**6. Conclusions**

In this paper, a design approach for the realization of a very low cost PMU is presented. The core of the instrument is an ARM microcontroller with integrated ADC and Ethernet controller. External devices are used to realize the voltage and current sensing stage, a simple GPS receiver is used to receive the PPS and the timestamp and, in particular, no GPS disciplined oscillator has been used.

Since a key feature of the PMU is the accurate synchronization with UTC, in order to meet this requirement with a su fficient accuracy, and without specific synchronization hardware, an e fficient signal processing synchronization technique has been used, which is able to lock the sampling frequency to the UTC. To verify the robustness of the proposed design approach, a very common phasor estimation algorithm, the IpDFT, has been implemented onboard the microcontroller. Its features are the ease to implement and the quite low computational complexity.

The cost of the prototype is very low, about 110 €, obtained in this way: (1) the transducer cost is about 90 €, (2) the cost of the microcontroller development board is about 10 €, (3) the cost of the GPS receiver and the antenna is lower than 10 € and (4) the electronic components of the conditioning circuit is lower than 1 €.

The realized prototype has been tested with a high performance PMU calibrator, the Fluke 6135A/PMUCAL, in several testing conditions reported in [2,3]. The maximum values for TVE, FE and RFE are below the standard limits practically for every testing condition, except for interharmonic, and for RFE in the frequency ramp test for Class M PMUs. However, comparing the obtained results with those obtained from an IpDFT implementation on a high performance measuring hardware [38], nearly the same issues have been found.

**Author Contributions:** Conceptualization, M.L.; methodology, C.L. and D.G.; software, A.D.F.; validation, M.L. and A.D.F.; formal analysis, C.L. and D.G.; investigation, M.L., C.L., D.G. and A.D.F.; resources, M.L. and C.L.; data curation, A.D.F.; writing—original draft preparation, M.L. and A.D.F.; writing—review and editing, C.L. and D.G.; supervision, C.L.; funding acquisition, M.L. and C.L.

**Funding:** The work presented in this paper was funded by European Metrology Programme for Innovation and Research (EMPIR), 17IND06 Future Grid II project, which is jointly funded by the EMPIR participating countries within EURopean Association of national METrology institutes (EURAMET) and the European Union.

**Conflicts of Interest:** The authors declare no conflict of interest.
