Control Strategies of Converter-Based Online Impedance Spectroscopy for Photovoltaic Panels

Impedance spectroscopy (IS) provides a powerful, non-destructive way to acquire photovoltaic (PV) panels' internal impedance over a wide frequency range. Compared with specific workstation-based IS, converter-based IS performed as an added function of the PV power converter provides an attractive mean to achieve online measurements. It can help reduce overall system costs and facilitate online applications, as no additional equipment is required. However, the control strategy of the power converter should be specifically designed. This work aims to provide a systematic design guideline for control strategies of converter-based IS implementation. Meanwhile, a cooperative control scheme between maximum power point tracking (MPPT) and IS modes is proposed to ensure quasi-maximum output power even during IS monitoring mode. For IS mode, two control strategies, named unified control and separated control are designed and compared based on three commonly used compensation controllers in linear system control, i.e., proportional (P), proportional-integral (PI) and quasi-proportional resonant (QPR) controllers. In the unified control strategy, a single PI controller controls the DC and AC components together to meet the control objectives. Meanwhile, in the separated control strategy, a segmented lower pass filter (LPF) with a variable cut-off frequency is designed to effectively separate the DC component of the PV current from the AC perturbation signals. Finally, an experimental study is conducted to verify that under both control strategies, reliable and accurate IS measurements can be achieved. Particularly, the unified control strategy based on a single PI controller is chosen for the cooperative control between MPPT and IS modes considering its control simplicity, high tracking accuracy (i.e., 99.5% and 93.7% for DC and AC components separately) and sufficient signal-to-noise ratio (SNR) (i.e., 6.9 dB). The effectiveness of the online IS monitoring based on the internal impedance of the PV panel is verified as well.