Multi-Frequency Impedance Characteristic of Long-Time Operating DC-Link Capacitor in HVDC System

The DC-link capacitor is one of the most vulnerable components of the modular multilevel converter (MMC) in an operating HVDC system. To assess the aging states of DC-link capacitors, the characteristics of capacitor impedance in a multi-frequency range are investigated and analyzed. First, the equivalent circuit model is established with consideration of the insulation resistance of the DC-link capacitor. Then, the multi-frequency impedance spectra of capacitors under five aging states are detected by a high-precision measuring bridge. The experimental result shows that as the aging degree of the capacitor increases, the inflection point of the capacitance spectrum in a multi-frequency range tends to move to a higher frequency. Meanwhile, the spectrum of the resistance in a multi-frequency range shows a growing slope at low frequency range (< 200 Hz), and the resistance value approaches a stable level at high frequency range. Further, the above findings can be verified based on the equivalent circuit method mentioned above. The multi-frequency characteristics of capacitors can not only evaluate the aging degree of the deteriorated capacitors, but also be utilized to identify deterioration causes including the capacitance loss, the conductive path resistance increase, the dielectric insulation deterioration, etc. The proposed method can provide a novel perspective on detection of DC-link capacitor aging.