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Passive damping is the most adopted method to guarantee the stability of LCL-filter based grid-converters. The method is simple and, if the switching and sampling frequencies are sufficiently high, the damping losses are negligible. This letter proposes the tuning of different passive damping methods and an analytical estimation of the damping losses allowing the choice of the minimum resistor value resulting in a stable current control and not compromising the LCL-filter effectiveness. Stability, including variations in the grid inductance, is studied through root locus analysis in the z-plane. The analysis is validated both with simulation and with experiments.

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Low Voltage Ride Through is an important feature for wind turbine systems to fulfill grid code requirements. In case of wind turbine technologies using doubly fed induction generators the reaction to grid voltage disturbances is sensitive. Hardware or software protection must be implemented to protect the converter from tripping during severe grid voltage faults. In this paper two methods for low voltage ride through of symmetrical grid voltage dips are investigated. As a basis, an analysis of the rotor voltages during grid fault is given. First, the conventional hardware method using a crowbar is introduced. Then the stator current reference feedback solution is presented. Both methods are investigated and compared by simulation results using 2 MW wind turbine system parameters. Measurement results on a 22 kW laboratory DFIG test bench show the effectiveness of the proposed control technique.

Three-phase active rectifiers guarantee sinusoidal input currents and unity power factor at the price of a high switching frequency ripple. To adopt an LCL-filter, instead of an L-filter, allows using reduced values for the inductances and so preserving dynamics. However, stability problems can arise in the current control loop if the present resonance is not properly damped. Passive damping simply adds resistors in series with the LCL-filter capacitors. This simplicity is at the expense of increased losses and encumbrances. Active damping modifies the control algorithm to attain stability without using dissipative elements but, sometimes, needing additional sensors. This solution has been addressed in many publications. The lead-lag network method is one of the first reported procedures and continues being in use. However, neither there is a direct tuning procedure (without trial and error) nor its rationale has been explained. Thus, in this paper a straightforward procedure is developed to tune the lead-lag network with the help of software tools. The rationale of this procedure, based on the capacitor current feedback, is elucidated. Stability is studied by means of the root locus analysis in z-plane. Selecting the lead-lag network for the maximum damping in the closed-loop poles uses a simple optimization algorithm. The robustness against the grid inductance variation is also analyzed. Simulations and experiments confirm the validity of the proposed design flow.

Design and analysis of PI state space control for gridconnected pulsewidth modulation (PWM) converters with LCL filters based on pole placement approach is addressed. State space control offers almost full controllability of system dynamic. However, pole placement design is difficult and usually requires much experience. In this paper, a suitable pole placement strategy is proposed, which ensures fulfilling the requirements, which are commonly specified with respect to rise time, overshoot, and proper resonance damping. Controller parameter expressions are derived in terms of system parameters and specified poles and zeros. Hence, straightforward controller tuning for a particular system setting is possible. Performance is analyzed by means of transfer function-based calculations, simulations with MATLAB, and experimental tests. Dynamic performance and robustness against grid impedance variations are addressed as well as harmonic rejection capability and other practical issues.