Compensation Voltage on Exciters

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

Description

No response

Additional information

No response

Start by reading the exciter and bus modeling code for the affected exciter, IEEET1, and GENROU models, then trace how E_C is currently computed and passed. The issue does not name files or tests and leaves the desired design unresolved; done criteria would need to be established for compensation parameters and conditional Bus variables before implementation.