Autorecloser fuse saving scheme - lowset based on load current or conductor ampacity

[kenth619]

Good day,

I am brainstorming a coordination problem whereby the inverse time overcurrent lowsetset pickup of an autorecloser has to be set for a 12kV feeder. The feeder has a small load of about 60A max expected. The smallest branch circuit fuse is the 20A K type Chance fuse with minimum melting pickup of 40A. The largest section (trunk) fuse is the 50A K type Chance fuse with minimum melting pickup of 100A. The feeder conductor is rated at 357A.

The question is should the lowset time overcurrent pickup be set based on the ampacity of the feeder conductor (357A) or based on the max expected load that the feeder is expected to have (just above 60A)? Two POCO engineers are giving me different answers.

(1)One is saying that the pickup should be just above 60A (about 90A to be exact) to check for overload, in which case the autorecloser will lockout after a sufficient amount of time (overload protection) without fuse operation. The definite time pickup is set at 1.5 times this pickup (135A) to discriminate between fault currents and overload and the definite time is set at 0 seconds so that on first trip for faults 135A and above the auto responds to the fault first before the fuse can blow. After reclosing if the fault persists the time overcurrent element is used to allow the fuse sufficient time to blow.

Disadvantage: with this method, the feeder is being limited to below its max rated current. There is the possibility of locking out the feeder below the max current the feeder can handle (between 90A and 100A)
Advantage: with this method, overload protection is possible as the pickup is set just above the max expected load current.

(2)The other engineer is saying that the fault levels are independent of the load current on the feeder and is only dependant on impedance from the generator down to the 12kV level. This means that although the feeder is serving a small load, there can still be fault currents of the order of thousands of amps. Also he is saying that the autorecloser's purpose is really to protect the feeder. Hence the lowset should be set at 357A the rated ampacity of the conductor. The definite time pickup should be set at 1.5 times this value (535A) for discrimination. The definite time is set at 0 seconds so that on first trip for faults 535A and above the auto responds to the fault first before the fuse can blow. After reclosing if the fault persists the time overcurrent element is used to allow the fuse sufficient time to blow.

Disadvantage: with this method, one can see that there will never be overload protection because the load is much smaller than the ampacity of the conductor.
Advantage: with this method, the feeder is not being limited to below what it can handle.

Method 2 has been implemented and the auto is tripping and reclosing before fuses melt for faults 535A and above. But I can't get over this feeling that for faults below 357A the fuses will blow. Most faults do occur at around 500A - 600A but in the last few days fuses have been blowing often. In some instances, the autorecloser operated before the fuse but there are more fuse blowing occurences than autorecloser operations. I would like an expert opinion on this please and if method 1 is to be used, should the 90A pickup be used (how is the pickup selected based on largest truck fuse and smallest branch fuse pickup)?

 

[GoldDigger]

Method 2 has been implemented and the auto is tripping and reclosing before fuses melt for faults 535A and above. But I can't get over this feeling that for faults below 357A the fuses will blow. Most faults do occur at around 500A - 600A but in the last few days fuses have been blowing often. In some instances, the autorecloser operated before the fuse but there are more fuse blowing occurences than autorecloser operation. I would like an expert opinion on this please and if method 1 is to be used, should the 90A pickup be used?

Is this some kind of test facility where you are expecting to generate faults frequently? If not, I am really curious why you accept fuses blowing nearly daily as a normal occurrence or one requiring coordination? Is there a lack of overcurrent protection on the secondary side for some frequent loads which the system cannot really handle??
Are there surges of some sort?

 

[kenth619]

Is this some kind of test facility where you are expecting to generate faults frequently? If not, I am really curious why you accept fuses blowing nearly daily as a normal occurrence or one requiring coordination? Is there a lack of overcurrent protection on the secondary side for some frequent loads which the system cannot really handle??
Are there surges of some sort?

GoldDigger thank you for responding.
The feeder area is heavily vegetated so tree branches tend to fall on the lines often. In some cases, branches remain on the line and the fault is persistent so the end result is blown fuses.

 

[GoldDigger]

GoldDigger thank you for responding.
The feeder area is heavily vegetated so tree branches tend to fall on the lines often. In some cases, branches remain on the line and the fault is persistent so the end result is blown fuses.

Thanks for the additional information. That does make it a "classic" situation for a recloser. Unfortunately I can't give you any help on sizing based on that.
I pass my turn to the next member.

Actually, I will mention that I have seen credible recommendations that the recloser current should be as high as safe for the conductors, to allow the branches to be burned out or blown away. But in that case, the fuse is going to blow instead for any fault which is downstream of a fuse. In a typical recloser situation that I have heard about, there is not any additional protection on the open feeder wires at a lower current level (except possibly for prolonged overloads), to avoid just that problem.
When a branch falls and does not clear, with the recloser set according to method 1, what do you end up doing?

 

[kenth619]

According to method 1 when a branch falls and does not clear, if the fault current is 135A or higher the definite time element of the recloser will operate causing it to trip and reclose before the fuse can blow. Then the time overcurrent element which is slower than the fuse operating curve allows time for the fuse to blow.

With method 2, the fault current has to be 535A or higher for the same to happen. The problem is this may be too high of a definite time pickup.

I have attached the curves for the two suggested schemes. I hope this will help to make things clearer.


fuse pickup = 100A
lowset time overcurrent pickup= 90A
definite time pickup 90*1.5 = 135A


fuse pickup = 100A
lowset time overcurrent pickup = 357A
definite time pickup = 357*1.5 = 535A

 

[kenth619]

A correction was made to method 2.

fuse pickup = 100A
lowset time overcurrent pickup = 357A
definite time pickup = 357*1.5 = 535A

 

[mbrooke]

I would agree with the first engineer more. A recloser should have over current protection in that a fallen tree branch or downed phase can cause an arcing fault high enough to mimic an over load but not high enough to go into the "magnetic" trip range. Slang for the short circuit protection range. You dont have to set the recloser to the maximum capacity of the line either. I dont have enough info about your situation but line impedance also plays a huge role setting trip values. Fault currents at the end of the line will be seen much lower to the recloser, and if fuse saving is still desired, requires lower values. Also keep in mind the fuses as well, some are not really meant to coordinate with reclosers like type Ts.

What kind of controller does this recloser have?

Cooper I know has good published trip curves:

https://www.eiseverywhere.com/file_...533bda0849d58140_3_4D_TimeCurves_R2809134.pdf

 

[kenth619]

Possible Solution

Possible Solution

I would agree with the first engineer more. A recloser should have over current protection in that a fallen tree branch or downed phase can cause an arcing fault high enough to mimic an over load but not high enough to go into the "magnetic" trip range. Slang for the short circuit protection range. You dont have to set the recloser to the maximum capacity of the line either. I dont have enough info about your situation but line impedance also plays a huge role setting trip values. Fault currents at the end of the line will be seen much lower to the recloser, and if fuse saving is still desired, requires lower values. Also keep in mind the fuses as well, some are not really meant to coordinate with reclosers like type Ts.

What kind of controller does this recloser have?

Cooper I know has good published trip curves:

https://www.eiseverywhere.com/file_...533bda0849d58140_3_4D_TimeCurves_R2809134.pdf

Please explain the principle behind these arcing faults resulting from fallen trees. My understanding was arcing faults were a result of an incipient breakdown in the insulation in contact with the conductor (pin or disc insulators). The second engineer is adamant about not having overcurrent protection because he says it is better for such insulators to burn out (it will be easy to find the faulty insulator) rather than the recloser locking out for overcurrent protection and the faulty insulator will be difficult to find.

We use K type (fast-acting) fuses to coordinate with the recloser as you correctly said T types (slow-acting) are not suitable for coordination.

We use Noja Power and ABB reclosers.

My take on the whole situation is that for fuse saving to be successful, the recloser must protect BOTH the conductor AND the fuse the lower rating will of course be the fuse.

Fuses have 2 characteristic curves a minimum melting time curve (the smallest time the fuse is expected to operate for a given fault current) and a maximum melting time curve (the largest time the fuse is expected to operate for a given fault current).

On first trip we check the minimum melting pickup of the smallest fuse which we want to save. This would be a tee-off (branch) fuse. The 20A K type Chance fuse is the smallest fuse we want to save. The minimum melting pickup of this fuse is 40A. This is below our load current of 60A. What I propose we do is calculate the low set pickup value by multiplying max expected load (60A) by 1.25 to get 75A. This will be our pickup value. This 75A is just under what the largest fuse minimum melting time pickup is at (80A) so we are guaranteed to save the largest fuse on the feeder which is the most important fuse imo (the first main line fuse). The definite time should also be set at 75A since we dont have overcurrent protection. We may not be able to save certain branch fuses but this is okay because outages will be contained to certain segments anyway. Please let me know what you think.