A42-Transformer Performance Parameters-Conversion Factors for 50 Hz to 60 Hz

1.???According to a 1975 US Report, out of 245 countries considered around the world, 75 % of countries operated their electrical grid at 50 Hz; 20 % operated at 60 Hz (included mighty US grid) and balance 5 % had both 50 & 60 Hz in parts of their grid. (eg. Japan). Transformer manufacturers sometimes have to supply transformers for a market with frequency different from the frequency of equipment available at their test lab. Indian manufacturers supplying 60 Hz transformers to US may not have test facilities to test transformers at 60 Hz. Then they will be testing transformers designed for 60 Hz at 50 Hz and applying conversion factors to get performance parameters for 60Hz. Some transformer factories do have facilities to test transformers both at 50 & 60 Hz frequencies. Others can use these conversion factors to get transformer performance parameters at different frequency than the tested frequency.

2.?IEC standard on transformers do not give the performance conversion factors for different frequencies. But IEEE standard gives such conversion factors in C57.12.90-2015 -Annexure B. These are based on the extensive study conducted around 2001 by the ABB team under Dr Ramis Girgis (St. Louis factory, USA)

Introduction to Annexure B of above standard mentions: “It is preferred to perform tests at the rated frequency of the transformer to be tested. However, tests cannot always be done at the rated frequency. In such cases, upon agreement between the purchaser and the manufacturer at the tender stage, the conversion factors given in this annex shall be used to convert the tested values from the frequency used for test to the required rated frequency. The purpose of the frequency conversion factors is to have uniformity among manufacturers when such cases arise. These conversion factors are intended for 50-Hz-to-60-Hz conversion; however, the 60-Hz-to-50-Hz conversion factors are essentially the reciprocal of the 50-Hz-to-60-Hz factors as shown in this annexure “

3.????Before we go to the conversion factors, let us see the implication of a 60 Hz transformer getting connected to 50 Hz supply. As per the well-known transformer equation,

V/N = 4.44 f Bm A where V/N =volts per turn; f=frequency Hz; Bm= maximum flux density in core T; A= effective area of core in square meter.

From the above it is clear that when we connect transformer to 50 Hz, Bm must increase by 20 % as frequency gets reduced by 20 %. This will result in core saturation unless Bm under 60 Hz is below 1.4T. Hence, we can use a 50 Hz transformer in 60 Hz supply (of course, %impedance will go up by 20 %) but the reverse may not be feasible in most cases. In such cases the voltages are to be reduced by 1.2 to keep the core flux density within limits. It also means rated kVA gets reduced by 1.2.

4.????Impact of frequency on percentage impedance

Leakage inductance of transformer windings depends on geometric dimensions of winding + number of turns. Hence inductance remains the same at both 50 & 60 Hz. Reactance in ohms= constant x inductance x frequency, varies as frequency. Percentage or per unit impedance = Actual impedance in ohms/ base impedance of transformer. Base impedance = impedance of rated load (kV x kV/MVA). Since base impedance (Rated Voltage/Rated current) remains the same under different frequencies, percentage impedance varies linearly with frequency for large transformers above 10 MVA. For smaller ratings X/R will be low and hence increase in impedance will be slightly lower.

5.????Conversion factors as per IEEE standard:

Notes:

1.????No-load loss and exciting current to be measured at 5/6 of the rated voltage for 60 Hz (To maintain the same core flux density)

2.????Percentage Impedance factor slightly less for < 10 MVA rating due to lower X/R ratio

3.????When sound level measured, apply 5/6 rated voltage at 50 Hz + coolers at 60 running at 60 Hz.

For additional information:

1)???R. S. Girgis, B. Beaster, and E. G. teNyenhuis, “Proposed standards for frequency conversion factors of transformer performance parameters,” in Proc. IEEE Power Eng. Soc. Transm. Dist. Conf. Expo., vol. 1, Oct. 28–Nov. 2, 2001, pp. 153–158.

2)???Ramsis Girgis, and Ed te Nyenhuis, 50 Hz to 60 Hz Conversion factors for transformer performance factors, Presentation at IEEE/PAS Spring 2002 Meeting

3)???IEEE Std C57.12.90-2015; IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformer- Annexure B- 50/60-Hz frequency conversion of measured performance parameters.

4)???K. R. M. Nair, Power & Distribution Transformers, Practical Design Guide, Section 30.7, Pages 408-411, CRC Press, 2021