Current And Power Control For LCL-Type Grid-Connected Inverters Under Weak Grid Condition

As the interface between the distributed power generation system and power grid,the gridconnected inverter plays an important role in converting the generated power into the high quality ac power and injecting it into the grid reliably.With the increasing penetration of the distributed generation sources,the grid can no longer be treated as the ideal voltage source,which imposes great challenges on the control of the grid-connected inverter.This dissertation is dedicated to the current and power control techniques for the LCL-type grid-connected inverter under weak grid condition,aiming to improve the power quality,stabilize the injected grid current,and participate in the grid frequency and voltage regulation to enhance safety and reliability of the power system.Capacitor-current-feedback active-damping method is an effective approach to damp the resonance of the LCL filter.However,the computation and pulse-width modulation(PWM)delays in the digital control system have a significant influence on the active-damping method,resulting in poor control robustness against the grid impedance variation.Meanwhile,these delays also reduce the control bandwidth greatly,and thus impose a severe limitation on the low-frequency gains of the grid current loop.This dissertation proposes a real-time computation method with dual sampling modes to remove the computation delay in the algorithem of the capacitor-current-feedback active-damping and grid current control,thus greatly improving the system robustness and control performance.Moreover,the time duration between the sampling instant and the switching instant of the inverter bridge is extended by the proposed method,which effectively prevents the switching ripple and high-frequency switching noise from distorting the sampled signals.Experimental results from a 6 kW LCL-type single-phase grid-connected inverter confirm the theoretical expectations and effectiveness of the proposed method.The ability of current harmonic rejection and the stability robustness of the grid connected inverter are depended on the magnitude and phase of its output impedance,respectively.This dissertation proposes an impedance shaping method by introducing the parallel and series virtual impedances.Specifically,the parallel virtual impedance is used to maximize the magnitude of the output impedance,while the series virtual inductance is used to boost its phase.With this proposed method,the grid-connected inverter can operate stably over a wide range of the grid impedance variation and exhibit strong rejection ability of the injected grid current harmonics that caused by the background harmonics of the grid.Experimental results from a 6 kW single-phase grid-connected inverter confirm the effectiveness of the proposed impedance shaping method.The imbalance between the produced and consumed active and reactive powers in the power system leads to the frequency and voltage variations.Therefore,the output powers of the generators should be regulated according to the load conditions.By investigating the regulation principle of the grid frequency and amplitude,the unified power source model and power load model are firstly proposed,which use an ideal power source/load and a parallel power impedance to describe the output power characteristics of the generators and load.Therefore,the model of the power system can be greatly simplified,and the influences of the grid-connected inverter on the grid frequency and voltage can be studied in a straight-forward way.It is revealed that the variations of the grid frequency and voltage can be suppressed by reducing the magnitudes of the grid-connected inverter's power impedances.Moreover,the grid-connected inverter's power impedance should be designed according to the capacity and allowed charging/discharging speed of the installed energy storage device,aiming to compensate the unbalanced power in different time span.Furthermore,the power stability criterions are derived for the overall inverter-grid system.Based on the above mentioned analysis,the general power control strategies are proposed,and its parameter design method and implementation are discussed.With the proposed power control strategies,the grid-connected inverter can support the weak grid as much as possible to reduce the frequency and voltage variations,and meet system stability requirements.Finally,a 15 kVA scale-down inverter-grid system,which includes the distributed generation source and weak grid,is designed and tested,and the experimental results confirm the effectiveness of the proposed power control strategies.