Low voltage coming from rectifier unit

we r facing a problem in our plant,we have s7-300 plc(CPU 315-2DP)and simovert master drive ,yesterday plc got damage,after that we replaced it with new one and download the program now we r face a problem that in the HMI(simatic panel sreen touch)it showing fault message'LOW VOLTAGE' as 127 V but from the rectifier unit it isgiving the supply of 420v to all the drive i.e to motors and the HMI through which we control the speed to motor. we download the parameter in rectifier unit but also it is showing the same fault so can u please help me.