In [31], the feature extraction from the combination of the wavelet transform and of a Fourier-based method is carried out for the monitoring purposes of the insulators. A comprehensive review of the methods employed for the fault detection, classification and location in distribution systems is presented in [32], where the feature extraction is the core of the fault detection. In this context, this paper proposes a methodology, based on Fast Fourier Transform (FFT), Wavelet Analysis (WA), HHT, and the Phase Coherence (PC), with the aim to support the Distribution System Operator (DSO) for the medium-term planning of the PDS having or not the grid-connected PV plants. The proposed methodology compares the different features extracted by the different methods applied to the same dataset of PDS, showing the most suitable analysis method--or a combination of several methods--to study the electrical behavior of the PDS. In this way, the DSO, after having classified the several PDS, can directly apply the most performing method/combination, without any preliminary evaluations. For these aims, we studied three different cases, having different characteristics: residential or commercial, containing or not PV plants. These PDS are operating, so the results are based on real data and not on simulated ones. A methodology able to classify the most suitable analysis method for each typology of the PDS under investigation is proposed. This paper gives a double contribute to the state of art: the phase coherence index to compare the similarity between two electrical signals and a methodology to choose the most effective analysis method for the PDS under investigation. In particular, the phase coherence index is a measure of coherence between two time dependent signals. In this paper, the phase coherence is used two times to accomplish two purposes. In the first time, the coherence degree between a reference signal and the signal under investigation is calculated, in order to evaluate the linearity of the electrical signal. This information is used to choice the most suitable analysis method of an electrical signal, as it will be explained later. In the second case, the phase coherence index is calculated between two electrical signals of different PDS (with and without PV plants), in order to estimate the different penetration level of the PV power. The monitoring of this value can be useful to prevent the major disturbances in distribution systems, as the voltage variation [33; 34] and the harmonic distortion [35]. Instead, the proposed methodology allows for selecting always the best analysis method (among HHT/FFT/WA) of the electrical signal. In fact, even if the FFT is the simplest and fastest method to use, it is able to extract all the operating modes of the electrical signal, only if it is linear and stationary. Otherwise, if the signal is linear but nonstationary, WA is more effective than FFT. Finally, if the signal is nonlinear, only HHT can extract all the operating modes. The paper is organized as follows. Section 2 details the procedure and introduces the Phase Coherence (PC) index. Section 3 describes a real case study--constituted by three different PDS-, applies the methods, discusses the results, and classifies the methods. The conclusions are reported in Section 4.