For synchronization applications, synchronous reference frame (SRF) phase-locked loop (PLL) is widely deployed. Its performance is excellent when the input voltage consists of only fundamental positive sequence (FPS) component. If the grid voltage is unbalanced or polluted with harmonics and dc offset, its performance degrades. Many modifications were proposed to address this issue. However, the removal of dc offset and fundamental negative sequence (FNS) component without compromising the dynamic performance still remains a challenging task. To this end, this paper presents a rapid Type-1 SRF PLL scheme with preloop filtering stage for tracking the attributes of grid voltage FPS component. Fixed sampling period sliding discrete Fourier transform (SDFT) and instantaneous symmetrical components method are employed in the preloop stage. With this modification, the dc offset, harmonics and the FNS component are rejected and only the FPS component enters the PLL. As a result, transients vanish quickly. However, when the grid frequency drifts, SDFT causes amplitude and phase errors, and Type-1 PLL introduces a steady-state tracking error in phase. These errors are compensated with the help of an error correction criteria. Robustness and the improved transient response of the proposed scheme are demonstrated with an experimental study involving real-time controller board (dSPACE DS1104) and three-phase programmable power source.

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The Gulf of Mannar (GoM), India, includes 21 coral islands (8°N; 79°E), covers an area of 10,500 sq. km and supports 94 species of corals belonging to 37 genera. Kappaphycus alvarezii, a Philippine-derived rhodophyte, has been introduced into the GoM for commercial cultivation in 2002. The ecological threat from this invasive alga to coral species in GoM was first indicated by Pereira and Verlecar (2005). After 6 years of its introduction, its bioinvasion on branching corals (Acropora species) in the Kurusadai island (9°15¢N; 79°12¢E) of GoM was reported in 2008 (Chandrasekaran et al. 2008). Consequently, commercial cultivation of this invasive alga was prohibited. A mechanical removal programme started in 2009 by the State Government at the invaded site could not deliver expected results. Thus, this alga enjoys a freedom of unrestricted spread and aggressive growth in GoM, predominantly on species of Acropora. On 28 April 2010, during our routine sampling visit at Kurusadai Island, unusual appearance of K. alvarezii, on the cup coral, Turbinaria sp. was observed (Fig. 1a). The space between the plates of Turbinaria sp. provides an ideal settlement surface for K. alvarezii, which protect them from the wave action and favour the profuse growth of secondary branches of K. alvarezii between the plates (Fig. 1b) in contrast to the smothering effect on the top of coral plates (Fig. 1c). This finding shows that K. alvarezii is capable of invading species of nonbranching corals as reported in Hawaii for Montipora capitata and Porites compressa (Conklin and Smith 2005). Therefore, perhaps this is the first report from India on bioinvasion of K. alvarezii on a non-branching coral (Turbinaria sp.) in the GoM.