Intro to three phase systems - 3 -

A symmetrical three-phase system is shown in the image. Considering positive sequence, V a n = 120 0 V_{an} = 120 \angle{0^{\circ}} and ignoring mutual impedance, what are the phasors for the phase voltage across the load?

Details and Assumptions:

  • f = 60 H z f = 60 Hz
  • α = 1 12 0 \alpha = 1\angle{120^{\circ}}
  • Positive sequence is the sequence ABC

116.21 2 [ 1 α α 2 ] 116.21 \angle{-2^{\circ}} \begin{bmatrix} 1 \\ \alpha \\ \alpha^2 \end{bmatrix} 116.21 2 [ 1 α 2 α ] 116.21 \angle{-2^{\circ}} \begin{bmatrix} 1 \\ \alpha^2 \\ \alpha \end{bmatrix} 122.84 10. 6 [ 1 α α 2 ] 122.84 \angle{-10.6^{\circ}} \begin{bmatrix} 1 \\ \alpha \\ \alpha^2 \end{bmatrix} 122.84 10. 6 [ 1 α 2 α ] 122.84 \angle{-10.6^{\circ}} \begin{bmatrix} 1 \\ \alpha^2 \\ \alpha \end{bmatrix}

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1 solution

Since the three-phase system is symmetrical, we need only consider one phase. The phase voltage across the load is given by:

V load = Z load Z line + Z load × V a n = 10 1 2 π × 60 × 531 × 1 0 6 j 0.5 + 2 π × 60 × 5.3 × 1 0 3 j + 10 1 2 π × 60 × 531 × 1 0 6 j × 120 = 10 5 j 0.5 + 2 j + 10 5 j × 120 = 600 ( 2 j ) 10.5 3 j = 1341.64 26.5 6 10.92 15.9 4 122.86 10. 6 \begin{aligned} V_{\text{load}} & = \frac {Z_{\text{load}}}{Z_{\text{line}}+Z_{\text{load}}}\times V_{an} \\ & = \frac {10-\frac 1{2\pi \times 60 \times 531 \times 10^{-6}}j}{0.5+2\pi \times 60 \times 5.3 \times 10^{-3}j + 10 -\frac 1{2\pi \times 60 \times 531 \times 10^{-6}}j} \times 120 \\ & = \frac {10-5j}{0.5+2j+10-5j} \times 120 \\ & = \frac {600(2-j)}{10.5-3j} \\ & = \frac {1341.64\angle -26.56^\circ}{10.92\angle -15.94^\circ} \\ & \approx 122.86 \angle -10.6^\circ \end{aligned}

Since the clockwise sequence is [ 1 α 2 α ] \begin{bmatrix} 1 \\ \alpha^2 \\ \alpha \end{bmatrix} , the phasor is 122.86 10. 6 [ 1 α 2 α ] \boxed{122.86 \angle -10.6^\circ \begin{bmatrix} 1 \\ \alpha^2 \\ \alpha \end{bmatrix}} .

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