Study On Condition Monitoring Technique Of DC-link Capacitors In Three-Phase AC/DC/AC PWM Power Converters

With the rapid development of renewable energy industries, the reliability of power electronic systems attract more attention than ever. The DC-Link Capacitors tend to be the weakest part in power electronic systems. The failure of DC-Link Capacitors could often result in damages of the electronic systems, which brought huge losses to the society. The scientists did many efforts to solve this problem these years, however there are still many problems existed in many applications.The DC-Link capacitor is a critical component in three phase AC/DC/AC systems which connect two terminal converters, balance the power between input and output, and absorb the instantaneous peak power from the grid. However, the DC-Link capacitor is one of the most fragile components, and the DC-Link capacitor in three phase AC/DC/AC systems is not directly connected with the input source or the load which make it more difficult to monitor the state of the DC-Link capacitors than that in DC-DC converters. The condition monitoring methods could classified into two categories: offline monitoring methods and online monitoring methods. The offline monitoring method could estimate the capacitor accurately but this method require the capacitors to be removed from the converters which could not be piratical in industry applications. The online monitoring method for DC-Link capacitors usually need complicated models and additional current sensor which are very costly and broke the main circuit in the systems.This paper propose several novel non-invasive condition monitoring method of DC-Link capacitor’s ESR and C for three phase AC/DC/AC PWM converters.First, this paper analyze the equivalent circuit of DC-Link capacitors, the character of impedance and the aging law of the capacitor parameter, and also compare the capability features, degradation mechanisms of different kinds of DC-Link capacitors(including the Al-Caps, MPPF-Caps, MLCC Caps). A double-closed-loop three phase AC/DC/AC PWM converter controlled by SVPWM is established in MATLAB Simulink. And then this paper proposes a time domain model of DC-Link capacitors with ESL, ESR, and C in this three phase system. A least square method and a recursive least square method(RLS) are designed to estimate the ESR, ESL, C of the time domain model of DC-Link capacitors in three phase systems.Second, to solve the drawbacks of the parameter identification method based on time-domain model of DC-Link capacitors, this paper proposed two novel non-invasive semi-online monitoring method for the DC-Link capacitors in three phase AC/DC/AC PWM converters. Depending on the operation principle of PWM converters, this paper reconstructs the capacitor current and monitors the bus voltage during the discharge of the DC-Link capacitor, and its capacitance and ESR is estimated.A variable discharge net is designed to estimate the C and ESR of the DC-Link capacitors in three phase AC/DC/AC PWM systems. During the discharge of the system, the DC-Link capacitors are discharged via the variable discharge net, and the capacitance C and equivalent resistance ESR are estimated accurately.Finally, to verify the semi-online monitoring method with the variable discharge net, a prototype has been made and tested. The experimental results show that the capacitance has been estimated within 3% error bound and the ESR has been estimated within 5% error bound.