one switch open while having closed since it will be feeding the loads better ?

There are several ways of creating a loop distribution system. I believe he is talking about one in which, at each building within a large property, there is a medium voltage feeder, a switch, and a transformer. The switch has three positions, each of which can be opened or closed independently of the others. There is an incoming MV feeder from the building to the left, an outgoing MV feeder to the building to the right, and a feeder to the transformer that is adjacent to the switch.

• With all three switch positions closed, you have power passing from left to right, and power feeding the adjacent transformer.
• You can open the switch position that serves the adjacent transformer, and keep the other two switch positions closed, and that will isolate the local transformer for maintenance while keeping the loop connection from the left side to the right side.
• You can open only the left switch, thus keeping the local transformer fed from that side, but breaking the loop connection. You can do the same for the right switch.
• If the switch itself fails, then in order to deenergize it and replace it you need to open the right side switch on the building to the left, and open the left side switch on the building to the right. That will remove all power from the local transformer.

As to keeping the loop closed at all times, that is an operational consideration. It may have some advantages perhaps, in terms of reliability. But it may create a situation in which the total fault current available from all parallel sources could exceed the ratings of the components.

 

[steve66]

3-Why do we need one switch open while having closed since it will be feeding the loads better ?
Sorry I couldnt get it..

You have lower fault currents with one switch open.

Plus, if there is a short circuit somewhere, leaving the loop open helps limit the damage to the other parts of the loop. Picture a short in the cable somewhere. If the short is fed from both directions, it is more likely to damage the cable on both sides of the short. Worst case, it damages both cables leaving your source, and then you can't refeed any of your facility until cables are replaced.

If the loop is closed, a short would also take the entire loop out of service. With one switch open, half of your transformers (or half the loop) may not even see an outtage.

 

[dbuckley]

Charlie explained almost all of it, thanks

3-Why do we need one switch open while having closed since it will be feeding the loads better ?

As I noted: The "ends" of your loop may come from different substations, so you don't want all the switches closed as that parallels the substations. Imagine the situation if the two different substations had a phase difference between them and yiou closed all the left and right switches - there would be massive circulating currents, right up till a breaker opened. Very ugly.

In these situations there is almost always a mechanical interlocking mechanism (often based on Kirk keys) to stop the ring being all closed.

Although these types of installations are often called loop or ring installations they strictly are not as the two ends of the "ring" originate at different places, not the same place. However, they use exactly the same piece of equipment, a ring switch, so they get lumped into the same category.

 

[electrics]

okay it is clear now, if you dont prefer to have synchronised sources u can use this system. But in configurations like small sources paralleled and any interruption of the power is not allowed I guess closed loop will do good...

 

[IslandSparky]

Closed loop gives redundancy; open loop enhances maintainability. Protection on a closed loop generally relies on directional relays at each segment to quickly isolate a single segment in a fault. Once that segment is isolated, the loop is now "open," and the damaged segment must be repaired.

One challenge with doing open loops in a NEC environment is that you often must de-energize the loop before changing the segmentation. If you get load-break switches, the cost premium of going closed-loop drops.

In general, loops seem to work best when you have a large number of transformers (6 or more) that are relatively small compared to the feeder capacity.

All protection for open loops is performed at the two input feeds, so this limits how sensitive the settings can be; you are just protecting the feeder.