In uninterruptible power supplies (UPS) using an automatic bypass switch, to reduce the transients due to the switch commutations, it is convenient that the voltage waveforms produced by the inverter are synchronized with the grid ones. In three-phase UPS, the inverter controller employs, as reference signals, the components of the grid voltage phasor, referred to a fix reference frame, suitably multiplied by an amplitude correction factor. Generally, the reference signals are affected by harmonics and amplitude unbalances. Therefore, improper synchronization between the inverter outputs and the grid voltages can arise. To improve the synchronization, many solutions based on a phase locked loop (PLL) system have been proposed in literature. The paper proposes a different PLL structure employing a Steady-State Linear Kalman Filter (SSLKF), based on a third-order linear and time-invariant observation model. Such filter is able to provide an accurate tracking of the grid voltage phasor also in critical operating conditions. The paper describes, at first, different PLL structures and then the proposed architecture based on a prediction-correction filter and its implementation on a DSP controller. Finally, a comparison, obtained by simulation, with traditional filters and some significant experimental results on a pre-production prototype are carried out. © 2008 IEEE

This paper presents a novel dedicated PWM technique for use with single-phase multi-level Cascaded H-Bridge Converters. The proposed modulation strategy aims to minimize the converter commutations and distribute them, for any amplitude of the voltage reference, amongst the different converter cells in order to evenly distribute the heating and stress on the power switches, improving their reliability without compromising the quality of the voltage waveform. Such characteristics are particularly important in high power grid-connected converters used as an interface for renewable energy sources or Smart Grid applications. The technique is described in detail along with the verification of the modulation effectiveness provided by a simulation comparison with other common modulation strategies. Finally, the proposed strategy is experimentally validated by means of different tests using a seven level Cascaded H-Bridge Converter

Microgrids represent one of the best candidates to redesign the architecture of the future distribution network. Such a model, in fact, promises to be able to properly manage a larger penetration of Distributed Energy Resources while improving the local production and utilization of energy. On the other hand, the control of a Microgrid is quite challenging, since the system has to be particularly reliable and flexible, due to the requested multiple and concurrent tasks to be performed. In this scenario the present paper proposes a novel solution able to synchronize the phase and the frequency of all the devices connected to the Microgrid in many different and complex ways. The proposed method, in particular, is based on the combined use of Phase Locked Loops and the 1 Pulse Per Second signal provided by any GPS device

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