This paper analyzes two modular power converter concepts, based on Cascaded H-Bridges (CHB) and Modular Multilevel Converter (MMC) topologies respectively, with special attention to the second design. Both have common characteristics and can provide the required functionalities for Power Electronic Transformers (PETs). Criteria for their analysis will include aspects like number of required cells, characteristics of the power devices, functionalities and potential uses. Conventional Line-Frequency Transformers (LFTs) are key elements in transmission and distribution systems to interface the different voltage levels in the grid. LFTs are a well established technology, they are relatively cheap, efficient and reliable. However, they have several limitations, including: voltage drop under load; sensitivity to harmonics, load imbalances and DC offsets; no overload protection and low efficiency when operate with low load levels or no load [1]. PETs, also called Solid State Transformers (SSTs), are envisioned as a semiconductor based alternative to LFTs. PETs are able to provide advanced functionalities such as power flow control, reactive power, harmonics and imbalances compensation, availability of low voltage DC (LVDC) link and smart protection. High switching frequencies of the semiconductors also enable a significant reduction of the volume and weight of the core material [1]-[4]. Generally speaking, PETs are expected to beat the LFTs in terms of power density and much superior functionalities, but would be inferior in terms of cost, efficiency (full load) and reliability. Based on this, there is a number of applications in which the use of PETs can be advantageous compared to standard LFTs. Smart-grid applications require an efficient integration of distributed generation and storage resources, flexible routing mechanisms, active filtering and protection mechanism. While power density might not be a key aspect for onshore applications, it can be of paramount importance for offshore applications [5]. Traction and subsea systems are also examples of space-critical applications in which the improved performances and power density of PETs compared to LFTs can be determinant [6], [7].

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The converse magnetoelectric (CME) coefficient of an artificial, multiferroic composite cylinder was determined for two interface boundary conditions; specifically epoxy-bonded and shrink-fit. The composite consists of two concentrically bonded rings with the inner and outer rings made from Terfenol-D and lead zirconate titanate, respectively. The diameter of the inner annulus was 25 mm, and the outer ring diameter was 30 mm. Electric fields ranging from 20 kV/m to 80 kV/m with AC components cycling at frequencies from 4 kHz to 50 kHz were applied for actuation of the composite. A magnetic bias field from 0 Oe to 2300 Oe was applied for enhancement of the CME coefficient. It has been found that the maximum CME for epoxy-bonded rings occurs at lower bias magnetic field than shrink-fitted rings. Resonance for the epoxy-bonded composite was found to be 36 kHz whereas the resonant frequency for the shrink-fit structure was 34 kHz. The maximum CME coefficients were approximately 535 mG/V at 100 Vpp and 330 mG/V at 400 Vpp for the epoxy-bonded and shrink-fit configurations, respectively.