Rerating a transformer to use smaller primary cables 1

[tumbleweed1]

Is it common that a transformer could be rerated with a new nameplate to a slightly smaller kVA in order to maintain code requirements for cable sizing?

For instance, a 5MVA transformer primary is a few amps more than the ampacity of a smaller cable. To bump up a size would be a ~$20k cost to the next size of cable. With the distance being ~1km to the transformer, I was thinking of rerating the transformer about 50kVA less to save the additional cable costs. This transformer is already ~45% oversized, so a few percent lost in that capacity is not a big deal compared to the cost savings of a smaller cable.

Is this something that is common/can be asked of the manufacturer to do?

 

[Sn00ze]

I don't know if i understand what the problem is. What part of the world are you in? For us, sizing the upstream fuse would dictate the size of the cables since we don't go by the size of the TX as much as the size of the load. Meaning: i would not oversize cables and fuse if the TX is only 50% loaded.

Re-rating a TX, I've never even heard of that. Hopefully one of the more experienced guys will be able to help you.

 

[waross]

A slightly lower rated temperature rise will give a slightly lower KVA rating.
Cosult the manufacturer.
It will also give a slightly lower percent impedance voltage, but won't change the available short circuit current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[krisys]

tumbleweed1,

If you are reducing the cable size, then you need not re-rate the transformer. You have declare the rating for that feeder and the system which it supplies. The load should be restricted due to the cable size and not due to the transformer rating.

Probably, you can install a modern feeder overcurrent relay. Now a days you also get the additional feature of overload protection in the same relay. Set it to limit the load to the cable rating.

 

[waross]

26-256 Overcurrent protection for dry-type transformer circuits rated 750 V or less (see
Appendix B)
.....................

(3) Notwithstanding Subrules (1) and (2), primary and secondary conductors shall be permitted to have an
ampacity rating not less than that required by the demand load, provided that they are protected in
accordance with Rules 14-100 and 14-104
14-104 Rating of overcurrent devices (see Appendix B)
(1) The rating or setting of overcurrent devices shall not exceed the allowable ampacity of the conductors that
they protect, except
(a) where a fuse or circuit breaker having a rating or setting of the same value as the ampacity of the
conductor is not available, the ratings or settings given in Table 13 shall be permitted to be used
within the maximum value of 600 A;
..........
Appendix B notes on rules.
Rule 26-256
Selection of overcurrent devices with too low a rating for the primary of a dry-type transformer can result in
unintended operation when the transformer is being energized (such as might occur after a power outage). To
avoid such operation, the overcurrent device should be able to carry
(a) 12 times the transformer rated primary full load current for 0.1 s; and
(b) 25 times the transformer rated primary full load current for 0.01 s.

Your challenge will be to select a breaker sized to support the demand load, that will also handle the inrush based on the full rated transformer current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wroggent]

The NEC requires that feeders over 600V at unsupervised locations be rated to carry the sum of the transformer nameplate ratings being served. That may be the basis of the OP's question.

 

[Sn00ze]

100% what waross said.

 

[tumbleweed1]

Waross, I agree. This is being fed from a 15kV breaker with a relay. Given code, minimum size conductor ampacity is 261A. Max CB setting is 627A. There'll be some coordination with breaker setting to allow for inrush, however its likely that the CB will need to be set greater than 125% FLA (261A).

If we choose a conductor rated 260A based on load factors(rule 3), then Table 13 allows an overcurrent device set at 300A.

Long question short:
Would 300A breaker setting be sufficient to allow inrush on the transformer? For me to figure out.

but at a high level:

What is the best method to size primary conductors of a medium voltage transformer protected with a CB and relay? Model it in an analysis program to determine breaker settings and then size cable based on those settings?

 

[waross]

By code the minimum ampacity is 125% of rated full load current of the transformer.If I was in your position I would discuss this with the authority having jurisdiction.
Be mindful of your place. The AHJ has the final say. You should adopt the attitude that you are asking, not telling.
This is always the first choice of selection.
Use this rule if you can. If special conditions make this an unattractive choice then it may be a challenge to find an acceptable alternate sizing scheme.
Re the breaker setting:
Use rule:
26-252 Overcurrent protection for power and distribution transformer circuits rated
over 750 V
(1) Except as permitted in Subrules (2), (3), and (4), each ungrounded conductor of the transformer feeder or
branch circuit supplying the transformer shall be provided with overcurrent protection
...........
(b) rated or set at not more than 300% of the rated primary current of the transformer in the case of
breakers.
A 600 Amp breaker will often have an instantaneous trip set at x 10 or 6000 Amps.
But rethinking the distance of ~ 1 km. Have you checked the voltage drop? Voltage drop may dictate a larger cable in any event.
And the impedance of ~ 1 km of cable should reduce the energization transient to a level that the 6000 Amp inst. setting will allow energization.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

A suggestion:
If you install a breaker on the transformer secondary rated at about 75% of the transformer rated secondary current, that should handle the demand load and may help convince the AHJ to accept a smaller cable on the primary.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

A 15kV vacuum breaker using a modern, properly set-up transformer protection relay should have no problem handling both the trip level requirements and the transformer inrush.

 

[waross]

One possible issue with down-rating a transformer is that although the maximum allowable full load current is lower, the inrush is the same.
The code limits the protection setting as a multiple of the rated full load current. This may not be enough to withstand the inrush.
However in your instance the impedance of ~1 km of feeder will more than offset the effect of moderate de-rating.
But, again, check your allowable voltage drop. This may well be the next issue with a long, undersized feeder.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GroovyGuy]

Howdy Tumbleweed,
If you are talking about a few amp, why sweat it. Why not simply rerate the cable? There are several recognized standards out there that will let you squeeze a few more amps out of a cable, given the right conditions. ie;
835-1994 - IEEE Standard Power Cable Ampacity Tables

BTW a modern transformer protection relay (TPR) will almost certainly include a 87 element. This won't work over the distances that you have indicated, even if you had a cable, the burden will kill you.

Even with a low PU on the TPR, you can always avoid the inrush current with the right selection of curve type and TD.

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[tumbleweed1]

As an update...

Contacted the manufacturer of the transformer that we received the quote from. They are fine with applying a lower kva nameplate value so we can meet the cutoff for the smaller cable. In theory- the unit is still the same, but the impedance pu value will change slightly as the base kVA rating is changed. All the same on the inside...

 

[prc]

Transformers are capable of continuous overload of 110% and short time overload of 150%. So you can limit the loading on transformer only through the protective relay or fuse setting on secondary or primary side. Why you want manufacturer's clearance to decide the loading that is below its rating? The inrush current will not change whether you load it to 50 % or 150 %. That is decided on 100 % rating on which it was designed.

 

[mvcjr]

"As an update...

Contacted the manufacturer of the transformer that we received the quote from. They are fine with applying a lower kva nameplate value so we can meet the cutoff for the smaller cable..."

From the above-mentioned statement, I assume the transformer is still to be manufactured. So why oversize the transformer capacity by 45% in the first place. You can reduce the oversizing to meet your requirements. Besides, you will only be paying extra for the unnecessary core losses in the transformer due to the oversized capacity.

 

[tumbleweed1]

mvcjr- this project is evolving and that 45% is quickly being eaten up. Currently sitting about 35% spare now.

 

[waross]

Hi prc.
You are correct. However the issue is not protecting the transformer. The issue is meeting local code requirements for minimum cable size and maximum allowable trip settings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PHovnanian]

"As an update...

All the same on the inside..."

So you may still have a problem coordinating the relay trip setting with the transformer inrush.