A phase-locked loop (PLL) based on the synchronous reference frame (SRF-PLL) is the most widely used synchronization technique for three-phase power systems. To synchronize with grid voltage, the magnitude, phase angle and frequency of the fundamental positive-sequence component (FPC) need to be known. To that end, one of the most important aspects to be considered in the control of these power converters is fast and exact synchronization with grid voltages at the point of common coupling (PCC), even if the grid voltage is unbalanced and severely distorted. WTGSs are only allowed to be disconnected from the grid when the grid voltage drops below a lower limit protection value, which means that a WTGS must has the ability of fault ride-through. For instance, in many countries a WTGS needs to be on grid and to provide reactive power to support the grid voltage even under grid fault conditions. On the other hand, the grid-interfaced converters used in DG systems should be carefully controlled to meet the strict grid codes. They are expected to be an effective solution to help ease the cost of energy, energy security and relevant environmental issues introduced by conventional large centralized power generators.
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Nowadays, more and more renewable energy-based distributed generation (DG) systems are being connected to utility grids through grid-interfaced converters -, such as wind turbine generator systems (WTGS), solar energy generating systems (SEGS), etc. Experimental results verify the excellent performance of the proposed synchronization method. To show the advantages of the HDN-FLL, a simulation comparison with other gird synchronization methods is carried out. In this paper, the transfer function expression of the general HDN is deduced and its stability is verified by the root locus method. However, the stability of the general HDN is seldom discussed. The structure of the HDN has been widely used as a prefilter in grid synchronization techniques.
In addition, the linearization and parameters tuning of the FLL is also discussed. The stability of the proposed FLL is strictly verified to be global asymptotically stable. The HDN-FLL is based on the harmonic decoupling network (HDN) consisting of multiple first order complex vector filters (FOCVF) with a frequency-locked loop (FLL), which makes the system frequency adaptive. This paper presents an alternative frequency adaptive grid synchronization technique named HDN-FLL, which can accurately extract the fundamental positive- and negative-sequence components and interested harmonics in adverse three-phase grid voltage.
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