A Back-to-Back Converter-based Electric Spring with an Improved Control Structure for Unbalanced Load Operations

This article proposes a back-to-back converter-based electric spring for the voltage and frequency regulation in a self-excited induction generator-based isolated generation system. The proposed electric spring topology gives the system seamless performance with linear, non-linear, and unbalanced load conditions. The electric spring in this work is equipped with a modified complex coefficient filter, which eliminates the harmonics and the unbalanced negative sequence component from the system currents caused by unbalanced loads. The performance of the proposed electric spring was evaluated with non-linear loads, unbalanced critical load, and unbalanced non-critical load. The proposed electric spring works effectively to regulate the system voltage and frequency within 2% tolerance limits. In the presence of non-linear loads, the THD of the generator current could be reduced to 1.97%, proving the electric spring’s efficacy as an active filter. The modified complex coefficient filter-based shunt side control strategy effectively eliminates the unbalanced current components from the system currents, which keeps the self-excited induction generator unaffected by load imbalances caused by both the critical load and non-critical load sides. The shunt side control strategy also effectively maintained the DC link voltage fluctuation within 16V, which is 6.4% of the rated value of 250V. The proposed minimum voltage injection control strategy to control the series converter was highly effective in maintaining a constant voltage of 230V during unbalanced non-critical load conditions. The real-time validation of the proposed work is done with OPAL-RT 4510 and 4520 platforms, and the detailed results and their analysis are presented in this work.