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Reducing frequency and increasing voltage accordingly for 50 Hz power system would have no effect as i know.b/c as you decrease the frequency level to 50Hz power system, the voltage level also increase accordingly to 220-230V.So there will no effect on operation of T/F but only increase core losses. You can operate the transformer at a higher frequency. However, the operating flux density will become less than its rated value. Therefore, derating will be necessary.. Further, iron losses will be more leading to lesser efficiency. Voltage regulation also will become more. Overall, the performance will be comparatively poor.