Investigation of acoustic noise sources in medium frequency, medium voltage transformers

Medium voltage, medium frequency transformers (MFTs) are much smaller in size and weight compared to conventional low frequency transformers. The MFTs are very attractive for applications where full control of the power flow and high power density are required, such as power electronic interfaces in smart grids and traction converter systems. Due to the limitation of high voltage semiconductor switches, the MFTs are usually operated in the kHz range, which results in acoustic noise radiated from the transformer. In this paper, the origins of acoustic noise associated with MFTs are investigated based on vibration and acoustic measurements. The investigation focuses on the core materials widely used for MFTs, i.e. the amorphous and the nanocrystalline alloy. Experimental results show that the nanocrystalline core features lower level of vibration and acoustic noise emission than the amorphous core. Comparing core shapes, the toroidal core has better vibration and acoustic performance than the U-shape core. The existence of air gap in case of a cut core leads to excessive vibrations and therefore higher acoustic noise level due to increased magnetic forces compared with uncut core. Based on analysis of measurement results, recommendations for low noise MFT design are proposed.