It is done and done regularly with some industrial customers and utilities. The manufacture's rating is conservative and it is not unusual to rate transformers dynamically based on the weather conditions. People do studies to try to determine how much slack they have based on present oil temperature, ambient temperature, and things like acceptable loss of life[pre][/pre].
It seems like North American standards require a 133% safety factor?
I have to find the other report, but I know for a fact some utilities will overload 50 and 60MVA transformers for the loss of units on the basis the overload will last a few hours and anticipating a 1% life reduction. I believe the number is 125% but have to find out.
One question I have for PRC: if I order a 100MVA ONAN only unit- will fans being added later make a difference and by how much? Basically what I'm asking is if an ONAN only unit is simply a OA/FA/FA2 trafo just without FA/FA2 listed and perhaps slightly larger radiators for a maybe 2% increase?
We purchased the low noise fans directly from the transformer manufacturer, so I do not have any specific recommendations.
No, it is not as simple as adding fans. As an example, we have one transformer that has a 76C winding rise on the 400 MVA ONAF2 rating but only a 58C winding rise on the 240 MVA ONAN rating. Manufacturers model all three stages of cooling to find out which case has the hottest temperatures.
As a side note, the factory testing is the only time an ONAN/ONAF/ONAF transformer operates at the ONAN design temperatures. In actual operation, the fans will turn on well before the ONAN rating is reached.
Why would an ONAF2 allow for higher winding temps than ONAN? Wouldn't it always be 76*C and fans/oil circulation/reduced loading/ect to make sure the wingdings never go above 76*C?
1) USBR document is a 30 year old document and IEEE/IEC revised Transformer Loading guides several times - latest IEC 60076-7-2018 & IEEE C57.91-2011.
2) If you order 100 MVA transformer, you will get only 100 MVA unit. Automatically you will not get overloading capacity. Of course you can over load transformers for short periods as indicated in above guides, but with loss of life.
3) It was experimentally found out that life of paper( ie tensile strength halves) is 20.5 years with a moisture level of 0.5 % for a continuous temperature of 98C or 110 C (with thermally upgraded paper) With every 6 C rise above these hot spots, life halves ie at with 108C it will be 10.25 years.
4) In service, paper absorbs moisture and with 1.5 % moisture and at 98 C also, life halves ie 10.25 years. With increased moisture, threshold temperature to start bubble release also comes down.ie overloading limit comes down.
5) Manufacturers(self being one for more than 50 years!)are not conservative or building up any extra margins during these days of cut throat competition.
6) Better to consult OEM before planning any continuous overloading to avoid premature failure of your costly transformer. It may or may not have overload capacity with increased cooler capacity.
7) OEMS design transformers for the same hot spot at various stages of cooling. But depending on various parameters, it can vary and for setting up WTI, maximum hot spot temperature is selected.
Will a 100MVA ONAN be heavier or larger than a 60/80/100MVA OA/FA/FA2 unit? And if so by how much? Thats where my curiosity lays. I've never seen an ONAN unit besides small ones 2.5MVA and under.
So errrr, can I ask a rather DIY question... Please forgive me... But does that mean I can load a 40/50/60MVA IEEE/ANSI transformer to 50 or 60MVA without fans?
Forgive me- the theory behind this is fascinating.
Two sets of numbers, one per IEEE and one per IEC.
The IEEE set of numbers will have both a lower MVA rating and a lower per unit impedance on the transformer base. The IEC numbers for MVA and impedance will both be higher.
Convert from one base to the other, or some third base, and you'll find the same they're the same. To get to the IEC base rating the fans have to be used. To get to the IEEE base rating you don't need to use the fans, but to get to the IEEE top rating, same as the IEC base rating, the fans are required.
Basically the IEC just ignores the lower ratings only listing the IEEE top rating? I know they do that with impedance- but with loading I'm still a bit murky.
As I understand IEEE/ANSI vs the IEC the IEEE standards force a larger core for the same base rating. Could be wrong though.
Let us quote from standard:
C57.12.10 - Recommended impedance for a 50/66.67/83.3 MVA 132 kV 550 BIL is 9.5 % with OLTC at 50 MVA base ie at 80 MVA, impedance will be 15.2 %
IEC 60076-5 - Recommended minimum impedance for a 50/80 MVA unit - 12.5 % on 80 MVA base. ie at 50 MVA 7.8 % impedance
In above case, IEEE unit will be lighter due to higher impedance ( less core, more copper); but if IEC unit were ordered with 15 % impedance at 80 MVA, active part weight will be almost same for both units.
The heat run tests I have seen indicate that OEMS do not design for the same hot spot temperature on each rating. For the 240/320/400 MVA example I mentioned above, the fans only added about 25/50 percent to the rating. The OEM seemed to size the cooling system for the worst case temperature rise, which was the ONAF2 rating. For the ONAN and ONAF1 ratings, the OEM just followed the IEEE requirements that the ratings be exactly 240/320/400 and the temperature rise be less than 65C/80C on each of the ratings.
Note that OEM has a strong incentive to be a little bit conservative in the thermal modeling because if the transformer exceeded the allowable rise, the transformer would have been rejected. After doing a heat run test at the factory, we asked for additional ratings to be calculated per IEEE C57.91 at various ambient temperatures ranging from 30C to 0C. Based on actual heat run data, the resulting ratings under nominal IEEE conditions of 30C were approximately 280/350/410 MVA. If we had asked for a self cooled 240 MVA transformer, it might have been more than 15% smaller.
Loss evaluation can also play into the size of core & coils versus how many radiators/fans are required. If losses are highly priced, the core/coils will be larger and less heat will need to be dissipated. Also, the weight difference between various bidders on the job often exceeds 15%.
