Grid-connected inverter time lag link

The control of grid-connected inverters is recently executed with digital microprocessors due to the advances in digital signal processing technology. However, the digital realisation has a drawback of the phase lag induced by the time-delay. This phase lag challenges the stability and robustness of the controller of the inverters.

When a grid-connected inverter system employs digital control, control delays are introduced, altering the characteristics of the capacitor current active damping. This results in the damping no longer being equivalent to a pure resistor in parallel with the capacitor. Instead, it becomes a complex impedance that varies with frequency.

In contrast, a short delay can be easily handled through various compensation techniques. In grid-connected inverters with LCL filter, the controller can be a voltage, current or direct power control or a hybrid of any of these controls in a cascaded loop with, either inner-loop or outer-loop structure.

However, the control bandwidth of the current controller is limited by the presence of time-delay in the control loop, especially in digital implementation. Furthermore, the control of grid-connected inverters is executed with digital microprocessors due to the advances in digital signal processing technology .

Active damping (generally by the feedback of capacitor current) is an effective solution to the resonance peak of LCL -type grid-connected inverters. However, the damping performance is significantly affected by the computation and pulse-width modulation delays in the digital control system, which results in poor system robustness.

In digital-controlled LCL-type grid-connected inverter systems, control delays affect the system stability, dynamic performance, and robustness. To address this, this paper proposed a delay compensation strategy that mitigates the adverse effects of delays, enhancing the robustness of the system against grid impedance variations.