Control of Capacitor-less Synchronous Machine VFDs under Weak Grid Conditions

A significant percentage of faults in Variable Frequency Drives (VFDs) are caused by the degradation of capacitors. Faster degradation, along with higher Equivalent Series Resistance (ESR) of the electrolytic capacitors, has led to VFDs shifting towards using film capacitors due to their longer lifespan. However, film capacitors require larger volumes to deliver similar capacitance as electrolytic capacitors. This shift to film capacitors has led to a decreased capacitance in the DC-link. The VFDs with small DC-link capacitors are referred to as capacitor-less drives.
In industrial applications, VFDs are required to operate in remote areas with weak grid connections. The reduced capacitance in the VFD along with the weaker grid connection lead to increased oscillations in the DC-link voltage. These oscillations can cause instability in the system leading to under and over-voltage in the DC-link capacitor, leading to its degradation. The thesis compares several algorithms found in literature to stabilize the DC-link by adding virtual impedance. The process of adding the virtual impedances has been presented as a three-step process with different alternatives discussed for each step. The different choices in the three steps are discussed mathematically and simulated in MATLAB Simulink. The effect of these choices on the DC-link stability, the DC-link voltage ripple, the grid current harmonics and the Interior Permanent Magnet Synchronous Machine (IPMSM) torque ripples are evaluated. It was concluded that the impedance placement proposed by Antonello et al. [1] is the most effective at decreasing the DC-link voltage ripples and input grid current harmonics. Using their virtual impedance placement, the DClink voltage ripples were decreased by 72 %, and the input grid current Total Harmonic Distortion (THD) was decreased to 36.4 %.