Regarding current sources, there seems to be some weird stuff being claimed. A current source is just that. A source that outputs whatever voltage is needed to maintain constant current in the face of varying impedances. Current sources are just as "real" as voltage sources. Take a bicycle generator. It can function as either, CVS or CCS. An ideal source of either type is impossible to make, but a non-ideal CCS can be made as easily as a non-ideal CVS.
A bike gen can be regulated by the pedaler for CVS or CCS. For CCS, if the handle bars are equipped with ammeter, and field current adjustment (wound rotor field), maintaining CCS function is as follows. Biker pedals until current on ammeter reads target value. Field current can be adjusted so that biker is producing a speed and torque that is comfortable. Say, the target is 1.0 amp, and the load is 10 ohm. Field current is adjusted so that a comfortable pedaling speed gives 1.0 amp. But then load changes from 10 to 20 ohm. To maintain 1.0 amp, power must double. If biker maintains torque, then field current must double. Or biker can maintain field current and maintain torque while doubling speed (rpm). But either approach results in constant current.
I know what critics are thinking - Claude all you describe is a "voltage" generator that allows adjustable voltage. But as we will see, a CVS is analogous to the CCS. To maintain a constant 10 volts, the biker pedals and observes a volt meter. With a 10 ohm load, the field current is adjusted for comfortable pedal speed/torque. Now drop the load to 5 ohms, and maintain 10 volts. What must be done? Increase field current or pedal speed or both. Torque will increase due to increased current. In the CCS case, load changing from 10 to 20 ohms dictates increased voltage.
The CVS generator at power plants operates much like this bike example. Generators do not naturally output constant voltage nor constant current. The stator inductance is very large. An ideal CVS has zero R, and most important zero L, or zero XL. As frequency increases, this is hard to achieve. In open circuit, no load condition, a CVS shaft speed and field current are adjusted for 10 volts. Assume the generator full power is 100 watt, so that 1.0 ohm is full load at 10 volts.
The inductive reactance for a typical generator, known as synchronous reactance, is between 0.50 and 2.50 ohms reactive (j0.50 to j2.50 ohm). When open, V = 10V, and the emf generated is 10V. Synchronous reactance XS is 1.50 ohm, typical. But fully load the output with 1.0 ohm resistive. The terminal voltage will drop like a rock. The new voltage at the terminals is 10V(1/(1.0 + j1.50)) = 5.547 volts. That is a 45% drop! To restore the 10 volts, we must increase field current and/or pedal with more torque.
By increasing torque we increased current. By increasing field current we compensated for the voltage dropped across XS. Generators must be controlled to produce constant voltage, it's not natural. The power company is so good at it, we don't see the effort it takes. Our wall outlet is 120 volts, 60 Hz, and stays that way whether we draw 1.0 milliamp, or 20 amps. It is very easy to slip into the heretical mindset that V causes I, or V is the independent variable.
A CCS is much like a CVS. Constant current from a generator is not natural, it is contrived and artificial, just like CVS. The bike generator has a 1.5 ohm reactance. Per Thevenin/Norton, we have a voltage source in series with 1.5 ohm, or a current source in parallel with 1.5 ohm. Full load is 1.0 ohm. The truth is, the generator makes a lousy CVS, and a lousy CCS. For load normalized to 1.0 ohm, a "good" CCS should have over 100 ohms of shunt impedance. That way a 1.0 ohm load gets almost the entire generator current, and little regulation if any would be needed. Likewise a CVS that is "good, would possess less than 0.010 ohms of series resistance, assuring that little voltage drops in the resistance, almost all reaches load.
But, that doesn't happen, so we artificially manipulate our filed current, speed and torque to maintain either CC or CV. Both are attainable to an extent, but neither ideally. Peace to all.
Claude