Main Transformer Loading 1

[kookypedia]

Guys,
I have a main transformer with the rating of 12/16/20MVA (KNAN/KNAF1/KNAF2).
My final load list shows 14MVA now. I know this transformer can handle that but how bad is to have normal running loads more than the base rating of transformer? (in my case 14MVA which is more than 12MVA)
Should I change the transformer size now?

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[waross]

No problem if you have the first set of fans installed and turned on.

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

 

[kookypedia]

But it looks like the fans should be on all the times right?

An investment in knowledge pays the best interest (Benjamin Franklin).

http://www.kookypedia.com

 

[waross]

The fans will be on a thermostat and only run when needed.
On colder days you may be able to deliver 14 MVA without the transformer getting hot enough to cycle on the fans.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[magoo2]

How bad is to have normal running loads more than the base rating of transformer? To me the transformer has a peak rating of 20 MVA and that is all I would worry about.

What you have so far looks fine to me, but what do you mean by your load list? Is that an estimated load or an actual measured load?

Questions that affect your decision to replace with a larger unit include:
- how many hours is your peak and is this a summer peak or a winter one?
- what is the load factor on peak days or what is the load shape on the peak days (i.e., hours at peak, MVA at peak, hours at off-peak and MVA at off-peak
- how much load growth do you expect and how rapid is it?
- what is your next size transformer that you would go to, a 15/20/25 MVA or a 25 MVA?

If it were me, I would look at the present loading compared to what the guidelines are in the loading guides like IEEE C57.91. You can expect some loss of life if you load it higher than the peak load and the hours given in the loading guide if you are higher than 20 MVA*factor. This factor depends on the initial loading (MVA) prior to peak, loading during peak, hours of peak load and ambient temperature. Right now you are in a good situation to evaluate this.

In the situations I've seen, our factor was 1.2 or greater with a summer peaking situation, so you wouldn't have to worry until the peak MVA got above 24 MVA (or 1.2 * 20). Obviously your situation and factor calc are probably different, but this will give you an idea of the thought process to follow.

 

[waross]

Heat is the enemy of transformers.
A 12/16/20MVA (KNAN/KNAF1/KNAF2) may have no fans, one bank of fans or two banks of fans.
With no fans (KNAN) it will be a 12 MVA transformer.
With one bank of fans (KNAF1) it will be a 16 MVA transformer.
With two banks of fans (KNAF2) it will be a 20 MVA transformer.
How many banks of fans are installed?

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

 

[kookypedia]

Bill,
I don't know how many banks of fans are installed.
Does that effect anything? (should I consider redundant Banks etc.?)

An investment in knowledge pays the best interest (Benjamin Franklin).

http://www.kookypedia.com

 

[cuky2000]

Power transformers are rated as follow:
• Average ambient temperature ≤ 30° C (86° F) in a 24-hour period.
• Maximum ambient temperature of 40° C (104° F).
• Average winding rise ≤65° C (149° F) at rated load, voltage (V), and frequency (Hz).
• Maximum hottest-spot winding temperature ≤80° C (176° F) above ambient.

Notice that 14 MVA can be achieved if the average temperature in 24 hrs is ~14oC (~57 oF)

Below is an illustration of the impact of exceeding the rated values on the transformer Insulation life expectancy and accelerated aging effect.

 

[prc]

You need not change the transformer until the peak load exceeds say 22 MVA. Normally transformer temperature indicator is programmed that automatically fans will be switched on once the top oil temperature exceeds the setting. So fans may come on at 10MVA during summer and may come on only at 15 MVA during winter.

 

[ScottyUK]

As an end-user / plant operator I'd prefer not to have fans running routinely if the option exists and there is a relatively small difference in cost. Fans consume energy and require maintenance, so the operating cost goes up while reliability goes down.

Also don't forget that ratings are normally stated at 40°C, so you need to consider how that relates to your actual operating environment.

 

[prc]

Transformer ratings are based on an ambient temperature of 20C and for every 6c rise above this paper nominal life will be halved.Similarly life will be gained for every 6 C below 20C. 40 C is the max ambient as per IEC.

 

[crshears]

waross (Electrical)20 Jan 17 00:33
The fans will be on a thermostat and only run when needed.
On colder days you may be able to deliver 14 MVA without the transformer getting hot enough to cycle on the fans.

Generally true...but there are always exceptions / deviations / oddities...

Saw a case where in addition to the above, there were auxiliary relays wired into the transformer CT secondary circuits that switched on banks of fans upon increase above a certain primary [low voltage side] current threshold, regardless of outdoor temperature...I don't know why it was done that way, but it was; go figure...maybe they didn't want to wait for the bank to heat up before the fans started...

CR

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

 

[prc]

The reason some users prefer cooler actuation by current settings is due to the long heating time constant for top oil (1-3 hours in power transformers). Disadvantage is more power loss by way of more running time for coolers.