ISSN: 2250-3005 - ijcer

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International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8previous one is (n−1). In Fig.(4), the minimum threshold (Kmin) of negative-sequence correction is defined according topractical demands, and a coefficient denoted by λ is introduced for smooth regulation when decreasing the output value of K VUF .Note that, for system protection, the current rating of the inverters is always checked before returningFig.(4) Flow chart of K VUF determination.B. Positive- and Negative-Sequence DetectionThe factor K VUF is essential to get the amplitude of negative-sequence currents. Thus the separation of sequencevoltages is central to get the value of K VUF , as well as to the synchronization with the utility grid. For unbalanced or distortedgrid voltages, a multi-variable filter was introduced in [12] for detecting the positive-sequence component in the stationaryframe. After modification, this filter is able to directly filter out the fundamental positive and negative -sequence vectors. Thefollowing mathematically demonstrates the multi- variable filter for symmetric sequence decomposition.For unbalanced distorted voltages, the positive- and negative-sequence components are in the α − β frame as expressed bywhere k denotes the harmonic number, ω1 denotes the fundamental radian frequency, and the superscript symbol “o” denotesconjugate.Let us look for a filter , which can damp all harmonic components of but the fundamental positive –sequencecomponent in the stationary frame. That is,where the “*” denotes a convolution product, andwithOtherwise stated the fundamental positive-sequence component of as defined in (2).By multiplying and with , respectively, which corresponds to a transformation to a positivesynchronous rotating frame (PSRF), we obtain from (2) and (3)(2)(3)(4)(5)||Issn 2250-3005(online)|| ||December||2012|| Page 203
International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8It can be seen that the fundamental positive-sequence voltage performs as a DC quantity in the PSRF. Therefore, a simplefirst order filter H (t) with= (6)where is the corner frequency, is sufficient to get from under the conditions of (4). This canbe expressed with(7)or, using Laplace(8)Substituting sinto (6) and (8), it follows that(9)From (3), we also have(10)Therefore, the filter we are looking for, in the stationary frame should be equal to(11)By expanding (10) to(12)the following equations are derived(13)Similarly, the fundamental negative-sequence component follows as(14)Where, (15)Orsimilar to (8) and (10), we have(16)Where,Correspondingly, the equations below are derived:||Issn 2250-3005(online)|| ||December||2012|| Page 204
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ISSN: 2250-3005 - ijcer

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