Hardware

Good afternoon~

My transformer can electrolysis the capacitor through rectifier bridge by output two 18V (three threads in here) and then give LM7812(this is its datasheet:http://www.kynix.com/uploadfiles/pdf9675/LM7812CT.pdf )and 7**2 electricity.Now my trouble is :
When the outputted voltage of transformer passing rectifier bridge ,the outputted voltage is 18V normally.However,when the voltage became odd after going through electrolytic capacitor--50V1000uf. The positive voltage rose into 28V from 18V. This situation leads to the 7812 burned.Why? How to solve ?

Thanks a lot!