1-ph Transformer primary connection

[Sn00ze]

Hello all,

I was wondering if someone could take a look at this picture and tell me their thoughts. I've been trying to understand what's happening here or if it's maybe just a mistake.

I have not been able to get a hold of the nameplate yet for the Station service TX. But is the primary connections right? they are getting a L-L voltage?
Should it be not a Line and Neutral connection? Also I don't understand where they got 3.15A fuse for the primary (I am in the CEC world).

I look forward to hearing your thoughts.

 

[waross]

As long as the transformer voltage matches the line to line voltage there is no problem.
As for the fuses, this rule may apply:
"(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."
or this rule:
"with the following exceptions:
(a) installations or equipment employed by an electric, communication, or community antenna distribution
system utility in the exercise of its function as a utility, as recognized by the regulatory authority having
jurisdiction, and located outdoors or in buildings or sections of buildings used for that purpose;

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

 

[crshears]

Should it be not a Line and Neutral connection?

Why should it have to be? The transformer information says it's designed for a primary supply of 23 kV; I've seen numerous cans with a pair of high voltage bushings on top like a pair of rabbit ears...

High side fuse kVA by my calculations is 72,450; low side on the other hand is 14,400...perhaps someone else will know why the discrepancy.

Add via edit: I guess I was too slow on the send button; what Bill says.

 

[Sn00ze]

Hi Waross,

I was using the 150% and I still don't get anywhere near 3.15A fuse. I am wondering if it is the "next higher fuse size". That would make sense then.

I've seen that exception for utilities before :

(a) installations or equipment employed by an electric, communication, or community antenna distribution
system utility in the exercise of its function as a utility, as recognized by the regulatory authority having
jurisdiction, and located outdoors or in buildings or sections of buildings used for that purpose;

but I don't see it as an exception for 26-250, which rule is that?

 

[Sn00ze]

Thanks Crshears,

Oh I don't mean it can't be that. I haven't seen any with this configuration. That's why I am here .

The kVA of this TX is 10 kVA, so I would get 0.65A with the 150%. I haven't looked into what the higher fuse size would be. Maybe it is 3.15A.?

 

[crshears]

Oh I don't mean it can't be that. I haven't seen any with this configuration. That's why I am here .

I recently heard someone say half-jokingly, "I knew what I meant when I said it..."

No problem.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Sn00ze]

I recently heard someone say half-jokingly, "I knew what I meant when I said it..." smile

Yeah for sure, this happens to me more times than i'd like to admit

 

[dpc]

Medium-voltage fuses with very low amp ratings are notoriously prone to nuisance operation and failure. I've experienced failures of 1.0 amp fuses for apparently no reason at all. In many substations, instrument voltage transformers are unfused on the primary.

 

[stevenal]

A little coordinating margin between the high and low side fuses is desirable. The same load is lost either way, but the 60A fuses are easier to service.

 

[Sn00ze]

Gotcha, yeah that's what I was thinking. probably because of inrush too.

Thanks for the input guys, I feel better about these set up now.

 

[waross]

Section 0 — Object, scope, and definitions
Under Scope.
The rule applies to the entire code.
Or the Code does not apply.

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

 

[Sn00ze]

Gotcha Waross, yes I remember that now. I find that clause very loose to follow. I understand the ideology behind it; in utilities or industrial settings the mentality behind that is that trained personnel will be the only ones accessing those facilities. Then you go into a rabbit hole of what constitutes a "trained personnel".

Regardless, I am always wary of following that clause since even though you could design something inferior than the code. Why would you, IF you can help it ?

 

[Bill West]

Hi Sn00ze.
I think the code's utility exemption was contemplating professional design and supervison more than just access only by trained personnel. Nonetheless we were quite strict that you had to be qualified under our corporate electric safety book, or be escorted by such a person, in order to get through the padlocked gate at any station. We viewed the fence as our locked electrical enclosure.

In the design area, for a utility the code is a good source of experienced data for the stuff on the low side of the station service transformer but it kinda of faints at designing 230kv buswork. What we had on the station service low side did indeed stick fairly well to the wisdom of the code.

Bill

 

[waross]

10,000 VA / 240 Volts = 41.7 Amps
41.7 x 150% = 62.5 Amps. (CEC Table 50;- 150%)
The transformer is adequately protected from overload and over current by the secondary fuses.
Downstream overcurrent and overload will be cleared by the secondary fuses.
The primary fuses will provide overcurrent protection in the event of catastrophic failure of the transformer.
Considering the experience of dpc;

Medium-voltage fuses with very low amp ratings are notoriously prone to nuisance operation and failure. I've experienced failures of 1.0 amp fuses for apparently no reason at all

The importance of maintaining station power,
The relative ease of servicing the secondary fuses;
The installation does not appear unsafe.
I believe that most engineers would consider this to be a reasonable use of the exceptions granted to utilities.
A station service transformer definitely complies with the terms of the exemption.

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

 

[Sn00ze]

Thanks Bill,

That along with how Waross explained it, ties everything together very logically. I am inclined to agree 100% Thank you.


Overall Thank you all very much for your great responses. Thank you also for providing very valid reasoning I always leave this forum feeling a little smarter.

 

[crshears]

I always leave this forum feeling a little smarter.

I personally tend to leave these fora feeling just as dumb as before, but also feeling better informed than I was prior...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

May be bit late- Single phase transformers getting line to line voltage in primary and feeding single phase line to neutral voltage at secondary is seen in two areas of application.
1) Distribution supply through pole mounted single phase transformers 11/ 0.433V

2) Rail Traction supply transformers - Primary 33- 400 KV ( 10-100 MVA) Secondary - 25 KV between pantograph and grounded rail

This type of line to line power intake instead line to ground on primary is done to balance the loading on grid. If at one point supply is taken from R-Y, next transformer down the line will take power from Y-B and then B-R etc.

Control transformers also take power from line to line when mid point of secondary is grounded.

Fuse selection based on primary current= single phase rating kVA /Line to line voltage