I'm tying to calculate the change in reactive power of an alternator with the change in its real power input.

P=EV/Xs sin(d) is the real power output of the generator.

Q=EV/Xs cos(d) - V^2/Xs is the reactive power output of the generator.

I can take derivative of these equations. dQ/dP to calculate change in reactive power with change in real power.

But I can also use the absolute equations to calculate reactive power before and after change in real power.I have applied both these methods, I'm getting different results.

What could be the problem ?

  • \$\begingroup\$ Show what you've done. \$\endgroup\$
    – user133493
    Commented Jul 9, 2017 at 6:17
  • \$\begingroup\$ Q is not a direct function of P, so you need to declare the common variable that's changing, say \$\small u\$, and then differentiate the product: \$\frac{dQ}{dP}=\frac{dQ}{du}.\frac{du}{dP}\$ \$\endgroup\$
    – Chu
    Commented Jul 9, 2017 at 7:32
  • \$\begingroup\$ that is how I solved. But the two methods yield different result. \$\endgroup\$
    – spaul
    Commented Jul 9, 2017 at 10:55


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