Real-Time Reliability Monitoring of DC-Link Capacitors in Back-to-Back Converters

Abstract

Electrolytic capacitors have large capacity, low price, and fast charge/discharge characteristics. Therefore, they are widely used in various power conversion devices. These electrolytic capacitors are mainly used for temporary storage and voltage stabilization of DC energy and have recently been used in the renewable energy field for linking AC/DC voltage and buffering charge/discharge energy. However, electrolytic capacitors continue to be disadvantageous in their reliability due to their structural weaknesses due to the use of electrolytes and very thin oxide and dielectric materials. Most capacitors are considered a failure when the capacitance has changed by 25% of its initial value. Accurate and fast monitoring or estimation techniques are essential to be used with low cost and no extra hardware. In order to achieve these objectives, an online, reliable, and high-quality technique that continuously monitors the DC-link capacitor condition in a three-phase back-to-back converter is proposed. In this paper, the particle swarm optimization (PSO)-based support vector regression (PSO-SVR) approach is employed for online capacitance estimation based on sensing or deriving the capacitor current. Because the SVR performance alone severely depends on the tuning of its parameters, the PSO algorithm is used, which enables a fast online-based approach with high-parameter estimation accuracy. Experimental results are provided to verify the validity of the method.Energies 2019, 12, 2369 2 of 11 increases and rushes evaporation of the capacitor's electrolyte. Therefore, the volume of the electrolyte is reduced so that the etched tunnels are not fully filled due to the electrolyte evaporation. Hence, the capacitance decreases as a result of the electrolyte volume reduction. The evaporation process will be higher whenever the ambient temperature increases; then, the capacitance value decreases and the lifetime deteriorated more.can be made thick, and as a result, the capacity per unit volume of the capacitor can be increased. Also shown also in Figure 2 is an oxide layer is formed on the cathode surface through a natural chemical reaction between the electrolyte and aluminum. Compared to the thickness of the anode surface oxidation layer formed by the manufacturing process, the capacitance on the cathode side is larger than that on the anode side. Therefore, since the anode and cathode capacitors can be regarded as connected in series, the total capacitance becomes similar to the capacitance value of the anode. Equivalent series resistance (Rs) represents the sum of all the resistances of the electrolyte, oxide layer, electrode, lead, and connection. The equivalent inductance (Ls) is the inductance formed by winding the coil in a cylindrical shape, which can usually be ignored in the frequency range of the power converter. Thus, the equivalent circuit can be simplified in the form of a series connection of the resistor and the capacitor.

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