I read that in large utility systems more and more generators are added to provide extra power. I'm lacking some basic knowledge here. So, what happens when we connect two generators in parallel? Is it like connecting 2 voltage sources in parallel with a resistor R? I'm trying to think it as a circuit but it doesn't help. Does the current in the load increase? And if so, how?
I think you look at each generator as if it is a DC battery. You can connect two DC batteries in parallel if they have the same voltage. Because they are DC. They provide direct current.
What if the voltage of one battery is greater than the voltage of the other battery? Current moves from the battery that have higher voltage to the battery of the lower voltage. And the battery of the lower voltage will blow or damage (unless it is a rechargeable battery).
generators do NOT provide DC voltage. They generate alternative voltage or current. I mean the value of the current changes depending on the time so, It has a frequency and maximum voltage value.
Imagine that there are two different generators operating separately, One of them generates the red voltage and the other generates the blue voltage as shown in figure:
Look at the point of time of (90 degree). The blue voltage is higher than the red one.
Look at the point of time of (180 degree). The red voltage is higher than the blue one.
So, at any time, The voltages of the two generators are not the same. As I said before, Current will flow from the generator of the higher voltage to the generator of the lower voltage and a damage will happen to the two generators. That's because a generator is supposed to produce current, Not to draw current so each generator will try to work as a motor and this causes damage.
How can we solve this issue?
A synchronisation process is the solution and a synchroniser is a device that controls the speed of the two generators so that they become in phase which means they should produce the same voltage in the same time.
This way we can connect millions of generators together to increase current capacity.
When two generators are connected in parallel, they must be first matched in frequency, phase relationship, and voltage. The voltage is initially set to prevent the added generator from supplying any current. Once the connection is made, the voltage can be increased so that it supplies the desired percentage of the load.
The output voltage of a wound-field synchronous generator are adjusted by adjusting the field excitation. The frequency and phase relationship must be adjusted by adjusting the speed of the prime mover. Speed also has an effect on voltage, but excitation voltage can be used to make voltage adjustments more quickly.
Load current is determine by the load itself, but it can be affected by supply voltage and frequency. However adding a parallel generator is made in a way that should not result in a significant change in load current.
When examining the circuit, it is necessary to consider the complex impedance of the individual generators, the complex impedances of the interconnecting transformers, transmission lines etc., as well as the complex impedance of the source viewed from the point of common coupling between the generators.