This paper addresses robust design of the active-power and dc-link control loops of powersynchronization control. Robustness is obtained by analytic gain selections which give large enough stability margins. The proposed design allows robust stability irrespective of the grid strength and of the operating point, the latter with one exception. The proposed design is compared to design based on the principle virtual synchronous machine. Experiments show that the time-domain results correlate well with the frequency-domain results.

This paper deals with state-feedback current control for power converters, which are equipped with an LCL filter and connected to a weak grid. The grid-side current is measured and other states needed by the current controller are estimated using a reduced-order observer. The control system is designed directly in the discrete-time domain. The gains of the control system are calculated using direct pole placement, assuming a strong grid. Recommendations for the nominal pole locations are given. The results show that the control system is robust against the unknown grid impedance, ranging from strong to very-weak grid conditions. The proposed design is validated by means of experiments. Index Terms-Grid converter, LCL filter, reduced-order observer, state-feedback current control, weak grid.

This paper deals with discrete-time statespace current controllers for three-phase grid converters equipped with an LCL filter. The integral action in the controller can be implemented either using an integrator or a disturbance observer. The results show that the disturbance-observer-based and integrator-based controllers become mathematically equal if the feedforward gains are selected to be equal, the feedforward zero is placed to cancel the pole originating from the integral action, and the closed-loop poles are placed identically. The equivalent performance in both designs is verified by means of analyses and experiments. The equivalence is also shown for double-frequency current controllers.

In this letter, an enhancement of powersynchronization control is proposed, whereby pole-zero cancellation in the closed-loop system is achieved. An effect thereof is that step-response ringing and overshoot are eliminated. For strong grids, the closed-loop bandwidth increases, allowing a shorter step-response rise time.

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