Performances of a PLL based digital filter for double-conversion UPS

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.