A new control algorithm for 3-phase PWM voltage source converters connected to the grid

This paper deals with the control of three-phase PWM voltage source converters connected to the grid. With the use of "field-oriented control", voltages and currents are usually transformed to the rotating d-q reference frame, where control becomes quite straightforward: because the current space vector in the rotating d-q reference frame is fixed, the PI controllers operate on dc, rather than sinusoidal signals. It is reported that one of the fundamental weakness of more classical control structures is the great difficulty in controlling the variables with sinusoidal references because of the limited ability of PI controllers to accurately track time varying inputs. This paper describes a new and simpler, but otherwise equivalent control technique which solves this problem without the burden of transforming time-dependent three-phase quantities into the time invariant d-q reference frame and vice versa. This new control structure is based on a complex controller (i.e. with real and imaginary parts), which achieves zero steady-state error by controlling the current space vector directly in the stationary d-q reference frame. The mathematical equivalence with field-oriented control is fully demonstrated. Simulation results are also presented which clearly show the indistinguishability of the two control systems