a. What are the advantages of using an inner current control loop for a voltage source converter rather than trying to directly control the voltage applied?

b. What is gained through the addition of the voltage feed-forward gain in a closed loop current regulator?

c. What are the advantages of implementing the inner current controllers for a three phase voltage source converter to act on variables in the synchronous d q reference frame as opposed to acting on them in the stationary a b c reference frame or stationary α-ß reference frame. How would the controller design be changed if either of the a b c or α-ß based implementation were used?

d. Why is synchronization circuit, such as a phase locked loop (PLL), needed for a grid connected voltage sourced converter? What does the output look like? Is the PLL needed for every type of controlled application?

e. The three phase grid connected applications often use a synchronous reference frame based PLL where Vsq (t) is regulated to zero. Sketch a system diagram and show Vs (t) is located in the system diagram? How does this simplify outer control loops for real and reactive power?