Analytical Control System Synthesis for Dual-Loop Cascaded Stationary Frame Voltage Regulators

This paper presents a control system synthesis strategy for a voltage regulated voltage source inverter (VSI) incorporating digital linear feedback regulators in a cascaded dual-loop architecture. Controller parameters have been synthesized using emulation design based on Laplace domain transfer functions that account for the second-order LC filter resonance, the current and voltage regulation stages, the inverter gain and transport delay associated with a regularly sampled pulse-width-modulator. Bode frequency analysis is applied to determine the control loop magnitude and phase response. It is shown that at the maximum current control loop bandwidth, an LC resonance condition must be enforced, which uniquely sets the voltage control loop parameters. This approach enables the voltage control loop bandwidth to be set at up to 2/3 times of the LC filter resonance frequency. The synthesis approach is validated using full switched digitally controlled simulation models.