Considering the continuous evolution of electricity grid and the impact on it of new players such as renewable generators, electric cars, etc., Power Quality phenomena are increasingly the subject of attention by Regulators and Network Operators due to their impact on equipment; this requires continuous and accurate monitoring of voltage and current signals on the electricity grid. The measurement equipments are connected to the network using instrument transformers called Current Transformers (CT) or Potential Transformers (PT). These transducers are already installed in enormous quantities in the Transmission and Distribution networks to meet the needs related to the protection and measurement functions. However, inductive instrument transformers present non-idealities due to the ferromagnetic core, in terms of linearity in amplitude, frequency and hysteresis, which can compromise their accuracy in frequency. To overcome this problem, an algorithm for compensating the non-idealities of inductive instrument transformers has been developed, based on the construction parameters (resistances and dispersion inductances of the windings, turns ratio and hysteresis cycle of the ferromagnetic material), capable of reconstructing the primary mains voltage accurately. This report presents an experimental validation of the first developed algorithm and its extension. Taking into account the availability and diffusion of new devices, called Stand-Alone Merging Unit (SAMU), designed to adapt the analog signals, output from the inductive measurement transformers, to the digital environment of the station and cabin based on the communication protocols of the IEC 61850 family, the project of a new prototype of SAMU-NG (Next Generation) with innovative functions is also reported, with particular reference to the possibility of implementing internally the compensation algorithms for the non- idealities of the developed instrument transformers. This would make it possible to make measurement data available that are as accurate as possible and aimed at faithfully reproducing voltages and currents present in the Medium and High Voltage network. Finally, the study of a new sensor for the measurement of mains voltage has been started, based on optical technology, free from the non-idealities of inductive instrument transformers and with a theoretical measurement bandwidth in the order of GHz, suitable for provide an accurate voltage measurement in the desired frequency ranges, using dedicated electronic cards.