Towards Customized Capacitor Design for an MMC Submodule

Metallized polypropylene film capacitors (MPPFC) are used in modular multilevel converters (MMC) for their high breakdown voltage, low loss, and high reliability. However, the low dielectric constant of commercially available polypropylene materials leads to large size and weight of the capacitors used in the MMC submodules. This paper pursues a method to reduce the capacitor by tailoring the dielectric material properties according to the expected duties: increasing the dielectric constant using barium titanate (BT) and boron nitride (BN) as nano-fillers. With suitable material composition, trade-off between material properties can be obtained to favor the specific application. Initial experimental results show that the relative dielectric constant of the film material can be increased from 2.298 to 2.821, without unacceptably reducing the breakdown strength or the power loss performance. Correspondingly, the capacitor volume can be reduced by 18.5%. To constrain the unfavorable temperature rise caused by the increase of power loss in this process, the relationship between the temperature rise and the capacitor’s structural parameters is also studied. The final designed capacitor is not only reduced in size, but also has a limited temperature rise. The potential of further improvement is discussed.