Open Access
Active damping based on the capacitor voltage positive-feedback for grid-connected power converters with LCL filter
(IEEE, 2019) Samanes Pascual, Javier; Gubía Villabona, Eugenio; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería Eléctrica, Electrónica y de Comunicación
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.
Open Access
Multisampled-capacitor-voltage active damping for parallel interleaved grid connected voltage source converters with LCL filter
(IEEE, 2017) Samanes Pascual, Javier; Gubía Villabona, Eugenio; Institute of Smart Cities - ISC; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
Parallel interleaved converters for high power renewable energy systems present stability issues at the LCL resonance frequency. A multisampled measurement and filtering strategy is proposed to stabilize the system based on the capacitor voltage derivative active damping, overcoming its limitations for low switching power converters. The effects of the delays on the stability of the active damping strategy would be analysed. The solution developed is robust against grid inductance variations, ensuring the fulfilment of the stringent harmonic grid codes.
Open Access
On the limits of the capacitor-voltage active damping for grid-connected power converters with LCL filter
(IEEE, 2018) Samanes Pascual, Javier; Gubía Villabona, Eugenio; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería Eléctrica, Electrónica y de Comunicación; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
Active damping strategies are widely extended to avoid stability issues at the LCL filter resonant poles in grid-connected voltage source converters. The capacitor voltage derivative active damping effectively damps the filter resonant poles without additional sensors, but it loses its effectiveness as the resonance frequency approaches the converter control Nyquist frequency, influenced by the existing delays in the control loop. To reduce this limitation, the delays can be reduced by performing a multisampled derivative, however, even though the stability limits might be extended, the oversampled approach might increase noise amplification problems. An appropriate filtering solution is required in the feedback path in order to reduce noise amplification and eliminate aliasing problems, without compromising the stability, as a result of a modification in the active damping feedback path. This work examines the limits of applicability of the capacitor voltage derivative strategy taking into account the filters used in the AD path, providing experimental results to validate the presented approach.
Open Access
Robust multisampled capacitor voltage active damping for grid-connected power converters
(Elsevier, 2019) Samanes Pascual, Javier; Urtasun Erburu, Andoni; Gubía Villabona, Eugenio; Petri, Alberto; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
The derivative feedback of the capacitor voltage is one of the most extended active damping strategies, used to eliminate stability problems in grid-connected power converters with an LCL filter. This strategy is equivalent to the implementation of a virtual impedance in parallel with the filter capacitor. This virtual impedance is strongly affected by the control loop delays and frequency, creating changes in the sign of the emulated virtual resistor, and raising instability regions where the active damping is ineffective. As a consequence, the LCL resonance frequency is restricted to vary, as the effective grid inductance changes, within the active damping stability region. This is an additional restriction imposed on the LCL filter design that can compromise the achievement of an optimised design. For this reason, in this work, a different strategy is presented; by adjusting the delay in the active damping feedback path, it becomes stable within the range where the LCL resonance frequency can be located for a given filter design, achieving a robust damping. Analytical expressions are provided to adjust this delay. To widen the stability region of the capacitor voltage derivative active damping, a multisampled derivative is implemented, overcoming its limitations close to the control Nyquist frequency. Experimental and simulation results validate the active damping strategy presented.
Open Access
MIMO based decoupling strategy for grid connected power converters controlled in the synchronous reference frame
(IEEE, 2018) Samanes Pascual, Javier; Gubía Villabona, Eugenio; López Taberna, Jesús; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería Eléctrica, Electrónica y de Comunicación; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
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.
Open Access
Sensorless active damping strategy for parallel interleaved voltage source power converters with LCL filter
(IEEE, 2017) Samanes Pascual, Javier; Gubía Villabona, Eugenio; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería Eléctrica, Electrónica y de Comunicación; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
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.