Here are some basic concepts about control loops for deriving a reference signal for PWM in DC-AC converters:

• Control loops: Control loops are used to regulate the output of a system to a desired value. They typically consist of a sensor, a controller, and an actuator. The sensor measures the output of the system and sends the signal to the controller. The controller compares the measured output to the desired value and generates a control signal. The actuator receives the control signal and uses it to change the input to the system.
• PI controllers: PI controllers are a type of controller that uses proportional and integral terms to regulate the output of a system. The proportional term is used to correct for instantaneous errors, while the integral term is used to correct for errors that accumulate over time.
• Reference signal: The reference signal is the desired value for the output of the system. It is typically generated by a human operator or by a computer program.

The fundamental idea behind using control loops to derive a reference signal for PWM is to use the sensor to measure the output of the system and to use the controller to generate a control signal that will drive the output of the system to the desired value. The PI controller is used to ensure that the output of the system tracks the reference signal closely.

Here are some good books and papers that you can read to learn more about control loops for deriving a reference signal for PWM in DC-AC converters:

"Control Systems Engineering" by Norman Nise

"Digital Control Systems" by Robert Dorf

"Modern Control Systems" by Katsuhiko Ogata

"Control Systems: Theory, Design, and Implementation" by John J. Craig

"Power Electronics and Control Systems" by M. H. Rashid