Bus Voltage Control With Zero Distortion and High Bandwidth for Single-Phase Solar Inverters

Single-phase inverters must include an energy storage device, typically a high-voltage bus capacitor, to match the inverter constant input power to its pulsating output power. Because of its increased cost, the size of this bus capacitor must be minimized. However, when the bus capacitor is small, the bus voltage includes a high ripple at the ac line second harmonic frequency, which causes harmonic distortion. The bus voltage controller must filter this ripple, while regulating the bus voltage efficiently during transients, and must therefore balance a tradeoff between two conflicting constraints, low-harmonic distortion and high bandwidth. This paper analyzes this tradeoff, and proposes a new control method for solving it without using addition hardware. Instead of reducing the distortion by lowering the loop gain, the new controller employs a digital FIR filter that samples the bus voltage at an integer multiple of the second harmonic frequency. The filter presents a notch that removes the second harmonic ripple, enabling a design that operates with zero distortion and high bandwidth simultaneously, and is suitable for inverters with small bus capacitors. The proposed controller is tested on a microinverter prototype with a 300-W photovoltaic panel and a 20-μF bus capacitor.