The transformer cannot be operated more than 10% higher than the rated voltage because if the primary is high therefore secondary also increases which leads to increase in power rating of a transformer. This increases the heat losses and may damage the transformer. The second one is that if the voltage is increased beyond the rated value at rated frequency then the flux will increased in the transformer
Flux = Voltage/frequency
The increase in flux leads to core saturation and heating affect and degrades the transformer performance.