17BEE0029_VL2019201007349_AST08.docx - Exp 8:Sizing and...

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Exp 8:Sizing and Selection of Neutral Link Surabhi Raman 17BEE0029 Theoretical Background: In a star-connected three-phase system, the current in the neutral conductor is the vector sum of the three line currents. With a balanced sinusoidal three-phase system of currents, this sum is zero at any point in time and the neutral current is therefore zero (Figure 1). In a three-phase power system feeding linear single-phase loads the current in the neutral conductor is rarely zero because the load on each phase is different. Typically the difference is small and is in any case far lower than the line currents (Figure 2). Where non-linear loads are being supplied, even when the load is well balanced across the phases, there is likely to be substantial current in the neutral conductor. With non-sinusoidal currents, the sum of the three line currents, even with the same rms value, may be different from zero. For example, currents with equal rms values and square shape will result in a significant neutral current (Figure 3).
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In fact, the third harmonic components (and all other harmonics where the order is a multiple of three - the sixth, ninth, etc.) of the line currents are all in phase with each other (i.e. they
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