I am an engineer working on generator regulators (AVR). I am used to working on numeric regulator but I have no clue about analogic regulators. I am not expecting you to make me a course but just a method where and what I should start with.
When I look at the drawing it's like there is amplifiers, resistors, capacitors everywhere. I asked some guys in my company but they actually don't know that much.
The basic concepts of a digital (numeric?) generator exciter regulator (AVR) are exactly the same as those for an analog generator exciter regulator: one has to control the field voltage/current to control the generator terminal voltage (during synchronizing) and to control the generator power factor/reactive current during loaded operation.
To do this, the systems use PTs (Potential Transformers) and in some cases, CTs (current transformers) as feedbacks to the regulator. And, in the case of the field voltage current there will be some kind of voltage sensing and/or current sensing (usually a current shunt of some sort). These are the very same feedbacks/inputs that digital control systems use!
In the analog world, what usually happened was that the AC voltage from the generator PTs and/or the bus (running) PTs was converted to a proportional DC voltage, which was compared to a reference signal (also a DC voltage), and the error was used to control the output of the regulator. So, most analog controls were converting some AC signals, and some DC signals (such as for field voltage and/or -current) to DC signals, and summing them with other DC signals (such as the reference which the operator set using some kind of switch to increase or decrease the reference value), and the error between the signals determined the generator exciter regulator output, controlling the field voltage/current to control the generator terminal voltage/power factor/reactive current.
All those resistors and capacitors and amplifiers are doing no more than creating DC signals which can be compared to derive a signal which is then used to control the exciter regulator output. The exact same things happen in a digital regulator--the difference is that most digital regulators scale the signals being compared in normal engineering units, instead of proportional DC signals. This is done using analog-to-digital converters to get the signals into the digital realm.
You need to find the instruction manual(s) for the analog controls; they won't teach you much about the resistors and capacitors and amplifiers--but they will tell you how to "calibrate" or "tune" the regulator to scale the feedback inputs properly for the system to work as it should.
This is the big difference between analog and digital (software-based) controls--the concepts are (must remain) the same, since the generators haven't changed. The device instruction manual will tell you where to hook your voltmeter leads to read the appropriate voltage(s) to check for proper operation, and what the voltages should be, etc. In digital control systems, the same inputs are converted to digital signals which are scaled into "normal" engineering units (VAC, VDC, Amperes, etc.) which are easier to recognize in the digital world.
Hope this helps!