A protection system for an electrical variable speed doubly fed induction machine (IM) having a stator (S) and a rotor (R), comprising: -instrument transformers (CTR1, CTR2, CTS1, CTS2, VTR1, VTR2) capable of measuring low frequency signals from the rotor and/or the stator; -a protection relay (R1, R2) configured to receive measurement values from the instrument transformers sampled at a constant sampling rate, wherein the protection relay comprises a frequency determination module for determining the frequency of the rotor or stator low frequency signals, and at least one protection module configured to implement an algorithm making use of root mean square values each calculated from a series of rotor or stator sampled measurement values within a variable size window size of which depends upon the determined frequency of the rotor or stator low frequency signals.
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