Small Transformer Protection Scheme

[Electriman]

Good morning everyone,

I am working on a project which I have small transformer (12.47kV/480V, 225 kVA) that feeds a 600A-480V switchgear. Utility feeds this transformer by 12.47 kV line.

I am wondering about the protection of this transformer. The client is concerned about the cost.

On the point of connection to utility, I have two options 1- fuse cut out 2- recloser if I use fuse cutout do I sacrifice anything?

For the transformer, can I use fuse cutout (or recloser) at the point of connection to utility for transfomer protection or do I need to use as seperate protection i.e SEL 751 for over current protection considering the secondary side of the transformer is 600A MCCB?

Do I need to watch for other protection i.e. ground fault protection.

If I need to use a seperate protection for transformer where do you put the CTs? I have seen it mostly primary side of the transformer and neutrual.

Please provide me your inputs.

Thank you in advance.

 

[templdl]

I'll kick this thing off.
Your concern seems to be the primary protective device. I doubt if a recloser will bring anything to the party if you look in to what their purpose is and their cost. Then you can use a simple basic fuse holder and fuse, a fused cutout, a primary disconnect in front of the fuse.
You we I'll have to consider the advantages of a fused cut out and a switch in combination with a fuse located on the load side. Remember that a switch may not be that simple as you would want it to be a load break and not a basic disconnect/isolation switch.
And select a medium voltage proprly ratwed fuse for use with transformer protection. Your goal is to take the transformer off line should the transformer fail.

 

[Electriman]

I'll kick this thing off.
Your concern seems to be the primary protective device. I doubt if a recloser will bring anything to the party if you look in to what their purpose is and their cost. Then you can use a simple basic fuse holder and fuse, a fused cutout, a primary disconnect in front of the fuse.
You we I'll have to consider the advantages of a fused cut out and a switch in combination with a fuse located on the load side. Remember that a switch may not be that simple as you would want it to be a load break and not a basic disconnect/isolation switch.
And select a medium voltage proprly ratwed fuse for use with transformer protection. Your goal is to take the transformer off line should the transformer fail.

Thanks. What about ground fault protection?

 

[don_resqcapt19]

Where is the service point and the service disconnect?

 

[Electriman]

Where is the service point and the service disconnect?

I am not sure if I understood your question. But the service point could be in rural area or urban area. the service discoonect would be in a few feet of utility metering pole (20'-50').

 

[templdl]

Where is the service point and the service disconnect?

Good point since this transformer is supplied by medium voltage which most likely is sorced from a POCO there must be a service drop. If so, how is it metered and describe the SE.

 

[Electriman]

Good point since this transformer is supplied by medium voltage which most likely is sorced from a POCO there must be a service drop. If so, how is it metered and describe the SE.

Nothing has been made because we are in design stage. But I am assuming everything will be utility arial standard.

 

[templdl]

Nothing has been made because we are in design stage. But I am assuming everything will be utility arial standard.

Interesting. Where is the 12,470 sourced from then? Is this an industrial facility? Remember that you are referring to the transformer as being small where small is a relative term. In trying too read between the lines this facility may be a large industrial where the SE may be at 12470 or at even a higher voltage and distributed by the industrial at 12470 within the facility.

 

[don_resqcapt19]

I am not sure if I understood your question. But the service point could be in rural area or urban area. the service discoonect would be in a few feet of utility metering pole (20'-50').

The service point is where the rules in the NEC start.

Service Point.
The point of connection between the facilities of the serving utility and the premises wiring.

Informational Note: The service point can be described as the point of demarcation between where the serving utility ends and the premises wiring begins. The serving utility generally specifies the location of the service point based on the conditions of service.

Assuming the service point is on the primary side of the transformer the service disconnect will be a 12.4kV switch. 230.208 requires short circuit protection to be on the load side of or a part of the service disconnect. Size that device to protect the transformer primary.

 

[Ingenieur]

Pad or pole mount?
utility feed or plant system?
assume delta-wye?

Primary current 10 A
secondary 270

goab with load side fusing on the primary, 20 A, maybe 15 with dual element td
if pad perhaps goab at pole and xfmr mntd prim fusing

molded case cb with electronic trip unit on the secondary
get a long-short adjustable one with integral gnd fault protection
a 400 A frame should be sufficient

coordination and fuse selection is important
make sure the cb goes first

 

[Electriman]

Interesting. Where is the 12,470 sourced from then? Is this an industrial facility? Remember that you are referring to the transformer as being small where small is a relative term. In trying too read between the lines this facility may be a large industrial where the SE may be at 12470 or at even a higher voltage and distributed by the industrial at 12470 within the facility.

Yes this is an industrial facility. The industrial facilities that I have seen they normally use couple MVA transformers. That's why I called this small.

 

[Electriman]

Pad or pole mount?
utility feed or plant system?
assume delta-wye?

Primary current 10 A
secondary 270

goab with load side fusing on the primary, 20 A, maybe 15 with dual element td
if pad perhaps goab at pole and xfmr mntd prim fusing

molded case cb with electronic trip unit on the secondary
get a long-short adjustable one with integral gnd fault protection
a 400 A frame should be sufficient

coordination and fuse selection is important
make sure the cb goes first

Thanks for the reply. It is a pad mounted transformer and utility provides the power only up to service point and feeding transformer is client responsibility. and The transformer is Delta to Wye.

So you recommend to use fuse than recloser. There is no need for CTs for protection.

 

[Electriman]

The service point is where the rules in the NEC start.

Assuming the service point is on the primary side of the transformer the service disconnect will be a 12.4kV switch. 230.208 requires short circuit protection to be on the load side of or a part of the service disconnect. Size that device to protect the transformer primary.

Thanks. What would you recommend for SC protection fuse or recloser?

 

[Ingenieur]

imo reclosers are more suitable for protection where a self clearing fault may occur
bird, wind, tree, etc on overhead lines
I'm not sure I would use one in this case where you might reclose on a fault
especially where personnel may be involved

a etu cb will give you the flexibility you need
imo a full blown relay scheme like an sel or multilin is required for the ckt capacity

 

[templdl]

imo reclosers are more suitable for protection where a self clearing fault may occur
bird, wind, tree, etc on overhead lines
I'm not sure I would use one in this case where you might reclose on a fault
especially where personnel may be involved

a etu cb will give you the flexibility you need
imo a full blown relay scheme like an sel or multilin is required for the ckt capacity

Exactly what I was thinking.

 

[templdl]

Thanks for the reply. It is a pad mounted transformer and utility provides the power only up to service point and feeding transformer is client responsibility. and The transformer is Delta to Wye.

So you recommend to use fuse than recloser. There is no need for CTs for protection.

Is this an existing transformer or are you going to order it? If you are going to order it you may be able to mount the HV fuses in the HV ATC and even a MCCB in the LV ATC.

 

[Ingenieur]

Is this an existing transformer or are you going to order it? If you are going to order it you may be able to mount the HV fuses in the HV ATC and even a MCCB in the LV ATC.

good idea

a full blown relay set-up with ct's, shunt/uv trip, etc is not required

just make sure isolation is provided on the primary so you can perform work safely on the system

 

[Iron_Ben]

Thanks for the reply. It is a pad mounted transformer and utility provides the power only up to service point and feeding transformer is client responsibility. and The transformer is Delta to Wye.

So you recommend to use fuse than recloser. There is no need for CTs for protection.

We used three fused cutouts and we also spec'd our padmounts with internal HV fusing. The combination gave us a full range of protection - well "left" of and "below" the transformer's damage curve. That's how we did it on all three phase padmounts up to 2500 kva (and a few that were 5000 kva). Our primary voltages were 13.2 and 34.5 kv. I can't imagine a recloser just ahead of a padmount transformer. That's not a situation where I would want to close back in on a permanent fault another one or two or three times.

 

[Electriman]

Is this an existing transformer or are you going to order it? If you are going to order it you may be able to mount the HV fuses in the HV ATC and even a MCCB in the LV ATC.

Good idea. But I still think I need to have some sort of protection at the service point like fuse cut-out. On the secondary side do you have a catalog number of the MCCB that also can protect against ground fault?

Second question when I have only fuse on the primary side of the transformer can I use that as a disconnect switch to isolate the transformer during maintenance and repair?

 

[Ingenieur]

Good idea. But I still think I need to have some sort of protection at the service point like fuse cut-out. On the secondary side do you have a catalog number of the MCCB that also can protect against ground fault?

Second question when I have only fuse on the primary side of the transformer can I use that as a disconnect switch to isolate the transformer during maintenance and repair?

Eaton mccb with a 310+ etu trip module
you can get gnd fault option plus adjustable long and a short time delay
others make similar products

eaton unit http://www.eaton.com/ecm/idcplg?Idc...tReleased&Rendition=Primary&&dDocName=6612C36