AIC ratings >200kA

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ron

Senior Member
Designing a new service entry in NYC. The utility company (Con Ed) refuses to give valid fault current available values for the primary of the service transformers, and states that an infinite primary calculation should be used.
(6) 2500kVA, secondary voltage=480V utility transformers in parallel.
Even at a relatively high impedance, the infinite primary calculation results in >200kAIC available at the multiple service switches.
I thought I may have seen certain fuses rated at 300kAIC, but never a switch to hold them rated that high.
Anyone know of service rated switches, 4000A, that are >200kAIC?
I really would prefer switchgear breakers for this application, but I can't find any of them >200kAIC either.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I don't know of any gear rated higher than 200KAIC, but two suggestions:

Tell the utility you are doing an arc flash analysis. Then they might give you a more realistic estimate of the fault current at the primary.

Or, get the data on the primary feeders for the transformer and the distance to the closest substation (or the next connection, if other customers might have loads that contribute to the fault current.) Then assume infinite current available at that end or the wire, and start your calculation there. That should reduce the fault current a little.

Steve
 

bob

Senior Member
Location
Alabama
Ron
I know that NY has an extensive UG network. What is the primary voltage
and are the transformers grouped close together?
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
ron said:
Designing a new service entry in NYC. The utility company (Con Ed) refuses to give valid fault current available values for the primary of the service transformers, and states that an infinite primary calculation should be used.
(6) 2500kVA, secondary voltage=480V utility transformers in parallel.
Even at a relatively high impedance, the infinite primary calculation results in >200kAIC available at the multiple service switches.
I thought I may have seen certain fuses rated at 300kAIC, but never a switch to hold them rated that high.
Anyone know of service rated switches, 4000A, that are >200kAIC?
I really would prefer switchgear breakers for this application, but I can't find any of them >200kAIC either.

What is the impedance of the transformers. Using 5.75% which is standard unless known otherwise, the infinite bus fault current is roughly 105kA, or roughly 52.3kA per transformer.

The impedance would have to be around 3% to make it 200kA. Who is going to supply/specify the transformers? If you are, then just make sure you specify an impedance that keeps the fault current down to the level you want. Worst case you can put in line reactors to choke it down, just have to watch voltage regulation.
 

iwire

Moderator
Staff member
Location
Massachusetts
A layman's question here.

Wouldn't taking the long way with the service conductors reduce the fault current?

It appears that is what is happening in my area, the pad mounts are moving further from the building.
 

bob

Senior Member
Location
Alabama
kingpb said:
What is the impedance of the transformers. Using 5.75% which is standard unless known otherwise, the infinite bus fault current is roughly 105kA, or roughly 52.3kA per transformer.

The impedance would have to be around 3% to make it 200kA.

King
According to Ron there are 6 transformers not 2. 6 x 52ka = 312 ka.
 

ron

Senior Member
Primary voltage is 13.8kV.
The UG network is very extensive. The six xfmrs are located underneath the sidewalk along the building edge :(
Distance from the common secondary bus to the service switches will be approximately 10 feet. :(
Con Ed in NYC has never been cooperative with regard to fault current values, and traditionally would say 200kA at the secondary of the xfmrs, when the bank was made up of (6) 2000kVA.
 

hmspe

Senior Member
Location
Temple, TX
Occupation
PE
ron said:
Designing a new service entry in NYC. The utility company (Con Ed) refuses to give valid fault current available values for the primary of the service transformers, and states that an infinite primary calculation should be used.
(6) 2500kVA, secondary voltage=480V utility transformers in parallel.
Even at a relatively high impedance, the infinite primary calculation results in >200kAIC available at the multiple service switches.
I thought I may have seen certain fuses rated at 300kAIC, but never a switch to hold them rated that high.
Anyone know of service rated switches, 4000A, that are >200kAIC?
I really would prefer switchgear breakers for this application, but I can't find any of them >200kAIC either.

Six 2500KVA transformers in parallel would be about 18,000 FLA on the secondary. I'd like to know a bit more about the network (number of services, how the services are paralleled on the secondary, etc.).
 

dbeasley488

Member
Location
Suwanee, GA
I've run into the same situation in other utilities across the country where the utility will not provide the actual maximum fault current.

Would the AHJ allow multiple services in your area. If so, you could use the six switch rule and lower per service AIC rating.
 

ron

Senior Member
hmpe,
Essentially the secondaries are paralleled with only network protectors between the actual secondary and the common bus. The network protectors have short circuit protection via a set of fuses per transformer secondary.
There are (6) service switches that are taken from the paralleled bus.

dbeasley488,
There will be (6) service disconnects, and the quantity will not effect the available AIC to any single one of them.
 

bob

Senior Member
Location
Alabama
Ron
I made a few caculations using a 5.5% Z transformer and assumed several available Primary Fault Capacity in MVA.


500 MVA(20.9 ka) -- 212 ka
400 MVA(16.5 ka) -- 195 ka
300 MVA(12.6 ka) -- 171 ka

The abiltiy of the system to deliver a 21 ka fault on the primary would see
remote but then I don't know the system.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
bob said:
King
According to Ron there are 6 transformers not 2. 6 x 52ka = 312 ka.

Oops, can't read! I read parallel, was thinking of two, and missed the (6).

Anyway,

I was doing a little research on this situation because it sounded odd/interesting. A few notes:

This is a network system (not distribution system) and in such a system the fault currents on the primary side are always considered an infinite bus. Also, the transformers and network protector are usually provided by the POCO because of the special nature of the application, e.g. submersible, system protection, etc.

A network protector goes on the secondary of each transformer. This is in place of, or addition to the switchgear main breaker/disconnect. This device is a heavy duty, air, power circuit breaker with an integral microprocessor which acts as a control device, as well as a fast acting protective device to disconnect the bad feeder.

Also, there should be normally closed tie breakers between the common bus sections, to isolate loads and minimize disruption of faults if they occur on individual sections.

From what I can ascertain, the situation where you would have a fault on a low voltage bus (common bus) the section that faults is destroyed anyway. The network protectors are going to operate, thus isolating and protecting the unfaulted portions. Therefore the equipment rating is based on maximum through fault, and not on maximum at any one point.

IMO, I would find a consultant that has specific experience in this area. The equipment is specialized, and the normal rules do not directly apply.
 

bob

Senior Member
Location
Alabama
kingpb said:
A network protector goes on the secondary of each transformer. This is in place of, or addition to the switchgear main breaker/disconnect. This device is a heavy duty, air, power circuit breaker with an integral microprocessor which acts as a control device, as well as a fast acting protective device to disconnect the bad feeder..
King
Where did you get this information. My experience with network protectors is
that they are not a breaker. The protector either opens or closes depending on power flow. The transformers are on different feeders with the secondary tied together. If one of the primary has a lower voltage you get a reverse
power flow back thru the transformer. If that occurs the protector opens.
If the network voltage is lower that the voltage at the protector it will close.

kingpb said:
From what I can ascertain, the situation where you would have a fault on a low voltage bus (common bus) the section that faults is destroyed anyway. The network protectors are going to operate, thus isolating and protecting the unfaulted portions.

That is true. The solution to the fault is to let the secondary conductors burn
til the fault stops. I have been told that in NY it is not uncommon to have 4 or 5 electrical fires on the network everyday.
 

ron

Senior Member
I would find a consultant that has specific experience in this area. The equipment is specialized, and the normal rules do not directly apply.

Kingpd,
Thanks for the recommendation. Unfortunately, the POCO will be handling everything upstream of the service switches, including the transformers, network protectors and common bus.
I'm handling the design from the service disconnects ..... and down.
I've done similar calculations that bob did, and have considered the network configuration of the transformer primary that is from the city grid, this is why I think that the fault current may be >200kA.
I will not consider the speed in which the network protectors operate when choosing duty levels of my equipment, as the POCO doesn't maintain there equipment until it burns.

This is a weird situation that I'm in, as outside of NYC, where I design similar loaded installations, I enter with medium voltage and distribute to substations in varied areas of the building. This is just an anomoly of NYC, that they don't seem to like offering medium voltage to the building.
 

bob

Senior Member
Location
Alabama
ron said:
Kingpd,
I will not consider the speed in which the network protectors operate when choosing duty levels of my equipment, as the POCO doesn't maintain there equipment until it burns.
Ron
Are you saying that these protectors have the ability to operate and interrupt
faults? As I said above, the ones I have delt with did not.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
kingpb said:
Using 5.75% which is standard unless known otherwise, the infinite bus fault current is ...52.3kA per transformer.
We could be using the same calculator, but isn't Xfmr-Z (5.75 / 6) = 0.96%, with six ~5.75% transformers in parrallel? When selecting the 15000 KVA xfmr (6 x 2500), at 0.96% on the same calculator, that ASCC shows 1,879,451 amps on the secondary bus, before the 10ft cable length.

My calculator's "Design considerations" Help feature mentions splitting this power between smaller switches, adding coils of extra-cable to the Maint. policy, among other things, but IMO there'd be less fires by isolating those xfmr banks and driving returns 25 feet down under, using Single wire earth return (SWER). With 5-10 ohm returns to local earth, thru 6m ground rods, SWER has resulted in equally safe, more reliable, and less cost than conventional transmission.
 
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bob

Senior Member
Location
Alabama
ramsy said:
We could be using the same calculator, but isn't Xfmr-Z (5.75 / 6) = 0.96%, with six ~5.75% transformers in parrallel? When selecting the 15000 KVA xfmr (6 x 2500), at 0.96% on the same calculator, that ASCC shows 1,879,451 amps on the secondary bus, before the 10ft cable length..
That may sound correct but it is not. When you have Z's in parallel your statement is correct. We do not have Z, we have %Z. Same rules do not apply.

ramsy said:
My calculator's "Design considerations" Help feature mentions splitting this power between smaller switches, adding coils of extra-cable to the Maint. policy, among other things, but IMO there'd be less fires by isolating those xfmr banks and driving returns 25 feet down under, using Single wire earth return (SWER). With 5-10 ohm returns to local earth, thru 6m ground rods, SWER has resulted in equally safe, more reliable, and less cost than conventional transmission.
The idea of a network is to maintain power if you have a failure of a primary feeder or transformer. Remember that these transformers are feeding many more custormers on the network. Ron's customer is only one of the many.
 
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ron

Senior Member
Bob,
I do not know the details about the protectors. I know that there are fuses in them (6000A), but I'm not sure about the automatic functions of them. They are installed/speced by the utility
 
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