Temperature Considerations for Online Health Monitoring of DC-Link Capacitors in Modular Multilevel Converters

Practical Online Condition Monitoring of DC-Link Capacitors in Modular Multilevel Converters: A Comparative Approach

“…The capacitance of MPPF capacitors has a negative temperature coefficient [21]. This means that at higher temperatures, the capacitance value drops.…”

“…For AE capacitors, the EoL is defined as 20% reduction in capacitance and/or 200% increase in ESR, while for the MLC capacitors, a 10% reduction in capacitance is reported as EoL [18]. Although degraded MPPF capacitors have also been shown to have an increase in their ESR [19], [20], this value is very small in high-current applications [21], and difficult to track accurately through online measurements. Nevertheless, the online estimation of capacitance must be sufficiently accurate in order to precisely detect the 5% drift in capacitance.…”

“…In [22], online tracking of the ESR has been experimentally shown. However, the level of ESRs in that study are orders of magnitude larger than what highpower capacitors typically have have [21]. A method based on detecting the time constant of resistive-capacitive (RC) discharge is presented in [23] where a modified operation of each submodule is necessary to assure accurate estimations, and a known resistive element must be introduced to realize the CM techniques.…”

“…That is the effect of temperature. The capacitance of MPPF capacitors are reported to have a negative temperature coefficient [21], while the capacitance of AE capacitors typically have a positive temperature coefficient [27]. Consequently, changes in the ambient temperature, as well as changes in the converter load may result in temperature variation of the capacitors.…”

“…If not compensated for, such variations may be wrongly attributed to degradation of the monitored capacitor. In [21], a dynamic margin is proposed to compensate for temperature variations, however, this method might cause overcompensation if the thermal model of the capacitor is not correctly modelled. In this paper, a novel SoH estimation approach is proposed that enables online compensation for temperature-related variations of the estimated capacitance values.…”