Passivation of Grid-Following VSCs: A Comparison Between Active Damping and Multi-Sampled PWM

This article compares different strategies used to enhance the stability properties of grid-following voltage-source converters (VSCs). Because of digital delays, VSC admittance exhibits a nonpassive zone, which introduces negative damping and may destabilize the grid-connected operation. It is shown that typically used active damping (AD) strategies only bring positive impact up to a certain frequency, while deteriorating admittance properties around and above the Nyquist frequency. Multi-sampled pulsewidth modulation (MS-PWM) greatly extends the passive admittance region, using only a single-loop current controller. Experimental admittance measurements are performed on a single-phase VSC, up to twice the switching frequency. Subsequently, different grid-connected scenarios are tested to show that MS-PWM retains stable operation, where AD methods cause instability. This article also offers analytic modeling and experimental measurements of noise propagation for compared strategies. It is shown that derivative-based AD is not highly sensitive; however, MS-PWM offers additional noise suppression

Sliding-mode and proportional-resonant based control strategy for three-phase two-leg T-type grid-connected inverters with LCL filter

In this study, sliding-mode and proportional-resonant based control strategy is proposed for three-phase two-leg T-type grid-connected inverter with LCL filter. The sliding surface function is formed by using the inverter current and capacitor voltage errors. When the inverter current and capacitor voltage feedbacks are included into the control loop, the active damping requirement is automatically resolved. The PR controllers are employed in cascaded manner to generate the references for inverter current and capacitor voltage. The use of PR controllers ensures zero steady-state error in the inverter current, capacitor voltage and grid current. In addition, since the proposed three-phase inverter has only two legs, the total switch count is reduced resulting in cheaper and reliable topology. The proposed system is validated through computer simulations which show that proposed control algorithm can achieve the control of grid currents. The total harmonic distortion level of the grid currents is in the limits of international standards

DISCRETE TIME QUASI-SLIDING MODE-BASED CONTROL OF LCL GRID INVERTERS

Application of a discrete time (DT) sliding mode controller (SMC) in the control structure of the primary controller of a three-phase LCL grid inverter is presented. The design of the inverter side current control loop is performed using a DT linear model of the grid inverter with LCL filter at output terminals. The DT quasi-sliding mode control was used due to its robustness to external and parametric disturbances. Additionally, in order to improve disturbance compensation, a disturbance compensator is also implemented. Also, a specific anti-windup mechanism has been implemented in the structure of the controller to prevent large overshoots in the inverter response in case of random disturbances of grid voltages, or sudden changes in the commanded power. The control of the grid inverter is realized in the reference system synchronized with the voltage of the power grid. The development of the digitally realized control subsystem is presented in detail, starting from theoretical considerations, through computer simulations to experimental tests. The experimental results confirm good static and dynamic performance

Analysis of Dead-time Harmonics in Single-phase Transformerless Full-bridge PV Inverters

A short period, called dead time, is usually implemented (e.g., through adding extra hardware in gate drivers or modifying pulse-width modulation schemes) for voltage source inverters to prevent shoot-through incidents. Clearly, larger dead time provides more safety, but may also degrade the injected currents from inverters. It thus requires sophisticated compensation schemes to meet certain stringent standards. For single-phase transformerless full-bridge PV inverters, different modulation schemes can be employed to suppress leakage currents, which in return may affect the distribution of the dead time harmonics. Thus, this drives the analysis of dead time harmonics in single-phase transformerless full-bridge inverters considering two modulation strategies: bipolar and unipolar modulation schemes. Effects of modulation on the dead time harmonics are observed in simulations and experimental tests. Furthermore, a periodic controller is adopted to mitigate the harmonics, which is independent of the modulation schemes