Keywords: grid-connected inverters, distributed generators, PWM, digital signal processing, DSP, pulse-width modulation, digital control, system stability, capacitor current observer, controller design
Digital control of a three-phase grid connected inverter
This paper presents the design and practical implementation of a digital current controller for a three-phase PWM voltage source inverter connected to the grid via an LCL filter to produce a high quality current injected into the grid. A two-feedback loops control system is proposed, with an outer grid current loop and an inner filter capacitor current loop. The controller also incorporates a grid voltage feedforward loop to compensate for the effect of the utility voltage disturbance. The paper also discusses the effect of the sampling and computational time delay both on system stability and current quality. This time delay is shown to reduce the stability of the inner loop. However, it is also shown that it is better, from the point of view of output current THD quality, to increase the inner loop time delay to a full one sampling period, so that the mains frequency envelope (nearly ripple free) current component is sampled at the peaks and troughs of the PWM, away from the switching instances of the transistors and associated switching noise. To ensure system stability, a capacitor current observer is proposed to compensate for the computational time delay. The design of the capacitor current observer and the controller are discussed. Simulation and practical results are presented to validate the design.