Paralleling Transformers

[brian john]

I have an existing facility with a single 750 KVA transformer 408 Delta 208/120 wye. The transformer was monitored for 30 days, primary 45, secondary 1050 amps (rough numbers without puling up the report). This is a 30 year old dry type transformer and the customer is budgeting for replacement. For the primary overcurrent protection there is a 1200 amp FPE PL fusible switch and the secondary is a 2000 amp switchboard with a single fusible main.

Here is the issue, a new 750KVA and/or a 500 KVA transformer will not fit down the hall or into the electric room, no how no way without doing major structural changes. What I have thought about would be paralleling 2-250 KVA transformers. But I am not clear on the NEC on this. Could I utilize a single overcurrent protection device on the primary or do I need separate OCP for each transformer? Could I legally parallel the secondary to the line side termination connections 2000 amp switchboard main if the existing OCP was properly sized.

In lieu of this I would have to install 2 separate transformers with individual OCP and secondary switchboards which goes back to the initial problem of getting this equipment into a facility and the resultant extended outage. This is a 24x7 facility.

 

[david luchini]

You can parallel transformers per 450.7, but the overcurrent protection for each transformer must meet the requirements of 450.3(B). I don't see how a single OCPD for both transformers would work the way you intend.

Have you considered using 3 separate 480-120V transformers (either 167 or 250 kVA) connected in a delta-wye configuration?

 

[RUWired]

You would have to meet the requirements of article 240.8 via a factory built and listed assembly.

Rick

 

[jim dungar]

Won't fit, wow.

The core and coil of a barrel wound 750kVA transformer is not very large when you take it out of its enclosure. The Square D Digest shows a Power-Zone Model III unit as 36"d x 60"w x 85"h in its enclosure which is extremely smaller than their traditional stand alone enclosed transformer.

 

[brian john]

Have you considered using 3 separate 480-120V transformers (either 167 or 250 kVA) connected in a delta-wye configuration?

There are space limitations and load distribution issues.

 

[brian john]

Won't fit, wow.

The core and coil of a barrel wound 750kVA transformer is not very large when you take it out of its enclosure. The Square D Digest shows a Power-Zone Model III unit as 36"d x 60"w x 85"h in its enclosure which is extremely smaller than their traditional stand alone enclosed transformer.

I spent two days with our rigger looking at all options we can think of to make this happen and weight and space limitation prohibit replacing the 750 kva with a like unit or a 500 kva.

 

[david luchini]

There are space limitations and load distribution issues.

I don't understand. I thought you wanted to install a new 500kVA transformer but couldn't get it through the building and into the electrical room.

Three single phase 167kVA xfmrs connected delta-wye would give you a 500kVA transformer, and would certainly be small enough to get through corridors and doorways.

 

[renosteinke]

If there's no room ... well, there's no room. I am encountering this sort of thing far more often that I would like - and that's what motivates several of my rants. A customers' poor planning and prior penny-pinching is no reason for me to lose sleep.

From experience ... code parsing aside ... there is no way you ever want to install two transformers in parallel without separate disconnects on both sides of each transformer. There WILL come a time when you will want to take one transformer out of the system, and you can't do that unless you have separate discos, both 'line AND 'load.' Ditto for OCPD's; cut corners here, and you're setting the stage for backfeeding nightmares.

 

[david luchini]

From experience ... code parsing aside ... there is no way you ever want to install two transformers in parallel without separate disconnects on both sides of each transformer. There WILL come a time when you will want to take one transformer out of the system, and you can't do that unless you have separate discos, both 'line AND 'load.' Ditto for OCPD's; cut corners here, and you're setting the stage for backfeeding nightmares.

Unless I'm misreading 450.7, you'd be required to have a single disconnect on the primary so that the paralleled xfmrs will be "switched as a unit." The code eliminates the backfeeding nightmares with this section.

 

[zbang]

Do you have the luxury of removing the old transformer before installing the new? Doesn't sound like it.

 

[brian john]

Do you have the luxury of removing the old transformer before installing the new? Doesn't sound like it.

We will set a temporary bypass on another floor to accommodate the load requirements during the cut over.

 

[brian john]

Here is my question, what does the code specifically say about paralleling transformers.

1. Single disconnect/CB with OCP on the primary?
2. Two disconnects/CBs with OCP on the primary?
3. If Two disconnects/CBs with OCP, do these need to be switched as a single unit
4. Single fusible disconnect on the secondary?
5. Not feasible to do per the NEC or with available equipment?

This job may never happen, but I need to give the customer some options.

 

[david luchini]

Here is my question, what does the code specifically say about paralleling transformers.

1. Single disconnect/CB with OCP on the primary?
2. Two disconnects/CBs with OCP on the primary?
3. If Two disconnects/CBs with OCP, do these need to be switched as a single unit
4. Single fusible disconnect on the secondary?
5. Not feasible to do per the NEC or with available equipment?

This job may never happen, but I need to give the customer some options.

Code says a single disconnecting means on the primary, but each transformer must have OCP in accordance with T450.3. If you could have a single OCP on each size that didn't exceed the ratings in 450.3 for each transformer, you could use it. But it doesn't sound feasible to me.

 

[brian john]

Single disconnect feeding two fusible disconnects sized for the transformer?
Secondary to the line side of a properly sized single fusible switch.

 

[Smart $]

Here is my question, what does the code specifically say about paralleling transformers.

1. Single disconnect/CB with OCP on the primary?
2. Two disconnects/CBs with OCP on the primary?
3. If Two disconnects/CBs with OCP, do these need to be switched as a single unit
4. Single fusible disconnect on the secondary?
5. Not feasible to do per the NEC or with available equipment?

This job may never happen, but I need to give the customer some options.

Switching as a unit is permitted, not required (450.7). If you do switch as a unit, primary ocp must be for each unit, not both together. One option if switched as a unit is to use a six-pole fusible disconnect.

In my opinion, you can combine outputs to one secondary ocpd. Otherwise, the secondaries would be considered secondary ties under 450.6?which says, "Conductors connecting the secondaries of transformers in accordance with 450.7 shall not be considered secondary ties."

 

[kwired]

No other place where the transformer could be installed, even outside if necessary?

Placing it in a different location could actually mean less down time when it is time to switch over to the new unit. You could have quite a bit of work done already before having to shut down. You also wouldn't need to remove the old unit until after things are up again.

 

[david luchini]

Switching as a unit is permitted, not required (450.7). If you do switch as a unit, primary ocp must be for each unit, not both together. One option if switched as a unit is to use a six-pole fusible disconnect.

I disagree with this. I read 450.7 as saying paralleling of transformers is "permitted" and paralleled transformers shall be switched as a unit to prevent backfeeding scenarios.

 

[Smart $]

I disagree with this. I read 450.7 as saying paralleling of transformers is "permitted" and paralleled transformers shall be switched as a unit to prevent backfeeding scenarios.

I understand how 450.7 can be interpretted as such, and you are probably correct in that interpretation. However, I believe if that is the intention, it needs to be better worded. As it is worded currently permits them to be switched as a unit and does not compel them to be switched as a unit...

Existing: 450.7 Parallel Operation. Transformers shall be permitted to be operated in parallel and switched as a unit, provided the overcurrent protection...

Improved 1: 450.7 Parallel Operation. Transformers shall be permitted to be operated in parallel when switched as a unit, provided the overcurrent protection...

Improved 2: 450.7 Parallel Operation. Transformers shall be permitted to be operated in parallel provided the transformers are switched as a unit and the overcurrent protection...

 

[david luchini]

I understand how 450.7 can be interpretted as such, and you are probably correct in that interpretation. However, I believe if that is the intention, it needs to be better worded. As it is worded currently permits them to be switched as a unit and does not compel them to be switched as a unit...

I would agree that the section could be better worded, but I disagree that it does not compel paralleled transformers to be switched as a unit.

What I read is Transformers shall be permitted to be: "Operated in parallel and switched as a unit"

Not,

Transformers shall be permitted to be: "Operated in parallel"
and
Transformers shall be permitted to be: "Switched as a unit"

In my mind, this compels transformers operated in parallel to be switched as a unit.

 

[Smart $]

I would agree that the section could be better worded, but I disagree that it does not compel paralleled transformers to be switched as a unit.

What I read is Transformers shall be permitted to be: "Operated in parallel and switched as a unit"

Not,

Transformers shall be permitted to be: "Operated in parallel"
and
Transformers shall be permitted to be: "Switched as a unit"

In my mind, this compels transformers operated in parallel to be switched as a unit.

Well I'm not going to argue a grammar issue when I agree your interpretation is the intent of the requirement.