Transformer MVA rating IEEE vs. IEC 2

[cuky2000]

There are difference between IEEE STD and IEC STD associated to power transformer rating as shown on the table below.
For a 100MVA unit with 30 degree C average ambient temperature in 24 hrs. What should be the maximum rating if specified per IEEE or the same maximum rating if designed based on the IEC std.?

 

[cranky108]

I don't know, as some of your temps appear to be in Kelvin.

 

[cuky2000]

Hi Cranky108, for temperature rise, the diferencial in Kevin or Celsius is the same.
The Celsius and Kelvin scales increase by the same increments, meaning that to increase the temperature by 1°C results in the same increase of 1 K. The major difference is that when water freezes, a Celsius thermometer will read 0°C, and a Kelvin thermometer will read 273.15 K.

 

[7anoter4]

If the transformer is built as per IEEE/ANSI or NEMA instructions -certain materials and dimensions-if the ambient temperature is lower you may overload in a way the transformer without reducing its life.
The same, on a transformer built as per IEC standard you have to reduce the load if the ambient is upper.
The calculation has to follow IEEE std. C57.91
and IEC 60076-2 and IEC 60354, in my opinion.

 

[dpc]

I'm not sure what "Life ambient temperature" means. For ANSI, my understanding is that the rating is based on a 30 deg C max ambient average over 24 hours.

 

[cuky2000]

The IEEE C57.12.00-2000 standard, power transformers are rated at the average ambient temperature not to exceed 30 degrees Celsius in any 24 hours.
The IEC standard 60076-2 specifies 20 oC for the average annual temperature and 30 oC for the average monthly hottest temperature.
Both standards limit the maximum ambient temperature to 40 oC.

QUESTION; what is the actual MVA net rating for a transformer built per the IEC standard rated for 100 MVA ONAN, operated in a tropical country with an annual average temperature of 30 oC, without exceeding 40 oC any day and 30 of C hottest monthly average temperature.

Below is an edited graph from the IEEE std that estimates the MVA net rating for different ambient temperatures. I was unable to estimate the above question based on the IEC but suspecting that the transformer's actual rating is less than 100 MVA. Please advise.

 

[prc]

Please see https://www.linkedin.com/pulse/rated-power-transformers-p-ramachandran

The temperatures you are referring to need to be corrected.

 

[cuky2000]

Thanks, prc. I found another link that provides a dereating curve per the IEC Standard practice. The example shows that a 30 MVA transformer built per IEC an operated at a 30 ambient temperature is equivalent to a 27 MVA unit. Therefore, appears that the transformer built per the IEEE standard performs better in terms of the MVA capacity at the same ambient temperatures than the one built per the IEC standard.

 

[prc]

Let me disagree.
IEC specifies
Temperature rises based on max ambient air temp of 40C, monthly average of 30C and annual average of 20C. In case any one or more of the parameters differs from above, temperature rises can be modified as specified in IEC std clause 6.3.2 and table 2.


As per ANSI C57.12.00-2006, the average winding temperature rise of 65C and hot spot winding temperature rise of 80C is specified based on a maximum ambient air temp of 40C and average daily ambient temperature of 30C ( 24 hour period)

For ambient conditions other than above, C57.12.91-2011 Loading guide, recommends as below for ambient temperatures from -30C to 50C.

Clause 6.3 &Table 3, for every degree reduction in average ambient temperature the rating can be increased by 1% of kVA rating and for every one degree rise in average ambient temperature, the load shall be decreased by 1.5 % of kVA rating. This is approximate variation in rating and for detailed calculation reference may be made to other clauses in C57.12.91

 

[cuky2000]

Yes, we can agree to disagree. Still, no response to the original question of whether or not a transformer built per IEC should be derated if operated above 20 C.

Several researcher consider that a transformer built per IEC should be derated if the transformer operate at ambient temperature above the standard parameters of 20 oC.

One of those researchers decrease transformer loading capacity at ambient temperatures above 20 °C as shown in the table below resulting that a 30MVA transformer loss 10.02% of its capacity went operated at ambient temperature of 30 oC or ~27 MVA. This is in agreement with the previous post.

 

[prc]

cuky, as per my under standing, the temperature rise difference between IEC and IEEE is only 2C. The reference ambient temperatures are same. IEC 20 C is the yearly average and not stipulated in IEEE.

As per IEC60076-2, the maximum permissible winding hot spot temperature rise is 78 C, over a max ambient of 40C, monthly max average of 30C over the hottest month, max 20C of yearly average. If any of the above ambient temperatures exceed for the site you are referring to, the winding hot spot temperature rise shall be reduced to that extent.

As per ANSI/IEEE C57.12.00, the maximum winding hot spot temperature rise is 80C over a maximum ambient temperature of 40C and the average maximum daily ambient temperature of 30C for any 24 hours.
The above temperature rises are for oil-immersed transformers.
As per IEC the rating is input to transformer. As per IEEE it is the output. These minor differences will not affect design or operation of transformer.

 

[cuky2000]

Here is a pictorial view of transformer ratings per IEEE and IEC from an old post

 

[Gr8blu]

@cuky from Saturday AM: In a tropical installation where max ambient is expected to be below 40 C and a (daily) average is expected to be below 30 C ... BUT the YEARLY AVERAGE is expected to exceed 20 C ... then yes, an IEC transformer needs to be derated where an North American (IEEE) one does not.

In a more temperate climate where the yearly average can be around 20 C (due to colder winter months, for example) - no derating required.

 

[cuky2000]

Hi PRC, I hearing what you said and thanks for your input. We are investigating the actual net rating of Power transformer operating in a tropical country with a fairly stable hot temperature with an average of 32 degree Celsius in contras with countries with cold winters and summers with extreme temperature variation.
We are observing that many transformers are operating beyond the nameplate rating and some of then have been failing beyond repair. Coincidentally the failure population is larger for IEC transformers than the one build for the IEEE standard.
We are suspecting that the root cause is because the IEEE ones are built with stronger parameter in line with the previous discussion.
In a similar manner we are finding that other equipments fallow the same pattern such as disconnect switches.
Will be great if other colleagues could bring some light to this issue.

 

[prc]

Cucky, as someone who has designed large power transformers according to IEC and IEEE standards for more than half a century, let me confirm that I was making the same cooling circuit regardless of the standard. The temperature rises (oil/winding/ hotspot ) are the same in both standards: IEC—60/65/78 C and IEEE—65/65/80 C. Tests were done to check temperature rises do not exceed these limits.
The site failures you reported may have nothing to do with the standard. Of course, there is one point—in the US, you are insisting on thermally upgraded paper for conductor covering while IEC uses standard kraft paper. Thermally upgraded paper is claimed to have a 10 C higher thermal withstand capacity for the same life span.

 

[cuky2000]

How it can be explained that transformers built per IEEE can be overloaded up to 200% and the one built per IEC only 150% as indicated in the table below. Considering this data along, we can infer that IEEE is built somehow stronger than the IEC and for the same price y will be incline to recommend that for application with continuos ambient temperature above 30 C. If you believe that IEC transformers are better under the above scenario, fill free to let us know your thoughts.

 

[cuky2000]

To facilitate the discussion of this interesting topic, enclosed is a graph with our interpretation of the transformer loading capability for normal operating conditions ONAN
& ONAF. The IEEE data is based on the reference in the curves and the IEC data is based on the table provided in the study referred in the previous post above.

If anyone has better data, please feel free to share that with us. Thanks.

 

[Gr8blu]

@cuky: "How it can be explained that transformers built per IEEE can be overloaded up to 200% and the one built per IEC only 150% ..."
Answer : Because the majority of the manufacturers build their equipment for a specific user base, and know how they tend to operate. This is not limited to transformers by the way - it applies to pretty much all forms of electrical and mechanical equipment. It is not uncommon for North American manufacturers to supply "heavy duty" designs for industrial applications, whereas European manufacturers tend to provide "light duty" equipment. This not only shows up in long term reliability and lifespan, but in allowable transient loading conditions.

Just one example: a North American manufacturer will put a safety factor of 4 on top of a "peak torque" rating of 10 per unit when designing a shaft for a reversing application. The European manufacturer will supply a safety factor of 2 on a peak torque of 4 per unit. Why? Because the European application is (typically) for cold rolling; the North American version is (typically) for hot rolling. Hot rolling is harder on the equipment, so it sees higher frequency of occurrence and higher peak values for transient overloads.

 

[cuky2000]

Hi Gr8blu,
Thanks for your input. Regarding the transformer design, both of us share the same idea that the one built per IEC need to be derated for operating average temperature above 20 oC that impact the hot spot limit of 98 oC by the standard while the one built per IEEE STD are more robust because the hot spot limit is higher up to 110 oC.
The graph below probably explains better the concept associated with the IEC rating for continuous load and overload conditions.
Note that 98 °C per IEC and 110 °C per IEEE are assumed when the insulation paper is used with mineral oil.

 

[7anoter4]

In IEC 60354/1991 standard it is a computer program for oil filled transformer ageing on paragraph 2.8

IEC 60354 standard Loading guide for oil-immersed power transformers

My edition is from 1991 and there are some tables.

In Table 6 is noted the results of calculation for a proposed transformer.