PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 93 NR 4/2017 1 Ukazuje się od 1919 roku 4'17 Organ Stowarzyszenia Elektryków Polskich  Wydawnictwo SIGMA-NOT Sp. z o.o. Tomasz GAJOWIK 1 , Krzysztof RAFAŁ 2 , Mariusz MALINOWSKI 1 Warsaw University of Technology, Institute of Control and Industrial Electronics (1), Institute of Heat Engineering (2) doi:10.15199/48.2017.04.01 Review of multilevel converters for application in solid state transformers Abstract. This paper presents an overview of AC-DC, DC-AC and DC-DC power converter topologies used in the context of the application of solid state transformers. It describes the features of individual solutions, with an emphasis on multi-level topologies connected to the medium voltage AC grid, what allows to apply commonly available and relatively inexpensive low-voltage semiconductor switches. Such systems maintain high efficiency as well as high switching frequency, resulting in reduction of converter passive elements volume and weight as well as overall price. Streszczenie. Niniejszy artykuł przedstawia przegląd topologii przekształtników energoelektronicznych AC-DC oraz DC-DC, pod kątem zastosowania w energoelektronicznych transformatorach inteligentnych. Opisano w nim cechy poszczególnych rozwiązań, ze szczególnym naciskiem na przekształtniki wielopoziomowe, które pracując wraz z siecią elektroenergetyczną średniego napięcia (SN), umożliwiają zastosowanie standardowych i stosunkowo tanich niskonapięciowych łączników półprzewodnikowych, a także zmniejszenie rozmiarów, wagi i ceny transformatora. (Przegląd topologii przekształtników wielopoziomowych pod kątem zastosowania w inteligentnym transformatorze). Keywords: power electronics, smart transformer, multi-level converters, SST. Słowa kluczowe: energoelektronika, inteligentny transformator, przekształtniki wielopoziomowe. Introduction Development of Smart Grid (SG) calls for flexible solutions to control power flow at a distribution level. Recently a group of devices called Custom Power Systems have emerged. By definition these are power electronic converters applied to distributions systems that are designed to control power flow and power quality. To improve SG functionality, there is a desire to implement power conversion and power control functionality in one device. Future trend is to replace conventional distribution transformers with “smart” solutions, based on power electronic converters. Therefore application of power electronic converters as a distribution transformer have become a wide area of interest. Increasing availability of medium voltage IGBT’s is making applications of power electronic converters directly with Medium Voltage (MV) grid technically and economically feasible. However, current state-of art semiconductor devices are still not ready to operate with MV directly. Common voltage classes of semiconductors are: 600V, 1200V, 1700V, 3.3kV and 6,5kV. Applicable solutions include: connection of a converter via LV/MV transformer, series connection of semiconductor devices or multilevel converter topologies. Although multilevel converters have been described in literature since 1980’s [1], they have gained wide interest in industry just lately. The falling price of standard semiconductor devices, the availability of advanced DSP and FPGA control platforms and the trend of reducing the overall dimensions of devices and passive components, as well as increasing the switching frequency and maintaining high performance are only some of a number of factors influencing the high interest in multi-level power electronic converter systems. Solid State Transformer concept SST is designed to replace conventional distribution transformer that is to convert MV AC to LV AC at power levels ranging from few hundred kVA up to few MVA. Typical arrangement of SST is presented in Fig. 1 SST performs multistage power conversion:  Medium Voltage AC-DC Converter (MVC) converts MV AC to MV DC,  DC-DC converter reduces DC voltage level from MV to LV. This converter incorporates medium frequency transformer (MFT), that provides galvanic isolation between circuits,  Low Voltage DC-AC converter (LVC) converts LV DC to LV AC. Fig. 1. Block scheme of SST with medium frequency intermediate transformer Presented structure is only one of possible solutions, however, this structure is the most advantageous [2]. Although SST structure is much more complex compared to conventional transformer it may eliminate some of the disadvantages and add a completely new functionality that was not yet available, such as:  The presence of DC-link allows to direct connection of DC grid as well as additional energy storage components, eliminating the need for application of additional power converters,