Efficiency-Oriented Control of LLC Resonant Converter for Grid-Connected Inverter

Abstract

This study proposes an efficiency-oriented control approach for an LLC resonant converter-based high-frequency-link grid-connected inverter. The proposed topology has two stages. In the first stage, the LLC resonant converter generates a rectified sine wave output synchronized with the grid voltage. However, achieving zero gain under this operating condition is a challenge for the LLC resonant converter. To address this, a combination of phase-shift modulation and pulse-frequency modulation is employed without relying on a topology-morphing strategy. This approach enables both zero gain and a wide gain range for the LLC resonant converter. However, integrating two different modulation strategies introduces another challenge. Multiple combinations of phase-shift angles and switching frequency values can yield the same gain. Effectively managing this is crucial to ensure both accurate gain control and optimal system efficiency. Therefore, in the proposed system, a PI controller generates the phase-shift angle value based on the current error. Then, the PSO algorithm dynamically finds the optimal switching frequency value for any instant to maximize efficiency according to the predetermined cost function. This novel efficiency-oriented approach effectively enhances the performance of LLC resonant converters. In the second stage, an unfolder inverter operating at the grid frequency generates a sine wave from the LLC output to achieve grid connection. Since the unfolder operates at the grid frequency, its switching losses are negligible. Experimental results demonstrate that the proposed system successfully tracks reference signals, even under abrupt changes, while maintaining high efficiency under various operating conditions. Additionally, the output current THD remains within internationally specified limits, ensuring compliance with relevant standards.