Analysis and Design of Fully Digital and Direct Current Controlled Voltage Source Converters Connected to the Grid

Direct current control has been used in many voltage source power converters connected to utility grid, such as static VAr generators (SVGs) and parallel active power filters (APFs). In their stationary coordinates control systems, there are usually two closed control loops. One is the current loop and the other is the voltage loop. In practical applications, because the voltage source converters are connected to the grid, the output current will be affected and very often a command-tracking error exists in the current control loop. In this paper, a detailed analysis and design considerations are carried out to investigate this command-tracking error caused by the source voltage. Based on the obtained current loop transfer function and block diagram of the system, analytical expressions of the current control error are derived. The variables that affect the current control errors are then identified. The relationship between the circuit/control parameters and the current control errors are extensively explored. The command-tracking error affects the peak value of the modulation wave. When we design a fully digital control system, the peak value of the modulation wave is smaller than the peak value of the carrier wave. Then, according to choosing the values of the variables that affect the command-tracking error, the methods to design a fully digital direct current controlled voltage source converters connected to the grid are presented. The computer simulation investigations and hardware experimental results verify the analysis and design considerations