Add signal bus model to phasor dynamics family

[pelesh]

Summary

Create a signal bus model that stores a scalar value, which can be overwritten by one (and only one) component model and which can be read by multiple components.

CC @lukelowry @reid-g

Rationale

Signal bus is needed to abstract connections between electrical machines and control devices such as governors and exciters. Machine would update sensor readings (e.g. machine speed), which would be read by control devices (e.g. governor). Control devices would update actuator signals (e.g. mechanical power required), which would be read by a machine.

Both the machine and the controller would only have access to the signal bus and would not need to know about each other internal properties.

Description

I think signal bus should inherit directly from Model::Evaluator class. It should store (memory address of) a single scalar value. Unlike electrical bus, it should discriminate between inlet and outlet. Inlet should be a non-constant accessor to the scalar value and should allow only one connection. Outlet is a const accessor.

To be able to test signal model, we should create constant signal source and signal reader blocks.

Additional information

No response

[pelesh]

CC @nkoukpaizan

[pelesh]

After experimanting with possible implementations I see an argument against signal bus storing the signal value. Instead, I believe the signal bus should only keep a pointer/reference to the signal value.

By looking at the Genrou model, for example, it may need to provide access to the generator speed, injection current, bus voltage (indirectly), etc. as the signal values. Which of these will be shared is a runtime decision based on how the user connects the components. If we decide to store those values in the signal bus, then the number of component's internal variables will have to change at the runtime, as well. This will in turn make computing Jacobian difficult because the same Jacobian entries will be computed with contributions from different components based on runtime decisions. We would like to compute components' Jacobian stamps at compile time.

For example, if the generator speed $\omega$ is a signal input to a governor, then it will become signal bus variable instead of generator variable. An implication of that is when we connect governor and generator with signal $\omega$, we would need to resize/reindex internal differential variables of the generator and recompute Jacobian stamps.

A better solution would be for signal bus only to acquire a reference to an internal variable which should be passed as a signal from one device to another. In that case the ownership of signal variables will remain with components and the signal bus would simply communicate those between devices.

CC @nkoukpaizan @lukelowry

[pelesh]

@lukelowry, in #181 I need to initialize generators before governors. I think the same is the case with the implementation in #173; is this correct?