Transformer primary protection - Can this be right? 1

[N1755L]

Evening Gents,

Does anyone else find this odd? Am I missing something here?

I'm looking at the Canadian Electrical Code, rule 26-252 (I'll paste the rule in its entirety at the end of this post), which states that I should protect the primary feeders with a circuit breaker rated at up to 300% of the rated primary current of the xformer.

Now if we were to take the example of a 3-phase 300KVA xformer, 4160/600V, then we'd calculate a rated primary current of 41.6A (300000/(4160*1.732)).

CEC rule 26-252 suggests that I can now protect that primary circuit with a 100A circuit breaker. The turns ratio of this xformer is 6.93:1... assuming that something goes wrong on the secondary side and that said secondary sees a current draw of 500A (or that someone gets overzealous and loads up the secondary), then the primary is, theoretically, ony seeing 72A of current (500A/6.93). So what is going to trip that 100A circuit breaker?

Now before anyone jumps too high, I do realize that the primary will see more current than the calculation suggests, due to the heat generated within the xformer, but then would that mean enough extra current on the primary to trip that 100A breaker?

I'm thinking of an example whereby one would want to install a Motor Control Center on the secondary side of this 300KVA xformer, conductors on this secondary would be installed at an ampacity of no less than 395A (500 MCM) (300000/(600*1.732)*1.25), so I'd expect the MCC busses would need to be rated for at least 400A. Here again, the primary fuse would appear to provide inadequate protection. Is there another code rule that would state that a disconnect needs to be installed on the secondary side, or that the primary breaker size needs to be reduced according to the secondary load?

There just has to be more to this.

Any ideas? Thanks.



Oh, almost forgot that rule:


Rule 26-252 Overcurrent Protection for Power and Distribution Transformers Rated Over 750 V

(1) Except as permitted in Subrules (2), (3), and (4), each transformer shall be protected by an individual overcurrent device on the primary side, which shall be rated at not more than 150% of the rated primary current of the transformer in the case of fuses, and which shall be set at not more than 300% of the rated primary current of the transformer in the case of circuit breakers.

(2) Where 150% of the rated primary current of the transformer does not correspond to a standard rating of a fuse, the next higher standard rating shall be permitted.

(3) An individual overcurrent device shall not be required where the feeder or branch circuit overcurrent device provides the protection specified in this Rule.

(4) A transformer having an overcurrent device on the secondary side rated or set at not more than the values in Table 50 or a transformer equipped with co-ordinated thermal overload protection by the manufacturer shall not be required to have an individual overcurrent device on the primary side, provided the primary feeder overcurrent device is rated or set at not more than the values in Table 50.

 

[cranky108]

Question 1: What would you do with the transformer after it were to fail?
Would you take it to a rewind shop, or take it to a scrap yard?

Question 2: Are you trying to remove a failed transformer, or protect it from an over load?
A 100 amp breaker is a common size that probally won't operate on inrush, and is easy to coordinate with.

Comment: I don't know anything about Canadian Electrical Code, rule 26-252.

 

[wareagle]

The NEC will allow you to fuse the primary at 300% but it requires the secondary OC device be set at 125%.

 

[davidbeach]

It is the secondary overcurrent that will protect the transformer against long term overloads. That 100A breaker will do just fine against fault currents.

 

[magoo2]

I think what you're missing is that the transformer has impedance. Suppose it is around 3%.

What is the fault current from a bolted 3-phase fault on the 600 V terminals? Neglecting source impedance, it works out to be about 1,388 A on the 4.16 kV side. That will be sufficient to trip a 100 A breaker.

If the fault in internal to the transformer, then the fault current will be even higher. Again, no problem for the 100 A breaker.

Let's calibrate our thinking. Full load amps as you say is 41.6 amps on the 4.16 kV side. It is also equal to 288.7 A on the 600 V side.

Since you assumed a 500 A fault on the secondary, this works out to be 1.73 times the secondary FLA (288.7 A), so it is also 1.732 times the primary FLA (41.6), so the primary amps under this fault assumption is 71.2 A. So you wouldn't expect the primary breaker (100 A) to trip for this condition.

What you need to do is install secondary protection to handle this condition.

 

[Elmir]

Table 50 of the CEC applied to rule 26-252.

The table lists the maximum permissable setting or rating of overcurrent device as a percentage of rated current of transformer.

Remember the rule pertains to maximum ratings permissable. Of course you will have to do a little more work to determine appropriate settings that protect the transfomer such as plotting damage curves with selected settings.

 

[N1755L]

Ok, thanks all,

I wasn't asking about fault protection, I was asking about what would protect the secondary conductors or, as in the example I'd mentioned, what would protect an MCC connected to the transformer, if a sustained current draw of some 500A or so were to occur on the secondary side.

Unless someone comes up with something else, I'm afraid Magoo and David's suggestion of another fused or breakered disconnect between the xformer and the MCC is likely the only solution for protecting that MCC.

I guess I just forgot to take into consideration the inrush current, as Cranky suggests, which is what would explain the high Amperage value of the breaker on the primary side.

Good enough.

Out of curiosity now, would a choke/reactor/inductor/coil, whatever you prefer calling it, would a choke on the primary side protect against (dampen) the inrush sufficiently so as to allow the installation of 60A or so breaker on the primary side?

Thanks for your time and assistance.

 

[davidbeach]

No, don't add a choke. The NEC would certainly require secondary overcurrent, can't really imagine that the CEC wouldn't. Been like that for ages; and for a good reason. Don't try to reinvent the wheel.

 

[waross]

Taking a code course may be a good idea if your duties include this type of planning and installations.

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

 

[rbulsara]

Heed the advice of davidbeach and waross.

Rafiq Bulsara

http://www.srengineersct.com

 

[N1755L]

Hehe...

Funny you should say that Rafiq, that is exactly where I am!

I'm finishing my electrical apprenticeship, on my last block in advanced here at school, and I cannot always get straight answers from the teachers when it comes to interpretation of the Canadian Electrical Code... everyone interprets it differently.

As for the choke mentioned previously, I know nothing of the sort is done in practice, I was only exploring in the realm of theory, as I'm sure you'll guess by now, I LOVE the theoretical realm, I like what-if scenarios and using the knowledge I have as tools for finding solutions to them.

For the scenario I presented above, I understand that would have to add overcurrent protection on the secondary of that transformer if I wanted to ensure a controlled level of current there.

I was curious to see if there might not be other possibilities. I think I can conclude there are not.

Thanks all for your suggestions.