Equivalent impedance per phase (3 Phase balanced systems)

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Hi, if a load is given in MVA and I work out the Real and reactive power, is the equivalent complex impedance per phase the values of real and reactive power divided by three in the following form:

Z (per phase) = Real power/3 + j Reactive power/3

Im not sure how to work out whether the complex number should be real term ± imaginary term? Thanks
 

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  • #2
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Hi, if a load is given in MVA and I work out the Real and reactive power, is the equivalent complex impedance per phase the values of real and reactive power divided by three in the following form:

Z (per phase) = Real power/3 + j Reactive power/3

Im not sure how to work out whether the complex number should be real term ± imaginary term? Thanks

RLC parallel circuits?
 
  • #3
818
67
Hi, if a load is given in MVA and I work out the Real and reactive power, is the equivalent complex impedance per phase the values of real and reactive power divided by three in the following form:

Z (per phase) = Real power/3 + j Reactive power/3
The RHS is the per-phase complex power for a balanced load. Since the LHS should be in units of ohms, you should reexamine what lead you to this equation.

Consider an impedance Z with known complex power S delivered to it:
[tex]
\mathbf{Z} = \frac{\mathbf{V}}{\mathbf{I}} \\
\mathbf{S} = \mathbf{V} \mathbf{I}^*
[/tex]
where V is the voltage across the impedance and I is the current through it.

This is 2 equations in 3 unknowns. This should tell you that you need more information to uniquely determine Z.

Since the angles of Z and S are equal, you could determine Z if you knew the magnitude of either V or I.
 

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