Failure Mechanism of Metallized Film Capacitors under DC Field Superimposed AC Harmonic

Researchers from the Research Institute, Nanjing, China, published an article on the degradation behaviour of metallised film capacitors, which are essential components for the stability of converter valves in flexible ultra-high-voltage direct current (HVDC) transmission systems.

The article was published by the Institution of Engineering and Technology.

Through systematic experimentation, the authors investigated the failure mechanisms of MFCs under HVDC fields with superimposed harmonics, considering both equipment and material perspectives.

The experiments subjected capacitors to 500 h of ageing under two conditions: a DC/AC-superimposed field with a constant DC component of 290 kV/mm and an AC ripple rate varying from 12% to 28%, and a control group aged solely under a DC field.

The findings indicate that capacitors aged under the DC/AC-superimposed field exhibited shorter lifespans and more significant capacitance loss than those aged under only the DC field. This difference in performance is primarily attributed to the distinct electrode loss behaviours observed under each ageing condition, which are key factors in the capacitors’ capacitance decay.

Moreover, the biaxially oriented polypropylene films in the DC-aged samples showed more severe deterioration, characterised by more noticeable molecular chain scission and reduced breakdown strength, compared to those aged under the DC/AC superimposed field. This difference is partly due to the moderate temperature increase caused by harmonics, which benefits the aggregation structure, and partly to the reduced molecular structure damage from the AC field.

Background of the Study

Metallised film capacitors (MFCs) have been found to have extensive applications in flexible DC transmission systems for voltage support and harmonic filtering. Compared with traditional power capacitors, the operation condition of the MFC in the flexible DC transmission system is much more stringent with the ultra-high DC field of more than 200 kV/mm superimposed complicated AC harmonics. However, there still is a shortage of systematic studies on the degradation behaviour and failure mechanism of the MFC under the superimposed harmonics conditions.

According to extensive studies about the ageing analysis under a single condition, the failure mechanism of the MFC under the sole DC or AC field was different. For the DC conditions, researchers found that the long-term DC field encouraged the injection of electrons into the biaxially oriented polypropylene (BOPP) films, causing localised degradation that resulted in the formation of “weak points.” Our previous work investigated the degradation mechanism of the MFC under the DC field strength, which was more than 200 kV/mm.

The results revealed that the high DC field resulted in the formation of weakness and the destruction of molecular structure, ultimately leading to degradation. For the traditional DC filter capacitor, the researchers found the continuous injection of electrons and molecular rearrangement were the main reasons for the decrease in electrical strength of the dielectric materials. For the sole AC field conditions, the existing research pointed out that the AC electrical force would make the PP films degrade, break down into small molecules, and produce large amounts of free radicals and that the peak value of the applied voltage had a significant effect on the lifetime of the dielectric materials, followed by the waveform and the mean square values of the voltage.

Besides, some research studies focused on the superimposed conditions of the impulse voltage. These studies revealed that the surface charge behaviour of polypropylene films was complex, resulting from a combination of the DC voltage and pulsed voltage and that electrochemical corrosion phenomena were observed in metallised films near the electrode edge. In general, extensive research had been conducted on the MFC under a single electric field, and the ageing mechanism for each condition had been clearly elucidated. Nevertheless, the failure mechanism of the MFC under the DC/AC-superimposed condition has become a challenge that requires further research.

In the present study, a comprehensive investigation was conducted to reveal the failure mechanism of the MFC from the capacitor equipment to the internal materials under the superimposed harmonic condition. The 500 h accelerated ageing test was performed on the MFC elements under the DC voltage superimposed at a different AC voltage, and the combined samples aged under the DC field of the same U_peak were involved as the reference.

The systemic studies, including the capacitance decay of the MFC equipment, the electrode degradation of metallised films, and the performance deterioration of BOPP base films, were carried on. From the aspect of capacitor equipment, the capacitance of the MFC under the DC/AC-superimposed conditions presented faster decay compared with the DC ageing capacitors. From the internal materials, the comparative results of metallised films revealed the mechanism of the shortened lifetime with the introduction of harmonics, while the studies on the based films provided the different impact of the DC/AC voltage on the molecular structures compared with the DC voltage.

This study could provide data and theoretical support for the experimental design and long-term performance of the MFC under the DC field superimposed harmonic conditions.

Discussions

Based on the presented results, it could be concluded that introducing harmonics would cause a faster and greater capacitance reduction compared to the single DC condition from the aspect of the equipment. The electrode degradation behaviour of internal metallised films could explain the difference between capacitance reduction and failure mechanism under the DC/AC field and the sole DC field for the capacitor equipment. However, the microstructure and insulating performance of internal dielectric materials did not deteriorate along with the same trend. The deterioration mechanism of dielectric materials required further discussion.

The decreased BDS was mostly attributed to the degradation of microstructures; therefore, the DMA test was employed to investigate the microstructures of BOPP films further. The T_g value of each sample increased with the rise in frequency. This is because the rearrangement rate of molecular chain segments did not synchronise completely with the frequency of dynamic external forces, necessitating additional external energy to complete the movement. The Tg value of both the DC- and DC/AC-aged samples decreased to less than 10°C, indicating the reduction in the ability of molecular chain segments to motion in amorphous regions.

The less DC voltage content would cause less charge injection, thus attenuating the electrical damage. On the other hand, the implementation of a harmonic field caused the temperature to increase by about 5°C. The temperature rise was caused by the gradual accumulation of Joule heat due to harmonics. A rise in temperature within a certain range would optimise the aggregation structure of polypropylene. This process can be summarised as the temperature promotes the movement of polymer chains, causing them to rearrange along a more ordered direction, leading to recrystallisation. The manifestation could be characterised as higher crystallinity and an increase in the activation energy of the molecular chain movement. It was worth saying that there was a threshold for the temperature rise caused by the introduction of ripple voltage, resulting in the actual temperature not exceeding 65°C, which was far away from the degradation temperature of PP.

During the ageing, the ultra-high DC voltage continuously injected electrons into the films, which caused the scission of molecular chains and the degradation of aggregation structure. This was reflected in the decrease in the crystallinity and the activation energy of molecular segment motion. All of these structural changes led to the reduction of the residual BDS. The structure degradation of the DC ageing samples was more severe than the DC/AC samples; therefore, the characteristic BDS of samples of the DC group was lower than that of the DC/AC group at the same U_peak. Besides, the limited harmonic ratio will cause a moderate temperature rise, which will encourage the rearrangement of molecular chains during ageing, resulting in a modified aggregation structure. Consequently, the BDS with the failure rate of 5% of the DC/AC group was much higher than the DC group since the encouraged molecular motion could somewhat optimise the weak points’ structure. It should be emphasised that this did not mean the higher the temperature the capacitor withstood, the less performance degradation it caused. Further research on the ageing mechanism of MFC under the influence of temperature is still needed.

Conclusions

In this study, we conducted accelerated ageing tests on MFCs under both DC/AC-superimposed conditions and solely under DC voltage for comparison. Through comprehensive monitoring of capacitor elements, along with structural and electrical analysis of dielectric films, we have arrived at the following key findings:

• (1)The presence of harmonics significantly accelerated the failure of MFCs, leading to a reduced lifespan for capacitors aged under DC/AC voltage compared to those subjected to the same peak DC voltage. The increase in the ripple ratio caused a notable “two-step” trend in capacitance decay, shifting from “slow-to-fast” to “fast-to-slow.” Consequently, capacitors aged under a C_1.6-DC/AC condition for 500 h exhibited a capacitance loss of 18.83%, 45% greater than those aged under a C_1.6-DC condition.
• (2)Under the DC/AC condition, the metallised film of MFCs experienced electrochemical erosion, displaying erosion spots with distinct morphologies compared to standard SH points. The elevated AC voltage exacerbated this electrochemical erosion, leading to a more rapid initial stage of capacitance loss in MFCs aged under DC/AC voltage compared to those aged under DC voltage.
• (3)The deterioration of BOPP was more severe under DC conditions than under DC/AC conditions, evidenced by more significant molecular chain scission and degraded aggregation structure. As a result, BOPP films aged under DC conditions had a lower residual BDS compared to those aged under DC/AC conditions.