A SOGI-based adaptive controller design for second-harmonic currents suppression of variable frequency loads in DC buildings

Abstract

DC buildings with integrated DC transformers (DCTs), as a typical representative of photovoltaics, energy storage, direct current, and flexibility (PEDF) building systems, are a crucial approach to reducing carbon emissions. Heating, ventilation, and air conditioning (HVAC) systems are typical loads in DC buildings that primarily aim to enhance the air quality and thermal comfort. However, the variable frequency drives (VFDs) used in HVAC systems can generate second-harmonic currents (SHC), which decrease equipment reliability and system power efficiency. To prevent SHC on the low-voltage side from flowing to the high-voltage bus, even in the grid, a second order generalized integrator (SOGI)-based adaptive controller for suppressing the SHC of variable frequency loads in DC buildings is proposed. The controller employs a phase-locked loop (PLL) to provide feedback on the VFD output frequency to the DCT control loop's notch filter, enabling adaptive suppression of the variable frequency SHC. Firstly, an accurate small-signal model of DCT based on Fourier series is established, which considers the inductor current. Then, the formula for DCT equivalent impedance is derived based on the model, and the feasibility of suppressing the SHC by increasing the equivalent impedance at variable frequency is analyzed. To enhance the stability of DC buildings, the parameters of the controller are designed to improve dynamic performance. Finally, the simulation results and experimental results verify the effectiveness and feasibility of the proposed adaptive controller compared to other suppression strategies.© 2017 Elsevier Inc. All rights reserved.