E1=4.44 f OmaxN1 (where, f the frequency of supply voltage, Omax  the peak flux in core produced by the magnetizing current, N1 is the primary turns) is a induced EMF equation for transformer primary, when voltage(V) is applied, and is in opposition to V(E2 = E1(N2/N1).  Thus if the turns and flux wave(magnetizing current) remains constant , the induced EMF will be directly praposnal  to frequency, hence for the rated current, KVA of transformer would be directly praposnal to frequency. No load Inductive reactance (X0 ) will change with frequency as X=2(pai)fL which will reduced the magnetizing current for the applied voltage V , hence  E1. Yet in power ferequency range the overall KVA capecity will be raised with increased in frequency.

See wikipedia T- or Г-equivalent circuit for transformer 

http://en.wikipedia.org/wiki/Transformer

http://www.allinterview.com/showanswers/117346/frequency-affect-kva-rating-transformer-if-500-400-v-60-hz-t-f-operated-50-chang.html

Please see this link

http://www.electricaltechnology.org/2013/07/how-to-calculatefind-rating-of.html

frequency is directly proportional to the kVA rating..

if we have 25kVA , 50Hz t/f and if we are intrested to operate it on 100Hz frequency then its kva rating will be 50kVA