Active damping based on the capacitor voltage positive-feedback for grid-connected power converters with LCL filter

The capacitor voltage positive-feedback is a widely extended active damping strategy. It can effectively damp the LCL output filter resonant poles for low ratios of resonance to sampling frequencies. However, the existing delays in the control loop limit the applicability of the capacitor-voltage positive-feedback. For high ratios of resonance to sampling frequencies, it becomes ineffective and can even destabilize the system. This limitation is overcome in this paper by adjusting the delay in the feedback path. With the delay adjustment, a robust damping can be achieved if the delays are properly considered, including the filters, and the grid impedance variations are taken into account. Simulation results validate the proposed active damping strategy.This work has been supported by the Spanish State Research Agency (AEI) and FEDER-UE under grant DPI2016-80641-R

Sensorless active damping strategy for parallel interleaved voltage source power converters with LCL filter

Grid-connected pulse-width modulation converters with LCL filters are widely extended as an interface for renewable energy generating systems. Those filters arise stability issues due to the filter resonance, which, must be damped, preferably by active damping methods, to avoid additional power losses. In some applications, such as offshore wind energy conversion systems, where high power converters are connected to low voltage networks, the commutation frequency is limited to low values to reduce the power losses and the resulting filter components are bulky. For this reason, in addition to its higher current handling capabilities, the use of power converters in parallel with the application of advanced modulation strategies, such as interleaving, is gaining importance. In this structure the filter components can be reduced, setting the filter resonance frequency at a higher value. As a consequence, the filter resonance frequency is close to the Nyquist frequency, complicating the implementation of some of the existing active damping solutions. In this work, an active damping approach based on a third order digital filter is proposed to stabilize the system. The robustness of the solution is investigated against grid inductance variations and the grid current harmonic content will be tested to comply with the most demanding grid codes. The proposed active damping strategy does not require extra sensors and can be easily designed by examining the open loop Bode plots. It is suitable to stabilize the LCL filter with a resonance frequency close to the Nyquist frequency, where some of the existing active damping approaches are unable to damp the system. The proposed method can be also an appropriate solution for existing grid connected power converters with stability issues due to changes in the effective grid impedance at which they are connected.The authors gratefully acknowledge Ingeteam Power Technology and the Spanish Ministry of Economy and Competitiveness under grant DPI2013-42853-R for its financial and ongoing support. This work was partially funded by the Public University of Navarre through a doctoral scholarship

MIMO based decoupling strategy for grid connected power converters controlled in the synchronous reference frame

Power converters are frequently connected to the grid through a LCL filter, controlling its power transfer through a current control loop in the synchronous reference frame. In this reference frame, cross coupling terms appear between the current and voltages of the passive components, which, without a proper decoupling strategy, penalize the converter transient response and the current control adjustment. In this work, an intuitive decoupling strategy is presented to improve the dynamic behavior, based on Multiple-Input-Multiple-Output systems theory. The approach developed is particularly interesting in extremely weak grids, allowing an easier adjustment of the main controller.This work has been supported by the Spanish State Research Agency (AEI) and FEDER-UE under grant DPI2016-80641-R, and partially funded by the Public University of Navarre through a doctoral scholarship