Rated Power of Transformers

During the initial years of transformer development, transformer power rating was specified as output, in kW (sometimes in HP) as the case with other electrical machines like motors and alternators. Typical transformer ratings in 1893 were 1.875- 3.75-7.5-11.25-15 kW with primary voltage of 2.4 kV (Reference: Book-Alternate current Transformers – R W Weekes) By around 1920, engineers appreciated that transformer performance (load losses, voltage regulation) is not dependent on kW but on kVA delivered at that power output. Since then, transformer rating was always referred as kVA or MVA. In British Standard BS171:1927 Transformer Specifications, it was defined:” the rating of the transformer is the output in kVA assigned to it by the maker, together with the associated conditions marked on the rating plate”. The recommended three phase kVA ratings were 5-7.5-10-15-20-25-30-40-50-75-100-150-200-250-300-400 kVA and multiples of 10 &100.

It was in the 1970 edition of British Standard BS 171 on Power Transformers that power rating was changed from output to input kVA. IEC world continues to follow this norm while IEEE standards and American world follow the old practice ie transformer rating refers to output kVA of the unit like in motors.

As per IEC 60076-1 ed3.0-2011, Power Transformers (General), “Rated Power is the conventional value of apparent power assigned to a winding which together with the rated voltage of the winding determines its rated current. (Clause 3.4.6) Both windings of a two-winding transformer have the same rated power.” For ratings up to 20 MVA, rated power should preferably be taken from the R10 series given in ISO 3:1973 Preferred Numbers-Series of preferred numbers. It means ratings between 1-10 is in ten steps each increment being multiplied by 1.26 (tenth root of 10) ie 1-1.25 -1.6 -2 - 2.5 -3.15 – 4-5 -6.3 - 8 -10 kVA or MVA and their multiples by 10 &100. In fact, this also results in a large variety and many utilities go for R5 series (5 steps in multiples of fifth root of 10=1.58) ie 1-1.6 - 2.5 - 4 - 6.3- 10 kVA or MVA and multiples of 10 &100 for higher ratings. Some utilities restrict the sizes still further by selecting only 2-3 ratings from the above band.

When transformer rating goes up, from say, 10 MVA to 20 MVA, the weights, losses and cost of transformer will not be doubled but increase by only 1.7 times (2 raised to 0.75) Hence for a fixed load, it is always economical to select as large a transformer power rating as possible instead of putting two in parallel. Of course, from the standby spare considerations, different selection criteria may be required. Normally (N-1) transformers should meet the anticipated peak load in a station where N is the total number of transformers in the station and the balance transformers will be able to meet the load even with the failure of one unit. The maximum peak load requirement at any station is decided by considering the total connected load with consideration of demand and diversity factors. When transformers are selected for step down mode, the maximum unit transformer size is limited by the short circuit current rating of the secondary breaker. This is the reason, in early days, several voltage step downs were necessary before feeding the distribution transformer near the user – 220 to 132 kV and then 132 kV to 66 kV and then finally 66 to 11 kV to feed distribution transformers. Thanks to the developments in MV breaker technology, today 400/33 kV direct stepdown is possible with 150-200 MVA transformer rating to meet concentrated heavy loads. eg 160 MVA 220/33 kV units to meet concentrated loads as in refineries; or 160 MVA 33/400 kV or 300 MVA 33-33/400 kV step up collector units in solar farms) In this way, number of voltage step-downs in transmission system can be reduced, bringing down the share of transformer loss  in the  total transmission loss.  

Rated power as per clause 6.4.1 of IEEE Standard C57.12.00-2015- “the rated kVA of a transformer shall be the output that can be delivered for the time specified at rated secondary voltage and rated frequency without exceeding the specified temperature rise limitations under prescribed conditions of test and within the limits of established standards.”  Preferred three phase ratings as per this standard are 1-1.5-2-2.5-3.75-5-7.5-10-12-15-20-25-30-37.5-50-60-80-100 MVA. As per IEEE (C57.12.10 -2017 Standard requirements of Liquid Immersed Power Transformers), the rated kVA of the transformer shall be based on its capacity at rated base kVA cooling stage (ONAN). When fans and/or pumps are added to the transformer (forced cooling- ONAF /OFAF), its rating shall be increased by a percentage. Increased ratings for forced air cooling (ONAF) is 125 % for 2.5-10MVA transformers. Increase in rating at forced cooled conditions, for higher ratings are 133 % of ONAN rating for ONAF cooled condition and 167 % for OFAF cooling. So, as per IEEE, efficiency, losses and impedances are also reported on ONAN rating while in IEC world, rated power is based on maximum MVA under forced cooling. In IEC, ONAN rating is expressed as a percentage (70 or 80%) of rated power ie maximum rating under forced cooled conditions. eg.70/100 MVA.

To sum up, if there is a 10/ 12.5 MVA ONAN/ONAF transformer as per IEC, then 12.5 MVA is the rating (input power) and all parameters (percentage impedance; losses at 750C; energy efficiency) are based on 12.5 MVA. As per IEEE, for the above transformer 10MVA is the rating (output power) of the transformer and the base for all parameters (percentage impedance, losses at 850C; energy efficiency) will be on 10 MVA base.

In case of distribution transformers and sub-transmission transformers, the kVA rating is decided as below:

Rating in kVA = Factor 1.10-1.25 (to take care of future demand increase) x total connected load x Demand Factor / Diversity Factor

Where Demand Factor= Maximum Peak Demand/ Total connected load < 1.0

          Diversity Factor= Sum of individual maximum demand of feeders/ Maximum Demand of the whole system > 1.0

When these transformers are with harmonic load (mainly due to power electronics), the power rating is to be reduced based on the extent of harmonic content in the load. Alternately transformer manufacturers can make special units to meet extra losses from the specified volume of harmonics. These are termed as K-rated transformers. K-factor is a weightage for the harmonic content in load according to their effects on transformer heating, as derived from ANSI/IEEE C57.110. A K-factor of 1.0 indicates a linear load (no harmonics). The higher the K-factor, the greater the harmonic heating effects. As an example, K- 17 transformer rating will be 28 % more than the nominal rating and hence load tested for 128 % rating.

When rating of generator step up transformers are selected for thermal power stations, MW and MVAR of Generator(unit) transformer is the net of generator out puts minus the requirements for auxiliary loads as met through auxiliary supply transformers. Hence a 200 MW set may use 240 or 250 MVA unit transformer and 500 & 800 MW sets may use 600 MVA and 1000 MVA transformer banks. But generator transformer ratings in hydro power stations cannot be standardized and will vary depending on the generation capacity based on water volume and head availability at each location.

Transformers connected to solar or wind farms will be subjected to full rating only part of the day. Hence nominal rating can be  less than the peak loading, roughly up to 20 % in case of solar and 10 % for wind.

It should be remembered that rated kVA of a transformer depends on the allowable maximum temperature rises for oil and winding. As per IEC, for an ambient condition of 20/30/40 C (yearly average/monthly average /maximum) allowed maximum temperature rises are 60/65/78 C (top oil / average winding/ winding hot spot) If the ambient conditions at any location is more than any of the above three ambient temperatures, all the three temperature rises are to be reduced by the same extent of rise in ambient temperature.   As an example, ambient temperature conditions in India are 32/40/50C and hence temperature rises are reduced as 48/53/66 C or rounded off to 50/55/ 66C as per IS. IEC Transformer Loading guides give directions for the allowable rating for such service situations. Under normal cyclic over loadings, the hot spot temperature inside winding shall not exceed 120 C under maximum ambient temperature or overload conditions.

Temperature rise of oil will go up at higher altitudes due to poor convection cooling from the lower density of air; ie transformer rating shall be reduced or more cooling shall be provided to limit temperature rises to the standard limits. As per IEC standards, permissible temperature rises are for a site elevation of 1000 meters or less. Temperature rises shall be reduced by 1 K for every interval of 400 meters by which site elevation exceeds 1000 meters above sea level for naturally cooled transformers (XXAN) and for every interval of 250 meters for forced air cooled transformers (XXAF)