In spite of existing modern estimating techniques, the parameter estimation of non-linear and time-variant processes with noisy measuring signals is difficult. This is the case with frequency measurement in power engineering. In order to achieve good results under these conditions, a recursive parameter estimation method is derived from the non-recursive Newton Raphson method. Additionally, the forgetting factor adaptation procedure is suggested in the new method, which improved both the algorithm convergency and accuracy. By computer simulation results of applying the new improved method for the purpose of frequency measurement in power system protection are obtained. For numerous input signals the new improved method was successfully applied and shows better results in comparison with the basic method.