8 3-phase loads and per unit system

# 8 3-phase loads and per unit system - Lecture 10 3 loads....

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Lecture 10 3 φ loads. Per unit system (p.u.) Consider the three phase loads of Figure 1 and Figure 2 . Figure 1 - Wye (Y) load Figure 2 - Delta ( ) load A load is balanced if the input voltages are balanced and the currents are also balanced. Alternatively if we apply balanced currents then the voltages are balanced. Mathematically, a balanced set of voltages are,

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2 1 a b abc c V VVa V Va   ==   Where 120 j ae = . Figure 3 - Phasor diagram Therefore a balanced set of currents would be of the form 2 1 a b abc c I I Ia I I a Some useful identities for a are: 3 21 2 1 10 a aa = = ++ = To show that the – load is balanced we apply balanced voltages and calculate the currents.
() ( ) ( ) 2 2 1 1 ab ab ac ac bc bc I VV Y a V Y I Y a V Y I Y aa V Y ∆∆ =− = = Then from Kirchhoff’s current law (KCL), ( ) 2 22 11 23 1 3 ba c II I a a V Y aaV Y Y VY  =+=−−   = Similarly (check as an exercise), 2 3 3 b c I aV Y Ia V Y = = Thus 2 1 3

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## This note was uploaded on 01/27/2011 for the course ECSE 361 taught by Professor Franciscodgaliana during the Spring '09 term at McGill.

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8 3-phase loads and per unit system - Lecture 10 3 loads....

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