Cause : Overvoltage comes when the voltage level in the DC bus (direct current bus) has exceeded the acceptable operating range. 

The A021 alarm in siemens master drive indicates that Overvoltage condition. This means the DC voltage within the drive has risen above its acceptable operating threshold. This can lead to various issues, including potential damage to the drive's internal components and malfunctioning operation.

1. Check the supply voltage.
2. The converter regenerates without regeneration possibility.