Quasi-Online Technique for Health Monitoring of Capacitor in Single-Phase Solar Inverter

Aluminium electrolytic capacitors (AECs) are used in dc-link of single-phase grid connected solar photovoltaic (PV) inverters to suppress the dc-link voltage oscillation. With aging its capacitance decreases and equivalent series resistance (ESR) increases, which may lead to increase in voltage ripple, thereby reducing average power extracted from solar PV and/or damage the semiconductor switches due to overvoltage. Further, prolonged use of the aged capacitor could lead to the open circuit of the dc-link due to vaporization of the electrolyte. Therefore, to enhance the reliability of the system by predictive maintenance, it is necessary to monitor the health of AEC. A quasi-online method for condition monitoring of the AECs in solar inverters is proposed in this paper. This method is active during the night when there is no solar irradiance. In this case, the current of various odd harmonic frequencies are injected by the inverter and dc-link capacitor impedances at various frequencies are evaluated. Using these impedance values, least mean square algorithm is used to estimate the ESR and capacitance values. The magnitude of each injected harmonic current is kept within the limits prescribed by IEEE 929-2000 standard. The key advantage of the technique is that it combines the estimation of both ESR and capacitance to determine the health. Further, it does not require significant processing capability and is implemented in the existing digital processor/controller used for inverter control. Detailed simulation studies are carried out using MATLAB-Simulink. Proposed scheme is verified experimentally on a scaled down version of a grid connected solar inverter. Puncturing based technique is used for accelerated aging of the capacitor. Estimated values of ESR and C are in agreement to that measured from LCR meter.