Check that overvoltage controller is on (parameter 2005 OVERVOLT CTRL).
Check brake chopper and resistor (if used). DC overvoltage control must be deactivated when brake chopper and resistor are used.
Check deceleration time (parameters 2203 DECELER TIME 1 and 2206 DECELER TIME 2).
Check input power line for static or transient overvoltage.
Retrofit frequency converter with brake chopper and brake resistor.