Condition Monitoring for Submodule Capacitors in Modular Multilevel Converters

Abstract

This letter proposes a method for the condition monitoring of submodule capacitors in modular multilevel converters (MMCs) without additional circuitry or computationally heavy algorithm. The proposed method leverages the discharging curve of submodule capacitors in connection with its parallel bleeding resistors. It is independent of the MMC control and modulation schemes. Moreover, it potentially does not require thermal and load related calibration for capacitor degradation monitoring. The principle, case study, and proof-ofconcept of the proposed method are presented.

Review of Health Monitoring Techniques for Capacitors Used in Power Electronics Converters

“…The reliability of the RSM method is mainly based on the appropriate selection of an RSM; thus, an accurate selection process is necessary. In [75,76], the SM from the operating MMC was isolated and the curve of the capacitor voltage in the discharging process was utilized to obtain the health condition of a capacitor. During the discharging process shown in Figure 14, the state of switch S 2 is ON, whereas that of switch S 1 is OFF.…”

“…Thus, the capacitor only discharges through the bleeding resistor R b , which absorbs the energy of the capacitor when the SM is not in operation. The health condition of the capacitor could be monitored through the decrease in discharging time, as follows [76]:…”

“…This method monitors the capacitor of the SM without necessitating heavy computation or requiring the arm current or present switching state. However, isolating an SM from an operating MMC could lead to an increase in the electrical stress acting on the remaining SMs; it can also lead to negative effects on the output voltage and current in the absence of redundant SMs or if the number of SMs is insufficient [76].…”

“…In [ 75 , 76 ], the SM from the operating MMC was isolated and the curve of the capacitor voltage in the discharging process was utilized to obtain the health condition of a capacitor. During the discharging process shown in Figure 14 , the state of switch is ON, whereas that of switch is OFF.…”

“…These errors could lead to incorrect decisions regarding maintenance, thereby increasing expenditure. Various studies [ 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 ], which are discussed herein, have reported on the health monitoring of capacitors. Capacitance monitoring methods are capable of estimating the capacitance of both Al-Caps and MPPF-Caps; therefore, they would be more beneficial and suitable for different applications.…”