Understanding the Effects of Exponentially Decaying DC Currents on the Dual dq Control of Power Converters in Systems with High X/R

This paper investigates the root causes of detrimental oscillations in the dc link voltage of an energy storage system using a dual dq controller, operating at a high-voltage ac grid with high reactance-resistance ratio. Dual dq controllers are recommended in the literature for power converters operating under unbalanced, fault, or reduced voltage conditions. They employ two separated rotating reference frames, one for the positive and one for the negative sequence. The causes of the oscillations are investigated both theoretically and by time-domain computer simulations. As a result of the simulations, the performance of two dual controllers used in the industry is compared. In the presence of exponentially decaying dc currents, the filtering techniques employed by the controllers affect differently the performance of the proportional-integral regulators and disturb the feed forwarding and dq decoupling schemes. Ultimately, this results in undesirable oscillations in the dc-link voltage. This paper sheds light on how a fundamental phenomenon of three-phase ac systems can critically affect the control of power electronic converters. It provides a valuable insight into a possible root cause of oscillations in large electrical system applications with a considerable power converter penetration, such as large industrial plants striving for reducing greenhouse gas emissions.