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Compensation Voltage on Exciters #518

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@lukelowry

Summary

Raised in #424

  • The 'Compensation voltage', $E_C$, typically comes from the machine model instead of the terminal Bus. This is because there is a parameter specified by the machine model (e.g.GENROU) that we do not currently support: $R_{comp}$ and $X_{comp}$. When these are both zero-valued, $E_C=|V_{bus}|$. Sometimes, the compensation reactance is configured to be nonzero specifically to decouple $E_C$ from $|V_{bus}|$ and improve numerical stability. I think there is a broader conversation to be had about how we can model sensed terminal voltage.
  • This exciter as well as IEEET1, locally compute $E_C=\sqrt{V_r^2+V_i^2}$, despite $E_C$ being documented as an external variable. This was a temporary design choice before we had signals.
  • For more advanced grid cases and models, the terminal voltage magnitude is consumed more than once. Therefore, I encourage us to introduce $V_{m}$ as a third internal algebraic variable of Bus, conditionally included based on consumption (otherwise we could introduce unused nonlinear algebraic equations incessantly!). Same for $\theta_{bus}$. I will soon need models that need the bus frequency (i.e., time derivative of $\theta_{bus}$).

Rationale

Will clean this up in a bit

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