Method for measuring the rate of change of frequency (ROCOF-CNT) of a waveform related to a converter unit of a wind turbine generator, said method comprising the steps of: - measuring an instantaneous frequency value of said waveform related to said converter unit, characterised in that the method further comprises the steps of: - calculating a first filtered value (ROCOF-F1) of a rate of change of frequency (ROCOF) based on said instantaneous frequency value, said first filtered value (ROCOF-F1) having a measurement error lower than a first predetermined measurement error (DB-ROCOF-F1), said first filtered value (ROCOF-F1) having a time response slower than a predetermined time response requirement, - calculating a second filtered value (ROCOF-F2) of said rate of change of frequency (ROCOF) based on said instantaneous frequency value, said second filtered value (ROCOF-F2) having a second measurement error (DB-ROCOF-F2) higher than said first predetermined measurement error (DB-ROCOF-F1), said second filtered value (ROCOF-F2) having a time response faster than said predetermined time response requirement, and - calculating a comparison value (Comp) between said first filtered value (ROCOF-F1) and said second filtered value (ROCOF-F2), and - determining that an output (ROCOF-CNT) value of said measured rate of change is zero when said comparison value (Comp) is equal to a predetermined threshold level, and - determining that an output (ROCOF-CNT) value of said measured rate of change is based on said first filtered value (ROCOF-F1) when said comparison value (Comp) stays lower or becomes lower than a predetermined threshold level, and - determining that said output (ROCOF-CNT) value of said measured rate of change is based on said second filtered value (ROCOF-F2) when said comparison value (Comp) becomes higher than said predetermined threshold level, wherein - said predetermined threshold is determined based on said second predetermined measurement error (DB-ROCOF-F2). - measuring an instantaneous frequency value of said waveform related to said converter unit, characterised in that the method further comprises the steps of: - calculating a first filtered value (ROCOF-F1) of a rate of change of frequency (ROCOF) based on said instantaneous frequency value, said first filtered value (ROCOF-F1) having a measurement error lower than a first predetermined measurement error (DB-ROCOF-F1), said first filtered value (ROCOF-F1) having a time response slower than a predetermined time response requirement, - calculating a second filtered value (ROCOF-F2) of said rate of change of frequency (ROCOF) based on said instantaneous frequency value, said second filtered value (ROCOF-F2) having a second measurement error (DB-ROCOF-F2) higher than said first predetermined measurement error (DB-ROCOF-F1), said second filtered value (ROCOF-F2) having a time response faster than said predetermined time response requirement, and - calculating a comparison value (Comp) between said first filtered value (ROCOF-F1) and said second filtered value (ROCOF-F2), and - determining that an output (ROCOF-CNT) value of said measured rate of change is zero when said comparison value (Comp) is equal to a predetermined threshold level, and - determining that an output (ROCOF-CNT) value of said measured rate of change is based on said first filtered value (ROCOF-F1) when said comparison value (Comp) stays lower or becomes lower than a predetermined threshold level, and - determining that said output (ROCOF-CNT) value of said measured rate of change is based on said second filtered value (ROCOF-F2) when said comparison value (Comp) becomes higher than said predetermined threshold level, wherein - said predetermined threshold is determined based on said second predetermined measurement error (DB-ROCOF-F2). - calculating a first filtered value (ROCOF-F1) of a rate of change of frequency (ROCOF) based on said instantaneous frequency value, said first filtered value (ROCOF-F1) having a measurement error lower than a first predetermined measurement error (DB-ROCOF-F1), said first filtered value (ROCOF-F1) having a time response slower than a predetermined time response requirement, - calculating a second filtered value (ROCOF-F2) of said rate of change of frequency (ROCOF) based on said instantaneous frequency value, said second filtered value (ROCOF-F2) having a second measurement error (DB-ROCOF-F2) higher than said first predetermined measurement error (DB-ROCOF-F1), said second filtered value (ROCOF-F2) having a time response faster than said predetermined time response requirement, and - calculating a comparison value (Comp) between said first filtered value (ROCOF-F1) and said second filtered value (ROCOF-F2), and - determining that an output (ROCOF-CNT) value of said measured rate of change is zero when said comparison value (Comp) is equal to a predetermined threshold level, and - determining that an output (ROCOF-CNT) value of said measured rate of change is based on said first filtered value (ROCOF-F1) when said comparison value (Comp) stays lower or becomes lower than a predetermined threshold level, and - determining that said output (ROCOF-CNT) value of said measured rate of change is based on said second filtered value (ROCOF-F2) when said comparison value (Comp) becomes higher than said predetermined threshold level, wherein - said predetermined threshold is determined based on said second predetermined measurement error (DB-ROCOF-F2).