Five papers at the IEEE CAMAD 2018!

Five papers from the CONNECT consortium have been included in the proceedings of the IEEE CAMAD 2018!

The first paper entitled “Frequency Estimation for Grid-connected Converters under Time-varying Harmonics and Inter-harmonics” by E. B. Kapisch, C. A. Duque, G. Tibola and J. L. Duarte, which presents an improved scheme for grid fundamental frequency tracking. A comparison under frequency variation and time-varying harmonic and inter-harmonic condition is also discussed in order to show the method robustness. The scheme is based on a frequency-locked loop operating integrated with a decomposition core, which utilizes a recursive implementation of a modified Hanning filter. The frequency value information is used to perform the modulation and demodulation required to make de decomposition into the core. The compared frequency estimation scheme is based on a zero-crossing with an interpolation correction. Simulation results show that the proposed scheme is able to estimate the fundamental component, including relative quadrature companion signal and the frequency value more accurately and stably than the scheme used to perform the comparison.

The second paper entitled “A CMOS RF-Powered Tag with Sensing and Localization Capabilities” by C. Tamburini et al., presents a battery-less CMOS integrated RF tag with sensing and localization capabilities. In order to allow tag addressing, communication, and localization by external readers, the IC performs a back-scattering modulation of a sequence of received UWB pulses according to specific spreading codes. For this purpose a UWB RF switch is designed and connected to the UWB antenna. The adoption of backscattering is integral to achieve ultra-low power wireless communication. The use of quasi-orthogonal spreading codes allows communication and localization with multiple tags at the same time. The IC includes a low-power sensing interface suitable for internal temperature sensing or external capacitive sensors. The power required for operation is scavenged from an UHF signal and converted by means of an internal RF rectifier and of a dynamically reconfigurable charge pump circuit. An on-chip micropower oscillator clocks a digital control circuit. The circuit is implemented in a 0.18 μm CMOS process. The architecture of the IC and preliminary test results are disclosed.

The third paper entitled “PV Cell Characteristic Extraction to Verify Power Transfer Efficiency in Indoor Harvesting System” by L. Perilli, L. Perugini, M. Pizzotti, E. F. Scarselli and R. Canegallo, proposes a method to verify the efficiency of low-power harvesting systems based on Photovoltaic (PV) cells for indoor applications and a Fractional Open-Circuit Voltage (FOCV) technique to track the Maximum Power Point (MPP). It relies on an algorithm to reconstruct the PV cell Power versus Voltage (P-V) characteristic measuring the open circuit voltage and the voltage/current operating point but not the short-circuit current as required by state-of-the-art algorithms. This way the characteristic is reconstructed starting from the two values corresponding to standard operation modes of dc-dc converters implementing the FOCV Maximum Power Point Tracking (MPPT) technique. The method is applied to a prototype system: an external board is connected between the transducer and the dc-dc converter to measure the open circuit voltage and the voltage/current operating values. Experimental comparisons between the reconstructed and the measured P-V characteristics validate the reconstruction algorithm. Experimental results show the method is able to clearly identify the error between the transducer operating point and the one corresponding to the maximum power transfer, whilst also suggesting corrective action on the programmable factor of the FOCV technique. The proposed technique therefore provides a possible way of estimating MPPT efficiency without sampling the full P-V characteristic.

The fourth paper entitled “Plug and Play DC-DC Converters for Smart DC Nanogrids with Advanced Control Ancillary Services” by G. Liu, A. Khodamoradi, P. Mattavelli, T. Caldognetto and P. Magnone, gives a general view of the control possibilities for dc-dc converters in dc nanogrids. A widely adopted control method is the droop control, which is able to achieve proportional load sharing among multiple sources and to stabilize the voltage of the dc distribution bus. Based on the droop control, several advanced control functions can be implemented. For example, power-based droop controllers allow dc-dc converters to operate with power flow control or droop control, whether the hosting nanogrid is operating connected to a strong upstream grid or it is operating autonomously (i.e., islanded). Converters can also be equipped with various supporting functions. Functions that are expected to play a crucial role in nanogrids that fully embrace the plug-and-play paradigm are those aiming at the monitoring and tuning of the key performance indices of the control loops. On-line stability monitoring tools respond to this need, by continuously providing estimates of the stability margins of the loops of interest; self-tuning can be eventually achieved on the basis of the obtained estimates. These control solutions can significantly enhance the operation and the plug-and-play feature of dc nanogrids, even with a variable number of hosted converters. Experimental results are reported to show the performance of the control approaches.

Finally, the fifth paper entitled “Why Use RF Energy Harvesting in Smart Grids” by F. M. Cruz, A. E. Molero, E. Castillo, M. Becherer, A. Rivadeneyra and D. P. Morales, presents an innovative technique for energy harvesting in the area of smart grid at the wireless sensor nodes through radio frequency (RF) electromagnetic waves: RF energy harvesting, making them energetically autonomous. Several specific use-cases and applications are proposed where nodes with RF energy harvesting have advantages respect batteries, wires or other methods. A general node hardware structure for their development and a special wireless communication protocol for energy optimization in this kind of nodes are proposed as well.